Kea Administrator Reference Manual

This is the reference guide for Kea version 1.5.0.

Abstract

Kea is an open source implementation of the Dynamic Host Configuration Protocol (DHCP) servers, developed and maintained by Internet Systems Consortium (ISC).

This is the reference guide for Kea version 1.5.0. The most up-to-date version of this document (in PDF, HTML, and plain text formats), along with other documents for Kea, can be found at kb.isc.org/docs.


Table of Contents

1. Introduction
1.1. Supported Platforms
1.2. Required Software at Run-time
1.3. Kea Software
2. Quick Start
2.1. Quick Start Guide for DHCPv4 and DHCPv6 Services
2.2. Running the Kea Servers Directly
3. Installation
3.1. Packages
3.2. Installation Hierarchy
3.3. Building Requirements
3.4. Installation from Source
3.4.1. Download Tar File
3.4.2. Retrieve from Git
3.4.3. Configure Before the Build
3.4.4. Build
3.4.5. Install
3.5. DHCP Database Installation and Configuration
3.5.1. Building with MySQL Support
3.5.2. Building with PostgreSQL support
3.5.3. Building with CQL (Cassandra) support
4. Kea Database Administration
4.1. Databases and Database Version Numbers
4.2. The kea-admin Tool
4.3. Supported Databases
4.3.1. memfile
4.3.2. MySQL
4.3.3. PostgreSQL
4.3.4. CQL (Cassandra)
4.3.5. Using Read-Only Databases with Host Reservations
4.3.6. Limitations Related to the use of SQL Databases
5. Kea Configuration
5.1. JSON Configuration
5.1.1. JSON Syntax
5.1.2. Simplified Notation
6. Managing Kea with keactrl
6.1. Overview
6.2. Command Line Options
6.3. The keactrl Configuration File
6.4. Commands
6.5. Overriding the Server Selection
7. Kea Control Agent
7.1. Overview
7.2. Configuration
7.3. Secure Connections
7.4. Starting Control Agent
7.5. Connecting to the Control Agent
8. The DHCPv4 Server
8.1. Starting and Stopping the DHCPv4 Server
8.2. DHCPv4 Server Configuration
8.2.1. Introduction
8.2.2. Lease Storage
8.2.3. Hosts Storage
8.2.4. Interface Configuration
8.2.5. Issues with Unicast Responses to DHCPINFORM
8.2.6. IPv4 Subnet Identifier
8.2.7. Configuration of IPv4 Address Pools
8.2.8. Standard DHCPv4 Options
8.2.9. Custom DHCPv4 options
8.2.10. DHCPv4 Private Options
8.2.11. DHCPv4 Vendor Specific Options
8.2.12. Nested DHCPv4 Options (Custom Option Spaces)
8.2.13. Unspecified Parameters for DHCPv4 Option Configuration
8.2.14. Stateless Configuration of DHCPv4 Clients
8.2.15. Client Classification in DHCPv4
8.2.16. DDNS for DHCPv4
8.2.17. Next Server (siaddr)
8.2.18. Echoing Client-ID (RFC 6842)
8.2.19. Using Client Identifier and Hardware Address
8.2.20. Authoritative DHCPv4 Server Behavior
8.2.21. DHCPv4-over-DHCPv6: DHCPv4 Side
8.2.22. Sanity checks in DHCPv4
8.3. Host Reservation in DHCPv4
8.3.1. Address Reservation Types
8.3.2. Conflicts in DHCPv4 Reservations
8.3.3. Reserving a Hostname
8.3.4. Including Specific DHCPv4 Options in Reservations
8.3.5. Reserving Next Server, Server Hostname and Boot File Name
8.3.6. Reserving Client Classes in DHCPv4
8.3.7. Storing Host Reservations in MySQL, PostgreSQL or Cassandra
8.3.8. Fine Tuning DHCPv4 Host Reservation
8.3.9. Global reservations in DHCPv4
8.4. Shared networks in DHCPv4
8.4.1. Local and relayed traffic in shared networks
8.4.2. Client classification in shared networks
8.4.3. Host reservations in shared networks
8.5. Server Identifier in DHCPv4
8.6. How the DHCPv4 Server Selects a Subnet for the Client
8.6.1. Using a Specific Relay Agent for a Subnet
8.6.2. Segregating IPv4 Clients in a Cable Network
8.7. Duplicate Addresses (DHCPDECLINE Support)
8.8. Statistics in the DHCPv4 Server
8.9. Management API for the DHCPv4 Server
8.10. Supported DHCP Standards
8.11. User contexts in IPv4
8.12. DHCPv4 Server Limitations
8.13. Kea DHCPv4 server examples
9. The DHCPv6 Server
9.1. Starting and Stopping the DHCPv6 Server
9.2. DHCPv6 Server Configuration
9.2.1. Introduction
9.2.2. Lease Storage
9.2.3. Hosts Storage
9.2.4. Interface Selection
9.2.5. IPv6 Subnet Identifier
9.2.6. Unicast Traffic Support
9.2.7. Subnet and Address Pool
9.2.8. Subnet and Prefix Delegation Pools
9.2.9. Prefix Exclude Option
9.2.10. Standard DHCPv6 Options
9.2.11. Common Softwire46 Options
9.2.12. Custom DHCPv6 Options
9.2.13. DHCPv6 Vendor-Specific Options
9.2.14. Nested DHCPv6 Options (Custom Option Spaces)
9.2.15. Unspecified Parameters for DHCPv6 Option Configuration
9.2.16. IPv6 Subnet Selection
9.2.17. Rapid Commit
9.2.18. DHCPv6 Relays
9.2.19. Relay-Supplied Options
9.2.20. Client Classification in DHCPv6
9.2.21. DDNS for DHCPv6
9.2.22. DHCPv4-over-DHCPv6: DHCPv6 Side
9.2.23. Sanity checks in DHCPv6
9.3. Host Reservation in DHCPv6
9.3.1. Address/Prefix Reservation Types
9.3.2. Conflicts in DHCPv6 Reservations
9.3.3. Reserving a Hostname
9.3.4. Including Specific DHCPv6 Options in Reservations
9.3.5. Reserving Client Classes in DHCPv6
9.3.6. Storing Host Reservations in MySQL, PostgreSQL or Cassandra
9.3.7. Fine Tuning DHCPv6 Host Reservation
9.3.8. Global reservations in DHCPv6
9.4. Shared networks in DHCPv6
9.4.1. Local and relayed traffic in shared networks
9.4.2. Client classification in shared networks
9.4.3. Host reservations in shared networks
9.5. Server Identifier in DHCPv6
9.6. Stateless DHCPv6 (Information-Request Message)
9.7. Support for RFC 7550 (being now part of RFC 8415)
9.8. Using Specific Relay Agent for a Subnet
9.9. Segregating IPv6 Clients in a Cable Network
9.10. MAC/Hardware Addresses in DHCPv6
9.11. Duplicate Addresses (DECLINE Support)
9.12. Statistics in the DHCPv6 Server
9.13. Management API for the DHCPv6 Server
9.14. User contexts in IPv6
9.15. Supported DHCPv6 Standards
9.16. DHCPv6 Server Limitations
9.17. Kea DHCPv6 server examples
10. Lease Expiration in DHCPv4 and DHCPv6
10.1. Lease Reclamation
10.2. Configuring Lease Reclamation
10.3. Configuring Lease Affinity
10.4. Default Configuration Values for Leases Reclamation
10.5. Reclaiming Expired Leases with Command
11. Congestion Handling in DHCPv4 and DHCPv6
11.1. What is Congestion?
11.2. Configuring Congestion Handling
12. The DHCP-DDNS Server
12.1. Overview
12.1.1. DNS Server selection
12.1.2. Conflict Resolution
12.1.3. Dual Stack Environments
12.2. Starting and Stopping the DHCP-DDNS Server
12.3. Configuring the DHCP-DDNS Server
12.3.1. Global Server Parameters
12.3.2. TSIG Key List
12.3.3. Forward DDNS
12.3.4. Reverse DDNS
12.3.5. User context in DDNS
12.3.6. Example DHCP-DDNS Server Configuration
12.4. DHCP-DDNS Server Limitations
13. The LFC process
13.1. Overview
13.2. Command Line Options
14. Client Classification
14.1. Client Classification Overview
14.2. Builtin Client Classes
14.3. Using Expressions In Classification
14.3.1. Logical operators
14.3.2. Substring
14.3.3. Concat
14.3.4. Ifelse
14.3.5. Hexstring
14.4. Configuring Classes
14.5. Using Static Host Reservations In Classification
14.6. Configuring Subnets With Class Information
14.7. Configuring Pools With Class Information
14.8. Using Classes
14.9. Classes and Hooks
14.10. Debugging Expressions
15. Hooks Libraries
15.1. Introduction
15.2. Installing Hook packages
15.3. Configuring Hooks Libraries
15.4. Available Hooks Libraries
15.4.1. user_chk: Checking User Access
15.4.2. legal_log: Forensic Logging Hooks
15.4.3. flex_id: Flexible Identifiers for Host Reservations
15.4.4. host_cmds: Host Commands
15.4.5. lease_cmds: Lease Commands
15.4.6. subnet_cmds: Subnet Commands
15.4.7. class_cmds: Class Commands
15.4.8. ha: High Availability
15.4.9. stat_cmds: Supplemental Statistics Commands
15.4.10. radius: RADIUS server support
15.4.11. host_cache: Caching Host Reservations
15.5. User contexts
16. Statistics
16.1. Statistics Overview
16.2. Statistics Lifecycle
16.3. Commands for Manipulating Statistics
16.3.1. statistic-get command
16.3.2. statistic-reset command
16.3.3. statistic-remove command
16.3.4. statistic-get-all command
16.3.5. statistic-reset-all command
16.3.6. statistic-remove-all command
17. Management API
17.1. Data Syntax
17.2. Using the Control Channel
17.3. Commands Supported by Both the DHCPv4 and DHCPv6 Servers
17.3.1. build-report
17.3.2. config-get
17.3.3. config-reload
17.3.4. config-test
17.3.5. config-write
17.3.6. leases-reclaim
17.3.7. libreload
17.3.8. list-commands
17.3.9. config-set
17.3.10. shutdown
17.3.11. dhcp-disable
17.3.12. dhcp-enable
17.3.13. version-get
17.4. Commands Supported by Control Agent
18. The libdhcp++ Library
18.1. Interface detection and Socket handling
19. Logging
19.1. Logging Configuration
19.1.1. Loggers
19.1.2. Logging Message Format
19.1.3. Logging During Kea Startup
20. The Kea Shell
20.1. Overview
20.2. Shell Usage
21. YANG/NETCONF Support
21.1. Overview
21.2. Installing NETCONF
21.2.1. Installing NETCONF on Ubuntu 18.04
21.2.2. Installing NETCONF on CentOS 7.5
21.3. Quick sysrepo overview
21.4. Supported YANG models
21.5. Using the Netconf agent
21.6. Configuration
21.7. Kea-netconf Configuration Example
21.8. Starting and Stopping the Netconf Agent
21.9. Step by Step Netconf Operation Example
21.9.1. Setup of Netconf Operation Example
21.9.2. Error handling in Netconf Operation Example
21.9.3. Netconf Operation Example with Two Pools
21.9.4. Netconf Operation Example with Two Subnets
21.9.5. Netconf Operation Example with Logging
A. API Reference
A.1. build-report reference
A.2. cache-clear reference
A.3. cache-get reference
A.4. cache-insert reference
A.5. cache-load reference
A.6. cache-remove reference
A.7. cache-write reference
A.8. class-add reference
A.9. class-del reference
A.10. class-get reference
A.11. class-list reference
A.12. class-update reference
A.13. config-get reference
A.14. config-reload reference
A.15. config-set reference
A.16. config-test reference
A.17. config-write reference
A.18. dhcp-disable reference
A.19. dhcp-enable reference
A.20. ha-continue reference
A.21. ha-heartbeat reference
A.22. ha-scopes reference
A.23. ha-sync reference
A.24. lease4-add reference
A.25. lease4-del reference
A.26. lease4-get reference
A.27. lease4-get-all reference
A.28. lease4-update reference
A.29. lease4-wipe reference
A.30. lease6-add reference
A.31. lease6-del reference
A.32. lease6-get reference
A.33. lease6-get-all reference
A.34. lease6-update reference
A.35. lease6-wipe reference
A.36. leases-reclaim reference
A.37. libreload reference
A.38. list-commands reference
A.39. network4-add reference
A.40. network4-del reference
A.41. network4-get reference
A.42. network4-list reference
A.43. network4-subnet-add reference
A.44. network4-subnet-del reference
A.45. network6-add reference
A.46. network6-del reference
A.47. network6-get reference
A.48. network6-list reference
A.49. network6-subnet-add reference
A.50. network6-subnet-del reference
A.51. reservation-add reference
A.52. reservation-del reference
A.53. reservation-get reference
A.54. shutdown reference
A.55. stat-lease4-get reference
A.56. stat-lease6-get reference
A.57. statistic-get reference
A.58. statistic-get-all reference
A.59. statistic-remove reference
A.60. statistic-remove-all reference
A.61. statistic-reset reference
A.62. statistic-reset-all reference
A.63. subnet4-add reference
A.64. subnet4-del reference
A.65. subnet4-get reference
A.66. subnet4-list reference
A.67. subnet6-add reference
A.68. subnet6-del reference
A.69. subnet6-get reference
A.70. subnet6-list reference
A.71. version-get reference
B. Acknowledgments

List of Tables

4.1. List of available backends
8.1. List of standard DHCPv4 options
8.2. List of standard DHCP option types
8.3. Default FQDN Flag Behavior
8.4. DHCPv4 Statistics
9.1. List of Standard DHCPv6 Options
9.2. List of Experimental DHCPv6 Options
9.3. Default FQDN Flag Behavior
9.4. DHCPv6 Statistics
12.1. Our example network
12.2. Forward DDNS Domains Needed
12.3. Reverse DDNS Domains Needed
14.1. List of Classification Values
14.2. List of Classification Expressions
15.1. List of available hooks libraries
15.2. Default behavior of the server in various HA states
19.1. List of loggers supported by Kea servers and hooks libraries shipped with Kea and premium packages

Chapter 1. Introduction

Kea is the next generation of DHCP software developed by ISC. It supports both DHCPv4 and DHCPv6 protocols along with their extensions, e.g. prefix delegation and dynamic updates to DNS.

Kea was initially developed as a part of the BIND 10 framework. In early 2014, ISC made the decision to discontinue active development of BIND 10 and continue development of Kea as standalone DHCP software.

This guide covers Kea version 1.5.0.

1.1. Supported Platforms

Kea is officially supported on CentOS, Fedora, Ubuntu, Debian and FreeBSD systems. It is also likely to work on many other platforms. Kea-1.5.0 builds have been tested on:

  • CentOS Linux — 7.1804 (aka 7.5)
  • Fedora — 28, 29
  • Ubuntu — 16.04, 18.04
  • Debian GNU/Linux — 7, 8, 9
  • FreeBSD — 11.0
  • macOS — 10.13, 10.14

There are currently no plans to port Kea to Windows platforms.

1.2. Required Software at Run-time

Running Kea uses various extra software which may not be provided in the default installation of some operating systems, nor in the standard package collections. You may need to install this required software separately. (For the build requirements, also see Section 3.3, “Building Requirements”.)

  • Kea supports two crypto libraries: Botan and OpenSSL. Only one of them is required to be installed during compilation. Kea uses the Botan crypto library for C++ (http://botan.randombit.net/), version 1.9 or later. Note that support for Botan versions earlier than 2.0 will be dropped in Kea 1.6.0 release. As an alternative to Botan, Kea can use the OpenSSL crypto library (http://www.openssl.org/), version 1.0.2 or later.
  • Kea uses the log4cplus C++ logging library (http://log4cplus.sourceforge.net/). It requires log4cplus version 1.0.3 or later.
  • Kea requires Boost system library (http://www.boost.org/) at run-time, as building with header only version of Boost is no longer recommended.
  • In order to store lease information in a MySQL database, Kea requires MySQL headers and libraries. This is an optional dependency in that Kea can be built without MySQL support.
  • In order to store lease information in a PostgreSQL database, Kea requires PostgreSQL headers and libraries. This is an optional dependency in that Kea can be built without PostgreSQL support.
  • In order to store lease information in a Cassandra database (CQL), Kea requires Cassandra headers and libraries. This is an optional dependency in that Kea can be built without Cassandra support.
  • As of Kea 1.4.0 release, the integration with RADIUS is provided in Kea via the hooks library available to our Support Customers. Use of this library requires FreeRadius-client library to be present on the system where Kea is running. This is an optional dependency in that Kea can be built without RADIUS support.
  • As of 1.5.0 release, Kea provides NETCONF interface with the kea-netconf agent. This Kea module is built optionally and requires Sysrepo software when used. Building Kea with NETCONF support requires many dependencies to be installed which are described in more details in Section 21.2, “Installing NETCONF”.

1.3. Kea Software

Kea is modular. Part of this modularity is accomplished using multiple cooperating processes which, together, provide the server functionality. The following software is included with Kea:

  • keactrl — Tool to start, stop, reconfigure, and report status for the Kea servers.
  • kea-dhcp4 — The DHCPv4 server process. This process responds to DHCPv4 queries from clients.
  • kea-dhcp6 — The DHCPv6 server process. This process responds to DHCPv6 queries from clients.
  • kea-dhcp-ddns — The DHCP Dynamic DNS process. This process acts as an intermediary between the DHCP servers and DNS servers. It receives name update requests from the DHCP servers and sends DNS Update messages to the DNS servers.
  • kea-admin — A useful tool for database backend maintenance (creating a new database, checking versions, upgrading etc.)
  • kea-lfc — This process removes redundant information from the files used to provide persistent storage for the memfile data base backend. While it can be run standalone, it is normally run as and when required by the Kea DHCP servers.
  • kea-ctrl-agent — Kea Control Agent (CA) is a daemon that exposes a RESTful control interface for managing Kea servers.
  • kea-netconf - Kea netconf is an agent that provides a YANG/NETCONF interface for the Kea environment.
  • kea-shell — Simple text client that uses REST interface to connect to Kea Control Agent.
  • perfdhcp — A DHCP benchmarking tool which simulates multiple clients to test both DHCPv4 and DHCPv6 server performance.

The tools and modules are covered in full detail in this guide. In addition, manual pages are also provided in the default installation.

Kea also provides C++ libraries and programmer interfaces for DHCP. These include detailed developer documentation and code examples.

Chapter 2. Quick Start

This section describes the basic steps needed to get Kea up and running. For further details, full customizations, and troubleshooting, see the respective chapters in the Kea guide.

2.1. Quick Start Guide for DHCPv4 and DHCPv6 Services

  1. Install required run-time and build dependencies. See Section 3.3, “Building Requirements” for details.
  2. Download Kea source tarball from ISC.org downloads page or ISC ftp server.

  3. Extract the tarball. For example:

    $ tar xvzf kea-1.5.0.tar.gz

  4. Go into the source directory and run the configure script:

    $ cd kea-1.5.0
    $ ./configure [your extra parameters]

  5. Build it:

    $ make

  6. Install it (by default it will be placed in /usr/local/, so it is likely that you will need root privileges for this step):

    # make install

  7. Edit the Kea configuration files which by default are installed in the [kea-install-dir]/etc/kea/ directory. These are: kea-dhcp4.conf, kea-dhcp6.conf, kea-dhcp-ddns.conf and kea-ctrl-agent.conf, for DHCPv4 server, DHCPv6 server, D2 and Control Agent respectively.

  8. In order to start the DHCPv4 server in background, run the following command (as root):

    # keactrl start -s dhcp4

    Or run the following command to start DHCPv6 server instead:

    # keactrl start -s dhcp6

    Note that it is also possible to start all servers simultaneously:

    $ keactrl start

  9. Verify that Kea server(s) are running:

    # keactrl status

    A server status of "inactive" may indicate a configuration error. Please check the log file (by default named [kea-install-dir]/var/kea/kea-dhcp4.log, [kea-install-dir]/var/kea/kea-dhcp6.log, [kea-install-dir]/var/kea/kea-ddns.log or [kea-install-dir]/var/kea/kea-ctrl-agent.log) for the details of the error.

  10. If the server has been started successfully, test that it is responding to DHCP queries and that the client receives a configuration from the server; for example, use the ISC DHCP client.

  11. Stop running the server(s):

    # keactrl stop

For instructions specific to your system, please read the system specific notes, available in the Kea section of ISC's Knowledgebase.

The details of keactrl script usage can be found in Chapter 6, Managing Kea with keactrl.

2.2. Running the Kea Servers Directly

The Kea servers can be started directly, without the need to use the keactrl. To start the DHCPv4 server run the following command:

# kea-dhcp4 -c /path/to/your/kea4/config/file.json

Similarly, to start the DHCPv6 server run the following command:

# kea-dhcp6 -c /path/to/your/kea6/config/file.json

Chapter 3. Installation

3.1. Packages

Some operating systems or software package vendors may provide ready-to-use, pre-built software packages for Kea. Installing a pre-built package means you do not need to install the software required only to build Kea and do not need to make the software.

3.2. Installation Hierarchy

The following is the directory layout of the complete Kea installation. (All directory paths are relative to the installation directory):

  • bin/ — utility programs.
  • etc/kea/ — configuration files.
  • include/ — C++ development header files.
  • lib/ — libraries.
  • lib/hooks — additional hooks libraries.
  • sbin/ — server software and commands used by the system administrator.
  • share/kea/ — configuration specifications and examples.
  • share/doc/kea/ — this guide, other supplementary documentation, and examples.
  • share/man/ — manual pages (online documentation).
  • var/kea/ — server identification, lease databases, and log files.

3.3. Building Requirements

In addition to the run-time requirements (listed in Section 1.2, “Required Software at Run-time”), building Kea from source code requires various development include headers and program development tools.

Note

Some operating systems have split their distribution packages into a run-time and a development package. You will need to install the development package versions, which include header files and libraries, to build Kea from the source code.

Building from source code requires the following software installed on the system:

  • Boost C++ Libraries (http://www.boost.org/). The oldest Boost version used for testing is 1.57 (it may work with older versions). Boost system library is required. Building boost header only is no longer recommended.

  • Botan (at least version 1.9) or OpenSSL (at least version 1.0.1). Note that Botan version 2 or later and OpenSSL version 1.0.2 or 1.1.0 or later are strongly recommended.

  • log4cplus (at least version 1.0.3) development include headers.

  • A C++ compiler (with C++11 support) and standard development headers. Kea builds have been tested with GCC g++ 4.7.2 4.7.3 4.8.2 4.8.4 4.8.5 4.9.3 4.9.4 5.3.1 5.4.0 6.3.0 6.3.1 clang-800.0.38 clang-802.0.42 clang-900.0.37

  • The development tools automake, libtool, pkg-config.

  • The MySQL client and the client development libraries, when using the --with-mysql configuration flag to build the Kea MySQL database backend. In this case an instance of the MySQL server running locally or on a machine reachable over a network is required. Note that running the unit tests requires a local MySQL server.

  • The PostgreSQL client and the client development libraries, when using the --with-pgsql configuration flag to build the Kea PostgreSQL database backend. In this case an instance of the PostgreSQL server running locally or on some other machine, reachable over the network from the machine running Kea, is required. Note that running the unit tests requires a local PostgreSQL server.

  • Cpp-driver from DataStax is needed when using the --with-cql configuration flag to build Kea with Cassandra database backend. In this case an instance of the Cassandra server running locally or on some other machine, reachable over the network from the machine running Kea, is required. Note that running the unit tests requires a local Cassandra server.

  • FreeRADIUS client library when --with-freeradius configuration flag used.

  • Sysrepo (version 0.7.6 or later) and libyang (version 0.16-r2 or later) when --with-sysrepo configuration flag used.

  • googletest (version 1.8 or later), when using the --with-gtest configuration option to build the unit tests.

  • The documentation generation tools elinks, docbook-xsl, libxslt and Doxygen, if using the --enable-generate-docs configuration option to create the documentation.

Visit ISC's Knowledgebase at https://kb.isc.org/docs/installing-kea for system-specific installation tips.

3.4. Installation from Source

Kea is open source software written in C++. It is freely available in source code form from ISC as a downloadable tar file. A copy of the Kea source code repository is accessible from Gitlab (https://gitlab.isc.org/isc-projects/kea). Kea may also be available in pre-compiled ready-to-use packages from operating system vendors.

3.4.1. Download Tar File

The Kea release tarballs may be downloaded from: http://ftp.isc.org/isc/kea/ (using FTP or HTTP).

3.4.2. Retrieve from Git

Downloading this "bleeding edge" code is recommended only for developers or advanced users. Using development code in a production environment is not recommended.

Note

When building from source code retrieved via Git, additional software will be required: automake (v1.11 or later), libtoolize, and autoconf (v2.69 or later). These may need to be installed.

The latest development code is available on Gitlab (see https://gitlab.isc.org/isc-projects/kea). The Kea source is public and development is done in the master branch.

The code can be checked out from https://gitlab.isc.org/isc-projects/kea.git:

$ git clone https://gitlab.isc.org/isc-projects/kea.git

The code checked out from the git repository does not include the generated configure script, Makefile.in files, nor their related build files. They can be created by running autoreconf with the --install switch. This will run autoconf, aclocal, libtoolize, autoheader, automake, and related commands.

Write access to the Kea repository is only granted to ISC staff. If you are a developer planning to contribute to Kea, please check our Contributor's Guide. The Kea Developer's Guide contains more information about the process, as well as describes the requirements for contributed code to be accepted by ISC.

3.4.3. Configure Before the Build

Kea uses the GNU Build System to discover build environment details. To generate the makefiles using the defaults, simply run:

$ ./configure

Run ./configure with the --help switch to view the different options. Some commonly-used options are:

--prefix
Define the installation location (the default is /usr/local).
--with-boost-include
Define the path to find the Boost headers.
--with-botan-config
Specify the path to the botan-config script to build with Botan for cryptographic functions.
--with-mysql
Build Kea with code to allow it to store leases and host reservations in a MySQL database.
--with-pgsql
Build Kea with code to allow it to store leases and host reservations in a PostgreSQL database.
--with-cql
Build Kea with code to allow it to store leases and host reservations in a Cassandra (CQL) database.
--with-gtest, --with-gtest-source
Enable the building of the C++ Unit Tests using the Google Test framework. This option specifies the path to the gtest source. (If the framework is not installed on your system, it can be downloaded from https://github.com/google/googletest.) from https://github.com/google/googletest.)
--with-benchmark, --with-benchmark-source
Enable the building of the database backend benchmarks using the Google Benchmark framework. This option specifies the path to the gtest source. (If the framework is not installed on your system, it can be downloaded from https://github.com/google/benchmark.)
--with-log4cplus
Define the path to find the Log4cplus headers and libraries.
--with-openssl
Replace Botan by the OpenSSL the cryptographic library. By default configure searches for a valid Botan installation: if one is not found, it searches for OpenSSL.

Note

For instructions concerning the installation and configuration of database backends for Kea, see Section 3.5, “DHCP Database Installation and Configuration”.

For example, the following command configures Kea to find the Boost headers in /usr/pkg/include, specifies that PostgreSQL support should be enabled, and sets the installation location to /opt/kea:

$ ./configure \
      --with-boost-include=/usr/pkg/include \
      --with-pgsql=/usr/local/bin/pg_config \
      --prefix=/opt/kea

If you have some problems with building Kea using the header-only Boost code or you'd like to use the Boost system library (assumed for the sake of this example to be located in /usr/pkg/lib):

$ ./configure \
      --with-boost-libs=-lboost_system \
      --with-boost-lib-dir=/usr/pkg/lib

If configure fails, it may be due to missing or old dependencies.

If configure succeeds, it displays a report with the parameters used to build the code. This report is saved into the file config.report and is also embedded into the executable binaries, e.g., kea-dhcp4.

3.4.4. Build

After the configure step is complete, build the executables from the C++ code and prepare the Python scripts by running the command:

$ make

3.4.5. Install

To install the Kea executables, support files, and documentation, issue the command:

$ make install

Do not use any form of parallel or job server options (such as GNU make's -j option) when performing this step: doing so may cause errors.

Note

The install step may require superuser privileges.

If required, run ldconfig as root with /usr/local/lib (or with prefix/lib if configured with --prefix) in /etc/ld.so.conf (or the relevant linker cache configuration file for your OS):

$ ldconfig

Note

If you do not run ldconfig where it is required, you may see errors like the following:

              program: error while loading shared libraries: libkea-something.so.1:
              cannot open shared object file: No such file or directory
            

3.5. DHCP Database Installation and Configuration

Kea stores its leases in a lease database. The software has been written in a way that makes it possible to choose which database product should be used to store the lease information. At present, Kea supports four database backends: MySQL, PostgreSQL, Cassandra and Memfile. To limit external dependencies, MySQL, PostgreSQL and Cassandra support are disabled by default and only Memfile is available. Support for the optional external database backend must be explicitly included when Kea is built. This section covers the building of Kea with one of the optional backends and the creation of the lease database.

Note

When unit tests are built with Kea (the --with-gtest configuration option is specified), the databases must be manually pre-configured for the unit tests to run. The details of this configuration can be found in the Kea Developer's Guide.

3.5.1. Building with MySQL Support

Install MySQL according to the instructions for your system. The client development libraries must be installed.

Build and install Kea as described in Chapter 3, Installation, with the following modification. To enable the MySQL database code, at the "configure" step (see Section 3.4.3, “Configure Before the Build”), the --with-mysql switch should be specified:

./configure [other-options] --with-mysql

If MySQL was not installed in the default location, the location of the MySQL configuration program "mysql_config" should be included with the switch, i.e.

./configure [other-options] --with-mysql=path-to-mysql_config

See Section 4.3.2.1, “First Time Creation of the MySQL Database” for details regarding MySQL database configuration.

3.5.2. Building with PostgreSQL support

Install PostgreSQL according to the instructions for your system. The client development libraries must be installed. Client development libraries are often packaged as "libpq".

Build and install Kea as described in Chapter 3, Installation, with the following modification. To enable the PostgreSQL database code, at the "configure" step (see Section 3.4.3, “Configure Before the Build”), the --with-pgsql switch should be specified:

./configure [other-options] --with-pgsql

If PostgreSQL was not installed in the default location, the location of the PostgreSQL configuration program "pg_config" should be included with the switch, i.e.

./configure [other-options] --with-pgsql=path-to-pg_config

See Section 4.3.3.1, “First Time Creation of the PostgreSQL Database” for details regarding PostgreSQL database configuration.

3.5.3. Building with CQL (Cassandra) support

Install Cassandra according to the instructions for your system. The Cassandra project website contains useful pointers: http://cassandra.apache.org.

If you have a cpp-driver package available as binary or as source simply install or build and install the package. Then build and install Kea as described in Chapter 3, Installation: To enable the Cassandra (CQL) database code, at the "configure" step (see Section 3.4.3, “Configure Before the Build”), do:

./configure [other-options] --with-cql=path-to-pkg-config

Note if pkg-config is at its standard location (and thus in the shell path) you do not need to supply its path. If it does not work (e.g. no pkg-config, package not available in pkg-config with the cassandra name), you can still use the cql_config script in tools/ as described below.

Download and compile cpp-driver from DataStax. For details regarding dependencies for building cpp-driver, see the project homepage https://github.com/datastax/cpp-driver. In June 2016, the following commands were used:

$ git clone https://github.com/datastax/cpp-driver
$ cd cpp-driver
$ mkdir build
$ cd build
$ cmake ..
$ make

As of June 2016, cpp-driver does not include cql_config script yet. Work is in progress to contribute such a script to the cpp-driver project but, until that is complete, intermediate steps need to be conducted. A cql_config script is present in the tools/ directory of the Kea sources. Before using it, please create a cql_config_defines.sh in the same directory (there is an example in cql_config_define.sh.sample available, you may copy it over to cql_config_defines.sh and edit path specified in it) and change the environment variable CPP_DRIVER_PATH to point to the directory, where cpp-driver sources are located. Make sure that appropriate access rights are set on this file. It should be executable by the system user building Kea.

Build and install Kea as described in Chapter 3, Installation, with the following modification. To enable the Cassandra (CQL) database code, at the "configure" step (see Section 3.4.3, “Configure Before the Build”), do:

./configure [other-options] --with-cql=path-to-cql_config

Chapter 4. Kea Database Administration

4.1. Databases and Database Version Numbers

Kea supports storing leases and host reservations (i.e. static assignments of addresses, prefixes and options) in one of the several supported databases. As future versions of Kea are released, the structure of those databases will change. For example, Kea currently only stores lease information and host reservations. Future versions of Kea will store additional data such as subnet definitions: the database structure will need to be updated to accommodate the extra information.

A given version of Kea expects a particular structure in the database and checks for this by examining the version of database it is using. Separate version numbers are maintained for backend databases, independent of the version of Kea itself. It is possible that the backend database version will stay the same through several Kea revisions: similarly, it is possible that the version of backend database may go up several revisions during a Kea upgrade. Versions for each database are independent, so an increment in the MySQL database version does not imply an increment in that of PostgreSQL.

Backend versions are specified in a major.minor format. The minor number is increased when there are backward compatible changes introduced. For example, the addition of a new index. It is desirable, but not mandatory to apply such a change; you can run on older database version if you want to. (Although, in the example given, running without the new index may be at the expense of a performance penalty.) On the other hand, the major number is increased when an incompatible change is introduced, for example an extra column is added to a table. If you try to run Kea software on a database that is too old (as signified by mismatched backend major version number), Kea will refuse to run: administrative action will be required to upgrade the database.

4.2. The kea-admin Tool

To manage the databases, Kea provides the kea-admin tool. It is able to initialize a new database, check its version number, perform a database upgrade, and dump lease data to a text file.

kea-admin takes two mandatory parameters: command and backend. Additional, non-mandatory options may be specified. Currently supported commands are:

  • lease-init — Initializes a new lease database. This is useful during a new Kea installation. The database is initialized to the latest version supported by the version of the software being installed.
  • lease-version — Reports the lease database version number. This is not necessarily equal to the Kea version number as each backend has its own versioning scheme.
  • lease-upgrade — Conducts a lease database upgrade. This is useful when upgrading Kea.
  • lease-dump — Dumps the contents of the lease database (for MySQL, PostgreSQL or CQL backends) to a CSV (comma separated values) text file. The first line of the file contains the column names. This is meant to be used as a diagnostic tool, so it provides a portable, human-readable form of the lease data.

backend specifies the backend type. Currently supported types are:

  • memfile — Lease information is stored on disk in a text file.
  • mysql — Lease information is stored in a MySQL relational database.
  • pgsql — Lease information is stored in a PostgreSQL relational database.
  • cql — Lease information is stored in a CQL database.

Additional parameters may be needed, depending on your setup and specific operation: username, password and database name or the directory where specific files are located. See the appropriate manual page for details (man 8 kea-admin).

4.3. Supported Databases

The following table presents the capabilities of available backends. Please refer to the specific sections dedicated to each backend to better understand their capabilities and limitations. Choosing the right backend may be essential for success or failure of your deployment.

Table 4.1. List of available backends

FeatureMemfileMySQLPostgreSQLCQL (Cassandra)
StatusStableStableStableExperimental
Data formatCSV fileSQL RMDBSQL RMDBNoSQL database (CQL)
Leasesyesyesyesyes
Host Reservationsnoyesyesyes
Options defined on per host basisnoyesyesyes


4.3.1. memfile

The memfile backend is able to store lease information, but is not able to store host reservation details: these must be stored in the configuration file. (There are no plans to add a host reservations storage capability to this backend.)

No special initialization steps are necessary for the memfile backend. During the first run, both kea-dhcp4 and kea-dhcp6 will create an empty lease file if one is not present. Necessary disk write permission is required.

4.3.1.1. Upgrading Memfile Lease Files from an Earlier Version of Kea

There are no special steps required to upgrade memfile lease files from an earlier version of Kea to a new version of Kea. During startup the servers will check the schema version of the lease files against their own. If there is a mismatch, the servers will automatically launch the LFC process to convert the files to the server's schema version. While this mechanism is primarily meant to ease the process of upgrading to newer versions of Kea, it can also be used for downgrading should the need arise. When upgrading, any values not present in the original lease files will be assigned appropriate default values. When downgrading, any data present in the files but not in the server's schema will be dropped. If you wish to convert the files manually, prior to starting the servers you may do so by running the LFC process yourself. See Chapter 13, The LFC process for more information.

4.3.2. MySQL

MySQL is able to store leases, host reservations and options defined on a per host basis. This section can be safely ignored if you chose to store the data in other backends.

4.3.2.1. First Time Creation of the MySQL Database

If you are setting the MySQL database for the first time, you need to create the database area within MySQL and set up the MySQL user ID under which Kea will access the database. This needs to be done manually: kea-admin is not able to do this for you.

To create the database:

  1. Log into MySQL as "root":

    $ mysql -u root -p
    Enter password:
    mysql>
    

  2. Create the MySQL database:

    mysql> CREATE DATABASE database-name;
    

    (database-name is the name you have chosen for the database.)

  3. Create the user under which Kea will access the database (and give it a password), then grant it access to the database tables:

    mysql> CREATE USER 'user-name'@'localhost' IDENTIFIED BY 'password';
    mysql> GRANT ALL ON database-name.* TO 'user-name'@'localhost';
    

    (user-name and password are the user ID and password you are using to allow Keas access to the MySQL instance. All apostrophes in the command lines above are required.)

  4. At this point, you may elect to create the database tables. (Alternatively, you can exit MySQL and create the tables using the kea-admin tool, as explained below.) To do this:

    mysql> CONNECT database-name;
    mysql> SOURCE path-to-kea/share/kea/scripts/mysql/dhcpdb_create.mysql
    

    (path-to-kea is the location where you installed Kea.)

  5. Exit MySQL:

    mysql> quit
    Bye
    $
    

If you elected not to create the tables in step 4, you can do so now by running the kea-admin tool:

$ kea-admin lease-init mysql -u database-user -p database-password -n database-name

(Do not do this if you did create the tables in step 4.) kea-admin implements rudimentary checks: it will refuse to initialize a database that contains any existing tables. If you want to start from scratch, you must remove all data manually. (This process is a manual operation on purpose to avoid possibly irretrievable mistakes by kea-admin.)

4.3.2.2. Upgrading a MySQL Database from an Earlier Version of Kea

Sometimes a new Kea version may use newer database schema, so there will be a need to upgrade the existing database. This can be done using the kea-admin lease-upgrade command.

To check the current version of the database, use the following command:

$ kea-admin lease-version mysql -u database-user -p database-password -n database-name

(See Section 4.1, “Databases and Database Version Numbers” for a discussion about versioning.) If the version does not match the minimum required for the new version of Kea (as described in the release notes), the database needs to be upgraded.

Before upgrading, please make sure that the database is backed up. The upgrade process does not discard any data but, depending on the nature of the changes, it may be impossible to subsequently downgrade to an earlier version. To perform an upgrade, issue the following command:

$ kea-admin lease-upgrade mysql -u database-user -p database-password -n database-name

4.3.3. PostgreSQL

PostgreSQL is able to store leases, host reservations and options defined on a per host basis. A PostgreSQL database must be set up if you want Kea to store lease and other information in PostgreSQL. This step can be safely ignored if you are using other database backends.

4.3.3.1. First Time Creation of the PostgreSQL Database

The first task is to create both the lease database and the user under which the servers will access it. A number of steps are required:

  1. Log into PostgreSQL as "root":

    $ sudo -u postgres psql postgres
    Enter password:
    postgres=#
    

  2. Create the database:

    postgres=# CREATE DATABASE database-name;
    CREATE DATABASE
    postgres=#
    

    (database-name is the name you have chosen for the database.)

  3. Create the user under which Kea will access the database (and give it a password), then grant it access to the database:

    postgres=# CREATE USER user-name WITH PASSWORD 'password';
    CREATE ROLE
    postgres=# GRANT ALL PRIVILEGES ON DATABASE database-name TO user-name;
    GRANT
    postgres=#
    

  4. Exit PostgreSQL:

    postgres=# \q
    Bye
    $
    

  5. At this point you are ready to create the database tables. This can be done using the kea-admin tool as explained in the next section (recommended), or manually. To create the tables manually enter the following command. Note that PostgreSQL will prompt you to enter the new user's password you specified in Step 3. When the command completes you will be returned to the shell prompt. You should see output similar to following:

    $ psql -d database-name -U user-name -f path-to-kea/share/kea/scripts/pgsql/dhcpdb_create.pgsql
    Password for user user-name:
    CREATE TABLE
    CREATE INDEX
    CREATE INDEX
    CREATE TABLE
    CREATE INDEX
    CREATE TABLE
    START TRANSACTION
    INSERT 0 1
    INSERT 0 1
    INSERT 0 1
    COMMIT
    CREATE TABLE
    START TRANSACTION
    INSERT 0 1
    COMMIT
    $
    

    (path-to-kea is the location where you installed Kea.)

    If instead you encounter an error like:

    psql: FATAL:  no pg_hba.conf entry for host "[local]", user "user-name", database "database-name", SSL off
    

    ... you will need to alter the PostgreSQL configuration. Kea uses password authentication when connecting to the database and must have the appropriate entries added to PostgreSQL's pg_hba.conf file. This file is normally located in the primary data directory for your PostgreSQL server. The precise path may vary but the default location for PostgreSQL 9.3 on Centos 6.5 is: /var/lib/pgsql/9.3/data/pg_hba.conf.

    Assuming Kea is running on the same host as PostgreSQL, adding lines similar to following should be sufficient to provide password-authenticated access to Kea's database:

    local   database-name    user-name                                 password
    host    database-name    user-name          127.0.0.1/32           password
    host    database-name    user-name          ::1/128                password
    

    These edits are primarily intended as a starting point not a definitive reference on PostgreSQL administration or database security. Please consult your PostgreSQL user manual before making these changes as they may expose other databases that you run. It may be necessary to restart PostgreSQL in order for these changes to take effect.

4.3.3.2. Initialize the PostgreSQL Database Using kea-admin

If you elected not to create the tables manually, you can do so now by running the kea-admin tool:

$ kea-admin lease-init pgsql -u database-user -p database-password -n database-name

Do not do this if you already created the tables in manually. kea-admin implements rudimentary checks: it will refuse to initialize a database that contains any existing tables. If you want to start from scratch, you must remove all data manually. (This process is a manual operation on purpose to avoid possibly irretrievable mistakes by kea-admin.)

4.3.3.3. Upgrading a PostgreSQL Database from an Earlier Version of Kea

The PostgreSQL database schema can be upgraded using the same tool and commands as described in Section 4.3.2.2, “Upgrading a MySQL Database from an Earlier Version of Kea”, with the exception that the "pgsql" database backend type must be used in the commands.

Use the following command to check the current schema version:

$ kea-admin lease-version pgsql -u database-user -p database-password -n database-name

Use the following command to perform an upgrade:

$ kea-admin lease-upgrade pgsql -u database-user -p database-password -n database-name

4.3.4. CQL (Cassandra)

Cassandra, or Cassandra Query Language (CQL), is the newest backend added to Kea. Since it was added recently and has not undergone as much testing as other backends, it is considered experimental. Please use with caution. The Cassandra backend is able to store leases, host reservations and options defined on a per host basis.

The CQL database must be properly set up if you want Kea to store information in CQL. This section can be safely ignored if you chose to store the data in other backends.

4.3.4.1. First Time Creation of the Cassandra Database

If you are setting up the CQL database for the first time, you need to create the keyspace area within CQL. This needs to be done manually: kea-admin is not able to do this for you.

To create the database:

  1. Export CQLSH_HOST environment variable:

    $ export CQLSH_HOST=localhost
    

  2. Log into CQL:

    $ cqlsh
    cql>
    

  3. Create the CQL keyspace:

    cql> CREATE KEYSPACE keyspace-name WITH replication = {'class' : 'SimpleStrategy','replication_factor' : 1};
    

    (keyspace-name is the name you have chosen for the keyspace)

  4. At this point, you may elect to create the database tables. (Alternatively, you can exit CQL and create the tables using the kea-admin tool, as explained below) To do this:

    cqslh -k keyspace-name -f path-to-kea/share/kea/scripts/cql/dhcpdb_create.cql
    

    (path-to-kea is the location where you installed Kea)

If you elected not to create the tables in step 4, you can do so now by running the kea-admin tool:

$ kea-admin lease-init cql -n database-name

(Do not do this if you did create the tables in step 4.) kea-admin implements rudimentary checks: it will refuse to initialize a database that contains any existing tables. If you want to start from scratch, you must remove all data manually. (This process is a manual operation on purpose to avoid possibly irretrievable mistakes by kea-admin)

4.3.4.2. Upgrading a CQL Database from an Earlier Version of Kea

Sometimes a new Kea version may use newer database schema, so there will be a need to upgrade the existing database. This can be done using the kea-admin lease-upgrade command.

To check the current version of the database, use the following command:

$ kea-admin lease-version cql -n database-name

(See Section 4.1, “Databases and Database Version Numbers” for a discussion about versioning) If the version does not match the minimum required for the new version of Kea (as described in the release notes), the database needs to be upgraded.

Before upgrading, please make sure that the database is backed up. The upgrade process does not discard any data but, depending on the nature of the changes, it may be impossible to subsequently downgrade to an earlier version. To perform an upgrade, issue the following command:

$ kea-admin lease-upgrade cql -n database-name

4.3.5. Using Read-Only Databases with Host Reservations

If a read-only database is used for storing host reservations, Kea must be explicitly configured to operate on the database in read-only mode. Sections Section 8.2.3.2, “Using Read-Only Databases for Host Reservations” and Section 9.2.3.2, “Using Read-Only Databases for Host Reservations” describe when such configuration may be required and how to configure Kea to operate using a read-only host database.

4.3.6. Limitations Related to the use of SQL Databases

4.3.6.1. Year 2038 issue

The lease expiration time is stored in the SQL database for each lease as a timestamp value. Kea developers observed that MySQL database doesn't accept timestamps beyond 2147483647 seconds (maximum signed 32-bit number) from the beginning of the epoch. At the same time, some versions of PostgreSQL do accept greater values but the value is altered when it is read back. For this reason the lease database backends put the restriction for the maximum timestamp to be stored in the database, which is equal to the maximum signed 32-bit number. This effectively means that the current Kea version can't store the leases which expiration time is later than 2147483647 seconds since the beginning of the epoch (around year 2038). This will be fixed when the database support for longer timestamps is available.

Chapter 5. Kea Configuration

Kea is using JSON structures to represent servers' configurations. The following sections describe how the configuration structures are organized.

5.1. JSON Configuration

JSON is notation used throughout the Kea project. The most obvious usage is for the configuration file, but it is also used for sending commands over Management API (see Chapter 17, Management API) and for communicating between DHCP servers and DDNS update daemon.

Typical usage assumes that the servers are started from the command line (either directly or using a script, e.g. keactrl). The configuration file is specified upon startup using the -c parameter.

5.1.1. JSON Syntax

Configuration files for DHCPv4, DHCPv6, DDNS, Control Agent and Netconf modules are defined in an extended JSON format. Basic JSON is defined in RFC 7159 and ECMA 404. Note that Kea 1.2.0 introduced a new parser which better adheres to the JSON specification. In particular, the only values allowed for boolean are true or false (all lowercase). The capitalized versions (True or False) are not accepted.

Kea components use an extended JSON with additional features allowed:

  • shell comments: any text after the hash (#) character is ignored.
  • C comments: any text after the double slashes (//) character is ignored.
  • Multiline comments: any text between /* and */ is ignored. This commenting can span multiple lines.
  • File inclusion: JSON files can include other JSON files. This can be done by using <?include "file.json"?>.

The configuration file consists of a single object (often colloquially called a map) started with a curly bracket. It comprises one or more of the "Dhcp4", "Dhcp6", "DhcpDdns", "Control-agent", "Netconf", and "Logging" objects. It is possible to define additional elements but they will be ignored.

A very simple configuration for DHCPv4 could look like this:

# The whole configuration starts here.
{
    # DHCPv4 specific configuration starts here.
    "Dhcp4": {
        "interfaces-config": {
            "interfaces": [ "eth0" ],
            "dhcp-socket-type": "raw"
        },
        "valid-lifetime": 4000,
        "renew-timer": 1000,
        "rebind-timer": 2000,
        "subnet4": [{
           "pools": [ { "pool": "192.0.2.1-192.0.2.200" } ],
           "subnet": "192.0.2.0/24"
        }]
    },

    # Logger configuration starts here.
    "Logging": {
       "loggers": [{
            "name": "*",
            "severity": "DEBUG"
        }]
    }

# The whole configuration structure ends here.
}

More examples are available in the installed share/doc/kea/examples directory.

Note

The "Logging" element will be removed in Kea 1.6.0 and its contents (the "loggers" object) moved inside the configuration objects (maps) for respective Kea modules. For example: the "Dhcp4" map will contain the "loggers" object specifying logging configuration for the DHCPv4 server. Backward compatibility will be maintained until at least Kea 1.7.0 release: it will be possible to specify "Logging" object at the top configuration level and "loggers" objects at module configuration level. Ultimately though, support for the top-level "Logging" object will be removed.

The specification of several supported elements (e.g. "Dhcp4", "Dhcp6") in a single configuration file can be confusing and works badly with the commands that fetch and write new configurations. Support for it will be removed in a future release of Kea, after which each component will require its own configuration file.

To avoid repetition of mostly similar structures, examples in the rest of this guide will showcase only the subset of parameters appropriate for a given context. For example, when discussing the IPv6 subnets configuration in DHCPv6, only subnet6 parameters will be mentioned. It is implied that the remaining elements (the global map that holds Dhcp6 and Logging) are present, but they are omitted for clarity. Usually, locations where extra parameters may appear are denoted by an ellipsis.

5.1.2. Simplified Notation

It is sometimes convenient to refer to a specific element in the configuration hierarchy. Each hierarchy level is separated by a slash. If there is an array, a specific instance within that array is referenced by a number in square brackets (with numbering starting at zero). For example, in the above configuration the valid-lifetime in the Dhcp4 component can be referred to as Dhcp4/valid-lifetime and the pool in the first subnet defined in the DHCPv4 configuration as Dhcp4/subnet4[0]/pool.

Chapter 6. Managing Kea with keactrl

6.1. Overview

keactrl is a shell script which controls the startup, shutdown and reconfiguration of the Kea servers (kea-dhcp4, kea-dhcp6, kea-dhcp-ddns, kea-ctrl-agent and kea-netconf). It also provides the means for checking the current status of the servers and determining the configuration files in use.

6.2. Command Line Options

keactrl is run as follows:

keactrl <command> [-c keactrl-config-file] [-s server[,server,..]]

<command> is the one of the commands described in Section 6.4, “Commands”.

The optional -c keactrl-config-file switch allows specification of an alternate keactrl configuration file. (--ctrl-config is a synonym for -c.) In the absence of -c, keactrl will use the default configuration file [kea-install-dir]/etc/kea/keactrl.conf.

The optional -s server[,server ...] switch selects the servers to which the command is issued. (--server is a synonym for -s.) If absent, the command is sent to all servers enabled in the keactrl configuration file. If multiple servers are specified, they should be separated by commas with no intervening spaces.

6.3. The keactrl Configuration File

Depending on requirements, not all of the available servers need be run. The keactrl configuration file sets which servers are enabled and which are disabled. The default configuration file is [kea-install-dir]/etc/kea/keactrl.conf, but this can be overridden on a per-command basis using the -c switch.

The contents of keactrl.conf are:

# This is a configuration file for keactrl script which controls
# the startup, shutdown, reconfiguration and gathering the status
# of the Kea's processes.

# prefix holds the location where the Kea is installed.
prefix=@prefix@

# Location of Kea configuration file.
kea_dhcp4_config_file=@sysconfdir@/@PACKAGE@/kea-dhcp4.conf
kea_dhcp6_config_file=@sysconfdir@/@PACKAGE@/kea-dhcp6.conf
kea_dhcp_ddns_config_file=@sysconfdir@/@PACKAGE@/kea-dhcp-ddns.conf
kea_ctrl_agent_config_file=@sysconfdir@/@PACKAGE@/kea-ctrl-agent.conf
kea_netconf_config_file=@sysconfdir@/@PACKAGE@/kea-netconf.conf


# Location of Kea binaries.
exec_prefix=@exec_prefix@
dhcp4_srv=@sbindir@/kea-dhcp4
dhcp6_srv=@sbindir@/kea-dhcp6
dhcp_ddns_srv=@sbindir@/kea-dhcp-ddns
ctrl_agent_srv=@sbindir@/kea-ctrl-agent
netconf_srv=@sbindir@/kea-netconf

# Start DHCPv4 server?
dhcp4=yes

# Start DHCPv6 server?
dhcp6=yes

# Start DHCP DDNS server?
dhcp_ddns=no

# Start Control Agent?
ctrl_agent=yes

# Start Netconf?
netconf=no

# Be verbose?
kea_verbose=no

The dhcp4, dhcp6, dhcp_ddns, ctrl_agent and netconf parameters set to "yes" will configure keactrl to manage (start, reconfigure) all servers, i.e. kea-dhcp4, kea-dhcp6, kea-dhcp-ddns, kea-ctrl-agent and kea-netconf. When any of these parameters is set to "no" the keactrl will ignore the corresponding server when starting or reconfiguring Kea. Some daemons (ddns and netconf) are disabled by default.

By default, Kea servers managed by keactrl are located in [kea-install-dir]/sbin. This should work for most installations. If the default location needs to be altered for any reason, the paths specified with the dhcp4_srv, dhcp6_srv, dhcp_ddns_srv ctrl_agent_srv and netconf_srv parameters should be modified.

The kea_verbose parameter specifies the verbosity of the servers being started. When kea_verbose is set to "yes" the logging level of the server is set to DEBUG. Modification of the logging severity in a configuration file, as described in Chapter 19, Logging, will have no effect as long as the kea_verbose is set to "yes". Setting it to "no" will cause the server to use the logging levels specified in the Kea configuration file for respective loggers. If no logging configuration is specified, the default settings will be used.

Note

The verbosity for the server is set when it is started. Once started, the verbosity can be only changed by stopping the server and starting it again with the new value of the kea_verbose parameter.

6.4. Commands

The following commands are supported by keactrl:

  • start - starts selected servers.
  • stop - stops all running servers.
  • reload - triggers reconfiguration of the selected servers by sending the SIGHUP signal to them.
  • status - returns the status of the servers (active or inactive) and the names of the configuration files in use.
  • version - prints out a version of the keactrl tool itself, but also the Kea daemons.

Typical output from keactrl when starting the servers looks similar to the following:

$ keactrl start
INFO/keactrl: Starting kea-dhcp4 -c /usr/local/etc/kea/kea-dhcp4.conf -d
INFO/keactrl: Starting kea-dhcp6 -c /usr/local/etc/kea/kea-dhcp6.conf -d
INFO/keactrl: Starting kea-dhcp-ddns -c /usr/local/etc/kea/kea-dhcp-ddns.conf -d
INFO/keactrl: Starting kea-ctrl-agent -c /usr/local/etc/kea/kea-ctrl-agent.conf -d
INFO/keactrl: Starting kea-netconf -c /usr/local/etc/kea/kea-netconf.conf -d

Kea's servers create PID files upon startup. These files are used by keactrl to determine whether or not a given server is running. If one or more servers are running when the start command is issued, the output will look similar to the following:

$ keactrl start
INFO/keactrl: kea-dhcp4 appears to be running, see: PID 10918, PID file: /usr/local/var/kea/kea.kea-dhcp4.pid.
INFO/keactrl: kea-dhcp6 appears to be running, see: PID 10924, PID file: /usr/local/var/kea/kea.kea-dhcp6.pid.
INFO/keactrl: kea-dhcp-ddns appears to be running, see: PID 10930, PID file: /usr/local/var/kea/kea.kea-dhcp-ddns.pid.
INFO/keactrl: kea-ctrl-agent appears to be running, see: PID 10931, PID file: /usr/local/var/kea/kea.kea-ctrl-agent.pid.
INFO/keactrl: kea-netconf appears to be running, see: PID 10123, PID file: /usr/local/var/kea/kea.kea-netconf.pid.

During normal shutdowns these PID files are deleted. They may, however, be left over as remnants following a system crash. It is possible, though highly unlikely, that upon system restart the PIDs they contain actually refer to processes unrelated to Kea. This condition will cause keactrl to decide that the servers are running, when in fact they are not. In such a case the PID files as listed in the keactrl output must be manually deleted.

The following command stops all servers:

$ keactrl stop
INFO/keactrl: Stopping kea-dhcp4...
INFO/keactrl: Stopping kea-dhcp6...
INFO/keactrl: Stopping kea-dhcp-ddns...
INFO/keactrl: Stopping kea-ctrl-agent...
INFO/keactrl: Stopping kea-netconf...

Note that the stop will attempt to stop all servers regardless of whether they are "enabled" in the keactrl.conf. If any of the servers are not running, an informational message is displayed as in the stop command output below.

$ keactrl stop
INFO/keactrl: kea-dhcp4 isn't running.
INFO/keactrl: kea-dhcp6 isn't running.
INFO/keactrl: kea-dhcp-ddns isn't running.
INFO/keactrl: kea-ctrl-agent isn't running.
INFO/keactrl: kea-netconf isn't running.

As already mentioned, the reconfiguration of each Kea server is triggered by the SIGHUP signal. The reload command sends the SIGHUP signal to the servers that are enabled in the keactrl configuration file and are currently running. When a server receives the SIGHUP signal it re-reads its configuration file and, if the new configuration is valid, uses the new configuration. A reload is executed as follows:

$ keactrl reload
INFO/keactrl: Reloading kea-dhcp4...
INFO/keactrl: Reloading kea-dhcp6...
INFO/keactrl: Reloading kea-dhcp-ddns...
INFO/keactrl: Reloading kea-ctrl-agent...

If any of the servers are not running, an informational message is displayed as in the reload command output below. Note that as of 1.5.0 kea-netconf does not support SIGHUP signal yet. If its configuration has changed, please stop and start it. This limitation will be removed in upcoming Kea releases.

$ keactrl stop
INFO/keactrl: kea-dhcp4 isn't running.
INFO/keactrl: kea-dhcp6 isn't running.
INFO/keactrl: kea-dhcp-ddns isn't running.
INFO/keactrl: kea-ctrl-agent isn't running.
INFO/keactrl: kea-netconf isn't running.

Note

NETCONF is an optional feature that is disabled by default and can be enabled during compilation. If Kea was compiled without NETCONF support, keactrl will do it's best to not bother the user with information about it. The netconf entries will still be present in the keactrl.conf file, but netconf status will not be shown and other commands will ignore it.

Note

Currently keactrl does not report configuration failures when the server is started or reconfigured. To check if the server's configuration succeeded the Kea log must be examined for errors. By default, this is written to the syslog file.

Sometimes it is useful to check which servers are running. The status reports this, typical output looking like:

$ keactrl status
DHCPv4 server: active
DHCPv6 server: inactive
DHCP DDNS: active
Control Agent: active
Netconf agent: inactive
Kea configuration file: /usr/local/etc/kea/kea.conf
Kea DHCPv4 configuration file: /usr/local/etc/kea/kea-dhcp4.conf
Kea DHCPv6 configuration file: /usr/local/etc/kea/kea-dhcp6.conf
Kea DHCP DDNS configuration file: /usr/local/etc/kea/kea-dhcp-ddns.conf
Kea Control Agent configuration file: /usr/local/etc/kea/kea-ctrl-agent.conf
Kea Netconf configuration file: /usr/local/etc/kea/kea-netconf.conf
keactrl configuration file: /usr/local/etc/kea/keactrl.conf

6.5. Overriding the Server Selection

The optional -s switch allows the selection of the servers to which keactrl command is issued. For example, the following instructs keactrl to stop the kea-dhcp4 and kea-dhcp6 servers and leave the kea-dhcp-ddns and kea-ctrl-agent running:

$ keactrl stop -s dhcp4,dhcp6

Similarly, the following will only start the kea-dhcp4 and kea-dhcp-ddns servers and not: kea-dhcp6, kea-ctrl-agent.

$ keactrl start -s dhcp4,dhcp_ddns

Note that the behavior of the -s switch with the start and reload commands is different to its behavior with the stop command. On start and reload, keactrl will check if the servers given as parameters to the -s switch are enabled in the keactrl configuration file: if not, the server will be ignored. For stop however, this check is not made: the command is applied to all listed servers, regardless of whether they have been enabled in the file.

The following keywords can be used with the -s command line option:

  • dhcp4 for kea-dhcp4.
  • dhcp6 for kea-dhcp6.
  • dhcp_ddns for kea-dhcp-ddns.
  • ctrl_agent for kea-ctrl-agent.
  • netconf for kea-netconf.
  • all for all servers (default).

Chapter 7. Kea Control Agent

7.1. Overview

Kea Control Agent (CA) is a daemon, first included in Kea 1.2, which exposes a RESTful control interface for managing Kea servers. The daemon can receive control commands over HTTP and either forward these commands to the respective Kea servers or handle these commands on its own. The determination whether the command should be handled by the CA or forwarded is made by checking the value of the 'service' parameter which may be included in the command from the controlling client. The details of the supported commands as well as their structures are provided in Chapter 17, Management API.

Hook libraries can be loaded by the CA to provide support for additional commands or custom behavior of existing commands. Such hook libraries must implement callouts for 'control_command_receive' hook point. Details about creating new hook libraries and supported hook points can be found in Kea Developer's Guide.

The CA processes received commands according to the following algorithm:

  • Pass command into any installed hooks (regardless of service value(s)). If the command is handled by a hook, return the response.
  • If the service specifies one more or services, the CA will forward the command to specified services and return the accumulated responses.
  • If service is not specified or is an empty list, the CA will handle the command if it supports it.

7.2. Configuration

The following example demonstrates the basic CA configuration.

{
    "Control-agent": {
        "http-host": "10.20.30.40",
        "http-port": 8080,

        "control-sockets": {
            "dhcp4": {
                "comment": "main server",
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v4"
            },
            "dhcp6": {
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v6",
                "user-context": { "version": 3 }
            }
        },

        "hooks-libraries": [
        {
            "library": "/opt/local/control-agent-commands.so",
            "parameters": {
                "param1": "foo"
            }
        } ]
    },

    "Logging": {
        "loggers": [ {
            "name": "kea-ctrl-agent",
            "severity": "INFO"
        } ]
    }
}

The http-host and http-port specify an IP address and port to which HTTP service will be bound. In case of the example configuration provided above, the RESTful service will be available under the URL of http://10.20.30.40:8080/. If these parameters are not specified, the default URL is http://127.0.0.1:8000/

It has been mentioned in the Section 7.1, “Overview” that CA can forward received commands to the specific Kea servers for processing. For example, config-get is sent to retrieve configuration of one of the Kea services. When CA receives this command, including a service parameter indicating that the client desires to retrieve configuration of the DHCPv4 server, the CA will forward this command to this server and then pass the received response back to the client. More about the service parameter and general structure of the commands can be found in Chapter 17, Management API.

The CA uses unix domain sockets to forward control commands and receive responses from other Kea services. The dhcp4, dhcp6 and d2 maps specify the files to which unix domain sockets are bound. In case of the configuration above, the CA will connect to the DHCPv4 server via /path/to/the/unix/socket-v4 to forward the commands to it. Obviously, the DHCPv4 server must be configured to listen to connections via this same socket. In other words, the command socket configuration for the DHCPv4 server and CA (for this server) must match. Consult the Section 8.9, “Management API for the DHCPv4 Server” and the Section 9.13, “Management API for the DHCPv6 Server” to learn how the socket configuration is specified for the DHCPv4 and DHCPv6 services.

Warning

We have renamed "dhcp4-server", "dhcp6-server" and "d2-server" to "dhcp4", "dhcp6" and "d2" respectively after release of Kea 1.2. If you are migrating from Kea 1.2 you need to tweak your CA config to use this new naming convention. We have made this incompatible change to facilitate future use cases where it will be possible to specify additional values of the "service" parameter to point to the particular instances of the Kea servers, e.g. "dhcp4/3" pointing to the 3rd instance of the DHCPv4 server in the multi-processed configuration. This is not yet supported but the current renaming lays the ground for it.

User contexts can store arbitrary data as long as it is valid JSON syntax and its top level element is a map (i.e. the data must be enclosed in curly brackets). Some hook libraries may expect specific formatting, though. Please consult specific hook library documentation for details.

User contexts can be specified on either global scope, control socket and loggers. One other useful usage is the ability to store comments or descriptions: the parser translates a "comment" entry into a user-context with the entry, this allows to attach a comment inside the configuration itself.

Hooks libraries can be loaded by the Control Agent just like to DHCPv4 and DHCPv6 servers. It currently supports one hook point 'control_command_receive' which makes it possible to delegate processing of some commands to the hooks library. The hooks-libraries list contains the list of hooks libraries that should be loaded by the CA, along with their configuration information specified with parameters.

Please consult Chapter 19, Logging for the details how to configure logging. The CA's root logger's name is kea-ctrl-agent as given in the example above.

7.3. Secure Connections

Control Agent doesn't natively support secure HTTP connections like SSL or TLS. In order to setup secure connection please use one of the available third party HTTP servers and configure it to run as a reverse proxy to the Control Agent. Kea has been tested with two major HTTP server implentations working as a reverse proxy: Apache2 and nginx. Example configurations including extensive comments are provided in the doc/examples/https/ directory.

The reverse proxy forwards HTTP requests received over secure connection to the Control Agent using (not secured) HTTP. Typically, the reverse proxy and the Control Agent are running on the same machine, but it is possible to configure them to run on separate machines as well. In this case, security depends on the protection of the communications between the reverse proxy and the Control Agent.

Apart from providing the encryption layer for the control channel, a reverse proxy server is also often used for authentication of the controlling clients. In this case, the client must present a valid certificate when it connects via reverse proxy. The proxy server authenticates the client by checking if the presented certificate is signed by the certificate authority used by the server.

To illustrate this, we provide a sample configuration for the nginx server running as a reverse proxy to the Kea Control Agent. The server enables authentication of the clients using certificates.

#   The server certificate and key can be generated as follows:
#
#   openssl genrsa -des3 -out kea-proxy.key 4096
#   openssl req -new -x509 -days 365 -key kea-proxy.key -out kea-proxy.crt
#
#   The CA certificate and key can be generated as follows:
#
#   openssl genrsa -des3 -out ca.key 4096
#   openssl req -new -x509 -days 365 -key ca.key -out ca.crt
#
#
#   The client certificate needs to be generated and signed:
#
#   openssl genrsa -des3 -out kea-client.key 4096
#   openssl req -new -key kea-client.key -out kea-client.csr
#   openssl x509 -req -days 365 -in kea-client.csr -CA ca.crt \
#           -CAkey ca.key -set_serial 01 -out kea-client.crt
#
#   Note that the 'common name' value used when generating the client
#   and the server certificates must differ from the value used
#   for the CA certificate.
#
#   The client certificate must be deployed on the client system.
#   In order to test the proxy configuration with 'curl' run
#   command similar to the following:
#
#   curl -k --key kea-client.key --cert kea-client.crt -X POST \
#        -H Content-Type:application/json -d '{ "command": "list-commands" }' \
#         https://kea.example.org/kea
#
#
#
#   nginx configuration starts here.

events {
}

http {
        #   HTTPS server
    server {
        #     Use default HTTPS port.
        listen 443 ssl;
        #     Set server name.
        server_name kea.example.org;

        #   Server certificate and key.
        ssl_certificate /path/to/kea-proxy.crt;
        ssl_certificate_key /path/to/kea-proxy.key;

        #   Certificate Authority. Client certificate must be signed by the CA.
        ssl_client_certificate /path/to/ca.crt;

        # Enable verification of the client certificate.
        ssl_verify_client on;

        # For URLs such as https://kea.example.org/kea, forward the
        # requests to http://127.0.0.1:8080.
        location /kea {
            proxy_pass http://127.0.0.1:8080;
        }
    }
}

Note

Note that the configuration snippet provided above is for testing purposes only. Consult security policies and best practices of your organization which apply to this setup.

