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

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

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

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

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

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

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

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:

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

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

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

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

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

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

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

$ make

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Kea components use an extended JSON with additional features allowed:

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

A very simple configuration for DHCPv4 could look like this:

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

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

# The whole configuration structure ends here.
}

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

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

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

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

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

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

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

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

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

} 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"Dhcp4": {
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/tmp/kea-leases4.csv",
        "lfc-interval": 1800,
        "max-row-errors": 100
    }
}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The values of the option are set as follows:

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

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

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

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

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

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

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

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

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

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

The definition used to decode a VSI option is:

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

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

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

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

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

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

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

Alternatively, the option can be specified using its code.

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

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

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

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

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

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

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

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

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

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

Finally, we can set values for the new options:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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:

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:

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

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

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

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

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

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

"Dhcp4": {
    "dhcp-ddns": {
        "hostname-char-set": "[^A-Za-z0-9.-]",
        "hostname-char-replacement": "x",
        ...
    },
    ...
}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The following configuration was used during some tests:

{

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

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

    "valid-lifetime": 4000,

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

    "dhcp4o6-port": 6767

},

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

}

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

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

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

There are five levels which are supported:

This feature is currently implemented for memfile backend.

An example configuration that sets this parameter looks as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Vendor specific options can be reserved in a similar manner:

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

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

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

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

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

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

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

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

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

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

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

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

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

An example configuration that disables reservation appears as follows:

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

An example configuration using global reservations is shown below:

"Dhcp4": {

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

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

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

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

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

If not specified, the default value is:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

{
    "client-classes": [
        {

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

} 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"Dhcp6": {
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/tmp/kea-leases6.csv",
        "lfc-interval": 1800,
        "max-row-errors": 100
    }
}

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

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

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

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

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

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

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

"Dhcp6": { "lease-database": { "port" : 12345, ... }, ... }

"Dhcp6": { "lease-database": { "connect-timeout" : timeout-in-seconds, ... }, ... }

"Dhcp6": { "lease-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

"Dhcp6": { "lease-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

"Dhcp6": { "lease-database": { "contact-points" : "192.0.2.1,192.0.2.2", ... }, ... }

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

Kea is also able to store information about host reservations in the database. The hosts database configuration uses the same syntax as the lease database. In fact, a Kea server opens independent connections for each purpose, be it lease or hosts information. This arrangement gives the most flexibility. Kea can be used to keep leases and host reservations separately, but can also point to the same database. Currently the supported hosts database types are MySQL and PostgreSQL. The Cassandra backend does not support host reservations yet.

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

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

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

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

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

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

"Dhcp6": { "hosts-database": { "port" : 12345, ... }, ... }

"Dhcp6": { "host-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

"Dhcp6": { "hosts-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

"Dhcp6": { "hosts-database": { "user": "user-name",
                               "password": "password",
                              ... },
           ... }

"Dhcp6": { "hosts-databases": [ { "type": "mysql", ... }, ... ], ... }

"Dhcp6": { "hosts-database": { "readonly": true, ... }, ... }

The DHCPv6 server has to be configured to listen on specific network interfaces. The simplest network interface configuration instructs the server to listen on all available interfaces:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "*" ]
    }
    ...
}

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

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ]
    },
    ...
}
  

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

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3", "*" ]
    },
    ...
}
  

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

As for the DHCPv4 server binding to specific addresses and disabling re-detection of interfaces are supported. But dhcp-socket-type is not because DHCPv6 uses UDP/IPv6 sockets only. The following example shows how to disable the interface detection:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "re-detect": false
    },
    ...
}
  

The loopback interfaces (i.e. the "lo" or "lo0" interface) are not configured by default, unles explicitely mentioned in the configration. Note Kea requires a link-local address which does not exist on all systems, or a specified unicast address as in:

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "lo/::1" ]
    },
    ...
}
  

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:

"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.

When the DHCPv6 server starts, by default it listens to the DHCP traffic sent to multicast address ff02::1:2 on each interface that it is configured to listen on (see Section 9.2.4, “Interface Selection”). In some cases it is useful to configure a server to handle incoming traffic sent to the global unicast addresses as well. The most common reason for this is to have relays send their traffic to the server directly. To configure the server to listen on a specific unicast address, nn interface name can be optionally followed by a slash, followed by the global unicast address on which the server should listen. The server listens to this address in addition to normal link-local binding and listening on ff02::1:2 address. The sample configuration below shows how to listen on 2001:db8::1 (a global address) configured on the eth1 interface.

"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ "eth1/2001:db8::1" ]
    },
    ...
    "option-data": [
        {
            "name": "unicast",
            "data": "2001:db8::1"
        } ],
    ...
}
 

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

Usually unicast support is associated with a server unicast option which allows clients to send unicast messages to the server. The example above includes a server unicast option specification which will cause the client to send messages to the specified unicast address.

