# The DHCPv6 Server

## Starting and Stopping the DHCPv6 Server

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

- `-c file` - specifies the configuration file. This is the only mandatory switch.

- `-d` - specifies whether the server logging should be switched to verbose mode. In verbose mode, the logging severity and debuglevel specified in the configuration file are ignored and "debug" severity and the maximum debuglevel (99) are assumed. The flag is convenient for temporarily switching the server into maximum verbosity, e.g. when debugging.

- `-p server-port` - specifies the local UDP port on which the server will listen. This is only useful during testing, as a DHCPv6 server listening on ports other than the standard ones will not be able to handle regular DHCPv6 queries.

- `-P client-port` - specifies the remote UDP port to which the server will send all responses. This is only useful during testing, as a DHCPv6 server sending responses to ports other than the standard ones will not be able to handle regular DHCPv6 queries.

- `-t file` - specifies a configuration file to be tested. Kea-dhcp6 will load it, check it, and exit. During the test, log messages are printed to standard output and error messages to standard error. The result of the test is reported through the exit code (0 = configuration looks ok, 1 = error encountered). The check is not comprehensive; certain checks are possible only when running the server.

- `-v` - displays the Kea version and exits.

- `-V` - displays the Kea extended version with additional parameters and exits. The listing includes the versions of the libraries dynamically linked to Kea.

- `-W` - displays the Kea configuration report and exits. The report is a copy of the `config.report` file produced by `./configure`; it is embedded in the executable binary.

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

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

- `localstatedir`: The value as passed into the build configure script; it defaults to "/usr/local/var". Note that this value may be overridden at runtime by setting the environment variable KEA_PIDFILE_DIR, although this is intended primarily for testing purposes.

- `conf name`: The configuration file name used to start the server, minus all preceding paths and the file extension. For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the portion used would be "myconf".

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

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

## DHCPv6 Server Configuration

### Introduction

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

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

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

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

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

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

    }

The following paragraphs provide a brief overview of the parameters in the above example, along 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`.

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

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

In the general case, the order in which those parameters appear does not matter, but there are two caveats. 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 DHCPv6 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.) The address will become deprecated in 3000 seconds, i.e. 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. To listen on two interfaces, the `interfaces-config` should 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. If there were 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 a minimum, a subnet definition has to have at least two parameters: `subnet` (which 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. If more than one were 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, so 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 first closing brace would be followed by a comma and another object definition.

### Lease Storage

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

#### Memfile - Basic Storage for Leases

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

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

- `persist`: controls whether the new leases and updates to existing leases are written to the file. It is strongly recommended that the value of this parameter is set to `true` at all times, during the server's normal operation. Not writing leases to disk means that if a server is restarted (e.g. after a power failure), it will not know what addresses have been assigned. As a result, it may hand out addresses to new clients that are already in use. The value of `false` is mostly useful for performance-testing purposes. The default value of the `persist` parameter is `true`, which enables writing lease updates to the lease file.

- `name`: specifies an absolute location of the lease file in which new leases and lease updates will be recorded. The default value for this parameter is `"[kea-install-dir]/var/kea/kea-leases6.csv"` .

- `lfc-interval`: specifies the interval, in seconds, at which the server will perform a lease file cleanup (LFC). This removes redundant (historical) information from the lease file and effectively reduces the lease file size. The cleanup process is described in more detailed fashion further in this section. The default value of the `lfc-interval` is `3600`. A value of 0 disables the LFC.

An example configuration of the Memfile backend is presented below:

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

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

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

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

Lease file cleanup is performed by a separate process (in the background) to avoid a performance impact on the server process. To avoid the conflicts between two processes both using the same lease files, the LFC process starts with Kea opening new lease file and the actual LFC process operates on the lease file that is no longer used by the server. There are also other files created as a side effect of the lease file cleanup. The detailed description of the LFC is located later in this Kea Administrator's Reference Manual: .

#### Lease Database Configuration

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

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

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

Next, the name of the database to hold the leases must be set; this is the name used when the database was created (see , , or ).

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

For Cassandra:

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

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

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

For Cassandra, multiple contact points can be provided:

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

Normally, the database will be on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

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

For Cassandra:

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

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

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

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

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

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

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

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

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

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

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

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

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

Note that host parameter is used by MySQL and PostgreSQL backends. Cassandra has a concept of contact points that could be used to contact the cluster, instead of a single IP or hostname. It takes a list of comma-separated IP addresses, which may be specified as:

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

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

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

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

#### Cassandra-Specific Parameters

The parameters are the same for DHCPv4 and DHCPv6. See for details.

### Hosts Storage

Kea is also able to store information about host reservations in the database. The hosts database configuration uses the same syntax as the lease database. In fact, a Kea server opens independent connections for each purpose, be it lease or hosts information. This arrangement gives the most flexibility. Kea can 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.

For example, the following configuration can be used to configure connection to MySQL:

    "Dhcp6": {
        "hosts-database": {
            "type": "mysql",
            "name": "kea",
            "user": "kea",
            "password": "secret123",
            "host": "localhost",
            "port": 3306
        }
    }

Note that depending on your database configuration, many of the parameters may be optional.

Please note that usage of hosts storage is optional. A user can define all host reservations in the configuration file, and that is the recommended way if the number of reservations is small. However, when the number of reservations grows, it is 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.

In fact, host information can be placed in multiple stores. Operations are performed on the stores in the order they are defined in the configuration file, although this leads to a restriction in ordering in the case of a host reservation addition; read-only stores must be configured after a (required) read-write store, or the addition will fail.

#### DHCPv6 Hosts Database Configuration

Hosts database configuration is controlled through the Dhcp6/hosts-database parameters. If enabled, the type of database must be set to "mysql" or "postgresql".

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

Next, the name of the database to hold the reservations must be set; this is the name used when the database was created (see for instructions on how to set up the desired database type).

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

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

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

Normally, the database will be on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

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

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

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

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

If the server is unable to reconnect to the database after making the maximum number of attempts the server will exit. A value of zero (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and Postgres only). For Cassandra, Kea uses a Cassandra interface that connects to all nodes in a cluster at the same time. Any connectivity issues should be handled by internal Cassandra mechanisms.

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

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

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

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

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

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

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

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

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

For additional Cassandra-specific parameters, see .

#### Using Read-Only Databases for Host Reservations

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

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

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

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

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

### Interface Configuration

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 a wildcard interface name (asterisk) concurrently with explicit interface names:

    "Dhcp6": {
        "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.

As with 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, unless explicitely mentioned in the configuration. Note that Kea requires a link-local address (which does not exist on all systems), or a specified unicast address as in:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "enp0s2/2001:db8::1234:abcd" ]
        },
        ...
    }

### IPv6 Subnet Identifier

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

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

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

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

    "Dhcp6": {
        "subnet6": [
            {
                "subnet": "2001:db8:1::/64",
                "id": 1024,
                ...
            }
        ]
    }

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

### Unicast Traffic Support

When the DHCPv6 server starts, by default it listens to the DHCP traffic sent to multicast address ff02::1:2 on each interface that it is configured to listen on (see ). In some cases it is useful to configure a server to handle incoming traffic sent to 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, the 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 the 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 names/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 was selected on purpose to allow the software to communicate over uncommon addresses if so desired.

### Subnet and Address Pool

The main role of a DHCPv6 server is address assignment. For this, the server must 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 decides 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.

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 a network address) address from that pool. For example, for pool 2001:db8:2::/64 the 2001:db8:2:: address may be assigned as well. To avoid this, use the "min-max" notation.

### Subnet and Prefix Delegation Pools

Subnets may also be configured to delegate prefixes, as defined in RFC 8415, section 6.3. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix (`prefix`) and a prefix length (`prefix-len`), as well as a delegated prefix length (`delegated-len`). The delegated length must not be shorter than (that is, it must be numerically greater than or equal to) 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
                    }
                ]
            }
        ],
        ...
    }

### Prefix Exclude Option

For each delegated prefix, the delegating router may choose to exclude a single prefix out of the delegated prefix as specified in 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 the 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
                    }
                ]
            }
        ]
    }

### Standard DHCPv6 Options

One of the major features of a DHCPv6 server is the ability 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 one of the most frequently used options, which supplies the address of DNS servers. 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 .) 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 . 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 hexadecimal string. The following commands configure the DNS-SERVERS option for all subnets with the following addresses: 2001:db8:1::cafe and 2001:db8:1::babe.

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

The value for the setting of the "data" element is split across two lines in this example for clarity; when entering the command, the whole string should be entered on the same line.

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

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in . Only one of name or code is required; 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 the response when the client requests them, as well as any options required by a protocol. An administrator can also specify that an option is always sent, even if a client did not specifically request it. 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; rather, you should set only subnet-specific ones.

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.

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 also be specified in a class of host reservation scope. The current Kea options precedence order is (from most important): host reservation, pool, subnet, shared network, class, global.

The currently supported standard DHCPv6 options are listed in . "Name" and "Code" are the values that should be used as a name/code in the option-data structures. "Type" designates the format of the data; the meanings of the various types is given in .

When a data field is a string, and that string contains the comma (,; U+002C) character, the comma must be escaped with double backslashes (\; 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, enabling clients to obtain the addresses of multiple DNS servers.

describes the configuration syntax to create custom option definitions (formats). Creation of custom definitions for standard options is generally not permitted, even if the definition being created matches the actual option format defined in the RFCs. There is an exception to 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 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 currently has no means to validate it.

