Kea Administrator Reference Manual

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

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

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

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

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

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

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

$ ./configure

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

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

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

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

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

$ make

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

$ make install

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

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

$ ldconfig

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

A PostgreSQL database must be set up if you want Kea to store lease and other information in PostgreSQL. This step can be safely ignored if you are using other database backends.

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

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

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

For example, a very simple configuration for both DHCPv4 and DHCPv6 could look like this:

# The whole configuration starts here.
{

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

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

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

# The whole configuration structure ends here.
}

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

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

It is sometimes convenient to refer to a specific element in the configuration hierarchy. Each hierarchy level is separated by a slash. If there is an array, a specific instance within that array is referenced by a number in square brackets (with numbering starting at zero). For example, in the above configuration the valid-lifetime in the Dhcp6 component can be referred to as Dhcp6/valid-lifetime and the pool in the first subnet defined in the DHCPv6 configuration as Dhcp6/subnet6[0]/pool.

This section explains how to configure the DHCPv4 server using the Kea configuration backend. (Kea configuration using any other backends is outside of scope of this document.) Before DHCPv4 is started, its configuration file has to be created. The basic configuration is as follows:

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

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

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

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

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

# DHCPv4 configuration ends with this line
}

} 

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

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

The configuration starts in the first line with the initial opening curly bracket (or brace). Each configuration consists of one or more objects. In this specific example, we have only one object called Dhcp4. This is a simplified configuration, as usually there will be additional objects, like Logging or DhcpDns, but we omit them now for clarity. The Dhcp4 configuration starts with the "Dhcp4": { line and ends with the corresponding closing brace (in the above example, the brace after the last comment). Everything defined between those lines is considered to be the Dhcp4 configuration.

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

Moving onto the DHCPv4 configuration elements, the very first few elements define some global parameters. valid-lifetime defines for how long the addresses (leases) given out by the server are valid. If nothing changes, a client that got an address is allowed to use it for 4000 seconds. (Note that integer numbers are specified as is, without any quotes around them.) renew-timer and rebind-timer are values that define T1 and T2 timers that govern when the client will begin the renewal and rebind procedures. Note that renew-timer and rebind-timer are optional. If they are not specified the client will select values for T1 and T2 timers according to the RFC 2131.

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

If it is desired that the server listens on multiple IPv4 addresses assigned to the same interface, multiple addresses can be specified for this interface as in the example below:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

One of the major features of the DHCPv4 server is to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requests them. The following example shows how to configure the addresses of DNS servers, which is one of the most frequently used options. Options specified in this way are considered global and apply to all configured subnets.

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

The name parameter specifies the option name. For a complete list of currently supported names, see Table 7.1, “List of standard DHCPv4 options” below. The code parameter specifies the option code, which must match one of the values from that list. The next line specifies the option space, which must always be set to "dhcp4" as these are standard DHCPv4 options. For other option spaces, including custom option spaces, see Section 7.2.9, “Nested DHCPv4 Options (Custom Option Spaces)”. The next line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. The sixth line gives the actual value to be sent to clients. Data is specified as normal text, with values separated by commas if more than one value is allowed.

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

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

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 7.2.10, “Unspecified parameters for DHCPv4 option configuration”.

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

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

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

The currently supported standard DHCPv4 options are listed in Table 7.1, “List of standard DHCPv4 options” and Table 7.2, “List of standard DHCPv4 options (continued)”. The "Name" and "Code" are the values that should be used as a name in the option-data structures. "Type" designates the format of the data: the meanings of the various types is given in Table 7.3, “List of standard DHCP option types”.

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

The Section 7.2.7, “Custom DHCPv4 options” describes the configuration syntax to create custom option definitions (formats). It is generally not allowed to create custom definitions for standard options, even if the definition being created matches the actual option format defined in the RFCs. There is an exception from this rule for standard options for which Kea does not provide a definition yet. In order to use such options, a server administrator must create a definition as described in Section 7.2.7, “Custom DHCPv4 options” in the 'dhcp4' option space. This definition should match the option format described in the relevant RFC but the configuration mechanism will allow any option format as it has no means to validate the format at the moment.

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

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

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

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

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

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

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

The type is set to "record" to indicate that the option contains multiple values of different types. These types are given as a comma-separated list in the record-types field and should be those listed in Table 7.3, “List of standard DHCP option types”.

The values of the option are set as follows:

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

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

Currently there are three option spaces defined: "dhcp4" (used by the DHCPv4 daemon) and "dhcp6" (for the DHCPv6 daemon); there is also "vendor-encapsulated-options-space", which is empty by default, but options can be defined in it. Those options are called vendor-specific information options. The following examples show how to define an option "foo" with code 1 that consists of an IPv4 address, an unsigned 16 bit integer and a string. The "foo" option is conveyed in a vendor specific information option.