It is possible to mix interface names, wildcards and interface name/addresses in the list of interfaces. It is not possible however to specify more than one unicast address on a given interface.

Care should be taken to specify proper unicast addresses. The server will attempt to bind to the addresses specified without any additional checks. This approach has selected on purpose to allow the software to communicate over uncommon addresses if so desired.

The main role of a DHCPv6 server is address assignment. For this, the server has to be configured with at least one subnet and one pool of dynamic addresses to be managed. For example, assume that the server is connected to a network segment that uses the 2001:db8:1::/64 prefix. The Administrator of that network has decided that addresses from range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6 server. Such a configuration can be achieved in the following way:

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

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

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

It is possible to define more than one pool in a subnet: continuing the previous example, further assume that 2001:db8:1:0:5::/80 should also be managed by the server. It could be written as 2001:db8:1:0:5:: to 2001:db8:1::5:ffff:ffff:ffff, but typing so many 'f's is cumbersome. It can be expressed more simply as 2001:db8:1:0:5::/80. Both formats are supported by Dhcp6 and can be mixed in the pool list. For example, one could define the following pools:

"Dhcp6": {
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" },
            { "pool": "2001:db8:1:05::/80" }
        ],
        ...
    }
    ]
}

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

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

The server may be configured to serve more than one subnet. To add a second subnet, use a command similar to the following:

"Dhcp6": {
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" }
        ]
    },
    {
        "subnet": "2001:db8:2::/64",
        "pools": [
            { "pool": "2001:db8:2::/64" }
        ]
    },

        ...
    ]
}

In this example, we allow the server to dynamically assign all addresses available in the whole subnet. Although rather wasteful, it is certainly a valid configuration to dedicate the whole /64 subnet for that purpose. Note that the Kea server does not preallocate the leases, so there is no danger in using gigantic address pools.

When configuring a DHCPv6 server using prefix/length notation, please pay attention to the boundary values. When specifying that the server can use a given pool, it will also be able to allocate the first (typically network address) address from that pool. For example, for pool 2001:db8:2::/64 the 2001:db8:2:: address may be assigned as well. If you want to avoid this, use the "min-max" notation.

Subnets may also be configured to delegate prefixes, as defined in RFC 8415, section 6.3. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix (prefix) and a prefix length (prefix-len), as well as a delegated prefix length (delegated-len). The delegated length must not be shorter (that is it must be numerically greater or equal) than the prefix length. If both the delegated and prefix lengths are equal, the server will be able to delegate only one prefix. The delegated prefix does not have to match the subnet prefix.

Below is a sample subnet configuration which enables prefix delegation for the subnet:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:d8b:1::/64",
            "pd-pools": [
                {
                    "prefix": "3000:1::",
                    "prefix-len": 64,
                    "delegated-len": 96
                }
            ]
        }
    ],
    ...
}

For each delegated prefix the delegating router may choose to exclude a single prefix out of the delegated prefix as specified in the RFC 6603. The requesting router must not assign the excluded prefix to any of its downstream interfaces and it is intended to be used on a link through which the delegating router exchanges DHCPv6 messages with the requesting router. The configuration example below demonstrates how to specify an excluded prefix within a prefix pool definition. The excluded prefix "2001:db8:1:babe:cafe:80::/72" will be sent to a requesting router which includes Prefix Exclude option in the ORO, and which is delegated a prefix from this pool.

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/48",
            "pd-pools": [
                {
                    "prefix": "2001:db8:1:8000::",
                    "prefix-len": 48,
                    "delegated-len": 64,
                    "excluded-prefix": "2001:db8:1:babe:cafe:80::",
                    "excluded-prefix-len": 72
                }
            ]
        }
    ]
}

One of the major features of a DHCPv6 server is to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requests them. The following example shows how to configure DNS servers, one of the most frequently used options. Options specified in this way are considered global and apply to all configured subnets.

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
           "csv-format": true,
           "data": "2001:db8::cafe, 2001:db8::babe"
        },
        ...
    ]
}

The option-data line creates a new entry in the option-data table. This table contains information on all global options that the server is supposed to configure in all subnets. The name line specifies the option name. (For a complete list of currently supported names, see Table 9.1, “List of Standard DHCPv6 Options”.) The next line specifies the option code, which must match one of the values from that list. The line beginning with space specifies the option space, which must always be set to "dhcp6" as these are standard DHCPv6 options. For other name spaces, including custom option spaces, see Section 9.2.14, “Nested DHCPv6 Options (Custom Option Spaces)”. The following line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. Finally, the data line gives the actual value to be sent to clients. Data is specified as normal text, with values separated by commas if more than one value is allowed.

