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Kea Administrator Reference Manual

Kea Administrator Reference Manual

This is the reference guide for Kea version 0.9.1-beta.

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 0.9.1-beta. 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 http://kea.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 Kea servers directly
3. Installation
3.1. Packages
3.2. Install 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. Selecting the Configuration Backend
3.6. DHCP Database Installation and Configuration
3.6.1. Building with MySQL Support
3.6.2. Building with PostgreSQL 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
5. Kea configuration
5.1. BUNDY configuration backend
5.2. JSON configuration backend
5.2.1. JSON syntax
5.2.2. 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. The DHCPv4 Server
7.1. Starting and Stopping the DHCPv4 Server
7.2. DHCPv4 Server Configuration
7.2.1. Introduction
7.2.2. Lease Storage
7.2.3. Interface configuration
7.2.4. IPv4 Subnet Identifier
7.2.5. Configuration of IPv4 Address Pools
7.2.6. Standard DHCPv4 options
7.2.7. Custom DHCPv4 options
7.2.8. DHCPv4 Vendor Specific Options
7.2.9. Nested DHCPv4 Options (Custom Option Spaces)
7.2.10. Unspecified parameters for DHCPv4 option configuration
7.2.11. Stateless Configuration of DHCPv4 clients
7.2.12. Client Classification in DHCPv4
7.2.13. Configuring DHCPv4 for DDNS
7.2.14. Next Server (siaddr)
7.2.15. Echoing Client-ID (RFC 6842)
7.3. Host reservation in DHCPv4
7.3.1. Address reservation types
7.3.2. Conflicts in DHCPv4 reservations
7.3.3. Reserving a hostname
7.3.4. Reserving specific options
7.3.5. Fine Tuning IPv4 Host Reservation
7.4. Server Identifier in DHCPv4
7.5. How the DHCPv4 Server Selects a Subnet for the Client
7.5.1. Using a Specific Relay Agent for a Subnet
7.5.2. Segregating IPv4 Clients in a Cable Network
7.6. Supported DHCP Standards
7.7. DHCPv4 Server Limitations
8. The DHCPv6 Server
8.1. Starting and Stopping the DHCPv6 Server
8.2. DHCPv6 Server Configuration
8.2.1. Introduction
8.2.2. Lease Storage
8.2.3. Interface selection
8.2.4. IPv6 Subnet Identifier
8.2.5. Unicast traffic support
8.2.6. Subnet and Address Pool
8.2.7. Subnet and Prefix Delegation Pools
8.2.8. Standard DHCPv6 options
8.2.9. Custom DHCPv6 options
8.2.10. DHCPv6 vendor specific options
8.2.11. Nested DHCPv6 options (custom option spaces)
8.2.12. Unspecified parameters for DHCPv6 option configuration
8.2.13. IPv6 Subnet Selection
8.2.14. DHCPv6 Relays
8.2.15. Client Classification in DHCPv6
8.2.16. Limiting access to IPv6 subnet to certain classes
8.2.17. Configuring DHCPv6 for DDNS
8.3. Host reservation in DHCPv6
8.3.1. Address/prefix reservation types
8.3.2. Conflicts in DHCPv6 reservations
8.3.3. Reserving a hostname
8.3.4. Reserving specific options
8.3.5. Fine Tuning IPv6 Host Reservation
8.4. Server Identifier in DHCPv6
8.5. Stateless DHCPv6 (Information-Request Message)
8.6. Using specific relay agent for a subnet
8.7. Segregating IPv6 clients in a cable network
8.8. MAC/Hardware addresses in DHCPv6
8.9. Supported DHCPv6 Standards
8.10. DHCPv6 Server Limitations
9. The DHCP-DDNS Server
9.1. Starting and Stopping the DHCP-DDNS Server
9.2. Configuring the DHCP-DDNS Server
9.2.1. Global Server Parameters
9.2.2. TSIG Key List
9.2.3. Forward DDNS
9.2.4. Reverse DDNS
9.2.5. Example DHCP-DDNS Server Configuration
9.3. DHCP-DDNS Server Limitations
10. Hooks Libraries
10.1. Introduction
10.2. Configuring Hooks Libraries
11. libdhcp++ library
11.1. Interface detection and Socket handling
12. Logging
12.1. Logging Configuration
12.1.1. Loggers
12.1.2. Output Options
12.1.3. Example Logger Configurations
12.2. Logging Message Format
12.3. Logging During Kea Startup
13. Acknowledgements

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 (http://bind10.isc.org). 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 0.9.1-beta.

1.1. Supported Platforms

Kea is officially supported on RedHat Enterprise Linux, CentOS, Fedora and FreeBSD systems. It is also likely to work on many other platforms: builds have been tested on (in no particular order) Debian GNU/Linux 6 and unstable, Ubuntu 9.10, NetBSD 5, Solaris 10 and 11, FreeBSD 7 and 8, CentOS Linux 5.3, MacOS 10.6 and 10.7, and OpenBSD 5.1. Non supported systems (especially non-Linux) are likely to have issues with directly connected DHCPv4 clients.

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 1.8 or later. As an alternative to Botan, Kea can use the OpenSSL crypto library (http://www.openssl.org/). It requires a version with SHA-2 support.
  • Kea uses the log4cplus C++ logging library (http://log4cplus.sourceforge.net/). It requires at least log4cplus version 1.0.3.
  • 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.

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 — DHCPv4 server process. This process responds to DHCPv4 queries from clients.
  • kea-dhcp6 — DHCPv6 server process. This process responds to DHCPv6 queries from clients.
  • kea-dhcp-ddns — DHCP-DDNS process. This process acts as an intermediary between the DHCP servers and DNS server. It receives name update requests from the DHCP servers and sends DNS Update messages to the DNS servers.
  • kea-admin — A tool useful for database backend maintenance (creating new database, checking versions, upgrading etc.)
  • perfdhcp — DHCP benchmarking tool which simulates multiple clients to test both DHCPv4 and DHCPv6 servers 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 quickly covers the standard steps for installing and deploying Kea. 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-0.9.1-beta.tar.gz

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

    $ cd kea-0.9.1-beta
    $ ./configure [your extra parameters]

  5. Build it:

    $ make

  6. Install it (by default the installation prefix is /usr/local/, so you likely need root privileges for that step):

    # make install

  7. Edit the configuration file which by default is installed in [kea-install-dir]/etc/kea/kea.conf and contains configuration for all Kea services. Configuration choices for DHCPv4 and DHCPv6 services are described in Section 7.2, “DHCPv4 Server Configuration” and Section 8.2, “DHCPv6 Server Configuration”, 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 both servers simultaneously:

    $ keactrl start

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

    # keactrl status

    If the server status is "inactive" may indicate a configuration error. Please check a log file (by default located in [kea-install-dir]/var/kea/kea.log) for the details of the error.

  10. If the server has been started sucessfully, 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 server(s):

    # keactrl stop

For more system specific installation procedures, you may want to visit System specific notes, available on Kea homepage.

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

2.2. Running Kea servers directly

Kea servers can be started directly (without a need to use keactrl). To start DHCPv4 server run the following commmand:

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

And, 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 build-only prerequisites and do not need to make the software.

FreeBSD ports, NetBSD pkgsrc, and Debian testing package collections provide all the prerequisite packages.

3.2. Install Hierarchy

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

  • etc/kea/ — configuration files.
  • include/ — C++ development header files.
  • lib/ — 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 build-time headers (http://www.boost.org/). At least Boost version 1.35 is required.

  • Botan (at least version 1.8) or OpenSSL.

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

  • A C++ compiler and standard development headers. Kea builds have been tested with GCC g++ 3.4.3, 4.1.2, 4.1.3, 4.2.1, 4.3.2, and 4.4.1; Clang++ 2.8; and Sun C++ 5.10.

  • The development tools "make".

Visit the user-contributed wiki at http://kea.isc.org/wiki/SystemSpecificNotes 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 or via Kea Git code revision control service. (It 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 (2.59 or later). These may need to be installed.

The latest development code (together with temporary experiments and un-reviewed code) is available via the Kea code revision control system. This is powered by Git and all the Kea development is public. The leading development is done in the master branch.

