# The DHCPv4 Server

## Starting and Stopping the DHCPv4 Server

It is recommended that the Kea DHCPv4 server be started and stopped using `keactl` (described in ). However, it is also possible to run the server directly: it accepts the following command-line switches:

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

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

- `-p port` - specifies UDP port the server will listen on. This is only useful during testing, as the DHCPv4 server listening on ports other than default DHCPv4 ports will not be able to handle regular DHCPv4 queries.

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

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

Since the DHCPv4 server opens privileged ports, it requires root access. Make sure you run this daemon as root.

## Configuring the DHCPv4 Server

### Introduction

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

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

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

    # Next 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",
                "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 general case, the order in which those parameters appear does not matter. There are two caveats here though. The first one is to remember that the configuration file must be a well formed JSON. That means that parameters for any given scope must be separate by a comma and there must not be a comma after the last parameter. When reordering configuration file, keep in mind that moving a parameter to or from the last position in a given scope may require moving the comma as well. The second caveat is that it is uncommon - although legal JSON - to repeat the same parameter multiple times. If that appears, 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 line defining `interfaces` parameter specifies a list of network interfaces on which the server should listen. Lists are opened and closed with square brackets, with elements separated by commas. Had we wanted to listen on two interfaces, the line could look like this:

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

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

A number of other parameters follow. `valid-lifetime` defines how long the addresses (leases) given out by the server are valid. If nothing changes, client that got the address is allowed to use it for 4000 seconds. (Note that integer numbers are specified as is, without any quotes around them.) `renew-timer` and `rebind-timer` are values that define T1 and T2 timers that govern when the client will begin renewal and rebind procedures.

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

Finally, we need to define a list of IPv4 subnets. This is the most important DHCPv4 configuration structure as the server uses that information to process clients' requests. It defines all subnets that the server is expected to receive DHCP requests from. The subnets are specified with the `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 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 `subnet4` parameter would be specified and separated by commas. For example, to define three subnets, the following syntax would be used:

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

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

### Lease Storage

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

#### Memfile - Basic Storage for Leases

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

The configuration of the file backend (Memfile) is controlled through the Dhcp4/lease-database parameters. The following configuration:

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

...sets the name of the lease file to /tmp/kea-leases4.csv.

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

#### Database Configuration

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

Database configuration is controlled through the Dhcp4/lease-database parameters. The type of the database must be set to "mysql" or "postgresql", e.g.

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

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

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

If the database is located on a different system to the DHCPv4 server, the database host name must also be specified (although it should be noted that this configuration may have a severe impact on server performance):

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

The usual state of affairs will be to have the database on the same machine as the DHCPv4 server. In this case, set the value to the empty string:

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

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

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

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

### Interface selection

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": ["*"], ... }

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

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

    "Dhcp4": { "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.

### IPv4 Subnet Identifier

The subnet identifier is a unique number associated with a particular subnet. In principle, it is used to associate clients' leases with 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 third is removed the last subnet will be assigned 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 the 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:

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

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

### Configuration of IPv4 Address Pools

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

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

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 in this example.

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": [
            "pool": [ "192.0.2.10-192.0.2.20", "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 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",
                "pool": [ "192.0.2.1 - 192.0.2.200" ],
                ...
            },
            {
                "subnet": "192.0.3.0/24",
                "pool": [ "192.0.3.100 - 192.0.3.200" ],
                ...
            },
            {
                "subnet": "192.0.4.0/24",
                "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 should use a given pool, it will be able to allocate also 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.

### Standard DHCPv4 options

One of the major features of the DHCPv4 server is to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requested 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 below. The `code` parameter specifies the option code, which must match one of the values from that list. The next line specifies option space, which must always be set to "dhcp4" as these are standard DHCPv4 options. For other option spaces, including custom option spaces, see . 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 a normal text, with values separated by commas if more than one value is allowed.