#### List of Standard DHCPv6 Options

Name | Code | Type | Array?
--- | --- | --- | ---
preference | 7 | uint8 | false
unicast | 12 | ipv6-address | false
vendor-opts | 17 | uint32 | false
-->
sip-server-dns | 21 | fqdn | true
sip-server-addr | 22 | ipv6-address | true
dns-servers | 23 | ipv6-address | true
domain-search | 24 | fqdn | true
nis-servers | 27 | ipv6-address | true
nisp-servers | 28 | ipv6-address | true
nis-domain-name | 29 | fqdn | true
nisp-domain-name | 30 | fqdn | true
sntp-servers | 31 | ipv6-address | true
information-refresh-time | 32 | uint32 | false
bcmcs-server-dns | 33 | fqdn | true
bcmcs-server-addr | 34 | ipv6-address | true
geoconf-civic | 36 | record (uint8, uint16, binary) | false
remote-id | 37 | record (uint32, binary) | false
subscriber-id | 38 | binary | false
client-fqdn | 39 | record (uint8, fqdn) | false
pana-agent | 40 | ipv6-address | true
new-posix-timezone | 41 | string | false
new-tzdb-timezone | 42 | string | false
ero | 43 | uint16 | true
lq-query (1) | 44 | record (uint8, ipv6-address) | false
client-data (1) | 45 | empty | false
clt-time (1) | 46 | uint32 | false
lq-relay-data (1) | 47 | record (ipv6-address, binary) | false
lq-client-link (1) | 48 | ipv6-address | true
v6-lost | 51 | fqdn | false
capwap-ac-v6 | 52 | ipv6-address | true
relay-id | 53 | binary | false
v6-access-domain | 57 | fqdn | false
sip-ua-cs-list | 58 | fqdn | true
bootfile-url | 59 | string | false
bootfile-param | 60 | tuple | true
client-arch-type | 61 | uint16 | true
nii | 62 | record (uint8, uint8, uint8) | false
aftr-name | 64 | fqdn | false
erp-local-domain-name | 65 | fqdn | false
rsoo | 66 | empty | false
pd-exclude | 67 | binary | false
rdnss-selection | 74 | record (ipv6-address, uint8, fqdn) | true
client-linklayer-addr | 79 | binary | false
link-address | 80 | ipv6-address | false
solmax-rt | 82 | uint32 | false
inf-max-rt | 83 | uint32 | false
dhcp4o6-server-addr | 88 | ipv6-address | true
s46-rule | 89 | record (uint8, uint8, uint8, ipv4-address, ipv6-prefix) | false
s46-br | 90 | ipv6-address | false
s46-dmr | 91 | ipv6-prefix | false
s46-v4v6bind | 92 | record (ipv4-address, ipv6-prefix) | false
s46-portparams | 93 | record(uint8, psid) | false
s46-cont-mape | 94 | empty | false
s46-cont-mapt | 95 | empty | false
s46-cont-lw | 96 | empty | false
v6-captive-portal | 103 | string | false
ipv6-address-andsf | 143 | ipv6-address | true

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 messages 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.

### Common Softwire46 Options

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

#### Softwire46 Container Options

S46 container options group rules and optional port parameters for a specified domain. There are three container options specified in the "dhcp6" (top-level) option space: the MAP-E Container option, the MAP-T Container option, and the S46 Lightweight 4over6 Container option. These options only contain encapsulated options specified below; they do not include any data fields.

To configure the server to send a specific container option along with all encapsulated options, the container option must be included in the server configuration as shown below:

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

This configuration will cause the server to include the MAP-E Container option to the client. Use "s46-cont-mapt" or "s46-cont-lw" for the MAP-T Container and S46 Lightweight 4over6 Container options, respectively.

All remaining Softwire options described below are included in one of the container options. Thus, they have to be included in appropriate option spaces by selecting a "space" name, which specifies in which option they are supposed to be included.

#### S46 Rule Option

The S46 Rule option is used for conveying the Basic Mapping Rule (BMR) and Forwarding Mapping Rule (FMR).

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

Another possible "space" value is "s46-cont-mapt-options".

The S46 Rule option conveys a number of parameters:

- `flags`, an unsigned 8-bit integer, with currently only the most-significant bit specified. It denotes whether the rule can be used for forwarding (128) or not (0).

- `ea-len`, an 8-bit-long Embedded Address length. Allowed values range from 0 to 48.

- `IPv4 prefix length`, 8 bits long; expresses the prefix length of the Rule IPv4 prefix specified in the ipv4-prefix field. Allowed values range from 0 to 32.

- `IPv4 prefix`, a fixed-length 32-bit field that specifies the IPv4 prefix for the S46 rule. The bits in the prefix after prefix4-len number of bits are reserved, and MUST be initialized to zero by the sender and ignored by the receiver.

- `IPv6 prefix`, in prefix/length notation that specifies the IPv6 domain prefix for the S46 rule. The field is padded on the right with zero bits up to the nearest octet boundary, when prefix6-len is not evenly divisible by 8.

#### S46 BR Option

The S46 BR option is used to convey the IPv6 address of the Border Relay. This option is mandatory in the MAP-E Container option and is not permitted in the MAP-T and S46 Lightweight 4over6 Container options.

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

Another possible "space" value is "s46-cont-lw-options".

#### S46 DMR Option

The S46 DMR option is used to convey values for the Default Mapping Rule (DMR). This option is mandatory in the MAP-T container option and is not permitted in the MAP-E and S46 Lightweight 4over6 Container options.

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

This option must not be included in other containers.

#### S46 IPv4/IPv6 Address Binding option.

The S46 IPv4/IPv6 Address Binding option may be used to specify the full or shared IPv4 address of the Customer Edge (CE). The IPv6 prefix field is used by the CE to identify the correct prefix to use for the tunnel source.

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

This option must not be included in other containers.

#### S46 Port Parameters

The S46 Port Parameters option specifies optional port-set information that MAY be provided to CEs.

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

Another possible "space" value is "s46-v4v6bind", to include this option in the S46 IPv4/IPv6 Address Binding option.

Note that the second value in the example above specifies the PSID and PSID-length fields in the format of PSID/PSID length. This is equivalent to the values of PSID-len=4 and PSID=12288 conveyed in the S46 Port Parameters option.

### Custom DHCPv6 Options

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

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

The "false" value of the `array` parameter determines that the option does NOT comprise an array of "uint32" values but 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. The `record-types` value should be non-empty if `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 and does not set its value(s).

Only 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 ones from those listed in .

The values of the options are set in a `option-data` statement 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 to `true` to indicate that the `data` field comprises a command-separated list of values. The values in `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.

In general, boolean values are specified as `true` or `false`, without quotes. Some specific boolean parameters may accept also `"true"`, `"false"`, `0`, `1`, `"0"`, and `"1"`.

### DHCPv6 Vendor-Specific Options

Currently there are two option spaces defined for the DHCPv6 daemon: "dhcp6" (for top-level DHCPv6 options) and "vendor-opts-space", which is empty by default, but in which options can be defined. Those options are 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, which 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"
            }
        ],
        ...
    }

### Nested DHCPv6 Options (Custom Option Spaces)

It is sometimes useful to define completely new option spaces. This is the case when a 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 the creation of a new option space is not required 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 ).

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 new 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 were required to carry a 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.

### Unspecified Parameters for DHCPv6 Option Configuration

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

- `name` - the server requires an option name or option code to identify an option. If this parameter is unspecified, the option code must be specified.

- `code` - the server requires an option name or option code to identify an option. This parameter may be left unspecified if the `name` parameter is specified. However, this also requires that the particular option have a definition (either as a standard option or an administrator-created definition for the option using an 'option-def' structure), as the option definition associates an option with a particular name. It is possible to configure an option for which there is no definition (unspecified option format). Configuration of such options requires the use of the option code.

- `space` - if the option space is unspecified it will default to 'dhcp6', which is an option space holding standard DHCPv6 options.

- `data` - if the option data is unspecified it defaults to an empty value. The empty value is mostly used for the options which have no payload (boolean options), but it is legal to specify empty values for some options which carry variable-length data and which the specification allows a length of 0. For such options, the data parameter may be omitted in the configuration.

- `csv-format` - if this value is not specified the server will assume that the option data is specified as a list of comma-separated values to be assigned to individual fields of the DHCP option.

### IPv6 Subnet Selection

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

In IPv4, the server can determine which of the configured subnets are local, as there is a reasonable expectation that the server will have a (global) IPv4 address configured on the interface, and it can use that information to detect whether a subnet is local. 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"
            }
        ],
        ...
    }

### Rapid Commit

The Rapid Commit option, described in RFC 8415, is supported by the Kea DHCPv6 server. However, support is disabled by default. It can be enabled on a per-subnet basis 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.

### DHCPv6 Relays

A DHCPv6 server with multiple subnets defined must select the appropriate subnet when it receives a request from a client. For clients connected via relays, two mechanisms are used:

The first uses the linkaddr field in the RELAY_FORW message. The name of this field is somewhat misleading in that it does not contain a link-layer address; instead, it holds an address (typically a global address) that is used to identify a link. The DHCPv6 server checks to see whether 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. For this to work successfully, the relay interface IDs must be unique within the network and the server configuration must match those values.

When configuring the DHCPv6 server, it should be noted that two similarly named parameters can be configured for a subnet:

- `interface` defines which local network interface can be used to access a given subnet.

- `interface-id` specifies the content of the interface-id option used by relays to identify the interface on the relay to which the response packet is sent.