The code can be checked out from git://git.kea.isc.org/kea:

$ git clone git://git.kea.isc.org/kea

The code checked out from the git repository doesn't 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.

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
To specific the path to the botan-config script to build with Botan for the crypto code.
--with-gtest
Enable the building of the C++ Unit Tests using the Google Test framework. Optionally this can define the path to the gtest header files and library. (If the framework is not already installed on your system, it can be downloaded from https://code.google.com/p/googletest.)
--with-log4cplus
Define the path to find the Log4cplus headers and libraries.
--with-openssl
Replace Botan by OpenSSL for the crypto library. The default is to try to find a working Botan then OpenSSL only if Botan is not found.
--without-werror
Disable the default use of the -Werror compiler flag so that compiler warnings do not result in build failures.

Note

For additional instructions concerning the building and installation of Kea for various databases, see Section 3.6, “DHCP Database Installation and Configuration”. For additional instructions concerning the configuration backends, see Section 3.5, “Selecting the Configuration Backend”.

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-dhcp-pgsql=/usr/local/bin/pg_config \
      --prefix=/opt/kea

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

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. Selecting the Configuration Backend

Kea 0.9 has introduced configuration backends that are switchable during the compilation phase. The backend is chosen using the --with-kea-config switch when running the configure script. It currently supports two values: BUNDY and JSON. JSON is the default.

BUNDY
BUNDY means that Kea is linked with the Bundy configuration backend that connects to the Bundy framework and in general works exactly the same as Kea 0.8 and earlier BIND10 versions. The benefits of that backend are uniform integration with the Bundy framework, easy on-line reconfiguration using bindctl, available RESTful API. On the other hand, it requires the whole heavy Bundy framework that requires Python3 to be present. That backend is likely to go away with the release of Kea 1.0.
JSON
JSON is the new default configuration backend that causes Kea to read JSON configuration files from disk. It does not require any framework and thus is considered more lightweight. It will allow dynamic on-line reconfiguration, but will lack remote capabilities (i.e. no RESTful API).

3.6. 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 three database backends: MySQL, PostgreSQL and Memfile. To limit external dependencies, both MySQL and PostgreSQL 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 MySQL and/or PostgreSQL and the creation of the lease database.

3.6.1. 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”), do:

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

Or specify the location of the MySQL configuration program "mysql_config" if MySQL was not installed in the default location:

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

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

3.6.2. 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”), do:

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

Or specify the location of the PostgreSQL configuration program "pg_config" if PostgreSQL was not installed in the default location:

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

See Section 4.3.3.1, “Manually Create the PostgreSQL Database and the Kea User” for details regarding PostgreSQL database configuration.

Chapter 4. Kea Database Administration

4.1. Databases and Database Version Numbers

Kea stores leases in one of 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: in the future, additional data - such as host reservation details - will also be stored.

A given version of Kea expects a particular structure in the database. It ensures this by checking the version of the 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. Likewise, 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 compatibile changes introduced. For example, the addition of a new index. It is desirable, but not mandatory to 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, and perform a database upgrade.

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. Useful during first Kea installation. The database is initialized to the latest version supported by the version of the software.
  • 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.

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.

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 appropriate manual page for details (man 8 kea-admin).

4.3. Supported Databases

4.3.1. memfile

There are no special initialization steps 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.

4.3.2. MySQL

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

4.3.2.1. First Time Creation of Kea 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 -d 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 -d 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 -d database-name

4.3.3. PostgreSQL

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. Manually Create the PostgreSQL Database and the Kea User

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=#
    postgres=# GRANT ALL PRIVILEGES ON DATABASE database-name TO user-name;
    GRANT
    postgres=#
    

  4. Exit PostgreSQL:

    postgres=# \q
    Bye
    $
    

  5. Create the database tables using the new user's credentials and the dhcpdb_create.pgsql script supplied with Kea. After entering the following command, you will be prompted for the new user's password. 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
    

    Please consult your PostgreSQL user manual before making these changes as they may expose your other databases that you run on the same system.

4.3.3.2. Initialize the PostgreSQL Database Using kea-admin

Support for PostgreSQL in kea-admin is currently not implemented.

Chapter 5. Kea configuration

Depending on the configuration backend chosen (see Section 3.5, “Selecting the Configuration Backend”), the configuration mechanisms are different. The following sections describe details of the different configuration backends. Note that only one configuration backend can be used and its selection is made when the configure script is run.

5.1. BUNDY configuration backend

This legacy configuration backend allows Kea to use the former BIND 10 framework. That framework and this Kea configuration backend is no longer supported by ISC. It is currently developed as part of the Bundy project (see Bundy homepage). See the Bundy project documentation regarding configuration.

5.2. JSON configuration backend

JSON is the default configuration backend. It assumes that the servers are started from the command line (either directly or using a script, e.g. keactrl). The JSON backend uses certain signals to influence Kea. The configuration file is specified upon startup using the -c parameter.

5.2.1. JSON syntax

Configuration files for DHCPv4, DHCPv6 and DDNS modules are defined in an extended JSON format. Basic JSON is defined in RFC 4627. Kea components use a slightly modified JSON, in that they allow shell-style comments in the file: lines with the hash (#) character in the first column are comment lines and are ignored.

The configuration file consists of a single object (often colloquially called a map) started with a curly bracket. It comprises the "Dhcp4", "Dhcp6", "DhcpDdns" and/or "Logging" objects. It is possible to define additional elements, but they will be ignored. For example, it is possible to define Dhcp4, Dhcp6 and Logging elements in a single configuration file that can be used to start both the DHCPv4 and DHCPv6 components. When starting, the DHCPv4 component will use Dhcp4 object to configure itself and the Logging object to configure logging parameters; it will ignore the Dhcp6 object.

For example, a very simple configuration for both DHCPv4 and DHCPv6 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"
    }],
    ...
},
# DHCPv4 specific configuration ends here.

# DHCPv6 specific configuration starts here.
"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1" ]
    },
    "preferred-lifetime": 3000,
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,
    "subnet6": [{
       "pools": [ { "pool": "2001:db8::/80" } ],
       "subnet": "2001:db8::/64"
    }],
    ...
},
# DHCPv6 specific configuration ends here.

# Logger parameters (that could be shared among several components) start here.
# This section is used by both the DHCPv4 and DHCPv6 servers.
"Logging": {
   "loggers": [{
        "name": "*",
        "severity": "DEBUG"
    }],
    ...
}
# Logger parameters end here.

# The whole configuration structure ends here.
}

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

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, Logging and possibly DhcpDdns) are present, but they are omitted for clarity. Usually, locations where extra parameters may appear are denoted with an ellipsis.

5.2.2. 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 Dhcp6 component can be referred to as Dhcp6/valid-lifetime and the pool in the first subnet defined in the DHCPv6 configuration as Dhcp6/subnet6[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 and kea-dhcp-ddns). 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 servers.

# prefix holds the location where the Kea is installed.
prefix=/usr/local

# Location of Kea configuration file.
kea_config_file=${prefix}/etc/kea/kea.conf

# Location of Kea binaries.
exec_prefix=${prefix}
dhcp4_srv=${exec_prefix}/sbin/kea/kea-dhcp4
dhcp6_srv=${exec_prefix}/sbin/kea/kea-dhcp6
dhcp_ddns_srv=${exec_prefix}/sbin/kea/kea-dhcp-ddns

# Start DHCPv4 server?
dhcp4=yes

# Start DHCPv6 server?
dhcp6=yes

# Start DHCP DDNS server?
dhcp_ddns=yes

# Be verbose?
kea_verbose=no

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

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 and dhcp_ddns_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. Otherwise, the default logging level is 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 to perform specific operations on the Kea servers:

  • 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.

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.conf
INFO/keactrl: Starting kea-dhcp6 -c /usr/local/etc/kea/kea.conf
INFO/keactrl: Starting kea-dhcp-ddns -c /usr/local/etc/kea/kea.conf

The following command stops all servers:

$ keactrl stop
INFO/keactrl: Skip sending signal 15 to process kea-dhcp6: process is not running

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 is not running, an informational message is displayed as in the stop command output above.

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

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
Kea configuration file: /usr/local/etc/kea/kea.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 server running:

$ keactrl stop -s dhcp4,dhcp6

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

$ 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.
  • all for all servers (default).

