# The DHCPv6 Server

## Starting and Stopping the DHCPv6 Server

It is recommended that the Kea DHCPv4 server be started and stopped using `keactl` (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.

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

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

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

On start-up, the server will detect available network interfaces and will attempt to open UDP sockets on all interfaces mentioned in the configuration file.

Since the DHCPv6 server opens privileged ports, it requires root access. Make sure you run this daemon as root.

## DHCPv6 Server Configuration

### Introduction

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

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

    # First we set up global values
        "interfaces": [ "eth0" ],
        "renew-timer": 1000,
        "rebind-timer": 2000,
        "preferred-lifetime": 3000,
        "valid-lifetime": 4000,

    # Next 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 and their format. Subsequent sections of this chapter go into much greater detail for these and other parameters.

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

The configuration starts in the first line with the initial opening curly bracket (or brace). Each configuration consists of one or more objects. In this specific example, we have only one object called Dhcp6. This is a simplified configuration, as usually there will be additional objects, like `Logging` or `DhcpDns`, but we omit them now for clarity. The Dhcp6 configuration starts with the `"Dhcp6": {` line and ends with the corresponding closing brace (in the above example, the brace after the last comment). Everything defined between those lines is considered to be the Dhcp6 configuration.

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

Moving onto the DHCPv6 configuration elements, the line defining `interfaces` parameter specifies a list of network interfaces on which the server should listen. Lists are opened and closed with square brackets, with elements separated by commas. Had we wanted to listen on two interfaces, the line could look like this:

    "interfaces": [ "eth0", "eth1" ],

As "`interfaces`" is not the last parameter in the configuration, a trailing comma is required.

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

The next couple 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 in-memory database and stores leases on disk in a CSV file. This is a very simple configuration. Usually, lease database configuration is more extensive and contains additional parameters. Note that `lease-database` is an object and opens up a new scope, using an opening brace. Its parameters (just one in this example -- `type`) follow. Had there been more than one, they would be separated by commas. This scope is closed with a closing brace. As more parameters follow, a trailing comma is present.

Finally, we need to define a list of IPv6 subnets. This is the most important DHCPv6 configuration structure as the server uses that information to process clients' requests. It defines all subnets that the server is expected to receive DHCP requests from. 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 is a structure and is opened and closed with braces. At minimum, a subnet definition has to have at least two parameters: `subnet` (that defines the whole subnet) and `pool` (which is a list of dynamically allocated pools that are governed by the DHCP server).

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

    "subnet6": [
        {
            "pools": [
            {
                "pool": "2001:db8:1::/112"
            }
            ],
            "subnet": "2001:db8:1::/64"
        },
        {
            "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
            "subnet": "2001:db8:2::/64",
            "interface": "eth0"
        }
    ]

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

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

Kea 0.9 does not have configuration syntax validation implemented yet. Such a feature is planned for the near future. For the time being, it is convenient to use on-line JSON validators and/or viewers to check whether the syntax is correct. One example of such a JSON validator is available at .

### Lease Storage

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

#### 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 use a CSV file rather than a database to store lease information. As well as requiring less administration, an advantage of using a file for storage is that it eliminates a dependency on third-party database software.

The configuration of the file backend (Memfile) is controlled through the Dhcp6/lease-database parameters. The following configuration:

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

...sets the name of the lease file to /tmp/kea-leases6.csv.

The "persist" parameter controls whether the leases are written to disk. It is strongly recommended that this parameter is set to true at all times during the normal operation of the server. (Not writing leases to disk will mean that if a server is restarted (e.g. after a power failure), it will not know what addresses have been assigned. As a result, it may hand out addresses to new clients that are already in use.)

#### Database Configuration

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

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

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

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

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

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

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

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

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

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

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

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

### Interface selection

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

    "Dhcp6": { "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": [ "eth1", "eth3" ], ... }

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

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

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

### IPv6 Subnet Identifier

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

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

The following configuration:

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

will assign the arbitrary subnet identifier to the newly configured subnet. 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 DHCPv6 server starts up, 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 the global unicast addresses as well. The most common reason for that is to have relays send their traffic to the server directly. To configure server to listen on specific unicast address, a notation to specify interfaces has been extended. Interface name can be optionally followed by a slash, followed by global unicast address that server should listen on. That will be done in addition to normal link-local binding + listening on ff02::1:2 address. The sample commands listed below show how to listen on 2001:db8::1 (a global address) configured on the eth1 interface.