Options can also be configured as hexadecimal values. If "csv-format" is set to false, the option data must be specified as a string of hexadecimal numbers. The following commands configure the DNS-SERVERS option for all subnets with the following addresses: 2001:db8:1::cafe and 2001:db8:1::babe.

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
           "csv-format": false,
           "data": "2001 0DB8 0001 0000 0000 0000 0000 CAFE
                    2001 0DB8 0001 0000 0000 0000 0000 BABE"
        },
        ...
    ]
}
        

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

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 9.2.15, “Unspecified Parameters for DHCPv6 Option Configuration”. Only one of name or code is required, so you don't need to specify both. Space has a default value of "dhcp6", so you can skip this as well if you define a regular (not encapsulated) DHCPv6 option. Finally, csv-format defaults to true, so it too can be skipped, unless you want to specify the option value as hexstring. Therefore the above example can be simplified to:

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe"
        },
        ...
    ]
}
        

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

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe",
           "always-send": true
        },
        ...
    ]
}
        

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

"Dhcp6": {
    "option-data": [
        {
           "name": "dns-servers",
           "data": "2001:db8::cafe, 2001:db8::babe",
           "always-send": true
        },
        ...
    ],
    "subnet6": [
        {
           "subnet": "2001:db8:1::/64",
           "option-data": [
               {
                   "name": "dns-servers",
                   "data": "2001:db8:1::cafe, 2001:db8:1::babe"
               },
               ...
           ],
           ...
        },
        ...
    ],
    ...
}
        

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

It is possible to override options on a per-subnet basis. If clients connected to most of your subnets are expected to get the same values of a given option, you should use global options: you can then override specific values for a small number of subnets. On the other hand, if you use different values in each subnet, it does not make sense to specify global option values (Dhcp6/option-data), rather you should set only subnet-specific values (Dhcp6/subnet[X]/option-data[Y]).

The following commands override the global DNS servers option for a particular subnet, setting a single DNS server with address 2001:db8:1::3.

"Dhcp6": {
    "subnet6": [
        {
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:1::3"
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

In some cases it is useful to associate some options with an address or prefix pool from which a client is assigned a lease. Pool specific option values override subnet specific and global option values. If the client is assigned multiple leases from different pools, the server will assign options from all pools from which the leases have been obtained. However, if the particular option is specified in multiple pools from which the client obtains the leases, only one instance of this option will be handed out to the client. The server's administrator must not try to prioritize assignment of pool specific options by trying to order pools declarations in the server configuration. Future Kea releases may change the order in which options are assigned from the pools without any notice.

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

"Dhcp6": {
    "subnet6": [
        {
            "pools": [
                {
                    "pool": "2001:db8:1::100-2001:db8:1::300",
                    "option-data": [
                        {
                            "name": "dns-servers",
                            "data": "2001:db8:1::10"
                        }
                    ]
                }
            ]
        },
        ...
    ],
    ...
}

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

The currently supported standard DHCPv6 options are listed in Table 9.1, “List of Standard DHCPv6 Options”. The "Name" and "Code" are the values that should be used as a name in the option-data structures. "Type" designates the format of the data: the meanings of the various types is given in Table 8.2, “List of standard DHCP option types”.

Experimental options (like standard options but with a code which was not assigned by IANA) are listed in Table 9.2, “List of Experimental DHCPv6 Options”.

When a data field is a string, and that string contains the comma (,; U+002C) character, the comma must be escaped with a reverse solidus character (\; U+005C). This double escape is required, because both the routine splitting CSV data into fields and JSON use the same escape character: a single escape (\,) would make the JSON invalid. For example, the string "EST5EDT4,M3.2.0/02:00,M11.1.0/02:00" would be represented as:

"Dhcp6": {
    "subnet6": [
        {
            "pools": [
                {
                    "option-data": [
                        {
                            "name": "new-posix-timezone",
                            "data": "EST5EDT4\,M3.2.0/02:00\,M11.1.0/02:00"
                        }
                    ]
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}

Some options are designated as arrays, which means that more than one value is allowed in such an option. For example the option dns-servers allows the specification of more than one IPv6 address, allowing clients to obtain the addresses of multiple DNS servers.

The Section 9.2.12, “Custom DHCPv6 Options” describes the configuration syntax to create custom option definitions (formats). It is generally not allowed to create custom definitions for standard options, even if the definition being created matches the actual option format defined in the RFCs. There is an exception from this rule for standard options for which Kea does not yes provide a definition. In order to use such options, a server administrator must create a definition as described in Section 9.2.12, “Custom DHCPv6 Options” in the 'dhcp6' option space. This definition should match the option format described in the relevant RFC but the configuration mechanism would allow any option format as it has no means to validate the format at the moment.

Options marked with (1) have option definitions, but the logic behind them is not implemented. That means that technically Kea knows how to parse them in incoming message or how to send them if configured to do so, but not what to do with them. Since the related RFCs require certain processing, the support for those options is non-functional. However, it may be useful in some limited lab testing, hence the definition formats are listed here.