Chapter 7. The DHCPv4 Server

7.1. 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 the server will listen on. This is only useful during testing, as the DHCPv4 server listening on ports other than default DHCPv4 ports will not be able to handle regular DHCPv4 queries.
  • -v - prints out Kea version and exits.
  • -V - prints out Kea extended version with additional parameters and exits.

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

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.

7.2. DHCPv4 Server Configuration

7.2.1. Introduction

This section explains how to configure the DHCPv4 server using the Kea configuration backend. (Kea configuration using any other backends is outside of scope of this document.) 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 this line
}

} 

The following paragraphs provide a brief overview of the parameters in the above example and 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 consists of one or more objects. In this specific example, we have only one object called Dhcp4. This is a simplified configuration, as usually there will be additional objects, like Logging or DhcpDns, but we omit them now 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 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.) 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. Note that renew-timer and rebind-timer are optional. If they are not specified the client will 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, 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 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"
    }
]

After all parameters are 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 additional components defined such as Logging or DhcpDdns, so the closing brace would be followed by a comma and another object definition.

7.2.2. Lease Storage

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

7.2.2.1. 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 7.2.2.2, “Database Configuration” describes this option. In typical smaller deployments though, the server will use a CSV file rather than a database to store lease information. 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 presents the remaining, not mandatory parameters, which 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 (Memfile backend) will perform a lease file cleanup (LFC), which removes the 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 0, which disables the LFC.

The 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 when 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 supersede the existing client's lease, as it would require the lookup of the specific lease entry, but simply appends the new lease information at the end of the lease 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 larger number of lease entries.

The lease file cleanup 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 renewals are, the lesser value of the 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, this 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 completing the cleanup before the new cleanup is triggered.

The LFC is performed by a separate process (in background) to avoid performance impact on the server process. In order to avoid the conflicts between the 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 on the Kea wiki: http://kea.isc.org/wiki/LFCDesign.

7.2.2.2. Database Configuration

Note

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.

Database configuration is controlled through the Dhcp4/lease-database parameters. The type of the database must be set to "mysql" or "postgresql", 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 lease database was created (see Section 4.3.2.1, “First Time Creation of Kea Database” or Section 4.3.3.1, “Manually Create the PostgreSQL Database and the Kea User”).

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

If the database is located on a different system to the DHCPv4 server, the database host name must also be specified (although 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" : "", ... }, ... }

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.)

7.2.3. 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 the 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" ]
    },
    ...
}
  

If it is desired that the server listens on multiple IPv4 addresses assigned to the same interface, multiple addresses can be specified for this 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 on the interface yet. 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, it is desired to configure the DHCP server to utilize the 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 brodcast 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 brodcast 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 the IP/UDP sockets.

7.2.4. 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.

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.

7.2.5. Configuration of IPv4 Address Pools

The essential role of DHCPv4 server is address assignment. 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 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 in this example.

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": [
        "pools": [
            { "pool": "192.0.2.10-192.0.2.20" },
            { "pool": "192.0.2.64/26" }
        ],
        ...
    ],
    ...
}

The number of pools is not limited, but for performance reasons it is recommended to use as few as possible. White space in pool definitions is ignored, so spaces before and after the hyphen are optional. They can be used to improve readability.

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.

7.2.6. Standard DHCPv4 options

One of the major features of the DHCPv4 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 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"
        },
        ...
    ]
}

The name parameter specifies the option name. For a complete list of currently supported names, see Table 7.1, “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 7.2.9, “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"
        },
        ...
    ],
    ...
}

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 7.2.10, “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"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

The currently supported standard DHCPv4 options are listed in Table 7.1, “List of standard DHCPv4 options” and Table 7.2, “List of standard DHCPv4 options (continued)”. 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 7.3, “List of standard DHCP option types”.

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 7.2.7, “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 does not provide a definition yet. In order to use such options, a server administrator must create a definition as described in Section 7.2.7, “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 has no means to validate the format at the moment.

Table 7.1. List of standard DHCPv4 options

NameCodeTypeArray?
subnet-mask1ipv4-addressfalse
time-offset2int32false
routers3ipv4-addresstrue
time-servers4ipv4-addresstrue
name-servers5ipv4-addressfalse
domain-name-servers6ipv4-addresstrue
log-servers7ipv4-addresstrue
cookie-servers8ipv4-addresstrue
lpr-servers9ipv4-addresstrue
impress-servers10ipv4-addresstrue
resource-location-servers11ipv4-addresstrue
host-name12stringfalse
boot-size13uint16false
merit-dump14stringfalse
domain-name15fqdnfalse
swap-server16ipv4-addressfalse
root-path17stringfalse
extensions-path18stringfalse
ip-forwarding19booleanfalse
non-local-source-routing20booleanfalse
policy-filter21ipv4-addresstrue
max-dgram-reassembly22uint16false
default-ip-ttl23uint8false
path-mtu-aging-timeout24uint32false
path-mtu-plateau-table25uint16true
interface-mtu26uint16false
all-subnets-local27booleanfalse
broadcast-address28ipv4-addressfalse
perform-mask-discovery29booleanfalse
mask-supplier30booleanfalse
router-discovery31booleanfalse
router-solicitation-address32ipv4-addressfalse
static-routes33ipv4-addresstrue
trailer-encapsulation34booleanfalse
arp-cache-timeout35uint32false
ieee802-3-encapsulation36booleanfalse
default-tcp-ttl37uint8false
tcp-keepalive-interval38uint32false
tcp-keepalive-garbage39booleanfalse


Table 7.2. List of standard DHCPv4 options (continued)

NameCodeTypeArray?
nis-domain40stringfalse
nis-servers41ipv4-addresstrue
ntp-servers42ipv4-addresstrue
vendor-encapsulated-options43emptyfalse
netbios-name-servers44ipv4-addresstrue
netbios-dd-server45ipv4-addresstrue
netbios-node-type46uint8false
netbios-scope47stringfalse
font-servers48ipv4-addresstrue
x-display-manager49ipv4-addresstrue
dhcp-requested-address50ipv4-addressfalse
dhcp-option-overload52uint8false
dhcp-message56stringfalse
dhcp-max-message-size57uint16false
vendor-class-identifier60binaryfalse
nwip-domain-name62stringfalse
nwip-suboptions63binaryfalse
tftp-server-name66stringfalse
boot-file-name67stringfalse
user-class77binaryfalse
fqdn81recordfalse
dhcp-agent-options82emptyfalse
authenticate90binaryfalse
client-last-transaction-time91uint32false
associated-ip92ipv4-addresstrue
subnet-selection118ipv4-addressfalse
domain-search119binaryfalse
vivco-suboptions124binaryfalse
vivso-suboptions125binaryfalse


Table 7.3. 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)
recordStructured data that may comprise any types (except "record" and "empty")
stringAny text
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


7.2.7. 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 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 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. 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.

Note

In the current release the default values are not propagated to the parser when the new configuration is being set. Therefore, all parameters must be specified at all times, even if their values are left blank.

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 those listed in Table 7.3, “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.

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 Kea versions will accept all those values for all boolean parameters.

7.2.8. DHCPv4 Vendor Specific Options

Currently there are three option spaces defined: "dhcp4" (used by the DHCPv4 daemon) and "dhcp6" (for the DHCPv6 daemon); there is also "vendor-encapsulated-options-space", which is empty by default, but options can be defined in it. Those options are called vendor-specific information options. The following examples show how to define an option "foo" with code 1 that consists of 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",
            "encapsulates": ""
        }
    ],
    ...
}

(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 set up a dummy value for "vendor-encapsulated-options", 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"
            "space": "dhcp4",
            "code": 43,
            "csv-format": false,
            "data": ""
        }
    ],
    ...
}

Note

With this version of Kea, the "vendor-encapsulated-options" option must be specified in the configuration although it has no configurable parameters. If it is not specified, the server will assume that it is not configured and will not send it to a client. In the future there will be no need to include this option in the configuration.