When you use an HTTP client without TLS support as kea-shell you can use an HTTP/HTTPS translator such as stunnel in client mode. A sample configuration is provided in the doc/examples/https/shell/ directory

7.4. Starting Control Agent

The CA is started by running its binary and specifying the configuration file it should use. For example:

$ ./kea-ctrl-agent -c /usr/local/etc/kea/kea-ctrl-agent.conf

It can be started by keactrl as well (see Chapter 6, Managing Kea with keactrl).

7.5. Connecting to the Control Agent

For an example of tool that can take advantage of the RESTful API, see Chapter 20, The Kea Shell.

Chapter 8. The DHCPv4 Server

Table of Contents

8.1. Starting and Stopping the DHCPv4 Server
8.2. DHCPv4 Server Configuration
8.2.1. Introduction
8.2.2. Lease Storage
8.2.3. Hosts Storage
8.2.4. Interface Configuration
8.2.5. Issues with Unicast Responses to DHCPINFORM
8.2.6. IPv4 Subnet Identifier
8.2.7. Configuration of IPv4 Address Pools
8.2.8. Standard DHCPv4 Options
8.2.9. Custom DHCPv4 options
8.2.10. DHCPv4 Private Options
8.2.11. DHCPv4 Vendor Specific Options
8.2.12. Nested DHCPv4 Options (Custom Option Spaces)
8.2.13. Unspecified Parameters for DHCPv4 Option Configuration
8.2.14. Stateless Configuration of DHCPv4 Clients
8.2.15. Client Classification in DHCPv4
8.2.16. DDNS for DHCPv4
8.2.17. Next Server (siaddr)
8.2.18. Echoing Client-ID (RFC 6842)
8.2.19. Using Client Identifier and Hardware Address
8.2.20. Authoritative DHCPv4 Server Behavior
8.2.21. DHCPv4-over-DHCPv6: DHCPv4 Side
8.2.22. Sanity checks in DHCPv4
8.3. Host Reservation in DHCPv4
8.3.1. Address Reservation Types
8.3.2. Conflicts in DHCPv4 Reservations
8.3.3. Reserving a Hostname
8.3.4. Including Specific DHCPv4 Options in Reservations
8.3.5. Reserving Next Server, Server Hostname and Boot File Name
8.3.6. Reserving Client Classes in DHCPv4
8.3.7. Storing Host Reservations in MySQL, PostgreSQL or Cassandra
8.3.8. Fine Tuning DHCPv4 Host Reservation
8.3.9. Global reservations in DHCPv4
8.4. Shared networks in DHCPv4
8.4.1. Local and relayed traffic in shared networks
8.4.2. Client classification in shared networks
8.4.3. Host reservations in shared networks
8.5. Server Identifier in DHCPv4
8.6. How the DHCPv4 Server Selects a Subnet for the Client
8.6.1. Using a Specific Relay Agent for a Subnet
8.6.2. Segregating IPv4 Clients in a Cable Network
8.7. Duplicate Addresses (DHCPDECLINE Support)
8.8. Statistics in the DHCPv4 Server
8.9. Management API for the DHCPv4 Server
8.10. Supported DHCP Standards
8.11. User contexts in IPv4
8.12. DHCPv4 Server Limitations
8.13. Kea DHCPv4 server examples

8.1. Starting and Stopping the DHCPv4 Server

It is recommended that the Kea DHCPv4 server be started and stopped using keactrl (described in Chapter 6, Managing Kea with keactrl). However, it is also possible to run the server directly: it accepts the following command-line switches:

  • -c file - specifies the configuration file. This is the only mandatory switch.
  • -d - specifies whether the server logging should be switched to debug/verbose mode. In verbose mode, the logging severity and debuglevel specified in the configuration file are ignored and "debug" severity and the maximum debuglevel (99) are assumed. The flag is convenient, for temporarily switching the server into maximum verbosity, e.g. when debugging.
  • -p port - specifies UDP port on which the server will listen. This is only useful during testing, as a DHCPv4 server listening on ports other than the standard ones will not be able to handle regular DHCPv4 queries.
  • -t file - specifies the configuration file to be tested. Kea-dhcp4 will attempt to load it, and will conduct sanity checks. Note that certain checks are possible only while running the actual server. The actual status is reported with exit code (0 = configuration looks ok, 1 = error encountered). Kea will print out log messages to standard output and error to standard error when testing configuration.
  • -v - prints out the Kea version and exits.
  • -V - prints out the Kea extended version with additional parameters and exits. The listing includes the versions of the libraries dynamically linked to Kea.
  • -W - prints out the Kea configuration report and exits. The report is a copy of the config.report file produced by ./configure: it is embedded in the executable binary.

The config.report may also be accessed more directly. The following command may be used to extract this information. The binary path may be found in the install directory or in the .libs subdirectory in the source tree. For example kea/src/bin/dhcp4/.libs/kea-dhcp4.

strings path/kea-dhcp4 | sed -n 's/;;;; //p'

On start-up, the server will detect available network interfaces and will attempt to open UDP sockets on all interfaces mentioned in the configuration file. Since the DHCPv4 server opens privileged ports, it requires root access. Make sure you run this daemon as root.

During startup the server will attempt to create a PID file of the form: localstatedir]/[conf name].kea-dhcp6.pid where:

  • localstatedir: The value as passed into the build configure script. It defaults to "/usr/local/var". (Note that this value may be overridden at run time by setting the environment variable KEA_PIDFILE_DIR. This is intended primarily for testing purposes.)
  • conf name: The configuration file name used to start the server, minus all preceding path and file extension. For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the portion used would be "myconf".

If the file already exists and contains the PID of a live process, the server will issue a DHCP4_ALREADY_RUNNING log message and exit. It is possible, though unlikely, that the file is a remnant of a system crash and the process to which the PID belongs is unrelated to Kea. In such a case it would be necessary to manually delete the PID file.

The server can be stopped using the kill command. When running in a console, the server can also be shut down by pressing ctrl-c. It detects the key combination and shuts down gracefully.

8.2. DHCPv4 Server Configuration

8.2.1. Introduction

This section explains how to configure the DHCPv4 server using a configuration file. Before DHCPv4 is started, its configuration file has to be created. The basic configuration is as follows:

{
# DHCPv4 configuration starts in this line
"Dhcp4": {

# First we set up global values
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,

# Next we setup the interfaces to be used by the server.
    "interfaces-config": {
        "interfaces": [ "eth0" ]
    },

# And we specify the type of lease database
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/var/kea/dhcp4.leases"
    },

# Finally, we list the subnets from which we will be leasing addresses.
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                {
                     "pool": "192.0.2.1 - 192.0.2.200"
                }
            ]
        }
    ]
# DHCPv4 configuration ends with the next line
}

} 

The following paragraphs provide a brief overview of the parameters in the above example together with their format. Subsequent sections of this chapter go into much greater detail for these and other parameters.

The lines starting with a hash (#) are comments and are ignored by the server; they do not impact its operation in any way.

The configuration starts in the first line with the initial opening curly bracket (or brace). Each configuration must contain an object specifying the configuration of the Kea module using it. In the example above this object is called Dhcp4.

Note

In the current Kea release it is possible to specify configurations of multiple modules within a single configuration file, but this is not recommended and support for it will be removed in the future releases. The only object, besides the one specifying module configuration, which can (and usually is) included in the same file is Logging. However, we don't include this object in the example above for clarity.

The Dhcp4 configuration starts with the "Dhcp4": { line and ends with the corresponding closing brace (in the above example, the brace after the last comment). Everything defined between those lines is considered to be the Dhcp4 configuration.

In the general case, the order in which those parameters appear does not matter. There are two caveats here though. The first one is to remember that the configuration file must be well formed JSON. That means that the parameters for any given scope must be separated by a comma and there must not be a comma after the last parameter. When reordering a configuration file, keep in mind that moving a parameter to or from the last position in a given scope may also require moving the comma. The second caveat is that it is uncommon — although legal JSON — to repeat the same parameter multiple times. If that happens, the last occurrence of a given parameter in a given scope is used while all previous instances are ignored. This is unlikely to cause any confusion as there are no real life reasons to keep multiple copies of the same parameter in your configuration file.

Moving onto the DHCPv4 configuration elements, the first few elements define some global parameters. valid-lifetime defines how long the addresses (leases) given out by the server are valid. If nothing changes, a client that got an address is allowed to use it for 4000 seconds. (Note that integer numbers are specified as is, without any quotes around them.) renew-timer and rebind-timer are values (also in seconds) that define T1 and T2 timers that govern when the client will begin the renewal and rebind procedures.

Note

Both renew-timer and rebind-timer are optional. The server will only send rebind-timer to the client, via DHPv4 option code 59, if it is less than valid-lifetime; and it will only send renew-timer, via DHCPv4 option code 58, if it is less than rebind-timer (or valid-lifetime if rebind-timer was not specified). In their absence, the client should select values for T1 and T2 timers according to the RFC 2131.

The interfaces-config map specifies the server configuration concerning the network interfaces, on which the server should listen to the DHCP messages. The interfaces parameter specifies a list of network interfaces on which the server should listen. Lists are opened and closed with square brackets, with elements separated by commas. Had we wanted to listen on two interfaces, the interfaces-config would look like this:

"interfaces-config": {
    "interfaces": [ "eth0", "eth1" ]
},

The next couple of lines define the lease database, the place where the server stores its lease information. This particular example tells the server to use memfile, which is the simplest (and fastest) database backend. It uses an in-memory database and stores leases on disk in a CSV file. This is a very simple configuration. Usually the lease database configuration is more extensive and contains additional parameters. Note that lease-database is an object and opens up a new scope, using an opening brace. Its parameters (just one in this example - type) follow. Had there been more than one, they would be separated by commas. This scope is closed with a closing brace. As more parameters for the Dhcp4 definition follow, a trailing comma is present.

Finally, we need to define a list of IPv4 subnets. This is the most important DHCPv4 configuration structure as the server uses that information to process clients' requests. It defines all subnets from which the server is expected to receive DHCP requests. The subnets are specified with the subnet4 parameter. It is a list, so it starts and ends with square brackets. Each subnet definition in the list has several attributes associated with it, so it is a structure and is opened and closed with braces. At a minimum, a subnet definition has to have at least two parameters: subnet (that defines the whole subnet) and pools (which is a list of dynamically allocated pools that are governed by the DHCP server).

The example contains a single subnet. Had more than one been defined, additional elements in the subnet4 parameter would be specified and separated by commas. For example, to define three subnets, the following syntax would be used:

"subnet4": [
    {
        "pools": [ { "pool":  "192.0.2.1 - 192.0.2.200" } ],
        "subnet": "192.0.2.0/24"
    },
    {
        "pools": [ { "pool": "192.0.3.100 - 192.0.3.200" } ],
        "subnet": "192.0.3.0/24"
    },
    {
        "pools": [ { "pool": "192.0.4.1 - 192.0.4.254" } ],
        "subnet": "192.0.4.0/24"
    }
]

Note that indentation is optional and is used for aesthetic purposes only. In some cases in may be preferable to use more compact notation.

After all the parameters have been specified, we have two contexts open: global and Dhcp4, hence we need two closing curly brackets to close them. In a real life configuration file there most likely would be an additional component defined such as Logging, so the closing brace would be followed by a comma and another object definition.

8.2.2. Lease Storage

All leases issued by the server are stored in the lease database. Currently there are four database backends available: memfile (which is the default backend), MySQL, PostgreSQL and Cassandra.

8.2.2.1. Memfile - Basic Storage for Leases

The server is able to store lease data in different repositories. Larger deployments may elect to store leases in a database. Section 8.2.2.2, “Lease Database Configuration” describes this option. In typical smaller deployments though, the server will store lease information in a CSV file rather than a database. As well as requiring less administration, an advantage of using a file for storage is that it eliminates a dependency on third-party database software.

The configuration of the file backend (Memfile) is controlled through the Dhcp4/lease-database parameters. The type parameter is mandatory and it specifies which storage for leases the server should use. The value of "memfile" indicates that the file should be used as the storage. The following list gives additional, optional, parameters that can be used to configure the Memfile backend.

  • persist: controls whether the new leases and updates to existing leases are written to the file. It is strongly recommended that the value of this parameter is set to true at all times, during the server's normal operation. Not writing leases to disk will mean that if a server is restarted (e.g. after a power failure), it will not know what addresses have been assigned. As a result, it may hand out addresses to new clients that are already in use. The value of false is mostly useful for performance testing purposes. The default value of the persist parameter is true, which enables writing lease updates to the lease file.
  • name: specifies an absolute location of the lease file in which new leases and lease updates will be recorded. The default value for this parameter is "[kea-install-dir]/var/kea/kea-leases4.csv" .
  • lfc-interval: specifies the interval in seconds, at which the server will perform a lease file cleanup (LFC). This removes redundant (historical) information from the lease file and effectively reduces the lease file size. The cleanup process is described in more detailed fashion further in this section. The default value of the lfc-interval is 3600. A value of 0 disables the LFC.

An example configuration of the Memfile backend is presented below:

"Dhcp4": {
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/tmp/kea-leases4.csv",
        "lfc-interval": 1800
    }
}

This configuration selects the /tmp/kea-leases4.csv as the storage for lease information and enables persistence (writing lease updates to this file). It also configures the backend perform the periodic cleanup of the lease files, executed every 30 minutes.

It is important to know how the lease file contents are organized to understand why the periodic lease file cleanup is needed. Every time the server updates a lease or creates a new lease for the client, the new lease information must be recorded in the lease file. For performance reasons, the server does not update the existing client's lease in the file, as it would potentially require rewriting the entire file. Instead, it simply appends the new lease information to the end of the file: the previous lease entries for the client are not removed. When the server loads leases from the lease file, e.g. at the server startup, it assumes that the latest lease entry for the client is the valid one. The previous entries are discarded. This means that the server can re-construct the accurate information about the leases even though there may be many lease entries for each client. However, storing many entries for each client results in bloated lease file and impairs the performance of the server's startup and reconfiguration as it needs to process a larger number of lease entries.

Lease file cleanup (LFC) removes all previous entries for each client and leaves only the latest ones. The interval at which the cleanup is performed is configurable, and it should be selected according to the frequency of lease renewals initiated by the clients. The more frequent the renewals, the smaller the value of lfc-interval should be. Note however, that the LFC takes time and thus it is possible (although unlikely) that new cleanup is started while the previous cleanup instance is still running, if the lfc-interval is too short. The server would recover from this by skipping the new cleanup when it detects that the previous cleanup is still in progress. But it implies that the actual cleanups will be triggered more rarely than configured. Moreover, triggering a new cleanup adds an overhead to the server which will not be able to respond to new requests for a short period of time when the new cleanup process is spawned. Therefore, it is recommended that the lfc-interval value is selected in a way that would allow for the LFC to complete the cleanup before a new cleanup is triggered.

Lease file cleanup is performed by a separate process (in background) to avoid a performance impact on the server process. In order to avoid the conflicts between two processes both using the same lease files, the LFC process operates on the copy of the original lease file, rather than on the lease file used by the server to record lease updates. There are also other files being created as a side effect of the lease file cleanup. The detailed description of the LFC is located in the Kea Administrator's Reference Manual: https://jenkins.isc.org/job/Kea_doc/guide/kea-guide.html#kea-lfc.

8.2.2.2. Lease Database Configuration

Note

Lease database access information must be configured for the DHCPv4 server, even if it has already been configured for the DHCPv6 server. The servers store their information independently, so each server can use a separate database or both servers can use the same database.

Lease database configuration is controlled through the Dhcp4/lease-database parameters. The type of the database must be set to "memfile", "mysql", "postgresql" or "cql", e.g.

"Dhcp4": { "lease-database": { "type": "mysql", ... }, ... }

Next, the name of the database to hold the leases must be set: this is the name used when the database was created (see Section 4.3.2.1, “First Time Creation of the MySQL Database”, Section 4.3.3.1, “First Time Creation of the PostgreSQL Database” or Section 4.3.4.1, “First Time Creation of the Cassandra Database”).

"Dhcp4": { "lease-database": { "name": "database-name" , ... }, ... }

For Cassandra:

"Dhcp4": { "lease-database": { "keyspace": "database-name" , ... }, ... }

If the database is located on a different system to the DHCPv4 server, the database host name must also be specified. (It should be noted that this configuration may have a severe impact on server performance.):

"Dhcp4": { "lease-database": { "host": "remote-host-name", ... }, ... }

The usual state of affairs will be to have the database on the same machine as the DHCPv4 server. In this case, set the value to the empty string:

"Dhcp4": { "lease-database": { "host" : "", ... }, ... }

Should the database use a port different than default, it may be specified as well:

"Dhcp4": { "lease-database": { "port" : 12345, ... }, ... }

Should the database be located on a different system, you may need to specify a longer interval for the connection timeout:

"Dhcp4": { "lease-database": { "connect-timeout" : timeout-in-seconds, ... }, ... }

The default value of five seconds should be more than adequate for local connections. If a timeout is given though, it should be an integer greater than zero.

The maxiumum number of times the server will automatically attempt to reconnect to the lease database after connectivity has been lost may be specified:

"Dhcp4": { "lease-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts the server will exit. A value of zero (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and Postgres only).

The number of milliseconds the server will wait in between attempts to reconnect to the lease database after connectivity has been lost may also be specified:

"Dhcp4": { "lease-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and Postgres is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity. The default value for Cassandra is 2000 ms.

Note

Automatic reconnect to database backends is configured individually per backend. This allows you to tailor the recovery parameters to the each backend you use. We do suggest that you either enable it for all backends or no backends so you have consistent behavior. Losing connectivity to a backend for which reconnect is disabled will result in the server shutting itself down. This includes the cases when lease database backend and hosts database backend is connected to the same database instance.

Finally, the credentials of the account under which the server will access the database should be set:

"Dhcp4": { "lease-database": { "user": "user-name",
                               "password": "password",
                              ... },
           ... }

If there is no password to the account, set the password to the empty string "". (This is also the default.)

8.2.2.3. Cassandra specific parameters

Cassandra backend is configured slightly differently. Cassandra has a concept of contact points that could be used to contact the cluster, instead of a single IP or hostname. It takes a list of comma separated IP addresses. This may be specified as:

"Dhcp4": {
    "lease-database": {
        "type": "cql",
        "contact-points": "ip-address1, ip-address2 [,...]",
        ...
    },
    ...
}

Cassandra also supports a number of optional parameters:

  • reconnect-wait-time - governs how long Kea waits before attempting to reconnect. Expressed in milliseconds. The default is 2000 [ms].
  • connect-timeout - sets the timeout for connecting to a node. Expressed in milliseconds. The default is 5000 [ms].
  • request-timeout - this parameter sets the timeout for waiting for a response from a node. Expressed in milliseconds. The default is 12000 [ms].
  • tcp-keepalive - This parameter governs the TCP keep-alive mechanism. Expressed in seconds of delay. If the parameter is not present, the mechanism is disabled.
  • tcp-nodelay - This parameter enables/disabled Nagle's algorithm on connections. The default is true.

For example, a complex Cassandra configuration with most parameters specified could look as follows:

"Dhcp4": {
  "lease-database": {
      "type": "cql",
      "keyspace": "keatest",
      "contact-points": "192.0.2.1, 192.0.2.2, 192.0.2.3",
      "port": 9042,
      "reconnect-wait-time": 2000,
      "connect-timeout": 5000,
      "request-timeout": 12000,
      "tcp-keepalive": 1,
      "tcp-nodelay": true
    },
    ...
}

Similar parameters can be specified for hosts database.

8.2.3. Hosts Storage

Kea is also able to store information about host reservations in the database. The hosts database configuration uses the same syntax as the lease database. In fact, a Kea server opens independent connections for each purpose, be it lease or hosts information. This arrangement gives the most flexibility. Kea can be used to keep leases and host reservations separately, but can also point to the same database. Currently the supported hosts database types are MySQL, PostgreSQL and Cassandra.

Please note that usage of hosts storage is optional. A user can define all host reservations in the configuration file. That is the recommended way if the number of reservations is small. However, when the number of reservations grows it's more convenient to use host storage. Please note that both storage methods (configuration file and one of the supported databases) can be used together. If hosts are defined in both places, the definitions from the configuration file are checked first and external storage is checked later, if necessary.

Version 1.4 extends the host storage to multiple storages. Operations are performed on host storages in the configuration order with a special case for addition: read-only storages must be configured after a required read-write storage, or host reservation addition will always fail.

8.2.3.1. DHCPv4 Hosts Database Configuration

Hosts database configuration is controlled through the Dhcp4/hosts-database parameters. If enabled, the type of the database must be set to "mysql" or "postgresql". Other hosts backends may be added in later versions of Kea.

"Dhcp4": { "hosts-database": { "type": "mysql", ... }, ... }

Next, the name of the database to hold the reservations must be set: this is the name used when the lease database was created (see Section 4.3, “Supported Databases” for instructions how to setup the desired database type).

"Dhcp4": { "hosts-database": { "name": "database-name" , ... }, ... }

If the database is located on a different system than the DHCPv4 server, the database host name must also be specified. (Again it should be noted that this configuration may have a severe impact on server performance.):

"Dhcp4": { "hosts-database": { "host": remote-host-name, ... }, ... }

The usual state of affairs will be to have the database on the same machine as the DHCPv4 server. In this case, set the value to the empty string:

"Dhcp4": { "hosts-database": { "host" : "", ... }, ... }

Should the database use a port different than default, it may be specified as well:

"Dhcp4": { "hosts-database": { "port" : 12345, ... }, ... }

The maxiumum number of times the server will automatically attempt to reconnect to the host database after connectivity has been lost may be specified:

"Dhcp4": { "hosts-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts the server will exit. A value of zero (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and Postgres only).

The number of milliseconds the server will wait in between attempts to reconnect to the host database after connectivity has been lost may also be specified:

"Dhcp4": { "hosts-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and Postgres is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity. The default value for Cassandra is 2000 ms.

Note

Automatic reconnect to database backends is configured individually per backend. This allows you to tailor the recovery parameters to the each backend you use. We do suggest that you either enable it for all backends or no backends so you have consistent behavior. Losing connectivity to a backend for which reconnect is disabled will result in the server shutting itself down. This includes the cases when lease database backend and hosts database backend is connected to the same database instance.

Finally, the credentials of the account under which the server will access the database should be set:

"Dhcp4": { "hosts-database": { "user": "user-name",
                               "password": "password",
                              ... },
           ... }

If there is no password to the account, set the password to the empty string "". (This is also the default.)

The multiple storage extension uses a similar syntax: a configuration is placed into a "hosts-databases" list instead of into a "hosts-database" entry as in:

"Dhcp4": { "hosts-databases": [ { "type": "mysql", ... }, ... ], ... }

For additional Cassandra specific parameters, see Section 8.2.2.3, “Cassandra specific parameters”.

8.2.3.2. Using Read-Only Databases for Host Reservations

In some deployments the database user whose name is specified in the database backend configuration may not have write privileges to the database. This is often required by the policy within a given network to secure the data from being unintentionally modified. In many cases administrators have inventory databases deployed, which contain substantially more information about the hosts than static reservations assigned to them. The inventory database can be used to create a view of a Kea hosts database and such view is often read only.

Kea host database backends operate with an implicit configuration to both read from and write to the database. If the database user does not have write access to the host database, the backend will fail to start and the server will refuse to start (or reconfigure). However, if access to a read only host database is required for retrieving reservations for clients and/or assign specific addresses and options, it is possible to explicitly configure Kea to start in "read-only" mode. This is controlled by the readonly boolean parameter as follows:

"Dhcp4": { "hosts-database": { "readonly": true, ... }, ... }

Setting this parameter to false would configure the database backend to operate in "read-write" mode, which is also a default configuration if the parameter is not specified.

Note

The readonly parameter is currently only supported for MySQL and PostgreSQL databases.

8.2.4. Interface Configuration

The DHCPv4 server has to be configured to listen on specific network interfaces. The simplest network interface configuration tells the server to listen on all available interfaces:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "*" ]
    }
    ...
},
  

The asterisk plays the role of a wildcard and means "listen on all interfaces". However, it is usually a good idea to explicitly specify interface names:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ]
    },
    ...
}
  

It is possible to use wildcard interface name (asterisk) concurrently with explicit interface names:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3", "*" ]
    },
    ...
}
  

It is anticipated that this form of usage will only be used when it is desired to temporarily override a list of interface names and listen on all interfaces.

Some deployments of DHCP servers require that the servers listen on the interfaces with multiple IPv4 addresses configured. In these situations, the address to use can be selected by appending an IPv4 address to the interface name in the following manner:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1/10.0.0.1", "eth3/192.0.2.3" ]
    },
    ...
}
  

Should the server be required to listen on multiple IPv4 addresses assigned to the same interface, multiple addresses can be specified for an interface as in the example below:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1/10.0.0.1", "eth1/10.0.0.2" ]
    },
    ...
}
  

Alternatively, if the server should listen on all addresses for the particular interface, an interface name without any address should be specified.

Kea supports responding to directly connected clients which don't have an address configured. This requires that the server injects the hardware address of the destination into the data link layer of the packet being sent to the client. The DHCPv4 server utilizes the raw sockets to achieve this, and builds the entire IP/UDP stack for the outgoing packets. The down side of raw socket use, however, is that incoming and outgoing packets bypass the firewalls (e.g. iptables). It is also troublesome to handle traffic on multiple IPv4 addresses assigned to the same interface, as raw sockets are bound to the interface and advanced packet filtering techniques (e.g. using the BPF) have to be used to receive unicast traffic on the desired addresses assigned to the interface, rather than capturing whole traffic reaching the interface to which the raw socket is bound. Therefore, in the deployments where the server doesn't have to provision the directly connected clients and only receives the unicast packets from the relay agents, the DHCP server should be configured to utilize IP/UDP datagram sockets instead of raw sockets. The following configuration demonstrates how this can be achieved:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "dhcp-socket-type": "udp"
    },
    ...
}
  

The dhcp-socket-type specifies that the IP/UDP sockets will be opened on all interfaces on which the server listens, i.e. "eth1" and "eth3" in our case. If the dhcp-socket-type is set to raw, it configures the server to use raw sockets instead. If the dhcp-socket-type value is not specified, the default value raw is used.

Using UDP sockets automatically disables the reception of broadcast packets from directly connected clients. This effectively means that the UDP sockets can be used for relayed traffic only. When using the raw sockets, both the traffic from the directly connected clients and the relayed traffic will be handled. Caution should be taken when configuring the server to open multiple raw sockets on the interface with several IPv4 addresses assigned. If the directly connected client sends the message to the broadcast address all sockets on this link will receive this message and multiple responses will be sent to the client. Hence, the configuration with multiple IPv4 addresses assigned to the interface should not be used when the directly connected clients are operating on that link. To use a single address on such interface, the "interface-name/address" notation should be used.

Note

Specifying the value raw as the socket type, doesn't guarantee that the raw sockets will be used! The use of raw sockets to handle the traffic from the directly connected clients is currently supported on Linux and BSD systems only. If the raw sockets are not supported on the particular OS, the server will issue a warning and fall back to use IP/UDP sockets.

In typical environment the DHCP server is expected to send back a response on the same network interface on which the query is received. This is the default behavior. However, in some deployments it is desired that the outbound (response) packets will be sent as regular traffic and the outbound interface will be determined by the routing tables. This kind of asymetric traffic is uncommon, but valid. Kea now supports a parameter called outbound-interface that controls this behavior. It supports two values. The first one, same-as-inbound, tells Kea to send back the response on the same inteface the query packet is received. This is the default behavior. The second one, use-routing tells Kea to send regular UDP packets and let the kernel's routing table to determine most appropriate interface. This only works when dhcp-socket-type is set to udp. An example configuration looks as follows:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "dhcp-socket-type": "udp",
        "outbound-interface": "use-routing"
    },
    ...
}

Interfaces are re-detected at each reconfiguration. This behavior can be disabled by setting re-detect value to false, for instance:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "re-detect": false
    },
    ...
}
  

Note interfaces are not re-detected during config-test.

Usually loopback interfaces (e.g. the "lo" or "lo0" interface) may not be configured but if a loopback interface is explicitely configured and IP/UDP sockets are specified the loopback interface is accepted.

It can be used for instance to run Kea in a FreeBSD jail having only a loopback interface, servicing relayed DHCP request:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "lo0" ],
        "dhcp-socket-type": "udp"
    },
    ...
}

8.2.5. Issues with Unicast Responses to DHCPINFORM

The use of UDP sockets has certain benefits in deployments where the server receives only relayed traffic; these benefits are mentioned in Section 8.2.4, “Interface Configuration”. From the administrator's perspective it is often desirable to configure the system's firewall to filter out the unwanted traffic, and the use of UDP sockets facilitates this. However, the administrator must also be aware of the implications related to filtering certain types of traffic as it may impair the DHCP server's operation.

In this section we are focusing on the case when the server receives the DHCPINFORM message from the client via a relay. According to RFC 2131, the server should unicast the DHCPACK response to the address carried in the "ciaddr" field. When the UDP socket is in use, the DHCP server relies on the low level functions of an operating system to build the data link, IP and UDP layers of the outgoing message. Typically, the OS will first use ARP to obtain the client's link layer address to be inserted into the frame's header, if the address is not cached from a previous transaction that the client had with the server. When the ARP exchange is successful, the DHCP message can be unicast to the client, using the obtained address.

Some system administrators block ARP messages in their network, which causes issues for the server when it responds to the DHCPINFORM messages, because the server is unable to send the DHCPACK if the preceding ARP communication fails. Since the OS is entirely responsible for the ARP communication and then sending the DHCP packet over the wire, the DHCP server has no means to determine that the ARP exchange failed and the DHCP response message was dropped. Thus, the server does not log any error messages when the outgoing DHCP response is dropped. At the same time, all hooks pertaining to the packet sending operation will be called, even though the message never reaches its destination.

Note that the issue described in this section is not observed when the raw sockets are in use, because, in this case, the DHCP server builds all the layers of the outgoing message on its own and does not use ARP. Instead, it inserts the value carried in the 'chaddr' field of the DHCPINFORM message into the link layer.

Server administrators willing to support DHCPINFORM messages via relays should not block ARP traffic in their networks or should use raw sockets instead of UDP sockets.

8.2.6. IPv4 Subnet Identifier

The subnet identifier is a unique number associated with a particular subnet. In principle, it is used to associate clients' leases with their respective subnets. When a subnet identifier is not specified for a subnet being configured, it will be automatically assigned by the configuration mechanism. The identifiers are assigned from 1 and are monotonically increased for each subsequent subnet: 1, 2, 3 ....

If there are multiple subnets configured with auto-generated identifiers and one of them is removed, the subnet identifiers may be renumbered. For example: if there are four subnets and the third is removed the last subnet will be assigned the identifier that the third subnet had before removal. As a result, the leases stored in the lease database for subnet 3 are now associated with subnet 4, something that may have unexpected consequences. It is planned to implement a mechanism to preserve auto-generated subnet ids in a future version of Kea. However, the only remedy for this issue at present is to manually specify a unique identifier for each subnet.

Note

Subnet IDs must be greater than zero and less than 4294967295.

The following configuration will assign the specified subnet identifier to the newly configured subnet:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "id": 1024,
            ...
        }
    ]
}

This identifier will not change for this subnet unless the "id" parameter is removed or set to 0. The value of 0 forces auto-generation of the subnet identifier.

8.2.7. Configuration of IPv4 Address Pools

The main role of a DHCPv4 server is address assignment. For this, the server has to be configured with at least one subnet and one pool of dynamic addresses for it to manage. For example, assume that the server is connected to a network segment that uses the 192.0.2.0/24 prefix. The Administrator of that network has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to be managed by the Dhcp4 server. Such a configuration can be achieved in the following way:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                { "pool": "192.0.2.10 - 192.0.2.20" }
            ],
            ...
        }
    ]
}

Note that subnet is defined as a simple string, but the pools parameter is actually a list of pools: for this reason, the pool definition is enclosed in square brackets, even though only one range of addresses is specified.

Each pool is a structure that contains the parameters that describe a single pool. Currently there is only one parameter, pool, which gives the range of addresses in the pool. Additional parameters will be added in future releases of Kea.

It is possible to define more than one pool in a subnet: continuing the previous example, further assume that 192.0.2.64/26 should be also be managed by the server. It could be written as 192.0.2.64 to 192.0.2.127. Alternatively, it can be expressed more simply as 192.0.2.64/26. Both formats are supported by Dhcp4 and can be mixed in the pool list. For example, one could define the following pools:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                { "pool": "192.0.2.10-192.0.2.20" },
                { "pool": "192.0.2.64/26" }
            ],
            ...
        }
    ],
    ...
}

White space in pool definitions is ignored, so spaces before and after the hyphen are optional. They can be used to improve readability.

The number of pools is not limited, but for performance reasons it is recommended to use as few as possible.

The server may be configured to serve more than one subnet:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.1 - 192.0.2.200" } ],
            ...
        },
        {
            "subnet": "192.0.3.0/24",
            "pools": [ { "pool": "192.0.3.100 - 192.0.3.200" } ],
            ...
        },
        {
            "subnet": "192.0.4.0/24",
            "pools": [ { "pool": "192.0.4.1 - 192.0.4.254" } ],
            ...
        }
    ]
}

When configuring a DHCPv4 server using prefix/length notation, please pay attention to the boundary values. When specifying that the server can use a given pool, it will also be able to allocate the first (typically network address) and the last (typically broadcast address) address from that pool. In the aforementioned example of pool 192.0.3.0/24, both 192.0.3.0 and 192.0.3.255 addresses may be assigned as well. This may be invalid in some network configurations. If you want to avoid this, please use the "min-max" notation.

8.2.8. Standard DHCPv4 Options

One of the major features of the DHCPv4 server is to provide configuration options to clients. Most of the options are sent by the server only if the client explicitly requests them using the Parameter Request List option. Those that do not require inclusion in the Parameter Request List option are commonly used options, e.g. "Domain Server", and options which require special behavior, e.g. "Client FQDN" is returned to the client if the client has included this option in its message to the server.

Table 8.1, “List of standard DHCPv4 options” comprises the list of the standard DHCPv4 options whose values can be configured using the configuration structures described in this section. This table excludes the options which require special processing and thus cannot be configured with some fixed values. The last column of the table indicates which options can be sent by the server even when they are not requested in the Parameter Request list option, and those which are sent only when explicitly requested.

The following example shows how to configure the addresses of DNS servers, which is one of the most frequently used options. Options specified in this way are considered global and apply to all configured subnets.

"Dhcp4": {
    "option-data": [
        {
           "name": "domain-name-servers",
           "code": 6,
           "space": "dhcp4",
           "csv-format": true,
           "data": "192.0.2.1, 192.0.2.2"
        },
        ...
    ]
}
        

Note that only one of name or code is required, you don't need to specify both. Space has a default value of "dhcp4", so you can skip this as well if you define a regular (not encapsulated) DHCPv4 option. Finally, csv-format defaults to true, so it too can be skipped, unless you want to specify the option value as hexstring. Therefore the above example can be simplified to:

"Dhcp4": {
    "option-data": [
        {
           "name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2"
        },
        ...
    ]
}
        

Defined options are added to response when the client requests them at a few exceptions which are always added. To enforce the addition of a particular option set the always-send flag to true as in:

"Dhcp4": {
    "option-data": [
        {
           "name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2",
           "always-send": true
        },
        ...
    ]
}
        

The effect is the same as if the client added the option code in the Parameter Request List option (or its equivalent for vendor options) so in:

"Dhcp4": {
    "option-data": [
        {
           "name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2",
           "always-send": true
        },
        ...
    ],
    "subnet4": [
        {
           "subnet": "192.0.3.0/24",
           "option-data": [
               {
                   "name": "domain-name-servers",
                   "data": "192.0.3.1, 192.0.3.2"
               },
               ...
           ],
           ...
        },
        ...
    ],
    ...
}
        

The Domain Name Servers option is always added to responses (the always-send is "sticky") but the value is the subnet one when the client is localized in the subnet.

The name parameter specifies the option name. For a list of currently supported names, see Table 8.1, “List of standard DHCPv4 options” below. The code parameter specifies the option code, which must match one of the values from that list. The next line specifies the option space, which must always be set to "dhcp4" as these are standard DHCPv4 options. For other option spaces, including custom option spaces, see Section 8.2.12, “Nested DHCPv4 Options (Custom Option Spaces)”. The next line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. The sixth line gives the actual value to be sent to clients. Data is specified as normal text, with values separated by commas if more than one value is allowed.

Options can also be configured as hexadecimal values. If csv-format is set to false, option data must be specified as a hexadecimal string. The following commands configure the domain-name-servers option for all subnets with the following addresses: 192.0.3.1 and 192.0.3.2. Note that csv-format is set to false.

"Dhcp4": {
    "option-data": [
        {
            "name": "domain-name-servers",
            "code": 6,
            "space": "dhcp4",
            "csv-format": false,
            "data": "C0 00 03 01 C0 00 03 02"
        },
        ...
    ],
    ...
}

Care should be taken to use proper encoding when using hexadecimal format as Kea's ability to validate data correctness in hexadecimal is limited.

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 8.2.13, “Unspecified Parameters for DHCPv4 Option Configuration”.

It is possible to specify or override options on a per-subnet basis. If clients connected to most of your subnets are expected to get the same values of a given option, you should use global options: you can then override specific values for a small number of subnets. On the other hand, if you use different values in each subnet, it does not make sense to specify global option values (Dhcp4/option-data), rather you should set only subnet-specific values (Dhcp4/subnet[X]/option-data[Y]).

The following commands override the global DNS servers option for a particular subnet, setting a single DNS server with address 192.0.2.3.

"Dhcp4": {
    "subnet4": [
        {
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.3"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

In some cases it is useful to associate some options with an address pool from which a client is assigned a lease. Pool specific option values override subnet specific and global option values. The server's administrator must not try to prioritize assignment of pool specific options by trying to order pools declarations in the server configuration. Future Kea releases may change the order in which options are assigned from the pools without any notice.

The following configuration snippet demonstrates how to specify the DNS servers option, which will be assigned to a client only if the client obtains an address from the given pool:

"Dhcp4": {
    "subnet4": [
        {
            "pools": [
                {
                    "pool": "192.0.2.1 - 192.0.2.200",
                    "option-data": [
                        {
                            "name": "domain-name-servers",
                            "data": "192.0.2.3"
                         },
                         ...
                    ],
                    ...
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

Options can be specified also in class of host reservation scope. In Kea 1.4 options precedence order is (from most important): host reservation, pool, subnet, shared network, class, global. In Kea 1.5 order will be changed to: host reservation, class, pool, subnet, shared network, global OR it will be fully configurable.

The currently supported standard DHCPv4 options are listed in Table 8.1, “List of standard DHCPv4 options”. The "Name" and "Code" are the values that should be used as a name in the option-data structures. "Type" designates the format of the data: the meanings of the various types is given in Table 8.2, “List of standard DHCP option types”.

When a data field is a string, and that string contains the comma (,; U+002C) character, the comma must be escaped with a double reverse solidus character (\; U+005C). This double escape is required, because both the routine splitting CSV data into fields and JSON use the same escape character: a single escape (\,) would make the JSON invalid. For example, the string "foo,bar" would be represented as:

"Dhcp4": {
    "subnet4": [
        {
            "pools": [
                {
                    "option-data": [
                        {
                            "name": "boot-file-name",
                            "data": "foo\\,bar"
                        }
                    ]
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

Some options are designated as arrays, which means that more than one value is allowed in such an option. For example the option time-servers allows the specification of more than one IPv4 address, so allowing clients to obtain the addresses of multiple NTP servers.

The Section 8.2.9, “Custom DHCPv4 options” describes the configuration syntax to create custom option definitions (formats). It is generally not allowed to create custom definitions for standard options, even if the definition being created matches the actual option format defined in the RFCs. There is an exception from this rule for standard options for which Kea currently does not provide a definition. In order to use such options, a server administrator must create a definition as described in Section 8.2.9, “Custom DHCPv4 options” in the 'dhcp4' option space. This definition should match the option format described in the relevant RFC but the configuration mechanism will allow any option format as it presently has no means to validate it.

Table 8.1. List of standard DHCPv4 options

NameCodeTypeArray?Returned if not requested?
time-offset2int32falsefalse
routers3ipv4-addresstruetrue
time-servers4ipv4-addresstruefalse
name-servers5ipv4-addresstruefalse
domain-name-servers6ipv4-addresstruetrue
log-servers7ipv4-addresstruefalse
cookie-servers8ipv4-addresstruefalse
lpr-servers9ipv4-addresstruefalse
impress-servers10ipv4-addresstruefalse
resource-location-servers11ipv4-addresstruefalse
boot-size13uint16falsefalse
merit-dump14stringfalsefalse
domain-name15fqdnfalsetrue
swap-server16ipv4-addressfalsefalse
root-path17stringfalsefalse
extensions-path18stringfalsefalse
ip-forwarding19booleanfalsefalse
non-local-source-routing20booleanfalsefalse
policy-filter21ipv4-addresstruefalse
max-dgram-reassembly22uint16falsefalse
default-ip-ttl23uint8falsefalse
path-mtu-aging-timeout24uint32falsefalse
path-mtu-plateau-table25uint16truefalse
interface-mtu26uint16falsefalse
all-subnets-local27booleanfalsefalse
broadcast-address28ipv4-addressfalsefalse
perform-mask-discovery29booleanfalsefalse
mask-supplier30booleanfalsefalse
router-discovery31booleanfalsefalse
router-solicitation-address32ipv4-addressfalsefalse
static-routes33ipv4-addresstruefalse
trailer-encapsulation34booleanfalsefalse
arp-cache-timeout35uint32falsefalse
ieee802-3-encapsulation36booleanfalsefalse
default-tcp-ttl37uint8falsefalse
tcp-keepalive-interval38uint32falsefalse
tcp-keepalive-garbage39booleanfalsefalse
nis-domain40stringfalsefalse
nis-servers41ipv4-addresstruefalse
ntp-servers42ipv4-addresstruefalse
vendor-encapsulated-options43emptyfalsefalse
netbios-name-servers44ipv4-addresstruefalse
netbios-dd-server45ipv4-addresstruefalse
netbios-node-type46uint8falsefalse
netbios-scope47stringfalsefalse
font-servers48ipv4-addresstruefalse
x-display-manager49ipv4-addresstruefalse
dhcp-option-overload52uint8falsefalse
dhcp-server-identifier54ipv4-addressfalsetrue
dhcp-message56stringfalsefalse
dhcp-max-message-size57uint16falsefalse
vendor-class-identifier60hexfalsefalse
nwip-domain-name62stringfalsefalse
nwip-suboptions63hexfalsefalse
nisplus-domain-name64stringfalsefalse
nisplus-servers65ipv4-addresstruefalse
tftp-server-name66stringfalsefalse
boot-file-name67stringfalsefalse
mobile-ip-home-agent68ipv4-addresstruefalse
smtp-server69ipv4-addresstruefalse
pop-server70ipv4-addresstruefalse
nntp-server71ipv4-addresstruefalse
www-server72ipv4-addresstruefalse
finger-server73ipv4-addresstruefalse
irc-server74ipv4-addresstruefalse
streettalk-server75ipv4-addresstruefalse
streettalk-directory-assistance-server76ipv4-addresstruefalse
user-class77hexfalsefalse
slp-directory-agent78record (boolean, ipv4-address)truefalse
slp-service-scope79record (boolean, string)falsefalse
nds-server85ipv4-addresstruefalse
nds-tree-name86stringfalsefalse
nds-context87stringfalsefalse
bcms-controller-names88fqdntruefalse
bcms-controller-address89ipv4-addresstruefalse
client-system93uint16truefalse
client-ndi94record (uint8, uint8, uint8)falsefalse
uuid-guid97record (uint8, hex)falsefalse
uap-servers98stringfalsefalse
geoconf-civic99hexfalsefalse
pcode100stringfalsefalse
tcode101stringfalsefalse
netinfo-server-address112ipv4-addresstruefalse
netinfo-server-tag113stringfalsefalse
default-url114stringfalsefalse
auto-config116uint8falsefalse
name-service-search117uint16truefalse
subnet-selection118ipv4-addressfalsefalse
domain-search119fqdntruefalse
vivco-suboptions124hexfalsefalse
vivso-suboptions125hexfalsefalse
pana-agent136ipv4-addresstruefalse
v4-lost137fqdnfalsefalse
capwap-ac-v4138ipv4-addresstruefalse
sip-ua-cs-domains142fqdntruefalse
rdnss-selection146record (uint8, ipv4-address, ipv4-address, fqdn)truefalse
v4-portparams159record (uint8, psid)falsefalse
v4-captive-portal160stringfalsefalse
option-6rd212record (uint8, uint8, ipv6-address, ipv4-address)truefalse
v4-access-domain213fqdnfalsefalse


Table 8.2. List of standard DHCP option types

NameMeaning
binaryAn arbitrary string of bytes, specified as a set of hexadecimal digits.
booleanBoolean value with allowed values true or false
emptyNo value, data is carried in suboptions
fqdnFully qualified domain name (e.g. www.example.com)
ipv4-addressIPv4 address in the usual dotted-decimal notation (e.g. 192.0.2.1)
ipv6-addressIPv6 address in the usual colon notation (e.g. 2001:db8::1)
ipv6-prefixIPv6 prefix and prefix length specified using CIDR notation, e.g. 2001:db8:1::/64. This data type is used to represent an 8-bit field conveying a prefix length and the variable length prefix value
psidPSID and PSID length separated by a slash, e.g. 3/4 specifies PSID=3 and PSID length=4. In the wire format it is represented by an 8-bit field carrying PSID length (in this case equal to 4) and the 16-bits long PSID value field (in this case equal to "0011000000000000b" using binary notation). Allowed values for a PSID length are 0 to 16. See RFC 7597 for the details about the PSID wire representation
recordStructured data that may be comprised of any types (except "record" and "empty"). The array flag applies to the last field only.
stringAny text
tupleA length encoded as a 8 (16 for DHCPv6) bit unsigned integer followed by a string of this length
uint88 bit unsigned integer with allowed values 0 to 255
uint1616 bit unsigned integer with allowed values 0 to 65535
uint3232 bit unsigned integer with allowed values 0 to 4294967295
int88 bit signed integer with allowed values -128 to 127
int1616 bit signed integer with allowed values -32768 to 32767
int3232 bit signed integer with allowed values -2147483648 to 2147483647


8.2.9. Custom DHCPv4 options

Kea supports custom (non-standard) DHCPv4 options. Assume that we want to define a new DHCPv4 option called "foo" which will have a code 222 and will convey a single unsigned 32 bit integer value. We can define such an option by using the following entry in the configuration file:

"Dhcp4": {
    "option-def": [
        {
            "name": "foo",
            "code": 222,
            "type": "uint32",
            "array": false,
            "record-types": "",
            "space": "dhcp4",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The false value of the array parameter determines that the option does NOT comprise an array of "uint32" values but is, instead, a single value. Two other parameters have been left blank: record-types and encapsulate. The former specifies the comma separated list of option data fields if the option comprises a record of data fields. This should be non-empty if the type is set to "record". Otherwise it must be left blank. The latter parameter specifies the name of the option space being encapsulated by the particular option. If the particular option does not encapsulate any option space it should be left blank. Note that the above set of comments define the format of the new option and do not set its values.

The name, code and type parameters are required, all others are optional. The array default value is false. The record-types and encapsulate default values are blank (i.e. ""). The default space is "dhcp4".

Once the new option format is defined, its value is set in the same way as for a standard option. For example the following commands set a global value that applies to all subnets.

"Dhcp4": {
    "option-data": [
        {
            "name": "foo",
            "code": 222,
            "space": "dhcp4",
            "csv-format": true,
            "data": "12345"
        }, ...
    ],
    ...
}

New options can take more complex forms than simple use of primitives (uint8, string, ipv4-address etc): it is possible to define an option comprising a number of existing primitives. Assume we want to define a new option that will consist of an IPv4 address, followed by an unsigned 16 bit integer, followed by a boolean value, followed by a text string. Such an option could be defined in the following way:

"Dhcp4": {
    "option-def": [
        {
            "name": "bar",
            "code": 223,
            "space": "dhcp4",
            "type": "record",
            "array": false,
            "record-types": "ipv4-address, uint16, boolean, string",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The type is set to "record" to indicate that the option contains multiple values of different types. These types are given as a comma-separated list in the record-types field and should be ones from those listed in Table 8.2, “List of standard DHCP option types”.

The values of the option are set as follows:

"Dhcp4": {
    "option-data": [
        {
            "name": "bar",
            "space": "dhcp4",
            "code": 223,
            "csv-format": true,
            "data": "192.0.2.100, 123, true, Hello World"
        }
    ],
    ...
}

csv-format is set to true to indicate that the data field comprises a command-separated list of values. The values in the data must correspond to the types set in the record-types field of the option definition.

When array is set to true and type is set to "record", the last field is an array, i.e., it can contain more than one value as in:

"Dhcp4": {
    "option-def": [
        {
            "name": "bar",
            "code": 223,
            "space": "dhcp4",
            "type": "record",
            "array": true,
            "record-types": "ipv4-address, uint16",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The new option content is one IPv4 address followed by one or more 16 bit unsigned integers.

Note

In the general case, boolean values are specified as true or false, without quotes. Some specific boolean parameters may accept also "true", "false", 0, 1, "0" and "1". Future versions of Kea will accept all those values for all boolean parameters.

Note

Numbers can be specified in decimal or hexadecimal format. The hexadecimal format can be either plain (e.g. abcd) or prefixed with 0x (e.g. 0xabcd).

8.2.10. DHCPv4 Private Options

Options with code between 224 and 254 are reserved for private use. They can be defined at the global scope or at client class local scope: this allows to use option definitions depending on context and to set option data accordingly. For instance to configure an old PXEClient vendor:

"Dhcp4": {
    "client-classes": [
        {
            "name": "pxeclient",
            "test": "option[vendor-class-identifier].text == 'PXEClient'",
            "option-def": [
                {
                    "name": "configfile",
                    "code": 209,
                    "type": "string"
                }
            ],
            ...
        }, ...
    ],
    ...
}

As the Vendor Specific Information option (code 43) has vendor specific format, i.e. can carry either raw binary value or sub-options, this mechanism is available for this option too.

In the following example taken from a real configuration two vendor classes use the option 43 for different and incompatible purposes:

"Dhcp4": {
    "option-def": [
        {
            "name": "cookie",
            "code": 1,
            "type": "string",
            "space": "APC"
        },
        {
            "name": "mtftp-ip",
            "code": 1,
            "type": "ipv4-address",
            "space": "PXE"
        },
        ...
    ],
    "client-classes": [
        {
            "name": "APC",
            "test": "(option[vendor-class-identifier].text == 'APC'",
            "option-def": [
                {
                    "name": "vendor-encapsulated-options",
                    "type": "empty",
                    "encapsulate": "APC"
                }
            ],
            "option-data": [
                {
                    "name": "cookie",
                    "space": "APC",
                    "data": "1APC"
                },
                {
                    "name": "vendor-encapsulated-options"
                },
                ...
            ],
            ...
        },
        {
            "name": "PXE",
            "test": "(option[vendor-class-identifier].text == 'PXE'",
            "option-def": [
                {
                    "name": "vendor-encapsulated-options",
                    "type": "empty",
                    "encapsulate": "PXE"
                }
            ],
            "option-data": [
                {
                    "name": "mtftp-ip",
                    "space": "PXE",
                    "data": "0.0.0.0"
                },
                {
                    "name": "vendor-encapsulated-options"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

The definition used to decode a VSI option is:

  1. The local definition of a client class the incoming packet belongs to

  2. If none, the global definition

  3. If none, the last resort definition described in the next section Section 8.2.11, “DHCPv4 Vendor Specific Options” (backward compatible with previous Kea versions).

Note

This last resort definition for the Vendor Specific Information option (code 43) is not compatible with a raw binary value. So when there are some known cases where a raw binary value will be used, a client class must be defined with a classification expression matching these cases and an option definition for the VSI option with a binary type and no encapsulation.

Note

Option definitions in client classes is allowed only for these limited option set (codes 43 and from 224 to 254), and only for DHCPv4.

8.2.11. DHCPv4 Vendor Specific Options

Currently there are two option spaces defined for the DHCPv4 daemon: "dhcp4" (for the top level DHCPv4 options) and "vendor-encapsulated-options-space", which is empty by default but in which options can be defined. Such options will be carried in the Vendor Specific Information option (code 43). The following examples show how to define an option "foo" in that space that has a code 1, and comprises an IPv4 address, an unsigned 16 bit integer and a string. The "foo" option is conveyed in a Vendor Specific Information option.

The first step is to define the format of the option:

"Dhcp4": {
    "option-def": [
        {
            "name": "foo",
            "code": 1,
            "space": "vendor-encapsulated-options-space",
            "type": "record",
            "array": false,
            "record-types": "ipv4-address, uint16, string",
            "encapsulate": ""
        }
    ],
    ...
}

(Note that the option space is set to "vendor-encapsulated-options-space".) Once the option format is defined, the next step is to define actual values for that option:

"Dhcp4": {
    "option-data": [
        {
            "name": "foo",
            "space": "vendor-encapsulated-options-space",
            "code": 1,
            "csv-format": true,
            "data": "192.0.2.3, 123, Hello World"
        }
    ],
    ...
}

We also include the Vendor Specific Information option, the option that conveys our sub-option "foo". This is required, else the option will not be included in messages sent to the client.

"Dhcp4": {
    "option-data": [
        {
            "name": "vendor-encapsulated-options"
        }
    ],
    ...
}

Alternatively, the option can be specified using its code.

"Dhcp4": {
    "option-data": [
        {
            "code": 43
        }
    ],
    ...
}

Another possibility, since Kea 1.3, is to redefine the option, see Section 8.2.10, “DHCPv4 Private Options”.

8.2.12. Nested DHCPv4 Options (Custom Option Spaces)

It is sometimes useful to define a completely new option space. This is the case when user creates new option in the standard option space ("dhcp4") and wants this option to convey sub-options. Since they are in a separate space, sub-option codes will have a separate numbering scheme and may overlap with the codes of standard options.

Note that creation of a new option space when defining sub-options for a standard option is not required, because it is created by default if the standard option is meant to convey any sub-options (see Section 8.2.11, “DHCPv4 Vendor Specific Options”).

Assume that we want to have a DHCPv4 option called "container" with code 222 that conveys two sub-options with codes 1 and 2. First we need to define the new sub-options:

"Dhcp4": {
    "option-def": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv4-address",
            "record-types": "",
            "array": false,
            "encapsulate": ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
            "array": false,
            "encapsulate": ""
        }
    ],
    ...
}

Note that we have defined the options to belong to a new option space (in this case, "isc").

The next step is to define a regular DHCPv4 option with our desired code and specify that it should include options from the new option space:

"Dhcp4": {
    "option-def": [
        ...,
        {
            "name": "container",
            "code": 222,
            "space": "dhcp4",
            "type": "empty",
            "array": false,
            "record-types": "",
            "encapsulate": "isc"
        }
    ],
    ...
}

The name of the option space in which the sub-options are defined is set in the encapsulate field. The type field is set to "empty" to indicate that this option does not carry any data other than sub-options.

Finally, we can set values for the new options:

"Dhcp4": {
    "option-data": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "data": "192.0.2.3"
        },
        }
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "data": "Hello world"
        },
        {
            "name": "container",
            "code": 222,
            "space": "dhcp4"
        }
    ],
    ...
}

Note that it is possible to create an option which carries some data in addition to the sub-options defined in the encapsulated option space. For example, if the "container" option from the previous example was required to carry an uint16 value as well as the sub-options, the type value would have to be set to "uint16" in the option definition. (Such an option would then have the following data structure: DHCP header, uint16 value, sub-options.) The value specified with the data parameter — which should be a valid integer enclosed in quotes, e.g. "123" — would then be assigned to the uint16 field in the "container" option.

8.2.13. Unspecified Parameters for DHCPv4 Option Configuration

In many cases it is not required to specify all parameters for an option configuration and the default values may be used. However, it is important to understand the implications of not specifying some of them as it may result in configuration errors. The list below explains the behavior of the server when a particular parameter is not explicitly specified:

  • name - the server requires an option name or option code to identify an option. If this parameter is unspecified, the option code must be specified.
  • code - the server requires an option name or option code to identify an option. This parameter may be left unspecified if the name parameter is specified. However, this also requires that the particular option has its definition (it is either a standard option or an administrator created a definition for the option using an 'option-def' structure), as the option definition associates an option with a particular name. It is possible to configure an option for which there is no definition (unspecified option format). Configuration of such options requires the use of option code.
  • space - if the option space is unspecified it will default to 'dhcp4' which is an option space holding DHCPv4 standard options.
  • data - if the option data is unspecified it defaults to an empty value. The empty value is mostly used for the options which have no payload (boolean options), but it is legal to specify empty values for some options which carry variable length data and which the specification allows for the length of 0. For such options, the data parameter may be omitted in the configuration.
  • csv-format - if this value is not specified the server will assume that the option data is specified as a list of comma separated values to be assigned to individual fields of the DHCP option. This behavior has changed in Kea 1.2. Older versions used additional logic to determine whether the csv-format should be true or false. That is no longer the case.

8.2.14. Stateless Configuration of DHCPv4 Clients

The DHCPv4 server supports the stateless client configuration whereby the client has an IP address configured (e.g. using manual configuration) and only contacts the server to obtain other configuration parameters, e.g. addresses of DNS servers. In order to obtain the stateless configuration parameters the client sends the DHCPINFORM message to the server with the "ciaddr" set to the address that the client is currently using. The server unicasts the DHCPACK message to the client that includes the stateless configuration ("yiaddr" not set).

The server will respond to the DHCPINFORM when the client is associated with a subnet defined in the server's configuration. An example subnet configuration will look like this:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24"
            "option-data": [ {
                "name": "domain-name-servers",
                "code": 6,
                "data": "192.0.2.200,192.0.2.201",
                "csv-format": true,
                "space": "dhcp4"
            } ]
        }
    ]
}

This subnet specifies the single option which will be included in the DHCPACK message to the client in response to DHCPINFORM. Note that the subnet definition does not require the address pool configuration if it will be used solely for the stateless configuration.

This server will associate the subnet with the client if one of the following conditions is met:

  • The DHCPINFORM is relayed and the giaddr matches the configured subnet.
  • The DHCPINFORM is unicast from the client and the ciaddr matches the configured subnet.
  • The DHCPINFORM is unicast from the client, the ciaddr is not set but the source address of the IP packet matches the configured subnet.
  • The DHCPINFORM is not relayed and the IP address on the interface on which the message is received matches the configured subnet.

8.2.15. Client Classification in DHCPv4

The DHCPv4 server includes support for client classification. For a deeper discussion of the classification process see Chapter 14, Client Classification.

In certain cases it is useful to differentiate between different types of clients and treat them accordingly. It is envisaged that client classification will be used for changing the behavior of almost any part of the DHCP message processing. In the current release of the software however, there are only some mechanisms that take advantage of client classification: private options and option 43 deferred unpacking, subnet selection, pool selection, assignment of different options, and, for cable modems, there are specific options for use with the TFTP server address and the boot file field.

Kea can be instructed to limit access to given subnets based on class information. This is particularly useful for cases where two types of devices share the same link and are expected to be served from two different subnets. The primary use case for such a scenario is cable networks. Here, there are two classes of devices: the cable modem itself, which should be handed a lease from subnet A and all other devices behind the modem that should get a lease from subnet B. That segregation is essential to prevent overly curious users from playing with their cable modems. For details on how to set up class restrictions on subnets, see Section 14.6, “Configuring Subnets With Class Information”.

When subnets belong to a shared network the classification applies to subnet selection but not to pools, e.g., a pool in a subnet limited to a particular class can still be used by clients which do not belong to the class if the pool they are expected to use is exhausted. So the limit access based on class information is also available at the pool level, see Section 14.7, “Configuring Pools With Class Information”, within a subnet. This is useful when to segregate clients belonging to the same subnet into different address ranges.

In a similar way a pool can be constrained to serve only known clients, i.e. clients which have a reservation, using the built-in "KNOWN" or "UNKNOWN" classes. One can assign addresses to registered clients without giving a different address per reservations, for instance when there is not enough available addresses. The determination whether there is a reservation for a given client is made after a subnet is selected. As such, it is not possible to use KNOWN/UNKNOWN classes to select a shared network or a subnet.

The process of doing classification is conducted in five steps. The first step is to assess an incoming packet and assign it to zero or more classes. The second step is to choose a subnet, possibly based on the class information. The next step is to evaluate class expressions depending on the built-in "KNOWN"/"UNKNOWN" classes after host reservation lookup, using them for pool selection and to assign classes from host reservations. After the list of required classes is built and each class of the list has its expression evaluated: when it returns true the packet is added as a member of the class. The last step is to assign options, again possibly based on the class information. More complete and detailed description is available in Chapter 14, Client Classification.

There are two main methods of doing classification. The first is automatic and relies on examining the values in the vendor class options or existence of a host reservation. Information from these options is extracted and a class name is constructed from it and added to the class list for the packet. The second allows for specifying an expression that is evaluated for each packet. If the result is true the packet is a member of the class.

Note

Care should be taken with client classification as it is easy for clients that do not meet class criteria to be denied any service altogether.

8.2.15.1. Setting Fixed Fields in Classification

It is possible to specify that clients belonging to a particular class should receive packets with specific values in certain fixed fields. In particular, three fixed fields are supported: next-server (that conveys an IPv4 address, which is set in the siaddr field), server-hostname (that conveys a server hostname, can be up to 64 bytes long and will be sent in the sname field) and boot-file-name (that conveys the configuration file, can be up to 128 bytes long and will be sent using file field).

Obviously, there are many ways to assign clients to specific classes, but for the PXE clients the client architecture type option (code 93) seems to be particularly suited to make the distinction. The following example checks if the client identifies itself as PXE device with architecture EFI x86-64, and sets several fields if it does. See Section 2.1 of RFC 4578) or the documentation of your client for specific values.

"Dhcp4": {
    "client-classes": [
        {
            "name": "ipxe_efi_x64",
            "test": "option[93].hex == 0x0009",
            "next-server": "192.0.2.254",
            "server-hostname": "hal9000",
            "boot-file-name": "/dev/null"
        },
        ...
    ],
    ...
          }

If there are multiple classes defined and an incoming packet is matched to multiple classes, the class which is evaluated first is used.

Note

In Kea versions prior to 1.4.0 the alphabetical order of class names was used. Starting from Kea 1.4.0 the classes are ordered as specified in the configuration.

8.2.15.2. Using Vendor Class Information in Classification

The server checks whether an incoming packet includes the vendor class identifier option (60). If it does, the content of that option is prepended with "VENDOR_CLASS_", it is interpreted as a class. For example, modern cable modems will send this option with value "docsis3.0" and as a result the packet will belong to class "VENDOR_CLASS_docsis3.0".

Note

Kea 1.0 and earlier versions performed special actions for clients that were in VENDOR_CLASS_docsis3.0. This is no longer the case in Kea 1.1 and later. In these versions the old behavior can be achieved by defining VENDOR_CLASS_docsis3.0 and setting its next-server and boot-file-name values appropriately.

This example shows a configuration using an automatically generated "VENDOR_CLASS_" class. The administrator of the network has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to be managed by the Dhcp4 server and only clients belonging to the docsis3.0 client class are allowed to use that pool.

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "VENDOR_CLASS_docsis3.0"
        }
    ],
    ...
}

8.2.15.3. Defining and Using Custom Classes

The following example shows how to configure a class using an expression and a subnet that makes use of the class. This configuration defines the class named "Client_foo". It is comprised of all clients who's client ids (option 61) start with the string "foo". Members of this class will be given addresses from 192.0.2.10 to 192.0.2.20 and the addresses of their DNS servers set to 192.0.2.1 and 192.0.2.2.

"Dhcp4": {
    "client-classes": [
        {
            "name": "Client_foo",
            "test": "substring(option[61].hex,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        },
        ...
    ],
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "Client_foo"
        },
        ...
    ],
    ...
}

8.2.15.4. Required Classification

In some cases it is useful to limit the scope of a class to a shared-network, subnet or pool. There are two parameters which are used to limit the scope of the class by instructing the server to perform evaluation of test expressions when required.

The first one is the per-class only-if-required flag which is false by default. When it is set to true the test expression of the class is not evaluated at the reception of the incoming packet but later and only if the class evaluation is required.

The second is the require-client-classes which takes a list of class names and is valid in shared-network, subnet and pool scope. Classes in these lists are marked as required and evaluated after selection of this specific shared-network/subnet/pool and before output option processing.

In this example, a class is assigned to the incoming packet when the specified subnet is used.

"Dhcp4": {
    "client-classes": [
       {
           "name": "Client_foo",
           "test": "member('ALL')",
           "only-if-required": true
       },
       ...
    ],
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "require-client-classes": [ "Client_foo" ],
            ...
        },
        ...
    ],
    ...
}

Required evaluation can be used to express complex dependencies, for example, subnet membership. It can also be used to reverse the precedence: if you set an option-data in a subnet it takes precedence over an option-data in a class. When you move the option-data to a required class and require it in the subnet, a class evaluated earlier may take precedence.

Required evaluation is also available at shared-network and pool levels. The order in which required classes are considered is: shared-network, subnet and pool, i.e. the opposite order option-data are processed.

8.2.16. DDNS for DHCPv4

As mentioned earlier, kea-dhcp4 can be configured to generate requests to the DHCP-DDNS server (referred to here as "D2" ) to update DNS entries. These requests are known as NameChangeRequests or NCRs. Each NCR contains the following information:

  1. Whether it is a request to add (update) or remove DNS entries

  2. Whether the change requests forward DNS updates (A records), reverse DNS updates (PTR records), or both.

  3. The FQDN, lease address, and DHCID

The parameters for controlling the generation of NCRs for submission to D2 are contained in the dhcp-ddns section of the kea-dhcp4 server configuration. The mandatory parameters for the DHCP DDNS configuration are enable-updates which is unconditionally required, and qualifying-suffix which has no default value and is required when enable-updates is set to true. The two (disabled and enabled) minimal DHCP DDNS configurations are:

"Dhcp4": {
    "dhcp-ddns": {
        "enable-updates": false
    },
    ...
}

and for example:

"Dhcp4": {
    "dhcp-ddns": {
        "enable-updates": true,
        "qualifying-suffix": "example."
    },
    ...
}

The default values for the "dhcp-ddns" section are as follows:

  • "server-ip": "127.0.0.1"
  • "server-port": 53001
  • "sender-ip": ""
  • "sender-port": 0
  • "max-queue-size": 1024
  • "ncr-protocol": "UDP"
  • "ncr-format": "JSON"
  • "override-no-update": false
  • "override-client-update": false
  • "replace-client-name": "never"
  • "generated-prefix": "myhost"
  • "hostname-char-set": ""
  • "hostname-char-replacement": ""

8.2.16.1. DHCP-DDNS Server Connectivity

In order for NCRs to reach the D2 server, kea-dhcp4 must be able to communicate with it. kea-dhcp4 uses the following configuration parameters to control this communication:

  • enable-updates - determines whether or not kea-dhcp4 will generate NCRs. By default, this value is false hence DDNS updates are disabled. To enable DDNS updates set this value to true:
  • server-ip - IP address on which D2 listens for requests. The default is the local loopback interface at address 127.0.0.1. You may specify either an IPv4 or IPv6 address.
  • server-port - port on which D2 listens for requests. The default value is 53001.
  • sender-ip - IP address which kea-dhcp4 should use to send requests to D2. The default value is blank which instructs kea-dhcp4 to select a suitable address.
  • sender-port - port which kea-dhcp4 should use to send requests to D2. The default value of 0 instructs kea-dhcp4 to select a suitable port.
  • max-queue-size - maximum number of requests allowed to queue waiting to be sent to D2. This value guards against requests accumulating uncontrollably if they are being generated faster than they can be delivered. If the number of requests queued for transmission reaches this value, DDNS updating will be turned off until the queue backlog has been sufficiently reduced. The intention is to allow the kea-dhcp4 server to continue lease operations without running the risk that its memory usage grows without limit. The default value is 1024.
  • ncr-protocol - socket protocol use when sending requests to D2. Currently only UDP is supported. TCP may be available in an upcoming release.
  • ncr-format - packet format to use when sending requests to D2. Currently only JSON format is supported. Other formats may be available in future releases.