The two are mutually exclusive; a subnet cannot be reachable both 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"
            }
        ],
        ...
    }

### Relay-Supplied Options

RFC 6422 defines a mechanism called Relay-Supplied DHCP Options. In certain cases relay agents are the only entities that may have specific information, and 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 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. (RSOO is the "Relay Supplied Options option.") 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 February 2019, only option 65 is RSOO-enabled by IANA. This option will always be treated as such, so there is no need to explicitly mark it. Also, when enabling standard options, it is possible to use their names, rather than option code, e.g. use `dns-servers` instead of `23`. See 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.

### Client Classification in DHCPv6

The DHCPv6 server includes support for client classification. For a deeper discussion of the classification process see .

In certain cases it is useful to configure the server to differentiate between DHCP client types and treat them accordingly. Client classification can be used to modify 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 client classification in DHCPv6: subnet selection, address pool selection, and DHCP options assignment.

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, where 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, which 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 .

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 on access based on class information is also available at the address/prefix pool level; see , within a subnet. This is useful when segregating 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 reservation, for instance when there are not enough available addresses. The determination whether there is a reservation for a given client is made after a subnet is selected, so it is not possible to use KNOWN/UNKNOWN classes to select a shared network or a subnet.

The process of classification is conducted in five steps. The first step is to assess an incoming packet and assign it to zero or more classes. Next, a subnet is chosen, possibly based on the class information. After that, class expressions are evaluated depending on the built-in "KNOWN"/"UNKNOWN" classes after host reservation lookup, using them for pool/pd-pool selection and assigning classes from host reservations. The list of required classes is then 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. Finally, options are assigned, again possibly based on the class information. More complete and detailed information is available in .

There are two main methods of classification. The first is automatic and relies on examining the values in the vendor class options or the 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 specifies an expression that is evaluated for each packet. If the result is "true", the packet is a member of the class.

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

#### Defining and Using Custom Classes

The following example shows how to configure a class using an expression and a subnet using that class. This configuration defines the class named "Client_enterprise". It is comprised of all clients whose client identifiers start with the given hex string (which would indicate a DUID based on an enterprise id of 0xAABBCCDD). They will be given an address from 2001:db8:1::0 to 2001:db8:1::FFFF and the addresses of their DNS servers set to 2001:db8:0::1 and 2001:db8:2::1.

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

This example shows a configuration using an automatically generated "VENDOR_CLASS_" class. The administrator of the network has decided that addresses in the range 2001:db8:1::1 to 2001:db8:1::ffff are to be managed by the DHCP6 server and that only clients belonging to the eRouter1.0 client class are allowed to use that pool.

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

#### Required Classification

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

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

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

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

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

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

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

### DDNS for DHCPv6

As mentioned earlier, kea-dhcp6 can be configured to generate requests to the DHCP-DDNS server (referred to here as "D2") to update DNS entries. These requests are known as Name Change Requests or NCRs. Each NCR contains the following information:

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

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

3. The Fully Qualified Domain Name (FQDN), lease address, and DHCID (information identifying the client associated with the FQDN)

The parameters controlling the generation of NCRs for submission to D2 are contained in the `dhcp-ddns` section of the kea-dhcp6 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:

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

and for example:

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

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

- `"server-ip": "127.0.0.1"`

- `"server-port": 53001`

- `"sender-ip": ""`

- `"sender-port": 0`

- `"max-queue-size": 1024`

- `"ncr-protocol": "UDP"`

- `"ncr-format": "JSON"`

- `"override-no-update": false`

- `"override-client-update": false`

- `"replace-client-name": "never"`

- `"generated-prefix": "myhost"`

- `"hostname-char-set": ""`

- `"hostname-char-replacement": ""`

#### DHCP-DDNS Server Connectivity

For NCRs to reach the D2 server, kea-dhcp6 must be able to communicate with it. kea-dhcp6 uses the following configuration parameters to control this communication:

- `enable-updates` - determines whether kea-dhcp6 will generate NCRs. If missing, this value is assumed to be false, so DDNS updates are disabled. To enable DDNS updates set this value to true.

- `server-ip` - IP address on which D2 listens for requests. The default is the local loopback interface at address 127.0.0.1. You may specify either an IPv4 or IPv6 address.

- `server-port` - port on which D2 listens for requests. The default value is 53001.

- `sender-ip` - IP address which kea-dhcp6 uses to send requests to D2. The default value is blank, which instructs kea-dhcp6 to select a suitable address.

- `sender-port` - port which kea-dhcp6 uses to send requests to D2. The default value of 0 instructs kea-dhcp6 to select a suitable port.

- `max-queue-size` - maximum number of requests allowed to queue waiting to be sent to D2. This value guards against requests accumulating uncontrollably if they are being generated faster than they can be delivered. If the number of requests queued for transmission reaches this value, DDNS updating will be turned off until the queue backlog has been sufficiently reduced. The intent is to allow kea-dhcp6 to continue lease operations without running the risk that its memory usage grows without limit. The default value is 1024.

- `ncr-protocol` - socket protocol to use when sending requests to D2. Currently only UDP is supported.

- `ncr-format` - packet format to use when sending requests to D2. Currently only JSON format is supported.

By default, kea-dhcp-ddns is assumed to be running on the same machine as kea-dhcp6, and all of the default values mentioned above should be sufficient. If, however, D2 has been configured to listen on a different address or port, these values must altered accordingly. For example, if D2 has been configured to listen on 2001:db8::5 port 900, the following configuration is required:

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

#### When Does the kea-dhcp6 Server Generate a DDNS Request?

kea-dhcp6 follows the behavior prescribed for DHCP servers in RFC 4704. It is important to keep in mind that kea-dhcp6 makes the initial decision of when and what to update and forwards that information to D2 in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are within the purview of D2 itself (). This section describes when kea-dhcp6 will generate NCRs and the configuration parameters that can be used to influence this decision. It assumes that the `enable-updates` parameter is true.

Currently the interface between kea-dhcp6 and D2 only supports requests which update DNS entries for a single IP address. If a lease grants more than one address, kea-dhcp6 will create the DDNS update request for only the first of these addresses.

In general, kea-dhcp6 will generate DDNS update requests when:

1. A new lease is granted in response to a DHCPREQUEST

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

3. An existing lease is released in response to a DHCPRELEASE

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

The decisions involved when granting a new lease the first case) are more complex. When a new lease is granted, kea-dhcp6 will generate a DDNS update request only if the DHCPREQUEST contains the FQDN option (code 39). By default, kea-dhcp6 will respect the FQDN N and S flags specified by the client as shown in the following table:

##### Default FQDN Flag Behavior

Client Flags:N-S | Client Intent | Server Response | Server Flags:N-S-O
--- | --- | --- | ---
0-0 | Client wants to do forward updates, server should do reverse updates | Server generates reverse-only request | 1-0-0
0-1 | Server should do both forward and reverse updates | Server generates request to update both directions | 0-1-0
1-0 | Client wants no updates done | Server does not generate a request | 1-0-0

The first row in the table above represents "client delegation". Here the DHCP client states that it intends to do the forward DNS updates and the server should do the reverse updates. By default, kea-dhcp6 will honor the client's wishes and generate a DDNS request to D2 to update only reverse DNS data. The parameter `override-client-update` can be used to instruct the server to override client delegation requests. When this parameter is true, kea-dhcp6 will disregard requests for client delegation and generate a DDNS request to update both forward and reverse DNS data. In this case, the N-S-O flags in the server's response to the client will be 0-1-1 respectively.

(Note that the flag combination N=1, S=1 is prohibited according to RFC 4702. If such a combination is received from the client, the packet will be dropped by kea-dhcp6.)

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

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

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter, `override-no-update`, can be used to instruct the server to disregard the client's wishes. When this parameter is true, kea-dhcp6 will generate DDNS update requests to kea-dhcp-ddns even if the client requests no updates be done. The N-S-O flags in the server's response to the client will be 0-1-1.

To override client delegation, issue the following commands:

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

#### kea-dhcp6 Name Generation for DDNS Update Requests

Each Name Change Request must of course include the fully qualified domain name whose DNS entries are to be affected. kea-dhcp6 can be configured to supply a portion or all of that name, based upon what it receives from the client in the DHCPREQUEST.

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

1. If the DHCPREQUEST contains the client FQDN option, take the candidate name from there.

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

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

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

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

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

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

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

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

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

For example, to instruct kea-dhcp6 to always generate the FQDN for a client, set the parameter `replace-client-name` to `always` as follows:

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

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

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

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

    "Dhcp6": {
        "dhcp-ddns": {
            "qualifying-suffix": "foo.example.org",
            ...
        },
        ...
    }

When qualifying a partial name, kea-dhcp6 will construct the name in the format:

[candidate-name].[qualifying-suffix].

where candidate-name is the partial name supplied in the DHCPREQUEST. For example, if the FQDN domain name value is "some-computer" and the qualifying-suffix "example.com", the generated FQDN is:

some-computer.example.com.

When generating the entire name, kea-dhcp6 will construct the name in the format:

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

where address-text is simply the lease IP address converted to a hyphenated string. For example, if the lease address is 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.

#### Sanitizing Client FQDN Names

It may be that some of your DHCP clients provide values in the name component of the FQDN option (Option code 39) that contain undesirable characters. It is possible to configure kea-dhcp6 to sanitize these values. The most typical use case is ensuring that only characters that are permitted by RFC 1035 be included: A-Z,a-z,0-9, and '-'.

This may be accomplished with the following two parameters:

- `hostname-char-set` - a regular expression describing the invalid character set. This can be any valid, regular expression using POSIX extended expression syntax. For example, "[^A-Za-z0-9-]" would replace any character other than the letters A through z, digits 0 through 9, and '-'. An empty string, the default value, disables sanitization.