7.2.9. Nested DHCPv4 Options (Custom Option Spaces)

It is sometimes useful to define completely new option space. This is the case when user creates new option in the standard option space ("dhcp4 or "dhcp6") 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 7.2.8, “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",
            "space": "isc",
            "code": 1,
            "csv-format": true,
            "data": "192.0.2.3"
        },
        }
            "name": "subopt2",
            "space": "isc",
            "code": 2,
            "csv-format": true,
            "data": "Hello world"
        },
        {
            "name": "container",
            "space": "dhcp4",
            "code": 222,
            "csv-format": true,
            "data": ""
        }
    ],
    ...
}

Even though the "container" option does not carry any data except sub-options, the "data" field must be explicitly set to an empty value. This is required because in the current version of Kea, the default configuration values are not propagated to the configuration parsers: if the "data" is not set the parser will assume that this parameter is not specified and an error will be reported.

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.

7.2.10. 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 specifing 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 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 and the definition for the particular option exists, 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. If the definition does not exist for this option, the server will assume that the data parameter contains the option payload in the binary format (represented as a string of hexadecimal digits). Note that not specifying this parameter doesn't imply that it defaults to a fixed value, but the configuration data interpretation also depends on the presence of the option definition. An administrator must be aware if the definition for the particular option exists when this parameter is not specified. It is generally recommended to not specify this parameter only for the options for which the definition exists, e.g. standard options. Setting csv-format to an explicit value will cause the server to strictly check the format of the option data specified.

7.2.11. 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. DNS servers' addresses. 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 the particular subnet defined in the server's configuration. The 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.

7.2.12. Client Classification in DHCPv4

Note

The DHCPv4 server has been extended to support limited client classification. Although the current capability is modest, it is expected to be expanded in the future. However, it is envisaged that the majority of client classification extensions will be using hooks extensions.

In certain cases it is useful to differentiate between different types of clients and treat them differently. The process of doing classification is conducted in two steps. The first step is to assess an incoming packet and assign it to zero or more classes. This classification is currently simple, but is expected to grow in capability soon. Currently 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_" then 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".

It is envisaged that the client classification will be used for changing the behavior of almost any part of the DHCP message processing, including assigning leases from different pools, assigning different options (or different values of the same options) etc. For now, there are only two mechanisms that are taking advantage of client classification: specific processing for cable modems and subnet selection.

For clients that belong to the VENDOR_CLASS_docsis3.0 class, the siaddr field is set to the value of next-server (if specified in a subnet). If there is a boot-file-name option specified, its value is also set in the file field in the DHCPv4 packet. For eRouter1.0 class, the siaddr is always set to 0.0.0.0. That capability is expected to be moved to an external hook library that will be dedicated to cable modems.

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. 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 7.2.12.1, “Limiting Access to IPv4 Subnet to Certain Classes”.

7.2.12.1. Limiting Access to IPv4 Subnet to Certain Classes

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given subnet. For details on client classes, see Section 7.2.12, “Client Classification in DHCPv4”. This is an extension of a previous example from Section 7.2.5, “Configuration of IPv4 Address Pools”. 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 VENDOR_CLASS_docsis3.0 are allowed to use this subnet. 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" } ],
            "client-class": "VENDOR_CLASS_docsis3.0"
        }
    ],
    ...
}

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.

7.2.13. Configuring DHCPv4 for DDNS

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": false
  • "generated-prefix": "myhost"

7.2.13.1. 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 how it communications with D2:

  • 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-format - socket protocol use when sending requests to D2. Currently only UDP is supported. TCP may be available in an upcoming release.
  • ncr-protocol - 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,
        ...
    },
    ...
}

7.2.13.2. 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 9, 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 and is discussed next.

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 7.4. 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 DHCP-DDNS 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 your 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 a DDNS update request 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.

7.2.13.3. 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 rules for determining the FQDN option are as follows:

  1. If configured to do, so ignore the REQUEST contents and generate a FQDN using a configurable prefix and suffix.

  2. If the REQUEST contains the client FQDN option, the candidate name is taken from there, otherwise it is taken from the Host Name option. The candidate name may then be modified:

    1. If the candidate name is a fully qualified domain name, use it.

    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 is a empty, generate a FQDN using a configurable prefix and suffix.

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

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

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.

7.2.14. Next Server (siaddr)

In some cases, clients want to obtain configuration from the 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.

"Dhcp4": {
    "next-server": "192.0.2.123",
    ...,
    "subnet4": {
        [
            "next-server": "192.0.2.234",
            ...
        ]
    }
}

7.2.15. 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 now states that the server must send client-id if the client sent it. That is the default behaviour that Kea offers. 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,
    ...
}

7.3. 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 specific, static address for exclusive use by a given client (host) ‐ returning client will receive the same address from the server every time, and other clients will generally not receive that address. 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 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 that has specific requirements, e.g. a printer that needs additional DHCP options. Yet another possible use case is to define unique names for hosts. Although not all of the presented use cases are implemented yet, Kea software will support them in the near future.

Hosts reservations are defined as parameters for each subnet. Each host has to be identified by its 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 reservrations for a single host. In particular, such a structure has to have an indentifer that uniquely identifies a host. In DHCPv4 context, such an identifier is a hardware or MAC address. In most cases, also an address will be specified. It is possible to specify a hostname. Additional capabilities are planned.

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

"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"
            },
            {
                "hw-address": "0a:0b:0c:0d:0e:0f",
                "ip-address": "192.0.2.100",
                "hostname": "alice-laptop"
            }
        ]
    }
]

The first entry reserves the 192.0.2.202 address for the client that uses MAC adress of 1a:1b:1c:1d:1e:1f. The second entry reserves the address 192.0.2.100 and the hostname of alice-laptop for client using MAC address 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.

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. It is a valid configuration, if such definitions are specified in different subnets, though.

Adding host reservation incurs a performance penalty. In principle, when the 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 (if there is a lease for it), but also whether the address could be used by someone else (if there is a reservation for it). That additional check incurs performance penalty.

7.3.1. 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, the 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. As of 0.9.1, 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.

7.3.2. Conflicts in DHCPv4 reservations

As reservations and lease information are stored separately, conflicts may arrise. Consider the following series of events. The server has configured dynamic pool of addresses from the range of 192.0.2.10 to 192.0.2.20. Host A requests an address and gets 19.0.2.10. Now the system administrator decides to reserve an address for host B. He decides to reserve 192.0.2.10 for that purpose. 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 the 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 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 Decline. Therefore in this situation, the server has to temporarily assign a different address (not matching what has been reserved) to host B.

When the 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 inform the host A that it is no longer allowed to use it by sending NAK message. The server will remove the lease, though, as there's small chance that the NAK may be lost if the network is lossy. If that happens, the client will not receive any responses, so it will retransmit its Request packet. Once the NAK is received by the host A, it will then 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 the address 192.0.2.10 will be no longer used. When host B tries to renew its temporary address, the server will detect that it has a valid lease, but there is a reservation for a different address. The server will send NAK to inform host B that its address is no longer usable, but will keep its least (again, the NAK may be lost, so the server will keep it, until the client gets back for a new address). The host B will revert to the server discovery phase and will eventually send a REQUEST 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 temporary address that the host B previously had.

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 propose a different address (when responding to DISCOVER) or would send NAK (when responding to REQUEST).

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 that having multiple reservations for the same address is not allowed).

7.3.3. Reserving a hostname

Reserving a hostname is currently not supported. It is possible to specify that information in the configuration file, but that data is not used by the server engine yet.

7.3.4. Reserving specific options

Currently it is not possible to specify options in host reservation. Such a feature will be added in the upcoming Kea releases.

7.3.5. Fine Tuning IPv4 Host Reservation

Note

reservation-mode configuration parameter in DHCPv4 server is accepted, but not used in the Kea 0.9.1 beta. Full implementation will be available in the upcoming releases.

Host reservation capability introduces additional restrictions for the allocation engine 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 be not 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's a reservation for this host, so the exisiting (dynamically allocated) address should be revoked and the reserved one be used instead.

Some of those checks may be unnecessary in certain deployments. 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 all host reservation types. This is the default value. This setting is the safest and the most flexible. It allows in-pool and out-of-pool reservations. As all checks are conducted, it is also the slowest.
  • 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 0.9.1 does not sanity check the reservations against reservation-mode. 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.

An example configuration that disables reservation looks like follows:

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

7.4. 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.

Currently there is no mechanism to override the default server identifiers by an administrator. In the future, the configuration mechanism will be used to specify the custom server identifier.