By default, kea-dhcp-ddns is assumed to be running on the same machine as kea-dhcp4, and all of the default values mentioned above should be sufficient. If, however, D2 has been configured to listen on a different address or port, these values must be altered accordingly. For example, if D2 has been configured to listen on 192.168.1.10 port 900, the following configuration would be required:

"Dhcp4": {
    "dhcp-ddns": {
        "server-ip": "192.168.1.10",
        "server-port": 900,
        ...
    },
    ...
}

8.2.16.2. When Does the kea-dhcp4 Server Generate DDNS Requests?

kea-dhcp4 follows the behavior prescribed for DHCP servers in RFC 4702. It is important to keep in mind that kea-dhcp4 provides the initial decision making of when and what to update and forwards that information to D2 in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are within the purview of D2 itself (Chapter 12, The DHCP-DDNS Server). This section describes when kea-dhcp4 will generate NCRs and the configuration parameters that can be used to influence this decision. It assumes that the enable-updates parameter is true.

In general, kea-dhcp4 will generate DDNS update requests when:

  1. A new lease is granted in response to a DHCP REQUEST

  2. An existing lease is renewed but the FQDN associated with it has changed.

  3. An existing lease is released in response to a DHCP RELEASE

In the second case, lease renewal, two DDNS requests will be issued: one request to remove entries for the previous FQDN and a second request to add entries for the new FQDN. In the last case, a lease release, a single DDNS request to remove its entries will be made.

The decision making involved when granting a new lease (the first case) is more involved. When a new lease is granted, kea-dhcp4 will generate a DDNS update request if the DHCP REQUEST contains either the FQDN option (code 81) or the Host Name option (code 12). If both are present, the server will use the FQDN option. By default kea-dhcp4 will respect the FQDN N and S flags specified by the client as shown in the following table:

Table 8.3. Default FQDN Flag Behavior

Client Flags:N-SClient IntentServer ResponseServer Flags:N-S-O
0-0 Client wants to do forward updates, server should do reverse updates Server generates reverse-only request1-0-0
0-1Server should do both forward and reverse updatesServer generates request to update both directions0-1-0
1-0Client wants no updates doneServer does not generate a request1-0-0

The first row in the table above represents "client delegation". Here the DHCP client states that it intends to do the forward DNS updates and the server should do the reverse updates. By default, kea-dhcp4 will honor the client's wishes and generate a DDNS request to the D2 server to update only reverse DNS data. The parameter override-client-update can be used to instruct the server to override client delegation requests. When this parameter is true, kea-dhcp4 will disregard requests for client delegation and generate a DDNS request to update both forward and reverse DNS data. In this case, the N-S-O flags in the server's response to the client will be 0-1-1 respectively.

(Note that the flag combination N=1, S=1 is prohibited according to RFC 4702. If such a combination is received from the client, the packet will be dropped by kea-dhcp4.)

To override client delegation, set the following values in the configuration file:

"Dhcp4": {
    "dhcp-ddns": {
        "override-client-update": true,
        ...
    },
    ...
}

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter, override-no-update, can be used to instruct the server to disregard the client's wishes. When this parameter is true, kea-dhcp4 will generate DDNS update requests to kea-dhcp-ddns even if the client requests that no updates be done. The N-S-O flags in the server's response to the client will be 0-1-1.

To override client delegation, the following values should be set in your configuration:

"Dhcp4": {
    "dhcp-ddns": {
        "override-no-update": true,
        ...
    },
    ...
}

kea-dhcp4 will always generate DDNS update requests if the client request only contains the Host Name option. In addition it will include an FQDN option in the response to the client with the FQDN N-S-O flags set to 0-1-0 respectively. The domain name portion of the FQDN option will be the name submitted to D2 in the DDNS update request.

8.2.16.3. kea-dhcp4 name generation for DDNS update requests

Each NameChangeRequest must of course include the fully qualified domain name whose DNS entries are to be affected. kea-dhcp4 can be configured to supply a portion or all of that name based upon what it receives from the client in the DHCP REQUEST.

The default rules for constructing the FQDN that will be used for DNS entries are:

  1. If the DHCPREQUEST contains the client FQDN option, the candidate name is taken from there, otherwise it is taken from the Host Name option.

  2. If the candidate name is a partial (i.e. unqualified) name then add a configurable suffix to the name and use the result as the FQDN.

  3. If the candidate name provided is empty, generate a FQDN using a configurable prefix and suffix.

  4. If the client provided neither option, then no DNS action will be taken.

These rules can amended by setting the replace-client-name parameter which provides the following modes of behavior:

  • never - Use the name the client sent. If the client sent no name, do not generate one. This is the default mode.

  • always - Replace the name the client sent. If the client sent no name, generate one for the client.

  • when-present - Replace the name the client sent. If the client sent no name, do not generate one.

  • when-not-present - Use the name the client sent. If the client sent no name, generate one for the client.

Note

Note that formerly, this parameter was a boolean and permitted only values of true and false. Boolean values have been deprecated and are no longer accepted. If you are currently using booleans, you must replace them with the desired mode name. A value of true maps to "when-present", while false maps to "never".

For example, To instruct kea-dhcp4 to always generate the FQDN for a client, set the parameter replace-client-name to always as follows:

"Dhcp4": {
    "dhcp-ddns": {
        "replace-client-name": "always",
        ...
    },
    ...
}

The prefix used in the generation of a FQDN is specified by the generated-prefix parameter. The default value is "myhost". To alter its value, simply set it to the desired string:

"Dhcp4": {
    "dhcp-ddns": {
        "generated-prefix": "another.host",
        ...
    },
    ...
}

The suffix used when generating a FQDN or when qualifying a partial name is specified by the qualifying-suffix parameter. This parameter has no default value, thus it is mandatory when DDNS updates are enabled. To set its value simply set it to the desired string:

"Dhcp4": {
    "dhcp-ddns": {
        "qualifying-suffix": "foo.example.org",
        ...
    },
    ...
}

When generating a name, kea-dhcp4 will construct name of the format:

[generated-prefix]-[address-text].[qualifying-suffix].

where address-text is simply the lease IP address converted to a hyphenated string. For example, if the lease address is 172.16.1.10, the qualifying suffix "example.com", and the default value is used for generated-prefix, the generated FQDN would be:

myhost-172-16-1-10.example.com.

8.2.16.4. Sanitizing Client Host Name and FQDN Names

It may be that some of your DHCP clients provide values in the Host Name option (Option code 12) or FQDN option (Option code 81), that contain undesirable characters. It is possible to configure kea-dhcp4 to sanitize these values. The most typical use case would be ensuring that only characters that are permitted by RFC 1035 be included: A-Z,a-z,0-9, and '-'. This may be accomplished with following two parameters:
  • hostname-char-set - a regular expression describing the invalid character set. This can be any valid, regular expression using POSIX extended expression syntax. For example, "[^A-Za-z0-9-]" would replace any character other then the letters A through z, digits 0 through 9, and '-'. An empty string, the default value, disables sanitization.
  • hostname-char-replacement - a string of zero or more characters with which to replace each invalid character in the host name. The default value is an empty string and will cause invalid characters to be OMITTED rather than replaced.
The following configuration, will replace anything other than a letter, digit, hyphen, or dot with the letter 'x':
"Dhcp4": {
    "dhcp-ddns": {
        "hostname-char-set": "[^A-Za-z0-9.-]",
        "hostname-char-replacement": "x",
        ...
    },
    ...
}
Thus, a client supplied value of "myhost-$[123.org" would become "myhost-xx123.org". Sanitizing is performed only on the portion of the name supplied by the client and it is performed before applying a qualifying suffix (if one is defined and needed).

Note

The following are some considerations to keep in mind:

Name sanitizing is meant to catch the more common cases of invalid characters through a relatively simple character replacement scheme. It is difficult to devise a scheme that works well in all cases, for both Host Name and FQDN options. If you find you have clients that are using odd, corner cases of character combinations that cannot be readily handled with this mechanism, you should consider writing a hook that can carry out sufficiently complex logic to address your needs.

If your clients include domain names in the Host Name option and you want these preserved, you will need to make sure that the dot, '.', is considered a valid character by the hostname-char-set expression, such as this: "[^A-Za-z0-9.-]". This will not affect dots in FQDN Option values. When scrubbing FQDNs, dots are treated as delimiters and used to separate the option value into individual domain labels that are scrubbed and then re-assembled.

If your clients are sending values that differ only by characters considered as invalid by your hostname-char-set, be aware that scrubbing them will yield identical values. In such cases, DDNS conflict rules will permit only one of them from registering the name.

Finally, given the latitude clients have in the values they send, it is virtually impossible to guarantee that a combination of these two parameters will always yield a name that is valid for use in DNS. For example, using an empty value for hostname-char-replacment could yield an empty domain label within a name, if that label consists only of invalid characters.

8.2.17. Next Server (siaddr)

In some cases, clients want to obtain configuration from a TFTP server. Although there is a dedicated option for it, some devices may use the siaddr field in the DHCPv4 packet for that purpose. That specific field can be configured using next-server directive. It is possible to define it in the global scope or for a given subnet only. If both are defined, the subnet value takes precedence. The value in subnet can be set to 0.0.0.0, which means that next-server should not be sent. It may also be set to an empty string, which means the same as if it was not defined at all, i.e. use the global value.

The server-hostname (that conveys a server hostname, can be up to 64 bytes long and will be sent in the sname field) and boot-file-name (that conveys the configuration file, can be up to 128 bytes long and will be sent using file field) directives are handled the same way as next-server.

"Dhcp4": {
    "next-server": "192.0.2.123",
    "boot-file-name": "/dev/null",
    ...,
    "subnet4": [
        {
            "next-server": "192.0.2.234",
            "server-hostname": "some-name.example.org",
            "boot-file-name": "bootfile.efi",
            ...
        }
    ]
}

8.2.18. Echoing Client-ID (RFC 6842)

The original DHCPv4 specification (RFC 2131) states that the DHCPv4 server must not send back client-id options when responding to clients. However, in some cases that confused clients that did not have MAC address or client-id; see RFC 6842. for details. That behavior has changed with the publication of RFC 6842 which updated RFC 2131. That update states that the server must send client-id if the client sent it. That is Kea's default behavior. However, in some cases older devices that do not support RFC 6842. may refuse to accept responses that include the client-id option. To enable backward compatibility, an optional configuration parameter has been introduced. To configure it, use the following configuration statement:

"Dhcp4": {
    "echo-client-id": false,
    ...
}

8.2.19. Using Client Identifier and Hardware Address

The DHCP server must be able to identify the client (and distinguish it from other clients) from which it receives the message. There are many reasons why this identification is required and the most important ones are:

  • When the client contacts the server to allocate a new lease, the server must store the client identification information in the lease database as a search key.
  • When the client is trying to renew or release the existing lease, the server must be able to find the existing lease entry in the database for this client, using the client identification information as a search key.
  • Some configurations use static reservations for the IP addresses and other configuration information. The server's administrator uses client identification information to create these static assignments.
  • In the dual stack networks there is often a need to correlate the lease information stored in DHCPv4 and DHCPv6 server for a particular host. Using common identification information by the DHCPv4 and DHCPv6 client allows the network administrator to achieve this correlation and better administer the network.

DHCPv4 makes use of two distinct identifiers which are placed by the client in the queries sent to the server and copied by the server to its responses to the client: "chaddr" and "client identifier". The former was introduced as a part of the BOOTP specification and it is also used by DHCP to carry the hardware address of the interface used to send the query to the server (MAC address for the Ethernet). The latter is carried in the Client-identifier option, introduced in RFC 2132.

RFC 2131 indicates that the server may use both of these identifiers to identify the client but the "client identifier", if present, takes precedence over "chaddr". One of the reasons for this is that "client identifier" is independent from the hardware used by the client to communicate with the server. For example, if the client obtained the lease using one network card and then the network card is moved to another host, the server will wrongly identify this host is the one which has obtained the lease. Moreover, RFC 4361 gives the recommendation to use a DUID (see RFC 8415, the DHCPv6 specification) carried as "client identifier" when dual stack networks are in use to provide consistent identification information of the client, regardless of the protocol type it is using. Kea adheres to these specifications and the "client identifier" by default takes precedence over the value carried in "chaddr" field when the server searches, creates, updates or removes the client's lease.

When the server receives a DHCPDISCOVER or DHCPREQUEST message from the client, it will try to find out if the client already has a lease in the database and will hand out that lease rather than allocate a new one. Each lease in the lease database is associated with the "client identifier" and/or "chaddr". The server will first use the "client identifier" (if present) to search the lease. If the lease is found, the server will treat this lease as belonging to the client even if the current "chaddr" and the "chaddr" associated with the lease do not match. This facilitates the scenario when the network card on the client system has been replaced and thus the new MAC address appears in the messages sent by the DHCP client. If the server fails to find the lease using the "client identifier" it will perform another lookup using the "chaddr". If this lookup returns no result, the client is considered as not having a lease and the new lease will be created.

A common problem reported by network operators is that poor client implementations do not use stable client identifiers, instead generating a new "client identifier" each time the client connects to the network. Another well known case is when the client changes its "client identifier" during the multi-stage boot process (PXE). In such cases, the MAC address of the client's interface remains stable and using "chaddr" field to identify the client guarantees that the particular system is considered to be the same client, even though its "client identifier" changes.

To address this problem, Kea includes a configuration option which enables client identification using "chaddr" only by instructing the server to disregard server to "ignore" the "client identifier" during lease lookups and allocations for a particular subnet. Consider the following simplified server configuration:

"Dhcp4": {
    ...
    "match-client-id": true,
    ...
    "subnet4": [
    {
        "subnet": "192.0.10.0/24",
        "pools": [ { "pool": "192.0.2.23-192.0.2.87" } ],
        "match-client-id": false
    },
    {
        "subnet": "10.0.0.0/8",
        "pools": [ { "pool": "10.0.0.23-10.0.2.99" } ],
    }
    ]
}

The match-client-id is a boolean value which controls this behavior. The default value of true indicates that the server will use the "client identifier" for lease lookups and "chaddr" if the first lookup returns no results. The false means that the server will only use the "chaddr" to search for client's lease. Whether the DHCID for DNS updates is generated from the "client identifier" or "chaddr" is controlled through the same parameter accordingly.

The match-client-id parameter may appear both in the global configuration scope and/or under any subnet declaration. In the example shown above, the effective value of the match-client-id will be false for the subnet 192.0.10.0/24, because the subnet specific setting of the parameter overrides the global value of the parameter. The effective value of the match-client-id for the subnet 10.0.0.0/8 will be set to true because the subnet declaration lacks this parameter and the global setting is by default used for this subnet. In fact, the global entry for this parameter could be omitted in this case, because true is the default value.

It is important to explain what happens when the client obtains its lease for one setting of the match-client-id and then renews when the setting has been changed. First consider the case when the client obtains the lease when the match-client-id is set to true. The server will store the lease information including "client identifier" (if supplied) and "chaddr" in the lease database. When the setting is changed and the client renews the lease the server will determine that it should use the "chaddr" to search for the existing lease. If the client hasn't changed its MAC address the server should successfully find the existing lease. The "client identifier" associated with the returned lease is ignored and the client is allowed to use this lease. When the lease is renewed only the "chaddr" is recorded for this lease according to the new server setting.

In the second case the client has the lease with only a "chaddr" value recorded. When the setting is changed to match-client-id set to true the server will first try to use the "client identifier" to find the existing client's lease. This will return no results because the "client identifier" was not recorded for this lease. The server will then use the "chaddr" and the lease will be found. If the lease appears to have no "client identifier" recorded, the server will assume that this lease belongs to the client and that it was created with the previous setting of the match-client-id. However, if the lease contains "client identifier" which is different from the "client identifier" used by the client the lease will be assumed to belong to another client and the new lease will be allocated.

8.2.20. Authoritative DHCPv4 Server Behavior

The original DHCPv4 specification (RFC 2131) states that if a clients requests an address in the INIT-REBOOT state of which, the server has no knowledge of, the server must remain silent, except if the server knows that the client requests an IP address from the wrong network. By default Kea follows the behavior of the ISC dhcpd instead of the specification and also remains silent, if the client requests an IP address from the wrong network, because configuration information about a given network segment is not known to be correct. Kea only rejects a client's DHCPREQUEST with a DHCPNAK message, if it already has a lease for the client, but with a different IP address. Administrators can override this behavior through the boolean authoritative (false by default) setting.

In authoritative mode, authoritative set to true, Kea always rejects INIT-REBOOT requests from unknown clients with DHCPNAK messages. The authoritative setting can be specified in global, shared-network, and subnet configuration scope and is automatically inherited from the parent scope, if not specified. All subnets in a shared-network must have the same authoritative setting.

8.2.21. DHCPv4-over-DHCPv6: DHCPv4 Side

The support of DHCPv4-over-DHCPv6 transport is described in RFC 7341 and is implemented using cooperating DHCPv4 and DHCPv6 servers. This section is about the configuration of the DHCPv4 side (the DHCPv6 side is described in Section 9.2.22, “DHCPv4-over-DHCPv6: DHCPv6 Side”).

Note

DHCPv4-over-DHCPv6 support is experimental and the details of the inter-process communication can change: both the DHCPv4 and DHCPv6 sides should be running the same version of Kea. For instance the support of port relay (RFC 8357) introduced such incompatible change.

The dhcp4o6-port global parameter specifies the first of the two consecutive ports of the UDP sockets used for the communication between the DHCPv6 and DHCPv4 servers (the DHCPv4 server is bound to ::1 on port + 1 and connected to ::1 on port).

With DHCPv4-over-DHCPv6 the DHCPv4 server does not have access to several of the identifiers it would normally use to select a subnet. In order to address this issue three new configuration entries have been added. The presence of any of these allows the subnet to be used with DHCPv4-over-DHCPv6. These entries are:

  • 4o6-subnet: Takes a prefix (i.e., an IPv6 address followed by a slash and a prefix length) which is matched against the source address.
  • 4o6-interface-id: Takes a relay interface ID option value.
  • 4o6-interface: Takes an interface name which is matched against the incoming interface name.

The following configuration was used during some tests:

{

# DHCPv4 conf
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eno33554984" ]
    },

    "lease-database": {
        "type": "memfile",
        "name": "leases4"
    },

    "valid-lifetime": 4000,

    "subnet4": [ {
        "subnet": "10.10.10.0/24",
        "4o6-interface": "eno33554984",
        "4o6-subnet": "2001:db8:1:1::/64",
        "pools": [ { "pool": "10.10.10.100 - 10.10.10.199" } ]
    } ],

    "dhcp4o6-port": 6767

},

"Logging": {
    "loggers": [ {
        "name": "kea-dhcp4",
        "output_options": [ {
            "output": "/tmp/kea-dhcp4.log"
        } ],
        "severity": "DEBUG",
        "debuglevel": 0
    } ]
}

}

8.2.22. Sanity checks in DHCPv4

An important aspect of a well running DHCP system is an assurance that the data remains consisent. However, in some cases it may be convenient to tolerate certain inconsistent data. For example, a network administrator that temporarily removed a subnet from a configuration wouldn't want all the leases associated with it disappear from the lease database. Kea 1.5 introduced a mechanism to better control sanity checks such as this. While currently the scope of configurable sanity checks is limited and their default value is set low, it is expected that over time the default settings will be set to more aggressive values and more parameters of similar nature will be added in the future.

Kea now supports a new configuration scope called sanity-checks. It currently allows only a single parameter called lease-checks. It governs what sort of verification is done when a new lease is being loaded from a lease file. With the introduction of sanity checks mechanism, it is now possible to tell Kea to try to correct inconsistent data.

Every subnet has a subnet-id value. This is how Kea internally identifies subnets. Each lease has a subnet-id parameter as well, which identifies which subnet it belongs to. However, if configuration has changed, it is possible that a lease could exist with a subnet-id without any subnet that matches it. Also, it may be possible that subnets configuration has changed and the subnet-id now belongs to a subnet that does not match the lease. Kea corrective algorithm first checks if there is a subnet with subnet-id specified by the lease. If there is, it checks whether the lease belongs to that subnet. If not, depending on the lease-checks setting, the lease is discarded, a warning is printed or a new subnet is selected for the lease that matches it topologically.

There are five levels which are supported:

  • none - do no special checks, accept the lease as is
  • warn - if problems are detected, a warning will be printed, but the lease data will be accepted anyway. This is the default value. If not explicitly configured to some other value, this level will be used.
  • fix - If data inconsistency is discovered, Kea will try to correct it. If the correction is not successful, the data will be inserted anyway.
  • fix-del - If data inconsistency is discovered, Kea will try to correct it. If the correction is not succesful, the lease will be rejected. This setting ensures the data correctness, but some incorrect data may be lost. Use with care.
  • del - This is the strictest mode. If any inconsistency is detected, the lease is rejected. Use with care.

This feature is currently implemented for memfile backend.

An example configuration that sets this parameter looks as follows:

"Dhcp4": {
    "sanity-checks": {
        // This parameter determines what to do when a new lease appears in the
        // system (i.e. either is read from disk during memfile startup or is
        // added via lease commands). There are five modes supported:
        // none - do nothing, accept them as is
        // warn - if subnet-id problems are detected, print a warning, but
        //        otherwise load the lease as is. This is the default value.
        // fix - attempt to fix the lease by finding appropriate subnet-id value.
        //       if there is no suitable subnet, the lease is loaded as is.
        // fix-del - attempt to fix the lease by findind appropriate subnet-id
        //           value. If there is no suitable subnet, the lease is deleted.
        // del - delete leases that have incorrect subnet-id values.
        "lease-checks": "fix-del"
    },
    ...
}

8.3. Host Reservation in DHCPv4

There are many cases where it is useful to provide a configuration on a per host basis. The most obvious one is to reserve a specific, static address for exclusive use by a given client (host) ‐ the returning client will receive the same address from the server every time, and other clients will generally not receive that address. Another example when the host reservations are applicable is when a host has specific requirements, e.g. a printer that needs additional DHCP options. Yet another possible use case is to define unique names for hosts.

Note that there may be cases when the new reservation has been made for the client for the address being currently in use by another client. We call this situation a "conflict". The conflicts get resolved automatically over time as described in subsequent sections. Once the conflict is resolved, the client will keep receiving the reserved configuration when it renews.

Host reservations are defined as parameters for each subnet. Each host has to be identified by an identifier, for example the hardware/MAC address. There is an optional reservations array in the Subnet4 element. Each element in that array is a structure that holds information about reservations for a single host. In particular, the structure has to have an identifier that uniquely identifies a host. In the DHCPv4 context, the identifier is usually a hardware or MAC address. In most cases an IP address will be specified. It is also possible to specify a hostname, host specific options or fields carried within DHCPv4 message such as siaddr, sname or file.

In Kea 1.0.0 it was only possible to create host reservations using client's hardware address. Host reservations by client identifier, DUID and circuit-id have been added in Kea 1.1.0.

The following example shows how to reserve addresses for specific hosts in a subnet:

"subnet4": [
    {
        "pools": [ { "pool":  "192.0.2.1 - 192.0.2.200" } ],
        "subnet": "192.0.2.0/24",
        "interface": "eth0",
        "reservations": [
            {
                "hw-address": "1a:1b:1c:1d:1e:1f",
                "ip-address": "192.0.2.202"
            },
            {
                "duid": "0a:0b:0c:0d:0e:0f",
                "ip-address": "192.0.2.100",
                "hostname": "alice-laptop"
            },
            {
                "circuit-id": "'charter950'",
                "ip-address": "192.0.2.203"
            },
            {
                "client-id": "01:11:22:33:44:55:66",
                "ip-address": "192.0.2.204"
            }
        ]
    }
]

The first entry reserves the 192.0.2.202 address for the client that uses a MAC address of 1a:1b:1c:1d:1e:1f. The second entry reserves the address 192.0.2.100 and the hostname of alice-laptop for the client using a DUID 0a:0b:0c:0d:0e:0f. (Note that if you plan to do DNS updates, it is strongly recommended for the hostnames to be unique.) The third example reserves address 192.0.3.203 to a client whose request would be relayed by a relay agent that inserts a circuit-it option with the value 'charter950'. The fourth entry reserves address 192.0.2.204 for a client that uses a client identifier with value 01:11:22:33:44:55:66.

The above example is used for illustrational purposes only and in actual deployments it is recommended to use as few types as possible (preferably just one). See Section 8.3.8, “Fine Tuning DHCPv4 Host Reservation” for a detailed discussion of this point.

Making a reservation for a mobile host that may visit multiple subnets requires a separate host definition in each subnet it is expected to visit. It is not allowed to define multiple host definitions with the same hardware address in a single subnet. Multiple host definitions with the same hardware address are valid if each is in a different subnet.

Adding host reservation incurs a performance penalty. In principle, when a server that does not support host reservation responds to a query, it needs to check whether there is a lease for a given address being considered for allocation or renewal. The server that also supports host reservation has to perform additional checks: not only if the address is currently used (i.e. if there is a lease for it), but also whether the address could be used by someone else (i.e. there is a reservation for it). That additional check incurs additional overhead.

8.3.1. Address Reservation Types

In a typical scenario there is an IPv4 subnet defined, e.g. 192.0.2.0/24, with certain part of it dedicated for dynamic allocation by the DHCPv4 server. That dynamic part is referred to as a dynamic pool or simply a pool. In principle, a host reservation can reserve any address that belongs to the subnet. The reservations that specify addresses that belong to configured pools are called "in-pool reservations". In contrast, those that do not belong to dynamic pools are called "out-of-pool reservations". There is no formal difference in the reservation syntax and both reservation types are handled uniformly. However, upcoming releases may offer improved performance if there are only out-of-pool reservations as the server will be able to skip reservation checks when dealing with existing leases. Therefore, system administrators are encouraged to use out-of-pool reservations if possible.

Beginning with Kea 1.5.0, there is now support for global host reservations. These are reservations that are specified at the global level within the configuration and that do not belong to any specific subnet. Kea will still match inbound client packets to a subnet as before, but when the subnet's reservation mode is set to "global", Kea will look for host reservations only among the global reservations defined. Typcially, such resrvations would be used to reserve hostnames for clients which may move from one subnet to another.

Note

You can reserve any ip-address in a global reservation. Just keep in mind that Kea will not do any sanity checking on the address and for Kea 1.5.0, support for global reservations should be considered experimental.

8.3.2. Conflicts in DHCPv4 Reservations

As the reservations and lease information are stored separately, conflicts may arise. Consider the following series of events. The server has configured the dynamic pool of addresses from the range of 192.0.2.10 to 192.0.2.20. Host A requests an address and gets 192.0.2.10. Now the system administrator decides to reserve address 192.0.2.10 for Host B. In general, reserving an address that is currently assigned to someone else is not recommended, but there are valid use cases where such an operation is warranted.

The server now has a conflict to resolve. Let's analyze the situation here. If Host B boots up and requests an address, the server is not able to assign the reserved address 192.0.2.10. A naive approach would to be immediately remove the existing lease for the Host A and create a new one for the Host B. That would not solve the problem, though, because as soon as the Host B gets the address, it will detect that the address is already in use by the Host A and would send the DHCPDECLINE message. Therefore, in this situation, the server has to temporarily assign a different address (not matching what has been reserved) to the Host B.

When Host A renews its address, the server will discover that the address being renewed is now reserved for another host - Host B. Therefore the server will inform the Host A that it is no longer allowed to use it by sending a DHCPNAK message. The server will not remove the lease, though, as there's small chance that the DHCPNAK may be lost if the network is lossy. If that happens, the client will not receive any responses, so it will retransmit its DHCPREQUEST packet. Once the DHCPNAK is received by Host A, it will revert to the server discovery and will eventually get a different address. Besides allocating a new lease, the server will also remove the old one. As a result, address 192.0.2.10 will become free . When Host B tries to renew its temporarily assigned address, the server will detect that it has a valid lease, but there is a reservation for a different address. The server will send DHCPNAK to inform Host B that its address is no longer usable, but will keep its lease (again, the DHCPNAK may be lost, so the server will keep it, until the client returns for a new address). Host B will revert to the server discovery phase and will eventually send a DHCPREQUEST message. This time the server will find out that there is a reservation for that host and the reserved address 192.0.2.10 is not used, so it will be granted. It will also remove the lease for the temporarily assigned address that Host B previously obtained.

This recovery will succeed, even if other hosts will attempt to get the reserved address. Had the Host C requested address 192.0.2.10 after the reservation was made, the server will either offer a different address (when responding to DHCPDISCOVER) or would send DHCPNAK (when responding to DHCPREQUEST).

This recovery mechanism allows the server to fully recover from a case where reservations conflict with the existing leases. This procedure takes time and will roughly take as long as the value set for of renew-timer. The best way to avoid such recovery is to not define new reservations that conflict with existing leases. Another recommendation is to use out-of-pool reservations. If the reserved address does not belong to a pool, there is no way that other clients could get this address.

Note

The conflict resolution mechanism does not work for global reservations. As of Kea 1.5.0, it is generally recommended to not use global reservations for addresses. If you want to use it anyway, you have to manually ensure that the reserved addressed are not in the dynamic pools.

8.3.3. Reserving a Hostname

When the reservation for a client includes the hostname, the server will return this hostname to the client in the Client FQDN or Hostname options. The server responds with the Client FQDN option only if the client has included Client FQDN option in its message to the server. The server will respond with the Hostname option if the client included Hostname option in its message to the server or when the client requested Hostname option using Parameter Request List option. The server will return the Hostname option even if it is not configured to perform DNS updates. The reserved hostname always takes precedence over the hostname supplied by the client or the autogenerated (from the IPv4 address) hostname.

The server qualifies the reserved hostname with the value of the qualifying-suffix parameter. For example, the following subnet configuration:

    {
        "subnet4": [ {
            "subnet": "10.0.0.0/24",
            "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
            "reservations": [
               {
                 "hw-address": "aa:bb:cc:dd:ee:ff",
                 "hostname": "alice-laptop"
               }
            ]
         }],
        "dhcp-ddns": {
            "enable-updates": true,
            "qualifying-suffix": "example.isc.org."
        }
    }

will result in assigning the "alice-laptop.example.isc.org." hostname to the client using the MAC address "aa:bb:cc:dd:ee:ff". If the qualifying-suffix is not specified, the default (empty) value will be used, and in this case the value specified as a hostname will be treated as fully qualified name. Thus, by leaving the qualifying-suffix empty it is possible to qualify hostnames for the different clients with different domain names:

    {
        "subnet4": [ {
            "subnet": "10.0.0.0/24",
            "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
            "reservations": [
               {
                 "hw-address": "aa:bb:cc:dd:ee:ff",
                 "hostname": "alice-laptop.isc.org."
               },
               {
                 "hw-address": "12:34:56:78:99:AA",
                 "hostname": "mark-desktop.example.org."
               }

            ]
         }],
        "dhcp-ddns": {
            "enable-updates": true,
        }
    }

8.3.4. Including Specific DHCPv4 Options in Reservations

Kea 1.1.0 introduced the ability to specify options on a per host basis. The options follow the same rules as any other options. These can be standard options (see Section 8.2.8, “Standard DHCPv4 Options”), custom options (see Section 8.2.9, “Custom DHCPv4 options”) or vendor specific options (see Section 8.2.11, “DHCPv4 Vendor Specific Options”). The following example demonstrates how standard options can be defined.

{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            "ip-address": "192.0.2.1",
            "option-data": [
            {
                "name": "cookie-servers",
                "data": "10.1.1.202,10.1.1.203"
            },
            {
                "name": "log-servers",
                "data": "10.1.1.200,10.1.1.201"
            } ]
        } ]
    } ]
}

Vendor specific options can be reserved in a similar manner:

{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            "ip-address": "10.0.0.7",
            "option-data": [
            {
                "name": "vivso-suboptions",
                "data": "4491"
            },
            {
                "name": "tftp-servers",
                "space": "vendor-4491",
                "data": "10.1.1.202,10.1.1.203"
            } ]
        } ]
    } ]
}

Options defined at host level have the highest priority. In other words, if there are options defined with the same type on global, subnet, class and host level, the host specific values will be used.

8.3.5. Reserving Next Server, Server Hostname and Boot File Name

BOOTP/DHCPv4 messages include "siaddr", "sname" and "file" fields. Even though, DHCPv4 includes corresponding options, such as option 66 and option 67, some clients may not support these options. For this reason, server administrators often use the "siaddr", "sname" and "file" fields instead.

With Kea, it is possible to make static reservations for these DHCPv4 message fields:

{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            "next-server": "10.1.1.2",
            "server-hostname": "server-hostname.example.org",
            "boot-file-name": "/tmp/bootfile.efi"
        } ]
    } ]
}

Note that those parameters can be specified in combination with other parameters for a reservation, e.g. reserved IPv4 address. These parameters are optional, i.e. a subset of them can specified, or all of them can be omitted.

8.3.6. Reserving Client Classes in DHCPv4

Section 14.3, “Using Expressions In Classification” explains how to configure the server to assign classes to a client based on the content of the options that this client sends to the server. Host reservations mechanisms also allow for statically assigning classes to the clients. The definitions of these classes should exist in the Kea configuration. The following configuration snippet shows how to specify that a client belongs to classes reserved-class1 and reserved-class2. Those classes are associated with specific options being sent to the clients which belong to them.

{
    "client-classes": [
    {
       "name": "reserved-class1",
       "option-data": [
       {
           "name": "routers",
           "data": "10.0.0.200"
       }
       ]
    },
    {
       "name": "reserved-class2",
       "option-data": [
       {
           "name": "domain-name-servers",
           "data": "10.0.0.201"
       }
       ]
    }
    ],
    "subnet4": [ {
        "subnet": "10.0.0.0/24",
        "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            
            "client-classes": [ "reserved-class1", "reserved-class2" ]
            
        }
        ]
    } ]
}

Static class assignments, as shown above, can be used in conjunction with classification using expressions. The "KNOWN" or "UNKNOWN" builtin class is added to the packet and any class depending on it directly or indirectly and not only-if-required is evaluated.

Note

If you want to force the evaluation of a class expression after the host reservation lookup, for instance because of a dependency on "reserved-class1" from the previous example, you should add a "member('KNOWN')" in the expression.

8.3.7. Storing Host Reservations in MySQL, PostgreSQL or Cassandra

It is possible to store host reservations in MySQL, PostgreSQL or Cassandra. See Section 9.2.3, “Hosts Storage” for information on how to configure Kea to use reservations stored in MySQL, PostgreSQL or Cassandra. Kea provides dedicated hook for managing reservations in a database, section Section 15.4.4, “host_cmds: Host Commands” provide detailed information. https://gitlab.isc.org/isc-projects/kea/wikis/designs/commands#23-host-reservations-hr-management provides some examples how to conduct common host reservation operations.

Note

In Kea maximum length of an option specified per host is arbitrarily set to 4096 bytes.

8.3.8. Fine Tuning DHCPv4 Host Reservation

The host reservation capability introduces additional restrictions for the allocation engine (the component of Kea that selects an address for a client) during lease selection and renewal. In particular, three major checks are necessary. First, when selecting a new lease, it is not sufficient for a candidate lease to not be used by another DHCP client. It also must not be reserved for another client. Second, when renewing a lease, additional check must be performed whether the address being renewed is not reserved for another client. Finally, when a host renews an address, the server has to check whether there is a reservation for this host, so the existing (dynamically allocated) address should be revoked and the reserved one be used instead.

Some of those checks may be unnecessary in certain deployments and not performing them may improve performance. The Kea server provides the reservation-mode configuration parameter to select the types of reservations allowed for the particular subnet. Each reservation type has different constraints for the checks to be performed by the server when allocating or renewing a lease for the client. Allowed values are:

  • all - enables both in-pool and out-of-pool host reservation types. This is the default value. This setting is the safest and the most flexible. As all checks are conducted, it is also the slowest. This does not check against global reservations.
  • out-of-pool - allows only out of pool host reservations. With this setting in place, the server may assume that all host reservations are for addresses that do not belong to the dynamic pool. Therefore it can skip the reservation checks when dealing with in-pool addresses, thus improving performance. Do not use this mode if any of your reservations use in-pool address. Caution is advised when using this setting: Kea does not sanity check the reservations against reservation-mode and misconfiguration may cause problems.
  • global - allows only global host reservations. With this setting in place, the server searches for reservations for a client only among the defined global reservations. If an address is specified, the server will skip the reservation checks done when dealing in other modes, thus improving performance. Caution is advised when using this setting: Kea does not sanity check the reservations when global and misconfiguration may cause problems.
  • disabled - host reservation support is disabled. As there are no reservations, the server will skip all checks. Any reservations defined will be completely ignored. As the checks are skipped, the server may operate faster in this mode.

The parameter can be specified at global, subnet and shared network levels.

An example configuration that disables reservation appears as follows:

"Dhcp4": {
    "subnet4": [
    {
        "subnet": "192.0.2.0/24",
        "reservation-mode": "disabled",
        ...
    }
    ]
}

An example configuration using global reservations is shown below:

"Dhcp4": {

    
    "reservation-mode": "global",
    "reservations": [
       {
        "hw-address": "01:bb:cc:dd:ee:ff",
        "hostname": "host-one"
       },
       {
        "hw-address": "02:bb:cc:dd:ee:ff",
        "hostname": "host-two"
       }
    ],
    
    "subnet4": [
    {
        "subnet": "192.0.2.0/24",
        ...
    }
    ]
}

For more details regarding global reservations, see Section 8.3.9, “Global reservations in DHCPv4”.

Another aspect of the host reservations are the different types of identifiers. Kea 1.1.0 supports four types of identifiers (hw-address, duid, client-id and circuit-id), but more identifier types are likely to be added in the future. This is beneficial from a usability perspective. However, there is a drawback. For each incoming packet Kea has to to extract each identifier type and then query the database to see if there is a reservation done by this particular identifier. If nothing is found, the next identifier is extracted and the next query is issued. This process continues until either a reservation is found or all identifier types have been checked. Over time with an increasing number of supported identifier types, Kea would become slower and slower.

To address this problem, a parameter called host-reservation-identifiers has been introduced. It takes a list of identifier types as a parameter. Kea will check only those identifier types enumerated in host-reservation-identifiers. From a performance perspective the number of identifier types should be kept to a minimum, ideally limited to one. If your deployment uses several reservation types, please enumerate them from most to least frequently used as this increases the chances of Kea finding the reservation using the fewest number of queries. An example of host reservation identifiers looks as follows:

"host-reservation-identifiers": [ "circuit-id", "hw-address", "duid", "client-id" ],
"subnet4": [
    {
        "subnet": "192.0.2.0/24",
        ...
    }
]

If not specified, the default value is:

"host-reservation-identifiers": [ "hw-address", "duid", "circuit-id", "client-id" ]

8.3.9. Global reservations in DHCPv4

In some deployments, such as mobile, clients can roam within the network and there is a desire to specify certain parameters regardless of the client's current location. To facilitate such a need, a global reservation mechanism has been implemented. The idea behind it is that regular host reservations are tied to specific subnets, by using specific subnet-id. Kea 1.5.0 introduced a new capability to specify global reservation that can be used in every subnet that has global reservations enabled.

This feature can be used to assign certain parameters, such as hostname or other dedicated, host-specific options. It can also be used to assign addresses. However, global reservations that assign addresses bypass the whole topology determination provided by DHCP logic implemented in Kea. It is very easy to misuse this feature and get configuration that is inconsistent. To give a specific example, imagine a global reservation for address 192.0.2.100 and two subnets 192.0.2.0/24 and 192.0.5.0/24. If global reservations are used in both subnets and a device matching global host reservations visits part of the network that is serviced by 192.0.5.0/24, it will get an IP address 192.0.2.100, a subnet 192.0.5.0 and a default router 192.0.5.1. Obviously such a configuration is unusable, as the client won't be able to reach its default gateway.

To use global host reservations a configuration similar to the following can be used:

"Dhcp4:" {
    // This specify global reservations. They will apply to all subnets that
    // have global reservations enabled.
    
    "reservations": [
    {
       "hw-address": "aa:bb:cc:dd:ee:ff",
       "hostname": "hw-host-dynamic"
    },
    {
       "hw-address": "01:02:03:04:05:06",
       "hostname": "hw-host-fixed",

       // Use of IP address is global reservation is risky. If used outside of
       // matching subnet, such as 192.0.1.0/24, it will result in a broken
       // configuration being handled to the client.
       "ip-address": "192.0.1.77"
    },
    {
       "duid": "01:02:03:04:05",
       "hostname": "duid-host"
    },
    {
       "circuit-id": "'charter950'",
       "hostname": "circuit-id-host"
    },
    {
       "client-id": "01:11:22:33:44:55:66",
       "hostname": "client-id-host"
    }
    ],
    "valid-lifetime": 600,
    "subnet4": [ {
        "subnet": "10.0.0.0/24",
        "reservation-mode": "global",
        "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ]
    } ]
}

When using database backends, the global host reservations are distinguished from regular reservations by using subnet-id value of zero.

8.4. Shared networks in DHCPv4

DHCP servers use subnet information in two ways. First, it is used to determine the point of attachment, or simply put, where the client is connected to the network. Second, the subnet information is used to group information pertaining to specific location in the network. This approach works well in general case, but the are scenarios where the boundaries are blurred. Sometimes it is useful to have more than one logical IP subnet being deployed on the same physical link. The need to understand that two or more subnets are used on the same link requires additional logic in the DHCP server. This capability has been added in Kea 1.3.0. It is called "shared networks" in Kea and ISC DHCP projects. It is sometimes also called "shared subnets". In Microsoft's nomenclature it is called "multinet".

There are many use cases where the feature is useful. This paragraph explains just a handful of the most common ones. The first and by far the most common use case is an existing network that has grown and is running out of available address space. Rather than migrating all devices to a new, larger subnet, it is easier to simply configure additional subnet on top of the existing one. Sometimes, due to address space fragmentation (e.g. only many disjoint /24s are available) this is the only choice. Also, configuring additional subnet has the advantage of not disrupting the operation of existing devices.

Another very frequent use case comes from cable networks. There are two types of devices in cable networks: cable modems and the end user devices behind them. It is a common practice to use different subnet for cable modems to prevent users from tinkering with their cable modems. In this case, the distinction is based on the type of device, rather than address space exhaustion.

A client connected to a shared network may be assigned an address from any of the address pools defined within the subnets belonging to the shared network. Internally, the server selects one of the subnets belonging to a shared network and tries to allocate an address from this subnet. If the server is unable to allocate an address from the selected subnet (e.g. due to address pools exhaustion) it will use another subnet from the same shared network and try to allocate an address from this subnet etc. Therefore, in the typical case, the server will allocate all addresses available for a given subnet before it starts allocating addresses from other subnets belonging to the same shared network. However, in certain situations the client can be allocated an address from the other subnets before the address pools in the first subnet get exhausted, e.g. when the client provides a hint that belongs to another subnet or the client has reservations in a different than default subnet.

Note

It is strongly discouraged for the Kea deployments to assume that the server doesn't allocate addresses from other subnets until it uses all the addresses from the first subnet in the shared network. Apart from the fact that hints, host reservations and client classification affect subnet selection, it is also foreseen that we will enhance allocation strategies for shared networks in the future versions of Kea, so as the selection of subnets within a shared network is equally probable (unpredictable).

In order to define a shared network an additional configuration scope is introduced:

{
"Dhcp4": {
    "shared-networks": [
        {
            // Name of the shared network. It may be an arbitrary string
            // and it must be unique among all shared networks.
            "name": "my-secret-lair-level-1",

            // Subnet selector can be specifed at the shared network level.
            // Subnets from this shared network will be selected for directly
            // connected clients sending requests to server's "eth0" interface.
            "interface": "eth0",

            // This starts a list of subnets in this shared network.
            // There are two subnets in this example.
            "subnet4": [
                {
                    "subnet": "10.0.0.0/8",
                    "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],
                },
                {
                    "subnet": "192.0.2.0/24",
                    "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ]
                }
            ],
        } ], // end of shared-networks

    // It is likely that in your network you'll have a mix of regular,
    // "plain" subnets and shared networks. It is perfectly valid to mix
    // them in the same config file.
    //
    // This is regular subnet. It's not part of any shared-network.
    "subnet4": [
        {
            "subnet": "192.0.3.0/24",
            "pools": [ { "pool":  "192.0.3.1 - 192.0.3.200" } ],
            "interface": "eth1"
        }
    ]

} // end of Dhcp4
}

As you see in the example, it is possible to mix shared and regular ("plain") subnets. Each shared network must have a unique name. This is similar to ID for subnets, but gives you more flexibility. This is used for logging, but also internally for identifying shared networks.

In principle it makes sense to define only shared networks that consist of two or more subnets. However, for testing purposes it is allowed to define a shared network with just one subnet or even an empty one. This is not a recommended practice in production networks, as the shared network logic requires additional processing and thus lowers server's performance. To avoid unnecessary performance degradation the shared subnets should only be defined when required by the deployment.

Shared networks provide an ability to specify many parameters in the shared network scope that will apply to all subnets within it. If necessary, you can specify a parameter on the shared network scope and then override its value in the subnet scope. For example:

"shared-networks": [
    {
        "name": "lab-network3",

        "interface": "eth0",

        // This applies to all subnets in this shared network, unless
        // values are overridden on subnet scope.
        "valid-lifetime": 600,

        // This option is made available to all subnets in this shared
        // network.
        "option-data": [ {
            "name": "log-servers",
            "data": "1.2.3.4"
        } ],

        "subnet4": [
            {
                "subnet": "10.0.0.0/8",
                "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],

                // This particular subnet uses different values.
                "valid-lifetime": 1200,
                "option-data": [
                {
                    "name": "log-servers",
                    "data": "10.0.0.254"
                },
                {
                    "name": "routers",
                    "data": "10.0.0.254"
                } ]
            },
            {
                 "subnet": "192.0.2.0/24",
                 "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ],

                 // This subnet does not specify its own valid-lifetime value,
                 // so it is inherited from shared network scope.
                 "option-data": [
                 {
                     "name": "routers",
                     "data": "192.0.2.1"
                 } ]
            }
        ]
    } ]

In this example, there is a log-servers option defined that is available to clients in both subnets in this shared network. Also, a valid lifetime is set to 10 minutes (600s). However, the first subnet overrides some of the values (valid lifetime is 20 minutes, different IP address for log-servers), but also adds its own option (router address). Assuming a client asking for router and log servers options is assigned a lease from this subnet, he will get a lease for 20 minutes and log-servers and routers value of 10.0.0.254. If the same client is assigned to the second subnet, he will get a 10 minutes long lease, log-servers value of 1.2.3.4 and routers set to 192.0.2.1.

8.4.1. Local and relayed traffic in shared networks

It is possible to specify interface name in the shared network scope to tell the server that this specific shared network is reachable directly (not via relays) using local network interface. It is sufficient to specify it once at the shared network level. As all subnets in a shared network are expected to be used on the same physical link, it is a configuration error to attempt to define a shared network using subnets that are reachable over different interfaces. It is allowed to specify interface parameter on each subnet, although its value must be the same for each subnet. Thus it's usually more convenient to specify it once at the shared network level.

"shared-networks": [
    {
        "name": "office-floor-2",

        // This tells Kea that the whole shared networks is reachable over
        // local interface. This applies to all subnets in this network.
        "interface": "eth0",

        "subnet4": [
            {
                "subnet": "10.0.0.0/8",
                "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],
                "interface": "eth0"
            },
            {
                 "subnet": "192.0.2.0/24",
                 "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ]

                 // Specifying a different interface name is configuration
                 // error:
                 // "interface": "eth1"
            }
        ]
    } ]

Somewhat similar to interface names, also relay IP addresses can be specified for the whole shared network. However, depending on your relay configuration, it may use different IP addresses depending on which subnet is being used. Thus there is no requirement to use the same IP relay address for each subnet. Here's an example:

"shared-networks": [
    {
        "name": "kakapo",
        "relay": {
            "ip-addresses": [ "192.3.5.6" ]
        },
        "subnet4": [
            {
                "subnet": "192.0.2.0/26",
                "relay": {
                    "ip-addresses": [ "192.1.1.1" ]
                },
                "pools": [ { "pool": "192.0.2.63 - 192.0.2.63" } ]
            },
            {
                "subnet": "10.0.0.0/24",
                "relay": {
                    "ip-addresses": [ "192.2.2.2" ]
                },
                "pools": [ { "pool": "10.0.0.16 - 10.0.0.16" } ]
            }
        ]
    }
]

In this particular case the relay IP address specified at network level doesn't have much sense, as it is overridden in both subnets, but it was left there as an example of how one could be defined at network level. Note that the relay agent IP address typically belongs to the subnet it relays packets from, but this is not a strict requirement. Therefore Kea accepts any value here as long as it is valid IPv4 address.

8.4.2. Client classification in shared networks

Sometimes it is desired to segregate clients into specific subnets based on some properties. This mechanism is called client classification and is described in Chapter 14, Client Classification. Client classification can be applied to subnets belonging to shared networks in the same way as it is used for subnets specified outside of shared networks. It is important to understand how the server selects subnets for the clients when client classification is in use, to assure that the desired subnet is selected for a given client type.

If a subnet is associated with a class, only the clients belonging to this class can use this subnet. If there are no classes specified for a subnet, any client connected to a given shared network can use this subnet. A common mistake is to assume that the subnet including a client class is preferred over subnets without client classes. Consider the following example:

{
    "client-classes": [
        {
            "name": "b-devices",
            "test": "option[93].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "interface": "eth0",
            "subnet4": [
                {
                    "subnet": "192.0.2.0/26",
                    "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                },
                {
                    "subnet": "10.0.0.0/24",
                    "pools": [ { "pool": "10.0.0.2 - 10.0.0.250" } ],
                    "client-class": "b-devices"
                }
            ]
        }
    ]
}

If the client belongs to "b-devices" class (because it includes option 93 with a value of 0x0002) it doesn't guarantee that the subnet 10.0.0.0/24 will be used (or preferred) for this client. The server can use any of the two subnets because the subnet 192.0.2.0/26 is also allowed for this client. The client classification used in this case should be pereceived as a way to restrict access to certain subnets, rather than a way to express subnet preference. For example, if the client doesn't belong to the "b-devices" class it may only use the subnet 192.0.2.0/26 and will never use the subnet 10.0.0.0/24.

A typical use case for client classification is in the cable network, where cable modems should use one subnet and other devices should use another subnet within the same shared network. In this case it is required to apply classification on all subnets. The following example defines two classes of devices. The subnet selection is made based on option 93 values.

{
    "client-classes": [
        {

            "name": "a-devices",
            "test": "option[93].hex == 0x0001"
        },
        {
            "name": "b-devices",
            "test": "option[93].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "interface": "eth0",
            "subnet4": [
                {
                    "subnet": "192.0.2.0/26",
                    "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                    "client-class": "a-devices"
                },
                {
                    "subnet": "10.0.0.0/24",
                    "pools": [ { "pool": "10.0.0.2 - 10.0.0.250" } ],
                    "client-class": "b-devices"
                }
            ]
        }
    ]
}

In this example each class has its own restriction. Only clients that belong to class "a-devices" will be able to use subnet 192.0.2.0/26 and only clients belonging to b-devices will be able to use subnet 10.0.0.0/24. Care should be taken to not define too restrictive classification rules, as clients that are unable to use any subnets will be refused service. Although, this may be a desired outcome if one desires to service only clients of known properties (e.g. only VoIP phones allowed on a given link).

Note that it is possible to achieve similar effect as presented in this section without the use of shared networks. If the subnets are placed in the global subnets scope, rather than in the shared network, the server will still use classification rules to pick the right subnet for a given class of devices. The major benefit of placing subnets within the shared network is that common parameters for the logically grouped subnets can be specified once, in the shared network scope, e.g. "interface" or "relay" parameter. All subnets belonging to this shared network will inherit those parameters.

8.4.3. Host reservations in shared networks

Subnets being part of a shared network allow host reservations, similar to regular subnets:

{
    "shared-networks": [
    {
        "name": "frog",
        "interface": "eth0",
        "subnet4": [
            {
                "subnet": "192.0.2.0/26",
                "id": 100,
                "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                "reservations": [
                    {
                        "hw-address": "aa:bb:cc:dd:ee:ff",
                        "ip-address": "192.0.2.28"
                    }
                ]
            },
            {
                "subnet": "10.0.0.0/24",
                "id": 101,
                "pools": [ { "pool": "10.0.0.1 - 10.0.0.254" } ],
                "reservations": [
                    {
                        "hw-address": "11:22:33:44:55:66",
                        "ip-address": "10.0.0.29"
                    }
                ]
            }
        ]
    }
    ]
}
  

It is worth noting that Kea conducts additional checks when processing a packet if shared networks are defined. First, instead of simply checking if there's a reservation for a given client in his initially selected subnet, it goes through all subnets in a shared network looking for a reservation. This is one of the reasons why defining a shared network may impact performance. If there is a reservation for a client in any subnet, that particular subnet will be picked for the client. Although it's technically not an error, it is considered a bad practice to define reservations for the same host in multiple subnets belonging to the same shared network.

While not strictly mandatory, it is strongly recommended to use explicit "id" values for subnets if you plan to use database storage for host reservations. If ID is not specified, the values for it be autogenerated, i.e. it will assign increasing integer values starting from 1. Thus, the autogenerated IDs are not stable across configuration changes.

8.5. Server Identifier in DHCPv4

The DHCPv4 protocol uses a "server identifier" to allow clients to discriminate between several servers present on the same link: this value is an IPv4 address of the server. The server chooses the IPv4 address of the interface on which the message from the client (or relay) has been received. A single server instance will use multiple server identifiers if it is receiving queries on multiple interfaces.

It is possible to override default server identifier values by specifying "dhcp-server-identifier" option. This option is only supported at the global, shared network and subnet level. It must not be specified on client class and host reservation level.

The following example demonstrates how to override server identifier for a subnet:

"subnet4": [
    {
        "subnet": "192.0.2.0/24",
        "option-data": [
            {
                "name": "dhcp-server-identifier",
                "data": "10.2.5.76"
            }
        ],
        ...
    }
]

8.6. How the DHCPv4 Server Selects a Subnet for the Client

The DHCPv4 server differentiates between the directly connected clients, clients trying to renew leases and clients sending their messages through relays. For directly connected clients, the server will check the configuration for the interface on which the message has been received and, if the server configuration doesn't match any configured subnet, the message is discarded.

Assuming that the server's interface is configured with the IPv4 address 192.0.2.3, the server will only process messages received through this interface from a directly connected client if there is a subnet configured to which this IPv4 address belongs, e.g. 192.0.2.0/24. The server will use this subnet to assign IPv4 address for the client.

The rule above does not apply when the client unicasts its message, i.e. is trying to renew its lease. Such a message is accepted through any interface. The renewing client sets ciaddr to the currently used IPv4 address. The server uses this address to select the subnet for the client (in particular, to extend the lease using this address).

If the message is relayed it is accepted through any interface. The giaddr set by the relay agent is used to select the subnet for the client.

It is also possible to specify a relay IPv4 address for a given subnet. It can be used to match incoming packets into a subnet in uncommon configurations, e.g. shared networks. See Section 8.6.1, “Using a Specific Relay Agent for a Subnet” for details.

Note

The subnet selection mechanism described in this section is based on the assumption that client classification is not used. The classification mechanism alters the way in which a subnet is selected for the client, depending on the classes to which the client belongs.

8.6.1. Using a Specific Relay Agent for a Subnet

A relay has to have an interface connected to the link on which the clients are being configured. Typically the relay has an IPv4 address configured on that interface that belongs to the subnet from which the server will assign addresses. In the typical case, the server is able to use the IPv4 address inserted by the relay (in the giaddr field of the DHCPv4 packet) to select the appropriate subnet.

However, that is not always the case. In certain uncommon — but valid — deployments, the relay address may not match the subnet. This usually means that there is more than one subnet allocated for a given link. The two most common examples where this is the case are long lasting network renumbering (where both old and new address space is still being used) and a cable network. In a cable network both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such a case, the DHCPv4 server needs additional information (the IPv4 address of the relay) to properly select an appropriate subnet.

The following example assumes that there is a subnet 192.0.2.0/24 that is accessible via a relay that uses 10.0.0.1 as its IPv4 address. The server will be able to select this subnet for any incoming packets that came from a relay that has an address in 192.0.2.0/24 subnet. It will also select that subnet for a relay with address 10.0.0.1.

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "relay": {
                "ip-addresses": [ "10.0.0.1" ]
            },
            ...
        }
    ],
    ...
}

If "relay" is specified, the "ip-addresses" parameter within it is mandatory.

Note

As of Kea 1.4, the "ip-address" parameter has been deprecated in favor of "ip-addresses" which supports specifying a list of addresses. Configuration parsing, will honor the singular form for now but users are encouraged to migrate.

8.6.2. Segregating IPv4 Clients in a Cable Network

In certain cases, it is useful to mix relay address information, introduced in Section 8.6.1, “Using a Specific Relay Agent for a Subnet” with client classification, explained in Chapter 14, Client Classification. One specific example is cable network, where typically modems get addresses from a different subnet than all devices connected behind them.

Let us assume that there is one CMTS (Cable Modem Termination System) with one CM MAC (a physical link that modems are connected to). We want the modems to get addresses from the 10.1.1.0/24 subnet, while everything connected behind modems should get addresses from another subnet (192.0.2.0/24). The CMTS that acts as a relay uses address 10.1.1.1. The following configuration can serve that configuration:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "10.1.1.0/24",
            "pools":  [ { "pool": "10.1.1.2 - 10.1.1.20" } ],
            "client-class" "docsis3.0",
            "relay": {
                "ip-addresses": [ "10.1.1.1 ]"
            }
        },
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "relay": {
                "ip-addresses": [ "10.1.1.1" ]
            }
        }
    ],
    ...
}

8.7. Duplicate Addresses (DHCPDECLINE Support)

The DHCPv4 server is configured with a certain pool of addresses that it is expected to hand out to the DHCPv4 clients. It is assumed that the server is authoritative and has complete jurisdiction over those addresses. However, due to various reasons, such as misconfiguration or a faulty client implementation that retains its address beyond the valid lifetime, there may be devices connected that use those addresses without the server's approval or knowledge.

Such an unwelcome event can be detected by legitimate clients (using ARP or ICMP Echo Request mechanisms) and reported to the DHCPv4 server using a DHCPDECLINE message. The server will do a sanity check (if the client declining an address really was supposed to use it), and then will conduct a clean up operation. Any DNS entries related to that address will be removed, the fact will be logged and hooks will be triggered. After that is done, the address will be marked as declined (which indicates that it is used by an unknown entity and thus not available for assignment to anyone) and a probation time will be set on it. Unless otherwise configured, the probation period lasts 24 hours. After that period, the server will recover the lease (i.e. put it back into the available state) and the address will be available for assignment again. It should be noted that if the underlying issue of a misconfigured device is not resolved, the duplicate address scenario will repeat. On the other hand, it provides an opportunity to recover from such an event automatically, without any sysadmin intervention.

To configure the decline probation period to a value other than the default, the following syntax can be used:

  "Dhcp4": {
    "decline-probation-period": 3600,
    "subnet4": [ ... ],
    ...
}

The parameter is expressed in seconds, so the example above will instruct the server to recycle declined leases after an hour.

There are several statistics and hook points associated with the Decline handling procedure. The lease4_decline hook is triggered after the incoming DHCPDECLINE message has been sanitized and the server is about to decline the lease. The declined-addresses statistic is increased after the hook returns (both global and subnet specific variants). (See Section 8.8, “Statistics in the DHCPv4 Server” and Chapter 15, Hooks Libraries for more details on DHCPv4 statistics and Kea hook points.)

Once the probation time elapses, the declined lease is recovered using the standard expired lease reclamation procedure, with several additional steps. In particular, both declined-addresses statistics (global and subnet specific) are decreased. At the same time, reclaimed-declined-addresses statistics (again in two variants, global and subnet specific) are increased.

Note about statistics: The server does not decrease the assigned-addresses statistics when a DHCPDECLINE is received and processed successfully. While technically a declined address is no longer assigned, the primary usage of the assigned-addresses statistic is to monitor pool utilization. Most people would forget to include declined-addresses in the calculation, and simply do assigned-addresses/total-addresses. This would have a bias towards under-representing pool utilization. As this has a potential for major issues, we decided not to decrease assigned addresses immediately after receiving DHCPDECLINE, but to do it later when we recover the address back to the available pool.

8.8. Statistics in the DHCPv4 Server

Note

This section describes DHCPv4-specific statistics. For a general overview and usage of statistics, see Chapter 16, Statistics.

The DHCPv4 server supports the following statistics:

Table 8.4. DHCPv4 Statistics

StatisticData TypeDescription
pkt4-receivedinteger Number of DHCPv4 packets received. This includes all packets: valid, bogus, corrupted, rejected etc. This statistic is expected to grow rapidly.
pkt4-discover-receivedinteger Number of DHCPDISCOVER packets received. This statistic is expected to grow. Its increase means that clients that just booted started their configuration process and their initial packets reached your server.
pkt4-offer-receivedinteger Number of DHCPOFFER packets received. This statistic is expected to remain zero at all times, as DHCPOFFER packets are sent by the server and the server is never expected to receive them. Non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPOFFER messages towards the server, rather back to the clients.
pkt4-request-receivedinteger Number of DHCPREQUEST packets received. This statistic is expected to grow. Its increase means that clients that just booted received server's response (DHCPOFFER), accepted it and now requesting an address (DHCPREQUEST).
pkt4-ack-receivedinteger Number of DHCPACK packets received. This statistic is expected to remain zero at all times, as DHCPACK packets are sent by the server and the server is never expected to receive them. Non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPACK messages towards the server, rather back to the clients.
pkt4-nak-receivedinteger Number of DHCPNAK packets received. This statistic is expected to remain zero at all times, as DHCPNAK packets are sent by the server and the server is never expected to receive them. Non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPNAK messages towards the server, rather back to the clients.
pkt4-release-receivedinteger Number of DHCPRELEASE packets received. This statistic is expected to grow. Its increase means that clients that had an address are shutting down or stop using their addresses.
pkt4-decline-receivedinteger Number of DHCPDECLINE packets received. This statistic is expected to remain close to zero. Its increase means that a client that leased an address, but discovered that the address is currently used by an unknown device in your network.
pkt4-inform-receivedinteger Number of DHCPINFORM packets received. This statistic is expected to grow. Its increase means that there are clients that either do not need an address or already have an address and are interested only in getting additional configuration parameters.
pkt4-unknown-receivedinteger Number of packets received of an unknown type. Non-zero value of this statistic indicates that the server received a packet that it wasn't able to recognize: either with unsupported type or possibly malformed (without message type option).
pkt4-sentinteger Number of DHCPv4 packets sent. This statistic is expected to grow every time the server transmits a packet. In general, it should roughly match pkt4-received, as most incoming packets cause server to respond. There are exceptions (e.g. DHCPRELEASE), so do not worry, if it is lesser than pkt4-received.
pkt4-offer-sentinteger Number of DHCPOFFER packets sent. This statistic is expected to grow in most cases after a DHCPDISCOVER is processed. There are certain uncommon, but valid cases where incoming DHCPDISCOVER is dropped, but in general this statistic is expected to be close to pkt4-discover-received.
pkt4-ack-sentinteger Number of DHCPACK packets sent. This statistic is expected to grow in most cases after a DHCPREQUEST is processed. There are certain cases where DHCPNAK is sent instead. In general, the sum of pkt4-ack-sent and pkt4-nak-sent should be close to pkt4-request-received.
pkt4-nak-sentinteger Number of DHCPNAK packets sent. This statistic is expected to grow when the server chooses to not honor the address requested by a client. In general, the sum of pkt4-ack-sent and pkt4-nak-sent should be close to pkt4-request-received.
pkt4-parse-failedinteger Number of incoming packets that could not be parsed. A non-zero value of this statistic indicates that the server received malformed or truncated packet. This may indicate problems in your network, faulty clients or a bug in the server.
pkt4-receive-dropinteger Number of incoming packets that were dropped. The exact reason for dropping packets is logged, but the most common reasons may be: an unacceptable packet type, direct responses are forbidden, or the server-id sent by the client does not match the server's server-id.
subnet[id].total-addressesintegerThe total number of addresses available for DHCPv4 management. In other words, this is the sum of all addresses in all configured pools. This statistic changes only during configuration changes. Note it does not take into account any addresses that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately. This statistic is reset during reconfiguration event.
subnet[id].assigned-addressesintegerThis statistic shows the number of assigned addresses in a given subnet. It increases every time a new lease is allocated (as a result of receiving a DHCPREQUEST message) and is decreased every time a lease is released (a DHCPRELEASE message is received) or expires. The id is the subnet-id of the subnet. This statistic is exposed for each subnet separately. This statistic is reset during reconfiguration event.
reclaimed-leasesintegerThis statistic is the number of expired leases that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed and is reset when the server is reconfigured.
subnet[id].reclaimed-leasesintegerThis statistic is the number of expired leases associated with a given subnet (id is the subnet-id) that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed and is reset when the server is reconfigured.
declined-addressesinteger This statistic shows the number of IPv4 addresses that are currently declined, so counting the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased. Ideally, this statistic should be zero. If this statistic is non-zero (or worse increasing), a network administrator should investigate if there is a misbehaving device in his network. This is a global statistic that covers all subnets.
subnet[id].declined-addressesinteger This statistic shows the number of IPv4 addresses that are currently declined in a given subnet, so is a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased. Ideally, this statistic should be zero. If this statistic is non-zero (or worse increasing), a network administrator should investigate if there is a misbehaving device in his network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
reclaimed-declined-addressesinteger This statistic shows the number of IPv4 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long term indicator of how many actual valid Declines were processed and recovered from. This is a global statistic that covers all subnets.
subnet[id].reclaimed-declined-addressesinteger This statistic shows the number of IPv4 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long term indicator of how many actual valid Declines were processed and recovered from. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.

8.9. Management API for the DHCPv4 Server

The management API allows the issuing of specific management commands, such as statistics retrieval, reconfiguration or shutdown. For more details, see Chapter 17, Management API. Currently the only supported communication channel type is UNIX stream socket. By default there are no sockets open. To instruct Kea to open a socket, the following entry in the configuration file can be used:

"Dhcp4": {
    "control-socket": {
        "socket-type": "unix",
        "socket-name": "/path/to/the/unix/socket"
    },

    "subnet4": [
        ...
    ],
    ...
}

The length of the path specified by the socket-name parameter is restricted by the maximum length for the unix socket name on your operating system, i.e. the size of the sun_path field in the sockaddr_un structure, decreased by 1. This value varies on different operating systems between 91 and 107 characters. Typical values are 107 on Linux and 103 on FreeBSD.