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

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

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

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

We also set up a dummy value for "vendor-encapsulated-options", the option that conveys our sub-option "foo". This is required else the option will not be included in messages sent to the client.

"Dhcp4": {
    "option-data": [
        {
            "name": "vendor-encapsulated-options",
            "space": "dhcp4",
            "code": 43,
            "csv-format": false,
            "data": ""
        }
    ],
    ...
}

It is sometimes useful to define completely new option space. This is the case when user creates new option in the standard option space ("dhcp4 or "dhcp6") and wants this option to convey sub-options. Since they are in a separate space, sub-option codes will have a separate numbering scheme and may overlap with the codes of standard options.

Note that creation of a new option space when defining sub-options for a standard option is not required, because it is created by default if the standard option is meant to convey any sub-options (see Section 7.2.8, “DHCPv4 Vendor Specific Options”).

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

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

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

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

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

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

Finally, we can set values for the new options:

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

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

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

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

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

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

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

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

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

In certain cases it is useful to differentiate between different types of clients and treat them differently. The process of doing classification is conducted in two steps. The first step is to assess an incoming packet and assign it to zero or more classes. This classification is currently simple, but is expected to grow in capability soon. Currently the server checks whether an incoming packet includes the vendor class identifier option (60). If it does, the content of that option is prepended with "VENDOR_CLASS_" then it is interpreted as a class. For example, modern cable modems will send this option with value "docsis3.0" and as a result the packet will belong to class "VENDOR_CLASS_docsis3.0".

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

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

Kea can be instructed to limit access to given subnets based on class information. This is particularly useful for cases where two types of devices share the same link and are expected to be served from two different subnets. The primary use case for such a scenario is cable networks. There are two classes of devices: the cable modem itself, which should be handed a lease from subnet A and all other devices behind the modem that should get a lease from subnet B. That segregation is essential to prevent overly curious users from playing with their cable modems. For details on how to set up class restrictions on subnets, see Section 7.2.12.1, “Limiting Access to IPv4 Subnet to Certain Classes”.

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

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

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

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

and for example:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

In a typical scenario there is an IPv4 subnet defined, e.g. 192.0.2.0/24, with certain part of it dedicated for dynamic allocation by the DHCPv4 server. That dynamic part is referred to as a dynamic pool or simply a pool. In principle, the host reservation can reserve any address that belongs to the subnet. The reservations that specify addresses that belong to configured pools are called in-pool reservations. In contrast, those that do not belong to dynamic pools are called out-of-pool reservations. There is no formal difference in the reservation syntax. As of 0.9.1, both reservation types are handled uniformly. However, upcoming releases may offer improved performance if there are only out-of-pool reservations as the server will be able to skip reservation checks when dealing with existing leases. Therefore, system administrators are encouraged to use out-of-pool reservations, if possible.

As the reservations and lease information are stored separately, conflicts may arise. Consider the following series of events. The server has configured the dynamic pool of addresses from the range of 192.0.2.10 to 192.0.2.20. The Host A requests an address and gets 19.0.2.10. Now the system administrator decides to reserve the address for the Host B. He decides to reserve 192.0.2.10 for that purpose. In general, reserving an address that is currently assigned to someone else is not recommended, but there are valid use cases where such an operation is warranted.

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

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

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

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

When the reservation for the client includes the hostname , the server will assign this hostname to the client and send it back in the Client FQDN or Hostname option, depending on which of them the client has sent to the server. The reserved hostname always takes precedence over the hostname supplied by the client or the autogenerated (from the IPv4 address) hostname.

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

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

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

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

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

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

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

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

An example configuration that disables reservation looks like follows:

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

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

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

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

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

In certain cases, it is useful to mix relay address information, introduced in Section 7.5.1, “Using a Specific Relay Agent for a Subnet” with client classification, explained in Section 7.2.12.1, “Limiting Access to IPv4 Subnet to Certain Classes”. One specific example is cable network, where typically modems get addresses from a different subnet than all devices connected behind them.

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

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

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

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

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

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

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

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

} 

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

When the DHCPv6 server starts, by default it listens to the DHCP traffic sent to multicast address ff02::1:2 on each interface that it is configured to listen on (see Section 8.2.3, “Interface selection”). In some cases it is useful to configure a server to handle incoming traffic sent to the global unicast addresses as well. The most common reason for that is to have relays send their traffic to the server directly. To configure the server to listen on a specific unicast address, the notation to specify interfaces has been extended. An interface name can be optionally followed by a slash, followed by the global unicast address on which the server should listen. This will be done in addition to normal link-local binding + listening on ff02::1:2 address. The sample configuration below shows how to listen on 2001:db8::1 (a global address) configured on the eth1 interface.