- `hostname-char-replacement` - a string of zero or more characters with which to replace each invalid character in the client value. The default value is an empty string and will cause invalid characters to be OMITTED rather than replaced.

The following configuration will replace anything other than a letter, digit, hyphen, or dot with the letter 'x':

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

Thus, a client supplied value of "myhost-$[123.org" would become "myhost-xx123.org". Sanitizing is performed only on the portion of the name supplied by the client, and it is performed before applying a qualifying suffix (if one is defined and needed).

The following are some considerations to keep in mind:

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

Do not include dots in your hostname-char-set expression. When scrubbing FQDNs, dots are treated as delimiters and used to separate the option value into individual domain labels that are scrubbed and then re-assembled.

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

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

### DHCPv4-over-DHCPv6: DHCPv6 Side

The support of DHCPv4-over-DHCPv6 transport is described in RFC 7341 and is implemented using cooperating DHCPv4 and DHCPv6 servers. This section is about the configuration of the DHCPv6 side (the DHCPv4 side is described in ).

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

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

Two other configuration entries are generally required: unicast traffic support (see ) 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
        } ]
    }

    }

Relayed DHCPv4-QUERY DHCPv6 messages are not supported.

### Sanity Checks in DHCPv6

An important aspect of a well-running DHCP system is an assurance that the data remains consistent. 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 has a mechanism to control sanity checks for situations such as this.

Kea supports a configuration scope called `sanity-checks`. It currently allows only a single parameter called `lease-checks`, which governs the verification carried out when a new lease is loaded from a lease file. This mechanism permits Kea to attempt 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 the configuration has changed, it is possible that a lease could exist with a subnet-id, but without any subnet that matches it. Also, it may be possible that the subnet's configuration has changed and the subnet-id now belongs to a subnet that does not match the lease. Kea's corrective algorithm first checks to see if there is a subnet with the subnet-id specified by the lease. If there is, it verifies whether the lease belongs to that subnet. If not, depending on the lease-checks setting, the lease is discarded, a warning is displayed, or a new subnet is selected for the lease that matches it topologically.

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

There are five levels which are supported:

- `none` - do no special checks; accept the lease as is

- `warn` - if problems are detected, a warning will be displayed, but the lease data will be accepted anyway. This is the default value.

- `fix` - If data inconsistency is discovered, Kea will try to correct it. If the correction is not successful, the incorrect data will be inserted anyway.

- `fix-del` - If a data inconsistency is discovered, Kea will try to correct it. If the correction is not succesful, the lease will be rejected. This setting ensures the data's correctness, but some incorrect data may be lost. Use with care.

- `del` - This is the strictest mode. If any inconsistency is detected, the lease is rejected. Use with care.

This feature is currently implemented for the memfile backend.

An example configuration that sets this parameter looks as follows:

    "Dhcp6": {
        "sanity-checks": {
            "lease-checks": "fix-del"
        },
        ...
    }

## Host Reservation in DHCPv6

There are many cases where it is useful to provide a configuration on a per-host basis. The most obvious one is to reserve a specific, static IPv6 address or/and prefix for exclusive use by a given client (host); the returning client will get the same address or/and prefix every time, and other clients will never get that address. Another example when host reservations are applicable is when a host has specific requirements, e.g. a printer that needs additional DHCP options or a cable modem that needs specific parameters. Yet another possible use case is to define unique names for hosts.

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

Host reservations are defined as parameters for each subnet. Each host must be identified by either DUID or its hardware/MAC address. See for details. There is an optional `reservations` array in the `subnet6` structure. Each element in that array is a structure that holds information about a single host. In particular, the structure has an identifier that uniquely identifies a host. In the DHCPv6 context, such an identifier is usually a DUID, but can also be a hardware or MAC address. One or more addresses or prefixes may also be specified, and it is possible to specify a hostname and DHCPv6 options for a given host.

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

    "subnet6": [
        {
            "subnet": "2001:db8:1::/48",
            "pools": [ { "pool": "2001:db8:1::/80" } ],
            "pd-pools": [
                {
                    "prefix": "2001:db8:1:8000::",
                    "prefix-len": 48,
                    "delegated-len": 64
                }
            ],
            "reservations": [
                {
                    "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                    "ip-addresses": [ "2001:db8:1::100" ]
                },
                {
                    "hw-address": "00:01:02:03:04:05",
                    "ip-addresses": [ "2001:db8:1::101", "2001:db8:1::102" ]
                },
                {
                    "duid": "01:02:03:04:05:06:07:08:09:0A",
                    "ip-addresses": [ "2001:db8:1::103" ],
                    "prefixes": [ "2001:db8:2:abcd::/64" ],
                    "hostname": "foo.example.com"
                }
            ]
        }
    ]

This example includes reservations for three different clients. The first reservation is for the address 2001:db8:1::100 for a client using DUID 01:02:03:04:05:0A:0B:0C:0D:0E. The second reservation is for two addresses 2001:db8:1::101 and 2001:db8:1::102, for a client using MAC address 00:01:02:03:04:05. Lastly, address 2001:db8:1::103 and prefix 2001:db8:2:abcd::/64 are reserved for a client using DUID 01:02:03:04:05:06:07:08:09:0A. The last reservation also assigns a hostname to this client.

Note that DHCPv6 allows a single client to lease multiple addresses and multiple prefixes at the same time. Therefore `ip-addresses` and `prefixes` are plural and are actually arrays. When the client sends multiple IA options (IA_NA or IA_PD), each reserved address or prefix is assigned to an individual IA of the appropriate type. If the number of IAs of a specific type is lower than the number of reservations of that type, the number of reserved addresses or prefixes assigned to the client is equal to the number of IA_NAs or IA_PDs sent by the client; that is, some reserved addresses or prefixes are not assigned. However, they still remain reserved for this client and the server will not assign them to any other client. If the number of IAs of a specific type sent by the client is greater than the number of reserved addresses or prefixes, the server will try to assign all reserved addresses or prefixes to the individual IAs and dynamically allocate addresses or prefixes to the remaining IAs. If the server cannot assign a reserved address or prefix because it is in use, the server will select the next reserved address or prefix and try to assign it to the client. If the server subsequently finds that there are no more reservations that can be assigned to the client at that moment, the server will try to assign leases dynamically.

Making a reservation for a mobile host that may visit multiple subnets requires a separate host definition in each subnet it is expected to visit. It is not possible to define multiple host definitions with the same hardware address in a single subnet. Multiple host definitions with the same hardware address are valid if each is in a different subnet. The reservation for a given host should include only one identifier, either DUID or hardware address. Defining both for the same host is considered a configuration error.

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

### Address/Prefix Reservation Types

In a typical scenario there is an IPv6 subnet defined, with a certain part of it dedicated for dynamic address allocation by the DHCPv6 server. There may be an additional address space defined for prefix delegation. Those dynamic parts are referred to as dynamic pools, address and prefix pools, or simply pools. In principle, a host reservation can reserve any address or prefix that belongs to the subnet. The reservations that specify addresses 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.

Kea supports 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.

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

### Conflicts in DHCPv6 Reservations

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

The server now has a conflict to resolve. If Host B boots up and requests 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 gets the address, it will detect that the address is already in use by someone else (Host A) and will send a DHCPDECLINE 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 the reservation, so it will release the current address Host B has and will create a new lease matching the reservation. As before, the server will send two addresses: the temporarily assigned one with zeroed lifetimes, and the new one that matches the reservation with proper lifetimes set.

This recovery will succeed, even if other hosts attempt to get the reserved address. If Host C requests the address 2001:db8::10 after the reservation is 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 the value set for renew-timer. The best way to avoid such recovery is not to 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 can get this address.

The conflict-resolution mechanism does not work for global reservations. As of Kea 1.5.0, it is generally recommended not to use global reservations for addresses or prefixes. If you choose to use them anyway, you must manually ensure that the reserved values are not in the dynamic pools.

### Reserving a Hostname

When the reservation for a 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 a fully qualified name. Thus, by leaving the `qualifying-suffix` empty it is possible to qualify hostnames for 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".

### Including Specific DHCPv6 Options in Reservations

Kea offers the ability to specify options on a per-host basis. These options follow the same rules as any other options. These can be standard options (see ), custom options (see ), or vendor-specific options (see ). 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.

### Reserving Client Classes in DHCPv6

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.

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

### Storing Host Reservations in MySQL, PostgreSQL, or Cassandra

It is possible to store host reservations in MySQL, PostgreSQL, or Cassandra. See for information on how to configure Kea to use reservations stored in MySQL, PostgreSQL, or Cassandra. Kea provides a dedicated hook for managing reservations in a database; section provides detailed information. The Kea wiki `https://gitlab.isc.org/isc-projects/kea/wikis/designs/commands#23-host-reservations-hr-management` provides some examples of how to conduct common host reservations operations.

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

### Fine Tuning DHCPv6 Host Reservation

The host reservation capability introduces additional restrictions for the allocation engine (the component of Kea that selects an address for a client) during lease selection and renewal. In particular, three major checks are necessary. First, when selecting a new lease, it is not sufficient for a candidate lease to simply not be in use by another DHCP client; it also must not be reserved for another client. Second, when renewing a lease, an additional check must be performed to see whether the address being renewed is reserved for another client. Finally, when a host renews an address or a prefix, the server must 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 a particular subnet. Each reservation type has different constraints for the checks to be performed by the server when allocating or renewing a lease for the client. Allowed values are:

- `all` - enables both in-pool and out-of-pool host reservation types. This setting is the default value, and is the safest and most flexible. However, as all checks are conducted, it is also the slowest. It does not check against global reservations.