    "Dhcp6": {
        "interfaces": [ "eth1/2001:db8::1"],
        ...
    }

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

It is possible to mix interface names, wildcards and interface name/addresses on the Dhcp6/interface list. It is not possible to specify more than one unicast address on a given interface.

Care should be taken to specify proper unicast addresses. The server will attempt to bind to those addresses specified, without any additional checks. That approach is selected on purpose, so in the software can be used to communicate over uncommon addresses if the administrator desires so.

### Subnet and Address Pool

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

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

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

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

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

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

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

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

            ...
        ]
    }

In this example, we allow 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 of 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 should use a given pool, it will be able to allocate also first (typically network address) address from that pool. For example for pool 2001:db8:2::/64 the 2001:db8:2:: address may be assigned as well. If you want to avoid this, use the "min-max" notation.

### Subnet and Prefix Delegation Pools

Subnets may also be configured to delegate prefixes, as defined in RFC 3633. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix and a prefix length, as well as a delegated prefix length. `delegated-len` must not be shorter (numerically greater) than `prefix-len`. If both `delegated-len` and `prefix-len` are equal, the server will be able to delegate only one prefix. A sample configuration is shown below:

    "Dhcp6": {
        "subnet6": [
            {
                "subnet": "2001:d8b:1::/64",
                "pd-pools": [
                    {
                        "prefix": "2001:db8:1::",
                        "prefix-len": 64,
                        "delegated-len": 96
                    }
                ]
            }
        ],
        ...
    }

### Standard DHCPv6 options

One of the major features of DHCPv6 server is to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requested them. The following example shows how to configure DNS servers, which is one of the most frequently used options. Numbers in the first column are added for easier reference and will not appear on screen. Options specified in this way are considered global and apply to all configured subnets.

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

The first line creates new entry in option-data table. It contains information on all global options that the server is supposed to configure in all subnets. The second line specifies option name. For a complete list of currently supported names, see . The third line specifies option code, which must match one of the values from that list. Line beginning with `space` specifies 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 fifth line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. The sixth line gives the actual value to be sent to clients. Data is specified as a normal text, with values separated by commas if more than one value is allowed.

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

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

The value for the setting of the "data" element is split across two lines in this document for clarity: when entering the command, the whole string should be entered on the same line. Care should be taken to use proper encoding when using hexadecimal format as Kea's ability to validate data correctness in hexadecimal is limited.

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

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

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

In future versions of Kea, it will not be necessary to specify the `code`, `space` and `csv-format` fields, as they will be set automatically.

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

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, so allowing clients to obtain the addresses of multiple DNS servers.

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

#### List of standard DHCPv6 options

Name | Code | Type | Array?
--- | --- | --- | ---
preference | 7 | uint8 | 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 | false
remote-id | 37 | record | false
subscriber-id | 38 | binary | false
client-fqdn | 39 | record | 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 | 44 | record | false
client-data | 45 | empty | false
clt-time | 46 | uint32 | false
lq-relay-data | 47 | record | false
lq-client-link | 48 | ipv6-address | true

### Custom DHCPv6 options

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

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

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

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

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

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

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

    "Dhcp6": {
        "option-def": [
            {
                "name": "bar",
                "code": 101,
                "space": "dhcp6",
                "type": "record",
                "array": false,
                "record-types": "ipv4-address, uint16, boolean, string",
                "encapsulate": ""
            }, ...
        ],
        ...
    }

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

The values of the option are set as follows:

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

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

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

### DHCPv6 vendor specific options

Currently there are three option spaces defined: dhcp4 (to be used in DHCPv4 daemon) and dhcp6 (for the DHCPv6 daemon); there is also vendor-opts-space, which is empty by default, but options can be defined in it. Those options are called vendor-specific information options. The following examples show how to define an option "foo" with code 1 that consists of an IPv6 address, an unsigned 16 bit integer and a string. The "foo" option is conveyed in a vendor specific information option. This option comprises a single uint32 value that is set to "12345". The sub-option "foo" follows the data field holding this value.

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

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

    "Dhcp6": {
        "option-data": [
            {
                "name": "foo"
                "space": "vendor-encapsulated-options-space",
                "code": 1,
                "csv-format": true,
                "data": "2001:db8:1::10, 123, Hello World"
            },
            ...
        ],
        ...
    }

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

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

### Nested DHCPv6 options (custom option spaces)

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

Note that it is not required to create new option space when defining sub-options for a standard option because it is by default created 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 isc option space:

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

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

Finally, we can set values for the new options:

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

Even though the "container" option does not carry any data except sub-options, the "data" field must be explicitly set to an empty value. This is required because in the current version of BIND 10 DHCP, the default configuration values are not propagated to the configuration parsers: if the "data" is not set the parser will assume that this parameter is not specified and an error will be reported.