Communication over control channel is conducted using JSON structures. See the Control Channel section in the Kea Developer's Guide for more details.

The DHCPv4 server supports the following operational commands:

  • build-report
  • config-get
  • config-reload
  • config-set
  • config-test
  • config-write
  • dhcp-disable
  • dhcp-enable
  • leases-reclaim
  • list-commands
  • shutdown
  • version-get

as described in Section 17.3, “Commands Supported by Both the DHCPv4 and DHCPv6 Servers”. In addition, it supports the following statistics related commands:

  • statistic-get
  • statistic-reset
  • statistic-remove
  • statistic-get-all
  • statistic-reset-all
  • statistic-remove-all

as described here Section 16.3, “Commands for Manipulating Statistics”.

8.10. Supported DHCP Standards

The following standards are currently supported:

  • Dynamic Host Configuration Protocol, RFC 2131: Supported messages are DHCPDISCOVER (1), DHCPOFFER (2), DHCPREQUEST (3), DHCPRELEASE (7), DHCPINFORM (8), DHCPACK (5), and DHCPNAK(6).
  • DHCP Options and BOOTP Vendor Extensions, RFC 2132: Supported options are: PAD (0), END(255), Message Type(53), DHCP Server Identifier (54), Domain Name (15), DNS Servers (6), IP Address Lease Time (51), Subnet mask (1), and Routers (3).
  • DHCP Relay Agent Information Option, RFC 3046: Relay Agent Information option is supported.
  • Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4, RFC 3925: Vendor-Identifying Vendor Class and Vendor-Identifying Vendor-Specific Information options are supported.
  • Client Identifier Option in DHCP Server Replies, RFC 6842: Server by default sends back client-id option. That capability may be disabled. See Section 8.2.18, “Echoing Client-ID (RFC 6842)” for details.

8.11. User contexts in IPv4

Kea allows loading hook libraries that sometimes could benefit from additional parameters. If such a parameter is specific to the whole library, it is typically defined as a parameter for the hook library. However, sometimes there is a need to specify parameters that are different for each pool.

User contexts can store arbitrary data as long as it is valid JSON syntax and its top level element is a map (i.e. the data must be enclosed in curly brackets). Some hook libraries may expect specific formatting, though. Please consult specific hook library documentation for details.

User contexts can be specified at either global scope, shared network, subnet, pool, client class, option data or definition level, and host reservation. One other useful usage is the ability to store comments or descriptions.

Let's consider an imaginary case of devices that have color LED lights. Depending on their location, they should glow red, blue or green. It would be easy to write a hook library that would send specific values as maybe a vendor option. However, the server has to have some way to specify that value for each pool. This need is addressed by user contexts. In essence, any user data can specified in the user context as long as it is a valid JSON map. For example, the forementioned case of LED devices could be configured in the following way:

"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ {
                "pool": "192.0.2.10 - 192.0.2.20",
                // This is pool specific user context
                "user-context": { "colour": "red" }
            } ],

            // This is a subnet specific user context. You can put whatever type
            // of information you want as long as it is a valid JSON.
            "user-context": {
                "comment": "network on the second floor",
                "last-modified": "2017-09-04 13:32",
                "description": "you can put here anything you like",
                "phones": [ "x1234", "x2345" ],
                "devices-registered": 42,
                "billing": false
            }
        },
        ...
    ],
    ...
}

It should be noted that Kea will not use that information, but will simply store and make it available to hook libraries. It is up to the hook library to extract that information and make use of it. The parser translates a "comment" entry into a user-context with the entry, this allows to attach a comment inside the configuration itself.

For more background information, see Section 15.5, “User contexts”.

8.12. DHCPv4 Server Limitations

These are the current limitations of the DHCPv4 server software. Most of them are reflections of the current stage of development and should be treated as not implemented yet, rather than actual limitations. However, some of them are implications of the design choices made. Those are clearly marked as such.

  • BOOTP (RFC 951) is not supported. This is a design choice: BOOTP support is not planned.
  • On Linux and BSD system families the DHCP messages are sent and received over the raw sockets (using LPF and BPF) and all packet headers (including data link layer, IP and UDP headers) are created and parsed by Kea, rather than the system kernel. Currently, Kea can only parse the data link layer headers with a format adhering to IEEE 802.3 standard and assumes this data link layer header format for all interfaces. Hence, Kea will fail to work on interfaces which use different data link layer header formats (e.g. Infiniband).
  • The DHCPv4 server does not verify that assigned address is unused. According to RFC 2131, the allocating server should verify that address is not used by sending ICMP echo request.

8.13. Kea DHCPv4 server examples

A collection of simple to use examples for DHCPv4 component of Kea is available with the sources. It is located in doc/examples/kea4 directory. At the time of writing this text there were 15 examples, but the number is growing slowly with each release.

Chapter 9. The DHCPv6 Server

Table of Contents

9.1. Starting and Stopping the DHCPv6 Server
9.2. DHCPv6 Server Configuration
9.2.1. Introduction
9.2.2. Lease Storage
9.2.3. Hosts Storage
9.2.4. Interface Selection
9.2.5. IPv6 Subnet Identifier
9.2.6. Unicast Traffic Support
9.2.7. Subnet and Address Pool
9.2.8. Subnet and Prefix Delegation Pools
9.2.9. Prefix Exclude Option
9.2.10. Standard DHCPv6 Options
9.2.11. Common Softwire46 Options
9.2.12. Custom DHCPv6 Options
9.2.13. DHCPv6 Vendor-Specific Options
9.2.14. Nested DHCPv6 Options (Custom Option Spaces)
9.2.15. Unspecified Parameters for DHCPv6 Option Configuration
9.2.16. IPv6 Subnet Selection
9.2.17. Rapid Commit
9.2.18. DHCPv6 Relays
9.2.19. Relay-Supplied Options
9.2.20. Client Classification in DHCPv6
9.2.21. DDNS for DHCPv6
9.2.22. DHCPv4-over-DHCPv6: DHCPv6 Side
9.2.23. Sanity checks in DHCPv6
9.3. Host Reservation in DHCPv6
9.3.1. Address/Prefix Reservation Types
9.3.2. Conflicts in DHCPv6 Reservations
9.3.3. Reserving a Hostname
9.3.4. Including Specific DHCPv6 Options in Reservations
9.3.5. Reserving Client Classes in DHCPv6
9.3.6. Storing Host Reservations in MySQL, PostgreSQL or Cassandra
9.3.7. Fine Tuning DHCPv6 Host Reservation
9.3.8. Global reservations in DHCPv6
9.4. Shared networks in DHCPv6
9.4.1. Local and relayed traffic in shared networks
9.4.2. Client classification in shared networks
9.4.3. Host reservations in shared networks
9.5. Server Identifier in DHCPv6
9.6. Stateless DHCPv6 (Information-Request Message)
9.7. Support for RFC 7550 (being now part of RFC 8415)
9.8. Using Specific Relay Agent for a Subnet
9.9. Segregating IPv6 Clients in a Cable Network
9.10. MAC/Hardware Addresses in DHCPv6
9.11. Duplicate Addresses (DECLINE Support)
9.12. Statistics in the DHCPv6 Server
9.13. Management API for the DHCPv6 Server
9.14. User contexts in IPv6
9.15. Supported DHCPv6 Standards
9.16. DHCPv6 Server Limitations
9.17. Kea DHCPv6 server examples

9.1. Starting and Stopping the DHCPv6 Server

It is recommended that the Kea DHCPv6 server be started and stopped using keactrl (described in Chapter 6, Managing Kea with keactrl). However, it is also possible to run the server directly: it accepts the following command-line switches:

  • -c file - specifies the configuration file. This is the only mandatory switch.
  • -d - specifies whether the server logging should be switched to verbose mode. In verbose mode, the logging severity and debuglevel specified in the configuration file are ignored and "debug" severity and the maximum debuglevel (99) are assumed. The flag is convenient, for temporarily switching the server into maximum verbosity, e.g. when debugging.
  • -p port - specifies UDP port on which the server will listen. This is only useful during testing, as a DHCPv6 server listening on ports other than the standard ones will not be able to handle regular DHCPv6 queries.
  • -t file - specifies the configuration file to be tested. Kea-dhcp6 will attempt to load it, and will conduct sanity checks. Note that certain checks are possible only while running the actual server. The actual status is reported with exit code (0 = configuration looks ok, 1 = error encountered). Kea will print out log messages to standard output and error to standard error when testing configuration.
  • -v - prints out the Kea version and exits.
  • -V - prints out the Kea extended version with additional parameters and exits. The listing includes the versions of the libraries dynamically linked to Kea.
  • -W - prints out the Kea configuration report and exits. The report is a copy of the config.report file produced by ./configure: it is embedded in the executable binary.

The config.report may also be accessed more directly. The following command may be used to extract this information. The binary path may be found in the install directory or in the .libs subdirectory in the source tree. For example kea/src/bin/dhcp6/.libs/kea-dhcp6.

strings path/kea-dhcp6 | sed -n 's/;;;; //p'

On start-up, the server will detect available network interfaces and will attempt to open UDP sockets on all interfaces mentioned in the configuration file. Since the DHCPv6 server opens privileged ports, it requires root access. Make sure you run this daemon as root.

During startup the server will attempt to create a PID file of the form: localstatedir]/[conf name].kea-dhcp6.pid where:

  • localstatedir: The value as passed into the build configure script. It defaults to "/usr/local/var". Note that this value may be overridden at run time by setting the environment variable KEA_PIDFILE_DIR. This is intended primarily for testing purposes.
  • conf name: The configuration file name used to start the server, minus all preceding path and file extension. For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the portion used would be "myconf".

If the file already exists and contains the PID of a live process, the server will issue a DHCP6_ALREADY_RUNNING log message and exit. It is possible, though unlikely, that the file is a remnant of a system crash and the process to which the PID belongs is unrelated to Kea. In such a case it would be necessary to manually delete the PID file.

The server can be stopped using the kill command. When running in a console, the server can be shut down by pressing ctrl-c. It detects the key combination and shuts down gracefully.

9.2. DHCPv6 Server Configuration

9.2.1. Introduction

This section explains how to configure the DHCPv6 server using a configuration file. Before DHCPv6 is started, its configuration file has to be created. The basic configuration is as follows:

{
# DHCPv6 configuration starts on the next line
"Dhcp6": {

# First we set up global values
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,
    "preferred-lifetime": 3000,

# Next we setup the interfaces to be used by the server.
    "interfaces-config": {
        "interfaces": [ "eth0" ]
    },

# And we specify the type of lease database
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/var/kea/dhcp6.leases"
    },

# Finally, we list the subnets from which we will be leasing addresses.
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ]
        }
    ]
# DHCPv6 configuration ends with the next line
}

} 

The following paragraphs provide a brief overview of the parameters in the above example together with their format. Subsequent sections of this chapter go into much greater detail for these and other parameters.

The lines starting with a hash (#) are comments and are ignored by the server; they do not impact its operation in any way.

The configuration starts in the first line with the initial opening curly bracket (or brace). Each configuration must contain an object specifying the configuration of the Kea module using it. In the example above this object is called Dhcp6.

Note

In the current Kea release it is possible to specify configurations of multiple modules within a single configuration file, but this is not recommended and support for it will be removed in the future releases. The only object, besides the one specifying module configuration, which can (and usually is) included in the same file is Logging. However, we don't include this object in the example above for clarity.

The Dhcp6 configuration starts with the "Dhcp6": { line and ends with the corresponding closing brace (in the above example, the brace after the last comment). Everything defined between those lines is considered to be the Dhcp6 configuration.

In the general case, the order in which those parameters appear does not matter. There are two caveats here though. The first one is to remember that the configuration file must be well formed JSON. That means that parameters for any given scope must be separated by a comma and there must not be a comma after the last parameter. When reordering a configuration file, keep in mind that moving a parameter to or from the last position in a given scope may also require moving the comma. The second caveat is that it is uncommon — although legal JSON — to repeat the same parameter multiple times. If that happens, the last occurrence of a given parameter in a given scope is used while all previous instances are ignored. This is unlikely to cause any confusion as there are no real life reasons to keep multiple copies of the same parameter in your configuration file.

Moving onto the DHCPv6 configuration elements, the very first few elements define some global parameters. valid-lifetime defines for how long the addresses (leases) given out by the server are valid. If nothing changes, a client that got an address is allowed to use it for 4000 seconds. (Note that integer numbers are specified as is, without any quotes around them.) The address will become deprecated in 3000 seconds (clients are allowed to keep old connections, but can't use this address for creating new connections). renew-timer and rebind-timer are values that define T1 and T2 timers that govern when the client will begin the renewal and rebind procedures.

The interfaces-config map specifies the server configuration concerning the network interfaces, on which the server should listen to the DHCP messages. The interfaces parameter specifies a list of network interfaces on which the server should listen. Lists are opened and closed with square brackets, with elements separated by commas. Had we wanted to listen on two interfaces, the interfaces-config would look like this:

"interfaces-config": {
    "interfaces": [ "eth0", "eth1" ]
},

The next couple of lines define the lease database, the place where the server stores its lease information. This particular example tells the server to use memfile, which is the simplest (and fastest) database backend. It uses an in-memory database and stores leases on disk in a CSV file. This is a very simple configuration. Usually the lease database configuration is more extensive and contains additional parameters. Note that lease-database is an object and opens up a new scope, using an opening brace. Its parameters (just one in this example - type) follow. Had there been more than one, they would be separated by commas. This scope is closed with a closing brace. As more parameters for the Dhcp6 definition follow, a trailing comma is present.

Finally, we need to define a list of IPv6 subnets. This is the most important DHCPv6 configuration structure as the server uses that information to process clients' requests. It defines all subnets from which the server is expected to receive DHCP requests. The subnets are specified with the subnet6 parameter. It is a list, so it starts and ends with square brackets. Each subnet definition in the list has several attributes associated with it, so it is a structure and is opened and closed with braces. At minimum, a subnet definition has to have at least two parameters: subnet (that defines the whole subnet) and pools (which is a list of dynamically allocated pools that are governed by the DHCP server).

The example contains a single subnet. Had more than one been defined, additional elements in the subnet6 parameter would be specified and separated by commas. For example, to define two subnets, the following syntax would be used:

"subnet6": [
    {
        "pools": [ { "pool": "2001:db8:1::/112" } ],
        "subnet": "2001:db8:1::/64"
    },
    {
        "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
        "subnet": "2001:db8:2::/64"
    }
]

Note that indentation is optional and is used for aesthetic purposes only. In some cases in may be preferable to use more compact notation.

After all parameters are specified, we have two contexts open: global and Dhcp6, hence we need two closing curly brackets to close them. In a real life configuration file there most likely would be additional components defined such as Logging, so the closing brace would be followed by a comma and another object definition.

9.2.2. Lease Storage

All leases issued by the server are stored in the lease database. Currently there are four database backends available: memfile (which is the default backend), MySQL, PostgreSQL and Cassandra.

9.2.2.1. Memfile - Basic Storage for Leases

The server is able to store lease data in different repositories. Larger deployments may elect to store leases in a database. Section 9.2.2.2, “Lease Database Configuration” describes this option. In typical smaller deployments though, the server will store lease information in a CSV file rather than a database. As well as requiring less administration, an advantage of using a file for storage is that it eliminates a dependency on third-party database software.

The configuration of the file backend (Memfile) is controlled through the Dhcp6/lease-database parameters. The type parameter is mandatory and it specifies which storage for leases the server should use. The value of "memfile" indicates that the file should be used as the storage. The following list gives additional, optional, parameters that can be used to configure the Memfile backend.

  • persist: controls whether the new leases and updates to existing leases are written to the file. It is strongly recommended that the value of this parameter is set to true at all times, during the server's normal operation. Not writing leases to disk will mean that if a server is restarted (e.g. after a power failure), it will not know what addresses have been assigned. As a result, it may hand out addresses to new clients that are already in use. The value of false is mostly useful for performance testing purposes. The default value of the persist parameter is true, which enables writing lease updates to the lease file.
  • name: specifies an absolute location of the lease file in which new leases and lease updates will be recorded. The default value for this parameter is "[kea-install-dir]/var/kea/kea-leases6.csv" .
  • lfc-interval: specifies the interval in seconds, at which the server will perform a lease file cleanup (LFC). This removes redundant (historical) information from the lease file and effectively reduces the lease file size. The cleanup process is described in more detailed fashion further in this section. The default value of the lfc-interval is 3600. A value of 0 disables the LFC.

An example configuration of the Memfile backend is presented below:

"Dhcp6": {
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/tmp/kea-leases6.csv",
        "lfc-interval": 1800
    }
}

This configuration selects the /tmp/kea-leases6.csv as the storage for lease information and enables persistence (writing lease updates to this file). It also configures the backend perform the periodic cleanup of the lease files, executed every 30 minutes.

It is important to know how the lease file contents are organized to understand why the periodic lease file cleanup is needed. Every time the server updates a lease or creates a new lease for the client, the new lease information must be recorded in the lease file. For performance reasons, the server does not update the existing client's lease in the file, as it would potentially require rewriting the entire file. Instead, it simply appends the new lease information to the end of the file: the previous lease entries for the client are not removed. When the server loads leases from the lease file, e.g. at the server startup, it assumes that the latest lease entry for the client is the valid one. The previous entries are discarded. This means that the server can re-construct the accurate information about the leases even though there may be many lease entries for each client. However, storing many entries for each client results in bloated lease file and impairs the performance of the server's startup and reconfiguration as it needs to process a larger number of lease entries.

Lease file cleanup (LFC) removes all previous entries for each client and leaves only the latest ones. The interval at which the cleanup is performed is configurable, and it should be selected according to the frequency of lease renewals initiated by the clients. The more frequent the renewals, the smaller the value of lfc-interval should be. Note however, that the LFC takes time and thus it is possible (although unlikely) that new cleanup is started while the previous cleanup instance is still running, if the lfc-interval is too short. The server would recover from this by skipping the new cleanup when it detects that the previous cleanup is still in progress. But it implies that the actual cleanups will be triggered more rarely than configured. Moreover, triggering a new cleanup adds an overhead to the server which will not be able to respond to new requests for a short period of time when the new cleanup process is spawned. Therefore, it is recommended that the lfc-interval value is selected in a way that would allow for the LFC to complete the cleanup before a new cleanup is triggered.

Lease file cleanup is performed by a separate process (in background) to avoid a performance impact on the server process. In order to avoid the conflicts between two processes both using the same lease files, the LFC process operates on the copy of the original lease file, rather than on the lease file used by the server to record lease updates. There are also other files being created as a side effect of the lease file cleanup. The detailed description of the LFC is located in the Kea Administrator's Reference Manual: https://jenkins.isc.org/job/Kea_doc/guide/kea-guide.html#kea-lfc.

9.2.2.2. Lease Database Configuration

Note

Lease database access information must be configured for the DHCPv6 server, even if it has already been configured for the DHCPv4 server. The servers store their information independently, so each server can use a separate database or both servers can use the same database.

Lease database configuration is controlled through the Dhcp6/lease-database parameters. The type of the database must be set to "memfile", "mysql", "postgresql" or "cql", e.g.

"Dhcp6": { "lease-database": { "type": "mysql", ... }, ... }

Next, the name of the database is to hold the leases must be set: this is the name used when the database was created (see Section 4.3.2.1, “First Time Creation of the MySQL Database”, Section 4.3.3.1, “First Time Creation of the PostgreSQL Database” or Section 4.3.4.1, “First Time Creation of the Cassandra Database”).

"Dhcp6": { "lease-database": { "name": "database-name" , ... }, ... }

For Cassandra:

"Dhcp6": { "lease-database": { "keyspace": "database-name" , ... }, ... }

If the database is located on a different system to the DHCPv6 server, the database host name must also be specified. (It should be noted that this configuration may have a severe impact on server performance.):

"Dhcp6": { "lease-database": { "host": "remote-host-name", ... }, ... }

For Cassandra, multiple contact points can be provided:

"Dhcp6": { "lease-database": { "contact-points": "remote-host-name[, ...]" , ... }, ... }

The usual state of affairs will be to have the database on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

"Dhcp6": { "lease-database": { "host" : "", ... }, ... }

For Cassandra:

"Dhcp6": { "lease-database": { "contact-points": "", ... }, ... }

Should the database use a port different than default, it may be specified as well:

"Dhcp6": { "lease-database": { "port" : 12345, ... }, ... }

Should the database be located on a different system, you may need to specify a longer interval for the connection timeout:

"Dhcp6": { "lease-database": { "connect-timeout" : timeout-in-seconds, ... }, ... }

The default value of five seconds should be more than adequate for local connections. If a timeout is given though, it should be an integer greater than zero.

The maxiumum number of times the server will automatically attempt to reconnect to the lease database after connectivity has been lost may be specified:

"Dhcp6": { "lease-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts the server will exit. A value of zero (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and Postgres only).

The number of milliseconds the server will wait in between attempts to reconnect to the lease database after connectivity has been lost may also be specified:

"Dhcp6": { "lease-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and Postgres is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity. The default value for Cassandra is 2000 ms.

Note

Automatic reconnect to database backends is configured individually per backend. This allows you to tailor the recovery parameters to the each backend you use. We do suggest that you either enable it for all backends or no backends so you have consistent behavior. Losing connectivity to a backend for which reconnect is disabled will result in the server shutting itself down. This includes the cases when lease database backend and hosts database backend is connected to the same database instance.

Note that host parameter is used by MySQL and PostgreSQL backends. Cassandra has a concept of contact points that could be used to contact the cluster, instead of a single IP or hostname. It takes a list of comma separated IP addresses. This may be specified as:

"Dhcp6": { "lease-database": { "contact-points" : "192.0.2.1,192.0.2.2", ... }, ... }

Finally, the credentials of the account under which the server will access the database should be set:

"Dhcp6": { "lease-database": { "user": "user-name",
                               "password": "password",
                              ... },
           ... }

If there is no password to the account, set the password to the empty string "". (This is also the default.)

9.2.2.3. Cassandra specific parameters

The parameters are the same for DHCPv4 and DHCPv6. See Section 8.2.2.3, “Cassandra specific parameters” for details.

9.2.3. Hosts Storage

Kea is also able to store information about host reservations in the database. The hosts database configuration uses the same syntax as the lease database. In fact, a Kea server opens independent connections for each purpose, be it lease or hosts information. This arrangement gives the most flexibility. Kea can be used to keep leases and host reservations separately, but can also point to the same database. Currently the supported hosts database types are MySQL and PostgreSQL. The Cassandra backend does not support host reservations yet.

Please note that usage of hosts storage is optional. A user can define all host reservations in the configuration file. That is the recommended way if the number of reservations is small. However, when the number of reservations grows it's more convenient to use host storage. Please note that both storage methods (configuration file and one of the supported databases) can be used together. If hosts are defined in both places, the definitions from the configuration file are checked first and external storage is checked later, if necessary.

Version 1.4 extends the host storage to multiple storages. Operations are performed on host storages in the configuration order with a special case for addition: read-only storages must be configured after a required read-write storage, or host reservation addition will always fail.

9.2.3.1. DHCPv6 Hosts Database Configuration

Hosts database configuration is controlled through the Dhcp6/hosts-database parameters. If enabled, the type of the database must be set to "mysql" or "postgresql". Other hosts backends may be added in later version of Kea.

"Dhcp6": { "hosts-database": { "type": "mysql", ... }, ... }

Next, the name of the database to hold the reservations must be set: this is the name used when the database was created (see Section 4.3, “Supported Databases” for instructions how to setup desired database type).

"Dhcp6": { "hosts-database": { "name": "database-name" , ... }, ... }

If the database is located on a different system than the DHCPv6 server, the database host name must also be specified. (Again it should be noted that this configuration may have a severe impact on server performance):

"Dhcp6": { "hosts-database": { "host": remote-host-name, ... }, ... }

The usual state of affairs will be to have the database on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

"Dhcp6": { "hosts-database": { "host" : "", ... }, ... }

"Dhcp6": { "hosts-database": { "port" : 12345, ... }, ... }

The maxiumum number of times the server will automatically attempt to reconnect to the host database after connectivity has been lost may be specified:

"Dhcp6": { "host-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts the server will exit. A value of zero (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and Postgres only).

The number of milliseconds the server will wait in between attempts to reconnect to the host database after connectivity has been lost may also be specified:

"Dhcp6": { "hosts-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and Postgres is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity. The default value for Cassandra is 2000 ms.

Note

Automatic reconnect to database backends is configured individually per backend. This allows you to tailor the recovery parameters to the each backend you use. We do suggest that you either enable it for all backends or no backends so you have consistent behavior. Losing connectivity to a backend for which reconnect is disabled will result in the server shutting itself down. This includes the cases when lease database backend and hosts database backend is connected to the same database instance.

Finally, the credentials of the account under which the server will access the database should be set:

"Dhcp6": { "hosts-database": { "user": "user-name",
                               "password": "password",
                              ... },
           ... }

If there is no password to the account, set the password to the empty string "". (This is also the default.)

The multiple storage extension uses a similar syntax: a configuration is placed into a "hosts-databases" list instead of into a "hosts-database" entry as in:

"Dhcp6": { "hosts-databases": [ { "type": "mysql", ... }, ... ], ... }

For additional Cassandra specific parameters, see Section 8.2.2.3, “Cassandra specific parameters”.

9.2.3.2. Using Read-Only Databases for Host Reservations

In some deployments the database user whose name is specified in the database backend configuration may not have write privileges to the database. This is often required by the policy within a given network to secure the data from being unintentionally modified. In many cases administrators have inventory databases deployed, which contain substantially more information about the hosts than static reservations assigned to them. The inventory database can be used to create a view of a Kea hosts database and such view is often read only.

Kea host database backends operate with an implicit configuration to both read from and write to the database. If the database user does not have write access to the host database, the backend will fail to start and the server will refuse to start (or reconfigure). However, if access to a read only host database is required for retrieving reservations for clients and/or assign specific addresses and options, it is possible to explicitly configure Kea to start in "read-only" mode. This is controlled by the readonly boolean parameter as follows:

"Dhcp6": { "hosts-database": { "readonly": true, ... }, ... }

Setting this parameter to false would configure the database backend to operate in "read-write" mode, which is also a default configuration if the parameter is not specified.

Note

The readonly parameter is currently only supported for MySQL and PostgreSQL databases.

9.2.4. Interface Selection

The DHCPv6 server has to be configured to listen on specific network interfaces. The simplest network interface configuration instructs the server to listen on all available interfaces:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "*" ]
    }
    ...
}

The asterisk plays the role of a wildcard and means "listen on all interfaces". However, it is usually a good idea to explicitly specify interface names:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ]
    },
    ...
}
  

It is possible to use wildcard interface name (asterisk) concurrently with the actual interface names:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3", "*" ]
    },
    ...
}
  

It is anticipated that this will form of usage only be used where it is desired to temporarily override a list of interface names and listen on all interfaces.

As for the DHCPv4 server binding to specific addresses and disabling re-detection of interfaces are supported. But dhcp-socket-type is not because DHCPv6 uses UDP/IPv6 sockets only. The following example shows how to disable the interface detection:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "re-detect": false
    },
    ...
}
  

The loopback interfaces (i.e. the "lo" or "lo0" interface) are not configured by default, unles explicitely mentioned in the configration. Note Kea requires a link-local address which does not exist on all systems, or a specified unicast address as in:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "lo/::1" ]
    },
    ...
}
  

9.2.5. IPv6 Subnet Identifier

The subnet identifier is a unique number associated with a particular subnet. In principle, it is used to associate clients' leases with their respective subnets. When a subnet identifier is not specified for a subnet being configured, it will be automatically assigned by the configuration mechanism. The identifiers are assigned from 1 and are monotonically increased for each subsequent subnet: 1, 2, 3 ....

If there are multiple subnets configured with auto-generated identifiers and one of them is removed, the subnet identifiers may be renumbered. For example: if there are four subnets and the third is removed the last subnet will be assigned the identifier that the third subnet had before removal. As a result, the leases stored in the lease database for subnet 3 are now associated with subnet 4, something that may have unexpected consequences. It is planned to implement a mechanism to preserve auto-generated subnet ids in a future version of Kea. However, the only remedy for this issue at present is to manually specify a unique identifier for each subnet.

Note

Subnet IDs must be greater than zero and less than 4294967295.

The following configuration will assign the specified subnet identifier to the newly configured subnet:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "id": 1024,
            ...
        }
    ]
}

This identifier will not change for this subnet unless the "id" parameter is removed or set to 0. The value of 0 forces auto-generation of the subnet identifier.

9.2.6. Unicast Traffic Support

When the DHCPv6 server starts, by default it listens to the DHCP traffic sent to multicast address ff02::1:2 on each interface that it is configured to listen on (see Section 9.2.4, “Interface Selection”). In some cases it is useful to configure a server to handle incoming traffic sent to the global unicast addresses as well. The most common reason for this is to have relays send their traffic to the server directly. To configure the server to listen on a specific unicast address, nn interface name can be optionally followed by a slash, followed by the global unicast address on which the server should listen. The server listens to this address in addition to normal link-local binding and listening on ff02::1:2 address. The sample configuration below shows how to listen on 2001:db8::1 (a global address) configured on the eth1 interface.

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1/2001:db8::1" ]
    },
    ...
    "option-data": [
        {
            "name": "unicast",
            "data": "2001:db8::1"
        } ],
    ...
}
 

This configuration will cause the server to listen on eth1 on the link-local address, the multicast group (ff02::1:2) and 2001:db8::1.

Usually unicast support is associated with a server unicast option which allows clients to send unicast messages to the server. The example above includes a server unicast option specification which will cause the client to send messages to the specified unicast address.

It is possible to mix interface names, wildcards and interface name/addresses in the list of interfaces. It is not possible however to specify more than one unicast address on a given interface.

Care should be taken to specify proper unicast addresses. The server will attempt to bind to the addresses specified without any additional checks. This approach has selected on purpose to allow the software to communicate over uncommon addresses if so desired.

9.2.7. Subnet and Address Pool

The main role of a DHCPv6 server is address assignment. For this, the server has to be configured with at least one subnet and one pool of dynamic addresses to be managed. For example, assume that the server is connected to a network segment that uses the 2001:db8:1::/64 prefix. The Administrator of that network has decided that addresses from range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6 server. Such a configuration can be achieved in the following way:

"Dhcp6": {
    "subnet6": [
       {
           "subnet": "2001:db8:1::/64",
           "pools": [
               {
                   "pool": "2001:db8:1::1-2001:db8:1::ffff"
               }
           ],
           ...
       }
    ]
}

Note that subnet is defined as a simple string, but the pools parameter is actually a list of pools: for this reason, the pool definition is enclosed in square brackets, even though only one range of addresses is specified.

Each pool is a structure that contains the parameters that describe a single pool. Currently there is only one parameter, pool, which gives the range of addresses in the pool. Additional parameters will be added in future releases of Kea.

It is possible to define more than one pool in a subnet: continuing the previous example, further assume that 2001:db8:1:0:5::/80 should also be managed by the server. It could be written as 2001:db8:1:0:5:: to 2001:db8:1::5:ffff:ffff:ffff, but typing so many 'f's is cumbersome. It can be expressed more simply as 2001:db8:1:0:5::/80. Both formats are supported by Dhcp6 and can be mixed in the pool list. For example, one could define the following pools:

"Dhcp6": {
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" },
            { "pool": "2001:db8:1:05::/80" }
        ],
        ...
    }
    ]
}

White space in pool definitions is ignored, so spaces before and after the hyphen are optional. They can be used to improve readability.

The number of pools is not limited, but for performance reasons it is recommended to use as few as possible.

The server may be configured to serve more than one subnet. To add a second subnet, use a command similar to the following:

"Dhcp6": {
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" }
        ]
    },
    {
        "subnet": "2001:db8:2::/64",
        "pools": [
            { "pool": "2001:db8:2::/64" }
        ]
    },

        ...
    ]
}

In this example, we allow the server to dynamically assign all addresses available in the whole subnet. Although rather wasteful, it is certainly a valid configuration to dedicate the whole /64 subnet for that purpose. Note that the Kea server does not preallocate the leases, so there is no danger in using gigantic address pools.

When configuring a DHCPv6 server using prefix/length notation, please pay attention to the boundary values. When specifying that the server can use a given pool, it will also be able to allocate the first (typically network address) address from that pool. For example, for pool 2001:db8:2::/64 the 2001:db8:2:: address may be assigned as well. If you want to avoid this, use the "min-max" notation.

9.2.8. Subnet and Prefix Delegation Pools

Subnets may also be configured to delegate prefixes, as defined in RFC 8415, section 6.3. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix (prefix) and a prefix length (prefix-len), as well as a delegated prefix length (delegated-len). The delegated length must not be shorter (that is it must be numerically greater or equal) than the prefix length. If both the delegated and prefix lengths are equal, the server will be able to delegate only one prefix. The delegated prefix does not have to match the subnet prefix.

Below is a sample subnet configuration which enables prefix delegation for the subnet:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:d8b:1::/64",
            "pd-pools": [
                {
                    "prefix": "3000:1::",
                    "prefix-len": 64,
                    "delegated-len": 96
                }
            ]
        }
    ],
    ...
}

9.2.9. Prefix Exclude Option

For each delegated prefix the delegating router may choose to exclude a single prefix out of the delegated prefix as specified in the RFC 6603. The requesting router must not assign the excluded prefix to any of its downstream interfaces and it is intended to be used on a link through which the delegating router exchanges DHCPv6 messages with the requesting router. The configuration example below demonstrates how to specify an excluded prefix within a prefix pool definition. The excluded prefix "2001:db8:1:babe:cafe:80::/72" will be sent to a requesting router which includes Prefix Exclude option in the ORO, and which is delegated a prefix from this pool.

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/48",
            "pd-pools": [
                {
                    "prefix": "2001:db8:1:8000::",
                    "prefix-len": 48,
                    "delegated-len": 64,
                    "excluded-prefix": "2001:db8:1:babe:cafe:80::",
                    "excluded-prefix-len": 72
                }
            ]
        }
    ]
}

9.2.10. Standard DHCPv6 Options

One of the major features of a DHCPv6 server is to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requests them. The following example shows how to configure DNS servers, one of the most frequently used options. Options specified in this way are considered global and apply to all configured subnets.

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
           "csv-format": true,
           "data": "2001:db8::cafe, 2001:db8::babe"
        },
        ...
    ]
}

The option-data line creates a new entry in the option-data table. This table contains information on all global options that the server is supposed to configure in all subnets. The name line specifies the option name. (For a complete list of currently supported names, see Table 9.1, “List of Standard DHCPv6 Options”.) The next line specifies the option code, which must match one of the values from that list. The line beginning with space specifies the option space, which must always be set to "dhcp6" as these are standard DHCPv6 options. For other name spaces, including custom option spaces, see Section 9.2.14, “Nested DHCPv6 Options (Custom Option Spaces)”. The following line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. Finally, the data line gives the actual value to be sent to clients. Data is specified as normal text, with values separated by commas if more than one value is allowed.

Options can also be configured as hexadecimal values. If "csv-format" is set to false, the option data must be specified as a string of hexadecimal numbers. The following commands configure the DNS-SERVERS option for all subnets with the following addresses: 2001:db8:1::cafe and 2001:db8:1::babe.

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
           "csv-format": false,
           "data": "2001 0DB8 0001 0000 0000 0000 0000 CAFE
                    2001 0DB8 0001 0000 0000 0000 0000 BABE"
        },
        ...
    ]
}
        

Note

The value for the setting of the "data" element is split across two lines in this example for clarity: when entering the command, the whole string should be entered on the same line.

Care should be taken to use proper encoding when using hexadecimal format as Kea's ability to validate data correctness in hexadecimal is limited.

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 9.2.15, “Unspecified Parameters for DHCPv6 Option Configuration”. Only one of name or code is required, so you don't need to specify both. Space has a default value of "dhcp6", so you can skip this as well if you define a regular (not encapsulated) DHCPv6 option. Finally, csv-format defaults to true, so it too can be skipped, unless you want to specify the option value as hexstring. Therefore the above example can be simplified to:

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe"
        },
        ...
    ]
}
        

Defined options are added to response when the client requests them at a few exceptions which are always added. To enforce the addition of a particular option set the always-send flag to true as in:

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe",
           "always-send": true
        },
        ...
    ]
}
        

The effect is the same as if the client added the option code in the Option Request Option (or its equivalent for vendor options) so in:

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe",
           "always-send": true
        },
        ...
    ],
    "subnet6": [
        {
           "subnet": "2001:db8:1::/64",
           "option-data": [
               {
                   "name": "dns-servers",
                   "data": "2001:db8:1::cafe, 2001:db8:1::babe"
               },
               ...
           ],
           ...
        },
        ...
    ],
    ...
}
        

The DNS Servers option is always added to responses (the always-send is "sticky") but the value is the subnet one when the client is localized in the subnet.

It is possible to override options on a per-subnet basis. If clients connected to most of your subnets are expected to get the same values of a given option, you should use global options: you can then override specific values for a small number of subnets. On the other hand, if you use different values in each subnet, it does not make sense to specify global option values (Dhcp6/option-data), rather you should set only subnet-specific values (Dhcp6/subnet[X]/option-data[Y]).

The following commands override the global DNS servers option for a particular subnet, setting a single DNS server with address 2001:db8:1::3.

"Dhcp6": {
    "subnet6": [
        {
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:1::3"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

In some cases it is useful to associate some options with an address or prefix pool from which a client is assigned a lease. Pool specific option values override subnet specific and global option values. If the client is assigned multiple leases from different pools, the server will assign options from all pools from which the leases have been obtained. However, if the particular option is specified in multiple pools from which the client obtains the leases, only one instance of this option will be handed out to the client. The server's administrator must not try to prioritize assignment of pool specific options by trying to order pools declarations in the server configuration. Future Kea releases may change the order in which options are assigned from the pools without any notice.

The following configuration snippet demonstrates how to specify the DNS servers option, which will be assigned to a client only if the client obtains an address from the given pool:

"Dhcp6": {
    "subnet6": [
        {
            "pools": [
                {
                    "pool": "2001:db8:1::100-2001:db8:1::300",
                    "option-data": [
                        {
                            "name": "dns-servers",
                            "data": "2001:db8:1::10"
                        }
                    ]
                }
            ]
        },
        ...
    ],
    ...
}

Options can be specified also in class of host reservation scope. In Kea 1.4 options precedence order is (from most important): host reservation, pool, subnet, shared network, class, global. In Kea 1.5 order will be changed to: host reservation, class, pool, subnet, shared network, global OR it will be fully configurable.

The currently supported standard DHCPv6 options are listed in Table 9.1, “List of Standard DHCPv6 Options”. The "Name" and "Code" are the values that should be used as a name in the option-data structures. "Type" designates the format of the data: the meanings of the various types is given in Table 8.2, “List of standard DHCP option types”.

Experimental options (like standard options but with a code which was not assigned by IANA) are listed in Table 9.2, “List of Experimental DHCPv6 Options”.

When a data field is a string, and that string contains the comma (,; U+002C) character, the comma must be escaped with a reverse solidus character (\; U+005C). This double escape is required, because both the routine splitting CSV data into fields and JSON use the same escape character: a single escape (\,) would make the JSON invalid. For example, the string "EST5EDT4,M3.2.0/02:00,M11.1.0/02:00" would be represented as:

"Dhcp6": {
    "subnet6": [
        {
            "pools": [
                {
                    "option-data": [
                        {
                            "name": "new-posix-timezone",
                            "data": "EST5EDT4\,M3.2.0/02:00\,M11.1.0/02:00"
                        }
                    ]
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

Some options are designated as arrays, which means that more than one value is allowed in such an option. For example the option dns-servers allows the specification of more than one IPv6 address, allowing clients to obtain the addresses of multiple DNS servers.

The Section 9.2.12, “Custom DHCPv6 Options” describes the configuration syntax to create custom option definitions (formats). It is generally not allowed to create custom definitions for standard options, even if the definition being created matches the actual option format defined in the RFCs. There is an exception from this rule for standard options for which Kea does not yes provide a definition. In order to use such options, a server administrator must create a definition as described in Section 9.2.12, “Custom DHCPv6 Options” in the 'dhcp6' option space. This definition should match the option format described in the relevant RFC but the configuration mechanism would allow any option format as it has no means to validate the format at the moment.

Table 9.1. List of Standard DHCPv6 Options

NameCodeTypeArray?
preference7uint8false
unicast12ipv6-addressfalse
vendor-opts17uint32false
sip-server-dns21fqdntrue
sip-server-addr22ipv6-addresstrue
dns-servers23ipv6-addresstrue
domain-search24fqdntrue
nis-servers27ipv6-addresstrue
nisp-servers28ipv6-addresstrue
nis-domain-name29fqdntrue
nisp-domain-name30fqdntrue
sntp-servers31ipv6-addresstrue
information-refresh-time32uint32false
bcmcs-server-dns33fqdntrue
bcmcs-server-addr34ipv6-addresstrue
geoconf-civic36record (uint8, uint16, hex)false
remote-id37record (uint32, hex)false
subscriber-id38hexfalse
client-fqdn39record (uint8, fqdn)false
pana-agent40ipv6-addresstrue
new-posix-timezone41stringfalse
new-tzdb-timezone42stringfalse
ero43uint16true
lq-query (1)44record (uint8, ipv6-address)false
client-data (1)45emptyfalse
clt-time (1)46uint32false
lq-relay-data (1)47record (ipv6-address, hex)false
lq-client-link (1)48ipv6-addresstrue
v6-lost51fqdnfalse
capwap-ac-v652ipv6-addresstrue
relay-id53hexfalse
v6-access-domain57fqdnfalse
sip-ua-cs-list58fqdntrue
bootfile-url59stringfalse
bootfile-param60tupletrue
client-arch-type61uint16true
nii62record (uint8, uint8, uint8)false
aftr-name64fqdnfalse
erp-local-domain-name65fqdnfalse
rsoo66emptyfalse
pd-exclude67hexfalse
rdnss-selection74record (ipv6-address, uint8, fqdn)true
client-linklayer-addr79hexfalse
link-address80ipv6-addressfalse
solmax-rt82uint32false
inf-max-rt83uint32false
dhcp4o6-server-addr88ipv6-addresstrue
s46-rule89record (uint8, uint8, uint8, ipv4-address, ipv6-prefix)false
s46-br90ipv6-addressfalse
s46-dmr91ipv6-prefixfalse
s46-v4v6bind92record (ipv4-address, ipv6-prefix)false
s46-portparams93record(uint8, psid)false
s46-cont-mape94emptyfalse
s46-cont-mapt95emptyfalse
s46-cont-lw96emptyfalse
v6-captive-portal103stringfalse
ipv6-address-andsf143ipv6-addresstrue


Options marked with (1) have option definitions, but the logic behind them is not implemented. That means that technically Kea knows how to parse them in incoming message or how to send them if configured to do so, but not what to do with them. Since the related RFCs require certain processing, the support for those options is non-functional. However, it may be useful in some limited lab testing, hence the definition formats are listed here.

Table 9.2. List of Experimental DHCPv6 Options

NameCodeTypeArray?
public-key701hexfalse
certificate702hexfalse
signature703record (uint8, uint8, hex)false
timestamp704hexfalse


9.2.11. Common Softwire46 Options

Softwire46 options are involved in IPv4 over IPv6 provisioning by means of tunneling or translation as specified in the RFC 7598. The following sections provide configuration examples of these options.

9.2.11.1. Softwire46 Container Options

S46 container options group rules and optional port parameters for a specified domain. There are three container options specified in the "dhcp6" (top level) option space: MAP-E Container option, MAP-T Container option and S46 Lightweight 4over6 Container option. These options only contain encapsulated options specified below. They do not include any data fields.

In order to configure the server to send specific container option along with all encapsulated options, the container option must be included in the server configuration as shown below:

"Dhcp6": {
    ...
    "option-data": [
        {
            "name": "s46-cont-mape"
        } ],
    ...
}

This configuration will cause the server to include MAP-E Container option to the client. Use "s46-cont-mapt" or "s46-cont-lw" for the MAP-T Container and S46 Lightweight 4over6 Container options respectively.

All remaining softwire options described below are included in one of the container options. Thus, they have to be included in appropriate option spaces by selecting a "space" name, which specifies in which option they are supposed to be included.

9.2.11.2. S46 Rule Option

The S46 Rule option is used for conveying the Basic Mapping Rule (BMR) and Forwarding Mapping Rule (FMR).

{
    "space": "s46-cont-mape-options",
    "name": "s46-rule",
    "data": "128, 0, 24, 192.0.2.0, 2001:db8:1::/64"
}

Other possible "space" value is "s46-cont-mapt-options".

The S46 Rule option conveys a number of parameters:

  • flags, an unsigned 8 bits integer, with currently only the most significant bit specified. It denotes whether the rule can be used for forwarding (128) or not (0).
  • ea-len, an 8 bits long Embedded Address length. Allowed values range from 0 to 48.
  • IPv4 prefix length, 8 bits long; expresses the prefix length of the Rule IPv4 prefix specified in the ipv4-prefix field. Allowed values range from 0 to 32.
  • IPv4 prefix, a fixed-length 32-bit field that specifies the IPv4 prefix for the S46 rule. The bits in the prefix after prefix4-len number of bits are reserved and MUST be initialized to zero by the sender and ignored by the receiver.
  • IPv6 prefix in prefix/length notation that specifies the IPv6 domain prefix for the S46 rule. The field is padded on the right with zero bits up to the nearest octet boundary when prefix6-len is not evenly divisible by 8.

9.2.11.3. S46 BR Option

The S46 BR option is used to convey the IPv6 address of the Border Relay. This option is mandatory in the MAP-E Container option and not permitted in the MAP-T and S46 Lightweight 4over6 Container options.

{
    "space": "s46-cont-mape-options",
    "name": "s46-br",
    "data": "2001:db8:cafe::1",
}

Other possible "space" value is "s46-cont-lw-options".

9.2.11.4. S46 DMR Option

The S46 DMR option is used to convey values for the Default Mapping Rule (DMR). This option is mandatory in the MAP-T container option and not permitted in the MAP-E and S46 Lightweight 4over6 Container options.

{
    "space": "s46-cont-mapt-options",
    "name": "s46-dmr",
    "data": "2001:db8:cafe::/64",
}

This option must not be included in other containers.

9.2.11.5. S46 IPv4/IPv6 Address Binding option.

The S46 IPv4/IPv6 Address Binding option may be used to specify the full or shared IPv4 address of the Customer Edge (CE). The IPv6 prefix field is used by the CE to identify the correct prefix to use for the tunnel source.

{
    "space": "s46-cont-lw",
    "name": "s46-v4v6bind",
    "data": "192.0.2.3, 2001:db8:1:cafe::/64"
}

This option must not be included in other containers.

9.2.11.6. S46 Port Parameters

The S46 Port Parameters option specifies optional port set information that MAY be provided to CEs

{
    "space": "s46-rule-options",
    "name": "s46-portparams",
    "data": "2, 3/4",
}

Other possible "space" value is "s46-v4v6bind" to include this option in the S46 IPv4/IPv6 Address Binding option.

Note that the second value in the example above specifies the PSID and PSID length fields in the format of PSID/PSID length. This is equivalent to the values of PSID-len=4 and PSID=12288 conveyed in the S46 Port Parameters option.

9.2.12. Custom DHCPv6 Options

It is possible to define options in addition to the standard ones. Assume that we want to define a new DHCPv6 option called "foo" which will have code 100 and which will convey a single unsigned 32 bit integer value. We can define such an option by using the following commands:

"Dhcp6": {
    "option-def": [
        {
            "name": "foo",
            "code": 100,
            "type": "uint32",
            "array": false,
            "record-types": "",
            "space": "dhcp6",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The "false" value of the array parameter determines that the option does NOT comprise an array of "uint32" values but rather a single value. Two other parameters have been left blank: record-types and encapsulate. The former specifies the comma separated list of option data fields if the option comprises a record of data fields. The record-types value should be non-empty if the type is set to "record". Otherwise it must be left blank. The latter parameter specifies the name of the option space being encapsulated by the particular option. If the particular option does not encapsulate any option space it should be left blank. Note that the above example only defines the format of the new option, it does not set its value(s).

The name, code and type parameters are required, all others are optional. The array default value is false. The record-types and encapsulate default values are blank (i.e. ""). The default space is "dhcp6".

Once the new option format is defined, its value is set in the same way as for a standard option. For example the following commands set a global value that applies to all subnets.

"Dhcp6": {
    "option-data": [
        {
            "name": "foo",
            "code": 100,
            "space": "dhcp6",
            "csv-format": true,
            "data": "12345"
        }, ...
    ],
    ...
}

New options can take more complex forms than simple use of primitives (uint8, string, ipv6-address etc): it is possible to define an option comprising a number of existing primitives.

For example, assume we want to define a new option that will consist of an IPv6 address, followed by an unsigned 16 bit integer, followed by a boolean value, followed by a text string. Such an option could be defined in the following way:

"Dhcp6": {
    "option-def": [
        {
            "name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
            "array": false,
            "record-types": "ipv6-address, uint16, boolean, string",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The "type" is set to "record" to indicate that the option contains multiple values of different types. These types are given as a comma-separated list in the "record-types" field and should be those listed in Table 8.2, “List of standard DHCP option types”.

The values of the option are set as follows:

"Dhcp6": {
    "option-data": [
        {
            "name": "bar",
            "space": "dhcp6",
            "code": 101,
            "csv-format": true,
            "data": "2001:db8:1::10, 123, false, Hello World"
        }
    ],
    ...
}

csv-format is set true to indicate that the data field comprises a command-separated list of values. The values in the "data" must correspond to the types set in the "record-types" field of the option definition.

When array is set to true and type is set to "record", the last field is an array, i.e., it can contain more than one value as in:

"Dhcp6": {
    "option-def": [
        {
            "name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
            "array": true,
            "record-types": "ipv6-address, uint16",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The new option content is one IPv6 address followed by one or more 16 bit unsigned integers.

Note

In the general case, boolean values are specified as true or false, without quotes. Some specific boolean parameters may accept also "true", "false", 0, 1, "0" and "1". Future versions of Kea will accept all those values for all boolean parameters.

9.2.13. DHCPv6 Vendor-Specific Options

Currently there are two option spaces defined for the DHCPv6 daemon: "dhcp6" (for top level DHCPv6 options) and "vendor-opts-space", which is empty by default, but in which options can be defined. Those options will be carried in the Vendor-Specific Information option (code 17). The following examples show how to define an option "foo" with code 1 that consists of an IPv6 address, an unsigned 16 bit integer and a string. The "foo" option is conveyed in a Vendor-Specific Information option. This option comprises a single uint32 value that is set to "12345". The sub-option "foo" follows the data field holding this value.

"Dhcp6": {
    "option-def": [
        {
            "name": "foo",
            "code": 1,
            "space": "vendor-opts-space",
            "type": "record",
            "array": false,
            "record-types": "ipv6-address, uint16, string",
            "encapsulate": ""
        }
    ],
    ...
}

(Note that the option space is set to vendor-opts-space.) Once the option format is defined, the next step is to define actual values for that option:

"Dhcp6": {
    "option-data": [
        {
            "name": "foo",
            "space": "vendor-opts-space",
            "data": "2001:db8:1::10, 123, Hello World"
        },
        ...
    ],
    ...
}

We should also define a value (enterprise-number) for the Vendor-specific Information option, that conveys our option "foo".

"Dhcp6": {
    "option-data": [
        ...,
        {
            "name": "vendor-opts",
            "data": "12345"
        }
    ],
    ...
}

Alternatively, the option can be specified using its code.

"Dhcp6": {
    "option-data": [
        ...,
        {
            "code": 17,
            "data": "12345"
        }
    ],
    ...
}

9.2.14. Nested DHCPv6 Options (Custom Option Spaces)

It is sometimes useful to define completely new option spaces. This is useful if the user wants their new option to convey sub-options that use a separate numbering scheme, for example sub-options with codes 1 and 2. Those option codes conflict with standard DHCPv6 options, so a separate option space must be defined.

Note that it is not required to create a new option space when defining sub-options for a standard option because it is created by default if the standard option is meant to convey any sub-options (see Section 9.2.13, “DHCPv6 Vendor-Specific Options”).

Assume that we want to have a DHCPv6 option called "container" with code 102 that conveys two sub-options with codes 1 and 2. First we need to define the new sub-options:

"Dhcp6": {
    "option-def": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv6-address",
            "record-types": "",
            "array": false,
            "encapsulate": ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
            "array": false
            "encapsulate": ""
        }
    ],
    ...
}

Note that we have defined the options to belong to a new option space (in this case, "isc").

The next step is to define a regular DHCPv6 option and specify that it should include options from the isc option space:

"Dhcp6": {
    "option-def": [
        ...,
        {
            "name": "container",
            "code": 102,
            "space": "dhcp6",
            "type": "empty",
            "array": false,
            "record-types": "",
            "encapsulate": "isc"
        }
    ],
    ...
}

The name of the option space in which the sub-options are defined is set in the encapsulate field. The type field is set to empty which limits this option to only carrying data in sub-options.

Finally, we can set values for the new options:

"Dhcp6": {
    "option-data": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "data": "2001:db8::abcd"
        },
        }
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "data": "Hello world"
        },
        {
            "name": "container",
            "code": 102,
            "space": "dhcp6"
        }
    ],
    ...
}

Note that it is possible to create an option which carries some data in addition to the sub-options defined in the encapsulated option space. For example, if the "container" option from the previous example was required to carry an uint16 value as well as the sub-options, the "type" value would have to be set to "uint16" in the option definition. (Such an option would then have the following data structure: DHCP header, uint16 value, sub-options.) The value specified with the "data" parameter — which should be a valid integer enclosed in quotes, e.g. "123" — would then be assigned to the uint16 field in the "container" option.

9.2.15. Unspecified Parameters for DHCPv6 Option Configuration

In many cases it is not required to specify all parameters for an option configuration and the default values can be used. However, it is important to understand the implications of not specifying some of them as it may result in configuration errors. The list below explains the behavior of the server when a particular parameter is not explicitly specified:

  • name - the server requires an option name or option code to identify an option. If this parameter is unspecified, the option code must be specified.
  • code - the server requires an option name or option code to identify an option. This parameter may be left unspecified if the name parameter is specified. However, this also requires that the particular option has its definition (it is either a standard option or an administrator created a definition for the option using an 'option-def' structure), as the option definition associates an option with a particular name. It is possible to configure an option for which there is no definition (unspecified option format). Configuration of such options requires the use of option code.
  • space - if the option space is unspecified it will default to 'dhcp6' which is an option space holding DHCPv6 standard options.
  • data - if the option data is unspecified it defaults to an empty value. The empty value is mostly used for the options which have no payload (boolean options), but it is legal to specify empty values for some options which carry variable length data and which spec allows for the length of 0. For such options, the data parameter may be omitted in the configuration.
  • csv-format - if this value is not specified the server will assume that the option data is specified as a list of comma separated values to be assigned to individual fields of the DHCP option. This behavior has changed in Kea 1.2. Older versions used additional logic to determine whether the csv-format should be true or false. That is no longer the case.

9.2.16. IPv6 Subnet Selection

The DHCPv6 server may receive requests from local (connected to the same subnet as the server) and remote (connecting via relays) clients. As the server may have many subnet configurations defined, it must select an appropriate subnet for a given request.

The server can not assume which of the configured subnets are local. In IPv4 it is possible as there is a reasonable expectation that the server will have a (global) IPv4 address configured on the interface, and can use that information to detect whether a subnet is local or not. That assumption is not true in IPv6: the DHCPv6 server must be able to operate while only using link-local addresses. Therefore an optional interface parameter is available within a subnet definition to designate that a given subnet is local, i.e. reachable directly over the specified interface. For example the server that is intended to serve a local subnet over eth0 may be configured as follows:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            "interface": "eth0"
        }
    ],
    ...
}

9.2.17. Rapid Commit

The Rapid Commit option, described in RFC 8415, is supported by the Kea DHCPv6 server. However, support is disabled by default for all subnets. It can be enabled for a particular subnet using the rapid-commit parameter as shown below:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "rapid-commit": true,
            "pools": [
                 {
                     "pool": "2001:db8:beef::1-2001:db8:beef::10"
                 }
             ],
        }
    ],
    ...
}

This setting only affects the subnet for which the rapid-commit is set to true. For clients connected to other subnets, the server will ignore the Rapid Commit option sent by the client and will follow the 4-way exchange procedure, i.e. respond with an Advertise for a Solicit containing a Rapid Commit option.

9.2.18. DHCPv6 Relays

A DHCPv6 server with multiple subnets defined must select the appropriate subnet when it receives a request from a client. For clients connected via relays, two mechanisms are used:

The first uses the linkaddr field in the RELAY_FORW message. The name of this field is somewhat misleading in that it does not contain a link-layer address: instead, it holds an address (typically a global address) that is used to identify a link. The DHCPv6 server checks if the address belongs to a defined subnet and, if it does, that subnet is selected for the client's request.

The second mechanism is based on interface-id options. While forwarding a client's message, relays may insert an interface-id option into the message that identifies the interface on the relay that received the message. (Some relays allow configuration of that parameter, but it is sometimes hardcoded and may range from the very simple (e.g. "vlan100") to the very cryptic: one example seen on real hardware was "ISAM144|299|ipv6|nt:vp:1:110"). The server can use this information to select the appropriate subnet. The information is also returned to the relay which then knows the interface to use to transmit the response to the client. In order for this to work successfully, the relay interface IDs must be unique within the network and the server configuration must match those values.

When configuring the DHCPv6 server, it should be noted that two similarly-named parameters can be configured for a subnet:

  • interface defines which local network interface can be used to access a given subnet.
  • interface-id specifies the content of the interface-id option used by relays to identify the interface on the relay to which the response packet is sent.

The two are mutually exclusive: a subnet cannot be both reachable locally (direct traffic) and via relays (remote traffic). Specifying both is a configuration error and the DHCPv6 server will refuse such a configuration.

The following example configuration shows how to specify an interface-id with a value of "vlan123".

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            "interface-id": "vlan123"
        }
    ],
    ...
}

9.2.19. Relay-Supplied Options

RFC 6422 defines a mechanism called Relay-Supplied DHCP Options. In certain cases relay agents are the only entities that may have specific information. They can insert options when relaying messages from the client to the server. The server will then do certain checks and copy those options to the response that will be sent to the client.

There are certain conditions that must be met for the option to be included. First, the server must not provide the option itself. In other words, if both relay and server provide an option, the server always takes precedence. Second, the option must be RSOO-enabled. IANA maintains a list of RSOO-enabled options here. However, there may be cases when system administrators want to echo other options. Kea can be instructed to treat other options as RSOO-enabled. For example, to mark options 110, 120 and 130 as RSOO-enabled, the following syntax should be used:

"Dhcp6": {
    "relay-supplied-options": [ "110", "120", "130" ],
    ...
}

As of March 2015, only option 65 is RSOO-enabled by IANA. This option will always be treated as such and there's no need to explicitly mark it. Also, when enabling standard options, it is possible to use their names, rather than option code, e.g. (e.g. use dns-servers instead of 23). See Table 9.1, “List of Standard DHCPv6 Options” for the names. In certain cases it could also work for custom options, but due to the nature of the parser code this may be unreliable and should be avoided.

9.2.20. Client Classification in DHCPv6

The DHCPv6 server includes support for client classification. For a deeper discussion of the classification process see Chapter 14, Client Classification.

In certain cases it is useful to configure the server to differentiate between DHCP clients types and treat them accordingly. It is envisaged that client classification will be used for modifying the behavior of almost any part of the DHCP message processing. In the current release of Kea, there are three mechanisms that take advantage of the client classification in DHCPv6: subnet selection, address pool selection and DHCP options assignment.

In certain cases it is useful to differentiate between different types of clients and treat them accordingly. It is envisaged that client classification will be used for changing the behavior of almost any part of the DHCP message processing. In the current release of the software however, there are only some mechanisms that take advantage of client classification: subnet selection, pool selection, and assignment of different options.

Kea can be instructed to limit access to given subnets based on class information. This is particularly useful for cases where two types of devices share the same link and are expected to be served from two different subnets. The primary use case for such a scenario is cable networks. Here, there are two classes of devices: the cable modem itself, which should be handed a lease from subnet A and all other devices behind the modem that should get a lease from subnet B. That segregation is essential to prevent overly curious users from playing with their cable modems. For details on how to set up class restrictions on subnets, see Section 14.6, “Configuring Subnets With Class Information”.

When subnets belong to a shared network the classification applies to subnet selection but not to pools, e.g., a pool in a subnet limited to a particular class can still be used by clients which do not belong to the class if the pool they are expected to use is exhausted. So the limit access based on class information is also available at the address/prefix pool level, see Section 14.7, “Configuring Pools With Class Information” within a subnet. This is useful when to segregate clients belonging to the same subnet into different address ranges.

In a similar way a pool can be constrained to serve only known clients, i.e. clients which have a reservation, using the built-in "KNOWN" or "UNKNOWN" classes. One can assign addresses to registered clients without giving a different address per reservations, for instance when there is not enough available addresses. The determination whether there is a reservation for a given client is made after a subnet is selected. As such, it is not possible to use KNOWN/UNKNOWN classes to select a shared network or a subnet.

The process of doing classification is conducted in five steps. The first step is to assess an incoming packet and assign it to zero or more classes. The second step is to choose a subnet, possibly based on the class information. The next step is to evaluate class expressions depending on the built-in "KNOWN"/"UNKNOWN" classes after host reservation lookup, using them for pool/pd-pool selection and to assign classes from host reservations. After the list of required classes is built and each class of the list has its expression evaluated: when it returns true the packet is added as a member of the class. The last step is to assign options again possibly based on the class information. More complete and detailed description is available in Chapter 14, Client Classification.

There are two main methods of doing classification. The first is automatic and relies on examining the values in the vendor class options or existence of a host reservation. Information from these options is extracted and a class name is constructed from it and added to the class list for the packet. The second allows for specifying an expression that is evaluated for each packet. If the result is true the packet is a member of the class.

Note

Care should be taken with client classification as it is easy for clients that do not meet class criteria to be denied any service altogether.

9.2.20.1. Defining and Using Custom Classes

The following example shows how to configure a class using an expression and a subnet making use of that class. This configuration defines the class named "Client_enterprise". It is comprised of all clients whose client identifiers start with the given hex string (which would indicate a DUID based on an enterprise id of 0xAABBCCDD). They will be given an address from 2001:db8:1::0 to 2001:db8:1::FFFF and the addresses of their DNS servers set to 2001:db8:0::1 and 2001:db8:2::1.

"Dhcp6": {
    "client-classes": [
        {
            "name": "Client_enterprise",
            "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD'",
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:0::1, 2001:db8:2::1"
                }
            ]
        },
        ...
    ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
            "client-class": "Client_enterprise"
        }
    ],
    ...
}

This example shows a configuration using an automatically generated "VENDOR_CLASS_" class. The Administrator of the network has decided that addresses from range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6 server and only clients belonging to the eRouter1.0 client class are allowed to use that pool.

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::-2001:db8:1::ffff"
                 }
             ],
            "client-class": "VENDOR_CLASS_eRouter1.0"
        }
    ],
    ...
}

9.2.20.2. Required classification

In some cases it is useful to limit the scope of a class to a shared-network, subnet or pool. There are two parameters which are used to limit the scope of the class by instructing the server to perform evaluation of test expressions when required.

The first one is the per-class only-if-required flag which is false by default. When it is set to true the test expression of the class is not evaluated at the reception of the incoming packet but later and only if the class evaluation is required.

The second is the require-client-classes which takes a list of class names and is valid in shared-network, subnet and pool scope. Classes in these lists are marked as required and evaluated after selection of this specific shared-network/subnet/pool and before output option processing.

In this example, a class is assigned to the incoming packet when the specified subnet is used.

"Dhcp6": {
    "client-classes": [
       {
           "name": "Client_foo",
           "test": "member('ALL')",
           "only-if-required": true
       },
       ...
    ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64"
            "pools": [
                 {
                     "pool": "2001:db8:1::-2001:db8:1::ffff"
                 }
             ],
            "require-client-classes": [ "Client_foo" ],
            ...
        },
        ...
    ],
    ...
}

Required evaluation can be used to express complex dependencies, for example, subnet membership. It can also be used to reverse the precedence: if you set an option-data in a subnet it takes precedence over an option-data in a class. When you move the option-data to a required class and require it in the subnet, a class evaluated earlier may take precedence.

Required evaluation is also available at shared-network and pool/pd-pool levels. The order in which required classes are considered is: shared-network, subnet and (pd-)pool, i.e. the opposite order option-data are processed.

9.2.21. DDNS for DHCPv6

As mentioned earlier, kea-dhcp6 can be configured to generate requests to the DHCP-DDNS server (referred to here as "D2") to update DNS entries. These requests are known as NameChangeRequests or NCRs. Each NCR contains the following information:

  1. Whether it is a request to add (update) or remove DNS entries

  2. Whether the change requests forward DNS updates (AAAA records), reverse DNS updates (PTR records), or both.

  3. The FQDN, lease address, and DHCID

The parameters controlling the generation of NCRs for submission to D2 are contained in the dhcp-ddns section of the kea-dhcp6 configuration. The mandatory parameters for the DHCP DDNS configuration are enable-updates which is unconditionally required, and qualifying-suffix which has no default value and is required when enable-updates is set to true. The two (disabled and enabled) minimal DHCP DDNS configurations are:

"Dhcp6": {
    "dhcp-ddns": {
        "enable-updates": false
    },
    ...
}

and for example:

"Dhcp6": {
    "dhcp-ddns": {
        "enable-updates": true,
        "qualifying-suffix": "example."
    },
    ...
}

The default values for the "dhcp-ddns" section are as follows:

  • "server-ip": "127.0.0.1"
  • "server-port": 53001
  • "sender-ip": ""
  • "sender-port": 0
  • "max-queue-size": 1024
  • "ncr-protocol": "UDP"
  • "ncr-format": "JSON"
  • "override-no-update": false
  • "override-client-update": false
  • "replace-client-name": "never"
  • "generated-prefix": "myhost"
  • "hostname-char-set": ""
  • "hostname-char-replacement": ""

9.2.21.1. DHCP-DDNS Server Connectivity

In order for NCRs to reach the D2 server, kea-dhcp6 must be able to communicate with it. kea-dhcp6 uses the following configuration parameters to control this communication:

  • enable-updates - determines whether or not kea-dhcp6 will generate NCRs. If missing, this value is assumed to be false hence DDNS updates are disabled. To enable DDNS updates set this value to true:
  • server-ip - IP address on which D2 listens for requests. The default is the local loopback interface at address 127.0.0.1. You may specify either an IPv4 or IPv6 address.
  • server-port - port on which D2 listens for requests. The default value is 53001.
  • sender-ip - IP address which kea-dhcp6 should use to send requests to D2. The default value is blank which instructs kea-dhcp6 to select a suitable address.
  • sender-port - port which kea-dhcp6 should use to send requests to D2. The default value of 0 instructs kea-dhcp6 to select a suitable port.
  • max-queue-size - maximum number of requests allowed to queue waiting to be sent to D2. This value guards against requests accumulating uncontrollably if they are being generated faster than they can be delivered. If the number of requests queued for transmission reaches this value, DDNS updating will be turned off until the queue backlog has been sufficiently reduced. The intent is to allow kea-dhcp6 to continue lease operations. The default value is 1024.
  • ncr-protocol - socket protocol use when sending requests to D2. Currently only UDP is supported. TCP may be available in an upcoming release.
  • ncr-format - packet format to use when sending requests to D2. Currently only JSON format is supported. Other formats may be available in future releases.