- `out-of-pool` - allows only out of pool host reservations. With this setting in place, the server may assume that all host reservations are for addresses that do not belong to the dynamic pool. Therefore, it can skip the reservation checks when dealing with in-pool addresses, thus improving performance. Do not use this mode if any of your reservations use in-pool addresses. Caution is advised when using this setting; Kea does not sanity-check the reservations against `reservation-mode` and misconfiguration may cause problems.

- `global` - allows only global host reservations. With this setting in place, the server searches for reservations for a client only among the defined global reservations. If an address is specified, the server will skip the reservation checks carried out when dealing in other modes, thus improving performance. Caution is advised when using this setting; Kea does not sanity-check the reservations when `global` and misconfiguration may cause problems.

- `disabled` - host reservation support is disabled. As there are no reservations, the server will skip all checks. Any reservations defined will be completely ignored. As the checks are skipped, the server may operate faster in this mode.

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

An example configuration that disables reservation looks as 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 .

Another aspect of the host reservations is the different types of identifiers. Kea currently supports two types of identifiers in DHCPv6: hardware address and DUID. This is beneficial from a usability perspective; however, there is one 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 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 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 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" ]

### Global Reservations in DHCPv6

In some deployments, such as mobile, clients can roam within the network and certain parameters must be specified 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 a specific subnet-id. Kea can specify a 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 a 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 unusable or, at the very least, riddled with issues, such as 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.

## Shared Networks in DHCPv6

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

There are many use cases where the feature is useful. The most common example in the IPv4 case is when the server is running out of available addresses in a subnet. This is less common in IPv6, but shared networks are still useful in IPv6. One of the use cases is an exhaustion of IPv6- delegated prefixes within a subnet. Another IPv6-specific example is an experiment with an addressing scheme. With the advent of IPv6 deployment and a vast address space, many organizations split the address space into subnets, deploy it, and then after a while discover that they want to split it differently. In the transition period, they want both old and new addressing to be available. Thus the need for more than one subnet on the same physical link.

Finally, the case of cable networks is directly applicable in IPv6. There are two types of devices in cable networks: cable modems and the end-user devices behind them. It is a common practice to use different subnets for cable modems to prevent users from tinkering with them. In this case, the distinction is based on the type of device, rather than address-space exhaustion.

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

Deployments should not assume that Kea waits until it has allocated all the addresses from the first subnet in a shared network before allocating addresses from other subnets.

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

    {
    "Dhcp6": {
        "shared-networks": [
            {
                // Name of the shared network. It may be an arbitrary string
                // and it must be unique among all shared networks.
                "name": "ipv6-lab-1",

                // The subnet selector can be specifed on the shared network level.
                // Subnets from this shared network will be selected for clients
                // communicating via relay agent having the specified IP address.
                "relay": {
                    "ip-addresses": [ "2001:db8:2:34::1" ]
                },

                // This starts a list of subnets in this shared network.
                // There are two subnets in this example.
                "subnet6": [
                    {
                        "subnet": "2001:db8::/48",
                        "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                    },
                    {
                        "subnet": "3ffe:ffe::/64",
                        "pools": [ { "pool":  "3ffe:ffe::/64" } ]
                    }
                ]
            } ], // end of shared-networks

        // It is likely that in your network you'll have a mix of regular,
        // "plain" subnets and shared networks. It is perfectly valid to mix
        // them in the same config file.
        //
        // This is regular subnet. It's not part of any shared-network.
        "subnet6": [
            {
                "subnet": "2001:db9::/48",
                "pools": [ { "pool":  "2001:db9::/64" } ],
                "relay": {
                    "ip-addresses": [ "2001:db8:1:2::1" ]
                }
            }
        ]

    } // end of Dhcp6
    }

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

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

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

    "shared-networks": [
        {
            "name": "lab-network3",
            "relay": {
                 "ip-addresses": [ "2001:db8:2:34::1" ]
            },

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

            // This option is made available to all subnets in this shared
            // network.
            "option-data": [ {
                "name": "dns-servers",
                "data": "2001:db8::8888"
            } ],

            "subnet6": [
                {
                    "subnet": "2001:db8:1::/48",
                    "pools": [ { "pool":  "2001:db8:1::1 - 2001:db8:1::ffff" } ],

                    // This particular subnet uses different values.
                    "valid-lifetime": 1200,
                    "option-data": [
                    {
                        "name": "dns-servers",
                        "data": "2001:db8::1:2"
                    },
                    {
                        "name": "unicast",
                        "data": "2001:abcd::1"
                    } ]
                },
                {
                     "subnet": "2001:db8:2::/48",
                     "pools": [ { "pool":  "2001:db8:2::1 - 2001:db8:2::ffff" } ],

                     // This subnet does not specify its own valid-lifetime value,
                     // so it is inherited from shared network scope.
                     "option-data": [
                     {
                         "name": "dns-servers",
                         "data": "2001:db8:cafe::1"
                     } ]
                }
            ],
        } ]

In this example, there is a dns-servers option defined that is available to clients in both subnets in this shared network. Also, a valid lifetime is set to 10 minutes (600s). However, the first subnet overrides some of the values (valid lifetime is 20 minutes, different IP address for dns-servers), but also adds its own option (unicast address). Assuming a client asking for a server unicast and dns servers options is assigned a lease from this subnet, it will get a lease for 20 minutes and dns-servers, and be allowed to use server unicast at address 2001:abcd::1. If the same client is assigned to the second subnet, it will get a 10-minute lease, a dns-servers value of 2001:db8:cafe::1, and no server unicast.

Some parameters must be the same in all subnets in the same shared network. This restriction applies to the `interface` and `rapid-commit` settings. The most convenient way is to define them on the shared network scope, but you may specify them for each subnet. However, care should be taken for each subnet to have the same value.

### Local and Relayed Traffic in Shared Networks

It is possible to specify an interface name in the shared network scope to tell the server that this specific shared network is reachable directly (not via relays) using a 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 possible to specify the interface parameter on each subnet, although its value must be the same for each subnet. Thus it is 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, relay IP addresses can also be specified for the whole shared network. However, depending on your relay configuration, it may use different IP addresses depending on which subnet is being used. Thus there is no requirement to use the same IP relay address for each subnet. Here's an example:

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

In this particular case the relay IP address specified at the 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 the 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. Kea accepts any value here as long as it is a valid IPv6 address.

### Client Classification in Shared Networks

Sometimes it is desirable to segregate clients into specific subnets based on certain properties. This mechanism is called client classification and is described in . 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 clients when client classification is in use, to ensure that the desired subnet is selected for a given client type.

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

    {
        "client-classes": [
            {
                "name": "b-devices",
                "test": "option[1234].hex == 0x0002"
            }
        ],
        "shared-networks": [
            {
                "name": "galah",
                "relay": {
                    "ip-address": [ "2001:db8:2:34::1" ]
                },
                "subnet6": [
                    {
                        "subnet": "2001:db8:1::/64",
                        "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ],
                    },
                    {
                        "subnet": "2001:db8:3::/64",
                        "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                        "client-class": "b-devices"
                    }
                ]
            }
        ]
    }

If the client belongs to the "b-devices" class (because it includes option 1234 with a value of 0x0002), that doesn't guarantee that the subnet 2001:db8:3::/64 will be used (or preferred) for this client. The server can use either of the two subnets because the subnet 2001:db8:1::/64 is also allowed for this client. The client classification used in this case should be perceived as a way to restrict access to certain subnets, rather than a way to express subnet preference. For example, if the client doesn't belong to the "b-devices" class it may only use the subnet 2001:db8:1::/64 and will never use the subnet 2001:db8:3::/64.

A typical use case for client classification is in a 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 necessary to apply classification on all subnets. The following example defines two classes of devices, and the subnet selection is made based on option 1234 values.

    {
        "client-classes": [
            {

                "name": "a-devices",
                "test": "option[1234].hex == 0x0001"
            },
            {
                "name": "b-devices",
                "test": "option[1234].hex == 0x0002"
            }
        ],
        "shared-networks": [
            {
                "name": "galah",
                "relay": {
                    "ip-addresses":  [ "2001:db8:2:34::1" ]
                },
                "subnet6": [
                    {
                        "subnet": "2001:db8:1::/64",
                        "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ],
                        "client-class": "a-devices"
                    },
                    {
                        "subnet": "2001:db8:3::/64",
                        "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                        "client-class": "b-devices"
                    }
                ]
            }
        ]
    }

In this example each class has its own restriction. Only clients that belong to class "a-devices" will be able to use subnet 2001:db8:1::/64 and only clients belonging to "b-devices" will be able to use subnet 2001:db8:3::/64. Care should be taken not to define too-restrictive classification rules, as clients that are unable to use any subnets will be refused service. However, this may be a desired outcome if one wishes to provide service only to clients with known properties (e.g. only VoIP phones allowed on a given link).

Note that it is possible to achieve an effect similar to the one 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. the "interface" or "relay" parameter. All subnets belonging to this shared network will inherit those parameters.

### Host Reservations in Shared Networks

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

    {
        "shared-networks": [
        {
            "name": "frog",
            "relay": {
                "ip-addresses": [ "2001:db8:2:34::1" ]
            },
            "subnet6": [
                {
                    "subnet": "2001:db8:1::/64",
                    "id": 100,
                    "pools": [ { "2001:db8:1::1 - 2001:db8:1::64" } ],
                    "reservations": [
                    {
                        "duid": "00:03:00:01:11:22:33:44:55:66",
                        "ip-addresses": [ "2001:db8:1::28" ]
                    }
                    ]
                },
                {
                    "subnet": "2001:db8:3::/64",
                    "id": 101,
                    "pools": [ { "pool": "2001:db8:3::1 - 2001:db8:3::64" } ],
                    "reservations": [
                        {
                            "duid": "00:03:00:01:aa:bb:cc:dd:ee:ff",
                            "ip-addresses": [ "2001:db8:2::28" ]
                        }
                    ]
                }
            ]
        }
        ]
    }

It is worth noting that Kea conducts additional checks when processing a packet if shared networks are defined. Instead of simply checking whether there's a reservation for a given client in its initially selected subnet, it looks through all subnets in a shared network 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 are autogenerated, i.e. it assigns increasing integer values starting from 1. Thus, the autogenerated IDs are not stable across configuration changes.