Options can also be configured as hexadecimal values. If `csv-format` is set to false, 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"
            },
            ...
        ],
        ...
    }

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

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

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

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

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

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

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

#### List of standard DHCPv4 options

Name | Code | Type | Array?
--- | --- | --- | ---
subnet-mask | 1 | ipv4-address | false
time-offset | 2 | int32 | false
routers | 3 | ipv4-address | true
time-servers | 4 | ipv4-address | true
name-servers | 5 | ipv4-address | false
domain-name-servers | 6 | ipv4-address | true
log-servers | 7 | ipv4-address | true
cookie-servers | 8 | ipv4-address | true
lpr-servers | 9 | ipv4-address | true
impress-servers | 10 | ipv4-address | true
resource-location-servers | 11 | ipv4-address | true
host-name | 12 | string | false
boot-size | 13 | uint16 | false
merit-dump | 14 | string | false
domain-name | 15 | fqdn | false
swap-server | 16 | ipv4-address | false
root-path | 17 | string | false
extensions-path | 18 | string | false
ip-forwarding | 19 | boolean | false
non-local-source-routing | 20 | boolean | false
policy-filter | 21 | ipv4-address | true
max-dgram-reassembly | 22 | uint16 | false
default-ip-ttl | 23 | uint8 | false
path-mtu-aging-timeout | 24 | uint32 | false
path-mtu-plateau-table | 25 | uint16 | true
interface-mtu | 26 | uint16 | false
all-subnets-local | 27 | boolean | false
broadcast-address | 28 | ipv4-address | false
perform-mask-discovery | 29 | boolean | false
mask-supplier | 30 | boolean | false
router-discovery | 31 | boolean | false
router-solicitation-address | 32 | ipv4-address | false
static-routes | 33 | ipv4-address | true
trailer-encapsulation | 34 | boolean | false
arp-cache-timeout | 35 | uint32 | false
ieee802-3-encapsulation | 36 | boolean | false
default-tcp-ttl | 37 | uint8 | false
tcp-keepalive-internal | 38 | uint32 | false
tcp-keepalive-garbage | 39 | boolean | false

#### List of standard DHCPv4 options (continued)

Name | Code | Type | Array?
--- | --- | --- | ---
nis-domain | 40 | string | false
nis-servers | 41 | ipv4-address | true
ntp-servers | 42 | ipv4-address | true
vendor-encapsulated-options | 43 | empty | false
netbios-name-servers | 44 | ipv4-address | true
netbios-dd-server | 45 | ipv4-address | true
netbios-node-type | 46 | uint8 | false
netbios-scope | 47 | string | false
font-servers | 48 | ipv4-address | true
x-display-manager | 49 | ipv4-address | true
dhcp-requested-address | 50 | ipv4-address | false
dhcp-option-overload | 52 | uint8 | false
dhcp-message | 56 | string | false
dhcp-max-message-size | 57 | uint16 | false
vendor-class-identifier | 60 | binary | false
nwip-domain-name | 62 | string | false
nwip-suboptions | 63 | binary | false
tftp-server-name | 66 | string | false
boot-file-name | 67 | string | false
user-class | 77 | binary | false
fqdn | 81 | record | false
dhcp-agent-options | 82 | empty | false
authenticate | 90 | binary | false
client-last-transaction-time | 91 | uint32 | false
associated-ip | 92 | ipv4-address | true
subnet-selection | 118 | ipv4-address | false
domain-search | 119 | binary | false
vivco-suboptions | 124 | binary | false
vivso-suboptions | 125 | binary | false

#### List of standard DHCP option types

Name | Meaning
--- | ---
binary | An arbitrary string of bytes, specified as a set of hexadecimal digits.
boolean | Boolean value with allowed values true or false
empty | No value, data is carried in suboptions
fqdn | Fully qualified domain name (e.g. www.example.com)
ipv4-address | IPv4 address in the usual dotted-decimal notation (e.g. 192.0.2.1)
ipv6-address | IPv6 address in the usual colon notation (e.g. 2001:db8::1)
record | Structured data that may comprise any types (except "record" and "empty")
string | Any text
uint8 | 8 bit unsigned integer with allowed values 0 to 255
uint16 | 16 bit unsigned integer with allowed values 0 to 65535
uint32 | 32 bit unsigned integer with allowed values 0 to 4294967295

### Custom DHCPv4 options

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

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

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

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

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

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

New options can take more complex forms than simple use of primitives (uint8, string, ipv4-address etc): it is possible to define an option comprising a number of existing primitives. Assume we want to define a new option that will consist of an IPv4 address, followed by 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 .