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

### IPv6 Subnet Selection

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

The server can not assume which of configured subnets are local. It is possible in IPv4, where there is reasonable expectation that the server will have a (global) IPv4 address configured on the interface, and can use that information to detect whether a subnet is local or not. That assumption is not true in IPv6, as the DHCPv6 must be able to operate with having link-local addresses only. Therefore an optional "interface" parameter is available within a subnet definition to designate that a given subnet is local, i.e. reachable directly over 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"
            }
        ],
        ...
    }

### DHCPv6 Relays

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

The first uses the linkaddr field in the RELAY_FORW message. The name of this field is somewhat misleading in that it does not contain a link-layer address: instead, it holds an address (typically a global address) that is used to identify a link. The DHCPv6 server checks if the address belongs to a defined subnet and, if it does, that subnet is selected for the client's request.

The second mechanism is based on interface-id options. While forwarding a client's message, relays may insert an interface-id option into the message that identifies the interface on the relay that received the message. (Some relays allow configuration of that parameter, but it is sometimes hardcoded and may range from the very simple (e.g. "vlan100") to the very cryptic: one example seen on real hardware was "ISAM144|299|ipv6|nt:vp:1:110"). The server can use this information to select the appropriate subnet. The information is also returned to the relay which then knows the interface to use to transmit the response to the client. In order for this to work successfully, the relay interface IDs must be unique within the network and the server configuration must match those values.

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

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

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

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

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

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

### Client Classification in DHCPv6

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

In certain cases it is useful to differentiate between different types of clients and treat them differently. The process of doing classification is conducted in two steps. The first step is to assess incoming packet and assign it to zero or more classes. This classification is currently simple, but is expected to grow in capability soon. Currently the server checks whether incoming packet has vendor class option (16). If it has, content of that option is prepended with "VENDOR_CLASS_" interpreted as a class. For example, modern cable modems will send this option with value "docsis3.0" and as a result the packet will belong to class "VENDOR_CLASS_docsis3.0".

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

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

### Limiting access to IPv6 subnet to certain classes

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given subnet. For details on client classes, see . This is an extension of a previous example from .

Let's assume that the server is connected to a network segment that uses the 2001:db8:1::/64 prefix. The Administrator of that network has decided that addresses from range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6 server. Only clients belonging to the eRouter1.0 client class are allowed to use that pool. Such a configuration can be achieved in the following way:

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

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

### Configuring DHCPv6 for DDNS

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

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

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

3. The FQDN, lease address, and DHCID

The parameters controlling the generation of NCRs for submission to D2 are contained in the "dhcp-ddns" section of kea-dhcp6 configuration. The default values for this section appears as follows:

    "Dhcp6": {
        "dhcp-ddns": {
            "enable-updates": true,
            "server-ip": "127.0.0.1",
            "server-port": 53001,
            "sender-ip": "",
            "sender-port: 0,
            "max-queue-size": 1024,
            "ncr-protocol": "UDP",
            "ncr-format": "JSON",
            "override-no-update": false,
            "override-client-update": false,
            "replace-client-name": false,
            "generated-prefix": "myhost",
            "qualifying-suffix": "example.com"
        },
        ...
    }

#### DHCP-DDNS Server Connectivity

In order for NCRs to reach the D2 server, kea-dhcp6 must be able to communicate with it. kea-dhcp6 uses the following configuration parameters to control how it communications with D2:

- `enable-updates` - determines whether or not kea-dhcp6 will generate NCRs. If missing, this value is assumed to be false hence DDNS updates are disabled. To enable DDNS updates set this value to true:

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

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

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

- `sender-port` - port which kea-dhcp6 should use to send requests to D2. The default value of 0 instructs kea-dhcp6 to select 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 allow kea-dhcp6 to continue lease operations. The default value is 1024.

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

- `ncr-protocol` - Packet format to use when sending requests to D2. Currently only JSON format is supported. Other formats may be available in future releases.

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

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

#### When does kea-dhcp6 generate DDNS request

kea-dhcp6 follows the behavior prescribed for DHCP servers in RFC 4704. It is important to keep in mind that kea-dhcp6 provides the initial decision making of when and what to update and forwards that information to D2 in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are the purview of D2 ().

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. Support for multiple address mappings may be provided in a future release.