Softwire46 options are involved in IPv4 over IPv6 provisioning by means of tunneling or translation as specified in the RFC 7598. The following sections provide configuration examples of these options.

"Dhcp6": {
    ...
    "option-data": [
        {
            "name": "s46-cont-mape"
        } ],
    ...
}

{
    "space": "s46-cont-mape-options",
    "name": "s46-rule",
    "data": "128, 0, 24, 192.0.2.0, 2001:db8:1::/64"
}

{
    "space": "s46-cont-mape-options",
    "name": "s46-br",
    "data": "2001:db8:cafe::1",
}

{
    "space": "s46-cont-mapt-options",
    "name": "s46-dmr",
    "data": "2001:db8:cafe::/64",
}

{
    "space": "s46-cont-lw",
    "name": "s46-v4v6bind",
    "data": "192.0.2.3, 2001:db8:1:cafe::/64"
}

{
    "space": "s46-rule-options",
    "name": "s46-portparams",
    "data": "2, 3/4",
}

It is possible to define options in addition to the standard ones. Assume that we want to define a new DHCPv6 option called "foo" which will have code 100 and which will convey a single unsigned 32 bit integer value. We can define such an option by using the following commands:

"Dhcp6": {
    "option-def": [
        {
            "name": "foo",
            "code": 100,
            "type": "uint32",
            "array": false,
            "record-types": "",
            "space": "dhcp6",
            "encapsulate": ""
        }, ...
    ],
    ...
}

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

The name, code and type parameters are required, all others are optional. The array default value is false. The record-types and encapsulate default values are blank (i.e. ""). The default space is "dhcp6".

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

"Dhcp6": {
    "option-data": [
        {
            "name": "foo",
            "code": 100,
            "space": "dhcp6",
            "csv-format": true,
            "data": "12345"
        }, ...
    ],
    ...
}

New options can take more complex forms than simple use of primitives (uint8, string, ipv6-address etc): it is possible to define an option comprising a number of existing primitives.

For example, assume we want to define a new option that will consist of an IPv6 address, followed by an unsigned 16 bit integer, followed by a boolean value, followed by a text string. Such an option could be defined in the following way:

"Dhcp6": {
    "option-def": [
        {
            "name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
            "array": false,
            "record-types": "ipv6-address, uint16, boolean, string",
            "encapsulate": ""
        }, ...
    ],
    ...
}

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

The values of the option are set as follows:

"Dhcp6": {
    "option-data": [
        {
            "name": "bar",
            "space": "dhcp6",
            "code": 101,
            "csv-format": true,
            "data": "2001:db8:1::10, 123, false, Hello World"
        }
    ],
    ...
}

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

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

"Dhcp6": {
    "option-def": [
        {
            "name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
            "array": true,
            "record-types": "ipv6-address, uint16",
            "encapsulate": ""
        }, ...
    ],
    ...
}

The new option content is one IPv6 address followed by one or more 16 bit unsigned integers.

Currently there are two option spaces defined for the DHCPv6 daemon: "dhcp6" (for top level DHCPv6 options) and "vendor-opts-space", which is empty by default, but in which options can be defined. Those options will be carried in the Vendor-Specific Information option (code 17). The following examples show how to define an option "foo" with code 1 that consists of an IPv6 address, an unsigned 16 bit integer and a string. The "foo" option is conveyed in a Vendor-Specific Information option. This option comprises a single uint32 value that is set to "12345". The sub-option "foo" follows the data field holding this value.

"Dhcp6": {
    "option-def": [
        {
            "name": "foo",
            "code": 1,
            "space": "vendor-opts-space",
            "type": "record",
            "array": false,
            "record-types": "ipv6-address, uint16, string",
            "encapsulate": ""
        }
    ],
    ...
}

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

"Dhcp6": {
    "option-data": [
        {
            "name": "foo",
            "space": "vendor-opts-space",
            "data": "2001:db8:1::10, 123, Hello World"
        },
        ...
    ],
    ...
}

We should also define a value (enterprise-number) for the Vendor-specific Information option, that conveys our option "foo".

"Dhcp6": {
    "option-data": [
        ...,
        {
            "name": "vendor-opts",
            "data": "12345"
        }
    ],
    ...
}

Alternatively, the option can be specified using its code.

"Dhcp6": {
    "option-data": [
        ...,
        {
            "code": 17,
            "data": "12345"
        }
    ],
    ...
}

It is sometimes useful to define completely new option spaces. This is useful if the user wants their new option to convey sub-options that use a separate numbering scheme, for example sub-options with codes 1 and 2. Those option codes conflict with standard DHCPv6 options, so a separate option space must be defined.