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

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

### DHCPv4 Vendor Specific Options

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

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

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

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

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

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

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

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

### Nested DHCPv4 Options (Custom Option Spaces)

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

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

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.

### Stateless Configuration of DHCPv4 clients

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

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

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

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

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

- The DHCPINFORM is relayed and the giaddr matches the configured subnet.

- The DHCPINFORM is unicast from the client and the ciaddr matches the configured subnet.

- The DHCPINFORM is unicast from the client, the ciaddr is not set but the source address of the IP packet matches the configured subnet.

- The DHCPINFORM is not relayed and the IP address on the interface on which the message is received matches the configured subnet.

### Client Classification in DHCPv4

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

In certain cases it is useful to differentiate between different types of clients and treat them differently. The process of doing classification is conducted in two steps. The first step is to assess incoming packet and assign it to zero or more classes. This classification is currently simple, but is expected to grow in capability soon. Currently the server checks whether incoming packet has vendor class identifier option (60). If it has, content of that option is prepended with "VENDOR_CLASS_" then 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 option (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 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 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 handled 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 .

#### Limiting Access to IPv4 Subnet to Certain Classes

In certain cases it beneficial to restrict access to certain subnets only to clients that belong to a given subnet. For details on client classes, see . This is an extension of a previous example from . Let's assume that the server is connected to a network segment that uses the 192.0.2.0/24 prefix. The Administrator of that network has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to be managed by the Dhcp4 server. Only clients belonging to client class VENDOR_CLASS_docsis3.0 are allowed to use this subnet. Such a configuration can be achieved in the following way:

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

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

### Configuring DHCPv4 for DDNS

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

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

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

3. The FQDN, lease address, and DHCID

The parameters for controlling the generation of NCRs for submission to the DHCP-DDNS server are contained in the `dhcp-ddns` section of the kea-dhcp4 server configuration. The default values for this section are as follows:

    "Dhcp4": {
        "dhcp-ddns": {
            "enable-updates": true,
            "server-ip": "127.0.0.1",
            "server-port": 53001,
            "sender-ip": "",
            "sender-port: 0,
            "max-queue-size": 1024,
            "ncr-protocol": "UDP",
            "ncr-format": "JSON",
            "override-no-update": false,
            "override-client-update": false,
            "replace-client-name": false,
            "generated-prefix": "myhost",
            "qualifying-suffix": "example.com"
        },
        ...
    }

#### DHCP-DDNS Server Connectivity

In order for NCRs to reach the DHCP-DDNS server, kea-dhcp4 must be able to communicate with it. kea-dhcp4 uses the following configuration parameters to control how it communications with DHCP-DDNS:

- `enable-updates` - determines whether or not kea-dhcp4 will generate NCRs. By default, this value is false hence DDNS updates are disabled. To enable DDNS updates set this value to true:

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

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

- `sender-ip` - IP address which kea-dhcp4 should use to send requests to the DHCP-DDNS server. The default value is blank which instructs kea-dhcp4 to select a suitable address.

- `sender-port` - port which kea-dhcp4 should use to send requests to the DHCP-DDNS server. The default value of 0 instructs kea-dhcp4 to select suitable port.

- `max-queue-size` - maximum number of requests allowed to queue waiting to be sent to the DHCP-DDNS server. This value guards against requests accumulating uncontrollably if they are being generated faster than they can be delivered. If the number of requests queued for transmission reaches this value, DDNS updating will be turned off until the queue backlog has been sufficiently reduced. The intention is allow the kea-dhcp4 server to continue lease operations without running the risk that its memory usage grows without limit. The default value is 1024.

- `ncr-format` - socket protocol use when sending requests to the DHCP-DDNS server. Currently only UDP is supported. TCP may be available in an upcoming release.

- `ncr-protocol` - packet format to use when sending requests to the DHCP-DDNS server. Currently only JSON format is supported. Other formats may be available in future releases.