By default, kea-dhcp-ddns is assumed to running on the same machine as kea-dhcp6, and all of the default values mentioned above should be sufficient. If, however, D2 has been configured to listen on a different address or port, these values must altered accordingly. For example, if D2 has been configured to listen on 2001:db8::5 port 900, the following configuration would be required:

"Dhcp6": {
    "dhcp-ddns": {
        "server-ip": "2001:db8::5",
        "server-port": 900,
        ...
    },
    ...
}

9.2.21.2. When Does kea-dhcp6 Generate a DDNS Request?

kea-dhcp6 follows the behavior prescribed for DHCP servers in RFC 4704. It is important to keep in mind that kea-dhcp6 provides the initial decision making of when and what to update and forwards that information to D2 in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are within the purview of D2 itself (Chapter 12, The DHCP-DDNS Server). This section describes when kea-dhcp6 will generate NCRs and the configuration parameters that can be used to influence this decision. It assumes that the enable-updates parameter is true.

Note

Currently the interface between kea-dhcp6 and D2 only supports requests which update DNS entries for a single IP address. If a lease grants more than one address, kea-dhcp6 will create the DDNS update request for only the first of these addresses. Support for multiple address mappings may be provided in a future release.

In general, kea-dhcp6 will generate DDNS update requests when:

  1. A new lease is granted in response to a REQUEST

  2. An existing lease is renewed but the FQDN associated with it has changed.

  3. An existing lease is released in response to a RELEASE

In the second case, lease renewal, two DDNS requests will be issued: one request to remove entries for the previous FQDN and a second request to add entries for the new FQDN. In the last case, a lease release, a single DDNS request to remove its entries will be made.

The decision making involved when granting a new lease the first case) is more involved. When a new lease is granted, kea-dhcp6 will generate a DDNS update request only if the REQUEST contains the FQDN option (code 39). By default kea-dhcp6 will respect the FQDN N and S flags specified by the client as shown in the following table:

Table 9.3. Default FQDN Flag Behavior

Client Flags:N-SClient IntentServer ResponseServer Flags:N-S-O
0-0 Client wants to do forward updates, server should do reverse updates Server generates reverse-only request1-0-0
0-1Server should do both forward and reverse updatesServer generates request to update both directions0-1-0
1-0Client wants no updates doneServer does not generate a request1-0-0

The first row in the table above represents "client delegation". Here the DHCP client states that it intends to do the forward DNS updates and the server should do the reverse updates. By default, kea-dhcp6 will honor the client's wishes and generate a DDNS request to D2 to update only reverse DNS data. The parameter, override-client-update, can be used to instruct the server to override client delegation requests. When this parameter is true, kea-dhcp6 will disregard requests for client delegation and generate a DDNS request to update both forward and reverse DNS data. In this case, the N-S-O flags in the server's response to the client will be 0-1-1 respectively.

(Note that the flag combination N=1, S=1 is prohibited according to RFC 4702. If such a combination is received from the client, the packet will be dropped by kea-dhcp6.)

To override client delegation, set the following values in the configuration:

"Dhcp6": {
    "dhcp-ddns": {
        "override-client-update": true,
        ...
    },
    ...
}

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter, override-no-update, can be used to instruct the server to disregard the client's wishes. When this parameter is true, kea-dhcp6 will generate DDNS update requests to kea-dhcp-ddns even if the client requests no updates be done. The N-S-O flags in the server's response to the client will be 0-1-1.

To override client delegation, issue the following commands:

"Dhcp6": {
    "dhcp-ddns": {
        "override-no-update": true,
        ...
    },
    ...
}

9.2.21.3. kea-dhcp6 Name Generation for DDNS Update Requests

Each NameChangeRequest must of course include the fully qualified domain name whose DNS entries are to be affected. kea-dhcp6 can be configured to supply a portion or all of that name based upon what it receives from the client.

The default rules for constructing the FQDN that will be used for DNS entries are:

  1. If the DHCPREQUEST contains the client FQDN option, the candidate name is taken from there.

  2. If the candidate name is a partial (i.e. unqualified) name then add a configurable suffix to the name and use the result as the FQDN.

  3. If the candidate name provided is empty, generate an FQDN using a configurable prefix and suffix.

  4. If the client provided neither option, then no DNS action will be taken.

These rules can amended by setting the replace-client-name parameter which provides the following modes of behavior:

  • never - Use the name the client sent. If the client sent no name, do not generate one. This is the default mode.

  • always - Replace the name the client sent. If the client sent no name, generate one for the client.

  • when-present - Replace the name the client sent. If the client sent no name, do not generate one.

  • when-not-present - Use the name the client sent. If the client sent no name, generate one for the client.

Note

Note that formerly, this parameter was a boolean and permitted only values of true and false. Boolean values have been deprecated and are no longer accepted. If you are currently using booleans, you must replace them with the desired mode name. A value of true maps to "when-present", while false maps to "never".

For example, To instruct kea-dhcp6 to always generate the FQDN for a client, set the parameter replace-client-name to always as follows:

"Dhcp6": {
    "dhcp-ddns": {
        "replace-client-name": "always",
        ...
    },
    ...
}

The prefix used in the generation of an FQDN is specified by the generated-prefix parameter. The default value is "myhost". To alter its value, simply set it to the desired string:

"Dhcp6": {
    "dhcp-ddns": {
        "generated-prefix": "another.host",
        ...
    },
    ...
}

The suffix used when generating an FQDN or when qualifying a partial name is specified by the qualifying-suffix parameter. This parameter has no default value, thus it is mandatory when DDNS updates are enabled. To set its value simply set it to the desired string:

"Dhcp6": {
    "dhcp-ddns": {
        "qualifying-suffix": "foo.example.org",
        ...
    },
    ...
}

When qualifying a partial name, kea-dhcp6 will construct a name with the format:

[candidate-name].[qualifying-suffix].

where candidate-name is the partial name supplied in the REQUEST. For example, if FQDN domain name value was "some-computer" and qualifying-suffix "example.com", the generated FQDN would be:

some-computer.example.com.

When generating the entire name, kea-dhcp6 will construct name of the format:

[generated-prefix]-[address-text].[qualifying-suffix].

where address-text is simply the lease IP address converted to a hyphenated string. For example, if lease address is 3001:1::70E, the qualifying suffix "example.com", and the default value is used for generated-prefix, the generated FQDN would be:

myhost-3001-1--70E.example.com.

9.2.21.4. Sanitizing Client FQDN Names

It may be that some of your DHCP clients provide values in the name component of the FQDN option (Option code 39), that contain undesirable characters. It is possible to configure kea-dhcp5 to sanitize these values. The most typical use case would be ensuring that only characters that are permitted by RFC 1035 be included: A-Z,a-z,0-9, and '-'. This may be accomplished with following two parameters:
  • hostname-char-set - a regular expression describing the invalid character set. This can be any valid, regular expression using POSIX extended expression syntax. For example, "[^A-Za-z0-9-]" would replace any character other then the letters A through z, digits 0 through 9, and '-'. An empty string, the default value, disables sanitization.
  • hostname-char-replacement - a string of zero or more characters with which to replace each invalid character in the client value. The default value is an empty string and will cause invalid characters to be OMITTED rather than replaced.
The following configuration, will replace anything other than a letter, digit, hyphen, or dot with the letter 'x':
"Dhcp4": {
    "dhcp-ddns": {
        "hostname-char-set": "[^A-Za-z0-9.-]",
        "hostname-char-replacement": "x",
        ...
    },
    ...
}
Thus, a client supplied value of "myhost-$[123.org" would become "myhost-xx123.org". Sanitizing is performed only on the portion of the name supplied by the client and it is performed before applying a qualifying suffix (if one is defined and needed).

Note

The following are some considerations to keep in mind:

Name sanitizing is meant to catch the more common cases of invalid characters through a relatively simple character replacement scheme. It is difficult to devise a scheme that works well in all cases and should you find you have clients that are using odd, corner cases of character combinations that cannot be readily handled with this mechanism, you should consider writing a hook that can carry out sufficiently complex logic to address your needs.

You do not account for dots ins your hostname-char-set expression. When scrubbing FQDNs, dots are treated as delimiters and used to separate the option value into individual domain labels that are scrubbed and then re-assembled.

If your clients are sending values that differ only by characters considered as invalid by your hostname-char-set, be aware that scrubbing them will yield identical values. In such cases, DDNS conflict rules will permit only one of them from registering the name.

Finally, given the latitude clients have in the values they send, it is virtually impossible to guarantee that a combination of these two parameters will always yield a name that is valid for use in DNS. For example, using an empty value for hostname-char-replacment could yield an empty domain label within a name, if that label consists only of invalid characters.

9.2.22. DHCPv4-over-DHCPv6: DHCPv6 Side

The support of DHCPv4-over-DHCPv6 transport is described in RFC 7341 and is implemented using cooperating DHCPv4 and DHCPv6 servers. This section is about the configuration of the DHCPv6 side (the DHCPv4 side is described in Section 8.2.21, “DHCPv4-over-DHCPv6: DHCPv4 Side”).

Note

DHCPv4-over-DHCPv6 support is experimental and the details of the inter-process communication can change: both the DHCPv4 and DHCPv6 sides should be running the same version of Kea. For instance the support of port relay (RFC 8357) introduced such such incompatible change.

There is only one specific parameter for the DHCPv6 side: dhcp4o6-port which specifies the first of the two consecutive ports of the UDP sockets used for the communication between the DHCPv6 and DHCPv4 servers (the DHCPv6 server is bound to ::1 on port and connected to ::1 on port + 1).

Two other configuration entries are in general required: unicast traffic support (see Section 9.2.6, “Unicast Traffic Support”) and DHCP 4o6 server address option (name "dhcp4o6-server-addr", code 88).

The following configuration was used during some tests:

{

# DHCPv6 conf
"Dhcp6": {

    "interfaces-config": {
        "interfaces": [ "eno33554984/2001:db8:1:1::1" ]
    },

    "lease-database": {
        "type": "memfile",
        "name": "leases6"
    },

    "preferred-lifetime": 3000,
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,

    "subnet6": [ {
        "subnet": "2001:db8:1:1::/64",
        "interface": "eno33554984",
        "pools": [ { "pool": "2001:db8:1:1::1:0/112" } ]
    } ],

    "dhcp4o6-port": 6767,

    "option-data": [ {
        "name": "dhcp4o6-server-addr",
        "code": 88,
        "space": "dhcp6",
        "csv-format": true,
        "data": "2001:db8:1:1::1"
    } ]

},

"Logging": {
    "loggers": [ {
        "name": "kea-dhcp6",
        "output_options": [ {
            "output": "/tmp/kea-dhcp6.log"
        } ],
        "severity": "DEBUG",
        "debuglevel": 0
    } ]
}

}

Note

Relayed DHCPv4-QUERY DHCPv6 messages are not yet supported.

9.2.23. Sanity checks in DHCPv6

An important aspect of a well running DHCP system is an assurance that the data remains consisent. However, in some cases it may be convenient to tolerate certain inconsistent data. For example, a network administrator that temporarily removed a subnet from a configuration wouldn't want all the leases associated with it disappear from the lease database. Kea 1.5 introduced a mechanism to better control sanity checks such as this. While currently the scope of configurable sanity checks is limited and their default value is set low, it is expected that over time the default settings will be set to more aggressive values and more parameters of similar nature will be added in the future.

Kea now supports a new configuration scope called sanity-checks. It currently allows only a single parameter called lease-checks. It governs what sort of verification is done when a new lease is being loaded from a lease file. With the introduction of sanity checks mechanism, it is now possible to tell Kea to try to correct inconsistent data.

Every subnet has a subnet-id value. This is how Kea internally identifies subnets. Each lease has a subnet-id parameter as well, which identifies which subnet it belongs to. However, if configuration has changed, it is possible that a lease could exist with a subnet-id without any subnet that matches it. Also, it may be possible that subnets configuration has changed and the subnet-id now belongs to a subnet that does not match the lease. Kea corrective algorithm first checks if there is a subnet with subnet-id specified by the lease. If there is, it checks whether the lease belongs to that subnet. If not, depending on the lease-checks setting, the lease is discarded, a warning is printed or a new subnet is selected for the lease that matches it topologically.

Since delegated prefixes do not have to belong to a subnet in which they're offered, there is no way to implement such a mechanism for IPv6 prefixes. As such, the mechanism works for IPv6 addresses only.

There are five levels which are supported:

  • none - do no special checks, accept the lease as is
  • warn - if problems are detected, a warning will be printed, but the lease data will be accepted anyway. This is the default value. If not explicitly configured to some other value, this level will be used.
  • fix - If data inconsistency is discovered, Kea will try to correct it. If the correction is not successful, the data will be inserted anyway.
  • fix-del - If data inconsistency is discovered, Kea will try to correct it. If the correction is not succesful, the lease will be rejected. This setting ensures the data correctness, but some incorrect data may be lost. Use with care.
  • del - This is the strictest mode. If any inconsistency is detected, the lease is rejected. Use with care.

This feature is currently implemented for memfile backend.

An example configuration that sets this parameter looks as follows:

"Dhcp6": {
    "sanity-checks": {
        // This parameter determines what to do when a new lease appears in the
        // system (i.e. either is read from disk during memfile startup or is
        // added via lease commands). There are five modes supported:
        // none - do nothing, accept them as is
        // warn - if subnet-id problems are detected, print a warning, but
        //        otherwise load the lease as is. This is the default value.
        // fix - attempt to fix the lease by finding appropriate subnet-id value.
        //       if there is no suitable subnet, the lease is loaded as is.
        // fix-del - attempt to fix the lease by findind appropriate subnet-id
        //           value. If there is no suitable subnet, the lease is deleted.
        // del - delete leases that have incorrect subnet-id values.
        "lease-checks": "fix-del"
    },
    ...
}

9.3. Host Reservation in DHCPv6

There are many cases where it is useful to provide a configuration on a per host basis. The most obvious one is to reserve specific, static IPv6 address or/and prefix for exclusive use by a given client (host) ‐ returning client will get the same address or/and prefix every time and other clients will never get that address. Note that there may be cases when the new reservation has been made for the client for the address or prefix being currently in use by another client. We call this situation a "conflict". The conflicts get resolved automatically over time as described in the subsequent sections. Once conflict is resolved, the client will keep receiving the reserved configuration when it renews.

Another example when the host reservations are applicable is when a host has specific requirements, e.g. a printer that needs additional DHCP options or a cable modem needs specific parameters. Yet another possible use case for host reservation is to define unique names for hosts.

Hosts reservations are defined as parameters for each subnet. Each host can be identified by either DUID or its hardware/MAC address. See Section 9.10, “MAC/Hardware Addresses in DHCPv6” for details. There is an optional reservations array in the subnet6 structure. Each element in that array is a structure, that holds information about a single host. In particular, the structure has an identifier that uniquely identifies a host. In the DHCPv6 context, such an identifier is usually a DUID, but can also be a hardware or MAC address. Also, either one or more addresses or prefixes may be specified. It is possible to specify a hostname and DHCPv6 options for a given host.

The following example shows how to reserve addresses and prefixes for specific hosts:

"subnet6": [
    {
        "subnet": "2001:db8:1::/48",
        "pools": [ { "pool": "2001:db8:1::/80" } ],
        "pd-pools": [
            {
                "prefix": "2001:db8:1:8000::",
                "prefix-len": 48,
                "delegated-len": 64
            }
        ],
        "reservations": [
            {
                "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                "ip-addresses": [ "2001:db8:1::100" ]
            },
            {
                "hw-address": "00:01:02:03:04:05",
                "ip-addresses": [ "2001:db8:1::101", "2001:db8:1::102" ]
            },
            {
                "duid": "01:02:03:04:05:06:07:08:09:0A",
                "ip-addresses": [ "2001:db8:1::103" ],
                "prefixes": [ "2001:db8:2:abcd::/64" ],
                "hostname": "foo.example.com"
            }
        ]
    }
]

This example includes reservations for three different clients. The first reservation is made for the address 2001:db8:1::100 for a client using DUID 01:02:03:04:05:0A:0B:0C:0D:0E. The second reservation is made for two addresses 2001:db8:1::101 and 2001:db8:1::102 for a client using MAC address 00:01:02:03:04:05. Lastly, address 2001:db8:1::103 and prefix 2001:db8:2:abcd::/64 are reserved for a client using DUID 01:02:03:04:05:06:07:08:09:0A. The last reservation also assigns a hostname to this client.

Note that DHCPv6 allows for a single client to lease multiple addresses and multiple prefixes at the same time. Therefore ip-addresses and prefixes are plural and are actually arrays. When the client sends multiple IA options (IA_NA or IA_PD), each reserved address or prefix is assigned to an individual IA of the appropriate type. If the number of IAs of specific type is lower than the number of reservations of that type, the number of reserved addresses or prefixes assigned to the client is equal to the number of IA_NAs or IA_PDs sent by the client, i.e. some reserved addresses or prefixes are not assigned. However, they still remain reserved for this client and the server will not assign them to any other client. If the number of IAs of specific type sent by the client is greater than the number of reserved addresses or prefixes, the server will try to assign all reserved addresses or prefixes to the individual IAs and dynamically allocate addresses or prefixes to remaining IAs. If the server cannot assign a reserved address or prefix because it is in use, the server will select the next reserved address or prefix and try to assign it to the client. If the server subsequently finds that there are no more reservations that can be assigned to the client at the moment, the server will try to assign leases dynamically.

Making a reservation for a mobile host that may visit multiple subnets requires a separate host definition in each subnet it is expected to visit. It is not allowed to define multiple host definitions with the same hardware address in a single subnet. Multiple host definitions with the same hardware address are valid if each is in a different subnet. The reservation for a given host should include only one identifier, either DUID or hardware address. Defining both for the same host is considered a configuration error, but as of 1.1.0, it is not rejected.

Adding host reservation incurs a performance penalty. In principle, when a server that does not support host reservation responds to a query, it needs to check whether there is a lease for a given address being considered for allocation or renewal. The server that also supports host reservation, has to perform additional checks: not only if the address is currently used (i.e. if there is a lease for it), but also whether the address could be used by someone else (i.e. if there is a reservation for it). That additional check incurs additional overhead.

9.3.1. Address/Prefix Reservation Types

In a typical scenario there is an IPv6 subnet defined with a certain part of it dedicated for dynamic address allocation by the DHCPv6 server. There may be an additional address space defined for prefix delegation. Those dynamic parts are referred to as dynamic pools, address and prefix pools or simply pools. In principle, the host reservation can reserve any address or prefix that belongs to the subnet. The reservations that specify an address that belongs to configured pools are called "in-pool reservations". In contrast, those that do not belong to dynamic pools are called "out-of-pool reservations". There is no formal difference in the reservation syntax and both reservation types are handled uniformly. However, upcoming releases may offer improved performance if there are only out-of-pool reservations as the server will be able to skip reservation checks when dealing with existing leases. Therefore, system administrators are encouraged to use out-of-pool reservations if possible.

Beginning with Kea 1.5.0, there is now support for global host reservations. These are reservations that are specified at the global level within the configuration and that do not belong to any specific subnet. Kea will still match inbound client packets to a subnet as before, but when the subnet's reservation mode is set to "global", Kea will look for host reservations only among the global reservations defined. Typcially, such reservations would be used to reserve hostnames for clients which may move from one subnet to another.

Note

You can reserve any ip-address or prefix in a global reservation. Just keep in mind that Kea will not do any sanity checking on the address or prefix and that for Kea 1.5.0, support for global reservations should be considered experimental.

9.3.2. Conflicts in DHCPv6 Reservations

As reservations and lease information are stored separately, conflicts may arise. Consider the following series of events. The server has configured the dynamic pool of addresses from the range of 2001:db8::10 to 2001:db8::20. Host A requests an address and gets 2001:db8::10. Now the system administrator decides to reserve address 2001:db8::10 for Host B. In general, reserving an address that is currently assigned to someone else is not recommended, but there are valid use cases where such an operation is warranted.

The server now has a conflict to resolve. Let's analyze the situation here. If Host B boots up and request an address, the server is not able to assign the reserved address 2001:db8::10. A naive approach would to be immediately remove the lease for Host A and create a new one for Host B. That would not solve the problem, though, because as soon as Host B get the address, it will detect that the address is already in use by someone else (Host A) and would send a Decline message. Therefore in this situation, the server has to temporarily assign a different address from the dynamic pool (not matching what has been reserved) to Host B.

When Host A renews its address, the server will discover that the address being renewed is now reserved for someone else (Host B). Therefore the server will remove the lease for 2001:db8::10, select a new address and create a new lease for it. It will send two addresses in its response: the old address with lifetime set to 0 to explicitly indicate that it is no longer valid and the new address with a non-zero lifetime. When Host B renews its temporarily assigned address, the server will detect that the existing lease does not match reservation, so it will release the current address Host B has and will create a new lease matching the reservation. Similar as before, the server will send two addresses: the temporarily assigned one with zeroed lifetimes, and the new one that matches reservation with proper lifetimes set.

This recovery will succeed, even if other hosts will attempt to get the reserved address. Had Host C requested address 2001:db8::10 after the reservation was made, the server will propose a different address.

This recovery mechanism allows the server to fully recover from a case where reservations conflict with existing leases. This procedure takes time and will roughly take as long as renew-timer value specified. The best way to avoid such recovery is to not define new reservations that conflict with existing leases. Another recommendation is to use out-of-pool reservations. If the reserved address does not belong to a pool, there is no way that other clients could get this address.

Note

The conflict resolution mechanism does not work for global reservations. As of Kea 1.5.0, it is generally recommended to not use global reservations for addresses or prefixes. If you want to use it anyway, you have to manually ensure that the reserved values are not in the dynamic pools.

9.3.3. Reserving a Hostname

When the reservation for the client includes the hostname, the server will assign this hostname to the client and send it back in the Client FQDN, if the client sent the FQDN option to the server. The reserved hostname always takes precedence over the hostname supplied by the client (via the FQDN option) or the autogenerated (from the IPv6 address) hostname.

The server qualifies the reserved hostname with the value of the qualifying-suffix parameter. For example, the following subnet configuration:

"subnet6": [
    {
        "subnet": "2001:db8:1::/48",
        "pools": [ { "pool": "2001:db8:1::/80" } ],
        "reservations": [
            {
                "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                "ip-addresses": [ "2001:db8:1::100" ]
                "hostname": "alice-laptop"
            }
        ]
    }
],
"dhcp-ddns": {
    "enable-updates": true,
    "qualifying-suffix": "example.isc.org."
}

will result in assigning the "alice-laptop.example.isc.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E". If the qualifying-suffix is not specified, the default (empty) value will be used, and in this case the value specified as a hostname will be treated as fully qualified name. Thus, by leaving the qualifying-suffix empty it is possible to qualify hostnames for the different clients with different domain names:

"subnet6": [
    {
        "subnet": "2001:db8:1::/48",
        "pools": [ { "pool": "2001:db8:1::/80" } ],
        "reservations": [
            {
                "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                "ip-addresses": [ "2001:db8:1::100" ]
                "hostname": "mark-desktop.example.org."
            }
        ]
    }
],
"dhcp-ddns": {
    "enable-updates": true,
}

The above example results in the assignment of the "mark-desktop.example.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E".

9.3.4. Including Specific DHCPv6 Options in Reservations

Kea 1.1.0 introduced the ability to specify options on a per host basis. The options follow the same rules as any other options. These can be standard options (see Section 9.2.10, “Standard DHCPv6 Options”), custom options (see Section 9.2.12, “Custom DHCPv6 Options”) or vendor specific options (see Section 9.2.13, “DHCPv6 Vendor-Specific Options”). The following example demonstrates how standard options can be defined.

"reservations": [
{
   "duid": "01:02:03:05:06:07:08",
   "ip-addresses": [ "2001:db8:1::2" ],
    "option-data": [
    {
        "option-data": [ {
            "name": "dns-servers",
            "data": "3000:1::234"
        },
        {
            "name": "nis-servers",
            "data": "3000:1::234"
        }
    } ]
} ]

Vendor specific options can be reserved in a similar manner:

"reservations": [
{
    "duid": "aa:bb:cc:dd:ee:ff",
    "ip-addresses": [ "2001:db8::1" ],
    "option-data": [
    {
        "name": "vendor-opts",
        "data": 4491
    },
    {
        "name": "tftp-servers",
        "space": "vendor-4491",
        "data": "3000:1::234"
    } ]
} ]

Options defined at host level have the highest priority. In other words, if there are options defined with the same type on global, subnet, class and host level, the host specific values will be used.

9.3.5. Reserving Client Classes in DHCPv6

The Section 14.3, “Using Expressions In Classification” explains how to configure the server to assign classes to a client based on the content of the options that this client sends to the server. Host reservations mechanisms also allow for the static assignment of classes to clients. The definitions of these classes are placed in the Kea configuration. The following configuration snippet shows how to specify that the client belongs to classes reserved-class1 and reserved-class2. Those classes are associated with specific options being sent to the clients which belong to them.

{
    "client-classes": [
    {
       "name": "reserved-class1",
       "option-data": [
       {
           "name": "dns-servers",
           "data": "2001:db8:1::50"
       }
       ]
   },
   {
       "name": "reserved-class2",
       "option-data": [
       {
           "name": "nis-servers",
           "data": "2001:db8:1::100"
       }
       ]
    }
    ],
    "subnet6": [
    {   "pools": [ { "pool": "2001:db8:1::/64" } ],
        "subnet": "2001:db8:1::/48",
        "reservations": [
        {
            "duid": "01:02:03:04:05:06:07:08",
            
            "client-classes": [ "reserved-class1", "reserved-class2" ]
            
         } ]
     } ]
 }

Static class assignments, as shown above, can be used in conjunction with classification using expressions. The "KNOWN" or "UNKNOWN" builtin class is added to the packet and any class depending on it directly or indirectly and not only-if-required is evaluated.

Note

If you want to force the evaluation of a class expression after the host reservation lookup, for instance because of a dependency on "reserved-class1" from the previous example, you should add a "member('KNOWN')" in the expression.

9.3.6. Storing Host Reservations in MySQL, PostgreSQL or Cassandra

It is possible to store host reservations in MySQL, PostgreSQL or Cassandra. See Section 9.2.3, “Hosts Storage” for information on how to configure Kea to use reservations stored in MySQL, PostgreSQL or Cassandra. Kea provides dedicated hook for managing reservations in a database, section Section 15.4.4, “host_cmds: Host Commands” provide detailed information. The Kea wiki https://gitlab.isc.org/isc-projects/kea/wikis/designs/commands#23-host-reservations-hr-management provides some examples how to conduct some common operations on host reservations.

Note

In Kea maximum length of an option specified per host is arbitrarily set to 4096 bytes.

9.3.7. Fine Tuning DHCPv6 Host Reservation

The host reservation capability introduces additional restrictions for the allocation engine (the component of Kea that selects an address for a client) during lease selection and renewal. In particular, three major checks are necessary. First, when selecting a new lease, it is not sufficient for a candidate lease to not be used by another DHCP client. It also must not be reserved for another client. Second, when renewing a lease, additional check must be performed whether the address being renewed is not reserved for another client. Finally, when a host renews an address or a prefix, the server has to check whether there is a reservation for this host, so the existing (dynamically allocated) address should be revoked and the reserved one be used instead.

Some of those checks may be unnecessary in certain deployments and not performing them may improve performance. The Kea server provides the reservation-mode configuration parameter to select the types of reservations allowed for the particular subnet. Each reservation type has different constraints for the checks to be performed by the server when allocating or renewing a lease for the client. Allowed values are:

  • all - enables both in-pool and out-of-pool host reservation types. This is the default value. This setting is the safest and the most flexible. As all checks are conducted, it is also the slowest. This does not check against global reservations.
  • out-of-pool - allows only out of pool host reservations. With this setting in place, the server may assume that all host reservations are for addresses that do not belong to the dynamic pool. Therefore it can skip the reservation checks when dealing with in-pool addresses, thus improving performance. Do not use this mode if any of your reservations use in-pool address. Caution is advised when using this setting. Kea does not sanity check the reservations against reservation-mode and misconfiguration may cause problems.
  • global - allows only global host reservations. With this setting in place, the server searches for reservations for a client only among the defined global reservations. If an address is specified, the server will skip the reservation checks done when dealing in other modes, thus improving performance. Caution is advised when using this setting: Kea does not sanity check the reservations when global and misconfiguration may cause problems.
  • disabled - host reservation support is disabled. As there are no reservations, the server will skip all checks. Any reservations defined will be completely ignored. As the checks are skipped, the server may operate faster in this mode.

The parameter can be specified at global, subnet and shared network levels.

An example configuration that disables reservation looks like follows:

"Dhcp6": {
    "subnet6": [
        {
        "subnet": "2001:db8:1::/64",
        "reservation-mode": "disabled",
        ...
        }
    ]
}
       

An example configuration using global reservations is shown below:

"Dhcp6": {

    
    "reservation-mode": "global",
    "reservations": [
       {
        "duid": "00:03:00:01:11:22:33:44:55:66",
        "hostname": "host-one"
       },
       {
        "duid": "00:03:00:01:99:88:77:66:55:44",
        "hostname": "host-two"
       }
    ],
    
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        ...
    }
    ]
}

For more details regarding global reservations, see Section 9.3.8, “Global reservations in DHCPv6”.

Another aspect of the host reservations are different types of identifiers. Kea 1.1.0 supports two types of identifiers in DHCPv6: hw-address and duid, but more identifier types are likely to be added in the future. This is beneficial from a usability perspective. However, there is a drawback. For each incoming packet Kea has to to extract each identifier type and then query the database to see if there is a reservation done by this particular identifier. If nothing is found, the next identifier is extracted and next query is issued. This process continues until either a reservation is found or all identifier types have been checked. Over time with an increasing number of supported identifier types, Kea would become slower and slower.

To address this problem, a parameter called host-reservation-identifiers has been introduced. It takes a list of identifier types as a parameter. Kea will check only those identifier types enumerated in host-reservation-identifiers. From a performance perspective the number of identifier types should be kept to minimum, ideally limited to one. If your deployment uses several reservation types, please enumerate them from most to least frequently used as this increases the chances of Kea finding the reservation using the fewest number of queries. An example of host reservation identifiers looks as follows:

"host-reservation-identifiers": [ "duid", "hw-address" ],
"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        ...
    }
]

If not specified, the default value is:

"host-reservation-identifiers": [ "hw-address", "duid" ]

9.3.8. Global reservations in DHCPv6

In some deployments, such as mobile, clients can roam within the network and there is a desire to specify certain parameters regardless of the client's current location. To facilitate such a need, a global reservation mechanism has been implemented. The idea behind it is that regular host reservations are tied to specific subnets, by using specific subnet-id. Kea 1.5.0 introduced a new capability to specify global reservation that can be used in every subnet that has global reservations enabled.

This feature can be used to assign certain parameters, such as hostname or other dedicated, host-specific options. It can also be used to assign addresses or prefixes. However, global reservations that assign either of these bypass the whole topology determination provided by DHCP logic implemented in Kea. It is very easy to misuse this feature and get configuration that is inconsistent. To give a specific example, imagine a global reservation for an address 2001:db8:1111::1 and two subnets 2001:db8:1111::/48 and 2001:db8:ffff::/48. If global reservations are used in both subnets and a device matching global host reservations visits part of the network that is covered by 2001:db8:ffff::/48, it will get an IP address 2001:db8:ffff::1, which will be outside of the prefix announced by its local router using Router Advertisements. Such a configuration would be unsuable or at the very least ridden with issues, such as the downlink traffic not reaching the device.

To use global host reservations a configuration similar to the following can be used:

"Dhcp6:" {
    // This specifies global reservations. They will apply to all subnets that
    // have global reservations enabled.
    
    "reservations": [
    {
       "hw-address": "aa:bb:cc:dd:ee:ff",
       "hostname": "hw-host-dynamic"
    },
    {
       "hw-address": "01:02:03:04:05:06",
       "hostname": "hw-host-fixed",

       // Use of IP address is global reservation is risky. If used outside of
       // matching subnet, such as 3001::/64, it will result in a broken
       // configuration being handled to the client.
       "ip-address": "2001:db8:ff::77"
    },
    {
       "duid": "01:02:03:04:05",
       "hostname": "duid-host"
    }
    ],
    "valid-lifetime": 600,
    "subnet4": [ {
        "subnet": "2001:db8:1::/64",
        "reservation-mode": "global",
        "pools": [ { "pool": "2001:db8:1::-2001:db8:1::100" } ]
    } ]
}

When using database backends, the global host reservations are distinguished from regular reservations by using subnet-id value of zero.

9.4. Shared networks in DHCPv6

DHCP servers use subnet information in two ways. First, it is used to determine the point of attachment, or simply put, where the client is connected to the network. Second, the subnet information is used to group information pertaining to specific location in the network. This approach works well in general case, but the are scenarios where the boundaries are blurred. Sometimes it is useful to have more than one logical IP subnet being deployed on the same physical link. The need to understand that two or more subnets are used on the same link requires additional logic in the DHCP server. This capability has been added in Kea 1.3.0. It is called "shared networks" in Kea and ISC DHCP projects. It is sometimes also called "shared subnets". In Microsoft's nomenclature it is called "multinet".

There are many use cases where the feature is useful. The most common example in the IPv4 case is when the server is running out of available addresses in a subnet. This is less common in IPv6, but the shared networks are still useful in IPv6. One of the use cases is an exhaustion of IPv6 delegated prefixes within a subnet. Another IPv6 specific example is an experiment with addressing scheme. With the advent of IPv6 deployment and vast address space, many organizations split the address space into subnets, then deploy it and after a while discover that they want to split it differently. In the transition period they want both old and new addressing to be available. Thus the need for more than one subnet on the same physical link.

Finally, the case of cable networks is directly applicable in IPv6. There are two types of devices in cable networks: cable modems and the end user devices behind them. It is a common practice to use different subnet for cable modems to prevent users from tinkering with their cable modems. In this case, the distinction is based on the type of device, rather than coming out of running out address space.

A client connected to a shared network may be assigned a lease (address or prefix) from any of the pools defined within the subnets belonging to the shared network. Internally, the server selects one of the subnets belonging to the shared network and tries to allocate a lease from this subnet. If the server is unable to allocate a lease from the selected subnet (e.g. due to pools exhaustion) it will use another subnet from the same shared network and try to allocate a lease from this subnet etc. Therefore, in the typical case, the server will allocate all leases available in a given subnet before it starts allocating leases from other subnets belonging to the same shared network. However, in certain situations the client can be allocated a lease from the other subnets before the pools in the first subnet get exhausted, e.g. when the client provides a hint that belongs to another subnet or the client has reservations in a different than default subnet.

Note

It is strongly discouraged for the Kea deployments to assume that the server doesn't allocate leases from other subnets until it uses all the leases from the first subnet in the shared network. Apart from the fact that hints, host reservations and client classification affect subnet selection, it is also foreseen that we will enhance allocation strategies for shared networks in the future versions of Kea, so as the selection of subnets within a shared network is equally probable (unpredictable).

In order to define a shared network an additional configuration scope is introduced:

{
"Dhcp6": {
    "shared-networks": [
        {
            // Name of the shared network. It may be an arbitrary string
            // and it must be unique among all shared networks.
            "name": "ipv6-lab-1",

            // Subnet selector can be specifed on the shared network level.
            // Subnets from this shared network will be selected for clients
            // communicating via relay agent having the specified IP address.
            "relay": {
                "ip-addresses": [ "2001:db8:2:34::1" ]
            },

            // This starts a list of subnets in this shared network.
            // There are two subnets in this example.
            "subnet6": [
                {
                    "subnet": "2001:db8::/48",
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                },
                {
                    "subnet": "3ffe:ffe::/64",
                    "pools": [ { "pool":  "3ffe:ffe::/64" } ]
                }
            ]
        } ], // end of shared-networks

    // It is likely that in your network you'll have a mix of regular,
    // "plain" subnets and shared networks. It is perfectly valid to mix
    // them in the same config file.
    //
    // This is regular subnet. It's not part of any shared-network.
    "subnet6": [
        {
            "subnet": "2001:db9::/48",
            "pools": [ { "pool":  "2001:db9::/64" } ],
            "relay": {
                "ip-addresses": [ "2001:db8:1:2::1" ]
            }
        }
    ]

} // end of Dhcp6
}

As you see in the example, it is possible to mix shared and regular ("plain") subnets. Each shared network must have a unique name. This is a similar concept to ID for subnets, but it offers more flexibility. This is used for logging, but also internally for identifying shared networks.

In principle it makes sense to define only shared networks that consist of two or more subnets. However, for testing purposes it is allowed to define a shared network with just one subnet or even an empty one. This is not a recommended practice in production networks, as the shared network logic requires additional processing and thus lowers server's performance. To avoid unnecessary performance degradation the shared subnets should only be defined when required by the deployment.

Shared networks provide an ability to specify many parameters in the shared network scope that will apply to all subnets within it. If necessary, you can specify a parameter in the shared network scope and then override its value on the subnet scope. For example:

"shared-networks": [
    {
        "name": "lab-network3",
        "relay": {
             "ip-addresses": [ "2001:db8:2:34::1" ]
        },

        // This applies to all subnets in this shared network, unless
        // values are overridden on subnet scope.
        "valid-lifetime": 600,

        // This option is made available to all subnets in this shared
        // network.
        "option-data": [ {
            "name": "dns-servers",
            "data": "2001:db8::8888"
        } ],

        "subnet6": [
            {
                "subnet": "2001:db8:1::/48",
                "pools": [ { "pool":  "2001:db8:1::1 - 2001:db8:1::ffff" } ],

                // This particular subnet uses different values.
                "valid-lifetime": 1200,
                "option-data": [
                {
                    "name": "dns-servers",
                    "data": "2001:db8::1:2"
                },
                {
                    "name": "unicast",
                    "data": "2001:abcd::1"
                } ]
            },
            {
                 "subnet": "2001:db8:2::/48",
                 "pools": [ { "pool":  "2001:db8:2::1 - 2001:db8:2::ffff" } ],

                 // This subnet does not specify its own valid-lifetime value,
                 // so it is inherited from shared network scope.
                 "option-data": [
                 {
                     "name": "dns-servers",
                     "data": "2001:db8:cafe::1"
                 } ]
            }
        ],
    } ]

In this example, there is a dns-servers option defined that is available to clients in both subnets in this shared network. Also, a valid lifetime is set to 10 minutes (600s). However, the first subnet overrides some of the values (valid lifetime is 20 minutes, different IP address for dns-servers), but also adds its own option (unicast address). Assuming a client asking for a server unicast and dns servers options is assigned a lease from this subnet, he will get a lease for 20 minutes and dns-servers and be allowed to use server unicast at address 2001:abcd::1. If the same client is assigned to the second subnet, he will get a 10 minutes long lease, dns-servers value of 2001:db8:cafe::1 and no server unicast.

Some parameters must be the same in all subnets in the same shared network. This restriction applies to interface and rapid-commit settings. The most convenient way is to define them on shared network scope, but you may specify them for each subnet. However, care should be taken for each subnet to have the same value.

9.4.1. Local and relayed traffic in shared networks

It is possible to specify interface name in the shared network scope to tell the server that this specific shared network is reachable directly (not via relays) using local network interface. It is sufficient to specify it once at the shared network level. As all subnets in a shared network are expected to be used on the same physical link, it is a configuration error to attempt to make a shared network out of subnets that are reachable over different interfaces. It is allowed to specify interface parameter on each subnet, although its value must be the same for each subnet. Thus it's usually more convenient to specify it once at the shared network level.

"shared-networks": [
    {
        "name": "office-floor-2",

        // This tells Kea that the whole shared networks is reachable over
        // local interface. This applies to all subnets in this network.
        "interface": "eth0",

        "subnet6": [
            {
                "subnet": "2001:db8::/64",
                "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ],
                "interface": "eth0"
            },
            {
                 "subnet": "3ffe:abcd::/64",
                 "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ]

                 // Specifying a different interface name is configuration
                 // error:
                 // "interface": "eth1"
            }
        ],
    } ]

Somewhat similar to interface names, also relay IP addresses can be specified for the whole shared network. However, depending on your relay configuration, it may use different IP addresses depending on which subnet is being used. Thus there is no requirement to use the same IP relay address for each subnet. Here's an example:

"shared-networks": [
    {
        "name": "kakapo",
        "relay": {
            "ip-addresses":  [ "2001:db8::abcd" ]
        },
        "subnet6": [
            {
                "subnet": "2001:db8::/64",
                "relay": {
                    "ip-addresses": [ "2001:db8::1234" ]
                },
                "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
            },
            {
                 "subnet": "3ffe:abcd::/64",
                 "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ],
                 "relay": {
                    "ip-addresses": [ "3ffe:abcd::cafe" ]
                 }
            }
        ]
    }
]

In this particular case the relay IP address specified at network level doesn't have much sense, as it is overridden in both subnets, but it was left there as an example of how one could be defined at network level. Note that the relay agent IP address typically belongs to the subnet it relays packets from, but this is not a strict requirement. Therefore Kea accepts any value here as long as it is valid IPv6 address.

9.4.2. Client classification in shared networks

Sometimes it is desired to segregate clients into specific subnets based on some properties. This mechanism is called client classification and is described in Chapter 14, Client Classification. Client classification can be applied to subnets belonging to shared networks in the same way as it is used for subnets specified outside of shared networks. It is important to understand how the server selects subnets for the clients when client classification is in use, to assure that the desired subnet is selected for a given client type.

If a subnet is associated with a class, only the clients belonging to this class can use this subnet. If there are no classes specified for a subnet, any client connected to a given shared network can use this subnet. A common mistake is to assume that the subnet including a client class is preferred over subnets without client classes. Consider the following example:

{
    "client-classes": [
        {
            "name": "b-devices",
            "test": "option[1234].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "relay": {
                "ip-address": [ "2001:db8:2:34::1" ]
            },
            "subnet6": [
                {
                    "subnet": "2001:db8:1::/64",
                    "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ],
                },
                {
                    "subnet": "2001:db8:3::/64",
                    "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                    "client-class": "b-devices"
                }
            ]
        }
    ]
}

If the client belongs to "b-devices" class (because it includes option 1234 with a value of 0x0002) it doesn't guarantee that the subnet 2001:db8:3::/64 will be used (or preferred) for this client. The server can use any of the two subnets because the subnet 2001:db8:1::/64 is also allowed for this client. The client classification used in this case should be pereceived as a way to restrict access to certain subnets, rather than a way to express subnet preference. For example, if the client doesn't belong to the "b-devices" class it may only use the subnet 2001:db8:1::/64 and will never use the subnet 2001:db8:3::/64.

A typical use case for client classification is in the cable network, where cable modems should use one subnet and other devices should use another subnet within the same shared network. In this case it is required to apply classification on all subnets. The following example defines two classes of devices. The subnet selection is made based on option 1234 values.

{
    "client-classes": [
        {

            "name": "a-devices",
            "test": "option[1234].hex == 0x0001"
        },
        {
            "name": "b-devices",
            "test": "option[1234].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "relay": {
                "ip-addresses":  [ "2001:db8:2:34::1" ]
            },
            "subnet6": [
                {
                    "subnet": "2001:db8:1::/64",
                    "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ],
                    "client-class": "a-devices"
                },
                {
                    "subnet": "2001:db8:3::/64",
                    "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                    "client-class": "b-devices"
                }
            ]
        }
    ]
}

In this example each class has its own restriction. Only clients that belong to class a-devices will be able to use subnet 2001:db8:1::/64 and only clients belonging to b-devices will be able to use subnet 2001:db8:3::/64. Care should be taken to not define too restrictive classification rules, as clients that are unable to use any subnets will be refused service. Although, this may be desired outcome if one desires to service only clients of known properties (e.g. only VoIP phones allowed on a given link).

Note that it is possible to achieve similar effect as presented in this section without the use of shared networks. If the subnets are placed in the global subnets scope, rather than in the shared network, the server will still use classification rules to pick the right subnet for a given class of devices. The major benefit of placing subnets within the shared network is that common parameters for the logically grouped subnets can be specified once, in the shared network scope, e.g. "interface" or "relay" parameter. All subnets belonging to this shared network will inherit those parameters.

9.4.3. Host reservations in shared networks

Subnets being part of a shared network allow host reservations, similar to regular subnets:

{
    "shared-networks": [
    {
        "name": "frog",
        "relay": {
            "ip-addresses": [ "2001:db8:2:34::1" ]
        },
        "subnet6": [
            {
                "subnet": "2001:db8:1::/64",
                "id": 100,
                "pools": [ { "2001:db8:1::1 - 2001:db8:1::64" } ],
                "reservations": [
                {
                    "duid": "00:03:00:01:11:22:33:44:55:66",
                    "ip-addresses": [ "2001:db8:1::28" ]
                }
                ]
            },
            {
                "subnet": "2001:db8:3::/64",
                "id": 101,
                "pools": [ { "pool": "2001:db8:3::1 - 2001:db8:3::64" } ],
                "reservations": [
                    {
                        "duid": "00:03:00:01:aa:bb:cc:dd:ee:ff",
                        "ip-addresses": [ "2001:db8:2::28" ]
                    }
                ]
            }
        ]
    }
    ]
}
  

It is worth noting that Kea conducts additional checks when processing a packet if shared networks are defined. First, instead of simply checking if there's a reservation for a given client in his initially selected subnet, it goes through all subnets in a shared network looking for a reservation. This is one of the reasons why defining a shared network may impact performance. If there is a reservation for a client in any subnet, that particular subnet will be picked for the client. Although it's technically not an error, it is considered a bad practice to define reservations for the same host in multiple subnets belonging to the same shared network.

While not strictly mandatory, it is strongly recommended to use explicit "id" values for subnets if you plan to use database storage for host reservations. If ID is not specified, the values for it be autogenerated, i.e. it will assign increasing integer values starting from 1. Thus, the autogenerated IDs are not stable across configuration changes.

9.5. Server Identifier in DHCPv6

The DHCPv6 protocol uses a "server identifier" (also known as a DUID) for clients to be able to discriminate between several servers present on the same link. RFC 8415 defines four DUID types: DUID-LLT, DUID-EN, DUID-LL and DUID-UUID. Future specifications may introduce new DUID types.

The Kea DHCPv6 server generates a server identifier once, upon the first startup, and stores it in a file. This identifier isn't modified across restarts of the server and so is a stable identifier.

Kea follows recommendation from RFC 8415 to use DUID-LLT as the default server identifier. However, we have received reports that some deployments require different DUID types, and there is a need to administratively select both DUID type and/or its contents.

The server identifier can be configured using parameters within the server-id map element in the global scope of the Kea configuration file. The following example demonstrates how to select DUID-EN as a server identifier:

"Dhcp6": {
    "server-id": {
        "type": "EN"
    },
    ...
}

Currently supported values for type parameter are: "LLT", "EN" and "LL", for DUID-LLT, DUID-EN and DUID-LL respectively.

When a new DUID type is selected the server will generate its value and replace any existing DUID in the file. The server will then use the new server identifier in all future interactions with the clients.

Note

If the new server identifier is created after some clients have obtained their leases, the clients using the old identifier will not be able to renew the leases: the server will ignore messages containing the old server identifier. Clients will continue sending Renew until they transition to the rebinding state. In this state they will start sending Rebind messages to multicast address without a server identifier. The server will respond to the Rebind messages with a new server identifier and the clients will associate the new server identifier with their leases. Although the clients will be able to keep their leases and will eventually learn the new server identifier, this will be at the cost of increased number of renewals and multicast traffic due to a need to rebind. Therefore it is recommended that modification of the server identifier type and value is avoided if the server has already assigned leases and these leases are still valid.

There are cases when an administrator needs to explicitly specify a DUID value rather than allow the server to generate it. The following example demonstrates how to explicitly set all components of a DUID-LLT.

"Dhcp6": {
    "server-id": {
        "type": "LLT",
        "htype": 8,
        "identifier": "A65DC7410F05",
        "time": 2518920166
    },
    ...
}

where:

  • htype is a 16-bit unsigned value specifying hardware type,
  • identifier is a link layer address, specified as a string of hexadecimal digits,
  • time is a 32-bit unsigned time value.

The hexadecimal representation of the DUID generated as a result of the configuration specified above will be:

 00:01:00:08:96:23:AB:E6:A6:5D:C7:41:0F:05
|type |htype|   time    |   identifier    |

It is allowed to use special value of 0 for "htype" and "time", which indicates that the server should use ANY value for these components. If the server already uses a DUID-LLT it will use the values from this DUID. If the server uses a DUID of a different type or doesn't use any DUID yet, it will generate these values. Similarly, if the "identifier" is assigned an empty string, the value of the identifier will be generated. Omitting any of these parameters is equivalent to setting them to those special values.

For example, the following configuration:

"Dhcp6": {
    "server-id": {
        "type": "LLT",
        "htype": 0,
        "identifier": "",
        "time": 2518920166
    },
    ...
}

indicates that the server should use ANY link layer address and hardware type. If the server is already using DUID-LLT it will use the link layer address and hardware type from the existing DUID. If the server is not using any DUID yet, it will use link layer address and hardware type from one of the available network interfaces. The server will use an explicit value of time. If it is different than a time value present in the currently used DUID, that value will be replaced, effectively causing modification of the current server identifier.

The following example demonstrates an explicit configuration of a DUID-EN:

"Dhcp6": {
    "server-id": {
        "type": "EN",
        "enterprise-id": 2495,
        "identifier": "87ABEF7A5BB545"
    },
    ...
}

where:

  • enterprise-id is a 32-bit unsigned value holding enterprise number,
  • identifier is a variable length identifier within DUID-EN.

The hexadecimal representation of the DUID-EN created according to the configuration above is:

 00:02:00:00:09:BF:87:AB:EF:7A:5B:B5:45
|type |  ent-id   |     identifier     |

As in the case of the DUID-LLT, special values can be used for the configuration of the DUID-EN. If enterprise-id is 0, the server will use a value from the existing DUID-EN. If the server is not using any DUID or the existing DUID has a different type, the ISC enterprise id will be used. When an empty string is used for identifier, the identifier from the existing DUID-EN will be used. If the server is not using any DUID-EN the new 6-bytes long identifier will be generated.

DUID-LL is configured in the same way as DUID-LLT with an exception that the time parameter has no effect for DUID-LL, because this DUID type only comprises a hardware type and link layer address. The following example demonstrates how to configure DUID-LL:

"Dhcp6": {
    "server-id": {
        "type": "LL",
        "htype": 8,
        "identifier": "A65DC7410F05"
    },
    ...
}

which will result in the following server identifier:

 00:03:00:08:A6:5D:C7:41:0F:05
|type |htype|   identifier    |

The server stores the generated server identifier in the following location: [kea-install-dir]/var/kea/kea-dhcp6-serverid.

In some uncommon deployments where no stable storage is available, the server should be configured not to try to store the server identifier. This choice is controlled by the value of persist boolean parameter:

"Dhcp6": {
    "server-id": {
        "type": "EN",
        "enterprise-id": 2495,
        "identifier": "87ABEF7A5BB545",
        "persist": false
    },
    ...
}

The default value of the "persist" parameter is true which configures the server to store the server identifier on a disk.

In the example above, the server is configured to not store the generated server identifier on a disk. But, if the server identifier is not modified in the configuration the same value will be used after server restart, because entire server identifier is explicitly specified in the configuration.

9.6. Stateless DHCPv6 (Information-Request Message)

Typically DHCPv6 is used to assign both addresses and options. These assignments (leases) have state that changes over time, hence their name, stateful. DHCPv6 also supports a stateless mode, where clients request configuration options only. This mode is considered lightweight from the server perspective as it does not require any state tracking; hence its name.

The Kea server supports stateless mode. Clients can send Information-Request messages and the server will send back answers with the requested options (providing the options are available in the server configuration). The server will attempt to use per-subnet options first. If that fails - for whatever reason - it will then try to provide options defined in the global scope.

Stateless and stateful mode can be used together. No special configuration directives are required to handle this. Simply use the configuration for stateful clients and the stateless clients will get just options they requested.

This usage of global options allows for an interesting case. It is possible to run a server that provides just options and no addresses or prefixes. If the options have the same value in each subnet, the configuration can define required options in the global scope and skip subnet definitions altogether. Here's a simple example of such a configuration:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "ethX" ]
    },
    "option-data": [ {
        "name": "dns-servers",
        "data": "2001:db8::1, 2001:db8::2"
    } ],
    "lease-database": { "type": "memfile" }
 }

This very simple configuration will provide DNS server information to all clients in the network, regardless of their location. Note the specification of the memfile lease database: this is needed as Kea requires a lease database to be specified even if it is not used.

9.7. Support for RFC 7550 (being now part of RFC 8415)

The RFC 7550 introduced some changes to the previous DHCPv6 specifications, RFC 3315 and RFC 3633, to resolve a few issues with the coexistence of multiple stateful options in the messages sent between the clients and servers. Those changes were later included in the most recent DHCPv6 protocol specification, RFC 8415, which obsoleted RFC 7550. Kea supports RFC 8415 along with these protocol changes, which are briefly described below.

When a client, such as a requesting router, requests an allocation of both addresses and prefixes during the 4-way (SARR) exchange with the server, and the server is not configured to allocate any prefixes but it can allocate some addresses, it will respond with the IA_NA(s) containing allocated addresses and the IA_PD(s) containing the NoPrefixAvail status code. According to the updated specifications, if the client can operate without prefixes it should accept allocated addresses and transition to the 'bound' state. When the client subsequently sends Renew/Rebind messages to the server, according to the T1 and T2 times, to extend the lifetimes of the allocated addresses, if the client is still interested in obtaining prefixes from the server, it may also include an IA_PD in the Renew/Rebind to request allocation of the prefixes. If the server still cannot allocate the prefixes it will respond with the IA_PD(s) containing NoPrefixAvail status code. However, if the server can allocate the prefixes it will allocate and send them in the IA_PD(s) to the client. Similar situation occurs when the server is unable to allocate addresses for the client but can delegate prefixes. The client may request allocation of the addresses while renewing the delegated prefixes. Allocating leases for other IA types while renewing existing leases is by default supported by the Kea DHCPv6 server, and the server provides no configuration mechanisms to disable this behavior.

The following are the other behaviors first introduced in the RFC 7550 (now being part of the RFC 8415) and supported by the Kea DHCPv6 server:

  • Set T1/T2 timers to the same value for all stateful (IA_NA and IA_PD) options to facilitate renewal of all client's leases at the same time (in a single message exchange),
  • NoAddrsAvail and NoPrefixAvail status codes are placed in the IA_NA and IA_PD options in the Advertise message, rather than as the top level options.

9.8. Using Specific Relay Agent for a Subnet

The relay has to have an interface connected to the link on which the clients are being configured. Typically the relay has a global IPv6 address configured on the interface that belongs to the subnet from which the server will assign addresses. In the typical case, the server is able to use the IPv6 address inserted by the relay (in the link-addr field in RELAY-FORW message) to select the appropriate subnet.

However, that is not always the case. The relay address may not match the subnet in certain deployments. This usually means that there is more than one subnet allocated for a given link. The two most common examples where this is the case are long lasting network renumbering (where both old and new address space is still being used) and a cable network. In a cable network both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such case, the DHCPv6 server needs additional information (like the value of interface-id option or IPv6 address inserted in the link-addr field in RELAY-FORW message) to properly select an appropriate subnet.

The following example assumes that there is a subnet 2001:db8:1::/64 that is accessible via a relay that uses 3000::1 as its IPv6 address. The server will be able to select this subnet for any incoming packets that came from a relay with an address in 2001:db8:1::/64 subnet. It will also select that subnet for a relay with address 3000::1.

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ],
             "relay": {
                 "ip-addresses": [ "3000::1" ]
             }
        }
    ]
}

If "relay" is specified, the "ip-addresses" parameter within it is mandatory.

Note

As of Kea 1.4, the "ip-address" parameter in "relay" has been deprecated in favor of "ip-addresses" which supports specifying a list of addresses. Configuration parsing, will honor the singular form for now but users are encouraged to migrate.

9.9. Segregating IPv6 Clients in a Cable Network

In certain cases, it is useful to mix relay address information, introduced in Section 9.8, “Using Specific Relay Agent for a Subnet” with client classification, explained in Chapter 14, Client Classification. One specific example is a cable network, where typically modems get addresses from a different subnet than all devices connected behind them.

Let's assume that there is one CMTS (Cable Modem Termination System) with one CM MAC (a physical link that modems are connected to). We want the modems to get addresses from the 3000::/64 subnet, while everything connected behind modems should get addresses from another subnet (2001:db8:1::/64). The CMTS that acts as a relay an uses address 3000::1. The following configuration can serve that configuration:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "3000::/64",
            "pools": [
                { "pool": "3000::2 - 3000::ffff" }
            ],
            "client-class": "VENDOR_CLASS_docsis3.0",
            "relay": {
                "ip-addresses": [ "3000::1" ]
            }
        },

        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ],
             "relay": {
                 "ip-addresses": [ "3000::1" ]
             }
        }
    ]
}

9.10. MAC/Hardware Addresses in DHCPv6

MAC/hardware addresses are available in DHCPv4 messages from the clients and administrators frequently use that information to perform certain tasks, like per host configuration, address reservation for specific MAC addresses and other. Unfortunately, the DHCPv6 protocol does not provide any completely reliable way to retrieve that information. To mitigate that issue a number of mechanisms have been implemented in Kea that attempt to gather it. Each of those mechanisms works in certain cases, but may fail in other cases. Whether the mechanism works or not in the particular deployment is somewhat dependent on the network topology and the technologies used.

Kea allows configuration of which of the supported methods should be used and in what order. This configuration may be considered a fine tuning of the DHCP deployment. In a typical deployment the default value of "any" is sufficient and there is no need to select specific methods. Changing the value of this parameter is the most useful in cases when an administrator wants to disable certain method, e.g. if the administrator trusts the network infrastructure more than the information provided by the clients themselves, the administrator may prefer information provided by the relays over that provided by the clients.

The configuration is controlled by the mac-sources parameter as follows:

"Dhcp6": {
    "mac-sources": [ "method1", "method2", "method3", ... ],

    "subnet6": [ ... ],

    ...
}

When not specified, a special value of "any" is used, which instructs the server to attempt to use all the methods in sequence and use value returned by the first one that succeeds. If specified, it has to have at least one value.

Supported methods are:

  • any - Not an actual method, just a keyword that instructs Kea to try all other methods and use the first one that succeeds. This is the default operation if no mac-sources are defined.
  • raw - In principle, a DHCPv6 server could use raw sockets to receive incoming traffic and extract MAC/hardware address information. This is currently not implemented for DHCPv6 and this value has no effect.
  • duid - DHCPv6 uses DUID identifiers instead of MAC addresses. There are currently four DUID types defined, with two of them (DUID-LLT, which is the default one and DUID-LL) convey MAC address information. Although RFC 8415 forbids it, it is possible to parse those DUIDs and extract necessary information from them. This method is not completely reliable, as clients may use other DUID types, namely DUID-EN or DUID-UUID.
  • ipv6-link-local - Another possible acquisition method comes from the source IPv6 address. In typical usage, clients are sending their packets from IPv6 link-local addresses. There is a good chance that those addresses are based on EUI-64, which contains MAC address. This method is not completely reliable, as clients may use other link-local address types. In particular, privacy extensions, defined in RFC 4941, do not use MAC addresses. Also note that successful extraction requires that the address's u-bit must be set to 1 and its g-bit set to 0, indicating that it is an interface identifier as per RFC 2373, section 2.5.1.
  • client-link-addr-option - One extension defined to alleviate missing MAC issues is client link-layer address option, defined in RFC 6939. This is an option that is inserted by a relay and contains information about client's MAC address. This method requires a relay agent that supports the option and is configured to insert it. This method is useless for directly connected clients. This parameter can also be specified as rfc6939, which is an alias for client-link-addr-option.
  • remote-id - RFC 4649 defines a remote-id option that is inserted by a relay agent. Depending on the relay agent configuration, the inserted option may convey the client's MAC address information. This parameter can also be specified as rfc4649, which is an alias for remote-id.
  • subscriber-id - Another option that is somewhat similar to the previous one is subscriber-id, defined in RFC 4580. It is, too, inserted by a relay agent that is configured to insert it. This parameter can also be specified as rfc4580, which is an alias for subscriber-id. This method is currently not implemented.
  • docsis-cmts - Yet another possible source of MAC address information are the DOCSIS options inserted by a CMTS that acts as a DHCPv6 relay agent in cable networks. This method attempts to extract MAC address information from suboption 1026 (cm mac) of the vendor specific option with vendor-id=4491. This vendor option is extracted from the relay-forward message, not the original client's message.
  • docsis-modem - Yet another possible source of MAC address information are the DOCSIS options inserted by the cable modem itself. This method attempts to extract MAC address information from suboption 36 (device id) of the vendor specific option with vendor-id=4491. This vendor option is extracted from the original client's message, not from any relay options.

Empty mac-sources is not allowed. If you do not want to specify it, either simply omit mac-sources definition or specify it with the "any" value which is the default.

9.11. Duplicate Addresses (DECLINE Support)

The DHCPv6 server is configured with a certain pool of addresses that it is expected to hand out to the DHCPv6 clients. It is assumed that the server is authoritative and has complete jurisdiction over those addresses. However, due to various reasons, such as misconfiguration or a faulty client implementation that retains its address beyond the valid lifetime, there may be devices connected that use those addresses without the server's approval or knowledge.

Such an unwelcome event can be detected by legitimate clients (using Duplicate Address Detection) and reported to the DHCPv6 server using a DECLINE message. The server will do a sanity check (if the client declining an address really was supposed to use it), then will conduct a clean up operation and confirm it by sending back a REPLY message. Any DNS entries related to that address will be removed, the fact will be logged and hooks will be triggered. After that is done, the address will be marked as declined (which indicates that it is used by an unknown entity and thus not available for assignment to anyone) and a probation time will be set on it. Unless otherwise configured, the probation period lasts 24 hours. After that period, the server will recover the lease (i.e. put it back into the available state) and the address will be available for assignment again. It should be noted that if the underlying issue of a misconfigured device is not resolved, the duplicate address scenario will repeat. On the other hand, it provides an opportunity to recover from such an event automatically, without any sysadmin intervention.

To configure the decline probation period to a value other than the default, the following syntax can be used:

  "Dhcp6": {
    "decline-probation-period": 3600,
    "subnet6": [ ... ],
    ...
}

The parameter is expressed in seconds, so the example above will instruct the server to recycle declined leases after an hour.

There are several statistics and hook points associated with the Decline handling procedure. The lease6_decline hook is triggered after the incoming Decline message has been sanitized and the server is about to decline the lease. The declined-addresses statistic is increased after the hook returns (both global and subnet specific variants). (See Section 8.8, “Statistics in the DHCPv4 Server” and Chapter 15, Hooks Libraries for more details on DHCPv4 statistics and Kea hook points.)

Once the probation time elapses, the declined lease is recovered using the standard expired lease reclamation procedure, with several additional steps. In particular, both declined-addresses statistics (global and subnet specific) are decreased. At the same time, reclaimed-declined-addresses statistics (again in two variants, global and subnet specific) are increased.

Note about statistics: The server does not decrease the assigned-addresses statistics when a DECLINE message is received and processed successfully. While technically a declined address is no longer assigned, the primary usage of the assigned-addresses statistic is to monitor pool utilization. Most people would forget to include declined-addresses in the calculation, and simply do assigned-addresses/total-addresses. This would have a bias towards under-representing pool utilization. As this has a potential for major issues, we decided not to decrease assigned addresses immediately after receiving Decline, but to do it later when we recover the address back to the available pool.

9.12. Statistics in the DHCPv6 Server

Note

This section describes DHCPv6-specific statistics. For a general overview and usage of statistics, see Chapter 16, Statistics.

The DHCPv6 server supports the following statistics:

Table 9.4. DHCPv6 Statistics

StatisticData TypeDescription
pkt6-receivedintegerNumber of DHCPv6 packets received. This includes all packets: valid, bogus, corrupted, rejected etc. This statistic is expected to grow rapidly.
pkt6-receive-dropintegerNumber of incoming packets that were dropped. The exact reason for dropping packets is logged, but the most common reasons may be: an unacceptable or not supported packet type, direct responses are forbidden, the server-id sent by the client does not match the server's server-id or the packet is malformed.
pkt6-parse-failedintegerNumber of incoming packets that could not be parsed. A non-zero value of this statistic indicates that the server received a malformed or truncated packet. This may indicate problems in your network, faulty clients, faulty relay agents or a bug in the server.
pkt6-solicit-receivedinteger Number of SOLICIT packets received. This statistic is expected to grow. Its increase means that clients that just booted started their configuration process and their initial packets reached your server.
pkt6-advertise-receivedinteger Number of ADVERTISE packets received. Advertise packets are sent by the server and the server is never expected to receive them. A non-zero value of this statistic indicates an error occurring in the network. One likely cause would be a misbehaving relay agent that incorrectly forwards ADVERTISE messages towards the server rather back to the clients.
pkt6-request-receivedintegerNumber of REQUEST packets received. This statistic is expected to grow. Its increase means that clients that just booted received the server's response (ADVERTISE), accepted it and are now requesting an address (REQUEST).
pkt6-reply-receivedintegerNumber of REPLY packets received. This statistic is expected to remain zero at all times, as REPLY packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards REPLY messages towards the server, rather back to the clients.
pkt6-renew-receivedintegerNumber of RENEW packets received. This statistic is expected to grow. Its increase means that clients received their addresses and prefixes and are trying to renew them.
pkt6-rebind-receivedintegerNumber of REBIND packets received. A non-zero value indicates that clients didn't receive responses to their RENEW messages (regular lease renewal mechanism) and are attempting to find any server that is able to take over their leases. It may mean that some server's REPLY messages never reached the clients.
pkt6-release-receivedintegerNumber of RELEASE packets received. This statistic is expected to grow when a device is being shut down in the network. It indicates that the address or prefix assigned is reported as no longer needed. Note that many devices, especially wireless, do not send RELEASE packets either because of design choice or due to the client moving out of range.
pkt6-decline-receivedinteger Number of DECLINE packets received. This statistic is expected to remain close to zero. Its increase means that a client leased an address, but discovered that the address is currently used by an unknown device in your network. If this statistic is growing, it may indicate a misconfigured server or devices that have statically assigned conflicting addresses.
pkt6-infrequest-receivedinteger Number of INFORMATION-REQUEST packets received. This statistic is expected to grow if there are devices that are using stateless DHCPv6. INFORMATION-REQUEST messages are used by clients that request stateless configuration, i.e. options and parameters other than addresses or prefixes.
pkt6-dhcpv4-query-receivedinteger Number of DHCPv4-QUERY packets received. This statistic is expected to grow if there are devices that are using DHCPv4-over-DHCPv6. DHCPv4-QUERY messages are used by DHCPv4 clients on an IPv6 only line which encapsulates the requests over DHCPv6.
pkt6-dhcpv4-response-receivedinteger Number of DHCPv4-RESPONSE packets received. This statistic is expected to remain zero at all times, as DHCPv4-RESPONSE packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPv4-RESPONSE message towards the server rather back to the clients.
pkt6-unknown-receivedintegerNumber of packets received of an unknown type. A non-zero value of this statistic indicates that the server received a packet that it wasn't able to recognize: either it had an unsupported type or was possibly malformed.
pkt6-sentintegerNumber of DHCPv6 packets sent. This statistic is expected to grow every time the server transmits a packet. In general, it should roughly match pkt6-received, as most incoming packets cause the server to respond. There are exceptions (e.g. server receiving a REQUEST with server-id matching other server), so do not worry, if it is lesser than pkt6-received.
pkt6-advertise-sentintegerNumber of ADVERTISE packets sent. This statistic is expected to grow in most cases after a SOLICIT is processed. There are certain uncommon, but valid cases where incoming SOLICIT is dropped, but in general this statistic is expected to be close to pkt6-solicit-received.
pkt6-reply-sentintegerNumber of REPLY packets sent. This statistic is expected to grow in most cases after a SOLICIT (with rapid-commit), REQUEST, RENEW, REBIND, RELEASE, DECLINE or INFORMATION-REQUEST is processed. There are certain cases where there is no response.
pkt6-dhcpv4-response-sentintegerNumber of DHCPv4-RESPONSE packets sent. This statistic is expected to grow in most cases after a DHCPv4-QUERY is processed. There are certain cases where there is no response.
subnet[id].total-nasinteger This statistic shows the total number of NA addresses available for DHCPv6 management for a given subnet. In other words, this is the sum of all addresses in all configured pools. This statistic changes only during configuration changes. Note that it does not take into account any addresses that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately and is reset during a reconfiguration event.
subnet[id].assigned-nasinteger This statistic shows the number of NA addresses in a given subnet that are assigned. This statistic increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately and is reset during a reconfiguration event.
subnet[id].total-pdsinteger This statistic shows the total number of PD prefixes available for DHCPv6 management for a given subnet. In other words, this is the sum of all prefixes in all configured pools. This statistic changes only during configuration changes. Note it does not take into account any prefixes that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately and is reset during a reconfiguration event.
subnet[id].assigned-pdsinteger This statistic shows the number of PD prefixes in a given subnet that are assigned. This statistic increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately and is reset during a reconfiguration event.
reclaimed-leasesinteger This statistic is the number of expired leases that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (it counts both NA and PD reclamations) and is reset when the server is reconfigured.
subnet[id].reclaimed-leasesintegerThis statistic is the number of expired leases associated with a given subnet ("id" is the subnet-id) that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (it counts both NA and PD reclamations) and is reset when the server is reconfigured.
declined-addressesinteger This statistic shows the number of IPv6 addresses that are currently declined and so counts the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased. Ideally, this statistic should be zero. If this statistic is non-zero (or worse, increasing), the network administrator should investigate if there is a misbehaving device in the network. This is a global statistic that covers all subnets.
subnet[id].declined-addressesinteger This statistic shows the number of IPv6 addresses that are currently declined in a given subnet. This statistic counts the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased. Ideally, this statistic should be zero. If this statistic is non-zero (or worse, increasing), a network administrator should investigate if there is a misbehaving device in the network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
reclaimed-declined-addressesinteger This statistic shows the number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long term indicator of how many actual valid Declines were processed and recovered from. This is a global statistic that covers all subnets.
subnet[id].reclaimed-declined-addressesinteger This statistic shows the number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long term indicator of how many actual valid Declines were processed and recovered from. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.