Note that it is not required to create a new option space when defining sub-options for a standard option because it is created by default if the standard option is meant to convey any sub-options (see Section 9.2.13, “DHCPv6 Vendor-Specific Options”).

Assume that we want to have a DHCPv6 option called "container" with code 102 that conveys two sub-options with codes 1 and 2. First we need to define the new sub-options:

"Dhcp6": {
    "option-def": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv6-address",
            "record-types": "",
            "array": false,
            "encapsulate": ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
            "array": false
            "encapsulate": ""
        }
    ],
    ...
}

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

The next step is to define a regular DHCPv6 option and specify that it should include options from the isc option space:

"Dhcp6": {
    "option-def": [
        ...,
        {
            "name": "container",
            "code": 102,
            "space": "dhcp6",
            "type": "empty",
            "array": false,
            "record-types": "",
            "encapsulate": "isc"
        }
    ],
    ...
}

The name of the option space in which the sub-options are defined is set in the encapsulate field. The type field is set to empty which limits this option to only carrying data in sub-options.

Finally, we can set values for the new options:

"Dhcp6": {
    "option-data": [
        {
            "name": "subopt1",
            "code": 1,
            "space": "isc",
            "data": "2001:db8::abcd"
        },
        }
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "data": "Hello world"
        },
        {
            "name": "container",
            "code": 102,
            "space": "dhcp6"
        }
    ],
    ...
}

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

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

The DHCPv6 server may receive requests from local (connected to the same subnet as the server) and remote (connecting via relays) clients. As the server may have many subnet configurations defined, it must select an appropriate subnet for a given request.

The server can not assume which of the configured subnets are local. In IPv4 it is possible as there is a reasonable expectation that the server will have a (global) IPv4 address configured on the interface, and can use that information to detect whether a subnet is local or not. That assumption is not true in IPv6: the DHCPv6 server must be able to operate while only using link-local addresses. Therefore an optional interface parameter is available within a subnet definition to designate that a given subnet is local, i.e. reachable directly over the specified interface. For example the server that is intended to serve a local subnet over eth0 may be configured as follows:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            "interface": "eth0"
        }
    ],
    ...
}

The Rapid Commit option, described in RFC 8415, is supported by the Kea DHCPv6 server. However, support is disabled by default for all subnets. It can be enabled for a particular subnet using the rapid-commit parameter as shown below:

"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "rapid-commit": true,
            "pools": [
                 {
                     "pool": "2001:db8:beef::1-2001:db8:beef::10"
                 }
             ],
        }
    ],
    ...
}

This setting only affects the subnet for which the rapid-commit is set to true. For clients connected to other subnets, the server will ignore the Rapid Commit option sent by the client and will follow the 4-way exchange procedure, i.e. respond with an Advertise for a Solicit containing a Rapid Commit option.

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:

The two are mutually exclusive: a subnet cannot be both reachable locally (direct traffic) and via relays (remote traffic). Specifying both is a configuration error and the DHCPv6 server will refuse such a configuration.

The following example configuration shows how to specify an interface-id with a value of "vlan123".

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

RFC 6422 defines a mechanism called Relay-Supplied DHCP Options. In certain cases relay agents are the only entities that may have specific information. They can insert options when relaying messages from the client to the server. The server will then do certain checks and copy those options to the response that will be sent to the client.

There are certain conditions that must be met for the option to be included. First, the server must not provide the option itself. In other words, if both relay and server provide an option, the server always takes precedence. Second, the option must be RSOO-enabled. IANA maintains a list of RSOO-enabled options here. However, there may be cases when system administrators want to echo other options. Kea can be instructed to treat other options as RSOO-enabled. For example, to mark options 110, 120 and 130 as RSOO-enabled, the following syntax should be used:

"Dhcp6": {
    "relay-supplied-options": [ "110", "120", "130" ],
    ...
}

As of March 2015, only option 65 is RSOO-enabled by IANA. This option will always be treated as such and there's no need to explicitly mark it. Also, when enabling standard options, it is possible to use their names, rather than option code, e.g. (e.g. use dns-servers instead of 23). See Table 9.1, “List of Standard DHCPv6 Options” for the names. In certain cases it could also work for custom options, but due to the nature of the parser code this may be unreliable and should be avoided.

The DHCPv6 server includes support for client classification. For a deeper discussion of the classification process see Chapter 14, Client Classification.

In certain cases it is useful to configure the server to differentiate between DHCP clients types and treat them accordingly. It is envisaged that client classification will be used for modifying the behavior of almost any part of the DHCP message processing. In the current release of Kea, there are three mechanisms that take advantage of the client classification in DHCPv6: subnet selection, address pool selection and DHCP options assignment.