## Server Identifier in DHCPv6

The DHCPv6 protocol uses a "server identifier" (also known as a DUID) to allow clients to discriminate between several servers present on the same link. RFC 8415 currently defines four DUID types: DUID-LLT, DUID-EN, DUID-LL, and DUID-UUID.

The Kea DHCPv6 server generates a server identifier once, upon the first startup, and stores it in a file. This identifier is not modified across restarts of the server and so is a stable identifier.

Kea follows the recommendation from RFC 8415 to use DUID-LLT as the default server identifier. However, we have received reports that some deployments require different DUID types, and there is a need to administratively select both DUID type and/or its contents.

The server identifier can be configured using parameters within the `server-id` map element in the global scope of the Kea configuration file. The following example demonstrates how to select DUID-EN as a server identifier:

    "Dhcp6": {
        "server-id": {
            "type": "EN"
        },
        ...
    }

Currently supported values for `type` parameter are: "LLT", "EN", and "LL", for DUID-LLT, DUID-EN, and DUID-LL respectively.

When a new DUID type is selected the server generates its value and replaces any existing DUID in the file. The server then uses the new server identifier in all future interactions with the clients.

If the new server identifier is created after some clients have obtained their leases, the clients using the old identifier are not able to renew the leases; the server will ignore messages containing the old server identifier. Clients will continue sending Renew until they transition to the rebinding state. In this state, they will start sending Rebind messages to the multicast address without a server identifier. The server will respond to the Rebind messages with a new server identifier, and the clients will associate the new server identifier with their leases. Although the clients will be able to keep their leases and will eventually learn the new server identifier, this will be at the cost of an increased number of renewals and multicast traffic due to a need to rebind. Therefore, it is recommended that modification of the server identifier type and value is avoided if the server has already assigned leases and these leases are still valid.

There are cases when an administrator needs to explicitly specify a DUID value rather than allow the server to generate it. The following example demonstrates how to explicitly set all components of a DUID-LLT.

    "Dhcp6": {
        "server-id": {
            "type": "LLT",
            "htype": 8,
            "identifier": "A65DC7410F05",
            "time": 2518920166
        },
        ...
    }

where:

- `htype` is a 16-bit unsigned value specifying hardware type,

- `identifier` is a link-layer address, specified as a string of hexadecimal digits, and

- `time` is a 32-bit unsigned time value.

The hexadecimal representation of the DUID generated as a result of the configuration specified above will be:

     00:01:00:08:96:23:AB:E6:A6:5D:C7:41:0F:05
    |type |htype|   time    |   identifier    |

A special value of 0 for "htype" and "time" is allowed, which indicates that the server should use ANY value for these components. If the server already uses a DUID-LLT it will use the values from this DUID; if the server uses a DUID of a different type or doesn't yet use any DUID, it will generate these values. Similarly, if the "identifier" is assigned an empty string, the value of the identifier will be generated. Omitting any of these parameters is equivalent to setting them to those special values.

For example, the following configuration:

    "Dhcp6": {
        "server-id": {
            "type": "LLT",
            "htype": 0,
            "identifier": "",
            "time": 2518920166
        },
        ...
    }

indicates that the server should use ANY link-layer address and hardware type. If the server is already using DUID-LLT, it will use the link-layer address and hardware type from the existing DUID. If the server is not yet using any DUID, it will use the link-layer address and hardware type from one of the available network interfaces. The server will use an explicit value of time; if it is different than a time value present in the currently used DUID, that value will be replaced, effectively modifying the current server identifier.

The following example demonstrates an explicit configuration of a DUID-EN:

    "Dhcp6": {
        "server-id": {
            "type": "EN",
            "enterprise-id": 2495,
            "identifier": "87ABEF7A5BB545"
        },
        ...
    }

where:

- `enterprise-id` is a 32-bit unsigned value holding an enterprise number, and

- `identifier` is a variable- length identifier within DUID-EN.

The hexadecimal representation of the DUID-EN created according to the configuration above is:

     00:02:00:00:09:BF:87:AB:EF:7A:5B:B5:45
    |type |  ent-id   |     identifier     |

As in the case of the DUID-LLT, special values can be used for the configuration of the DUID-EN. If the `enterprise-id` is 0, the server will use a value from the existing DUID-EN. If the server is not using any DUID or the existing DUID has a different type, the ISC enterprise id will be used. When an empty string is entered for `identifier`, the identifier from the existing DUID-EN will be used. If the server is not using any DUID-EN, a new 6-byte-long identifier will be generated.

DUID-LL is configured in the same way as DUID-LLT except that the `time` parameter has no effect for DUID-LL, because this DUID type only comprises a hardware type and link-layer address. The following example demonstrates how to configure DUID-LL:

    "Dhcp6": {
        "server-id": {
            "type": "LL",
            "htype": 8,
            "identifier": "A65DC7410F05"
        },
        ...
    }

which will result in the following server identifier:

     00:03:00:08:A6:5D:C7:41:0F:05
    |type |htype|   identifier    |

The server stores the generated server identifier in the following location: [kea-install-dir]/var/kea/kea-dhcp6-serverid.

In some uncommon deployments where no stable storage is available, the server should be configured not to try to store the server identifier. This choice is controlled by the value of the `persist` boolean parameter:

    "Dhcp6": {
        "server-id": {
            "type": "EN",
            "enterprise-id": 2495,
            "identifier": "87ABEF7A5BB545",
            "persist": false
        },
        ...
    }

The default value of the "persist" parameter is `true`, which configures the server to store the server identifier on a disk.

In the example above, the server is configured not to store the generated server identifier on a disk. But if the server identifier is not modified in the configuration, the same value will be used after server restart, because the entire server identifier is explicitly specified in the configuration.

## Stateless DHCPv6 (Information-Request Message)

Typically DHCPv6 is used to assign both addresses and options. These assignments (leases) have a state that changes over time, hence their name, stateful. DHCPv6 also supports a stateless mode, where clients request configuration options only. This mode is considered lightweight from the server perspective, as it does not require any state tracking, and carries the stateless name.

The Kea server supports stateless mode. Clients can send Information-Request messages and the server sends back answers with the requested options (providing the options are available in the server configuration). The server attempts to use per-subnet options first. If that fails - for whatever reason - it then tries to provide options defined in the global scope.

Stateless and stateful mode can be used together. No special configuration directives are required to handle this; simply use the configuration for stateful clients and the stateless clients will get just the options they requested.

This usage of global options allows for an interesting case. It is possible to run a server that provides only options and no addresses or prefixes. If the options have the same value in each subnet, the configuration can define required options in the global scope and skip subnet definitions altogether. Here's a simple example of such a configuration:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "ethX" ]
        },
        "option-data": [ {
            "name": "dns-servers",
            "data": "2001:db8::1, 2001:db8::2"
        } ],
        "lease-database": {
            "type": "memfile"
        }
     }

This very simple configuration will provide DNS server information to all clients in the network, regardless of their location. Note the specification of the memfile lease database; this is needed as Kea requires a lease database to be specified even if it is not used.

## Support for RFC 7550 (now part of RFC 8415)

RFC 7550 introduced some changes to the previous DHCPv6 specifications, RFC 3315 and RFC 3633, to resolve a few issues with the coexistence of multiple stateful options in the messages sent between the clients and servers. Those changes were later included in the most recent DHCPv6 protocol specification, RFC 8415, which obsoleted RFC 7550. Kea supports RFC 8415 along with these protocol changes, which are briefly described below.

When a client, such as a requesting router, requests an allocation of both addresses and prefixes during the 4-way (SARR) exchange with the server, and the server is not configured to allocate any prefixes but it can allocate some addresses, it will respond with the IA_NA(s) containing allocated addresses and the IA_PD(s) containing the NoPrefixAvail status code. According to the updated specifications, if the client can operate without prefixes it should accept allocated addresses and transition to the "bound" state. When the client subsequently sends Renew/Rebind messages to the server, according to the T1 and T2 times, to extend the lifetimes of the allocated addresses, and if the client is still interested in obtaining prefixes from the server, it may also include an IA_PD in the Renew/Rebind to request allocation of the prefixes. If the server still cannot allocate the prefixes, it will respond with the IA_PD(s) containing the NoPrefixAvail status code. However, if the server can allocate the prefixes it will allocate and send them in the IA_PD(s) to the client. A similar situation occurs when the server is unable to allocate addresses for the client but can delegate prefixes. The client may request allocation of the addresses while renewing the delegated prefixes. Allocating leases for other IA types while renewing existing leases is by default supported by the Kea DHCPv6 server, and the server provides no configuration mechanisms to disable this behavior.

The following are the other behaviors first introduced in RFC 7550 (now part of RFC 8415) and supported by the Kea DHCPv6 server:

- Set T1/T2 timers to the same value for all stateful (IA_NA and IA_PD) options to facilitate renewal of all a client's leases at the same time (in a single message exchange),

- Place NoAddrsAvail and NoPrefixAvail status codes in the IA_NA and IA_PD options in the Advertise message, rather than as the top-level options.