9.13. Management API for the DHCPv6 Server

The management API allows the issuing of specific management commands, such as statistics retrieval, reconfiguration or shutdown. For more details, see Chapter 17, Management API. Currently the only supported communication channel type is UNIX stream socket. By default there are no sockets open. To instruct Kea to open a socket, the following entry in the configuration file can be used:

"Dhcp6": {
    "control-socket": {
        "socket-type": "unix",
        "socket-name": "/path/to/the/unix/socket"
    },

    "subnet6": [
        ...
    ],
    ...
}

The length of the path specified by the socket-name parameter is restricted by the maximum length for the unix socket name on your operating system, i.e. the size of the sun_path field in the sockaddr_un structure, decreased by 1. This value varies on different operating systems between 91 and 107 characters. Typical values are 107 on Linux and 103 on FreeBSD.

Communication over control channel is conducted using JSON structures. See the Control Channel section in the Kea Developer's Guide for more details.

The DHCPv6 server supports the following operational commands:

  • build-report
  • config-get
  • config-reload
  • config-set
  • config-test
  • config-write
  • dhcp-disable
  • dhcp-enable
  • leases-reclaim
  • list-commands
  • shutdown
  • version-get

as described in Section 17.3, “Commands Supported by Both the DHCPv4 and DHCPv6 Servers”. In addition, it supports the following statistics related commands:

  • statistic-get
  • statistic-reset
  • statistic-remove
  • statistic-get-all
  • statistic-reset-all
  • statistic-remove-all

as described here Section 16.3, “Commands for Manipulating Statistics”.

9.14. User contexts in IPv6

Kea allows loading hook libraries that sometimes could benefit from additional parameters. If such a parameter is specific to the whole library, it is typically defined as a parameter for the hook library. However, sometimes there is a need to specify parameters that are different for each pool.

User contexts can store arbitrary data as long as it is valid JSON syntax and its top level element is a map (i.e. the data must be enclosed in curly brackets). Some hook libraries may expect specific formatting, though. Please consult specific hook library documentation for details.

User contexts can be specified at either global scope, shared network, subnet, pool, client class, option data or definition level, and host reservation. One other useful usage is the ability to store comments or descriptions.

Let's consider a lightweight 4over6 deployment as an example. It is an IPv6 transition technology that allows mapping IPv6 prefix into full or parts of IPv4 addresses. In DHCP context, these are certain parameters that are supposed to be delivered to clients in form of additional options. Values of those options are correlated to delegated prefixes, so it is reasonable to keep those parameters together with the PD pool. On the other hand, lightweight 4over6 is not a commonly used feature, so it is not a part of the base Kea code. The solution to this problem is to use user context. For each PD pool that is expected to be used for lightweight 4over6, user context with extra parameters is defined. Those extra parameters will be used by hook library that would be loaded only when dynamic calculation of the lightweight 4over6 option is actually needed. An example configuration looks as follows:

"Dhcp6": {
    "subnet6": [ {
        "pd-pools": [
        {
            "prefix":  "2001:db8::",
            "prefix-len": 56,
            "delegated-len": 64,

            // This is a pool specific context.
            "user-context": {
                "threshold-percent": 85,
                "v4-network": "192.168.0.0/16",
                "v4-overflow": "10.0.0.0/16",
                "lw4over6-sharing-ratio": 64,
                "lw4over6-v4-pool": "192.0.2.0/24",
                "lw4over6-sysports-exclude": true,
                "lw4over6-bind-prefix-len": 56
            }
        } ],
        "subnet": "2001:db8::/32",

        // This is a subnet specific context. You can put any type of
        // information here as long as it is a valid JSON.
        "user-context": {
            "comment": "Those v4-v6 migration technologies are tricky.",
            "experimental": true,
            "billing-department": 42,
            "contact-points": [ "Alice", "Bob" ]
        }
    } ],
    ...
}

Kea does not interpret or use the content of the user context: it just stores it, making it available to the hook libraries. It is up to each hook library to extract the information and make use of it. The parser translates a "comment" entry into a user-context with the entry, this allows to attach a comment inside the configuration itself.

For more background information, see Section 15.5, “User contexts”.

9.15. Supported DHCPv6 Standards

The following standards are currently supported:

  • Dynamic Host Configuration Protocol for IPv6, RFC 3315: Supported messages are SOLICIT, ADVERTISE, REQUEST, RELEASE, RENEW, REBIND, INFORMATION-REQUEST, CONFIRM and REPLY.
  • IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6, RFC 3633: Supported options are IA_PD and IA_PREFIX. Also supported is the status code NoPrefixAvail.
  • DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 3646: Supported option is DNS_SERVERS.
  • The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Relay Agent Remote-ID Option, RFC 4649: REMOTE-ID option is supported.
  • The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option, RFC 4704: Supported option is CLIENT_FQDN.
  • Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite, RFC 6334: the AFTR-Name DHCPv6 Option is supported.
  • Relay-Supplied DHCP Options, RFC 6422: Full functionality is supported: OPTION_RSOO, ability of the server to echo back the options, checks whether an option is RSOO-enabled, ability to mark additional options as RSOO-enabled.
  • Prefix Exclude Option for DHCPv6-based Prefix Delegation, RFC 6603: Prefix Exclude option is supported.
  • Client Link-Layer Address Option in DHCPv6, RFC 6939: Supported option is client link-layer address option.
  • Issues and Recommendations with Multiple Stateful DHCPv6 Options, RFC 7550: All recommendations related to the DHCPv6 server operation are supported.
  • DHCPv6 Options for Configuration of Softwire Address and Port-Mapped Clients, RFC 7598: All options specified in this specification are supported by the DHCPv6 server.
  • Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 8415: New DHCPv6 protocol specification which obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283 and RFC 7550

9.16. DHCPv6 Server Limitations

These are the current limitations of the DHCPv6 server software. Most of them are reflections of the early stage of development and should be treated as not implemented yet, rather than actual limitations.

  • The server will allocate, renew or rebind a maximum of one lease for a particular IA option (IA_NA or IA_PD) sent by a client. RFC 8415 allows for multiple addresses or prefixes to be allocated for a single IA.
  • Temporary addresses are not supported.
  • Client reconfiguration (RECONFIGURE) is not yet supported.

9.17. Kea DHCPv6 server examples

A collection of simple to use examples for DHCPv6 component of Kea is available with the sources. It is located in doc/examples/kea6 directory. At the time of writing this text there were 18 examples, but the number is growing slowly with each release.

Chapter 10. Lease Expiration in DHCPv4 and DHCPv6

The primary role of the DHCP server is to assign addresses and/or delegate prefixes to DHCP clients. These addresses and prefixes are often referred to as "leases". Leases are typically assigned to clients for a finite amount of time, known as the "valid lifetime". DHCP clients who wish to continue using their assigned leases, will periodically renew them by sending the appropriate message to the DHCP server. The DHCP server records the time when these leases are renewed and calculates new expiration times for them.

If the client does not renew a lease before its valid lifetime elapses, the lease is considered expired. There are many situations when the client may cease lease renewals. A common scenario is when the machine running the client shuts down for an extended period of time.

The process through which the DHCP server makes expired leases available for reassignment is referred to as "lease reclamation" and expired leases returned to availability through this process are referred to as "reclaimed". The DHCP server attempts to reclaim an expired lease as soon as it detects that it has expired. One way in which the server detects expiration occurs when it is trying to allocate a lease to a client and finds this lease already present in the database but expired. Another way is by periodically querying the lease database for them. Regardless of how an expired lease is detected, before it may assigned to a client, it must be reclaimed.

This chapter explains how to configure the server to periodically query for the expired leases and how to minimize the impact of the periodic lease reclamation process on the server's responsiveness. Finally, it explains "lease affinity", which provides the means to assign the same lease to a returning client after its lease has expired.

Although, all configuration examples in this section are provided for the DHCPv4 server, the same parameters may be used for the DHCPv6 server configuration.

10.1. Lease Reclamation

Lease reclamation is the process through which an expired lease becomes available for assignment to the same or different client. This process involves the following steps for each reclaimed lease:

  • Invoke callouts for the lease4_expire or lease6_expire hook points if hook libraries supporting those callouts are currently loaded.
  • Update DNS, i.e. remove any DNS entries associated with the expired lease.
  • Update lease information in the lease database to indicate that the lease is now available for re-assignment.
  • Update counters on the server, which includes increasing the number of reclaimed leases and decreasing the number of assigned addresses or delegated prefixes.

Please refer to Chapter 12, The DHCP-DDNS Server to see how to configure DNS updates in Kea, and to Chapter 15, Hooks Libraries for information about using hooks libraries.

10.2. Configuring Lease Reclamation

Kea can be configured to periodically detect and reclaim expired leases. During this process the lease entries in the database are modified or removed. While this is happening the server will not process incoming DHCP messages to avoid issues with concurrent access to database information. As a result, the server will be unresponsive while lease reclamation is performed and DHCP queries will accumulate; responses will be sent once the leases reclamation cycle is complete.

In deployments where response time is critical, administrators may wish to minimize the interruptions in service caused by lease reclamation. Toward this end, Kea provides configuration parameters to control: the frequency of lease reclamation cycles, the maximum number of leases processed in a single reclamation cycle, and the maximum amount of time a single reclamation cycle is allowed to run before being interrupted. The following examples demonstrate how these parameters can be used:

"Dhcp4": {
    ...

    "expired-leases-processing": {
        "reclaim-timer-wait-time": 5,
        "max-reclaim-leases": 0,
        "max-reclaim-time": 0,
        "flush-reclaimed-timer-wait-time": 0,
    },

    ...
}

The first parameter is expressed in seconds and specifies an interval between the two consecutive lease reclamation cycles. This is explained by the following diagram.


|  c1  |            | c2 |            |c3|            | c4 |
|<---->|<---------->|<-->|<---------->|<>|<---------->|<-->|
---------------------------------------------------------------->
|      |     5s     |    |     5s     |  |     5s     |    | time

This diagram shows four lease reclamation cycles (c1 through c4) of variable duration. Note that the duration of the reclamation cycle depends on the number of expired leases detected and processed in the particular cycle. This duration is also usually significantly shorter than the interval between the cycles.

According to the reclaim-timer-wait-time the server keeps fixed intervals of five seconds between the end of one cycle and the start of the next cycle. This guarantees the presence of 5s long periods during which the server remains responsive to DHCP queries and does not perform lease reclamation. The max-reclaim-leases and max-reclaim-time are set to 0, which sets no restriction on the maximum number of leases reclaimed in the particular cycle, or on the maximum duration of each cycle.

In deployments with high lease pool utilization, relatively short valid lifetimes, and frequently disconnecting clients which allow leases to expire, the number of expired leases requiring reclamation at any given time may rise significantly. In this case it is often desirable to apply restrictions on the maximum duration of a reclamation cycle or the maximum number of leases reclaimed in a cycle. The following configuration demonstrates how this can be done:

"Dhcp4": {
    ...

    "expired-leases-processing": {
        "reclaim-timer-wait-time": 3,
        "max-reclaim-leases": 100,
        "max-reclaim-time": 50,
        "unwarned-reclaim-cycles": 10,
    },

    ...
}

The max-reclaim-leases parameter limits the number of leases reclaimed in a single cycle to 100. The max-reclaim-time limits the maximum duration of each cycle to 50ms. The lease reclamation cycle will be interrupted if either of these limitations is reached. The reclamation of all unreclaimed leases will be attempted in subsequent cycles.

The following diagram illustrates the behavior of the system in the presence of many expired leases, when the limits are applied for the reclamation cycles.


| c1 |                | c2 |                | c3 |                | c4 |
|<-->|<-------------->|<-->|<-------------->|<-->|<-------------->|<-->|<--
------------------------------------------------------------------------------>
|50ms|       3s       |50ms|       3s       |50ms|       3s       |50ms|  time

The diagram demonstrates the case when each reclamation cycle would take more than 50ms, and thus is interrupted according to the value of the max-reclaim-time. This results in equal durations of all reclamation cycles over time. Note that in this example the limitation of maximum 100 leases is not reached. This may be the case when database transactions are slow or callouts in the hook libraries attached to the server are slow. Regardless, the choosing values for either the maximum number of leases or a maximum cycle time strongly depends on the particular deployment, lease database backend being used, and any hooks libraries etc. Administrators may need to experiment to tune the system to suit the dynamics of their deployment.

It is important to realize that with the use of these limits, there is a risk that expired leases will accumulate faster than the server can reclaim them. This should not be the problem if the server is dealing with a temporary burst of expirations, because it should be able to eventually deal with them over time. However, if leases expire at a high rate for a longer period of time, the unreclaimed leases will pile up in the database. In order to notify the administrator that the current configuration does not satisfy the needs for reclamation of expired leases, the server issues a warning message in the log if it was unable to reclaim all leases within the last couple of reclamation cycles. The number of cycles after which such warning is issued is specified with the unwarned-reclaim-cycles configuration parameter.

Setting the reclaim-timer-wait-time to 0 disables periodic reclamation of the expired leases.

10.3. Configuring Lease Affinity

Suppose that a laptop goes to a sleep mode after a period of user inactivity. While the laptop is in sleep mode, its DHCP client will not renew leases obtained from the server and these leases will eventually expire. When the laptop wakes up, it is often desirable for it to continue using its previous assigned IP addresses. In order to facilitate this, the server needs to correlate returning clients with their expired leases When the client returns, the server will first check for those leases and re-assign them if they have not been assigned to another client. The ability of the server to re-assign the same lease to a returning client is referred to as "lease affinity".

When lease affinity is enabled, the server will still reclaim leases according to the parameters described in Section 10.2, “Configuring Lease Reclamation”, but the reclaimed leases will be held in the database (rather than removed) for the specified amount of time. When the client returns, the server will first check if there are any reclaimed leases associated with this client and re-assign them if possible. However, it is important to note that any reclaimed lease may be assigned to another client if that client specifically asks for it. Therefore, the lease affinity does not guarantee that the reclaimed lease will be available for the client who used it before; it merely increases the chances for the client to be assigned the same lease. If the lease pool is small (this mostly applies to DHCPv4 for which address space is small), there is an increased likelihood that the expired lease will be assigned to another client.

Consider the following configuration:

"Dhcp4": {
    ...

    "expired-leases-processing": {
        "reclaim-timer-wait-time": 3,
        "hold-reclaimed-time": 1800,
        "flush-reclaimed-timer-wait-time": 5
    },

    ...
}

The hold-reclaim-time specifies how many seconds after an expiration a reclaimed lease should be held in the database for re-assignment to the same client. In the example given above, reclaimed leases will be held for 30 minutes (1800s) after their expiration. During this time, the server will likely be able to re-assign the same lease to the returning client, unless another client requests this lease and the server assigns it.

The server must periodically remove reclaimed leases for which the time indicated by hold-reclaim-time has elapsed. The flush-reclaimed-timer-wait-time controls how often the server removes such leases. In the example provided above, the server will initiate removal of such leases 5 seconds after the previous removal attempt was completed. Setting this value to 0 disables lease affinity, in which case leases will be removed from the lease database when they are reclaimed. If lease affinity is enabled, it is recommended that hold-reclaim-time be set to a value significantly higher than the reclaim-timer-wait-time, as timely removal of expired-reclaimed leases is less critical while the removal process may impact server responsiveness.

10.4. Default Configuration Values for Leases Reclamation

The following list presents all configuration parameters pertaining to processing expired leases with their default values:

  • reclaim-timer-wait-time = 10 [seconds]
  • flush-reclaimed-timer-wait-time = 25 [seconds]
  • hold-reclaimed-time = 3600 [seconds]
  • max-reclaim-leases = 100
  • max-reclaim-time = 250 [milliseconds]
  • unwarned-reclaim-cycles = 5

The default value for any parameter is used when this parameter not explicitly specified in the configuration. Also, the expired-leases-processing map may be omitted entirely in the configuration, in which case the default values are used for all parameters listed above.

10.5. Reclaiming Expired Leases with Command

The leases-reclaim command can be used to trigger leases reclamation at any time. Please consult the Section 17.3.6, “leases-reclaim” for the details about using this command.

Chapter 11. Congestion Handling in DHCPv4 and DHCPv6

11.1. What is Congestion?

Congestion occurs when servers are subjected to client queries faster than they can be fulfilled. Subsequently, the servers begin accumulating a backlog of pending queries. The longer the high rate of traffic continues the farther behind the servers fall. Depending on the client implementations, those that fail to get leases either give up or simply continue to retry forever. In the former case, the server may eventually recover. The latter case is vicious cycle from which the server is unable escape.

In a well-planned deployment, the number and capacity of servers is matched to the maximum client loads expected. As long as capacity is matched to load, congestion does not occur. If the load is routinely too heavy, then the deployment needs to be re-evaluated. Congestion typically occurs when there is a network event that causes overly large numbers of clients to simultaneously need leases such as recovery after a network outage.

The goal of Congestion handling is to help the servers mitigate the peak in traffic by fulfilling as many of the most relevant requests as possible until it subsides.

Prior to Kea 1.5, kea-dhcp4 and kea-dhcp4 read inbound packets directly from the interface sockets in the main application thread. This meant that packets waiting to be processed were held in socket buffers themselves. Once these buffers fill any new packets are discarded. Under swamped conditions the servers can end up processing client packets that may no longer be relevant, or worse are redundant. In other words, the packets waiting in the FIFO socket buffers become increasingly stale.

11.2. Configuring Congestion Handling

Kea 1.5 introduces a new feature referred to as Congestion Handling. Congestion handling offers the ability to configure the server to use a separate thread to read packets from the interface socket buffers. As the thread reads packets from the buffers they are added to an internal packet queue. The server's main application thread process packets from this queue rather than the socket buffers. By structuring it this way, we've introduced a configurable layer which can make decisions on which packets to process, how to store them, and the order in which they are processed by the server.

The default packet queue implemenation for both kea-dhcp4 and kea-dhcp6 is a simple ring buffer. Once it reaches capacity, new packets get added to the back of queue by discarding packets from the front of queue. Rather than always discarding the newest packets, we now always discard the oldest packets. The capacity of the buffer, (i.e the maximum number of packets the buffer can contain) is configurable. A reasonable starting point would be to match the capacity to the number of leases per second your installation of Kea can handle. Please note this figure varies widely depending on the specifics of your deployment. We anticipate adding more knobs as we learn from experience, testing, and user feedback.

As there is no one algorithm that will best handle the dynamics of all sites, and because over time new approaches will evolve, the packet queue is implemented as plug-in, which can replaced by a custom queue implementation via hook library. This should make it straight forward for interested parties to experiment with their own solutions. (Developers may refer to isc::dhcp::PacketQueue and isc::dhcp::PacketQueueMgr, in our Developer's guide).

Packet queue behavior is configured in both kea-dhcp4 and kea-dhcp6 servers through an optional, top level configuration element, 'dhcp-queue-control' (Omitting this element disables packet queueing):

   "dhcp-queue-control": {
       "enable-queue": true|false,
       "queue-type": "queue type",
       "capacity" : n
   }

where:

  • enable-queue true|false. Enables or disables packet queueing. When true, the server will process packets from the packet queue, which is filled by a separate thread. When false, the server will process packets directly from the socket buffers in the main thread (as done in all releases prior Kea 1.5). It is disabled by default.
  • queue-type name of the queue implementation to use. This value exists such that custom implementations can be registered (via hook lib) and then selected. There is a default packet queue implementation that is pre-registered during server start up: "kea-ring4" for kea-dhcp4 and "kea-ring6" for kea-dhcp6.
  • capacity = n [packets]. This is the maximum number of packets the packet queue can hold before packets are discarded. The optimal value for this is extremely site dependent. The default value for is 500 for both kea-ring4 and kea-ring6.

The following example enables the default packet queue for kea-dhcp4, with a queue capacity of 250 packets:

"Dhcp4":
{
    ...
   "dhcp-queue-control": {
       "enable-queue": true,
       "queue-type": "kea-ring4",
       "capacity" : 250
    },
    ...
}

The following example enables the default packet queue for kea-dhcp6, with a queue capacity of 300 packets:

"Dhcp6":
{
    ...
   "dhcp-queue-control": {
       "enable-queue": true,
       "queue-type": "kea-ring6",
       "capacity" : 300
    },
    ...
}

The number of parameters and plug-ins is expected to grow over time.

Chapter 12. The DHCP-DDNS Server

12.1. Overview

The DHCP-DDNS Server (kea-dhcp-ddns, known informally as D2) conducts the client side of the DDNS protocol (defined in RFC 2136) on behalf of the DHCPv4 and DHCPv6 servers (kea-dhcp4 and kea-dhcp6 respectively). The DHCP servers construct DDNS update requests, known as NameChangeRequests (NCRs), based upon DHCP lease change events and then post these to D2. D2 attempts to match each such request to the appropriate DNS server(s) and carry out the necessary conversation with those servers to update the DNS data.

12.1.1. DNS Server selection

In order to match a request to the appropriate DNS servers, D2 must have a catalog of servers from which to select. In fact, D2 has two such catalogs, one for forward DNS and one for reverse DNS; these catalogs are referred to as DDNS Domain Lists. Each list consists of one or more named DDNS Domains. Further, each DDNS Domain has a list of one or more DNS servers that publish the DNS data for that domain.

When conducting forward domain matching, D2 will compare the FQDN in the request against the name of each forward DDNS Domain. The domain whose name matches the longest portion of the FQDN is considered the best match. For example, if the FQDN is "myhost.sample.example.com.", and there are two forward domains in the catalog: "sample.example.com." and "example.com.", the former is regarded as the best match. In some cases, it may not be possible to find a suitable match. Given the same two forward domains there would be no match for the FQDN, "bogus.net", so the request would be rejected. Finally, if there are no forward DDNS Domains defined, D2 will simply disregard the forward update portion of requests.

When conducting reverse domain matching, D2 constructs a reverse FQDN from the lease address in the request and compare that against the name of each reverse DDNS Domain. Again, the domain whose name matches the longest portion of the FQDN is considered the best match. For instance, if the lease address is "172.16.1.40" and there are two reverse domains in the catalog: "1.16.172.in-addr.arpa." and "16.172.in-addr.arpa", the former is the best match. As with forward matching, it is possible to not find a suitable match. Given the same two domains, there would be no match for the lease address, "192.168.1.50", and the request would be rejected. Finally, if there are no reverse DDNS Domains defined, D2 will simply disregard the reverse update portion of requests.

12.1.2. Conflict Resolution

D2 implements the conflict resolution strategy prescribed by RFC 4703. Conflict resolution is intended to prevent different clients from mapping to the same FQDN at the same time. To make this possible, the RFC requires that forward DNS entries for a given FQDN must be accompanied by a DHCID resource record (RR). This record contains a client identifier that uniquely identifies the client to whom the name belongs. Furthermore, any DNS updater who wishes to update or remove existing forward entries for an FQDN may only do so if their client matches that of the DHCID RR.

In other words, the DHCID RR maps an FQDN to the client to whom it belongs and thereafter only changes to that mapping should only be done by or at the behest of that client.

Currently, conflict detection is always performed. Future releases may offer alternative behavior.

12.1.3. Dual Stack Environments

RFC 4703, sec. 5.2, describes issues that may arise with dual stack clients. These are clients that wish to have have both IPv4 and IPv6 mappings for the same FQDN. In order for this work properly, such clients are required to embed ther IPv6 DUID within their IPv4 client identifier option as described in RFC 4703. In this way, DNS updates for both IPv4 and IPv6 can be managed under the same DHCID RR. Support for this does not yet exist in Kea but is called for in the ticket, http://oldkea.isc.org/ticket/4519, which we hope to include in a future release.

12.2. Starting and Stopping the DHCP-DDNS Server

kea-dhcp-ddns is the Kea DHCP-DDNS server and, due to the nature of DDNS, it is run alongside either the DHCPv4 or DHCPv6 components (or both). Like other parts of Kea, it is a separate binary that can be run on its own or through keactrl (see Chapter 6, Managing Kea with keactrl). In normal operation, controlling kea-dhcp-ddns with keactrl is recommended. However, it is also possible to run the DHCP-DDNS server directly. It accepts the following command-line switches:

  • -c file - specifies the configuration file. This is the only mandatory switch.
  • -d - specifies whether the server logging should be switched to debug/verbose mode. In verbose mode, the logging severity and debuglevel specified in the configuration file are ignored and "debug" severity and the maximum debuglevel (99) are assumed. The flag is convenient, for temporarily switching the server into maximum verbosity, e.g. when debugging.
  • -v - prints out Kea version and exits.
  • -W - prints out the Kea configuration report and exits. The report is a copy of the config.report file produced by ./configure: it is embedded in the executable binary.
  • -t file specifies the configuration file to be tested. Kea-dhcp-ddns will attempt to load it, and will conduct sanity checks. Note that certain checks are possible only while running the actual server. The actual status is reported with exit code (0 = configuration looks ok, 1 = error encountered). Kea will print out log messages to standard output and error to standard error when testing configuration.

The config.report may also be accessed more directly. The following command may be used to extract this information. The binary path may be found in the install directory or in the .libs subdirectory in the source tree. For example kea/src/bin/d2/.libs/kea-dhcp-ddns.

strings path/kea-dhcp-ddns | sed -n 's/;;;; //p'

Upon start up the module will load its configuration and begin listening for NCRs based on that configuration.

During startup the server will attempt to create a PID file of the form: [localstatedir]/[conf name].kea-dhcp-ddns.pid where:

  • localstatedir: The value as passed into the build configure script. It defaults to "/usr/local/var". Note that this value may be overridden at run time by setting the environment variable KEA_PIDFILE_DIR. This is intended primarily for testing purposes.
  • conf name: The configuration file name used to start the server, minus all preceding path and file extension. For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the portion used would be "myconf".

If the file already exists and contains the PID of a live process, the server will issue a DHCP_DDNS_ALREADY_RUNNING log message and exit. It is possible, though unlikely, that the file is a remnant of a system crash and the process to which the PID belongs is unrelated to Kea. In such a case it would be necessary to manually delete the PID file.

12.3. Configuring the DHCP-DDNS Server

Before starting kea-dhcp-ddns module for the first time, a configuration file needs to be created. The following default configuration is a template that can be customized to your requirements.

"DhcpDdns": {
    "ip-address": "127.0.0.1",
    "port": 53001,
    "dns-server-timeout": 100,
    "ncr-protocol": "UDP",
    "ncr-format": "JSON",
    "tsig-keys": [ ],
    "forward-ddns": {
        "ddns-domains": [ ]
    },
    "reverse-ddns": {
        "ddns-domains": [ ]
    }
}

The configuration can be divided as follows, each of which is described in its own section:

  • Global Server Parameters - values which control connectivity and global server behavior
  • TSIG Key Info - defines the TSIG keys used for secure traffic with DNS servers
  • Forward DDNS - defines the catalog of Forward DDNS Domains
  • Reverse DDNS - defines the catalog of Forward DDNS Domains

12.3.1. Global Server Parameters

  • ip-address - IP address on which D2 listens for requests. The default is the local loopback interface at address 127.0.0.1. You may specify either an IPv4 or IPv6 address.
  • port - Port on which D2 listens for requests. The default value is 53001.
  • dns-server-timeout - The maximum amount of time in milliseconds, that D2 will wait for a response from a DNS server to a single DNS update message.
  • ncr-protocol - Socket protocol to use when sending requests to D2. Currently only UDP is supported. TCP may be available in a future release.
  • ncr-format - Packet format to use when sending requests to D2. Currently only JSON format is supported. Other formats may be available in future releases.

D2 must listen for change requests on a known address and port. By default it listens at 127.0.0.1 on port 53001. The following example illustrates how to change D2's global parameters so it will listen at 192.168.1.10 port 900:

"DhcpDdns": {
    "ip-address": "192.168.1.10",
    "port": 900,
    ...
    }
}

Warning

It is possible for a malicious attacker to send bogus NameChangeRequests to the DHCP-DDNS server. Addresses other than the IPv4 or IPv6 loopback addresses (127.0.0.1 or ::1) should only be used for testing purposes, but note that local users may still communicate with the DHCP-DDNS server. A future version of Kea will implement authentication to guard against such attacks.

Note

If the ip-address and port are changed, it will be necessary to change the corresponding values in the DHCP servers' "dhcp-ddns" configuration section.

12.3.2. TSIG Key List

A DDNS protocol exchange can be conducted with or without TSIG (defined in RFC 2845). This configuration section allows the administrator to define the set of TSIG keys that may be used in such exchanges.

To use TSIG when updating entries in a DNS Domain, a key must be defined in the TSIG Key List and referenced by name in that domain's configuration entry. When D2 matches a change request to a domain, it checks whether the domain has a TSIG key associated with it. If so, D2 will use that key to sign DNS update messages sent to and verify responses received from the domain's DNS server(s). For each TSIG key required by the DNS servers that D2 will be working with there must be a corresponding TSIG key in the TSIG Key list.

As one might gather from the name, the tsig-key section of the D2 configuration lists the TSIG keys. Each entry describes a TSIG key used by one or more DNS servers to authenticate requests and sign responses. Every entry in the list has three parameters:

  • name - a unique text label used to identify this key within the list. This value is used to specify which key (if any) should be used when updating a specific domain. So long as it is unique its content is arbitrary, although for clarity and ease of maintenance it is recommended that it match the name used on the DNS server(s). It cannot be blank.
  • algorithm - specifies which hashing algorithm should be used with this key. This value must specify the same algorithm used for the key on the DNS server(s). The supported algorithms are listed below:
    • HMAC-MD5
    • HMAC-SHA1
    • HMAC-SHA224
    • HMAC-SHA256
    • HMAC-SHA384
    • HMAC-SHA512
    This value is not case sensitive.
  • digest-bits - is used to specify the minimum truncated length in bits. The default value 0 means truncation is forbidden, non-zero values must be an integral number of octets, be greater than 80 and the half of the full length. Note in BIND9 this parameter is appended after a dash to the algorithm name.
  • secret - is used to specify the shared secret key code for this key. This value is case sensitive and must exactly match the value specified on the DNS server(s). It is a base64-encoded text value.

As an example, suppose that a domain D2 will be updating is maintained by a BIND9 DNS server which requires dynamic updates to be secured with TSIG. Suppose further that the entry for the TSIG key in BIND9's named.conf file looks like this:

   :
   key "key.four.example.com." {
       algorithm hmac-sha224;
       secret "bZEG7Ow8OgAUPfLWV3aAUQ==";
   };
   :

By default, the TSIG Key list is empty:

"DhcpDdns": {
   "tsig-keys": [ ],
   ...
}

We must extend the list with a new key:

"DhcpDdns": {
    "tsig-keys": [
        {
            "name": "key.four.example.com.",
            "algorithm": "HMAC-SHA224",
            "secret": "bZEG7Ow8OgAUPfLWV3aAUQ=="
        }
    ],
    ...
}

These steps would be repeated for each TSIG key needed. Note that the same TSIG key can be used with more than one domain.

12.3.3. Forward DDNS

The Forward DDNS section is used to configure D2's forward update behavior. Currently it contains a single parameter, the catalog of forward DDNS Domains, which is a list of structures.

"DhcpDdns": {
    "forward-ddns": {
        "ddns-domains": [ ]
    },
    ...
}

By default, this list is empty, which will cause the server to ignore the forward update portions of requests.

12.3.3.1. Adding Forward DDNS Domains

A forward DDNS Domain maps a forward DNS zone to a set of DNS servers which maintain the forward DNS data (i.e. name to address mapping) for that zone. You will need one forward DDNS Domain for each zone you wish to service. It may very well be that some or all of your zones are maintained by the same servers. You will still need one DDNS Domain per zone. Remember that matching a request to the appropriate server(s) is done by zone and a DDNS Domain only defines a single zone.

This section describes how to add Forward DDNS Domains. Repeat these steps for each Forward DDNS Domain desired. Each Forward DDNS Domain has the following parameters:

  • name - The fully qualified domain name (or zone) that this DDNS Domain can update. This is value used to compare against the request FQDN during forward matching. It must be unique within the catalog.
  • key-name - If TSIG is used with this domain's servers, this value should be the name of the key from within the TSIG Key List to use. If the value is blank (the default), TSIG will not be used in DDNS conversations with this domain's servers.
  • dns-servers - A list of one or more DNS servers which can conduct the server side of the DDNS protocol for this domain. The servers are used in a first to last preference. In other words, when D2 begins to process a request for this domain it will pick the first server in this list and attempt to communicate with it. If that attempt fails, it will move to next one in the list and so on until the it achieves success or the list is exhausted.

To create a new forward DDNS Domain, one must add a new domain element and set its parameters:

"DhcpDdns": {
    "forward-ddns": {
        "ddns-domains": [
            {
                "name": "other.example.com.",
                "key-name": "",
                "dns-servers": [
                ]
            }
        ]
    }
}

It is permissible to add a domain without any servers. If that domain should be matched to a request, however, the request will fail. In order to make the domain useful though, we must add at least one DNS server to it.

12.3.3.1.1. Adding Forward DNS Servers

This section describes how to add DNS servers to a Forward DDNS Domain. Repeat them for as many servers as desired for a each domain.

Forward DNS Server entries represent actual DNS servers which support the server side of the DDNS protocol. Each Forward DNS Server has the following parameters:

  • hostname - The resolvable host name of the DNS server. This value is not yet implemented.
  • ip-address - The IP address at which the server listens for DDNS requests. This may be either an IPv4 or an IPv6 address.
  • port - The port on which the server listens for DDNS requests. It defaults to the standard DNS service port of 53.

To create a new forward DNS Server, one must add a new server element to the domain and fill in its parameters. If for example the service is running at "172.88.99.10", then set it as follows:

"DhcpDdns": {
    "forward-ddns": {
        "ddns-domains": [
            {
                "name": "other.example.com.",
                "key-name": "",
                "dns-servers": [
                    {
                        "hostname": "",
                        "ip-address": "172.88.99.10",
                        "port": 53
                    }
                ]
            }
        ]
    }
}

Note

As stated earlier, "hostname" is not yet supported so, the parameter "ip-address" must be set to the address of the DNS server.

12.3.4. Reverse DDNS

The Reverse DDNS section is used to configure D2's reverse update behavior, and the concepts are the same as for the forward DDNS section. Currently it contains a single parameter, the catalog of reverse DDNS Domains, which is a list of structures.

"DhcpDdns": {
    "reverse-ddns": {
        "ddns-domains": [ ]
    }
    ...
}

By default, this list is empty, which will cause the server to ignore the reverse update portions of requests.

12.3.4.1. Adding Reverse DDNS Domains

A reverse DDNS Domain maps a reverse DNS zone to a set of DNS servers which maintain the reverse DNS data (address to name mapping) for that zone. You will need one reverse DDNS Domain for each zone you wish to service. It may very well be that some or all of your zones are maintained by the same servers; even then, you will still need one DDNS Domain entry for each zone. Remember that matching a request to the appropriate server(s) is done by zone and a DDNS Domain only defines a single zone.

This section describes how to add Reverse DDNS Domains. Repeat these steps for each Reverse DDNS Domain desired. Each Reverse DDNS Domain has the following parameters:

  • name - The fully qualified reverse zone that this DDNS Domain can update. This is the value used during reverse matching which will compare it with a reversed version of the request's lease address. The zone name should follow the appropriate standards: for example, to to support the IPv4 subnet 172.16.1, the name should be. "1.16.172.in-addr.arpa.". Similarly, to support an IPv6 subnet of 2001:db8:1, the name should be "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa." Whatever the name, it must be unique within the catalog.
  • key-name - If TSIG should be used with this domain's servers, then this value should be the name of that key from the TSIG Key List. If the value is blank (the default), TSIG will not be used in DDNS conversations with this domain's servers. Currently this value is not used as TSIG has not been implemented.
  • dns-servers - a list of one or more DNS servers which can conduct the server side of the DDNS protocol for this domain. Currently the servers are used in a first to last preference. In other words, when D2 begins to process a request for this domain it will pick the first server in this list and attempt to communicate with it. If that attempt fails, it will move to next one in the list and so on until the it achieves success or the list is exhausted.

To create a new reverse DDNS Domain, one must add a new domain element and set its parameters. For example, to support subnet 2001:db8:1::, the following configuration could be used:

"DhcpDdns": {
    "reverse-ddns": {
        "ddns-domains": [
            {
                "name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
                "key-name": "",
                "dns-servers": [
                ]
            }
        ]
    }
}

It is permissible to add a domain without any servers. If that domain should be matched to a request, however, the request will fail. In order to make the domain useful though, we must add at least one DNS server to it.

12.3.4.1.1. Adding Reverse DNS Servers

This section describes how to add DNS servers to a Reverse DDNS Domain. Repeat them for as many servers as desired for each domain.

Reverse DNS Server entries represents a actual DNS servers which support the server side of the DDNS protocol. Each Reverse DNS Server has the following parameters:

  • hostname - The resolvable host name of the DNS server. This value is currently ignored.
  • ip-address - The IP address at which the server listens for DDNS requests.
  • port - The port on which the server listens for DDNS requests. It defaults to the standard DNS service port of 53.

To create a new reverse DNS Server, one must first add a new server element to the domain and fill in its parameters. If for example the service is running at "172.88.99.10", then set it as follows:

"DhcpDdns": {
    "reverse-ddns": {
        "ddns-domains": [
            {
                "name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
                "key-name": "",
                "dns-servers": [
                    {
                        "hostname": "",
                        "ip-address": "172.88.99.10",
                        "port": 53
                    }
                ]
            }
        ]
    }
}

Note

As stated earlier, "hostname" is not yet supported so, the parameter "ip-address" must be set to the address of the DNS server.

12.3.5. User context in DDNS

Note

User contexts were designed for hook libraries which are not yet supported for DHCP-DDNS server configuration.

User contexts can store arbitrary data as long as it is valid JSON syntax and its top level element is a map (i.e. the data must be enclosed in curly brackets).

User contexts can be specified on either global scope, ddns domain, dns server, tsig key and loggers. One other useful usage is the ability to store comments or descriptions: the parser translates a "comment" entry into a user-context with the entry, this allows to attach a comment inside the configuration itself.

12.3.6. Example DHCP-DDNS Server Configuration

This section provides an example DHCP-DDNS server configuration based on a small example network. Let's suppose our example network has three domains, each with their own subnet.

Table 12.1. Our example network

DomainSubnetForward DNS ServersReverse DNS Servers
four.example.com192.0.2.0/24172.16.1.5, 172.16.2.5172.16.1.5, 172.16.2.5
six.example.com2001:db8:1::/643001:1::503001:1::51
example.com192.0.0.0/16172.16.2.5172.16.2.5


We need to construct three forward DDNS Domains:

Table 12.2. Forward DDNS Domains Needed

#DDNS Domain NameDNS Servers
1.four.example.com.172.16.1.5, 172.16.2.5
2.six.example.com.3001:1::50
3.example.com.172.16.2.5


As discussed earlier, FQDN to domain matching is based on the longest match. The FQDN, "myhost.four.example.com.", will match the first domain ("four.example.com") while "admin.example.com." will match the third domain ("example.com"). The FQDN, "other.example.net." will fail to match any domain and would be rejected.

The following example configuration specified the Forward DDNS Domains.


"DhcpDdns": {
    "comment": "example configuration: forward part",
    "forward-ddns": {
        "ddns-domains": [
            {
                "name": "four.example.com.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "172.16.1.5" },
                    { "ip-address": "172.16.2.5" }
                ]
            },
            {
                "name": "six.example.com.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "2001:db8::1" }
                ]
            },
            {
                "name": "example.com.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "172.16.2.5" }
                ],
                "user-context": { "backup": false }
            },

        ]
    }
}

Similarly, we need to construct the three reverse DDNS Domains:

Table 12.3. Reverse DDNS Domains Needed

#DDNS Domain NameDNS Servers
1.2.0.192.in-addr.arpa.172.16.1.5, 172.16.2.5
2.1.0.0.0.8.d.b.0.1.0.0.2.ip6.arpa.3001:1::50
3.0.182.in-addr.arpa.172.16.2.5


An address of "192.0.2.150" will match the first domain, "2001:db8:1::10" will match the second domain, and "192.0.50.77" the third domain.

These Reverse DDNS Domains are specified as follows:


"DhcpDdns": {
    "comment": "example configuration: reverse part",
    "reverse-ddns": {
        "ddns-domains": [
            {
                "name": "2.0.192.in-addr.arpa.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "172.16.1.5" },
                    { "ip-address": "172.16.2.5" }
                ]
            }
            {
                "name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "2001:db8::1" }
                ]
            }
            {
                "name": "0.192.in-addr.arpa.",
                "key-name": "",
                "dns-servers": [
                    { "ip-address": "172.16.2.5" }
                ]
            }
        ]
    }
}

12.4. DHCP-DDNS Server Limitations

The following are the current limitations of the DHCP-DDNS Server.

  • Requests received from the DHCP servers are placed in a queue until they are processed. Currently all queued requests are lost when the server shuts down.

Chapter 13. The LFC process

13.1. Overview

kea-lfc is a service process that removes redundant information from the files used to provide persistent storage for the memfile data base backend. This service is written to run as a stand alone process.

While kea-lfc can be started externally, there is usually no need to do this. kea-lfc is run on a periodic basis by the Kea DHCP servers.

The process operates on a set of files, using them for input and output of the lease entries and to indicate where it is in the process in case of an interruption. Currently the caller must supply names for all of the files, in the future this requirement may be relaxed with the process getting the names from either the configuration file or from defaults.

13.2. Command Line Options

kea-lfc is run as follows:

kea-lfc [-4 | -6] -c config-file -p pid-file -x previous-file -i copy-file -o output-file -f finish-file

The argument -4 or -6 selects the protocol version of the lease files.

The -c argument specifies the configuration file. This is required, but not currently used by the process.

The -p argument specifies the PID file. When the kea-lfc process starts it attempts to determine if another instance of the process is already running by examining the pid file. If one is already running the new process is terminated. If one isn't running it writes its pid into the pid file.

The other filenames specify where the kea-lfc process should look for input, write its output and use for bookkeeping.

  • previous — When kea-lfc starts this is the result of any previous run of kea-lfc. When kea-lfc finishes it is the result of this run. If kea-lfc is interrupted before completing, this file may not exist.
  • input — Before the DHCP server invokes kea-lfc it will move the current lease file here and then call kea-lfc with this file.
  • output — The temporary file kea-lfc should use to write the leases. Upon completion of writing this file, it will be moved to the finish file (see below).
  • finish — Another temporary file kea-lfc uses for bookkeeping. When kea-lfc completes writing the outputfile it moves it to this file name. After kea-lfc finishes deleting the other files (previous and input) it moves this file to previous lease file. By moving the files in this fashion the kea-lfc and the DHCP server processes can determine the correct file to use even if one of the processes was interrupted before completing its task.

There are several additional arguments mostly for debugging purposes. -d Sets the logging level to debug. -v and -V print out version stamps with -V providing a longer form. -h prints out the usage string.

Chapter 14. Client Classification

14.1. Client Classification Overview

In certain cases it is useful to differentiate between different types of clients and treat them accordingly. Common reasons include:

  • The clients represent different pieces of topology, e.g. a cable modem is different to the clients behind that modem.

  • The clients have different behavior, e.g. a smart phone behaves differently to a laptop.

  • The clients require different values for some options, e.g. a docsis3.0 cable modem requires different settings to docsis2.0 cable modem.

Conversely, different clients can be grouped into a client class to get a common option.

An incoming packet can be associated with a client class in serveral ways:

  • Implicitly, using a vendor class option or another builtin condition.

  • Using an expression which evaluates to true.

  • Using static host reservations, a shared network, a subnet, etc.

  • Using a hook.

It is envisaged that client classification will be used for changing the behavior of almost any part of the DHCP message processing. In the current release of the software however, there are only five mechanisms that take advantage of client classification: subnet selection, pool selection, definition of DHCPv4 private (codes 224-254) and code 43 options, assignment of different options and, for DHCPv4 cable modems, the setting of specific options for use with the TFTP server address and the boot file field.

The process of doing classification is conducted in several steps:

  1. The ALL class is associated with the incoming packet.

  2. Vendor class options are processed.

  3. Classes with matching expressions and not marked for later ("on request" or depending on the KNOWN/UNKNOWN builtin classes) evaluation are processed in the order they are defined in the configuration: the boolean expression is evaluated and when it returns true ("match") the incoming packet is associated to the class.

  4. If a private or code 43 DHCPv4 option is received, decoding it following its client class or global (or for option 43 last resort) definition.

  5. Choose a subnet, possibly based on the class information when some subnets are guarded. More precisely: when choosing a subnet, the server will iterate over all of the subnets that are feasible given the information found in the packet (client address, relay address etc). It will use the first subnet it finds that either doesn't have a class associated with it or that has a class which matches one of the packet's classes.

  6. Host reservations are looked for. If an identifier from the incoming packet matches a host reservation in the subnet or shared network, the packet is associated with the KNOWN class and all classes of the host reservation. If a reservation is not found, the packet is assigned to UNKNOWN class.

  7. Classes with matching expressions using directly or indirectly the KNOWN/UNKNOWN builtin classes and not marked for later ("on request") evaluation are processed in the order they are defined in the configuration: the boolean expression is evaluated and when it returns true ("match") the incoming packet is associated to the class. The determination whether there is a reservation for a given client is made after a subnet is selected. As such, it is not possible to use KNOWN/UNKNOWN classes to select a shared network or a subnet.

  8. If needed, addresses and prefixes from pools are assigned, possibly based on the class information when some pools are reserved to class members.

  9. Evaluate classes marked as "required" in the order in which they are listed as required: first shared network, then the subnet and to finally pools assigned resources belong too.

  10. Assign options, again possibly based on the class information in order classes were associated with the incoming packet. For DHCPv4 private and code 43 options this includes class local option definitions.

Note

Beginning with Kea 1.4.0 release, client classes follow the order in which they are specified in the configuration (vs. alphabetical order in previous releases). Required classes follow the order in which they are required.

When determining which options to include in the response, the server will examine the union of options from all of the assigned classes. In case when two or more classes include the same option, the value from the first class examined will be used, and classes are examined in the order they were associated so ALL is always the first class and matching required classes are last.

As an example, imagine that an incoming packet matches two classes. Class "foo" defines values for an NTP server (option 42 in DHCPv4) and an SMTP server (option 69 in DHCPv4) while class "bar" defines values for an NTP server and a POP3 server (option 70 in DHCPv4). The server will examine the three options NTP, SMTP and POP3 and return any of them that the client requested. As the NTP server was defined twice the server will choose only one of the values for the reply: the class from which the value is obtained is unspecified.

Note

Care should be taken with client classification as it is easy for clients that do not meet class criteria to be denied any service altogether.

14.2. Builtin Client Classes

Some classes are builtin so do not need to be defined. The main example uses Vendor Class information: The server checks whether an incoming DHCPv4 packet includes the vendor class identifier option (60) or an incoming DHCPv6 packet includes the vendor class option (16). If it does, the content of that option is prepended with "VENDOR_CLASS_" and the result is interpreted as a class. For example, modern cable modems will send this option with value "docsis3.0" and so the packet will belong to class "VENDOR_CLASS_docsis3.0".

The "HA_" prefix is used by the High Availability hooks library to designate certain servers to process DHCP packets as a result of load balancing. The class name is constructed by prepending the "HA_" prefix to the name of the server which should process the DHCP packet. This server will use appropriate pool or subnet to allocate IP addresses (and/or prefixes) from, based on the assigned client classes. The details can be found in Section 15.4.8, “ha: High Availability”.

Other examples are: the ALL class which all incoming packets belong to, and the KNOWN class assigned when host reservations exist for the particular client. By convention, builtin classes' names begin with all capital letters.

Currently recognized builtin class names are ALL, KNOWN and UNKNOWN, and prefixes VENDOR_CLASS_, HA_, AFTER_ and EXTERNAL_. The AFTER_ prefix is a provision for a not yet written hook, the EXTERNAL_ prefix can be freely used: builtin classes are implicitly defined so never raise warnings if they do not appear in the configuration.

14.3. Using Expressions In Classification

The expression portion of classification contains operators and values. All values are currently strings and operators take a string or strings and return another string. When all the operations have completed the result should be a value of "true" or "false". The packet belongs to the class (and the class name is added to the list of classes) if the result is "true". Expressions are written in standard format and can be nested.

Expressions are pre-processed during the parsing of the configuration file and converted to an internal representation. This allows certain types of errors to be caught and logged during parsing. Examples of these errors include an incorrect number or types of arguments to an operator. The evaluation code will also check for this class of error and generally throw an exception, though this should not occur in a normally functioning system.

Other issues, for example the starting position of a substring being outside of the substring or an option not existing in the packet, result in the operator returning an empty string.

Expressions are a work in progress and the supported operators and values are limited. The expectation is that additional operators and values will be added over time, however the basic mechanisms will remain the same.

Dependencies between classes are checked too: for instance forward dependencies are rejected when the configuration is parsed: an expression can only depend on already defined classes (including builtin classes) and which are evaluated in a previous or the same evaluation phase. This does not apply to the KNOWN or UNKNOWN classes.

Table 14.1. List of Classification Values

NameExample expressionExample valueDescription
String literal'example''example'A string
Hexadecimal string literal0x5a7d'Z}'A hexadecimal string
IP address literal10.0.0.10x0a000001An IP address
Integer literal123'123'A 32 bit unsigned integer value
Binary content of the optionoption[123].hex'(content of the option)'The value of the option with given code from the packet as hex
Option existenceoption[123].exists'true'If the option with given code is present in the packet "true" else "false"
Client class membershipmember('foobar')'true'If the packet belongs to the given client class "true" else "false"
Known clientknownmember('KNOWN')If there is a host reservation for the client "true" else "false"
Unknown clientunknownnot member('KNOWN')If there is a host reservation for the client "false" else "true"
DHCPv4 relay agent sub-optionrelay4[123].hex'(content of the RAI sub-option)'The value of sub-option with given code from the DHCPv4 Relay Agent Information option (option 82)
DHCPv6 Relay Optionsrelay6[nest].option[code].hex(value of the option)The value of the option with code "code" from the relay encapsulation "nest"
DHCPv6 Relay Peer Addressrelay6[nest].peeraddr2001:DB8::1The value of the peer address field from the relay encapsulation "nest"
DHCPv6 Relay Link Addressrelay6[nest].linkaddr2001:DB8::1The value of the link address field from the relay encapsulation "nest"
Interface name of packetpkt.ifaceeth0The name of the incoming interface of a DHCP packet.
Source address of packetpkt.src10.1.2.3The IP source address of a DHCP packet.
Destination address of packetpkt.dst10.1.2.3The IP destination address of a DHCP packet.
Length of packetpkt.len513The length of a DHCP packet (UDP header field), expressed as a 32 bit unsigned integer.
Hardware address in DHCPv4 packetpkt4.mac0x010203040506The value of the chaddr field of the DHCPv4 packet, hlen (0 to 16) bytes
Hardware length in DHCPv4 packetpkt4.hlen6The value of the hlen field of the DHCPv4 packet padded to 4 bytes
Hardware type in DHCPv4 packetpkt4.htype6The value of the htype field of the DHCPv4 packet padded to 4 bytes
ciaddr field in DHCPv4 packetpkt4.ciaddr192.0.2.1The value of the ciaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
giaddr field in DHCPv4 packetpkt4.giaddr192.0.2.1The value of the giaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
yiaddr field in DHCPv4 packetpkt4.yiaddr192.0.2.1The value of the yiaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
siaddr field in DHCPv4 packetpkt4.siaddr192.0.2.1The value of the siaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
Message Type in DHCPv4 packetpkt4.msgtype1The value of the message type field in the DHCPv4 packet (expressed as a 32 bit unsigned integer).
Transaction ID (xid) in DHCPv4 packetpkt4.transid12345The value of the transaction id in the DHCPv4 packet (expressed as a 32 bit unsigned integer).
Message Type in DHCPv6 packetpkt6.msgtype1The value of the message type field in the DHCPv6 packet (expressed as a 32 bit unsigned integer).
Transaction ID in DHCPv6 packetpkt6.transid12345The value of the transaction id in the DHCPv6 packet (expressed as a 32 bit unsigned integer).
Vendor option existence (any vendor)vendor[*].existstrueReturns whether a vendor option from any vendor is present ('true') or absent ('false').
Vendor option existence (specific vendor)vendor[4491].existstrueReturns whether a vendor option from specified vendor (determined by its enterprise-id) is present ('true') or absent ('false').
Enterprise-id from vendor optionvendor.enterprise4491If the vendor option is present, it returns the value of the enterprise-id field padded to 4 bytes. Returns "" otherwise.
Vendor sub-option existencevendor[4491].option[1].existstrueReturns 'true' if there is vendor option with specified enterprise-id and given sub-option is present. Returns 'false' otherwise.
Vendor sub-option contentvendor[4491].option[1].hexdocsis3.0Returns content of the specified sub-option of a vendor option with specified enterprise id. Returns '' if no such option or sub-option is present.
Vendor class option existence (any vendor)vendor-class[*].existstrueReturns whether a vendor class option from any vendor is present ('true') or absent ('false').
Vendor class option existence (specific vendor)vendor-class[4491].existstrueReturns whether a vendor class option from specified vendor (determined by its enterprise-id) is present ('true') or absent ('false').
Enterprise-id from vendor class optionvendor-class.enterprise4491If the vendor option is present, it returns the value of the enterprise-id field padded to 4 bytes. Returns "" otherwise.
First data chunk from vendor class optionvendor-class[4491].datadocsis3.0Returns content of the first data chunk from the vendor class option with specified enterprise-id. Returns "" if missing.
Specific data chunk from vendor class optionvendor-class[4491].data[3]docsis3.0Returns content of the specified data chunk of a vendor class option with specified enterprise id. Returns '' if no such option or data chunk is present.


Notes:

  • Hexadecimal strings are converted into a string as expected. The starting "0X" or "0x" is removed and if the string is an odd number of characters a "0" is prepended to it.

  • IP addresses are converted into strings of length 4 or 16. IPv4, IPv6, and IPv4 embedded IPv6 (e.g., IPv4 mapped IPv6) addresses are supported.

  • Integers in an expression are converted to 32 bit unsigned integers and are represented as four-byte strings. For example 123 is represented as 0x0000007b. All expressions that return numeric values use 32-bit unsigned integers, even if the field in the packet is smaller. In general it is easier to use decimal notation to represent integers, but it is also possible to use hex notation. When using hex notation to represent an integer care should be taken to make sure the value is represented as 32 bits, e.g. use 0x00000001 instead of 0x1 or 0x01. Also, make sure the value is specified in network order, e.g. 1 is represented as 0x00000001.

  • "option[code].hex" extracts the value of the option with the code "code" from the incoming packet. If the packet doesn't contain the option, it returns the empty string. The string is presented as a byte string of the option payload without the type code or length fields.

  • "option[code].exists" checks if an option with the code "code" is present in the incoming packet. It can be used with empty options.

  • "member('foobar')" checks if the packet belongs to the client class "foobar". To avoid dependency loops the configuration file parser checks if client classes were already defined or are built-in, i.e., beginning by "VENDOR_CLASS_", "AFTER__" (for the to come "after" hook) and "EXTERNAL_" or equal to "ALL", "KNOWN", "UNKNOWN"etc.

    "known" and "unknown" are short hands for "member('KNOWN')" and "not member('KNOWN')". Note the evaluation of any expression using directly or indirectly the "KNOWN" class is deferred after the host reservation lookup (i.e. when the "KNOWN" or "UNKNOWN" partition is determined).

  • "relay4[code].hex" attempts to extract the value of the sub-option "code" from the option inserted as the DHCPv4 Relay Agent Information (82) option. If the packet doesn't contain a RAI option, or the RAI option doesn't contain the requested sub-option, the expression returns an empty string. The string is presented as a byte string of the option payload without the type code or length fields. This expression is allowed in DHCPv4 only.

  • "relay4" shares the same representation types as "option", for instance "relay4[code].exists" is supported.

  • "relay6[nest]" allows access to the encapsulations used by any DHCPv6 relays that forwarded the packet. The "nest" level specifies the relay from which to extract the information, with a value of 0 indicating the relay closest to the DHCPv6 server. Negative values allow to specify relays counted from the DHCPv6 client, -1 indicating the relay closest to the client. In general negative "nest" level is the same as the number of relays + "nest" level. If the requested encapsulation doesn't exist an empty string "" is returned. This expression is allowed in DHCPv6 only.

  • "relay6[nest].option[code]" shares the same representation types as "option", for instance "relay6[nest].option[code].exists" is supported.

  • Expressions starting with "pkt4" can be used only in DHCPv4. They allows access to DHCPv4 message fields.

  • "pkt6" refers to information from the client request. To access any information from an intermediate relay use "relay6". "pkt6.msgtype" and "pkt6.transid" output a 4 byte binary string for the message type or transaction id. For example the message type SOLICIT will be "0x00000001" or simply 1 as in "pkt6.msgtype == 1".

  • Vendor option means Vendor-Identifying Vendor-specific Information option in DHCPv4 (code 125, see Section 4 of RFC 3925) and Vendor-specific Information Option in DHCPv6 (code 17, defined in Section 21.17 of RFC 8415). Vendor class option means Vendor-Identifying Vendor Class Option in DHCPv4 (code 124, see Section 3 of RFC 3925) in DHCPv4 and Class Option in DHCPv6 (code 16, see Section 21.16 of RFC 8415). Vendor options may have sub-options that are referenced by their codes. Vendor class options do not have sub-options, but rather data chunks, which are referenced by index value. Index 0 means the first data chunk, Index 1 is for the second data chunk (if present), etc.

  • In the vendor and vendor-class constructs Asterisk (*) or 0 can be used to specify a wildcard enterprise-id value, i.e. it will match any enterprise-id value.

  • Vendor Class Identifier (option 60 in DHCPv4) can be accessed using option[60] expression.

  • RFC 3925 and RFC 8415 allow for multiple instances of vendor options to appear in a single message. The client classification code currently examines the first instance if more than one appear. For vendor.enterprise and vendor-class.enterprise expressions, the value from the first instance is returned. Please submit a feature request on Kea website if you need support for multiple instances.

Table 14.2. List of Classification Expressions

NameExampleDescription
Equal'foo' == 'bar'Compare the two values and return "true" or "false"
Notnot ('foo' == 'bar')Logical negation
And('foo' == 'bar') and ('bar' == 'foo')Logical and
Or('foo' == 'bar') or ('bar' == 'foo')Logical or
Substringsubstring('foobar',0,3)Return the requested substring
Concatconcat('foo','bar')Return the concatenation of the strings
Ifelseifelse('foo' == 'bar','us','them')Return the branch value according to the condition
Hexstringhexstring('foo', '-')Converts the value to a hexadecimal string, e.g. 0a:1b:2c:3e


14.3.1. Logical operators

The Not, And and Or logical operators are the common operators. Not has the highest precedence and Or the lowest. And and Or are (left) associative, parentheses around a logical expression can be used to enforce a specific grouping, for instance in "A and (B or C)" (without parentheses "A and B or C" means "(A and B) or C").

14.3.2. Substring

The substring operator "substring(value, start, length)" accepts both positive and negative values for the starting position and the length. For "start", a value of 0 is the first byte in the string while -1 is the last byte. If the starting point is outside of the original string an empty string is returned. "length" is the number of bytes to extract. A negative number means to count towards the beginning of the string but doesn't include the byte pointed to by "start". The special value "all" means to return all bytes from start to the end of the string. If length is longer than the remaining portion of the string then the entire remaining portion is returned. Some examples may be helpful:
        substring('foobar', 0, 6) == 'foobar'
        substring('foobar', 3, 3) == 'bar'
        substring('foobar', 3, all) == 'bar'
        substring('foobar', 1, 4) == 'ooba'
        substring('foobar', -5, 4) == 'ooba'
        substring('foobar', -1, -3) == 'oba'
        substring('foobar', 4, -2) == 'ob'
        substring('foobar', 10, 2) == ''
          

14.3.3. Concat

The concat function "concat(string1, string2)" returns the concatenation of its two arguments. For instance:
        concat('foo', 'bar') == 'foobar'
          

14.3.4. Ifelse

The ifelse function "ifelse(cond, iftrue, ifelse)" returns the "iftrue" or "ifelse" branch value following the boolean condition "cond". For instance:
         ifelse(option[230].exists, option[230].hex, 'none')
           

14.3.5. Hexstring

The hexstring function "hexstring(binary, separator)" returns the binary value as its hexadecimal string representation: pairs of hexadecimal digits separated by the separator, e.g ':', '-', '' (empty separator).
          hexstring(pkt4.mac, ':')
            

Note

The expression for each class is executed on each packet received. If the expressions are overly complex, the time taken to execute them may impact the performance of the server. If you need complex or time consuming expressions you should write a hook to perform the necessary work.

14.4. Configuring Classes

A class contains five items: a name, a test expression, option data, option definition and only-if-required flag. The name must exist and must be unique amongst all classes. The test expression, option data and definition, and only-if-required flag are optional.

The test expression is a string containing the logical expression used to determine membership in the class. The entire expression is in double quotes.

The option data is a list which defines any options that should be assigned to members of this class.

The option definition is for DHCPv4 option 43 (Section 8.2.11, “DHCPv4 Vendor Specific Options” and DHCPv4 private options (Section 8.2.10, “DHCPv4 Private Options”).

Usually the test expression is evaluated before subnet selection but in some cases it is useful to evaluate it later when the subnet, shared-network or pools are known but output option processing not yet done. The only-if-required flag, false by default, allows to perform the evaluation of the test expression only when it was required, i.e. in a require-client-classes list of the selected subnet, shared-network or pool.

The require-client-classes list which is valid for shared-network, subnet and pool scope specifies the classes which are evaluated in the second pass before output option processing. The list is built in the reversed precedence order of option data, i.e. an option data in a subnet takes precedence on one in a shared-network but required class in a subnet is added after one in a shared-network. The mechanism is related to the only-if-required flag but it is not mandatory that the flag was set to true.

In the following example the class named "Client_foo" is defined. It is comprised of all clients whose client ids (option 61) start with the string "foo". Members of this class will be given 192.0.2.1 and 192.0.2.2 as their domain name servers.

"Dhcp4": {
    "client-classes": [
        {
            "name": "Client_foo",
            "test": "substring(option[61].hex,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        },
        ...
    ],
    ...
}

This example shows a client class being defined for use by the DHCPv6 server. In it the class named "Client_enterprise" is defined. It is comprised of all clients who's client identifiers start with the given hex string (which would indicate a DUID based on an enterprise id of 0xAABBCCDD). Members of this class will be given an 2001:db8:0::1 and 2001:db8:2::1 as their domain name servers.

"Dhcp6": {
    "client-classes": [
        {
            "name": "Client_enterprise",
            "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD'",
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:0::1, 2001:db8:2::1"
                }
            ]
        },
        ...
    ],
    ...
}

14.5. Using Static Host Reservations In Classification

Classes can be statically assigned to the clients using techniques described in Section 8.3.6, “Reserving Client Classes in DHCPv4” and Section 9.3.5, “Reserving Client Classes in DHCPv6”.

14.6. Configuring Subnets With Class Information

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given class, using the "client-class" keyword when defining the subnet.

Let's assume that the server is connected to a network segment that uses the 192.0.2.0/24 prefix. The Administrator of that network has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to be managed by the DHCP4 server. Only clients belonging to client class Client_foo are allowed to use this subnet. Such a configuration can be achieved in the following way:

"Dhcp4": {
    "client-classes": [
        {
            "name": "Client_foo",
            "test": "substring(option[61].hex,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        },
        ...
    ],
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "Client_foo"
        },
        ...
    ],,
    ...
}

The following example shows restricting access to a DHCPv6 subnet. This configuration will restrict use of the addresses 2001:db8:1::1 to 2001:db8:1::FFFF to members of the "Client_enterprise" class.

"Dhcp6": {
    "client-classes": [
        {
            "name": "Client_enterprise",
            "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD'",
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:0::1, 2001:db8:2::1"
                }
            ]
        },
        ...
    ], 
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
            "client-class": "Client_enterprise"
        }
    ],
    ...
}

14.7. Configuring Pools With Class Information

Similar to subnets in certain cases access to certain address or prefix pools must be restricted to only clients that belong to a given class, using the "client-class" when defining the pool.