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

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

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

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

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

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

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

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

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

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

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

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

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

        ...
    ]
}

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

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

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

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

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

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

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

The option-data> line creates a new entry in the option-data table. This table contains information on all global options that the server is supposed to configure in all subnets. The name line specifies the option name. (For a complete list of currently supported names, see Table 8.1, “List of standard DHCPv6 options”.) The next line specifies the option code, which must match one of the values from that list. The line beginning with space specifies the option space, which must always be set to "dhcp6" as these are standard DHCPv6 options. For other name spaces, including custom option spaces, see Section 8.2.11, “Nested DHCPv6 options (custom option spaces)”. The next line specifies the format in which the data will be entered: use of CSV (comma separated values) is recommended. The data line gives the actual value to be sent to clients. Data is specified as normal text, with values separated by commas if more than one value is allowed.

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

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

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

Most of the parameters in the "option-data" structure are optional and can be omitted in some circumstances as discussed in the Section 8.2.12, “Unspecified parameters for DHCPv6 option configuration”.

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

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

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

The currently supported standard DHCPv6 options are listed in Table 8.1, “List of standard DHCPv6 options”. The "Name" and "Code" are the values that should be used as a name in the option-data structures. "Type" designates the format of the data: the meanings of the various types is given in Table 7.3, “List of standard DHCP option types”.

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

The Section 8.2.9, “Custom DHCPv6 options” describes the configuration syntax to create custom option definitions (formats). It is generally not allowed to create custom definitions for standard options, even if the definition being created matches the actual option format defined in the RFCs. There is an exception from this rule for standard options for which Kea does not provide a definition yet. In order to use such options, a server administrator must create a definition as described in Section 8.2.9, “Custom DHCPv6 options” in the 'dhcp6' option space. This definition should match the option format described in the relevant RFC but the configuration mechanism would allow any option format as it has no means to validate the format at the moment.

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

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

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

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

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

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

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

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

The "type" is set to "record" to indicate that the option contains multiple values of different types. These types are given as a comma-separated list in the "record-types" field and should be those listed in Table 7.3, “List of standard DHCP option types”.

The values of the option are set as follows:

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

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

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"
        }
    ],
    ...
}

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

Note that it is not required to create a new option space when defining sub-options for a standard option because it is created by default if the standard option is meant to convey any sub-options (see Section 8.2.10, “DHCPv6 vendor specific options”).

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

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

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

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

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

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

Finally, we can set values for the new options:

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

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

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

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

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

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

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

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

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

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

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

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

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"
        }
    ],
    ...
}

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

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

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

As of March 2015, only option 65 is RSOO-enabled by IANA. This option will always be treated as such and there's no need to explicitly mark it. Also, when enabling standard options, it is possible to use their names, rather than option code, e.g. (e.g. use dns-servers instead of 23). See Table 8.1, “List of standard DHCPv6 options” for the names. In certain cases it could also work for custom options, but due to the nature of the parser code this may be unreliable and should be avoided.

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

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

Kea can be instructed to limit access to given subnets based on class information. This is particularly useful for cases where two types of devices share the same link and are expected to be served from two different subnets. The primary use case for such a scenario are cable networks. There are two classes of devices: the cable modem itself, which should be handed a lease from subnet A and all other devices behind modems that should get a lease from subnet B. That segregation is essential to prevent overly curious users from playing with their cable modems. For details on how to set up class restrictions on subnets, see Section 8.2.17, “Limiting access to IPv6 subnet to certain classes”.

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given class. For details on client classes, see Section 8.2.16, “Client Classification in DHCPv6”. This is an extension of a previous example from Section 8.2.6, “Subnet and Address Pool”. Let's assume that the server is connected to a network segment that uses the 2001:db8:1::/64 prefix. The Administrator of that network has decided that addresses from range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6 server. Only clients belonging to the eRouter1.0 client class are allowed to use that pool. Such a configuration can be achieved in the following way:

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

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

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

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

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

and for example:

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

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

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

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

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

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

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

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

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

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

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

some-computer.example.com.

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

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

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

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

In a typical scenario there's an IPv6 subnet defined with a certain part of it dedicated for dynamic address allocation by the DHCPv6 server. There may be an additional address space defined for prefix delegation. Those dynamic parts are referred to as dynamic pools, address and prefix pools or simply pools. In principle, the host reservation can reserve any address or prefix that belongs to the subnet. The reservations that specify an address that belongs to configured pools are called in-pool reservations. In contrast, those that do not belong to dynamic pools are called out-of-pool reservations. There is no formal difference in the reservation syntax. As of 0.9.1, both reservation types are handled uniformly. However, upcoming releases may offer improved performance if there are only out-of-pool reservations as the server will be able to skip reservation checks when dealing with existing leases. Therefore, system administrators are encouraged to use out-of-pool reservations, if possible.