## Using a Specific Relay Agent for a Subnet

The relay must have an interface connected to the link on which the clients are being configured. Typically the relay has a global IPv6 address configured on the interface that belongs to the subnet from which the server will assign addresses. Normally, the server is able to use the IPv6 address inserted by the relay (in the link-addr field in RELAY-FORW message) to select the appropriate subnet.

However, that is not always the case. The relay address may not match the subnet in certain deployments. This usually means that there is more than one subnet allocated for a given link. The two most common examples where this is the case are long-lasting network renumbering (where both old and new address space is still being used) and a cable network. In a cable network, both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such a case, the DHCPv6 server needs additional information (like the value of the interface-id option or the IPv6 address inserted in the link-addr field in the RELAY-FORW message) to properly select an appropriate subnet.

The following example assumes that there is a subnet 2001:db8:1::/64 that is accessible via a relay that uses 3000::1 as its IPv6 address. The server is able to select this subnet for any incoming packets that come from a relay with an address in 2001:db8:1::/64 subnet. It also selects that subnet for a relay with address 3000::1.

    "Dhcp6": {
        "subnet6": [
            {
                "subnet": "2001:db8:1::/64",
                "pools": [
                     {
                         "pool": "2001:db8:1::1-2001:db8:1::ffff"
                     }
                 ],
                 "relay": {
                     "ip-addresses": [ "3000::1" ]
                 }
            }
        ]
    }

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

The current version of Kea uses the "ip-addresses" parameter, which supports specifying a list of addresses.

## Segregating IPv6 Clients in a Cable Network

In certain cases, it is useful to mix relay address information, introduced in , with client classification, explained in . One specific example is in a 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 3000::/64 subnet, while everything connected behind the modems should get addresses from another subnet (2001:db8:1::/64). The CMTS that acts as a relay uses address 3000::1. The following configuration can serve that configuration:

    "Dhcp6": {
        "subnet6": [
            {
                "subnet": "3000::/64",
                "pools": [
                    { "pool": "3000::2 - 3000::ffff" }
                ],
                "client-class": "VENDOR_CLASS_docsis3.0",
                "relay": {
                    "ip-addresses": [ "3000::1" ]
                }
            },

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

## MAC/Hardware Addresses in DHCPv6

MAC/hardware addresses are available in DHCPv4 messages from the clients, and administrators frequently use that information to perform certain tasks like per-host configuration and address reservation for specific MAC addresses. Unfortunately, the DHCPv6 protocol does not provide any completely reliable way to retrieve that information. To mitigate that issue, a number of mechanisms have been implemented in Kea. Each of these mechanisms works in certain cases, but may fail in others. Whether the mechanism works in a particular deployment is somewhat dependent on the network topology and the technologies used.

Kea allows specification of which of the supported methods should be used and in what order. This configuration may be considered a fine tuning of the DHCP deployment. In a typical deployment the default value of `"any"` is sufficient and there is no need to select specific methods. Changing the value of this parameter is the most useful in cases when an administrator wants to disable certain methods; for example, if the administrator trusts the network infrastructure more than the information provided by the clients themselves, they may prefer information provided by the relays over that provided by the clients.

The configuration is controlled by the `mac-sources` parameter as follows:

    "Dhcp6": {
        "mac-sources": [ "method1", "method2", "method3", ... ],

        "subnet6": [ ... ],

        ...
    }

When not specified, a special value of "any" is used, which instructs the server to attempt to try all the methods in sequence and use the value returned by the first one that succeeds. If specified, it must have at least one value.

Supported methods are:

- `any` - Not an actual method, just a keyword that instructs Kea to try all other methods and use the first one that succeeds. This is the default operation if no `mac-sources` are defined.

- `raw` - In principle, a DHCPv6 server could use raw sockets to receive incoming traffic and extract MAC/hardware address information. This is currently not implemented for DHCPv6 and this value has no effect.

- `duid` - DHCPv6 uses DUID identifiers instead of MAC addresses. There are currently four DUID types defined, and two of them (DUID-LLT, which is the default, and DUID-LL) convey MAC address information. Although RFC 8415 forbids it, it is possible to parse those DUIDs and extract necessary information from them. This method is not completely reliable, as clients may use other DUID types, namely DUID-EN or DUID-UUID.

- `ipv6-link-local` - Another possible acquisition method comes from the source IPv6 address. In typical usage, clients are sending their packets from IPv6 link-local addresses. There is a good chance that those addresses are based on EUI-64, which contains a MAC address. This method is not completely reliable, as clients may use other link-local address types. In particular, privacy extensions, defined in RFC 4941, do not use MAC addresses. Also note that successful extraction requires that the address's u-bit must be set to 1 and its g-bit set to 0, indicating that it is an interface identifier as per RFC 2373, section 2.5.1.

- `client-link-addr-option` - One extension defined to alleviate missing MAC issues is the client link-layer address option, defined in RFC 6939. This is an option that is inserted by a relay and contains information about a client's MAC address. This method requires a relay agent that supports the option and is configured to insert it. This method is useless for directly connected clients. This parameter can also be specified as `rfc6939`, which is an alias for `client-link-addr-option`.

- `remote-id` - RFC 4649 defines a remote-id option that is inserted by a relay agent. Depending on the relay agent configuration, the inserted option may convey the client's MAC address information. This parameter can also be specified as `rfc4649`, which is an alias for `remote-id`.

- `subscriber-id` - Another option that is somewhat similar to the previous one is subscriber-id, defined in RFC 4580. It, too, is inserted by a relay agent that is configured to insert it. This parameter can also be specified as `rfc4580`, which is an alias for `subscriber-id`. This method is currently not implemented.

- `docsis-cmts` - Yet another possible source of MAC address information are the DOCSIS options inserted by a CMTS that acts as a DHCPv6 relay agent in cable networks. This method attempts to extract MAC address information from suboption 1026 (cm mac) of the vendor-specific option with vendor-id=4491. This vendor option is extracted from the relay-forward message, not the original client's message.

- `docsis-modem` - The final possible source of MAC address information are the DOCSIS options inserted by the cable modem itself. This method attempts to extract MAC address information from suboption 36 (device id) of the vendor-specific option with vendor-id=4491. This vendor option is extracted from the original client's message, not from any relay options.

Empty mac-sources is not allowed. If you do not want to specify it, either simply omit the mac-sources definition or specify it with the "any" value which is the default.

## Duplicate Addresses (DECLINE Support)

The DHCPv6 server is configured with a certain pool of addresses that it is expected to hand out to DHCPv6 clients. It is assumed that the server is authoritative and has complete jurisdiction over those addresses. However, for various reasons, such as misconfiguration or a faulty client implementation that retains its address beyond the valid lifetime, there may be devices connected that use those addresses without the server's approval or knowledge.

Such an unwelcome event can be detected by legitimate clients (using Duplicate Address Detection) and reported to the DHCPv6 server using a DHCPDECLINE message. The server will do a sanity check (to see whether the client declining an address really was supposed to use it), and then will conduct a clean-up operation and confirm it by sending back a REPLY message. Any DNS entries related to that address will be removed, the fact will be logged, and hooks will be triggered. After that is complete, the address will be marked as declined (which indicates that it is used by an unknown entity and thus not available for assignment) and a probation time will be set on it. Unless otherwise configured, the probation period lasts 24 hours; after that period, the server will recover the lease (i.e. put it back into the available state) and the address will be available for assignment again. It should be noted that if the underlying issue of a misconfigured device is not resolved, the duplicate-address scenario will repeat. If reconfigured correctly, this mechanism provides an opportunity to recover from such an event automatically, without any system administrator intervention.

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

      "Dhcp6": {
        "decline-probation-period": 3600,
        "subnet6": [ ... ],
        ...
    }

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

There are several statistics and hook points associated with the Decline handling procedure. The lease6_decline hook is triggered after the incoming DHCPDECLINE message has been sanitized and the server is about to decline the lease. The declined-addresses statistic is increased after the hook returns (both global and subnet-specific variants). (See and for more details on DHCPv6 statistics and Kea hook points.)

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

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

## Statistics in the DHCPv6 Server

This section describes DHCPv6-specific statistics. For a general overview and usage of statistics, see .