Let's assume that the server is connected to a network segment that uses the 192.0.2.0/24 prefix. The Administrator of that network has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to be managed by the DHCP4 server. Only clients belonging to client class Client_foo are allowed to use this pool. Such a configuration can be achieved in the following way:

"Dhcp4": {
    "client-classes": [
        {
            "name": "Client_foo",
            "test": "substring(option[61].hex,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        },
        ...
    ],
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                {
                    "pool": "192.0.2.10 - 192.0.2.20",
                    "client-class": "Client_foo"
                }
            ]
        },
        ...
    ],,

}

The following example shows restricting access to an address pool. This configuration will restrict use of the addresses 2001:db8:1::1 to 2001:db8:1::FFFF to members of the "Client_enterprise" class.

"Dhcp6": {
    "client-classes": [
        {
            "name": "Client_enterprise_",
            "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD'",
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:0::1, 2001:db8:2::1"
                }
            ]
        },
        ...
    ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            
            "pools": [
                {
                    "pool": "2001:db8:1::-2001:db8:1::ffff",
                    "client-class": "Client_foo"
                }
            ]
        },
        ...
    ],
    ...
}

14.8. Using Classes

Currently classes can be used for two functions. They can supply options to the members of the class and they can be used to choose a subnet from which an address will be assigned to the class member.

When supplying options, options defined as part of the class definition are considered "class globals". They will override any global options that may be defined and in turn will be overridden by any options defined for an individual subnet.

14.9. Classes and Hooks

You may use a hook to classify your packets. This may be useful if the expression would either be complex or time consuming and be easier or better to write as code. Once the hook has added the proper class name to the packet the rest of the classification system will work as normal in choosing a subnet and selecting options. For a description of hooks see Chapter 15, Hooks Libraries, for a description on configuring classes see Section 14.4, “Configuring Classes” and Section 14.6, “Configuring Subnets With Class Information”.

14.10. Debugging Expressions

While you are constructing your classification expressions you may find it useful to enable logging see Chapter 19, Logging for a more complete description of the logging facility.

To enable the debug statements in the classification system you will need to set the severity to "DEBUG" and the debug level to at least 55. The specific loggers are "kea-dhcp4.eval" and "kea-dhcp6.eval".

In order to understand the logging statements, one must understand a bit about how expressions are evaluated; for a more complete description refer to the design document at https://gitlab.isc.org/isc-projects/kea/wikis/design%20documents. In brief there are two structures used during the evaluation of an expression: a list of tokens which represent the expressions and a value stack which represents the values being manipulated.

The list of tokens is created when the configuration file is processed with most expressions and values being converted to a token. The list is organized in reverse Polish notation. During execution, the list will be traversed in order. As each token is executed it will be able to pop values from the top of the stack and eventually push its result on the top of the stack. Imagine the following expression:

       "test": "substring(option[61].hex,0,3) == 'foo'",
       

This will result in the following tokens:

       option, number (0), number (3), substring, text ('foo'), equals
       

In this example the first three tokens will simply push values onto the stack. The substring token will then remove those three values and compute a result that it places on the stack. The text option also places a value on the stack and finally the equals token removes the two tokens on the stack and places its result on the stack.

When debug logging is enabled, each time a token is evaluated it will emit a log message indicating the values of any objects that were popped off of the value stack and any objects that were pushed onto the value stack.

The values will be displayed as either text if the command is known to use text values or hexadecimal if the command either uses binary values or can manipulate either text or binary values. For expressions that pop multiple values off the stack, the values will be displayed in the order they were popped. For most expressions this won't matter but for the concat expression the values are displayed in reverse order from how they are written in the expression.

Let us assume that the following test has been entered into the configuration. This example skips most of the configuration to concentrate on the test.

       "test": "substring(option[61].hex,0,3) == 'foo'",
       

The logging might then resemble this:

       2016-05-19 13:35:04.163 DEBUG [kea.eval/44478] EVAL_DEBUG_OPTION Pushing option 61 with value 0x666F6F626172
       2016-05-19 13:35:04.164 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string '0'
       2016-05-19 13:35:04.165 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string '3'
       2016-05-19 13:35:04.166 DEBUG [kea.eval/44478] EVAL_DEBUG_SUBSTRING Popping length 3, start 0, string 0x666F6F626172 pushing result 0x666F6F
       2016-05-19 13:35:04.167 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string 'foo'
       2016-05-19 13:35:04.168 DEBUG [kea.eval/44478] EVAL_DEBUG_EQUAL Popping 0x666F6F and 0x666F6F pushing result 'true'
       

Note

The debug logging may be quite verbose if you have a number of expressions to evaluate. It is intended as an aid in helping you create and debug your expressions. You should plan to disable debug logging when you have your expressions working correctly. You also may wish to include only one set of expressions at a time in the configuration file while debugging them in order to limit the log statements. For example when adding a new set of expressions you might find it more convenient to create a configuration file that only includes the new expressions until you have them working correctly and then add the new set to the main configuration file.

Chapter 15. Hooks Libraries

15.1. Introduction

Although Kea offers a lot of flexibility, there may be cases where its behavior needs customization. To accommodate this possibility, Kea includes the idea of "Hooks". This feature lets Kea load one or more dynamically-linked libraries (known as "hooks libraries") and, at various points in its processing ("hook points"), call functions in them. Those functions perform whatever custom processing is required.

The hooks concept also allows keeping the core Kea code reasonably small by moving features that some, but not all users find useful to external libraries. People who don't need specific functionality simply don't load the libraries.

Hooks libraries are loaded by individual Kea processes, not to Kea as a whole. This means (for example) that it is possible to associate one set of libraries with the DHCP4 server and a different set to the DHCP6 server.

Another point to note is that it is possible for a process to load multiple libraries. When processing reaches a hook point, Kea calls the hooks library functions attached to it. If multiple libraries have attached a function to a given hook point, Kea calls all of them, in the order in which the libraries are specified in the configuration file. The order may be important: consult the documentation of the libraries to see if this is the case.

The next section describes how to configure hooks libraries. If you are interested in writing your own hooks library, information can be found in the Kea Developer's Guide.

Note that some libraries are available under different licenses.

Note that some libraries may require additional dependencies and/or compilation switches to be enabled, e.g. Radius library introduced in Kea 1.4 requires FreeRadius-client library to be present. If --with-free-radius option is not specified, the Radius library will not be built.

15.2. Installing Hook packages

Note

The installation procedure has changed in 1.4.0. Kea 1.3.0 and earlier needed special switches passed to configure script to detect the hook libraries. Please see this KB article: https://kb.isc.org/article/AA-01587 .

Some hook packages are included in the base Kea sources. There is no need to do anything special to compile or install them, they are covered by the usual building and installation procedure. ISC also provides several additional hooks in form of various packages. All of those packages follow the same installation procedure that is similar to base Kea, but has several additional steps. For your convenience, the whole procedure is described here. Please refer to Chapter 3, Installation for general overview.

1. Download the package. You will receive detailed instructions how to get it separately. This will be a file with a name similar to kea-premium-1.5.0.tar.gz. Your name may differ depending on which package you got.

2. If you have the sources for the corresponding version of the open-source Kea package still on your system (from when you installed Kea), skip this step. Otherwise extract the Kea source from the original tarball you downloaded. For example, if you downloaded Kea 1.5.0., you should have a tarball called kea-1.5.0.tar.gz on your system. Unpack this tarball:

$ tar zxvf kea-1.5.0.tar.gz

This will unpack the tarball into the kea-1.5.0 subdirectory of your current working directory.

3. Unpack the Kea premium tarball into the directory into which Kea was unpacked. For example, assuming that you followed step 2 and that Kea 1.5.0 has been unpacked into a kea-1.5.0 subdirectory and that the Kea premium tarball is in your current directory, the following steps will unpack the premium tarball into the correct location:

  $ cd kea-1.5.0
  $ tar xvf ../kea-premium-1.5.0.tar.gz

Note that unpacking the Kea premium package will put the files into a directory named premium. Regardless of the name of your package, the directory will always be called premium, just its content may vary.

4. Run autoreconf tools. This step is necessary to update Kea's build script to include additional directory. If this tool is not already available on your system, you need to install automake and autoconf tools. To generate configure script, please use:

  $ autoreconf -i

5. Rerun configure, using the same configure options as you used when originally building Kea. You can check if configure has detected the premium package by inspecting the summary printed when it exits. The first section of the output should look something like:

Package:
  Name:            kea
  Version:         1.5.0
  Extended version:1.5.0 (tarball)
  OS Family:       Linux
  Using GNU sed:   yes
  Premium package: yes
  Included Hooks:  forensic_log flex_id host_cmds

The last line indicates which specific hooks were detected. Note that some hooks may require its own dedicated switches, e.g. radius hook requires extra switches for FreeRADIUS. Please consult later sections of this chapter for details.

6. Rebuild Kea

  $ make

If your machine has multiple CPU cores, interesting option to consider here is -j X, where X is the number of available cores.

7. Install Kea sources together with hooks:

$ sudo make install

Note that as part of the installation procedure, the install script will eventually venture into premium/ directory and will install additional hook libraries and associated files.

The installation location of the hooks libraries depends whether you specified --prefix parameter to the configure script. If you did not, the default location will be /usr/local/lib/hooks. You can verify the libraries are installed properly with this command:

$ ls -l /usr/local/lib/hooks/*.so
/usr/local/lib/hooks/libdhcp_class_cmds.so
/usr/local/lib/hooks/libdhcp_flex_id.so
/usr/local/lib/hooks/libdhcp_host_cmds.so
/usr/local/lib/hooks/libdhcp_lease_cmds.so
/usr/local/lib/hooks/libdhcp_legal_log.so
/usr/local/lib/hooks/libdhcp_subnet_cmds.so

The exact list you see will depend on the packages you have. If you specified directory via --prefix, the hooks libraries will be located in {prefix directory}/lib/hooks.

15.3. Configuring Hooks Libraries

The hooks libraries for a given process are configured using the hooks-libraries keyword in the configuration for that process. (Note that the word "hooks" is plural). The value of the keyword is an array of map structures, each structure corresponding to a hooks library. For example, to set up two hooks libraries for the DHCPv4 server, the configuration would be:

"Dhcp4": {
    :
    "hooks-libraries": [
        {
            "library": "/opt/charging.so"
        },
        {
            "library": "/opt/local/notification.so",
            "parameters": {
                "mail": "spam@example.com",
                "floor": 13,
                "debug": false,
                "users": [ "alice", "bob", "charlie" ],
                "languages": {
                    "french": "bonjour",
                    "klingon": "yl'el"
                }
            }
        }
    ]
    :
}

Note

This is a change to the syntax used in Kea 0.9.2 and earlier, where hooks-libraries was a list of strings, each string being the name of a library. The change was made in Kea 1.0 to facilitate the specification of library-specific parameters, a capability available in Kea 1.1.0 onwards. Libraries should allow a parameter entry where to put comments as it is done for many configuration scopes with comment and user context.

Note

The library reloading behavior has changed in Kea 1.1.0. Libraries are reloaded, even if their list hasn't changed. Kea does that, because the parameters specified for the library (or the files those parameters point to) may have changed.

Libraries may have additional parameters. Those are not mandatory in the sense that there may be libraries that don't require them. However, for specific library there is often specific requirement for specify certain set of parameters. Please consult the documentation for your library for details. In the example above, the first library has no parameters. The second library has five parameters, specifying mail (string parameter), floor (integer parameter), debug (boolean parameter) and even lists (list of strings) and maps (containing strings). Nested parameters could be used if the library supports it. This topic is explained in detail in the Hooks Developer's Guide in the "Configuring Hooks Libraries" section.

Notes:

  • The full path to each library should be given.

  • As noted above, order may be important - consult the documentation for each library.

  • An empty list has the same effect as omitting the hooks-libraries configuration element all together.

    Note

    There is one case where this is not true: if Kea is running with a configuration that contains a hooks-libraries item, and that item is removed and the configuration reloaded, the removal will be ignored and the libraries remain loaded. As a workaround, instead of removing the hooks-libraries item, change it to an empty list. This will be fixed in a future version of Kea.

At the present time, only the kea-dhcp4 and kea-dhcp6 processes support hooks libraries.

15.4. Available Hooks Libraries

As described above, the hooks functionality provides a way to customize a Kea server without modifying the core code. ISC has chosen to take advantage of this feature to provide functions that may only be useful to a subset of Kea users. To this end ISC has created some hooks libraries; these discussed in the following sections.

Note

Some of these libraries will be available with the base code while others will be shared with organizations supporting development of Kea , possibly as a 'benefit' or 'thank you' for helping to sustain the larger Kea project. If you would like to get access to those libraries, please consider taking out a support contract: this includes professional support, advance security notifications, input into our roadmap planning, and many other benefits, while helping making Kea sustainable in the long term.

The following table provides a list of libraries currently available from ISC. It is important to pay attention to which libraries may be loaded by which Kea processes. It is a common mistake to configure the kea-ctrl-agent process to load libraries that should, in fact, be loaded by the kea-dhcp4 or kea-dhcp6 processes. If a library from ISC doesn't work as expected, please make sure that it has been loaded by the correct process per the table below.

Warning

While the Kea Control Agent includes the "hooks" functionality, (i.e. hooks libraries can be loaded by this process), none of ISC's current hooks libraries should be loaded by the Control Agent.

Table 15.1. List of available hooks libraries

NameAvailabilitySinceLoad by processDescription
user_chkKea sourcesKea 0.8
kea-dhcp4
kea-dhcp6
Reads known users list from a file. Unknown users will be assigned a lease from the last subnet defined in the configuration file, e.g. to redirect them a captive portal. This demonstrates how an external source of information can be used to influence the Kea allocation engine. This hook is part of the Kea source code and is available in the src/hooks/dhcp/user_chk directory.
Forensic LoggingSupport customersKea 1.1.0
kea-dhcp4
kea-dhcp6
This library provides hooks that record a detailed log of lease assignments and renewals into a set of log files. In many legal jurisdictions companies, especially ISPs, must record information about the addresses they have leased to DHCP clients. This library is designed to help with that requirement. If the information that it records is sufficient it may be used directly. If your jurisdiction requires that you save a different set of information, you may use it as a template or example and create your own custom logging hooks.
Flexible IdentifierSupport customersKea 1.2.0
kea-dhcp4
kea-dhcp6
Kea software provides a way to handle host reservations that include addresses, prefixes, options, client classes and other features. The reservation can be based on hardware address, DUID, circuit-id or client-id in DHCPv4 and using hardware address or DUID in DHCPv6. However, there are sometimes scenarios where the reservation is more complex, e.g. uses other options that mentioned above, uses part of specific options or perhaps even a combination of several options and fields to uniquely identify a client. Those scenarios are addressed by the Flexible Identifiers hook application. It allows defining an expression, similar to the one used in client classification, e.g. substring(relay6[0].option[37],0,6). Each incoming packet is evaluated against that expression and its value is then searched in the reservations database.
Host CommandsSupport customersKea 1.2.0
kea-dhcp4
kea-dhcp6
Kea provides a way to store host reservations in a database. In many larger deployments it is useful to be able to manage that information while the server is running. This library provides management commands for adding, querying and deleting host reservations in a safe way without restarting the server. In particular, it validates the parameters, so an attempt to insert incorrect data, e.g. add a host with conflicting identifier in the same subnet will be rejected. Those commands are exposed via command channel (JSON over unix sockets) and Control Agent (JSON over RESTful interface). Additional commands and capabilities related to host reservations will be added in the future.
Subnet CommandsSupport customersKea 1.3.0
kea-dhcp4
kea-dhcp6
In deployments in which subnet configuration needs to be frequently updated, it is a hard requirement that such updates be performed without the need for a full DHCP server reconfiguration or restart. This hooks library allows for incremental changes to the subnet configuration such as: adding a subnet, removing a subnet. It also allows for listing all available subnets and fetching detailed information about a selected subnet. The commands exposed by this library do not affect other subnets or configuration parameters currently used by the server.
Lease CommandsKea sourcesKea 1.3.0
kea-dhcp4
kea-dhcp6
The lease commands hook library offers a number of new commands used to manage leases. Kea provides a way to store lease information in various backends: memfile, MySQL, PostgreSQL and Cassandra. This library provides a unified interface that can manipulate leases in an unified, safe way. In particular, it allows: manipulate leases in memfile while Kea is running, sanity check changes, check lease existence and remove all leases belonging to specific subnet. It can also catch more obscure errors, like adding a lease with subnet-id that does not exist in the configuration or configuring a lease to use an address that is outside of the subnet to which it is supposed to belong. It provides a way to manage user contexts associated with leases.
High AvailabilityKea sourcesKea 1.4.0
kea-dhcp4
kea-dhcp6
Minimizing a risk of DHCP service unavailability is achieved by setting up a pair of the DHCP servers in a network. Two modes of operation are supported. The first one is called load balancing and is sometimes referred to as active-active. Each server can handle selected group of clients in this network or all clients, if it detects that its partner has became unavailable. It is also possible to designate one server to serve all DHCP clients, and leave another server as "standby". This mode is called hot standby and is sometimes referenced to as active-passive. This server will activate its DHCP function when it detects that its partner is not available. Such cooperation between the DHCP servers requires that these servers constantly communicate with each other to send updates about allocated leases and to periodically test whether their partners are still operational. The hook library also provides an ability to send lease updates to external backup server, making it much easier to have a replacement that is almost up to date. The "libdhcp_ha" library provides such functionality for Kea DHCP servers.
Statistics CommandsKea sourcesKea 1.4.0
kea-dhcp4
kea-dhcp6
The Statistics Commands library provides additional commmands for retrieving accurate DHCP lease statistics for Kea DHCP servers that share the same lease database. This setup is common in deployments where DHCP service redundancy is required and a shared lease database is used to avoid lease data replication between the DHCP servers. A feature was introduced in Kea 1.4.0 that allows tracking lease allocations within the lease database, thus making the statistics accessible to all connected DHCP servers. The Statistics Commands hooks library utilizes this feature and returns lease statistics for all subnets respectively.
RadiusSupport customersKea 1.4.0
kea-dhcp4
kea-dhcp6
The RADIUS Hook library allows Kea to interact with the RADIUS servers using access and accounting mechanisms. The access mechanism may be used for access control, assigning specific IPv4 or IPv6 addresses reserved by RADIUS, dynamically assigning addresses from designated pools chosen by RADIUS or rejecting the client's messages altogether. The accounting mechanism allows RADIUS server to keep track of device activity over time.
Host CacheSupport customersKea 1.4.0
kea-dhcp4
kea-dhcp6
Some of the database backends, such as RADIUS, are considered slow and may take a long time to respond. Since Kea in general is synchronous, the backend performance directly affects the DHCP performance. To minimize the impact and improve performance, the Host Cache library provides a way to cache responses from other hosts. This includes negative caching, i.e. the ability to remember that there is no client information in the database.
Class CommandsSupport customersKea 1.5.0
kea-dhcp4
kea-dhcp6
This Class Cmds hooks library allows for adding, updating deleting and fetching configured DHCP client classes without the need to restart the DHCP server.


ISC hopes to see more hooks libraries become available as time progresses, both developed internally and externally. Since this list may evolve dynamically, we decided to keep it on a wiki page, available at this link: https://gitlab.isc.org/isc-projects/kea/wikis/Hooks-available. If you are a developer or are aware of any hooks libraries not listed there, please send a note to the kea-users or kea-dev mailing lists and someone will update it.

The libraries developed by ISC are described in detail in the following sections.

15.4.1. user_chk: Checking User Access

The user_chk library is the first hooks library published by ISC. It attempts to serve several purposes:

  • To assign "new" or "unregistered" users to a restricted subnet, while "known" or "registered" users are assigned to unrestricted subnets.

  • To allow DHCP response options or vendor option values to be customized based upon user identity.

  • To provide a real time record of the user registration activity which can be sampled by an external consumer.

  • To serve as a demonstration of various capabilities possible using the hooks interface.

Once loaded, the library allows segregating incoming requests into known and unknown clients. For known clients, the packets are processed mostly as usual, except it is possible to override certain options being sent. That can be done on a per host basis. Clients that are not on the known hosts list will be treated as unknown and will be assigned to the last subnet defined in the configuration file.

As an example of use, this behavior may be used to put unknown users into a separate subnet that leads to a walled garden, where they can only access a registration portal. Once they fill in necessary data, their details are added to the known clients file and they get a proper address after their device is restarted.

Note

This library was developed several years before the host reservation mechanism has become available. Currently host reservation is much more powerful and flexible, but nevertheless the user_chk capability to consult and external source of information about clients and alter Kea's behavior is useful and remains of educational value.

The library reads the /tmp/user_chk_registry.txt file while being loaded and each time an incoming packet is processed. The file is expected to have each line contain a self-contained JSON snippet which must have the following two entries:

  • type, whose value is "HW_ADDR" for IPv4 users or "DUID" for IPv6 users

  • id, whose value is either the hardware address or the DUID from the request formatted as a string of hex digits, with or without ":" delimiters.

and may have the zero or more of the following entries:

  • bootfile whose value is the pathname of the desired file

  • tftp_server whose value is the hostname or IP address of the desired server

A sample user registry file is shown below:

{ "type" : "HW_ADDR", "id" : "0c:0e:0a:01:ff:04", "bootfile" : "/tmp/v4bootfile" }
{ "type" : "HW_ADDR", "id" : "0c:0e:0a:01:ff:06", "tftp_server" : "tftp.v4.example.com" }
{ "type" : "DUID", "id" : "00:01:00:01:19:ef:e6:3b:00:0c:01:02:03:04", "bootfile" : "/tmp/v6bootfile" }
{ "type" : "DUID", "id" : "00:01:00:01:19:ef:e6:3b:00:0c:01:02:03:06", "tftp_server" : "tftp.v6.example.com" }

As with any other hooks libraries provided by ISC, internals of the user_chk code are well documented. You can take a look at the Kea Developer's Guide section dedicated to the user_chk library that discusses how the code works internally. That, together with our general entries in Hooks Framework section should give you some pointers how to extend this library and perhaps even write your own from scratch.

15.4.2. legal_log: Forensic Logging Hooks

This section describes the forensic log hooks library. This library provides hooks that record a detailed log of lease assignments and renewals into a set of log files. Currently this library is only available to ISC customers with a support contract.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

In many legal jurisdictions companies, especially ISPs, must record information about the addresses they have leased to DHCP clients. This library is designed to help with that requirement. If the information that it records is sufficient it may be used directly. If your jurisdiction requires that you save a different set of information you may use it as a template or example and create your own custom logging hooks.

This logging is done as a set of hooks to allow it to be customized to any particular need. Modifying a hooks library is easier and safer than updating the core code. In addition by using the hooks features those users who don't need to log this information can leave it out and avoid any performance penalties.

15.4.2.1. Log File Naming

The names for the log files have the following form:

path/base-name.CCYYMMDD.txt

The "path" and "base-name" are supplied in the configuration as described below see Section 15.4.2.4, “Configuring the Forensic Log Hooks”. The next part of the name is the date the log file was started, with four digits for year, two digits for month and two digits for day. The file is rotated on a daily basis.

Note

When running Kea servers for both DHCPv4 and DHCPv6 the log names must be distinct. See the examples in Section 15.4.2.4, “Configuring the Forensic Log Hooks”.

15.4.2.2. DHCPv4 Log Entries

For DHCPv4 the library creates entries based on DHCPREQUEST messages and corresponding DHCPv4 leases intercepted by lease4_select (for new leases) and lease4_renew (for renewed leases) hooks.

An entry is a single string with no embedded end-of-line markers, a prepended timestamp and has the following sections:

timestamp address duration device-id {client-info} {relay-info} {user-context}

Where:

  • timestamp - the current date and time the log entry was written in "%Y-%m-%d %H:%M:%S %Z" strftime format ("%Z" is the time zone name).

  • address - the leased IPv4 address given out and whether it was assigned or renewed.

  • duration - the lease lifetime expressed in days (if present), hours, minutes and seconds. A lease lifetime of 0xFFFFFFFF will be denoted with the text "infinite duration".

  • device-id - the client's hardware address shown as numerical type and hex digit string.

  • client-info - the DHCP client id option (61) if present, shown as a hex string.

  • relay-info - for relayed packets the giaddr and the RAI circuit-id, remote-id and subscriber-id options (option 82 sub options: 1, 2 and 6) if present. The circuit id and remote id are presented as hex strings

  • user-context - the optional user context associated to the lease.

For instance (line breaks added for readability, they would not be present in the log file).

2018-01-06 01:02:03 CET Address: 192.2.1.100 has been renewed for 1 hrs 52 min 15 secs to a device with hardware address:
hwtype=1 08:00:2b:02:3f:4e, client-id: 17:34:e2:ff:09:92:54 connected via relay at address: 192.2.16.33,
identified by circuit-id: 68:6f:77:64:79 and remote-id: 87:f6:79:77:ef

In addition to logging lease activity driven by DHCPv4 client traffic, it also logs entries for the following lease management control channel commands: lease4-add, lease4-update, and lease4-del. Each entry is a single string with no embedded end-of-line markers and they will typically have the following forms:

lease4-add:

*timestamp* Administrator added a lease of address: *address* to a device with hardware address: *device-id*

Dependent on the arguments of the add command, it may also include the client-id and duration.

Example:

2018-01-06 01:02:03 CET Administrator added a lease of address: 192.0.2.202 to a device with hardware address:
1a:1b:1c:1d:1e:1f for 1 days 0 hrs 0 mins 0 secs

lease4-update:

*timestamp* Administrator updated information on the lease of address: *address* to a device with hardware address: *device-id*

Dependent on the arguments of the update command, it may also include the client-id and lease duration.

Example:

2018-01-06 01:02:03 CET Administrator updated information on the lease of address: 192.0.2.202 to a device
with hardware address: 1a:1b:1c:1d:1e:1f, client-id: 1234567890

lease4-del: Deletes have two forms, one by address and one by identifier and identifier type:

*timestamp* Administrator deleted the lease for address: *address*

or

*timestamp* Administrator deleted a lease for a device identified by: *identifier-type* of *identifier*

Currently only a type of @b hw-address (hardware address) is supported.

Examples:

2018-01-06 01:02:03 CET Administrator deleted the lease for address: 192.0.2.202

2018-01-06 01:02:12 CET Administrator deleted a lease for a device identified by: hw-address of 1a:1b:1c:1d:1e:1f

15.4.2.3. DHCPv6 Log Entries

For DHCPv6 the library creates entries based on lease management actions intercepted by the lease6_select (for new leases), lease6_renew (for renewed leases) and lease6_rebind (for rebound leases).

An entry is a single string with no embedded end-of-line markers, a prepended timestamp and has the following sections:

timestamp address duration device-id {relay-info}* {user-context}

Where:

  • timestamp - the current date and time the log entry was written in "%Y-%m-%d %H:%M:%S %Z" strftime format ("%Z" is the time zone name).

  • address - the leased IPv6 address or prefix given out and whether it was assigned or renewed.

  • duration - the lease lifetime expressed in days (if present), hours, minutes and seconds. A lease lifetime of 0xFFFFFFFF will be denoted with the text "infinite duration".

  • device-id - the client's DUID and hardware address (if present).

  • relay-info - for relayed packets the content of relay agent messages, remote-id (code 37), subscriber-id (code 38) and interface-id (code 18) options if present. Note that interface-id option, if present, identifies the whole interface the relay agent received the message on. This typically translates to a single link in your network, but it depends on your specific network topology. Nevertheless, this is useful information to better scope down the location of the device, so it is being recorded, if present.

  • user-context - the optional user context associated to the lease.

For instance (line breaks added for readability, they would not be present in the log file).

2018-01-06 01:02:03 PST Address:2001:db8:1:: has been assigned for 0 hrs 11 mins 53 secs
to a device with DUID: 17:34:e2:ff:09:92:54 and hardware address: hwtype=1 08:00:2b:02:3f:4e
(from Raw Socket) connected via relay at address: fe80::abcd for client on link address: 3001::1,
hop count: 1, identified by remote-id: 01:02:03:04:0a:0b:0c:0d:0e:0f and subscriber-id: 1a:2b:3c:4d:5e:6f

In addition to logging lease activity driven by DHCPv6 client traffic, it also logs entries for the following lease management control channel commands: lease6-add, lease6-update, and lease6-del. Each entry is a single string with no embedded end-of-line markers and they will typically have the following forms:

lease6-add:

*timestamp* Administrator added a lease of address: *address* to a device with DUID: *DUID*

Dependent on the arguments of the add command, it may also include the hardware address and duration.

Example:

2018-01-06 01:02:03 PST Administrator added a lease of address: 2001:db8::3 to a device with DUID:
1a:1b:1c:1d:1e:1f:20:21:22:23:24 for 1 days 0 hrs 0 mins 0 secs

lease6-update:

*timestamp* Administrator updated information on the lease of address: *address* to a device with DUID: *DUID*

Dependent on the arguments of the update command, it may also include the hardware address and lease duration.

Example:

2018-01-06 01:02:03 PST Administrator updated information on the lease of address: 2001:db8::3 to a device with
DUID: 1a:1b:1c:1d:1e:1f:20:21:22:23:24, hardware address: 1a:1b:1c:1d:1e:1f

lease6-del: Deletes have two forms, one by address and one by identifier and identifier type:

*timestamp* Administrator deleted the lease for address: *address*

or

*timestamp* Administrator deleted a lease for a device identified by: *identifier-type* of *identifier*

Currently only a type of DUID is supported.

Examples:

2018-01-06 01:02:03 PST Administrator deleted the lease for address: 2001:db8::3

2018-01-06 01:02:11 PST Administrator deleted a lease for a device identified by: duid of 1a:1b:1c:1d:1e:1f:20:21:22:23:24

15.4.2.4. Configuring the Forensic Log Hooks

To use this functionality the hook library must be included in the configuration of the desired DHCP server modules. The legal_log library is installed alongside the Kea libraries in [kea-install-dir]/lib where kea-install-dir is determined by the "--prefix" option of the configure script. It defaults to /usr/local. Assuming the default value then, configuring kea-dhcp4 to load the legal_log library could be done with the following Kea4 configuration:

"Dhcp4": { 
    "hooks-libraries": [
        {
            "library": "/usr/local/lib/libdhcp_legal_log.so",
            "parameters": {
                "path": "/var/kea/var",
                "base-name": "kea-forensic4"
            }
        },
        ...
    ] 
}

To configure it for kea-dhcp6, the commands are simply as shown below:

"Dhcp6": { 
    "hooks-libraries": [
        {
            "library": "/usr/local/lib/libdhcp_legal_log.so",
            "parameters": {
                "path": "/var/kea/var",
                "base-name": "kea-forensic6"
            }
        },
        ...
    ] 
}

Two Hook Library parameters are supported:

  • path - the directory in which the forensic file(s) will be written. The default value is [prefix]/kea/var. The directory must exist.

  • base-name - an arbitrary value which is used in conjunction with the current system date to form the current forensic file name. It defaults to kea-legal.

If it is desired to restrict forensic logging to certain subnets, the "legal-logging" boolean parameter can be specified within a user context of these subnets. For example:

"Dhcpv4" {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                {
                     "pool": "192.0.2.1 - 192.0.2.200"
                }
            ],
            "user-context": {
                "legal-logging": false
            }
        }
    ]
}

disables legal logging for the subnet "192.0.2.0/24". If this parameter is not specified, it defaults to 'true', which enables legal logging for the subnet.

The following example demonstrates how to selectively disable legal logging for an IPv6 subnet.

"Dhcpv6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
            ],
            "user-context": {
                "legal-logging": false
            }
        }
    ]
}

See Section 8.11, “User contexts in IPv4” and Section 9.14, “User contexts in IPv6” to learn more about user contexts in Kea configuration.

15.4.2.5. Database backend

Log entries can be inserted into a database when Kea is configured with database backend support: a table named 'logs' is used with a timestamp (timeuuid for Cassandra CQL) generated by the database software and a text log with the same format than for files without the timestamp.

Please refer to Section 4.3.2, “MySQL” for MySQL, to Section 4.3.3, “PostgreSQL” for PostgreSQL or to Section 4.3.4, “CQL (Cassandra)” for Cassandra CQL. The logs table is part of the Kea database schemas.

Configuration parameters are extended by standard lease database parameters as defined in Section 8.2.2.2, “Lease Database Configuration”. The "type" parameter should be "mysql", "postgresql", "cql" or be "logfile". When it is absent or set to "logfile" files are used.

This database feature is experimental and will be likely improved, for instance to add an address / prefix index (currently the only index is the timestamp). No specific tools is provided to operate the database but standard tools are applicable, for instance to dump the logs table from a CQL database:

$ echo 'SELECT dateOf(timeuuid), log FROM logs;' | cqlsh -k database-name

 system.dateof(timeuuid)         | log
---------------------------------+---------------------------------------
 2018-01-06 01:02:03.227000+0000 | Address: 192.2.1.100 has been renewed ...
 ...
(12 rows)
$

15.4.3. flex_id: Flexible Identifiers for Host Reservations

This section describes a hook application dedicated to generate flexible identifiers for host reservation. Kea software provides a way to handle host reservations that include addresses, prefixes, options, client classes and other features. The reservation can be based on hardware address, DUID, circuit-id or client-id in DHCPv4 and using hardware address or DUID in DHCPv6. However, there are sometimes scenarios where the reservation is more complex, e.g. uses other options that mentioned above, uses part of specific options or perhaps even a combination of several options and fields to uniquely identify a client. Those scenarios are addressed by the Flexible Identifiers hook application.

Currently this library is only available to ISC customers with a support contract.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

The library allows for defining an expression, using notation initially used for client classification only. See Section 14.3, “Using Expressions In Classification” for detailed description of the syntax available. One notable difference is that for client classification the expression currently has to evaluate to either true or false, while the flexible identifier expression is expected to evaluate to a string that will be used as identifier. It is a valid case for the expression to evaluate to empty string (e.g. in cases where a client does not sent specific options). This expression is then evaluated for each incoming packet. This evaluation generates an identifier that is used to identify the client. In particular, there may be host reservations that are tied to specific values of the flexible identifier.

The library can be loaded in similar way as other hook libraries. It takes a mandatory parameter identifier-expression and optional boolean parameter replace-client-id:

"Dhcp6": { 
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_flex_id.so",
            "parameters": {
                "identifier-expression": "expression",
                "replace-client-id": "false"
            }
        },
        ...
    ] 
}

The flexible identifier library supports both DHCPv4 and DHCPv6.

EXAMPLE: Let's consider a case of an IPv6 network that has an independent interface for each of the connected customers. Customers are able to plug in whatever device they want, so any type of identifier (e.g. a client-id) is unreliable. Therefore the operator may decide to use an option inserted by a relay agent to differentiate between clients. In this particular deployment, the operator verified that the interface-id is unique for each customer facing interface. Therefore it is suitable for usage as reservation. However, only the first 6 bytes of the interface-id are interesting, because remaining bytes are either randomly changed or not unique between devices. Therefore the customer decided to use first 6 bytes of the interface-id option inserted by the relay agent. After adding "flex-id" host-reservation-identifiers goal can be achieved by using the following configuration:

"Dhcp6": {
    "subnet6": [{ ..., // subnet definition starts here
    "reservations": [
        "flex-id": "'port1234'", // value of the first 8 bytes of the interface-id
        "ip-addresses": [ "2001:db8::1" ]
    ],
    }], // end of subnet definitions
    "host-reservation-identifiers": ["duid", "flex-id"], // add "flex-id" to reservation identifiers
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_flex_id.so",
            "parameters": {
                "identifier-expression": "substring(relay6[0].option[18].hex,0,8)"
            }
        },
        ...
    ]
}

NOTE: Care should be taken when adjusting the expression. If the expression changes, then all the flex-id values may change, possibly rendering all reservations based on flex-id unusable until they're manually updated. Therefore it is strongly recommended to start with the expression and a handful reservations, adjust the expression as needed and only after it was confirmed the expression does exactly what is expected out of it go forward with host reservations on any broader scale.

flex-id values in host reservations can be specified in two ways. First, they can be expressed as hex string, e.g. bar string can be represented as 626174. Alternatively, it can be expressed as quoted value (using double and single quotes), e.g. "'bar'". The former is more convenient for printable characters, while hex string values are more convenient for non-printable characters and does not require the use of the hexstring operator.

"Dhcp6": {
    "subnet6": [{ ..., // subnet definition starts here
    "reservations": [
        "flex-id": "01:02:03:04:05:06", // value of the first 8 bytes of the interface-id
        "ip-addresses": [ "2001:db8::1" ]
    ],
    }], // end of subnet definitions
    "host-reservation-identifiers": ["duid", "flex-id"], // add "flex-id" to reservation identifiers
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_flex_id.so",
            "parameters": {
                "identifier-expression": "vendor[4491].option[1026].hex"
            }
        },
        ...
    ]
}

When "replace-client-id" is set to false (which is the default setting), the flex-id hook library uses evaluated flexible identifier solely for identifying host reservations, i.e. searching for reservations within a database. This is a functional equivalent of other identifiers, similar to hardware address or circuit-id. However, this mode of operation has an implication that if a client device is replaced, it may cause a conflict between an existing lease (allocated for old device) and the new lease being allocated for the new device. The conflict arises because the same flexible identifier is computed for the replaced device and the server will try to allocate the same lease. The mismatch between client identifiers sent by new device and old device causes the server to refuse this new allocation until the old lease expires. A manifestation of this problem is dependant on specific expression used as flexible identifier and is likely to appear if you only use options and other parameters that are identifying where the device is connected (e.g. circuit-id), rather than the device identification itself (e.g. MAC address).

The flex-id library offers a way to overcome the problem with lease conflicts by dynamically replacing client identifier (or DUID in DHCPv6 case) with a value derived from flexible identifier. The server processes the client's query as if flexible identifier was sent in the client identifier (or DUID) option. This guarantees that returning client (for which the same flexible identifier is evaluated) will be assigned the same lease despite the client identifier and/or MAC address change.

The following is a stub configuration that enables this behavior:

"Dhcp4": { 
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_flex_id.so",
            "parameters": {
                "identifier-expression": "expression",
                "replace-client-id": "true"
            }
        },
        ...
    ] 
}

In the DHCPv4 case, the value derived from the flexible identifier is formed by prepending 1 byte with a value of zero to flexible identifier. In the IPv6 case, it is formed by prepanding two zero bytes before the flexible identifier.

Note that for this mechanism to take effect, the DHCPv4 server must be configured to respect the client identifier option value during lease allocation, i.e. "match-client-id" must be set to true. See Section 8.2.19, “Using Client Identifier and Hardware Address” for details. No additional settings are required for DHCPv6.

If "replace-client-id" option is set to true, the value of "echo-client-id" parameter (that governs whether to send back a client-id option or not) is ignored.

The Section 15.4.5, “lease_cmds: Lease Commands” section describes commands used to retrieve, update and delete leases using various identifiers, e.g. "hw-address", "client-id". The lease_cmds library doesn't natively support querying for leases by flexible identifier. However, when "replace-client-id" is set to true, it makes it possible to query for leases using a value derived from the flexible identifier. In the DHCPv4 case, the query will look similar to this:

{
    "command": "lease4-get",
    "arguments": {
        "identifier-type": "client-id",
        "identifier": "00:54:64:45:66",
        "subnet-id": 44
    }
}

where hexadecimal value of "54:64:45:66" is a flexible identifier computed for the client.

In the DHCPv6 case, the corresponding query will look similar to this:

{
    "command": "lease6-get",
    "arguments": {
        "identifier-type": "duid",
        "identifier": "00:00:54:64:45:66",
        "subnet-id": 10
    }
}

15.4.4. host_cmds: Host Commands

This section describes a hook application that offers a number of new commands used to query and manipulate host reservations. Kea provides a way to store host reservations in a database. In many larger deployments it is useful to be able to manage that information while the server is running. This library provides management commands for adding, querying and deleting host reservations in a safe way without restarting the server. In particular, it validates the parameters, so an attempt to insert incorrect data e.g. add a host with conflicting identifier in the same subnet will be rejected. Those commands are exposed via command channel (JSON over unix sockets) and Control Agent (JSON over RESTful interface). Additional commands and capabilities related to host reservations will be added in the future.

Currently this library is only available to ISC customers with a support contract.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

Currently three commands are supported: reservation-add (which adds new host reservation), reservation-get (which returns existing reservation if specified criteria are matched) and reservation-del (which attempts to delete a reservation matching specified criteria). To use commands that change the reservation information (currently these are reservation-add and reservation-del, but this rule applies to other commands that may be implemented in the future), hosts database must be specified (see hosts-databases description in Section 8.2.3.1, “DHCPv4 Hosts Database Configuration” and Section 9.2.3.1, “DHCPv6 Hosts Database Configuration”) and it must not operate in read-only mode. If the hosts-databases are not specified or are running in read-only mode, the host_cmds library will load, but any attempts to use reservation-add or reservation-del will fail.

Additional host reservation commands are planned in the future. For a description of envisaged commands, see Control API Requirements document.

All commands are using JSON syntax. They can be issued either using control channel (see Chapter 17, Management API) or via Control Agent (see Chapter 7, Kea Control Agent).

The library can be loaded in similar way as other hook libraries. It does not take any parameters. It supports both DHCPv4 and DHCPv6 servers.

"Dhcp6": { 
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_host_cmds.so"
        }
        ...
    ] 
}

15.4.4.1. The subnet-id parameter

Prior to diving into the individual commands, it is worth discussing the parameter, subnet-id. Currently it is mandatory for all of the commands supplied by this library. Prior to Kea 1.5.0, reservations had to belong to specific subnet. Beginning with Kea 1.5.0, reservations may now be specified globally. In other words, they are not specific to any subnet. When reservations are supplied via the configuration file, the ID of the containing subnet (or lack thereof) is implicit in the configuration structure. However, when managing reservations using the host commands, it is necessary to explicitly identify the scope to which the reservation belongs. This is done via the subnet-id parameter. For global reservations, use a value of zero (0). For reservations scoped to a specific subnet, use that subnet's ID.

15.4.4.2. reservation-add command

reservation-add allows for the insertion of a new host. It takes a set of arguments that vary depending on the nature of the host reservation. Any parameters allowed in the configuration file that pertain to host reservation are permitted here. For details regarding IPv4 reservations, see Section 8.3, “Host Reservation in DHCPv4” and Section 9.3, “Host Reservation in DHCPv6”. The subnet-id is manadatory. Use a value of zero (0) to add a global reservation, or the id of the subnet to which the reservation should be added. An example command can be as simple as:

{
    "command": "reservation-add",
    "arguments": {
        "reservation": {
            "subnet-id": 1,
            "hw-address": "1a:1b:1c:1d:1e:1f",
            "ip-address": "192.0.2.202"
        }
    }
}

but can also take many more parameters, for example:

{
    "command": "reservation-add",
    "arguments": {
        "reservation":
            {
                "subnet-id":1,
                "client-id": "01:0a:0b:0c:0d:0e:0f",
                "ip-address": "192.0.2.205",
                "next-server": "192.0.2.1",
                "server-hostname": "hal9000",
                "boot-file-name": "/dev/null",
                "option-data": [
                    {
                        "name": "domain-name-servers",
                        "data": "10.1.1.202,10.1.1.203"
                    }
                ],
                "client-classes": [ "special_snowflake", "office" ]
            }
    }
}

Here is an example of complex IPv6 reservation:

{
    "command": "reservation-add",
    "arguments": {
        "reservation":
            {
                "subnet-id":1,
                "duid": "01:02:03:04:05:06:07:08:09:0A",
                "ip-addresses": [ "2001:db8:1:cafe::1" ],
                "prefixes": [ "2001:db8:2:abcd::/64" ],
                "hostname": "foo.example.com",
                "option-data": [
                    {
                        "name": "vendor-opts",
                        "data": "4491"
                    },
                    {
                        "name": "tftp-servers",
                        "space": "vendor-4491",
                        "data": "3000:1::234"
                    }
                ]
            }
    }
}

The command returns a status that indicates either a success (result 0) or a failure (result 1). Failed command always includes text parameter that explains the cause of failure. Example results:

{ "result": 0, "text": "Host added." }

Example failure:

{ "result": 1, "text": "Mandatory 'subnet-id' parameter missing." }

As reservation-add is expected to store the host, hosts-databases parameter must be specified in your configuration and databases must not run in read-only mode. In the future versions it will be possible to modify the reservations read from a configuration file. Please contact ISC if you are interested in this functionality.

15.4.4.3. reservation-get command

reservation-get can be used to query the host database and retrieve existing reservations. There are two types of parameters this command supports: (subnet-id, address) or (subnet-id, identifier-type, identifier). The first type of query is used when the address (either IPv4 or IPv6) is known, but the details of the reservation aren't. One common use case of this type of query is to find out whether a given address is reserved or not. The second query uses identifiers. For maximum flexibility, Kea stores the host identifying information as a pair of values: type and the actual identifier. Currently supported identifiers are "hw-address", "duid", "circuit-id", "client-id" and "flex-id", but additional types may be added in the future. If any new identifier types are defined in the future, reservation-get command will support them automatically. The subnet-id is manadatory. Use a value of zero (0) to fetch a global reservation, or the id of the subnet to which the reservation belongs.

An example command for getting a host reservation by (subnet-id, address) pair looks as follows:

{
    "command": "reservation-get",
    "arguments": {
        "subnet-id": 1,
        "ip-address": "192.0.2.202"
    }
}

An example query by (subnet-id, identifier-type, identifier) looks as follows:

{
    "command": "reservation-get",
    "arguments": {
        "subnet-id": 4,
        "identifier-type": "hw-address",
        "identifier": "01:02:03:04:05:06"
    }
}

reservation-get typically returns result 0 when the query was conducted properly. In particular, 0 is returned when the host was not found. If the query was successful a number of host parameters will be returned. An example of a query that did not find the host looks as follows:

{ "result": 0, "text": "Host not found." }

An example result returned when the host was found:

{
  "arguments": {
    "boot-file-name": "bootfile.efi",
    "client-classes": [

    ],
    "hostname": "somehost.example.org",
    "hw-address": "01:02:03:04:05:06",
    "ip-address": "192.0.2.100",
    "next-server": "192.0.0.2",
    "option-data": [

    ],
    "server-hostname": "server-hostname.example.org"
  },
  "result": 0,
  "text": "Host found."
}

An example result returned when the query was malformed:

{ "result": 1, "text": "No 'ip-address' provided and 'identifier-type'
                        is either missing or not a string." }

15.4.4.4. reservation-del command

reservation-del can be used to delete a reservation from the host database. There are two types of parameters this command supports: (subnet-id, address) or (subnet-id, identifier-type, identifier). The first type of query is used when the address (either IPv4 or IPv6) is known, but the details of the reservation aren't. One common use case of this type of query is to remove a reservation (e.g. you want a specific address to no longer be reserved). The second query uses identifiers. For maximum flexibility, Kea stores the host identifying information as a pair of values: type and the actual identifier. Currently supported identifiers are "hw-address", "duid", "circuit-id", "client-id" and "flex-id", but additional types may be added in the future. If any new identifier types are defined in the future, reservation-get command will support them automatically. The subnet-id is manadatory. Use a value of zero (0) to delete a global reservation, or the id of the subnet from which the reservation should be deleted.

An example command for deleting a host reservation by (subnet-id, address) pair looks as follows:

{
    "command": "reservation-del",
    "arguments": {
        "subnet-id": 1,
        "ip-address": "192.0.2.202"
    }
}

An example deletion by (subnet-id, identifier-type, identifier) looks as follows:

{
    "command": "reservation-del",
    "arguments":
        "subnet-id": 4,
        "identifier-type": "hw-address",
        "identifier": "01:02:03:04:05:06"
    }
}

reservation-del returns result 0 when the host deletion was successful or 1 if it was not. A descriptive text is provided in case of error. Example results look as follows:

{
    "result": 1,
    "text": "Host not deleted (not found)."
}

{
    "result": 0,
    "text": "Host deleted."
}

{
    "result": 1,
    "text": "Unable to delete a host because there is no hosts-database
             configured."
}

15.4.5. lease_cmds: Lease Commands

This section describes the hook library that offers a number of new commands used to manage leases. Kea provides a way to store lease information in several backends (memfile, MySQL, PostgreSQL and Cassandra). This library provides a unified interface that can manipulate leases in an unified, safe way. In particular, it allows things previously impossible: manipulate leases in memfile while Kea is running, sanity check changes, check lease existence and remove all leases belonging to specific subnet. It can also catch more obscure errors, like adding a lease with subnet-id that does not exist in the configuration or configuring a lease to use an address that is outside of the subnet to which it is supposed to belong. It provides a non programmatic way to manage user contexts associated to leases.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

There are many use cases when an administrative command may be useful: during migration between servers (possibly even between different vendors), when a certain network is being retired, when a device has been disconnected and the sysadmin knows for sure that it will not be coming back. The "get" queries may be useful for automating certain management and monitoring tasks. They can also act as preparatory steps for lease updates and removals.

This library provides the following commands:

  • lease4-add - adds new IPv4 lease;

  • lease6-add - adds new IPv6 lease;

  • lease4-get - checks if an IPv4 lease with the specified parameters exists and returns it if it does;

  • lease6-get - checks if an IPv6 lease with the specified parameters exists and returns it if it does;

  • lease4-get-all - returns all IPv4 leases or IPv4 leases for specified subnets;

  • lease6-get-all - returns all IPv6 leases or IPv6 leases for specified subnets;

  • lease4-get-page - returns a page of all IPv4 leases; by iterating over pages it is possible to retrieve all leases in chunks;

  • lease6-get-page - returns a page of all IPv6 leases; by iterating over pages it is possible to retrieve all leases in chunks;

  • lease4-del - attempts to delete an IPv4 lease with the specified parameters;

  • lease6-del - attempts to delete an IPv6 lease with the specified parameters;

  • lease4-update - updates an IPv4 lease;

  • lease6-update - updates an IPv6 lease;

  • lease4-wipe - removes all leases from a specific IPv4 subnet or all subnets;

  • lease6-wipe - removes all leases from a specific IPv6 subnet or all subnets;

Lease commands library is part of the open source code and is available to every Kea user.

All commands are using JSON syntax. They can be issued either using control channel (see Chapter 17, Management API) or via Control Agent (see Chapter 7, Kea Control Agent).

The library can be loaded in the same way as other hook libraries. It does not take any parameters. It supports both DHCPv4 and DHCPv6 servers.

"Dhcp6": { 
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_lease_cmds.so"
        }
        ...
    ] 
}

15.4.5.1. lease4-add, lease6-add commands

lease4-add and lease6-add commands allow for the creation of a new lease. Typically Kea creates a lease on its own, when it first sees a new device. However, sometimes it may be convenient to create the lease administratively. The lease4-add command requires at least two parameters: an IPv4 address, and an identifier: hardware (MAC) address. A third parameter, subnet-id, was mandatory in Kea 1.4.0, but is optional beginning with Kea 1.5.0.. If not specified or specified value is 0, Kea will try to determine the value by running a subnet selection procedure. If specified, however, its value must match the existing subnet. The simplest successful call might look as follows:

{
    "command": "lease4-add",
    "arguments": {
        "ip-address": "192.0.2.202",
        "hw-address": "1a:1b:1c:1d:1e:1f"
    }
}

lease6-add command requires four parameters: an IPv6 address, and IAID value (identity association identifier, a value sent by clients) and a DUID. The same principle for subnet-id as in lease4-add applies. If not specified or specified value is 0, Kea will try to determine the value own by running a subnet selection procedure. If specified, however, its value must match the existing subnet. For example:

{
    "command": "lease6-add",
    "arguments": {
        "subnet-id": 66,
        "ip-address": "2001:db8::3",
        "duid": "1a:1b:1c:1d:1e:1f:20:21:22:23:24",
        "iaid": 1234
    }
}

lease6-add can be also used to add leases for IPv6 prefixes. In this case there are three additional parameters that must be specified: subnet-id, type (set to value of "IA_PD") and a prefix length. The actual prefix is set using ip-address field. Note that Kea is not able to guess subnet-id values for prefixes, so it must be specified explicitly. For example, to configure a lease for prefix 2001:db8:abcd::/48, the following command can be used:

{
    "command": "lease6-add",
    "arguments": {
        "subnet-id": 66,
        "type": "IA_PD",
        "ip-address": "2001:db8:abcd::",
        "prefix-len": 48,
        "duid": "1a:1b:1c:1d:1e:1f:20:21:22:23:24",
        "iaid": 1234
    }
}

The commands can take a number of additional optional parameters:

  • valid-lft - specifies the lifetime of the lease, expressed in seconds. If not specified, the value configured in the subnet related to specified subnet-id is used.

  • expire - timestamp of the lease expiration time, expressed in unix format (seconds since 1 Jan 1970). If not specified, the default value is now + valid lifetime.

  • fqdn-fwd - specifies whether the lease should be marked as if forward DNS update was conducted. Note this only affects the lease parameter and the actual DNS update will not be conducted at the lease insertion time. If configured, a DNS update to remove the A or AAAA records will be conducted when the lease is removed due to expiration or being released by a client. If not specified, the default value is false. Hostname parameter must be specified in fqdn-fwd is set to true.

  • fqdn-rev - specifies whether the lease should be marked as if reverse DNS update was conducted. Note this only affects the lease parameter and the actual DNS update will not be conducted at the lease insertion time. If configured, a DNS update to remove the PTR record will be conducted when the lease is removed due to expiration or being released by a client. If not specified, the default value is false. Hostname parameter must be specified in fqdn-fwd is set to true.

  • hostname - specifies the hostname to be associated with this lease. Its value must be non-empty if either fqdn-fwd or fwdn-rev are set to true. If not specified, the default value is an empty string.

  • hw-address - hardware (MAC) address can be optionally specified for IPv6 lease. It is mandatory parameter for IPv4 lease.

  • client-id - client identifier is an optional parameter that can be specified for IPv4 lease.

  • preferred-lft - Preferred lifetime is an optional parameter for IPv6 leases. If not specified, the value configured for the subnet corresponding to the specified subnet-id is used. This parameter is not used in IPv4.

  • user-context - specifies the user context to be associated with this lease. It must be a JSON map.

Here's an example of more complex lease addition:

{
    "command": "lease6-add",
    "arguments": {
        "subnet-id": 66,
        "ip-address": "2001:db8::3",
        "duid": "01:02:03:04:05:06:07:08",
        "iaid": 1234,
        "hw-address": "1a:1b:1c:1d:1e:1f",
        "preferred-lft": 500,
        "valid-lft": 1000,
        "expire": 12345678,
        "fqdn-fwd": true,
        "fqdn-rev": true,
        "hostname": "urania.example.org",
        "user-context": { "version": 1 }
    }
}

The command returns a status that indicates either a success (result 0) or a failure (result 1). Failed command always includes text parameter that explains the cause of failure. Example results:

{ "result": 0, "text": "Lease added." }

Example failure:

{ "result": 1, "text": "missing parameter 'ip-address' (<string>:3:19)" }

15.4.5.1.1. lease4-get, lease6-get commands

lease4-get or lease6-get can be used to query the lease database and retrieve existing leases. There are two types of parameters the lease4-get supports: (address) or (subnet-id, identifier-type, identifier). There are two types for lease6-get: (address,type) or (subnet-id, identifier-type, identifier, IAID, type). The first type of query is used when the address (either IPv4 or IPv6) is known, but the details of the lease aren't. One common use case of this type of query is to find out whether a given address is being used or not. The second query uses identifiers. Currently supported identifiers for leases are: "hw-address" (IPv4 only), "client-id" (IPv4 only) and "duid" (IPv6 only).

An example lease4-get command for getting a lease by an IPv4 address looks as follows:

{
    "command": "lease4-get",
    "arguments": {
        "ip-address": "192.0.2.1"
    }
}

An example of the lease6-get query looks as follows:

{
  "command": "lease6-get",
  "arguments": {
    "ip-address": "2001:db8:1234:ab::",
    "type": "IA_PD"
  }
}

An example query by "hw-address" for IPv4 lease looks as follows:

{
    "command": "lease4-get",
    "arguments": {
        "identifier-type": "hw-address",
        "identifier": "08:08:08:08:08:08",
        "subnet-id": 44
    }
}

An example query by "client-id" for IPv4 lease looks as follows:

{
    "command": "lease4-get",
    "arguments": {
        "identifier-type": "client-id",
        "identifier": "01:01:02:03:04:05:06",
        "subnet-id": 44
    }
}

An example query by (subnet-id, identifier-type, identifier, iaid, type) for IPv6 lease looks as follows:

{
    "command": "lease4-get",
    "arguments": {
        "identifier-type": "duid",
        "identifier": "08:08:08:08:08:08",
        "iaid": 1234567,
        "type": "IA_NA",
        "subnet-id": 44
    }
}

The type is an optional parameter. Supported values are: IA_NA (non-temporary address) and IA_PD (IPv6 prefix) are supported. If not specified, IA_NA is assumed.

leaseX-get returns a result that indicates a result of the operation and lease details, if found. It has one of the following values: 0 (success), 1 (error) or 2 (empty). The empty result means that a query has been completed properly, but the object (a lease in this case) has not been found. The lease parameters, if found, are returned as arguments.

An example result returned when the host was found:

{
  "arguments": {
    "client-id": "42:42:42:42:42:42:42:42",
    "cltt": 12345678,
    "fqdn-fwd": false,
    "fqdn-rev": true,
    "hostname": "myhost.example.com.",
    "hw-address": "08:08:08:08:08:08",
    "ip-address": "192.0.2.1",
    "state": 0,
    "subnet-id": 44,
    "valid-lft": 3600
  },
  "result": 0,
  "text": "IPv4 lease found."
}

15.4.5.1.2. lease4-get-all, lease6-get-all commands

lease4-get-all and lease6-get-all are used to retrieve all IPv4 or IPv6 leases or all leases for the specified set of subnets. All leases are returned when there are no arguments specified with the command as in the following example:

{
    "command": "lease4-get-all"
}

If the arguments are provided, it is expected that they contain "subnets" parameter, being a list of subnet identifiers for which the leases should be returned. For example, in order to retrieve all IPv6 leases belonging to the subnets with identifiers 1, 2, 3 and 4:

{
    "command": "lease6-get-all",
    "arguments": {
        "subnets": [ 1, 2, 3, 4 ]
    }
}

The returned response contains a detailed list of leases in the following format:

{
    "arguments": {
        "leases": [
            {
                "cltt": 12345678,
                "duid": "42:42:42:42:42:42:42:42",
                "fqdn-fwd": false,
                "fqdn-rev": true,
                "hostname": "myhost.example.com.",
                "hw-address": "08:08:08:08:08:08",
                "iaid": 1,
                "ip-address": "2001:db8:2::1",
                "preferred-lft": 500,
                "state": 0,
                "subnet-id": 44,
                "type": "IA_NA",
                "valid-lft": 3600
            },
            {
                "cltt": 12345678,
                "duid": "21:21:21:21:21:21:21:21",
                "fqdn-fwd": false,
                "fqdn-rev": true,
                "hostname": "",
                "iaid": 1,
                "ip-address": "2001:db8:0:0:2::",
                "preferred-lft": 500,
                "prefix-len": 80,
                "state": 0,
                "subnet-id": 44,
                "type": "IA_PD",
                "valid-lft": 3600
            }
        ]
    },
    "result": 0,
    "text": "2 IPv6 lease(s) found."
}

Warning

The lease4-get-all and lease6-get-all commands may result in very large responses. This may have negative impact on the DHCP server responsiveness while the response is generated and transmitted over the control channel, as the server imposes no restriction on the number of leases returned as a result of this command.

15.4.5.1.3. lease4-get-page, lease6-get-page commands

The lease4-get-all and lease6-get-all commands may result in very large responses. Generating such a response may consume CPU bandwith as well as memory. It may even cause the server to become unresponsive. In case of large lease databases it is usually better to retrieve leases in chunks, using paging mechanism. The lease4-get-page and lease6-get-page implement paging mechanism for DHCPv4 and DHCPv6 servers respectively. The following command retrieves first 1024 IPv4 leases:

{
    "command": "lease4-get-page",
    "arguments": {
        "from": "start",
        "limit": 1024
    }
}

The keyword start denotes that the first page of leases should be retrieved. Alternatively, an IPv4 zero address can be specified to retrieve the first page:

{
    "command": "lease4-get-page",
    "arguments": {
        "from": "0.0.0.0",
        "limit": 1024
    }
}

Similaraly, the IPv6 zero address can be specified in the lease6-get-page command:

{
    "command": "lease6-get-page",
    "arguments": {
        "from": "::",
        "limit": 6
    }
}

The response has the following structure:

{
    "arguments": {
        "leases": [
            {
                "ip-address": "2001:db8:2::1",
                ...
            },
            {
                "ip-address": "2001:db8:2::9",
                ...
            },
            {
                "ip-address": "2001:db8:3::1",
                ...
            },
            {
                "ip-address": "2001:db8:5::3",
                ...
            }
            {
                "ip-address": "2001:db8:4::1",
                ...
            },
            {
                "ip-address": "2001:db8:2::7",
                ...
            }

        ],
        "count": 6
    },
    "result": 0,
    "text": "6 IPv6 lease(s) found."
}

Note that the leases' details were excluded from the response above for brevity.

Generally, the returned list isn't sorted in any particular order. Some lease database backends may sort leases in ascending order of addresses, but the controlling client must not rely on this behavior. In case of highly distributed databases, such as Cassandra, ordering may be inefficient or even impossible.

The count parameter contains a number of returned leases on the page.

In order to fetch next page the client must use the last address of the current page as an input to the next lease4-get-page or lease6-get-page command call. In this example it is going to be:

{
    "command": "lease6-get-page",
    "arguments": {
        "from": "2001:db8:2::7",
        "count": 6
    }
}

because the 2001:db8:2::7 is the last address on the current page.

The client may assume that it has reached the last page when the count value is lower than specified in the command, including the case when the count is equal to 0, which means that no leases were found.

15.4.5.1.4. lease4-del, lease6-del commands

leaseX-del can be used to delete a lease from the lease database. There are two types of parameters this command supports, similar to leaseX-get commands: (address) for both v4 and v6, (subnet-id, identifier-type, identifier) for v4 and (subnet-id, identifier-type, identifier, type, IAID) for v6. The first type of query is used when the address (either IPv4 or IPv6) is known, but the details of the lease are not. One common use case of this type of query is to remove a lease (e.g. you want a specific address to no longer be used, no matter who may use it). The second query uses identifiers. For maximum flexibility, this interface uses identifiers as a pair of values: type and the actual identifier. Currently supported identifiers are "hw-address" (IPv4 only), "client-id" (IPv4 only) and "duid" (IPv6 only), but additional types may be added in the future.

An example command for deleting a host reservation by address looks as follows:

{
    "command": "lease4-del",
    "arguments": {
        "ip-address": "192.0.2.202"
    }
}

An example IPv4 lease deletion by "hw-address" looks as follows:

{
  "command": "lease4-del",
  "arguments": {
    "identifier": "08:08:08:08:08:08",
    "identifier-type": "hw-address",
    "subnet-id": 44
  }
}

leaseX-del returns a result that indicates a outcome of the operation. It has one of the following values: 0 (success), 1 (error) or 3 (empty). The empty result means that a query has been completed properly, but the object (a lease in this case) has not been found.

15.4.5.1.5. lease4-update, lease6-update commands

lease4-update and lease6-update commands can be used to update existing leases. Since all lease database backends are indexed by IP addresses, it is not possible to update an address. All other fields may be updated. If an address needs to be changed, please use leaseX-del followed by leaseX-add commands.

Subnet-id parameter, used to be mandatory in Kea 1.4.0, but it is optional since 1.5.0. If not specified or specified value is 0, Kea will try to determine its value by running a subnet selection procedure. If specified, however, its value must match the existing subnet.

The optional boolean parameter "force-create" specifies if the lease should be created if it doesn't exist in the database. It defaults to false, which indicates that the lease is not created if it doesn't exist. In such case, an error is returned as a result of trying to update a non-existing lease. If the "force-create" parameter is set to true and the updated lease doesn't exist, the new lease is created as a result of receiving the leaseX-update.

An example command updating IPv4 lease looks as follows:

{
  "command": "lease4-update",
  "arguments": {
    "ip-address": "192.0.2.1",
    "hostname": "newhostname.example.org",
    "hw-address": "1a:1b:1c:1d:1e:1f",
    "subnet-id": 44,
    "force-create": true
  }
}

An example command updating IPv6 lease looks as follows:

{
  "command": "lease6-update",
  "arguments": {
    "ip-address": "2001:db8::1",
    "duid": "88:88:88:88:88:88:88:88",
    "iaid": 7654321,
    "hostname": "newhostname.example.org",
    "subnet-id": 66,
    "force-create": false
  }
}

15.4.5.1.6. lease4-wipe, lease6-wipe commands

lease4-wipe and lease6-wipe are designed to remove all leases associated with a given subnet. This administrative task is expected to be used when existing subnet is being retired. Note that the leases are not properly expired, there are no DNS updates conducted, no log messages and hooks are not called for leases being removed.

An example of lease4-wipe looks as follows:

{
  "command": "lease4-wipe",
  "arguments": {
    "subnet-id": 44
  }
}

An example of lease6-wipe looks as follows:

{
  "command": "lease6-wipe",
  "arguments": {
    "subnet-id": 66
  }
}

The commands return a textual description of the number of leases removed and 0 (success) status code if any leases were removed and 2 (empty) if there were no leases. Status code 1 (error) may be returned in case the parameters are incorrect or some other exception is encountered.

The subnet-id 0 has special meaning. It tells Kea to delete leases from all configured subnets. Also, the subnet-id parameter may be omitted. If not specified, leases from all subnets are wiped.

Note: not all backends support this command.

15.4.6. subnet_cmds: Subnet Commands

This section describes a hook application that offers a number of new commands used to query and manipulate subnet and shared network configurations in Kea. This application is very useful in deployments with a large number of subnets being managed by the DHCP servers and when the subnets are frequently updated. The commands offer lightweight approach for manipulating subnets without a need to fully reconfigure the server and without affecting existing servers' configurations. An ability to manage shared networks (listing, retrieving details, adding new ones, removing existing ones, adding subnets to and removing from shared networks) is also provided.

Currently this library is only available to ISC customers with a support contract.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

The following commands are currently supported:

  • subnet4-list/subnet6-list: lists all configured subnets
  • subnet4-get/subnet6-get: retrieves detailed information about a specified subnet
  • subnet4-add/subnet6-add: adds new subnet into server's configuration
  • subnet4-del/subnet6-del: removes a subnet from the server's configuration
  • network4-list/network6-list: lists all configured shared networks
  • network4-get/network6-get: retrieves detailed information about specified shared network
  • network4-add/network6-add: adds a new shared network to the server's configuration
  • network4-del/network6-del: removes a shared network from the server's configuration
  • network4-subnet-add/network6-subnet-add: adds existing subnet to existing shared network
  • network4-subnet-del/network6-subnet-del: removes a subnet from existing shared network and demotes it to a plain subnet.

15.4.6.1. subnet4-list command

This command is used to list all currently configured subnets. The subnets are returned in a brief form, i.e. a subnet identifier and subnet prefix is included for each subnet. In order to retrieve the detailed information about the subnet the subnet4-get should be used.

This command has the simple structure:

{
    "command": "subnet4-list"
}

The list of subnets returned as a result of this command is returned in the following format:

{
    "result": 0,
    "text": "2 IPv4 subnets found",
    "arguments": {
    "subnets": [
        {
            "id": 10,
            "subnet": "10.0.0.0/8"
        },
        {
            "id": 100,
            "subnet": "192.0.2.0/24"
        }
    ]
}

If no IPv4 subnets are found, an error code is returned along with the error description.

15.4.6.2. subnet6-list command

This command is used to list all currently configured subnets. The subnets are returned in a brief form, i.e. a subnet identifier and subnet prefix is included for each subnet. In order to retrieve the detailed information about the subnet the subnet6-get should be used.