In certain cases it is useful to differentiate between different types of clients and treat them accordingly. It is envisaged that client classification will be used for changing the behavior of almost any part of the DHCP message processing. In the current release of the software however, there are only some mechanisms that take advantage of client classification: subnet selection, pool selection, and assignment of different options.

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

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

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

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

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

"Dhcp6": {
    "client-classes": [
        {
            "name": "Client_enterprise",
            "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD'",
            "option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
                    "space": "dhcp6",
                    "csv-format": true,
                    "data": "2001:db8:0::1, 2001:db8:2::1"
                }
            ]
        },
        ...
    ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
            "client-class": "Client_enterprise"
        }
    ],
    ...
}

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

"Dhcp6": {
    "client-classes": [
       {
           "name": "Client_foo",
           "test": "member('ALL')",
           "only-if-required": true
       },
       ...
    ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64"
            "pools": [
                 {
                     "pool": "2001:db8:1::-2001:db8:1::ffff"
                 }
             ],
            "require-client-classes": [ "Client_foo" ],
            ...
        },
        ...
    ],
    ...
}

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:

The parameters controlling the generation of NCRs for submission to D2 are contained in the dhcp-ddns section of the kea-dhcp6 configuration. The mandatory parameters for the DHCP DDNS configuration are enable-updates which is unconditionally required, and qualifying-suffix which has no default value and is required when enable-updates is set to true. The two (disabled and enabled) minimal DHCP DDNS configurations are:

"Dhcp6": {
    "dhcp-ddns": {
        "enable-updates": false
    },
    ...
}

and for example:

"Dhcp6": {
    "dhcp-ddns": {
        "enable-updates": true,
        "qualifying-suffix": "example."
    },
    ...
}

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

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

"Dhcp6": {
    "dhcp-ddns": {
        "override-client-update": true,
        ...
    },
    ...
}

"Dhcp6": {
    "dhcp-ddns": {
        "override-no-update": true,
        ...
    },
    ...
}

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

"Dhcp6": {
    "dhcp-ddns": {
        "generated-prefix": "another.host",
        ...
    },
    ...
}

"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.

"Dhcp4": {
    "dhcp-ddns": {
        "hostname-char-set": "[^A-Za-z0-9.-]",
        "hostname-char-replacement": "x",
        ...
    },
    ...
}

The support of DHCPv4-over-DHCPv6 transport is described in RFC 7341 and is implemented using cooperating DHCPv4 and DHCPv6 servers. This section is about the configuration of the DHCPv6 side (the DHCPv4 side is described in Section 8.2.21, “DHCPv4-over-DHCPv6: DHCPv4 Side”).

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

Two other configuration entries are in general required: unicast traffic support (see Section 9.2.6, “Unicast Traffic Support”) and DHCP 4o6 server address option (name "dhcp4o6-server-addr", code 88).

The following configuration was used during some tests:

{

# DHCPv6 conf
"Dhcp6": {

    "interfaces-config": {
        "interfaces": [ "eno33554984/2001:db8:1:1::1" ]
    },

    "lease-database": {
        "type": "memfile",
        "name": "leases6"
    },

    "preferred-lifetime": 3000,
    "valid-lifetime": 4000,
    "renew-timer": 1000,
    "rebind-timer": 2000,

    "subnet6": [ {
        "subnet": "2001:db8:1:1::/64",
        "interface": "eno33554984",
        "pools": [ { "pool": "2001:db8:1:1::1:0/112" } ]
    } ],

    "dhcp4o6-port": 6767,

    "option-data": [ {
        "name": "dhcp4o6-server-addr",
        "code": 88,
        "space": "dhcp6",
        "csv-format": true,
        "data": "2001:db8:1:1::1"
    } ]

},

"Logging": {
    "loggers": [ {
        "name": "kea-dhcp6",
        "output_options": [ {
            "output": "/tmp/kea-dhcp6.log"
        } ],
        "severity": "DEBUG",
        "debuglevel": 0
    } ]
}

}

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

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

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

Since delegated prefixes do not have to belong to a subnet in which they're offered, there is no way to implement such a mechanism for IPv6 prefixes. As such, the mechanism works for IPv6 addresses only.

There are five levels which are supported:

This feature is currently implemented for memfile backend.