The DHCPv6 server supports the following statistics:

### DHCPv6 Statistics

Statistic | Data Type | Description
--- | --- | ---
pkt6-received | integer | Number of DHCPv6 packets received. This includes all packets: valid, bogus, corrupted, rejected, etc. This statistic is expected to grow rapidly.
pkt6-receive-drop | integer | Number of incoming packets that were dropped. The exact reason for dropping packets is logged, but the most common reasons may be: an unacceptable or not supported packet type is received, direct responses are forbidden, the server-id sent by the client does not match the server's server-id, or the packet is malformed.
pkt6-parse-failed | integer | Number of incoming packets that could not be parsed. A non-zero value of this statistic indicates that the server received a malformed or truncated packet. This may indicate problems in your network, faulty clients, faulty relay agents, or a bug in the server.
pkt6-solicit-received | integer | Number of SOLICIT packets received. This statistic is expected to grow; its increase means that clients that just booted started their configuration process and their initial packets reached your Kea server.
pkt6-advertise-received | integer | Number of ADVERTISE packets received. Advertise packets are sent by the server and the server is never expected to receive them. A non-zero value of this statistic indicates an error occurring in the network. One likely cause would be a misbehaving relay agent that incorrectly forwards ADVERTISE messages towards the server, rather than back to the clients.
pkt6-request-received | integer | Number of DHCPREQUEST packets received. This statistic is expected to grow. Its increase means that clients that just booted received the server's response (DHCPADVERTISE) and accepted it, and are now requesting an address (DHCPREQUEST).
pkt6-reply-received | integer | Number of REPLY packets received. This statistic is expected to remain zero at all times, as REPLY packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards REPLY messages towards the server, rather than back to the clients.
pkt6-renew-received | integer | Number of RENEW packets received. This statistic is expected to grow; its increase means that clients received their addresses and prefixes and are trying to renew them.
pkt6-rebind-received | integer | Number of REBIND packets received. A non-zero value indicates that clients didn't receive responses to their RENEW messages (through the regular lease-renewal mechanism) and are attempting to find any server that is able to take over their leases. It may mean that some servers' REPLY messages never reached the clients.
pkt6-release-received | integer | Number of RELEASE packets received. This statistic is expected to grow when a device is being shut down in the network; it indicates that the address or prefix assigned is reported as no longer needed. Note that many devices, especially wireless, do not send RELEASE packets either because of design choice or due to the client moving out of range.
pkt6-decline-received | integer | Number of DECLINE packets received. This statistic is expected to remain close to zero. Its increase means that a client leased an address, but discovered that the address is currently used by an unknown device in your network. If this statistic is growing, it may indicate a misconfigured server or devices that have statically assigned conflicting addresses.
pkt6-infrequest-received | integer | Number of INFORMATION-REQUEST packets received. This statistic is expected to grow if there are devices that are using stateless DHCPv6. INFORMATION-REQUEST messages are used by clients that request stateless configuration, i.e. options and parameters other than addresses or prefixes.
pkt6-dhcpv4-query-received | integer | Number of DHCPv4-QUERY packets received. This statistic is expected to grow if there are devices that are using DHCPv4-over-DHCPv6. DHCPv4-QUERY messages are used by DHCPv4 clients on an IPv6-only line which encapsulates the requests over DHCPv6.

pkt6-dhcpv4-response-received

integer

Number of DHCPv4-RESPONSE packets received. This statistic is expected to remain zero at all times, as DHCPv4-RESPONSE packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPv4-RESPONSE message towards the server rather than back to the clients.

pkt6-unknown-received | integer | Number of packets received of an unknown type. A non-zero value of this statistic indicates that the server received a packet that it wasn't able to recognize; either it had an unsupported type or was possibly malformed.
pkt6-sent | integer | Number of DHCPv6 packets sent. This statistic is expected to grow every time the server transmits a packet. In general, it should roughly match pkt6-received, as most incoming packets cause the server to respond. There are exceptions (e.g. server receiving a REQUEST with server-id matching other server), so do not worry if it is less than pkt6-received.
pkt6-advertise-sent | integer | Number of ADVERTISE packets sent. This statistic is expected to grow in most cases after a SOLICIT is processed. There are certain uncommon, but valid, cases where incoming SOLICIT packets are dropped, but in general this statistic is expected to be close to pkt6-solicit-received.
pkt6-reply-sent | integer | Number of REPLY packets sent. This statistic is expected to grow in most cases after a SOLICIT (with rapid-commit), REQUEST, RENEW, REBIND, RELEASE, DECLINE, or INFORMATION-REQUEST is processed. There are certain cases where there is no response.
pkt6-dhcpv4-response-sent | integer | Number of DHCPv4-RESPONSE packets sent. This statistic is expected to grow in most cases after a DHCPv4-QUERY is processed. There are certain cases where there is no response.

subnet[id].total-nas

integer

Total number of NA addresses available for DHCPv6 management for a given subnet; in other words, this is the sum of all addresses in all configured pools. This statistic changes only during configuration changes. Note that it does not take into account any addresses that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.

subnet[id].assigned-nas

integer

Number of NA addresses in a given subnet that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.

subnet[id].total-pds

integer

Total number of PD prefixes available for DHCPv6 management for a given subnet; in other words, this is the sum of all prefixes in all configured pools. This statistic changes only during configuration changes. Note it does not take into account any prefixes that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.

subnet[id].assigned-pds

integer

Number of PD prefixes in a given subnet that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.

reclaimed-leases | integer | Number of expired leases that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (counting both NA and PD reclamations) and is reset when the server is reconfigured.
subnet[id].reclaimed-leases | integer | Number of expired leases associated with a given subnet ("id" is the subnet-id) that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (counting both NA and PD reclamations) and is reset when the server is reconfigured.

declined-addresses

integer

Number of IPv6 addresses that are currently declined; a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased; ideally, this statistic should be zero. If this statistic is non-zero or increasing, a network administrator should investigate whether there is a misbehaving device in the network. This is a global statistic that covers all subnets.

subnet[id].declined-addresses

integer

Number of IPv6 addresses that are currently declined in a given subnet; a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased; ideally, this statistic should be zero. If this statistic is non-zero or increasing, a network administrator should investigate whether there is a misbehaving device in the network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.

reclaimed-declined-addresses | integer | Number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long-term indicator of how many actual valid Declines were processed and recovered from. This is a global statistic that covers all subnets.
subnet[id].reclaimed-declined-addresses | integer | Number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long-term indicator of how many actual valid Declines were processed and recovered from. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.

## Management API for the DHCPv6 Server

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

    "Dhcp6": {
        "control-socket": {
            "socket-type": "unix",
            "socket-name": "/path/to/the/unix/socket"
        },

        "subnet6": [
            ...
        ],
        ...
    }

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

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

The DHCPv6 server supports the following operational commands:

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

as described in . In addition, it supports the following statistics-related commands:

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

as described in .

## User Contexts in IPv6

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

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

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

Let's consider a lightweight 4over6 deployment as an example. It is an IPv6 transition technology that allows mapping IPv6 prefixes into full or partial IPv4 addresses. In the DHCP context, these are specific parameters that are supposed to be delivered to clients in the form of additional options. Values of these options are correlated to delegated prefixes, so it is reasonable to keep these parameters together with the PD pool. On the other hand, lightweight 4over6 is not a commonly used feature, so it is not a part of the base Kea code. The solution to this problem is to use user context. For each PD pool that is expected to be used for lightweight 4over6, a user context with extra parameters is defined. Those extra parameters will be used by a hook library that would be loaded only when dynamic calculation of the lightweight 4over6 option is actually needed. An example configuration looks as follows:

    "Dhcp6": {
        "subnet6": [ {
            "pd-pools": [
            {
                "prefix":  "2001:db8::",
                "prefix-len": 56,
                "delegated-len": 64,

                // This is a pool specific context.
                "user-context": {
                    "threshold-percent": 85,
                    "v4-network": "192.168.0.0/16",
                    "v4-overflow": "10.0.0.0/16",
                    "lw4over6-sharing-ratio": 64,
                    "lw4over6-v4-pool": "192.0.2.0/24",
                    "lw4over6-sysports-exclude": true,
                    "lw4over6-bind-prefix-len": 56
                }
            } ],
            "subnet": "2001:db8::/32",

            // This is a subnet specific context. You can put any type of
            // information here as long as it is a valid JSON.
            "user-context": {
                "comment": "Those v4-v6 migration technologies are tricky.",
                "experimental": true,
                "billing-department": 42,
                "contact-points": [ "Alice", "Bob" ]
            }
        } ],
        ...
    }

Kea does not interpret or use the content of the user context; it simply stores it, making it available to the hook libraries. It is up to each hook library to extract the information and use it. The parser translates a "comment" entry into a user context with the entry, which allows a comment to be attached inside the configuration itself.

For more background information, see .

## Supported DHCPv6 Standards

The following standards are currently supported:

- Dynamic Host Configuration Protocol for IPv6, RFC 3315: Supported messages are SOLICIT, ADVERTISE, REQUEST, RELEASE, RENEW, REBIND, INFORMATION-REQUEST, CONFIRM and REPLY.

- IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6, RFC 3633: Supported options are IA_PD and IA_PREFIX. Also supported is the status code NoPrefixAvail.

- DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 3646: Supported option is DNS_SERVERS.

- The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Relay Agent Remote-ID Option, RFC 4649: REMOTE-ID option is supported.

- The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option, RFC 4704: Supported option is CLIENT_FQDN.

- Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite, RFC 6334: the AFTR-Name DHCPv6 Option is supported.

- Relay-Supplied DHCP Options, RFC 6422: Full functionality is supported: OPTION_RSOO, ability of the server to echo back the options, checks whether an option is RSOO-enabled, ability to mark additional options as RSOO-enabled.

- Prefix Exclude Option for DHCPv6-based Prefix Delegation, RFC 6603: Prefix Exclude option is supported.

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

- Issues and Recommendations with Multiple Stateful DHCPv6 Options, RFC 7550: All recommendations related to the DHCPv6 server operation are supported.

- DHCPv6 Options for Configuration of Softwire Address and Port-Mapped Clients, RFC 7598: All options indicated in this specification are supported by the DHCPv6 server.

- Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 8415: New DHCPv6 protocol specification which obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550

## DHCPv6 Server Limitations

These are the current limitations of the DHCPv6 server software. Most of them are reflections of the current stage of development and should be treated as not implemented yet, rather than actual limitations.

- The server will allocate, renew, or rebind a maximum of one lease for a particular IA option (IA_NA or IA_PD) sent by a client. RFC 8415 allows for multiple addresses or prefixes to be allocated for a single IA.

- Temporary addresses are not supported.

- Client reconfiguration (RECONFIGURE) is not yet supported.

## Kea DHCPv6 server examples

A collection of simple-to-use examples for the DHCPv6 component of Kea is available with the source files, located in the doc/examples/kea6 directory.