7.5. 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 the 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 subnets. See Section 7.5.1, “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.

7.5.1. Using a 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 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 — 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-address": "10.0.0.1"
            },
            ...
        }
    ],
    ...
}

7.5.2. Segregating IPv4 Clients in a Cable Network

In certain cases, it is useful to mix relay address information, introduced in Section 7.5.1, “Using a Specific Relay Agent for a Subnet” with client classification, explained in Section 7.2.12.1, “Limiting Access to IPv4 Subnet to Certain Classes”. One specific example is 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 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-address": "10.1.1.1"
            }
        },
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "relay": {
                "ip-address": "10.1.1.1"
            }
        }
    ],
    ...
}

7.6. Supported DHCP Standards

The following standards are currently supported:

  • Dynamic Host Configuration Protocol, RFC 2131: Supported messages are DISCOVER (1), OFFER (2), REQUEST (3), RELEASE (7), INFORM (8), ACK (5), and NAK(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 7.2.15, “Echoing Client-ID (RFC 6842)” for details.

7.7. 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.

  • Removal of a subnet during server reconfiguration may cause renumbering of auto-generated subnet identifiers, as described in section Section 7.2.4, “IPv4 Subnet Identifier”.
  • Host reservation (static addresses) is not supported yet.
  • Full featured client classification is not supported yet.
  • 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.
  • Address duplication report (DECLINE) is not supported yet.
  • The server doesn't act upon expired leases. In particular, when a lease expires, the server doesn't request the removal of the DNS records associated with it. Expired leases can be recycled.

Chapter 8. The DHCPv6 Server

8.1. 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 default DHCPv6 ports will not be able to handle regular DHCPv6 queries.
  • -v - prints out Kea version and exits.
  • -V - prints out Kea extended version with additional parameters and exits.

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

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.

8.2. DHCPv6 Server Configuration

8.2.1. Introduction

This section explains how to configure the DHCPv6 server using the Kea configuration backend. (Kea configuration using any other backends is outside of scope of this document.) Before DHCPv6 is started, its configuration file has to be created. The basic configuration looks as follows:

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

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

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

# And we specify the type of a 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 and 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 consists of one or more objects. In this specific example, we have only one object called Dhcp6. This is a simplified configuration, as usually there will be additional objects, like Logging or DhcpDns, but we omit them now 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 require moving the comma as well. 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, 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 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 pool (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",
        "interface": "eth0"
    }
]

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 or DhcpDdns, 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 three database backends available: memfile (which is the default backend), MySQL and PostgreSQL.

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, “Database Configuration” describes this option. In typical smaller deployments though, the server will use a CSV file rather than a database to store lease information. 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 presents the remaining, not mandatory parameters, which 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 (Memfile backend) will perform a lease file cleanup (LFC), which removes the 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 0, which disables the LFC.

The example configuration of the Memfile backend is presented below:

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

It is important to know how the lease file contents are organized to understand why the periodic lease file cleanup is needed. Every time when 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 supersede the existing client's lease, as it would require the lookup of the specific lease entry, but simply appends the new lease information at the end of the lease 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 larger number of lease entries.

The lease file cleanup 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 renewals are, the lesser value of the 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, this 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 completing the cleanup before the new cleanup is triggered.

The LFC is performed by a separate process (in background) to avoid performance impact on the server process. In order to avoid the conflicts between the 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 on the Kea wiki: http://kea.isc.org/wiki/LFCDesign.

8.2.2.2. Database Configuration

Note

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.

Database configuration is controlled through the Dhcp6/lease-database parameters. The type of the database must be set to "mysql" or "postgresql", 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 lease database was created (see Section 4.3.2.1, “First Time Creation of Kea Database” or Section 4.3.3.1, “Manually Create the PostgreSQL Database and the Kea User”).

"Dhcp6": { "lease-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 (although it should be noted that this configuration may have a severe impact on server performance):

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

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.)

8.2.3. 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.

8.2.4. 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 respective subnets. When the 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, which may have unexpected consequences. In the future it is planned to implement a mechanism to preserve auto-generated subnet ids upon removal of one of the subnets. Currently, the only remedy for this issue is to manually specify a unique subnet identifier for each subnet.

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.

8.2.5. 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 8.2.3, “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 that is to have relays send their traffic to the server directly. To configure the server to listen on a specific unicast address, the notation to specify interfaces has been extended. An interface name can be optionally followed by a slash, followed by the global unicast address on which the server should listen. This will be done in addition to normal link-local binding + 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" ]
    },
    ...
}
 

This configuration will cause the server to listen on eth1 on link-local address, multicast group (ff02::1:2) and 2001:db8::1.

It is possible to mix interface names, wildcards and interface name/addresses on the list of interfaces. It is not possible 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 those addresses specified, without any additional checks. This approach is selected on purpose, so the software can be used to communicate over uncommon addresses if the administrator so desires.

8.2.6. Subnet and Address Pool

The essential 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 pool 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" }
        ],
        ...
    }
    ]
}

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.

8.2.7. Subnet and Prefix Delegation Pools

Subnets may also be configured to delegate prefixes, as defined in RFC 3633. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix and a prefix length, as well as a delegated prefix length. delegated-len must not be shorter (that is it must be numerically greater or equal) than prefix-len. If both delegated-len and prefix-len 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
                }
            ]
        }
    ],
    ...
}

8.2.8. 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, which is one of the most frequently used options. Numbers in the first column are added for easier reference and will not appear on screen. 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 8.1, “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 8.2.11, “Nested DHCPv6 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 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"
        },
        ...
    ]
}

The value for the setting of the "data" element is split across two lines in this document 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 8.2.12, “Unspecified parameters for DHCPv6 option configuration”.

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"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

The currently supported standard DHCPv6 options are listed in Table 8.1, “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 7.3, “List of standard DHCP option types”.

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 8.2.9, “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 provide a definition yet. In order to use such options, a server administrator must create a definition as described in Section 8.2.9, “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 8.1. List of standard DHCPv6 options

NameCodeTypeArray?
preference7uint8false
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-civic36recordfalse
remote-id37recordfalse
subscriber-id38binaryfalse
client-fqdn39recordfalse
pana-agent40ipv6-addresstrue
new-posix-timezone41stringfalse
new-tzdb-timezone42stringfalse
ero43uint16true
lq-query44recordfalse
client-data45emptyfalse
clt-time46uint32false
lq-relay-data47recordfalse
lq-client-link48ipv6-addresstrue


8.2.9. Custom DHCPv6 options

It is also possible to define options other than the standard ones. Assume that we want to define a new DHCPv6 option called "foo" which will have code 100 and 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-fields" 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 set of comments define the format of the new option and do not set its values.

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.

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": "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 those listed in Table 7.3, “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.

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 Kea versions will accept all those values for all boolean parameters.

8.2.10. DHCPv6 vendor specific options

Currently there are three option spaces defined: dhcp4 (to be used in DHCPv4 daemon) and dhcp6 (for the DHCPv6 daemon); there is also vendor-opts-space, which is empty by default, but options can be defined in it. Those options are called vendor-specific information options. 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-encapsulated-options-space",
            "type": "record",
            "array": false,
            "record-types": "ipv6-address, uint16, string",
            "encapsulates": ""
        }
    ],
    ...
}

(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-encapsulated-options-space",
            "code": 1,
            "csv-format": true,
            "data": "2001:db8:1::10, 123, Hello World"
        },
        ...
    ],
    ...
}

We should also define values for the vendor-opts, that will convey our option foo.

"Dhcp6": {
    "option-data": [
        ...,
        {
            "name": "vendor-encapsulated-options"
            "space": "dhcp6",
            "code": 17,
            "csv-format": true,
            "data": "12345"
        }
    ],
    ...
}

8.2.11. Nested DHCPv6 options (custom option spaces)

It is sometimes useful to define completely new option spaces. This is useful if the user wants his 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 8.2.10, “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",
            "space": "isc",
            "code": 1,
            "csv-format": true,
            "data": "2001:db8::abcd"
        },
        }
            "name": "subopt2",
            "space": "isc",
            "code": 2,
            "csv-format": true,
            "data": "Hello world"
        },
        {
            "name": "container",
            "space": "dhcp6",
            "code": 102,
            "csv-format": true,
            "data": ""
        }
    ],
    ...
}

Even though the "container" option does not carry any data except sub-options, the "data" field must be explicitly set to an empty value. This is required because in the current version of Kea, the default configuration values are not propagated to the configuration parsers: if the "data" is not set the parser will assume that this parameter is not specified and an error will be reported.