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

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

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

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

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

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

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

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

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

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

will result in assigning the "mark-desktop.example.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E".

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

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

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

An example configuration that disables reservation looks like follows:

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

D2 must listen for change requests on a known address and port. By default it listens at 127.0.0.1 on port 53001. The following example illustrates how to change D2's global parameters so it will listen at 192.168.1.10 port 900:

"DhcpDdns": {
    "ip_address": "192.168.1.10",
    "port": 900,
    ...
    }
}

A DDNS protocol exchange can be conducted with or without TSIG (defined in RFC 2845). This configuration section allows the administrator to define the set of TSIG keys that may be used in such exchanges.

To use TSIG when updating entries in a DNS Domain, a key must be defined in the TSIG Key List and referenced by name in that domain's configuration entry. When D2 matches a change request to a domain, it checks whether the domain has a TSIG key associated with it. If so, D2 will use that key to sign DNS update messages sent to and verify responses received from the domain's DNS server(s). For each TSIG key required by the DNS servers that D2 will be working with there must be a corresponding TSIG key in the TSIG Key list.

As one might gather from the name, the tsig_key section of the D2 configuration lists the TSIG keys. Each entry describes a TSIG key used by one or more DNS servers to authenticate requests and sign responses. Every entry in the list has three parameters:

As an example, suppose that a domain D2 will be updating is maintained by a BIND9 DNS server which requires dynamic updates to be secured with TSIG. Suppose further that the entry for the TSIG key in BIND9's named.conf file looks like this:

   :
   key "key.four.example.com." {
       algorithm hmac-sha224;
       secret "bZEG7Ow8OgAUPfLWV3aAUQ==";
   };
   :

By default, the TSIG Key list is empty:

"DhcpDdns": {
   "tsig_keys": [ ],
   ...
}

We must extend the list with a new key:

"DhcpDdns": {
    "tsig_keys": [
        {
	    "name": "key.four.example.com.",
	    "algorithm": "HMAC-SHA224",
	    "secret": "bZEG7Ow8OgAUPfLWV3aAUQ=="
	}
    ],
    ...
}

These steps would be repeated for each TSIG key needed. Note that the same TSIG key can be used with more than one domain.

The Forward DDNS section is used to configure D2's forward update behavior. Currently it contains a single parameter, the catalog of forward DDNS Domains, which is a list of structures.

"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [ ]
    },
    ...
}

By default, this list is empty, which will cause the server to ignore the forward update portions of requests.

"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    {
		"name": "other.example.com.",
		"key_name": "",
		"dns_servers": [
		]
	    }
	]
    }
}

"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    {
		"name": "other.example.com.",
		"key_name": "",
		"dns_servers": [
		    {
			"hostname": "",
			"ip_address": "172.88.99.10",
			"port": 53
		    }
		]
	    }
	]
    }
}

The Reverse DDNS section is used to configure D2's reverse update behavior, and the concepts are the same as for the forward DDNS section. Currently it contains a single parameter, the catalog of reverse DDNS Domains, which is a list of structures.

"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [ ]
    }
    ...
}

By default, this list is empty, which will cause the server to ignore the reverse update portions of requests.

"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    {
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		]
	    }
	]
    }
}

"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    {
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		    {
			"hostname": "",
			"ip_address": "172.88.99.10",
			"port": 53
		    }
		]
	    }
	]
    }
}

This section provides an example DHCP-DDNS server configuration based on a small example network. Let's suppose our example network has three domains, each with their own subnet.

We need to construct three forward DDNS Domains:

As discussed earlier, FQDN to domain matching is based on the longest match. The FQDN, "myhost.four.example.com.", will match the first domain ("four.example.com") while "admin.example.com." will match the third domain ("example.com"). The FQDN, "other.example.net." will fail to match any domain and would be rejected.

The following example configuration specified the Forward DDNS Domains.

"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    {
		"name": "four.example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.1.5" },
		    { "ip_address": "172.16.2.5" }
		]
	    },
	    {
		"name": "six.example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "2001:db8::1" }
		]
	    },
	    {
		"name": "example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.2.5" }
		]
	    },

	]
    }
}

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

An address of "192.0.2.150" will match the first domain, "2001:db8:1::10" will match the second domain, and "192.0.50.77" the third domain.

These Reverse DDNS Domains are specified as follows:

"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    {
		"name": "2.0.192.in-addr.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.1.5" },
		    { "ip_address": "172.16.2.5" }
		]
	    }
	    {
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "2001:db8::1" }
		]
	    }
	    {
		"name": "0.192.in-addr.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.2.5" }
		]
	    }
	]
    }
}

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

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

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

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

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

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

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

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