By default, the DHCP-DDNS server is assumed to running on the same machine as kea-dhcp4, and all of the default values mentioned above should be sufficient. If, however, the DHCP-DDNS server has been configured to listen on a different address or port, these values must altered accordingly. For example, if the DHCP-DDNS server has been configured to listen on 192.168.1.10 port 900, the following configuration would be required:

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

#### When Does the kea-dhcp4 Server Generate DDNS Requests?

kea-dhcp4 follows the behavior prescribed for DHCP servers in RFC 4702. It is important to keep in mind that kea-dhcp4 provides the initial decision making of when and what to update and forwards that information to the DHCP-DDNS server in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are within the purview of the DHCP-DDNS server itself (). This section describes when kea-dhcp4 will generate NCRs and the configuration parameters that can be used to influence this decision. It assumes that the "enable-updates" parameter is true.

In general, kea-dhcp4 will generate DDNS update requests when:

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

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

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

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

When a new lease is granted, kea-dhcp4 will generate a DDNS update request if the DHCP REQUEST contains either the FQDN option (code 81) or the Host Name option (code 12). If both are present, the server will use the FQDN option. By default kea-dhcp4 will respect the FQDN N and S flags specified by the client as shown in the following table:

##### Default FQDN Flag Behavior

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

The first row in the table above represents "client delegation". Here the DHCP client states that it intends to do the forward DNS updates and the server should do the reverse updates. By default, kea-dhcp4 will honor the client's wishes and generate a DDNS request to the DHCP-DDNS server to update only reverse DNS data. The parameter `override-client-update` can be used to instruct the server to override client delegation requests. When this parameter is true, kea-dhcp4 will disregard requests for client delegation and generate a DDNS request to update both forward and reverse DNS data. In this case, the N-S-O flags in the server's response to the client will be 0-1-1 respectively.

(Note that the flag combination N=1, S=1 is prohibited according to RFC 4702. If such a combination is received from the client, the packet will be dropped by kea-dhcp4.)

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

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

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter, `override-no-update`, can be used to instruct the server to disregard the client's wishes. When this parameter is true, kea-dhcp4 will generate DDNS update request to the DHCP-DDNS server even if the client requests that no updates be done. The N-S-O flags in the server's response to the client will be 0-1-1.

To override client delegation, the following values should be set in your configuration:

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

kea-dhcp4 will always generate DDNS update requests if the client request only contains the Host Name option. In addition it will include an FQDN option in the response to the client with the FQDN N-S-O flags set to 0-1-0 respectively. The domain name portion of the FQDN option will be the name submitted to D2 in the DDNS update request.

#### kea-dhcp4 name generation for DDNS update requests

Each NameChangeRequest must of course include the fully qualified domain name whose DNS entries are to be affected. kea-dhcp4 can be configured to supply a portion or all of that name based upon what it receives from the client in the DHCP REQUEST.

The rules for determining the FQDN option are as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

### Next Server (siaddr)

In some cases, clients want to obtain configuration from the TFTP server. Although there is a dedicated option for it, some devices may use 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 global scope or for a given subnet only. If both are defined, 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 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",
                ...
            ]
        }
    }

### Echoing Client-ID (RFC 6842)

The original DHCPv4 specification (RFC 2131) states that the DHCPv4 server must not send back client-id options when responding to clients. However, in some cases that confused clients that did not have MAC address or client-id; see RFC 6842. for details. That behavior has changed with the publication of RFC 6842. which updated RFC 2131. That update now states that the server must send client-id if 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 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,
        ...
    }

## Server Identifier in DHCPv4

The DHCPv4 protocol uses a "server identifier" to allow clients to discriminate between several servers present on the same link: this value is an IPv4 address of the server. The server chooses the IPv4 address of the interface on which the message from the client (or relay) has been received. A single server instance will use multiple server identifiers if it is receiving queries on multiple interfaces.

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

## How the DHCPv4 Server Selects a Subnet for the Client

The DHCPv4 server differentiates between the directly connected clients, clients trying to renew leases and clients sending their messages through relays. For the directly connected clients the server will check the configuration of the interface on which the message has been received, and if the server configuration doesn't match any configured subnet the message is discarded.