An example configuration that sets this parameter looks as follows:

"Dhcp6": {
    "sanity-checks": {
        // This parameter determines what to do when a new lease appears in the
        // system (i.e. either is read from disk during memfile startup or is
        // added via lease commands). There are five modes supported:
        // none - do nothing, accept them as is
        // warn - if subnet-id problems are detected, print a warning, but
        //        otherwise load the lease as is. This is the default value.
        // fix - attempt to fix the lease by finding appropriate subnet-id value.
        //       if there is no suitable subnet, the lease is loaded as is.
        // fix-del - attempt to fix the lease by findind appropriate subnet-id
        //           value. If there is no suitable subnet, the lease is deleted.
        // del - delete leases that have incorrect subnet-id values.
        "lease-checks": "fix-del"
    },
    ...
}

In a typical scenario there is an IPv6 subnet defined with a certain part of it dedicated for dynamic address allocation by the DHCPv6 server. There may be an additional address space defined for prefix delegation. Those dynamic parts are referred to as dynamic pools, address and prefix pools or simply pools. In principle, the host reservation can reserve any address or prefix that belongs to the subnet. The reservations that specify an address that belongs to configured pools are called "in-pool reservations". In contrast, those that do not belong to dynamic pools are called "out-of-pool reservations". There is no formal difference in the reservation syntax and both reservation types are handled uniformly. However, upcoming releases may offer improved performance if there are only out-of-pool reservations as the server will be able to skip reservation checks when dealing with existing leases. Therefore, system administrators are encouraged to use out-of-pool reservations if possible.

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

As reservations and lease information are stored separately, conflicts may arise. Consider the following series of events. The server has configured the dynamic pool of addresses from the range of 2001:db8::10 to 2001:db8::20. Host A requests an address and gets 2001:db8::10. Now the system administrator decides to reserve address 2001:db8::10 for Host B. In general, reserving an address that is currently assigned to someone else is not recommended, but there are valid use cases where such an operation is warranted.

The server now has a conflict to resolve. Let's analyze the situation here. If Host B boots up and request an address, the server is not able to assign the reserved address 2001:db8::10. A naive approach would to be immediately remove the lease for Host A and create a new one for Host B. That would not solve the problem, though, because as soon as Host B get the address, it will detect that the address is already in use by someone else (Host A) and would send a Decline message. Therefore in this situation, the server has to temporarily assign a different address from the dynamic pool (not matching what has been reserved) to Host B.

When Host A renews its address, the server will discover that the address being renewed is now reserved for someone else (Host B). Therefore the server will remove the lease for 2001:db8::10, select a new address and create a new lease for it. It will send two addresses in its response: the old address with lifetime set to 0 to explicitly indicate that it is no longer valid and the new address with a non-zero lifetime. When Host B renews its temporarily assigned address, the server will detect that the existing lease does not match reservation, so it will release the current address Host B has and will create a new lease matching the reservation. Similar as before, the server will send two addresses: the temporarily assigned one with zeroed lifetimes, and the new one that matches reservation with proper lifetimes set.

This recovery will succeed, even if other hosts will attempt to get the reserved address. Had Host C requested address 2001:db8::10 after the reservation was made, the server will propose a different address.

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

When the reservation for the client includes the hostname, the server will assign this hostname to the client and send it back in the Client FQDN, if the client sent the FQDN option to the server. The reserved hostname always takes precedence over the hostname supplied by the client (via the FQDN option) or the autogenerated (from the IPv6 address) hostname.

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

"subnet6": [
    {
        "subnet": "2001:db8:1::/48",
        "pools": [ { "pool": "2001:db8:1::/80" } ],
        "reservations": [
            {
                "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                "ip-addresses": [ "2001:db8:1::100" ]
                "hostname": "alice-laptop"
            }
        ]
    }
],
"dhcp-ddns": {
    "enable-updates": true,
    "qualifying-suffix": "example.isc.org."
}

will result in assigning the "alice-laptop.example.isc.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E". If the qualifying-suffix is not specified, the default (empty) value will be used, and in this case the value specified as a hostname will be treated as fully qualified name. Thus, by leaving the qualifying-suffix empty it is possible to qualify hostnames for the different clients with different domain names:

"subnet6": [
    {
        "subnet": "2001:db8:1::/48",
        "pools": [ { "pool": "2001:db8:1::/80" } ],
        "reservations": [
            {
                "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                "ip-addresses": [ "2001:db8:1::100" ]
                "hostname": "mark-desktop.example.org."
            }
        ]
    }
],
"dhcp-ddns": {
    "enable-updates": true,
}

The above example results in the assignment of the "mark-desktop.example.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E".

Kea 1.1.0 introduced the ability to specify options on a per host basis. The options follow the same rules as any other options. These can be standard options (see Section 9.2.10, “Standard DHCPv6 Options”), custom options (see Section 9.2.12, “Custom DHCPv6 Options”) or vendor specific options (see Section 9.2.13, “DHCPv6 Vendor-Specific Options”). The following example demonstrates how standard options can be defined.