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.

8.2.12. 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 specifing 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 and the definition for the particular option exists, 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. If the definition does not exist for this option, the server will assume that the data parameter contains the option payload in the binary format (represented as a string of hexadecimal digits). Note that not specifying this parameter doesn't imply that it defaults to a fixed value, but the configuration data interpretation also depends on the presence of the option definition. An administrator must be aware if the definition for the particular option exists when this parameter is not specified. It is generally recommended to not specify this parameter only for the options for which the definition exists, e.g. standard options. Setting csv-format to an explicit value will cause the server to strictly check the format of the option data specified.

8.2.13. 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 having 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"
        }
    ],
    ...
}

8.2.14. 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.

To specify interface-id with value "vlan123", the following commands can be used:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            "interface-id": "vlan123"
        }
    ],
    ...
}

8.2.15. Client Classification in DHCPv6

Note

DHCPv6 server has been extended to support limited client classification. Although the current capability is modest, it is expected to be expanded in the future. It is envisaged that the majority of client classification extensions will be using hooks extensions.

In certain cases it is useful to differentiate between different types of clients and treat them differently. The process of doing classification is conducted in two steps. The first step is to assess an incoming packet and assign it to zero or more classes. This classification is currently simple, but is expected to grow in capability soon. Currently the server checks whether the incoming packet includes vendor class option (16). If it has, the content of that option is prepended with "VENDOR_CLASS_" then 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".

It is envisaged that the client classification will be used for changing behavior of almost any part of the DHCP engine processing, including assigning leases from different pools, assigning different option (or different values of the same options) etc. For now, there is only one mechanism that is taking advantage of client classification: subnet selection.

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 are cable networks. There are two classes of devices: the cable modem itself, which should be handed a lease from subnet A and all other devices behind modems 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 8.2.16, “Limiting access to IPv6 subnet to certain classes”.

8.2.16. Limiting access to IPv6 subnet to certain classes

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given class. For details on client classes, see Section 8.2.15, “Client Classification in DHCPv6”. This is an extension of a previous example from Section 8.2.6, “Subnet and Address Pool”. Let's 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. Only clients belonging to the eRouter1.0 client class are allowed to use that pool. Such a configuration can be achieved in the following way:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::-2001:db8:1::ffff"
                 }
             ],
            "client-class": "VENDOR_CLASS_eRouter1.0"
        }
    ],
    ...
}

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.

8.2.17. Configuring DHCPv6 for DDNS

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 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": false
  • "generated-prefix": "myhost"

8.2.17.1. 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 how it communications with D2:

  • 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-format - Socket protocol use when sending requests to D2. Currently only UDP is supported. TCP may be available in an upcoming release.
  • ncr-protocol - 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 commands would be required:

"Dhcp6": {
    "dhcp-ddns": {
        "server-ip": "2001:db8::5",
        "server-port": 900,
        ...
    },
    ...
}

8.2.17.2. When does kea-dhcp6 generate 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 the purview of D2 (Chapter 9, 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 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 is more involved and is discussed next.

kea-dhcp6 will generate a DDNS update request only if the DHCP 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 8.2. 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, issue the following commands:

"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,
        ...
    },
    ...
}

8.2.17.3. 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 in the DHCP REQUEST.

The rules for determining the FQDN option are as follows:

  1. If configured to do so ignore the REQUEST contents and generate a FQDN using a configurable prefix and suffix.

  2. Otherwise, using the domain name value from the client FQDN option as the candidate name:

    1. If the candidate name is a fully qualified domain name then use it.

    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 is a empty then generate a FQDN using a configurable prefix and suffix.

To instruct kea-dhcp6 to always generate a FQDN, set the parameter "replace-client-name" to true:

"Dhcp6": {
    "dhcp-ddns": {
        "replace-client-name": true,
        ...
    },
    ...
}

The prefix used when generating 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:

"Dhcp6": {
    "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:

"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.

8.3. 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 that 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. Although not all of the presented use cases are are implemented yet, Kea software will support them in the near future.

Hosts reservations are defined as parameters for each subnet. Each host can be identified by either DUID or its hardware/MAC address. See Section 8.8, “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, such a structure has to have an indentifer that uniquely identifies a host. In DHCPv6 context, such an identifier is a hardware (MAC) address or a DUID. Also, either one or more addresses or prefixes should be specified. It is possible to specify a hostname. Additional capabilities are planned.

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": 56,
                "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" ]
            },
            {
                "duid": "01:02:03:04:05:06:07:08:09:0A",
                "ip-addresses": [ "2001:db8:1::102" ],
                "prefixes": [ "2001:db8:2:abcd::/64" ],
                "hostname": "foo.example.com"
            }
        ]
    }
]

This example makes 3 reservations. The first one reserves 2001:db8:1::100 address for the client using DUID 01:02:03:04:05:0A:0B:0C:0D:0E. The second one also reserves an address, but does so using MAC or hardware address, rather than DUID. The third example is most advanced. It reserves an address, a prefix and a hostname at the same time.

Note that DHCPv6 allows for a single client to lease multiple addresses and multiple prefixes at the same time. In the upcoming Kea releases, it will be possible to have multiple addresses and prefixes reserved for a single host. Therefore ip-addresses and prefixes are plural and are actually arrays. As of 0.9.1 having more than one IPv6 address or prefix is only partially supported.

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. It is a valid configuration, if such definitions are specified in different subnets, though. The reservation for a given host should include only one identifier, either DUID or hwardware address. Defining both for the same host is considered a configuration error, but as of 0.9.1 beta, it is not rejected.

Adding host reservation incurs a performance penalty. In principle, when the 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 (if there is a lease for it), but also whether the address could be used by someone else (if there is a reservation for it). That additional check incurs performance penalty.

8.3.1. Address/prefix reservation types

In a typical scenario there's 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. As of 0.9.1, 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.

8.3.2. Conflicts in DHCPv6 reservations

As reservations and lease information are stored in different places, conflicts may arrise. 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 an address for host B. He decides to reserve 2001:db8::10 for that purpose. 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 Decline. 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 the 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 and select a new address and will create a new lease for it. It will send two addresses in its response: the old address with lifetimes set to 0 to explicitly indicate that it is no longer valid and a new address with non-zero lifetimes. When the 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 the 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 that having multiple reservations for the same address is not allowed).

8.3.3. Reserving a hostname

Reserving a hostname is currently not supported. It is possible to specify that information in the configuration file, but that data is not used by the server engine yet.

8.3.4. Reserving specific options

Currently it is not possible to specify options in host reservation. Such a feature will be added in the upcoming Kea releases.

8.3.5. Fine Tuning IPv6 Host Reservation

Note

reservation-mode in the DHCPv6 server is implemented in Kea 0.9.1 beta, but has not been tested and is considered experimental.

Host reservation capability introduces additional restrictions for the allocation engine 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 be not 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's 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. 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 all host reservation types. This is the default value. This setting is the safest and the most flexible. It allows in-pool and out-of-pool reservations. As all checks are conducted, it is also the slowest.
  • 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 0.9.1 does not sanity check the reservations against reservation-mode. 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.

An example configuration that disables reservation looks like follows:

"Dhcp6": {
    "subnet6": [
        "subnet": "2001:db8:1::/64",
        "reservation-mode": "disabled",
        ...
    ]
}

8.4. 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. There are several types of DUIDs defined, but RFC 3315 instructs servers to use DUID-LLT if possible. This format consists of a link-layer (MAC) address and a timestamp. When started for the first time, the DHCPv6 server will automatically generate such a DUID and store the chosen value to a file. That file is read by the server and the contained value used whenever the server is subsequently started.

It is unlikely that this parameter should ever need to be changed. However, if such a need arises, stop the server, edit the file and restart the server. (The file is named kea-dhcp6-serverid and by default is stored in the "var" subdirectory of the directory in which Kea is installed. This can be changed when Kea is built by using "--localstatedir" on the "configure" command line.) The file is a text file that contains double digit hexadecimal values separated by colons. This format is similar to typical MAC address format. Spaces are ignored. No extra characters are allowed in this file.