Assuming that the server's interface is configured with the IPv4 address 192.0.2.3, the server will only process messages received through this interface from a directly connected client if there is a subnet configured to which this IPv4 address belongs, e.g. 192.0.2.0/24. The server will use this subnet to assign IPv4 address for the client.

The rule above does not apply when the client unicasts its message, i.e. is trying to renew its lease. Such message is accepted through any interface. The renewing client sets ciaddr to the currently used IPv4 address. The server uses this address to select the subnet for the client (in particular, to extend the lease using this address).

If the message is relayed it is accepted through any interface. The giaddr set by the relay agent is used to select the subnet for the client.

It is also possible to specify a relay IPv4 address for a given subnet. It can be used to match incoming packets into a subnet in uncommon configurations, e.g. shared subnets. See for details.

The subnet selection mechanism described in this section is based on the assumption that client classification is not used. The classification mechanism alters the way in which subnet is selected for the client, depending on the classes that the client belongs to.

### Using a Specific Relay Agent for a Subnet

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

However, that is not always the case. In certain uncommon - but valid - deployments, the relay address may not match the subnet. This usually means that there is more than one subnet allocated for a given link. The two most common examples where this is the case are long lasting network renumbering (where both old and new address space is still being used) and a cable network. In a cable network both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such 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 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",
                "pool": [ "192.0.2.10 - 192.0.2.20" ],
                "relay": {
                    "ip-address": "10.0.0.1"
                },
                ...
            }
        ],
        ...
    }

### Segregating IPv4 Clients in a Cable Network

In certain cases, it is useful to mix relay address information, introduced in with client classification, explained in . One specific example is cable network, where typically modems get addresses from a different subnet than all devices connected behind them.

Let's assume that there is one CMTS (Cable Modem Termination System) with one CM MAC (a physical link that modems are connected to). We want the modems to get addresses from the 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",
                "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",
                "pool": [ "192.0.2.10 - 192.0.2.20" ],
                "relay": {
                    "ip-address": "10.1.1.1"
                }
            }
        ],
        ...
    }

## Supported Standards

The following standards and draft standards are currently supported:

- RFC 2131: Supported messages are DISCOVER (1), OFFER (2), REQUEST (3), RELEASE (7), INFORM (8), ACK (5), and NAK(6).

- RFC 2132: Supported options are: PAD (0), END(255), Message Type(53), DHCP Server Identifier (54), Domain Name (15), DNS Servers (6), IP Address Lease Time (51), Subnet mask (1), and Routers (3).

- RFC 3046: Relay Agent Information option is supported.

- RFC 3925: Vendor-Identifying Vendor Class and Vendor-Identifying Vendor-Specific Information option are supported.

- RFC 6842: Server by default sends back client-id option. That capability may be disabled. See for details.

## DHCPv4 Server Limitations

These are the current limitations of the DHCPv4 server software. Most of them are reflections of the current stage of development and should be treated as not implemented yet, rather than actual limitations. However, some of them are implications of the design choices made. Those are clearly marked as such.

- Removal of a subnet during server reconfiguration may cause renumbering of auto-generated subnet identifiers, as described in section .

- Host reservation (static addresses) is not supported yet.

- Full featured client classification is not supported yet.

- BOOTP (RFC 951) is not supported. This is a design choice. BOOTP support is not planned.

- On Linux and BSD system families the DHCP messages are sent and received over the raw sockets (using LPF and BPF) and all packet headers (including data link layer, IP and UDP headers) are created and parsed by Kea, rather than the system kernel. Currently, Kea can only parse the data link layer headers with a format adhering to IEEE 802.3 standard and assumes this data link layer header format for all interfaces. Hence, Kea will fail to work on interfaces which use different data link layer header formats (e.g. Infiniband).

- The DHCPv4 server does not verify that assigned address is unused. According to RFC 2131, the allocating server should verify that address is not used by sending ICMP echo request.

- Address duplication report (DECLINE) is not supported yet.

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