"reservations": [
{
   "duid": "01:02:03:05:06:07:08",
   "ip-addresses": [ "2001:db8:1::2" ],
    "option-data": [
    {
        "option-data": [ {
            "name": "dns-servers",
            "data": "3000:1::234"
        },
        {
            "name": "nis-servers",
            "data": "3000:1::234"
        }
    } ]
} ]

Vendor specific options can be reserved in a similar manner:

"reservations": [
{
    "duid": "aa:bb:cc:dd:ee:ff",
    "ip-addresses": [ "2001:db8::1" ],
    "option-data": [
    {
        "name": "vendor-opts",
        "data": 4491
    },
    {
        "name": "tftp-servers",
        "space": "vendor-4491",
        "data": "3000:1::234"
    } ]
} ]

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

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

{
    "client-classes": [
    {
       "name": "reserved-class1",
       "option-data": [
       {
           "name": "dns-servers",
           "data": "2001:db8:1::50"
       }
       ]
   },
   {
       "name": "reserved-class2",
       "option-data": [
       {
           "name": "nis-servers",
           "data": "2001:db8:1::100"
       }
       ]
    }
    ],
    "subnet6": [
    {   "pools": [ { "pool": "2001:db8:1::/64" } ],
        "subnet": "2001:db8:1::/48",
        "reservations": [
        {
            "duid": "01:02:03:04:05:06:07:08",
            
            "client-classes": [ "reserved-class1", "reserved-class2" ]
            
         } ]
     } ]
 }

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

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

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

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

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

An example configuration that disables reservation looks like follows:

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

An example configuration using global reservations is shown below:

"Dhcp6": {

    
    "reservation-mode": "global",
    "reservations": [
       {
        "duid": "00:03:00:01:11:22:33:44:55:66",
        "hostname": "host-one"
       },
       {
        "duid": "00:03:00:01:99:88:77:66:55:44",
        "hostname": "host-two"
       }
    ],
    
    "subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        ...
    }
    ]
}

For more details regarding global reservations, see Section 9.3.8, “Global reservations in DHCPv6”.

Another aspect of the host reservations are different types of identifiers. Kea 1.1.0 supports two types of identifiers in DHCPv6: hw-address and duid, but more identifier types are likely to be added in the future. This is beneficial from a usability perspective. However, there is a drawback. For each incoming packet Kea has to to extract each identifier type and then query the database to see if there is a reservation done by this particular identifier. If nothing is found, the next identifier is extracted and next query is issued. This process continues until either a reservation is found or all identifier types have been checked. Over time with an increasing number of supported identifier types, Kea would become slower and slower.

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

"host-reservation-identifiers": [ "duid", "hw-address" ],
"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        ...
    }
]

If not specified, the default value is:

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

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

This feature can be used to assign certain parameters, such as hostname or other dedicated, host-specific options. It can also be used to assign addresses or prefixes. However, global reservations that assign either of these bypass the whole topology determination provided by DHCP logic implemented in Kea. It is very easy to misuse this feature and get configuration that is inconsistent. To give a specific example, imagine a global reservation for an address 2001:db8:1111::1 and two subnets 2001:db8:1111::/48 and 2001:db8:ffff::/48. If global reservations are used in both subnets and a device matching global host reservations visits part of the network that is covered by 2001:db8:ffff::/48, it will get an IP address 2001:db8:ffff::1, which will be outside of the prefix announced by its local router using Router Advertisements. Such a configuration would be unsuable or at the very least ridden with issues, such as the downlink traffic not reaching the device.

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

"Dhcp6:" {
    // This specifies global reservations. They will apply to all subnets that
    // have global reservations enabled.
    
    "reservations": [
    {
       "hw-address": "aa:bb:cc:dd:ee:ff",
       "hostname": "hw-host-dynamic"
    },
    {
       "hw-address": "01:02:03:04:05:06",
       "hostname": "hw-host-fixed",

       // Use of IP address is global reservation is risky. If used outside of
       // matching subnet, such as 3001::/64, it will result in a broken
       // configuration being handled to the client.
       "ip-address": "2001:db8:ff::77"
    },
    {
       "duid": "01:02:03:04:05",
       "hostname": "duid-host"
    }
    ],
    "valid-lifetime": 600,
    "subnet4": [ {
        "subnet": "2001:db8:1::/64",
        "reservation-mode": "global",
        "pools": [ { "pool": "2001:db8:1::-2001:db8:1::100" } ]
    } ]
}

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

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

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

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

        "subnet6": [
            {
                "subnet": "2001:db8::/64",
                "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ],
                "interface": "eth0"
            },
            {
                 "subnet": "3ffe:abcd::/64",
                 "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ]

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

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

"shared-networks": [
    {
        "name": "kakapo",
        "relay": {
            "ip-addresses":  [ "2001:db8::abcd" ]
        },
        "subnet6": [
            {
                "subnet": "2001:db8::/64",
                "relay": {
                    "ip-addresses": [ "2001:db8::1234" ]
                },
                "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
            },
            {
                 "subnet": "3ffe:abcd::/64",
                 "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ],
                 "relay": {
                    "ip-addresses": [ "3ffe:abcd::cafe" ]
                 }
            }
        ]
    }
]