8.5. 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 required since, as of version 0.9.1, Kea requires a lease database to be specified even if it is not used.

8.6. 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 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 that has 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-address": "3000::1"
	     }
        }
    ]
}

8.7. Segregating IPv6 clients in a cable network

In certain cases, it is useful to mix relay address information, introduced in Section 8.6, “Using specific relay agent for a subnet” with client classification, explained in Section 8.2.16, “Limiting access to IPv6 subnet to certain classes”. 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-address": "3000::1"
            }
        },	    

        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ],
	     "relay": {
	         "ip-address": "3000::1"
	     }
        }
    ]
}

8.8. MAC/Hardware addresses in DHCPv6

MAC/hardware addesses are available in DHCPv4 messages from the clients and administrators frequently use that information to perform certain tasks, like per host configuration, address reserveration for specific MAC addresses and other. Unfortunately, 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 that information. 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 for configuration which of the supported methods should be used and in which 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 format of this parameter is 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.

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 RFC3315 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 aquisition method comes from the source IPv6 address. In typical usage, clients are sending their packets from IPv6 link-local addresses. There's 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 RFC4941, do not use MAC addresses.
  • 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 remote-id option that is inserted by a relay agent. Depending on the relay agent configuration, the inserted option may convey 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 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 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.

8.9. 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, 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.
  • Client Link-Layer Address Option in DHCPv6, RFC 6939: Supported option is client link-layer address option.

8.10. DHCPv6 Server Limitations

These are the current limitations and known problems with 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.

  • On-line configuration has some limitations. Adding new subnets or modifying existing ones work, as is removing the last subnet from the list. However, removing non-last (e.g. removing subnet 1,2 or 3 if there are 4 subnets configured) will cause issues. The problem is caused by simplistic subnet-id assignment. The subnets are always numbered, starting from 1. That subnet-id is then used in leases that are stored in the lease database. Removing non-last subnet will cause the configuration information to mismatch data in the lease database. It is possible to manually update subnet-id fields in MySQL or PostgreSQL database, but it is awkward and error prone process. A better reconfiguration support is planned.

  • 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 3315 and RFC 3633 allow for multiple addresses or prefixes to be allocated for a single IA.
  • Temporary addresses are not supported.
  • Duplication report (DECLINE) and client reconfiguration (RECONFIGURE) are not yet supported.
  • The server doesn't act upon expired leases. In particular, when a lease expires, the server doesn't request removal of the DNS records associated with it.

Chapter 9. The DHCP-DDNS Server

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.

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.

9.1. 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.
  • -V - prints out Kea extended version with additional parameters and exits.

Upon start up the module will load its configuration and begin listening for NCRs based on that configuration.

9.2. 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 customised 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

9.2.1. 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 - Packet format to use when sending requests to D2. Currently only JSON format is supported. Other formats may be available in future releases.
  • ncr_format - Socket protocol to use when sending requests to D2. Currently only UDP is supported. TCP may be available in an upcoming release.

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.

9.2.2. 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, not 0 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.

9.2.3. 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.

9.2.3.1. 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.

9.2.3.1.1. 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.

9.2.4. 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.

9.2.4.1. 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 subent 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.

9.2.4.1.1. 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.

9.2.5. 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 9.1. 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 9.2. 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": {
    "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" }
		]
	    },

	]
    }
}

Similarly, we need to construct the three reverse DDNS Domains:

Table 9.3. 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": {
    "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" }
		]
	    }
	]
    }
}

9.3. 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 10. Hooks Libraries

10.1. Introduction

Although Kea offers a lot of flexibility, there may be cases where its behavior needs customisation. 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.

Hooks libraries are attached to 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.

10.2. 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 strings, each string corresponding to a hooks library. For example, to set up two hooks libraries for the DHCPv4 server, the configuration would be:

"Dhcp4": {
    :
    "hooks-libraries": [
       "/opt/charging.so",
       "/opt/local/notification.so"
    ]
    :
}

Note

At present, the libraries are specified as a simple list. A future version of Kea will support the capability of specifying a set of parameters for each library. When that is added, it is likely that the syntax for specifying hooks libraries will change.

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.

At the present time, only the kea-dhcp4 and kea-dhcp6 processes support hooks libraries.

Chapter 11. libdhcp++ library

libdhcp++ is a common library written in C++ that handles many DHCP-related tasks, including:

  • DHCPv4 and DHCPv6 packets parsing, manipulation and assembly
  • Option parsing, manipulation and assembly
  • Network interface detection
  • Socket operations such as creation, data transmission and reception and socket closing.

While this library is currently used by Kea, it is designed to be a portable, universal library, useful for any kind of DHCP-related software.

11.1. Interface detection and Socket handling

Both the DHCPv4 and DHCPv6 components share network interface detection routines. Interface detection is currently supported on Linux, all BSD family (FreeBSD, NetBSD, OpenBSD), Mac OS X and Solaris 11 systems.

DHCPv4 requires special raw socket processing to send and receive packets from hosts that do not have IPv4 address assigned yet. Support for this operation is implemented on Linux, FreeBSD, NetBSD and OpenBSD. It is likely that DHCPv4 component will not work in certain cases on other systems.

Chapter 12. Logging

12.1. Logging Configuration

During its operation Kea may produce many messages. They differ in severity (some are more important than others) and source (some are produced by specific components, e.g. hooks). It is useful to understand which log messages are needed and which are not and configure your logging appropriately. For example, debug level messages can be safely ignored in a typical deployment. They are, however, very useful when debugging a problem.

The logging system in Kea is configured through the Logging structure in your configuration file. All daemons (e.g. DHCPv4 and DHCPv6 servers) will use the configuration in the Logging structure to see what should be logged and to where. This allows for sharing identical logging configuration between daemons.

12.1.1. Loggers

Within Kea, a message is logged through an entity called a "logger". Different parts of the code log messages through different loggers, and each logger can be configured independently of one another. For example there are different components that deal with hooks ("hooks" logger) and with DHCP engine ("dhcpsrv" logger).

In the Logging structure in a configuration file you can specify the configuration for zero or more loggers. If there are no loggers specified, the code will use default values which cause Kea to log messages on at least INFO severity to standard output.

The three most important elements of a logger configuration are the name (the component that is generating the messages), the severity (what to log), and the output_options (where to log).

12.1.1.1. name (string)

Each logger in the system has a name, the name being that of the component binary file using it to log messages. For instance, if you want to configure logging for the DHCPv4 server, you add an entry for a logger named kea-dhcp4. This configuration will then be used by the loggers in the DHCPv4 server, and all the libraries used by it (unless a library defines its own logger and there is specific logger configuration that applies to that logger).

If you want to specify logging for one specific library within a daemon, you set the name to daemon.library. For example, the logger used by the code from libdhcpsrv used in kea-dhcp4 binary has the full name of kea-dhcp4.dhcpsrv. If there is no entry in Logging for a particular library, it will use the configuration given for the whole daemon.

To illustrate this, suppose you want the dhcpsrv library to log messages of severity DEBUG, and the rest of the DHCPv4 server code to log messages of severity INFO. To achieve this you specify two loggers, one with the name kea-dhcp4 and severity INFO, and one with the name kea-dhcp4.dhcpsrv with severity DEBUG. As there are no entries for other libraries, they will use the configuration for the daemon (kea-dhcp4), so giving the desired behavior.

If there are multiple logger specifications in the configuration that might match a particular logger, the specification with the more specific logger name takes precedence. For example, if there are entries for both kea-dhcp4 and kea-dhcp4.dhcpsrv, the DHCPv4 server — and all libraries it uses that are not dhcpsrv — will log messages according to the configuration in the first entry (kea-dhcp4).

One final note about the naming. When specifying the daemon name within a logger, use the name of the binary file, e.g. kea-dhcp4 for the DHCPv4 server, kea-dhcp6 for the DHCPv6 server, etc. When the message is logged, the message will include the name of the process (e.g. kea-dhcp4) followed by the specific component in that process, e.g. hooks. It is possible to specify either just the process name (kea-dhcp4, will apply to everything logged within that process) or process name followed by specific logger, e.g. kea-dhcp4.hooks. That will apply only to messages originating from that component.