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

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

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

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

In the second case, lease renewal, two DDNS requests will be issued: one request to remove entries for the previous FQDN and a second request to add entries for the new FQDN. In the last case, a lease release, a single DDNS request to remove its entries will be made. The decision making involved when granting a new lease is more involved and is discussed next.

kea-dhcp6 will generate a DDNS update request only if the DHCP REQUEST contains the FQDN option (code 39). By default kea-dhcp6 will respect the FQDN N and S flags specified by the client as shown in the following table:

##### 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, issue the following commands:

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

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter, "override-no-update", can be used to instruct the server to disregard the client's wishes. When this parameter is true, kea-dhcp6 will generate DDNS update request to D2 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 NameChangeRequest must of course include the fully qualified domain name whose DNS entries are to be affected. kea-dhcp6 can be configured to supply a portion or all of that name based upon what it receives from the client in the DHCP REQUEST.

The rules for determining the FQDN option are as follows:

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

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

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

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

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

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

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

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

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

The suffix used when generating a FQDN or when qualifying a partial name is specified by the `qualifying-suffix` parameter. There is no default value. To set its value simply set it to the desired string:

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

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

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

where candidate-name is the partial name supplied in the REQUEST. For example, if FQDN domain name value was "some-computer" and assuming the default value for qualifying-suffix, the generated FQDN would be:

some-computer.example.com.

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

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

where address-text is simply the lease IP address converted to a hyphenated string. For example, if lease address is 3001:1::70E and assuming default values for generated-prefix and qualifying-suffix, the generated FQDN would be:

myhost-3001-1--70E.example.com.

## Server Identifier in DHCPv6

The DHCPv6 protocol uses a "server identifier" (also known as a DUID) for clients to be able to discriminate between several servers present on the same link. There are several types of DUIDs defined, but RFC 3315 instructs servers to use DUID-LLT if possible. This format consists of a link-layer (MAC) address and a timestamp. When started for the first time, the DHCPv6 server will automatically generate such a DUID and store the chosen value to a file. That file is read by the server and the contained value used whenever the server is subsequently started.

It is unlikely that this parameter should ever need to be changed. However, if such a need arises, stop the server, edit the file and restart the server. (The file is named kea-dhcp6-serverid and by default is stored in the "var" subdirectory of the directory in which BIND 10 is installed. This can be changed when BIND 10 is built by using "--localstatedir" on the "configure" command line.) The file is a text file that contains double digit hexadecimal values separated by colons. This format is similar to typical MAC address format. Spaces are ignored. No extra characters are allowed in this file.

## Using specific relay agent for a subnet

The relay has to have an interface connected to the link on which the clients are being configured. Typically the relay has a global IPv6 address configured on that interface that belongs to the subnet that the server will assign addresses from. In such typical case, the server is able to use IPv6 address inserted by the relay (in link-addr field in RELAY-FORW message) to select 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. Two most common examples where this is the case are long lasting network renumbering (where both old and new address space is still being used) and a cable network. In a cable network both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such case, the DHCPv6 server needs additional information (like the value of interface-id option or IPv6 address inserted in the link-addr field in RELAY-FORW message) to properly select an appropriate subnet.

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

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

## 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 cable network, where typically modems get addresses from a different subnet than all devices connected behind them.

Let's assume that there is one CMTS (Cable Modem Termination System) with one CM MAC (a physical link that modems are connected to). We want the modems to get addresses from the 3000::/64 subnet, while everything connected behind modems should get addresses from another subnet (2001:db8:1::/64). The CMTS that acts as a relay an uses address 3000::1. The following configuration can serve that configuration:

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

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

## Supported Standards

The following standards and draft standards are currently supported:

- RFC 3315: Supported messages are SOLICIT, ADVERTISE, REQUEST, RELEASE, RENEW, REBIND and REPLY.

- RFC 3633: Supported options are IA_PD and IA_PREFIX. Also supported is the status code NoPrefixAvail.

- RFC 3646: Supported option is DNS_SERVERS.

- RFC 4704: Supported option is CLIENT_FQDN.

## DHCPv6 Server Limitations

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

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

- The server will allocate, renew or rebind a maximum of one lease for a particular IA option (IA_NA or IA_PD) sent by a client. RFC 3315 and RFC 3633 allow for multiple addresses or prefixes to be allocated for a single IA.

- Temporary addresses are not supported.

- Duplication report (DECLINE), stateless configuration (INFORMATION-REQUEST) and client reconfiguration (RECONFIGURE) are not yet supported.

- The server doesn't act upon expired leases. In particular, when a lease expires, the server doesn't request removal of the DNS records associated with it.