This command has the simple structure:

{
    "command": "subnet6-list"
}

The list of subnets returned as a result of this command is returned in the following format:

{
    "result": 0,
    "text": "2 IPv6 subnets found",
    "arguments": {
    "subnets": [
        {
            "id": 11,
            "subnet": "2001:db8:1::/64"
        },
        {
            "id": 233,
            "subnet": "3000::/16"
        }
    ]
}

If no IPv6 subnets are found, an error code is returned along with the error description.

15.4.6.3. subnet4-get command

This command is used to retrieve detailed information about the specified subnet. This command usually follows the subnet4-list, which is used to discover available subnets with their respective subnet identifiers and prefixes. Any of those parameters can be then used in subnet4-get to fetch subnet information:

{
    "command": "subnet4-get",
    "arguments": {
        "id": 10
    }
}

or

{
    "command": "subnet4-get",
    "arguments": {
        "subnet": "10.0.0.0/8"
    }
}

If the subnet exists the response will be similar to this:

{
    "result": 0,
    "text": "Info about IPv4 subnet 10.0.0.0/8 (id 10) returned",
    "arguments": {
        "subnets": [
            {
                "subnet": "10.0.0.0/8",
                "id": 1,
                "option-data": [
                    ....
                ]
                ...
            }
        ]
    }
}

15.4.6.4. subnet6-get command

This command is used to retrieve detailed information about the specified subnet. This command usually follows the subnet6-list, which is used to discover available subnets with their respective subnet identifiers and prefixes. Any of those parameters can be then used in subnet6-get to fetch subnet information:

{
    "command": "subnet6-get",
    "arguments": {
        "id": 11
    }
}

or

{
    "command": "subnet6-get",
    "arguments": {
        "subnet": "2001:db8:1::/64"
    }
}

If the subnet exists the response will be similar to this:

{
    "result": 0,
    "text": "Info about IPv6 subnet 2001:db8:1::/64 (id 11) returned",
    "arguments": {
        "subnets": [
            {
                "subnet": "2001:db8:1::/64",
                "id": 1,
                "option-data": [
                    ...
                ]
                ....
            }
        ]
    }
}

15.4.6.5. subnet4-add

This command is used to create and add new subnet to the existing server configuration. This operation has no impact on other subnets. The subnet identifier must be specified and must be unique among all subnets. If the identifier or a subnet prefix is not unique an error is reported and the subnet is not added.

The subnet information within this command has the same structure as the subnet information in the server configuration file with the exception that static host reservations must not be specified within subnet4-add. The commands described in Section 15.4.4, “host_cmds: Host Commands” should be used to add, remove and modify static reservations.

{
    "command": "subnet4-add",
    "arguments": {
        "subnets": [ {
            "id": 123,
            "subnet": "10.20.30.0/24",
            ...
        } ]
    }
}

The response to this command has the following structure:

{
    "result": 0,
    "text": "IPv4 subnet added",
    "arguments": {
        "subnets": [
            {
                "id": 123,
                "subnet": "10.20.30.0/24"
            }
        ]
    }
}

15.4.6.6. subnet6-add

This command is used to create and add new subnet to the existing server configuration. This operation has no impact on other subnets. The subnet identifier must be specified and must be unique among all subnets. If the identifier or a subnet prefix is not unique an error is reported and the subnet is not added.

The subnet information within this command has the same structure as the subnet information in the server configuration file with the exception that static host reservations must not be specified within subnet6-add. The commands described in Section 15.4.4, “host_cmds: Host Commands” should be used to add, remove and modify static reservations.

{
    "command": "subnet6-add",
    "arguments": {
        "subnet6": [ {
            "id": 234,
            "subnet": "2001:db8:1::/64",
            ...
        } ]
    }
}

The response to this command has the following structure:

{
    "result": 0,
    "text": "IPv6 subnet added",
    "arguments": {
        "subnet6": [
            {
                "id": 234,
                "subnet": "2001:db8:1::/64"
            }
        ]
    }
}

It is recommended, but not mandatory to specify subnet id. If not specified, Kea will try to assign the next subnet-id value. This automatic ID value generator is simple. It returns a previously automatically assigned value increased by 1. This works well, unless you manually create a subnet with a value bigger than previously used. For example, if you call subnet4-add five times, each without id, Kea will assign IDs: 1,2,3,4 and 5 and it will work just fine. However, if you try to call subnet4-add five times, with the first subnet having subnet-id of value 3 and remaining ones having no subnet-id, it will fail. The first command (with explicit value) will use subnet-id 3, the second command will create a subnet with id of 1, the third will use value of 2 and finally the fourth will have the subnet-id value auto-generated as 3. However, since there is already a subnet with that id, it will fail.

The general recommendation is to either: never use explicit values (so the auto-generated values will always work) or always use explicit values (so the auto-generation is never used). You can mix those two approaches only if you understand how the internal automatic subnet-id generation works.

Note

Subnet IDs must be greater than zero and less than 4294967295.

15.4.6.7. subnet4-del command

This command is used to remove a subnet from the server's configuration. This command has no effect on other configured subnets but removing a subnet has certain implications which the server's administrator should be aware of.

In most cases the server has assigned some leases to the clients belonging to the subnet. The server may also be configured with static host reservations which are associated with this subnet. The current implementation of the subnet4-del removes neither the leases nor host reservations associated with a subnet. This is the safest approach because the server doesn't loose track of leases assigned to the clients from this subnet. However, removal of the subnet may still cause configuration errors and conflicts. For example: after removal of the subnet, the server administrator may add a new subnet with the ID used previously for the removed subnet. This means that the existing leases and static reservations will be in conflict with this new subnet. Thus, we recommend that this command is used with extreme caution.

This command can also be used to completely delete an IPv4 subnet that is part of a shared network. If you want to simply remove the subnet from a shared network and keep the subnet configuration, use network4-subnet-del command instead.

The command has the following structure:

{
    "command": "subnet4-del",
    "arguments": {
        "id": 123
    }
}

The example successful response may look like this:

{
    "result": 0,
    "text": "IPv4 subnet 192.0.2.0/24 (id 123) deleted",
    "arguments": {
        "subnets": [
            {
                "id": 123,
                "subnet": "192.0.2.0/24"
            }
        ]
    }
}

15.4.6.8. subnet6-del command

This command is used to remove a subnet from the server's configuration. This command has no effect on other configured subnets but removing a subnet has certain implications which the server's administrator should be aware of.

In most cases the server has assigned some leases to the clients belonging to the subnet. The server may also be configured with static host reservations which are associated with this subnet. The current implementation of the subnet6-del removes neither the leases nor host reservations associated with a subnet. This is the safest approach because the server doesn't loose track of leases assigned to the clients from this subnet. However, removal of the subnet may still cause configuration errors and conflicts. For example: after removal of the subnet, the server administrator may add a new subnet with the ID used previously for the removed subnet. This means that the existing leases and static reservations will be in conflict with this new subnet. Thus, we recommend that this command is used with extreme caution.

This command can also be used to completely delete an IPv6 subnet that is part of a shared network. If you want to simply remove the subnet from a shared network and keep the subnet configuration, use network6-subnet-del command instead.

The command has the following structure:

{
    "command": "subnet6-del",
    "arguments": {
        "id": 234
    }
}

The example successful response may look like this:

{
    "result": 0,
    "text": "IPv6 subnet 2001:db8:1::/64 (id 234) deleted",
    "subnets": [
        {
            "id": 234,
            "subnet": "2001:db8:1::/64"
        }
    ]
}

15.4.6.9. network4-list, network6-list commands

These commands are used to retrieve full list of currently configured shared networks. The list contains only very basic information about each shared network. If more details are needed, please use network4-get or network6-get to retrieve all information available. This command does not require any parameters and its invocation is very simple:

{
    "command": "network4-list"
}

An example response for network4-list looks as follows:

{
    "arguments": {
        "shared-networks": [
            { "name": "floor1" },
            { "name": "office" }
        ]
    },
    "result": 0,
    "text": "2 IPv4 network(s) found"
}

network6-list follows exactly the same syntax for both the query and the response.

15.4.6.10. network4-get, network6-get commands

These commands are used to retrieve detailed information about shared networks, including subnets currently being part of a given network. Both commands take one mandatory parameter name, which specify the name of shared network. An example command to retrieve details about IPv4 shared network with a name "floor13" looks as follows:

{
    "command": "network4-get",
    "arguments": {
        "name": "floor13"
    }
}

An example response could look as follows:

{
    "result": 0,
    "text": "Info about IPv4 shared network 'floor13' returned",
    "arguments": {
        "shared-networks": [
        {
            "match-client-id": true,
            "name": "floor13",
            "option-data": [ ],
            "rebind-timer": 90,
            "relay": {
                "ip-address": "0.0.0.0"
            },
            "renew-timer": 60,
            "reservation-mode": "all",
            "subnet4": [
                {
                    "subnet": "192.0.2.0/24",
                    "id": 5,
                    // many other subnet specific details here
                },
                {
                    "id": 6,
                    "subnet": "192.0.3.0/31",
                    // many other subnet specific details here
                }
            ],
            "valid-lifetime": 120
        }
        ]
    }
}

Note that actual response contains many additional fields that are omitted here for clarity. The response format is exactly the same as used in config-get, just is limited to returning shared networks information.

15.4.6.11. network4-add, network6-add commands

These commands are used to add a new shared network. New network has to have unique name. This command requires one parameter shared-networks, which is a list and should contain exactly one entry that defines the network. The only mandatory element for a network is its name. Although it does not make operational sense, it is allowed to add an empty shared network that does not have any subnets in it. That is allowed for testing purposes, but having empty networks (or with only one subnet) is discouraged in production environments. For details regarding syntax, see Section 8.4, “Shared networks in DHCPv4” and Section 9.4, “Shared networks in DHCPv6”.

Note

As opposed to parameter inheritance during full new configuration processing, this command does not fully handle parameter inheritance and any missing parameters will be filled with default values, rather than inherited from global scope.

An example that showcases how to add a new IPv4 shared network looks as follows:

{
    "command": "network4-add",
    "arguments": {
        "shared-networks": [ {
            "name": "floor13",
            "subnet4": [
            {
                "id": 100,
                "pools": [ { "pool": "192.0.2.2-192.0.2.99" } ],
                "subnet": "192.0.2.0/24",
                "option-data": [
                    {
                        "name": "routers",
                        "data": "192.0.2.1"
                    }
                ]
            },
            {
                "id": 101,
                "pools": [ { "pool": "192.0.3.2-192.0.3.99" } ],
                "subnet": "192.0.3.0/24",
                "option-data": [
                    {
                        "name": "routers",
                        "data": "192.0.3.1"
                    }
                ]
            } ]
        } ]
    }
}

Assuming there was no shared network with a name floor13 and no subnets with id 100 and 101 previously configured, the command will be successful and will return the following response:

{
    "arguments": {
        "shared-networks": [ { "name": "floor13" } ]
    },
    "result": 0,
    "text": "A new IPv4 shared network 'floor13' added"
}

The network6-add uses the same syntax for both the query and the response. However, there are some parameters that are IPv4-only (e.g. match-client-id) and some are IPv6-only (e.g. interface-id). The same applies to subnets within the network.

15.4.6.12. network4-del, network6-del commands

These commands are used to delete existing shared networks. Both commands take exactly one parameter 'name' that specifies the name of the network to be removed. An example invocation of network4-del command looks as follows:

{
    "command": "network4-del",
    "arguments": {
        "name": "floor13"
    }
}

Assuming there was such a network configured, the response will look similar to the following:

{
    "arguments": {
        "shared-networks": [
            {
                "name": "floor13"
            }
        ]
    },
    "result": 0,
    "text": "IPv4 shared network 'floor13' deleted"
}

The network6-del command uses exactly the same syntax for both the command and the response.

If there are any subnets belonging to the shared network being deleted, they will be demoted to a plain subnet. There is an optional parameter called subnets-action that, if specified, takes one of two possible values: keep (which is the default) and delete. It controls whether the subnets be demoted to plain subnets or removed. An example usage in network6-del command that deletes the shared network and all subnets in it could looks like as follows:

{
    "command": "network4-del",
    "arguments": {
        "name": "floor13",
        "subnets-action": "delete"
    }
}

Alternatively, if you want to completely remove the subnets, you may use subnet4-del or subnet6-del commands.

15.4.6.13. network4-subnet-add, network6-subnet-add commands

These commands are used to add existing subnets to existing shared networks. There are several ways to add new shared network. System administrator can add the whole shared network at once, either by editing a configuration file or by calling network4-add or network6-add commands with desired subnets in it. This approach works better for completely new shared subnets. However, there may be cases when an existing subnet is running out of addresses and needs to be extended with additional address space. In other words another subnet has to be added on top of it. For this scenario, a system administrator can use network4-add or network6-add and then add existing subnet to this newly created shared network using network4-subnet-add or network6-subnet-add.

The network4-subnet-add and network6-subnet-add commands take two parameters: id, which is an integer and specifies subnet-id of existing subnet to be added to a shared network; and name, which specifies name of the shared network the subnet will be added to. The subnet must not belong to any existing network. In case you want to reassign a subnet from one shared network to another, please use network4-subnet-del or network6-subnet-del commands first.

An example invocation of network4-subnet-add command looks as follows:

{
    "command": "network4-subnet-add",
    "arguments": {
        "name": "floor13",
        "id": 5
    }
}

Assuming there is a network named 'floor13', there is a subnet with subnet-id 5 and it is not a part of existing network, the command will return a response similar to the following:

{
    "result": 0,
    "text": "IPv4 subnet 10.0.0.0/8 (id 5) is now part of shared network 'floor1'"
}

The network6-subnet-add command uses exactly the same syntax for both the command and the response.

Note

As opposed to parameter inheritance during full new configuration processing or when adding a new shared network with new subnets, this command does not fully handle parameter inheritance and any missing parameters will be filled with default values, rather than inherited from global scope or from the shared network.

15.4.6.14. network4-subnet-del, network6-subnet-del commands

These commands are used to remove a subnet that is part of existing shared network and demote it to a plain, stand-alone subnet. If you want to remove a subnet completely, use subnet4-del or subnet6-del commands instead. The network4-subnet-del and network6-subnet-del commands take two parameters: id, which is an integer and specifies subnet-id of existing subnet to be removed from a shared network; and name, which specifies name of the shared network the subnet will be removed from.

An example invocation of the network4-subnet-del command looks as follows:

 {
    "command": "network4-subnet-del",
    "arguments": {
        "name": "floor13",
        "id": 5
    }
 }

Assuming there was a subnet with subnet-id equal to 5 that was part of a shared network named 'floor13', the response would look similar to the following:

{
    "result": 0,
    "text": "IPv4 subnet 10.0.0.0/8 (id 5) is now removed from shared network 'floor13'"
}

The network6-subnet-del command uses exactly the same syntax for both the command and the response.

15.4.7. class_cmds: Class Commands

This section describes the Class Commands hooks library, which exposes several control commands for manipulating client classes, being a part of the Kea DHCP servers' configurations, without the need to restart those servers. Using these commands it is possible to add, update, delete and list client classes configured for a given server.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

The Class Commands hooks library is available to premium Kea customers only.

15.4.7.1. class-add command

The class-add command is used to add a new client class to the DHCP server configuration. This class is appended at the end of the list of classes used by the server, i.e. this class may depend on any of the already configured client classes.

The following example demonstrates how to add a new client class to the DHCPv4 server configuration:

{
    "command": "class-add",
    "arguments": {
        "client-classes": [
            {
                "name": "ipxe_efi_x64",
                "test": "option[93].hex == 0x0009",
                "next-server": "192.0.2.254",
                "server-hostname": "hal9000",
                "boot-file-name": "/dev/null"
            }
        ]
    }
}

Note that the client-classes parameter is a JSON list, but it allows only a single client class to be present. In the future this hooks library may be extended to support specification of multiple client classes within a single class-add command.

The following is the example response to the class-add command:

{
    "result": 0,
    "text": "Class 'ipxe_efi_x64' added."
}

15.4.7.2. class-update command

The class-update command is used to update an existing client class in the DHCP server configuration. If the client class with the given name doesn't exist, the server returns result code of 3. In such case, the server configuration is not modified (client class is not created). The class-add command must be used instead to create the new client class.

The class-update command has the same arguments structure as class-add command:

{
    "command": "class-update",
    "arguments": {
        "client-classes": [
            {
                "name": "ipxe_efi_x64",
                "test": "option[93].hex == 0x0017",
                "next-server": "0.0.0.0",
                "server-hostname": "xfce",
                "boot-file-name": "/dev/null"
            }
        ]
    }
}

The following is the example response:

{
    "result": 0,
    "text": "Class 'ipxe_efi_x64' updated."
}

Any parameter of the client class can be modified with this command, except name. There is currently no way to rename the class, because the class name is used as a key for searching the class to be updated. In order to achieve similar effect to renaming the class, an existing class should be removed with class-del command and then added again with a different name using class-add. Note however, that the class with the new name will be added at the end of the list of configured classes.

15.4.7.3. class-del command

The class-del is used to remove a particular class from the server configuration. The class to be removed is identified by name. The class won't be removed if there are other classes dependening on it. In order to remove such class, the dependent classes must be removed first.

The following is the example command removing ipxe_efi_x64 class:

{
    "command": "class-del",
    "arguments": {
        {
            "name": "ipxe_efi_x64"
        }
    }
}

The following is the sample response to the class-del command when the specified client class has been found:

{
    "result": 0,
    "text": "Class 'ipxe_efi_x64' deleted."
}

If the class doesn't exist, the result of 3 is returned.

15.4.7.4. class-list command

The class-list is used to retrieve a list of all client classes. This command includes no arguments:

{
    "command": "class-list"
}

The following is the example response of the server including the list of client classes:

{
    "result": 0,
    "text": "2 classes found",
    "arguments": {
        "client-classes": [
            {
                "name": "ipxe_efi_x64"
            },
            {
                "name": "pxeclient"
            }
        ]
    }
}

Note that the returned list does not contain full class definitions, but merely class names. In order to retrieve full class information, the class-get command should be used.

15.4.7.5. class-get command

The class-get is used to retrieve detail information about specified class. The command structure is very simple:

{
    "command": "class-get",
    "arguments": {
        "name": "pxeclient"
    }
}

If the class with the specified name doesn't exist, the status code of 3 is returned. If the specified client class exists, the class details are returned in the following format:

{
    "result": 0,
    "text": "Class 'pxeclient' definition returned",
    "arguments": {
        "client-classes": [
            {
                "name": "pxeclient",
                "only-if-required": true,
                "test": "option[vendor-class-identifier].text == 'PXEClient'",
                "option-def": [
                    {
                        "name": "configfile",
                        "code": 209,
                        "type": "string"
                    }
                ],
                "option-data": [ ],
                "next-server": "0.0.0.0",
                "server-hostname": "xfce",
                "boot-file-name": "/dev/null"
            }
        ]
    }
}

Note that the example above is DHCPv4 specific. The last three parameters are only returned by the DHCPv4 server and never returned by the DHCPv6 server. Also, some of the parameters provided in this example may not be returned if they are not specified for the class. Specifically, only-if-required, test and the option-def are not returned if they are not specified for the class.

15.4.8. ha: High Availability

This section describes the High Availability hooks library, which can be loaded on a pair of DHCPv4 or DHCPv6 servers to increase reliability of the DHCP service in case of outage of one of the servers. This library used to be only available to ISC customers, but is now part of the open source Kea, available to all users.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

High Availability (HA) of the DHCP service is provided by running multiple cooperating server instances. If any of these instances becomes unavailable for whatever reason (DHCP software crash, Control Agent software crash, power outage, hardware failure), a surviving server instance can continue providing the reliable service to the clients. Many DHCP servers implementations include "DHCP Failover" protocol, which most significant features are: communication between the servers, partner failure detection and leases synchronization between the servers. However, the DHCPv4 failover standardization process was never completed at IETF. The DHCPv6 failover standard (RFC 8156) was published, but it is complex, difficult to use, has significant operational constraints and is different than its v4 counterpart. Although it may be useful for some users to use a "standard" failover protocol, it seems that most of the Kea users are simply interested in a working solution which guarantees high availability of the DHCP service. Therefore, Kea HA hook library derives major concepts from the DHCP Failover protocol but uses its own solutions for communication, configuration and its own state machine, which greatly simplifies its implementation and generally better fits into Kea. Also, it provides the same features in both DHCPv4 and DHCPv6. This document purposely uses the term "High Availability" rather than "Failover" to emphasize that it is not the Failover protocol implementation.

The following sections describe the configuration and operation of the Kea HA hook library.

15.4.8.1. Supported Configurations

The Kea HA hook library supports two configurations also known as HA modes: load balancing and hot standby. In the load balancing mode, there are two servers responding to the DHCP requests. The load balancing function is implemented as described in RFC3074, with each server responding to 1/2 of received DHCP queries. When one of the servers allocates a lease for a client, it notifies the partner server over the control channel (RESTful API), so as the partner can save the lease information in its own database. If the communication with the partner is unsuccessful, the DHCP query is dropped and the response is not returned to the DHCP client. If the lease update is successful, the response is returned to the DHCP client by the server which has allocated the lease. By exchanging the lease updates, both servers get a copy of all leases allocated by the entire HA setup and any of the servers can be switched to handle the entire DHCP traffic if its partner becomes unavailable.

In the load balancing configuration, one of the servers must be designated as "primary" and the other server is designated as "secondary". Functionally, there is no difference between the two during the normal operation. This distniction is required when the two servers are started at (nearly) the same time and have to synchronize their lease databases. The primary server synchronizes the database first. The secondary server waits for the primary server to complete the lease database synchronization before it starts the synchronization.

In the hot standby configuration one of the servers is designated as "primary" and the second server is designated as "secondary". During the normal operation, the primary server is the only one that responds to the DHCP requests. The secodary server receives lease updates from the primary over the control channel. However, it does not respond to any DHCP queries as long as the primary is running or, more accurately, until the secondary considers the primary to be offline. When the secondary server detects the failure of the primary, it starts responding to all DHCP queries.

In the configurations described above, the primary, secondary and standby are referred to as "active" servers, because they receive lease updates and can automatically react to the partner's failures by responding to the DHCP queries which would normally be handled by the partner. The HA hook library supports another server type (role) - backup server. The use of the backup servers is optional. They can be used in both load balancing and hot standby setup, in addition to the active servers. There is no limit on the number of backup servers in the HA setup. However, the presence of the backup servers increases latency of the DHCP responses, because not only do active servers send lease updates to each other, but also to the backup servers.

15.4.8.2. Clocks on Active Servers

Synchronized clocks are essential for the HA setup to operate reliably. The servers share lease information via lease updates and during synchronization of the databases. The lease information includes the time when the lease has been allocated and when it expires. Some clock skew between the servers participating the HA setup would usually exist. This is acceptable as long as the clock skew is relatively low, comparing to the lease lifetimes. However, if the clock skew becomes too high, the different notions of time for the lease expiration by different servers may cause the HA system to malfuction. For example, one server may consider a valid lease to be expired. As a consequence, the lease reclamation process may remove a name associated with this lease from the DNS, even though the lease may later get renewed by a client.

Each active server monitors the clock skew by comparing its current time with the time returned by its partner in response to the heartbeat command. This gives a good approximation of the clock skew, although it doesn't take into account the time between sending the response by the partner and receiving this response by the server which sent the heartbeat command. If the clock skew exceeds 30 seconds, a warning log message is issued. The administrator may correct this problem by synchronizing the clocks (e.g. using NTP). The servers should notice the clock skew correction and stop issuing the warning

If the clock skew is not corrected and it exceeds 60 seconds, the HA service on each of the servers is terminated, i.e. the state machine enters the terminated state. The servers will continue to respond to the DHCP clients (as in the load-balancing or hot-standby mode), but will neither exchange lease updates nor heartbeats and their lease databases will diverge. In this case, the administrator should synchronize the clocks and restart the servers.

15.4.8.3. Server States

The DHCP server operating within an HA setup runs a state machine and the state of the server can be retrieved by its peers using the ha-heartbeat command sent over the RESTful API. If the partner server doesn't respond to the ha-heartbeat command longer than configured amount of time, the communication is considered interrupted and the server may (depending on the configuration) use additional measures (desribed further in this document) to verify if the partner is still operating. If it finds that the partner is not operating, the server transitions to the partner-down state to handle the entire DHCP traffic directed to the system.

In this case, the surviving server continues to send the ha-heartbeat command to detect when the partner wakes up. The partner synchronizes the lease database and when it is finally ready to operate, the surviving server returns to the normal operation, i.e. load-balancing or hot-standby state.

The following is the list of all possible states into which the servers may transition:

  • backup - normal operation of the backup server. In this state it receives lease updates from the active servers.

  • hot-standby - normal operation of the active server running in the hot standby mode. Both primary and standby server are in this state during their normal operation. The primary server is responding to the DHCP queries and sends lease updates to the standby server and to the backup servers, if any backup servers are present.

  • load-balancing - normal operation of the active server running in the load balancing mode. Both primary and secondary server are in this state during their normal operation. Both servers are responding to the DHCP queries and send lease updates to each other and to the backup servers, if any backup servers are present.

  • partner-down - an active server transitions to this state after detecting that its partner (another active server) is offline. The server doesn't transition to this state if any of the backup servers is unavailable. In the partner-down state the server responds to all DHCP queries, so also those queries which are normally handled by the active server which is now unavailable.

  • ready - an active server transitions to this state after synchronizing its lease database with an active partner. This state is to indicate to the partner (likely being in the partner-down state that it may return to the normal operation. When it does, the server being in the ready state will also start normal operation.

  • syncing - an active server transitions to this state to fetch leases from the active partner and update the local lease database. When it this state, it issues the dhcp-disable to disable the DHCP service of the partner from which the leases are fetched. The DHCP servie is disabled for the maximum time of 60 seconds, after which it is automatically enabled, in case the syncing partner has died again failing to re-enable the service. If the synchronization is completed the syncing server issues the dhcp-enable to re-enable the DHCP service of the partner. The syncing operation is synchronous. The server is waiting for an answer from the partner and is not doing anything else while the leases synchronization takes place. A server which is configured to not synchronize its database with the partner, i.e. when the sync-leases configuration parameter is set to false, will never transition to this state. Instead, it will transition directly from the waiting to ready state.

  • terminated - an active server transitions to this state when the High Availability hooks library is unable to further provide reliable service and a manual intervention of the administrator is required to correct the problem. It is envisaged that various issues with the HA setup may cause the server to transition to this state in the future. As of Kea 1.4.0 release, the only issue causing the HA service to terminate is unacceptably high clock skew between the active servers, i.e. if the clocks on respective servers are more than 60 seconds apart. While in this state, the server will continue responding to the DHCP clients based on the HA mode selected (load balancing or hot standby), but the lease updates won't be exchanged and the heartbeats won't be sent. Once a server has entered the "terminated" state it will remain in this state until it is restarted. The administrator must correct the issue which caused this situation prior to restarting the server (e.g. synchronize clocks). Otherwise, the server will return to the "terminated" state as soon as it finds that the clock skew is still too high.

  • waiting - each started server instance enters this state. The backup server will transition directly from this state to the backup state. An active server will send heartbeat to its partner to check its state. If the partner appears to be unavailable the server will transition to the partner-down, otherwise it will transition to the syncing or ready state (depending on the setting of the sync-leases configuration parameter). If both servers appear to be in the waiting state (concurrent startup) the primary server will transition to the next state first. The secondary or standby server will remain in the waiting state until the primary transitions to the ready state.

Note

Currently, restarting the HA service being in the terminated state requires restarting the DHCP server or reloading its configuration. In the future, we will provide a command to restart the HA service.

Whether the server responds to the DHCP queries and which queries it responds to is a matter of the server's state, if no administrative action is performed to configure the server otherwise. The following table provides the default behavior for various states.

The DHCP Server Scopes denotes what group of received DHCP queries the server responds to in the given state. The in-depth explanation what the scopes are can be found below.

Table 15.2. Default behavior of the server in various HA states

StateServer TypeDHCP ServiceDHCP Service Scopes
backupbackup serverdisablednone
hot-standbyprimary or standby (hot standby mode)enabledHA_server1 if primary, none otherwise
load-balancingprimary or secondary (load balancing mode)enabledHA_server1 or HA_server2
partner-downactive serverenabledall scopes
readyactive serverdisablednone
syncingactive serverdisablednone
terminatedactive serverenabledsame as in the load-balancing or hot-standby state
waitingany serverdisablednone


The DHCP service scopes require some explanation. The HA configuration must specify a unique name for each server within the HA setup. This document uses the following convention within provided examples: server1 for a primary server, server2 for the secondary or standby server and server3 for the backup server. In the real life any names can be used as long as they remain unique.

In the load balancing mode there are two scopes named after the active servers: HA_server1 and HA_server2. The DHCP queries load balanced to the server1 belong to the HA_server1 scope and the queries load balanced to the server2 belong to the HA_server2 scope. If any of the servers is in the partner-down state, it is responsible for serving both scopes.

In the hot standby mode, there is only one scope HA_server1 because only the server1 is responding to the DHCP queries. If that server becomes unavailable, the server2 becomes responsible for this scope.

The backup servers do not have their own scopes. In some cases they can be used to respond to the queries belonging to the scopes of the active servers. Also, a server which is neither in the partner-down state nor in the normal operation serves no scopes.

The scope names can be used to associate pools, subnets and networks with certain servers, so as only these servers can allocate addresses or prefixes from those pools, subnets or network. This is done via the client classification mechanism (see below).

15.4.8.4. Scope Transition in Partner Down Case

When one of the servers finds that its partner is unavailble, it will start serving clients from its own scope and the scope of the partner which is considered unavailable. This is straight forward for the new clients, i.e. sending DHCPDISCOVER (DHCPv4) or Solicit (DHCPv6), because those requests are not sent to any particular server. The available server will respond to all such queries when it is in the partner-down state.

When the client is renewing a lease, it will send its DHCPREQUEST (DHCPv4) or Renew (DHCPv6) message directly to the server which has allocated the lease being renewed. Because this server is unavailable, the client will not get any response. In that case, the client continues to use its lease and re-tries to renew until the rebind timer (T2) elapses. The client will now enter the rebinding phase, in which it will send DHCPREQUEST (DHCPv4) or Rebind (DHCPv6) message to any available server. The surviving server will receive the rebinding request and will (typically) extend the lifetime of the lease. The client will continue to contact that new server to renew its lease as appropriate.

When the other server becomes available, both active servers will eventually transition to the load-balancing or hot-standby state, in which they will be responsible for their own scopes. Some clients belonging to the scope of the started server will be trying to renew their leases via the surviving server. This server will not respond to them anymore and the client will eventually transition back to the right server via rebinding mechanism again.

15.4.8.5. Load Balancing Configuration

The following is the configuration snippet which enables high availability on the primary server within the load balancing configuration. The same configuration should be applied on the secondary and the backup server, with the only difference that the this-server-name should be set to server2 and server3 on those servers respectively.

{
"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "load-balancing",
                    "heartbeat-delay": 10000,
                    "max-response-delay": 10000,
                    "max-ack-delay": 5000,
                    "max-unacked-clients": 5,
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary",
                            "auto-failover": true
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "secondary",
                            "auto-failover": true
                        },
                        {
                            "name": "server3",
                            "url": "http://192.168.56.99:8080/",
                            "role": "backup",
                            "auto-failover": false
                        }
                    ]
                } ]
            }
        }
    ],

    "subnet4": [
        {
            "subnet": "192.0.3.0/24",
            "pools": [
                {
                    "pool": "192.0.3.100 - 192.0.3.150",
                    "client-class": "HA_server1"
                },
                {
                    "pool": "192.0.3.200 - 192.0.3.250",
                    "client-class": "HA_server2"
                }
            ],

            "option-data": [
                {
                    "name": "routers",
                    "data": "192.0.3.1"
                }
            ],

            "relay": { "ip-address": "10.1.2.3" }
        }
    ],

    ...

}

}

Two hook libraries must be loaded to enable HA: libdhcp_lease_cmds.so and libdhcp_ha.so. The latter provides the implemenation of the HA feature. The former enables control commands required by HA to fetch and manipulate leases on the remote servers. In the example provided above, it is assumed that Kea libraries are installed in the /usr/lib directory. If Kea is not installed in the /usr directory, the hook libraries locations must be updated accordingly.

The HA configuration is specified within the scope of the libdhcp_ha.so. Note that the top level parameter high-availability is a list, even though it currently contains only one entry. In the future this configuration is likely to be extended to contain more entries, if the particular server can participate in more than one HA relationships.

The following are the global parameters which control the server's behavior with respect to HA:

  • this-server-name - is a unique identifier of the server within this HA setup. It must match with one of the servers specified within peers list.

  • mode - specifies a HA mode of operation. Currently supported modes are load-balancing and hot-standby.

  • heartbeat-delay - specifies a duration in milliseconds between the last heartbeat (or other command sent to the partner) and sending the next heartbeat. The heartbeats are sent periodically to gather the status of the partner and to verify whether the partner is still operating. The default value of this parameter is 10000 ms.

  • max-response-delay - specifies a duration in milliseconds since the last successful communication with the partner, after which the server assumes that the communication with the partner is interrupted. This duration should be greater than the heartbeat-delay. Usually it is a greater than the duration of multiple heartbeat-delay values. When the server detects that the communication is interrupted, it may transition to the partner-down state (when max-unacked-clients is 0) or trigger failure detection procedure using the values of the two parameters below. The default value of this parameter is 60000.

  • max-ack-delay - is one of the parameters controlling partner failure detection. When the communication with the partner is interrupted, the server examines values of the secs field (DHCPv4) or Elapsed Time option (DHCPv6) which denote how long the DHCP client has been trying to communicate with the DHCP server. This parameter specifies the maximum time in milliseconds for the client to try to communicate with the DHCP server, after which this server assumes that the client failed to communicate with the DHCP server (is "unacked"). The default value of this parameter is 10000.

  • max-unacked-clients - specifies how many "unacked" clients are allowed (see max-ack-delay) before this server assumes that the partner is offline and transitions to the partner-down state. The special value of 0 is allowed for this parameter which disables failure detection mechanism. In this case, the server which can't communicate with the partner over the control channel assumes that the partner server is down and transitions to the partner-down state immediately. The default value of this parameter is 10.

The values of max-ack-delay and max-unacked-clients must be selected carefully, taking into account specifics of the network in which DHCP servers are operating. Note that the server in question may not respond to some of the DHCP clients because these clients are not to be serviced by this server (per administrative policy). The server may also drop malformed queries from the clients. Therefore, selecting too low value for the max-unacked-clients may result in transitioning to the partner-down state even though the partner is still operating. On the other hand, selecting too high value may result in never transitioning to the partner-down state if the DHCP traffic in the network is very low (e.g. night time), because the number of distinct clients trying to communicate with the server could be lower than max-unacked-clients.

In some cases it may be useful to disable the failure detection mechanism altogether, if the servers are located very close to each other and the network partitioning is unlikely, i.e. failure to respond to heartbeats is only possible when the partner is offline. In such cases, set the max-unacked-clients to 0.

The peers parameter contains a list of servers within this HA setup. In this configuration it must contain at least one primary and one secondary server. It may also contain unlimited number of backup servers. In this example there is one backup server which receives lease updates from the active servers.

There are the following parameters specified for each of the peers within this list:

  • name - specifies unique name for the server.

  • url - specifies URL to be used to contact this server over the control channel. Other servers use this URL to send control commands to that server.

  • role - denotes the role of the server in the HA setup. The following roles are supported in the load balancing configuration: primary, secondary and backup. There must be exactly one primary and one secondary server in the load balancing setup.

  • auto-failover - a boolean value which denotes whether the server detecting a partner's failure should automatically start serving partner's clients. The default value of this parameter is true.

In our example configuration, both active servers can allocate leases from the subnet "192.0.3.0/24". This subnet contains two address pools: "192.0.3.100 - 192.0.3.150" and "192.0.3.200 - 192.0.3.250", which are associated with HA servers scopes using client classification. When the server1 processes a DHCP query it will use the first pool for the lease allocation. Conversely, when the server2 is processing the DHCP query it will use the second pool. When any of the servers is in the partner-down state, it can serve leases from both pools and it will select the pool which is appropriate for the received query. In other words, if the query would normally be processed by the server2, but this server is not available, the server1 will allocate the lease from the pool of "192.0.3.200 - 192.0.3.250".

15.4.8.6. Load Balancing with Advanced Classification

In the previous section we have provided an example which demonstrated the load balancing configuration with the client classification limited to the use of HA_server1 and HA_server2 classes, which are dynamically assigned to the received DHCP queries. In many cases it will be required to use HA in deployments which already use some client classification.

Suppose there is a system which classifies devices into two groups: phones and laptops, based on some classification criteria specified in Kea configuration file. Both types of devices are allocated leases from different address pools. Introducing HA in the load balancing mode is expected to result in further split of each of those pools, so as each of the servers can allocate leases for some part of the phones and part of the laptops. This requires that each of the existing pools should be split between the HA_server1 and HA_server2, so we end up with the following classes:

  • phones_server1
  • laptops_server1
  • phones_server2
  • laptops_server2

The corresponding server configuration using advanced classification (and member expression) is provided below. For brevity the HA hook library configuration has been removed from this example.

{
"Dhcp4": {

    "client-classes": [
        {
            "name": "phones",
            "test": "substring(option[60].hex,0,6) == 'Aastra'",
        },
        {
            "name": "laptops",
            "test": "not member('phones')"
        },
        {
            "name": "phones_server1",
            "test": "member('phones') and member('HA_server1')"
        },
        {
            "name": "phones_server2",
            "test": "member('phones') and member('HA_server2')"
        },
        {
            "name": "laptops_server1",
            "test": "member('laptops') and member('HA_server1')"
        },
        {
            "name": "laptops_server2",
            "test": "member('laptops') and member('HA_server2')"
        }
    ],

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {

                    ...

                } ]
            }
        }
    ],

    "subnet4": [
        {
            "subnet": "192.0.3.0/24",
            "pools": [
                {
                    "pool": "192.0.3.100 - 192.0.3.125",
                    "client-class": "phones_server1"
                },
                {
                    "pool": "192.0.3.126 - 192.0.3.150",
                    "client-class": "laptops_server1"
                },
                {
                    "pool": "192.0.3.200 - 192.0.3.225",
                    "client-class": "phones_server2"
                },
                {
                    "pool": "192.0.3.226 - 192.0.3.250",
                    "client-class": "laptops_server2"
                }
            ],

            "option-data": [
                {
                    "name": "routers",
                    "data": "192.0.3.1"
                }
            ],

            "relay": { "ip-address": "10.1.2.3" }
        }
    ],

    ...

}

}

The configuration provided above splits the address range into four pools. Two pools are dedicated to server1 and two are dedicated for server2. Each server can assign leases to both phones and laptops. Both groups of devices are assigned addresses from different pools. The HA_server1 and HA_server2 are builtin classes (see Section 14.2, “Builtin Client Classes”) and they don't need to be declared. They are assigned dynamically by the HA hook library as a result of load balancing algorithm. The phones_* and laptop_* evaluate to "true" when the query belongs to a given combination of other classes, e.g. HA_server1 and phones. The pool will be selected accordingly as a result of such evaluation.

Consult Chapter 14, Client Classification for details on how to use member expression and about class dependencies.

15.4.8.7. Hot Standby Configuration

The following is the example configuration of the primary server in the hot standby configuration:

{
"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "hot-standby",
                    "heartbeat-delay": 10000,
                    "max-response-delay": 10000,
                    "max-ack-delay": 5000,
                    "max-unacked-clients": 5,
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary",
                            "auto-failover": true
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "standby",
                            "auto-failover": true
                        },
                        {
                            "name": "server3",
                            "url": "http://192.168.56.99:8080/",
                            "role": "backup",
                            "auto-failover": false
                        }
                    ]
                } ]
            }
        }
    ],

    "subnet4": [
        {
            "subnet": "192.0.3.0/24",
            "pools": [
                {
                    "pool": "192.0.3.100 - 192.0.3.250",
                    "client-class": "HA_server1"
                }
            ],

            "option-data": [
                {
                    "name": "routers",
                    "data": "192.0.3.1"
                }
            ],

            "relay": { "ip-address": "10.1.2.3" }
        }
    ],

    ...

}

}

This configuration is very similar to the load balancing configuration described Section 15.4.8.5, “Load Balancing Configuration”, with a few notable differences.

The mode is now set to hot-standby, in which only one server is responding to the DHCP clients. If the primary server is online, the primary server is responding to all DHCP queries. The standby server takes over the entire DHCP traffic when it discovers that the primary is unavailable.

In this mode, the non-primary active server is called standby and that's what the role of the second active server is set to.

Finally, because there is always one server responding to the DHCP queries, there is only one scope HA_server1 in use within pools definitions. In fact, the client-class parameter could be removed from this configuration without harm, because there are no conflicts in lease allocations by different servers as they do not allocate leases concurrently. The client-class is left in this example mostly for demonstration purposes, to highlight the differences between the hot standby and load balancing mode of operation.

15.4.8.8. Lease Information Sharing

An HA enabled server informs its active partner about allocated or renewed leases by sending appropriate control commands. The partner updates the lease information in its own database. When the server starts up for the first time or recovers after a failure it synchronizes its lease database with the partner. These two mechanisms guarantee consistency of the lease information between the servers and allow for designating one of the servers to handle the entire DHCP traffic in case the other server becomes unavailable.

In some cases, though, it is desired to disable lease updates and/or database synchronization between the active servers if the exchange of information about the allocated leases is performed using some other mechanism. Kea supports various types of databases to be used as a storage for leases, e.g. MySQL, Postgres, Cassandra. Those databases include builtin solutions for data replication which are often used by Kea users to provide redundancy.

The HA hook library supports such scenarios by allowing to disable lease updates over the control channel and/or lease database synchronization, leaving the server to rely on the database replication mechanism. This is controlled by the two boolean parameters: send-lease-updates and sync-leases, which values default to true:

{
"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "load-balancing",
                    "send-lease-updates": false,
                    "sync-leases": false,
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary"
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "secondary"
                        }
                    ]
                } ]
            }
        }
    ],

    ...

}

In the most typical use case, both parameters are set to the same value, i.e. both are false if the database replication is in use, or both are true otherwise. Introducing two separate parameters to control lease updates and lease database synchronization is aimed at possible special use cases, e.g. synchronization is performed by copying a lease file (therefore the sync-leases is set to false), but lease updates should be conducted as usual (send-lease-updates set to true). It should be noted that Kea doesn't natively support such use case, but users may develop their own scripts and tools around Kea to provide such mechanisms. The HA hooks library configuration is designed to maximize the administration flexibility.

15.4.8.9. Controlling Lease Page Size Limit

An HA enabled server initiates synchronization of the lease database after down time or upon receiving ha-sync command. The server uses commands described in Section 15.4.5.1.3, “lease4-get-page, lease6-get-page commands” to fetch leases from the partner server (lease queries). The size of the results page (maximum number of leases to be returned in a single response to one of these commands) can be controlled via HA hooks library configuration. Increasing the page size decreases the number of lease queries sent to the partner server, but it causes the partner server to generate larger responses, which lengthens transmission time as well as memory and CPU utilization on both servers. Decreasing the page size helps to decrease resources utilization but requires more lease queries to be issued to fetch the entire lease database.

The default value of the sync-page-limit controlling the page size is 10000. This means that the entire lease database can be fetched with a single command if the size of this database is equal or lower than 10000.

15.4.8.10. Discussion about Timeouts

In deployments with large number of clients connected to the network, lease database synchronization after the server failure may be a time consuming operation. The synchronizing server needs to gather all leases from the partner which yields a large response over the RESTful interface. The server receives leases using the paging mechanism described in Section 15.4.8.9, “Controlling Lease Page Size Limit”. Before the page of leases is fetched, the synchronizing server sends a dhcp-disable command to disable the DHCP service on the partner server. If the service is already disabled, this command will reset the timeout for the DHCP service being disabled. This timeout value is by default set to 60 seconds. If fetching a single page of leases takes longer, the partner server will assume that the synchronizing server died and will resume its DHCP service. The connection of the synchronizing server with its partner is also guarded with the timeout. If the synchronization of a single page of leases takes longer than 60 seconds the synchronizing server terminates the connection and the synchronization fails. Both timeout values are controlled by a single configuration parameter sync-timeout. The following configuration snippet deminstrates how to modify the timeout for automatic re-enabling of the DHCP service on the partner server and the timeout for fetching a single page of leases from 60 seconds to 90 seconds:

{
"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "load-balancing",
                    "sync-timeout": 90000,
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary"
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "secondary"
                        }
                    ]
                } ]
            }
        }
    ],

    ...

}

It is important to note that extending this value may sometimes be insufficient to prevent issues with timeouts during lease database synchronization. The control commands travel via Control Agent, which also monitors incoming (with synchronizing server) and outgoing (with DHCP server) connections for timeouts. The DHCP server also monitors the connection from the Control Agent for timeouts. Those timeouts can't be currently modified via configuration. Extending these timeouts is only possible by modifing them in the Kea code and recompiling the server. The relevant constants are located in the Kea sources: src/lib/config/timeouts.h.

15.4.8.11. Pausing HA State Machine

The high availability state machine includes many different states described in detail in Section 15.4.8.3, “Server States”. The server enters each state when certain conditions are met, most often taking into account the partner server's state. In some states the server performs specific actions, e.g. synchronization of the lease database in the syncing state or responding to DHCP queries according to the configured mode of operation in the load-balancing and hot-standby states.

By default, transitions between the states are performed automatically and the server administrator has no direct control when the transitions take place and, in most cases, the administrator doesn't need such control. In some situations, however, the administrator may want to "pause" the HA state machine in a selected state to perform some additional administrative actions before the server transitions to the next state.

Consider a server failure which results in a loss of the entire lease database. Typically, the server will rebuild its lease database when it enters the syncing state by querying the partner server for leases, but it is possible that the partner was also experiencing a failure and lacks lease information. In this case, it may be required to reconstruct lease databases on both servers from some external source, e.g. a backup server. If the lease database is going to be reconstructed via RESTful API, the servers should be started in the initial, i.e. waiting state and remain in this state while leases are being added. In particular, the servers should not attempt to synchronize their lease databases nor start serving DHCP clients.

The HA hooks library provides configuration parameters and a command to control when the HA state machine should be paused and resumed. The following configuration will cause the HA state machine to pause in the waiting state after server startup.

"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "load-balancing",
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary"
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "secondary"
                        }
                    ],
                    "state-machine": {
                        "states":  [
                            {
                                "state": "waiting",
                                "pause": "once"
                            }
                        ]
                    }
                } ]
            }
        }
    ],

    ...

}

The pause parameter value once denotes that the state machine should be paused upon the first transition to the waiting state. Later transitions to this state won't cause the state machine to pause. Two other supported values of the pause parameter are: always and never. The latter is the default value for each state, which instructs the server to never pause the state machine.

In order to "unpause" the state machine the ha-continue command must be sent to the paused server. This command does not take any arguments. See Section 15.4.8.13, “Control Commands for High Availability” for details about commands specific for HA hooks library.

It is possible to configure the state machine to pause in more than one state. Consider the following configuration.

"Dhcp4": {

    ...

    "hooks-libraries": [
        {
            "library": "/usr/lib/hooks/libdhcp_lease_cmds.so",
            "parameters": { }
        },
        {
            "library": "/usr/lib/hooks/libdhcp_ha.so",
            "parameters": {
                "high-availability": [ {
                    "this-server-name": "server1",
                    "mode": "load-balancing",
                    "peers": [
                        {
                            "name": "server1",
                            "url": "http://192.168.56.33:8080/",
                            "role": "primary"
                        },
                        {
                            "name": "server2",
                            "url": "http://192.168.56.66:8080/",
                            "role": "secondary"
                        }
                    ],
                    "state-machine": {
                        "states": [
                            {
                                "state": "ready",
                                "pause": "always"
                            },
                            {
                                "state": "partner-down",
                                "pause": "once"
                            }
                        ]
                    }
                } ]
            }
        }
    ],

    ...

}

This configuration instructs the server to pause the state machine every time it transitions to the ready state and upon the first transition to the partner-down state.

Refer to the Section 15.4.8.3, “Server States” for a complete of list of server states. The state machine can be paused in any of the supported states, however it is not practical for the backup and terminated states because the server never transitions out of these states anyway.

Note

In the syncing state the server is paused before it makes an attempt to synchronize lease database with a partner. In order to pause the state machine after lease database synchronization, use the ready state instead.

Note

The state of the HA state machine depends on the state of the cooperating server. Therefore, it must be taken into account that pausing the state machine of one server may affect the operation of the partner server. For example: if the primary server is paused in the waiting state the partner server will also remain in the waiting state until the state machine of the primary server is resumed and that server transitions to the ready state.

15.4.8.12. Control Agent Configuration

The Chapter 7, Kea Control Agent describes in detail the Kea deamon which provides RESTful interface to control Kea servers. The same functionality is used by High Availability hook library to establish communication between the HA peers. Therefore, the HA library requires that Control Agent is started for each DHCP instance within HA setup. If the Control Agent is not started the peers will not be able to communicate with the particular DHCP server (even if the DHCP server itself is online) and may eventually consider this server to be offline.

The following is the example configuration for the CA running on the same machine as the primary server. This configuration is valid for both load balancing and hot standby cases presented in previous sections.

{
"Control-agent": {
    "http-host": "192.168.56.33",
    "http-port": 8080,

    "control-sockets": {
        "dhcp4": {
            "socket-type": "unix",
            "socket-name": "/tmp/kea-dhcp4-ctrl.sock"
        },
        "dhcp6": {
            "socket-type": "unix",
            "socket-name": "/tmp/kea-dhcp6-ctrl.sock"
        }
    }
}
}

15.4.8.13. Control Commands for High Availability

Even though the HA hook library is designed to automatically resolve issues with DHCP service interruptions by redirecting the DHCP traffic to a surviving server and synchronizing the lease database when required, it may be useful for the administrator to have control over the server behavior. In particular, it may be useful be able to trigger lease database synchronization on demand. It may also be useful to manually set the HA scopes that are being served.

Note that the backup server can sometimes be used to handle the DHCP traffic in case if both active servers are down. The backup servers do not perform failover function automatically. Hence, in order to use the backup server to respond to the DHCP queries, the server administrator must enable this function manually.

The following sections describe commands supported by the HA hook library which are available for the administrator.

15.4.8.13.1. ha-sync command

The ha-sync is issued to instruct the server to synchronize its local lease database with the selected peer. The server fetches all leases from the peer and updates those locally stored leases which are older comparing to those fetched. It also creates new leases when any of those fetched do not exist in the local database. All leases that are not returned by the peer but are in the local database are preserved. The database synchronization is unidirectional, i.e. only the database on the server to which the command has been sent is updated. In order to synchronize the peer's database a separate ha-sync has to be issued to that peer.

The database synchronization may be triggered for both active and backup server type. The ha-sync has the following structure (DHCPv4 server case):

{
    "command": "ha-sync",
    "service": [ "dhcp4 "],
    "arguments": {
        "server-name": "server2",
        "max-period": 60
    }
}

When the server receives this command it first disables the DHCP service of the server from which it will be fetching leases, i.e. sends dhcp-disable command to that server. The max-period parameter specifies the maximum duration (in seconds) for which the DHCP service should be disabled. If the DHCP service is successfully disabled, the synchronizing server will fetch leases from the remote server by issuing one or more lease4-get-page commands. When the lease database synchronization is complete, the synchronizing server sends the dhcp-enable to the peer to re-enable its DHCP service.

The max-period value should be sufficiently long to guarantee that it doesn't elapse before the synchronization is completed. Otherwise, the DHCP server will automatically enable its DHCP function while the synchronization is still in progress. If the DHCP server subsequently allocates any leases during the synchronization, those new (or updated) leases will not be fetched by the synchronizing server leading to database inconsistencies.

15.4.8.13.2. ha-scopes command

This command allows for modifying the HA scopes that the server is serving. Consult Section 15.4.8.5, “Load Balancing Configuration” and Section 15.4.8.7, “Hot Standby Configuration” to learn what scopes are available for different HA modes of operation. The ha-scopes command has the following structure (DHCPv4 server case):

{
    "command": "ha-scopes",
    "service": [ "dhcp4" ],
    "arguments": {
        "scopes": [ "HA_server1", "HA_server2" ]
    }
}

This command configures the server to handle traffic from both HA_server1 and HA_server2 scopes. In order to disable all scopes specify an empty list:

{
    "command": "ha-scopes",
    "service": [ "dhcp4 "],
    "arguments": {
        "scopes": [ ]
    }
}

15.4.8.13.3. ha-continue command

This command is used to resume the operation of the paused HA state machine as described in the Section 15.4.8.11, “Pausing HA State Machine”. It takes no arguments, so the command structure is as simple as:

{
    "command": "ha-continue"
}

15.4.9. stat_cmds: Supplemental Statistics Commands

This library provides additional statistics commands for retrieving lease statistics from Kea DHCP servers. These commands were added to address an issue with obtaining accurate lease statistics in deployments running multiple Kea servers that use shared lease back end. The in-memory statistics kept by individual servers only track lease changes made by that server. Thus in a deployment with multiple servers (e.g. two kea-dhcp6 servers using the same PostgreSQL database for lease storage), these statistics are incomplete. In Kea 1.4, the MySQL and PostgreSQL back ends were modified to track lease allocation changes as they occur via database triggers. Additionally, all four lease back ends were extended to support retrieving lease statistics for all subnets, a single subnet, or a range of subnets. Finally, this library was constructed to provide commands for retrieving these statistics. Additional statistics commands may be added to this library in future releases.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

The commands currently provided by this library are:

  • stat-lease4-get - fetches DHCPv4 lease statistics

  • stat-lease6-get - fetches DHCPv6 lease statistics

The Stat commands library is part of the open source code and is available to every Kea user.

All commands use JSON syntax and can be issued either directly to the servers via the control channel (see Chapter 17, Management API) or via Control Agent (see Chapter 7, Kea Control Agent).

This library may be loaded by both kea-dhcp4 and kea-dhcp6 servers. It is loaded in the same way as other libraries and currently has no parameters:

"Dhcp6": { 
    "hooks-libraries": [
        {
            "library": "/path/libdhcp_stat_cmds.so"
        }
        ...
    ] 
}

In a deployment with multiple Kea DHCP servers sharing a common lease storage, it may be loaded by any or all of the servers. However, one thing to keep in mind is that a server's response to a stat-lease{4/6}-get command will only contain data for subnets known to that server. In other words, if subnet does not appear in a server's configuration, it will not retrieve statistics for it.

15.4.9.1. stat-lease4-get, stat-lease6-get commands

The stat-lease4-get and stat-lease6-get commands fetch lease statistics for a range of known subnets. The range of subnets is determined through the use of optional command input parameters:

  • subnet-id - ID of the subnet for which lease statistics should be fetched. Use this to get statistics for a single subnet. If the subnet does not exist the command result code will be 3 (i.e. CONTROL_RESULT_EMPTY).

  • subnet-range - A pair of subnet IDs which describe an inclusive range of subnets for which statistics should be retrieved. Note that fuzzy values are supported thus allowing for a query for statistics using approximate ID values. If the range does not include any known subnets, the command result code will be 3 (i.e. CONTROL_RESULT_EMPTY).

    • first-subnet-id - ID of the first subnet in the range.

    • last-subnet-id - ID of the first subnet in the range.

The use of subnet-id and subnet-range are mutually exclusive. If no parameters are given, the result will contain data for all known subnets. Note that in configurations with large numbers of subnets, this can be result in a large response.

The following command would fetch lease statistics for all known subnets from kea-dhcp4 server:

{
  "command": "stat-lease4-get"
}

The following command would fetch lease statistcis for subnet ID 10 from kea-dhcp6 server:

{
  "command": "stat-lease6-get",
  "arguments": {
    "subnet-id" : 10
  }
}

The following command would fetch lease statistcis for all subnets from subnet ID 10 through 50 from kea-dhcp4 server:

{
  "command": "stat-lease4-get",
  "arguments": {
    "subnet-range" {
      "first-subnet-id": 10,
      "last-subnet-id": 50,
    }
  }
}

The response to the either command will contain three elements:

  • result - numeric value indicating the outcome of the command where:

    • 0 - command was successful
    • 1 - an error occurred, an explanation will be the "text" element
    • 2 - indicates the fetch found no matching data

  • text - an explanation of the command outcome. When the command succeeds it will contain the command name along with the number of rows returned.

  • arguments - a map containing the data returned by the command as the element "result-set", which patterned after SQL statement responses:

    • columns - a list of text column labels. The columns returned for DHCPv4 are:

      • subnet-id - ID of the subnet
      • total-addresses - total number of addresses available for DHCPv4 management in the subnet. In other words, this is the sum of all addresses in all the configured pools in the subnet.
      • assigned-addresses - number of addresses in the subnet that are currently assigned to a client.
      • declined-addresses - number of addresses in the subnet that are currently declined and are thus unavailable for assignment.
    • The columns returned for DHCPv6 are:

      • subnet-id - ID of the subnet
      • total-nas - number of NA addresses available for DHCPv6 management in the subnet. In other words, this is the sum of all the NA addresses in the all configured NA pools in the subnet.
      • assigned-nas - number of NA addresses in a the subnet that are currently assigned to a client.
      • declined-nas - number of NA addresses that are currently declined and are thus unavailable for assignment.
      • total-pds - total number of prefixes available of DHCPv6 management in the subnet. In other words, this is the sum of all prefixes in all the configured prefix pools in the subnet.
      • assigned-pds - number of prefixes in the subnet that are currently assigned to a client.
    • rows - a list of rows, one per subnet ID. Each row contains a data value for corresponding to and in the same order as each column listed in "columns" for a given subnet.

    • timestamp - textual date and time the data was fetched, expressed as GMT

The response to a DHCPv4 command might look as follows:

  {
    "result": 0,
    "text": "stat-lease4-get: 2 rows found",
    "arguments": {
      "result-set": {
        "columns": [ "subnet-id", "total-addresses", "assigned-addresses", "declined-addresses" ]
        "rows": [
          [ 10, 256, 111, 0 ],
          [ 20, 4098, 2034, 4 ]
        ],
      "timestamp": "2018-05-04 15:03:37.000000"
      }
    }
  }

The response to a DHCPv6 command might look as follows (subnet 10 has no prefix pools, subnet 20 has no NA pools, and subnet 30 has both NA and PD pools):

  {
    "result": 0,
    "text": "stat-lease6-get: 2 rows found",
    "arguments": {
      "result-set": {
        "columns": [ "subnet-id", "total-nas", "assigned-nas", "declined-nas", "total-pds", "assigned-pds" ]
        "rows": [
          [ 10, 4096, 2400, 3, 0, 0],
          [ 20, 0, 0, 0, 1048, 233 ]
          [ 30, 256, 60, 0, 1048, 15 ]
        ],
      "timestamp": "2018-05-04 15:03:37.000000"
      }
    }
  }

15.4.10. radius: RADIUS server support

The RADIUS hook library allows Kea to interact with two types of RADIUS servers: access and accounting. Although the most common DHCP and RADIUS integration is done on DHCP relay agent level (DHCP clients send DHCP packets to DHCP relays; relays contact RADIUS server and depending on the response either send the packet to the DHCP server or drop it), it does require a DHCP relay hardware to support RADIUS communication. Also, even if the relay has necessary support it is often not flexible enough to send and receive additional RADIUS attributes. As such, the alternative looks more appealing: to extend DHCP server to talk to RADIUS directly. That is the goal this library intends to fulfill.

Note

This library may only be loaded by kea-dhcp4 or kea-dhcp6 process.

The major feature of the library is the ability to use RADIUS authorization. When a DHCP packet is received, the Kea server will send send Access-Request to the RADIUS server and will await a response. The server will then send back either Access-Accept with specific client attributes or Access-Reject. There are two cases supported here. First, the Access-Accept includes Framed-IP-Address (for DHCPv4) or Framed-IPv6-Address (for DHCPv6), which will be interpreted by Kea as an instruction to assign that specified IPv4 or IPv6 address. This effectively means RADIUS can act as address reservation database.

The second case supported is the ability to assign clients to specific pools based on RADIUS response. In this case RADIUS server sends back Access-Accept with Framed-Pool (IPv4) or Framed-IPv6-Pool (IPv6). In both cases, Kea will interpret those attributes as client classes. With the recent addition of the ability to limit access to pools to specific classes (see Section 14.7, “Configuring Pools With Class Information”), it can be used to force client to be assigned a dynamic address from specific pool. Furthermore, the same mechanism can be used to control what kind of options the client will get (if there are DHCP options specified for a particular class).

15.4.10.1. Compilation and Installation of RADIUS Hook

The following section describes how to compile and install the software on CentOS 7.0. Other systems may differ slightly.

STEP 1: Install dependencies

Several tools are needed to build dependencies and Kea itself. The following commands should install them:

$ sudo rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
$ sudo yum install gcc-g++ openssl-devel log4cplus-devel wget git

STEP 2: FreeRADIUS installation.

Kea RADIUS hook library uses FreeRadius client library to conduct RADIUS communication. Unfortunately, the standard 1.1.7 release available from the project website http://freeradius.org/sub_projects/ has several serious deficiencies. ISC engineers observed a segmentation fault during testing. Also, the base version of the library does not offer asynchronous transmissions, which is essential for effective accounting implementation. Both of these issues were addressed by ISC engineers. The changes have been reported to FreeRadius client project. Acceptance of those changes is outside of ISC responsibilities. Until those are processed, it is strongly recommended to use FreeRadius client with ISC patches. To download and compile this version, please use the following steps:

$ git clone https://github.com/fxdupont/freeradius-client.git
$ cd freeradius-client/
$ git checkout iscdev
$ ./configure
$ make
$ sudo make install

You may pass additional parameters to configure script, if you need to. Once installed, the FreeRADIUS client will be installed in /usr/local. This is the default path where Kea will be looking for it. You may install it in a different directory. If you choose to do so, make sure you pass that path to configure script when compiling kea.

STEP 3: Install recent BOOST version

Kea requires reasonably recent Boost version. Unfortunately, the version available in CentOS 7 is too old. Therefore a newer Boost version is necessary. Furthermore, CentOS 7 has an old version of g++ compiler that does not handle latest Boost versions. Fortunately, Boost 1.65 meets both requirements: is recent enough for Kea and is still able to be compiled using the g++ 4.8 version in CentOS.

To download and compile Boost 1.65, please use the following commands:

$ wget -nd https://dl.bintray.com/boostorg/release/1.65.1/source/boost_1_65_1.tar.gz
$ tar zxvf boost_1_65_1.tar.gz
$ cd boost_1_65_1/
$ ./bootstrap.sh
$ ./b2 --without-python
$ sudo ./b2 install

Note that b2 script may optionally take extra parameters. One of them specify the destination path where the sources are to be compiled.

STEP 4: Compile and Install Kea

Obtain Kea sources either by downloading it from git repository or extract the tarball:

# Use one of those commands to obtain Kea sources:

# Choice 1: get from github
$ git clone https://github.com/isc-projects/kea

# Get a tarball and extract it
$ tar zxvf kea-1.5.0.tar.gz

The next step is to extract premium Kea package that contains Radius repository into the Kea sources. After the tarball is extracted, the Kea sources should have a premium/ subdirectory.

  $ cd kea
  $ tar zxvf ../kea-premium-radius-1.5.0.tar.gz

Once this is done, make sure the kea sources look similar to this:

$ ls -l
total 952
-rw-r--r--   1 thomson  staff    6192 Apr 25 17:38 AUTHORS
-rw-r--r--   1 thomson  staff   29227 Apr 25 17:38 COPYING
-rw-r--r--   1 thomson  staff  360298 Apr 25 20:00 ChangeLog
-rw-r--r--   1 thomson  staff     645 Apr 25 17:38 INSTALL
-rw-r--r--   1 thomson  staff    5015 Apr 25 17:38 Makefile.am
-rw-r--r--   1 thomson  staff     587 Apr 25 17:38 README
drwxr-xr-x   5 thomson  staff     170 Apr 26 19:04 compatcheck
-rw-r--r--   1 thomson  staff   62323 Apr 25 17:38 configure.ac
-rw-r--r--   1 thomson  staff     300 Apr 25 17:38 dns++.pc.in
drwxr-xr-x  12 thomson  staff     408 Apr 26 19:04 doc
drwxr-xr-x   7 thomson  staff     238 Apr 25 17:38 examples
drwxr-xr-x   5 thomson  staff     170 Apr 26 19:04 ext
drwxr-xr-x   8 thomson  staff     272 Apr 26 19:04 m4macros
drwxr-xr-x  20 thomson  staff     680 Apr 26 11:22 premium
drwxr-xr-x  10 thomson  staff     340 Apr 26 19:04 src
drwxr-xr-x  14 thomson  staff     476 Apr 26 19:04 tools

The makefiles have to be regenerated using autoreconf.

The next step is to configure Kea. There are several essential steps necessary here. Running autoreconf -if is necessary to compile premium package that contains RADIUS. Also, --with-freeradius option is necessary to tell Kea where the FreeRADIUS client sources can be found. Also, since the non-standard boost is used, the path to it has to be specified.

$ autoreconf -i
$ ./configure --with-freeradius=/path/to/freeradius --with-boost-include=/path/to/boost --with-boost-lib-dir=/path/to/boost/state/lib

For example, assuming FreeRadius client was installed in the default directory (/usr/local) and Boost 1.65 sources were compiled in /home/thomson/devel/boost1_65_1, the configure path should look as follows:

./configure --with-freeradius=/usr/local \
            --with-boost-include=/home/thomson/devel/boost_1_65_1 \
            --with-boost-lib-dir=/home/thomson/devel/boost_1_65_1/stage/lib

After some checks, the configure script should print a report similar to the following:


       Kea source configure results:
    -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

Package:
  Name:              kea
  Version:           1.5.0
  Extended version:  1.5.0 (tarball)
  OS Family:         Linux

  Hooks directory:   /usr/local/lib/hooks
  Premium hooks:     yes
  Included Hooks:    forensic_log flex_id host_cmds subnet_cmds radius host_cache

C++ Compiler:
  CXX:             g++ --std=c++11
  CXX_VERSION:     g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16)
  CXX_STANDARD:    201103
  DEFS:            -DHAVE_CONFIG_H
  CPPFLAGS:         -DOS_LINUX  -DBOOST_ASIO_HEADER_ONLY
  CXXFLAGS:        -g -O2
  LDFLAGS:          -lpthread
  KEA_CXXFLAGS:     -Wall -Wextra -Wnon-virtual-dtor -Wwrite-strings -Woverloaded-virtual -Wno-sign-compare -pthread -Wno-missing-field-initializers -fPIC

Python:
  PYTHON_VERSION:  not needed (because kea-shell is disabled)

Boost:
  BOOST_VERSION:   1.65.1
  BOOST_INCLUDES:  -I/home/thomson/devel/boost_1_65_1
  BOOST_LIBS:      -L/home/thomson/devel/boost_1_65_1/stage/lib  -lboost_system

OpenSSL:
  CRYPTO_VERSION:  OpenSSL 1.0.2k  26 Jan 2017
  CRYPTO_CFLAGS:
  CRYPTO_INCLUDES:
  CRYPTO_LDFLAGS:
  CRYPTO_LIBS:     -lcrypto

Botan: no

Log4cplus:
  LOG4CPLUS_VERSION:  1.1.3
  LOG4CPLUS_INCLUDES: -I/usr/include
  LOG4CPLUS_LIBS