Currently defined loggers are:

  • kea-dhcp4 - this is the root logger for the DHCPv4 server. All components used by the DHCPv4 server inherit the settings from this logger if there is no specialized logger provided.
  • kea-dhcp4.dhcp4 - this is the logger used solely by the DHCPv4 server deamon. This logger does not specify logging settings for libraries used by the deamon.
  • kea-dhcp4.dhcpsrv - this logger is used by the libdhcpsrv library. This covers mostly DHCP engine (the lease allocation and renewal process), database operations and configuration.
  • kea-dhcp4.hooks - this logger is used during DHCPv4 hooks operation, i.e. anything related to user libraries will be logged using this logger.
  • kea-dhcp6 - this is the root logger for the DHCPv6 server. All components used by the DHCPv6 server inherit the settings from this logger if there is no specialized logger provided.
  • kea-dhcp6.dhcp6 - this is the logger used solely by the DHCPv6 server deamon. This logger does not specify logging settings for libraries used by the daemon.
  • kea-dhcp6.dhcpsrv - this logger is used by the libdhcpsrv library. This covers mostly DHCP engine (the lease allocation and renewal process), database operations and configuration.
  • kea-dhcp6.hooks - this logger is used during DHCPv6 hooks operation, i.e. anything related to user libraries will be logged using this logger.
  • kea-dhcp-ddns - this is the root logger for the kea-dhcp-ddns deamon. All components used by this deamon inherit the settings from this logger if there is no specialized logger provided.
  • kea-dhcp-ddns.dhcpddns - this is the logger used solely by the kea-dhcp-ddns deamon. This logger does not specify logging settings for libraries used by the deamon.

Additional loggers may be defined in the future. The easiest way to find out the logger name is to configure all logging to go to a single destination and look for specific logger names. See Section 12.2, “Logging Message Format” for details.

12.1.1.2. severity (string)

This specifies the category of messages logged. Each message is logged with an associated severity which may be one of the following (in descending order of severity):

  • FATAL
  • ERROR
  • WARN
  • INFO
  • DEBUG

When the severity of a logger is set to one of these values, it will only log messages of that severity, and the severities above it. The severity may also be set to NONE, in which case all messages from that logger are inhibited.

12.1.1.3. output_options (list)

Each logger can have zero or more output_options. These specify where log messages are sent. These are explained in detail below.

The other options for a logger are:

12.1.1.4. debuglevel (integer)

When a logger's severity is set to DEBUG, this value specifies what debug messages should be printed. It ranges from 0 (least verbose) to 99 (most verbose).

If severity for the logger is not DEBUG, this value is ignored.

12.1.2. Output Options

The main settings for an output option are the destination and a value called output, the meaning of which depends on the destination that is set.

12.1.2.1. destination (string)

The destination is the type of output. It can be one of:

  • console
  • file
  • syslog

12.1.2.2. output (string)

This value determines the type of output. There are several special values allowed here: stdout (messages are printed on standard output), stderr (messages are printed on stderr), syslog (messages are logged to syslog using default name, syslog:name (messages are logged to syslog using specified name). Any other value is interpreted as a filename that the logs should be written to.

The other options for output_options are:

12.1.2.2.1. maxsize (integer)

Only relevant when destination is file, this is maximum file size of output files in bytes. When the maximum size is reached, the file is renamed and a new file opened. (For example, a ".1" is appended to the name — if a ".1" file exists, it is renamed ".2", etc.)

If this is 0, no maximum file size is used.

Note

Due to a limitation of the underlying logging library (log4cplus), rolling over the log files (from ".1" to ".2", etc) may show odd results: There can be multiple small files at the timing of roll over. This can happen when multiple processes try to roll over the files simultaneously. Version 1.1.0 of log4cplus solved this problem, so if this or higher version of log4cplus is used to build Kea, it shouldn't happen. Even for older versions it is normally expected to happen rarely unless the log messages are produced very frequently by multiple different processes.

12.1.2.2.2. maxver (integer)

Maximum number of old log files to keep around when rolling the output file. Only relevant when output is file.

12.1.3. Example Logger Configurations

In this example we want to set the global logging to write to the console using standard output.


"Logging": {
    "loggers": [
        {
            "name": "kea-dhcp4",
            "output_options": [
                {
                    "output": "stdout"
                }
            ], 
            "severity": "WARN"
        }
    ]
}

In this second example, we want to store debug log messages in a file that is at most 2MB and keep up to 8 copies of old logfiles. Once the logfile grows to 2MB, it will be renamed and a new file file be created.


"Logging": {
    "loggers": [
        {
            "name": "kea-dhcp6",
            "output_options": [
                {
                    "output": "/var/log/kea-debug.log",
                    "maxver": 8,
                    "maxsize": 204800,
                    "destination": "file"
                }
            ], 
            "severity": "DEBUG",
            "debuglevel": 99
        }
   ]
}

12.2. Logging Message Format

Each message written to the configured logging destinations comprises a number of components that identify the origin of the message and, if the message indicates a problem, information about the problem that may be useful in fixing it.

Consider the message below logged to a file:

2014-04-11 12:58:01.005 INFO  [kea-dhcp4.dhcpsrv/27456]
    DHCPSRV_MEMFILE_DB opening memory file lease database: type=memfile universe=4

Note: the layout of messages written to the system logging file (syslog) may be slightly different. This message has been split across two lines here for display reasons; in the logging file, it will appear on one line.

The log message comprises a number of components:

2014-04-11 12:58:01.005

The date and time at which the message was generated.

INFO

The severity of the message.

[kea-dhcp4.dhcpsrv/27456]

The source of the message. This comprises two elements: the Kea process generating the message (in this case, kea-dhcp4) and the component within the program from which the message originated (which is the name of the common library used by DHCP server implementations). The number after the slash is a process id (pid).

DHCPSRV_MEMFILE_DB

The message identification. Every message in Kea has a unique identification, which can be used as an index into the Kea Messages Manual (http://kea.isc.org/docs/kea-messages.html) from which more information can be obtained.

opening memory file lease database: type=memfile universe=4

A brief description. Within this text, information relating to the condition that caused the message to be logged will be included. In this example, the information is logged that the in-memory lease database backend will be used to store DHCP leases.

12.3. Logging During Kea Startup

The logging configuration is specified in the configuration file. However, when Kea starts, the file is not read until some way into the initialization process. Prior to that, the logging settings are set to default values, although it is possible to modify some aspects of the settings by means of environment variables. Note that in the absence of any logging configuration in the configuration file, the settings of (possibly modified) default configuration will persist while the program is running.

The following environment variables can be used to control the behavio of logging during startup:

KEA_LOCKFILE_DIR

Specifies a directory where the logging system should create its lock file. If not specified, it is prefix/var/run/kea, where prefix defaults to /usr/local. This variable must not end with a slash. There is one special value: "none", which instructs Kea to not create lock file at all. This may cause issues if several processes log to the same file.

KEA_LOGGER_DESTINATION

Specifies logging output. There are several special values.

stdout

Log to standard output.

stderr

Log to standard error.

syslog[:fac]

Log via syslog. The optional fac (which is separated from the word "syslog" by a colon) specifies the facility to be used for the log messages. Unless specified, messages will be logged using the facility "local0".

Any other value is treated as a name of the output file. If not specified otherwise, Kea will log to standard output.

Chapter 13. Acknowledgements

Kea is primarily designed, developed, and maintained by Internet Systems Consortium, Inc. It is an open source project and contributions are welcomed.

Support for the development of the DHCPv4, DHCPv6 and DHCP-DDNS components was provided by Comcast.

Kea was initially implemented as a collection of applications within the BIND 10 framework. Hence, Kea development would not be possible without the generous support of past BIND 10 project sponsors.

JPRS and CIRA were Patron Level BIND 10 sponsors.

AFNIC, CNNIC, CZ.NIC, DENIC eG, Google, RIPE NCC, Registro.br, .nz Registry Services, and Technical Center of Internet were past BIND 10 sponsors.

Afilias, IIS.SE, Nominet, and SIDN were founding sponsors of the BIND 10 project.