Configuring a Dynamic Host Configuration Protocol (DHCP) server

DHCP options

The DHCP server can configure a client with any of the options specified in RFC 2132. DHCP uses the same request and reply packet format as BOOTP so configuration options are carried in the portion of the BOOTP packet reserved for vendor extensions. DHCP terms this the ``options'' field and expands it from 64 bytes to 312 bytes.

Multiple sets of DHCP options can be specified in the DHCP configuration database. ``Option sets and precedence'' shows the possible types of option sets and their precedence.

Option sets and precedence

Option set type Precedence Description

Client Highest Generally used to specify a static IP address for an individual client. The DHCP server automatically assigns this address to the client rather than using one obtained from the address pool assigned to the subnet. You can also create a client entry without a static address, if you need to specify unique option values for the client but still want its IP address to be assigned dynamically. A DHCP client identifies itself to the server using its network hardware (MAC) address, or using an identifier if it is an opaque client. As many sets of client options can be created as are needed.

User class Offered to any client that specifies the user class identifier in the ``discover'' message. As many sets of user class options can be created as are needed.

Vendor class Offered to any client that specifies the vendor class identifier in the ``discover'' message. As many sets of vendor class options can be created as are needed.

Subnet Includes the name of the address pool from which the DHCP server draws IP addresses to allocate to clients, the default and maximum leases for these addresses, and lease renewal and rebind times, along with a set of DHCP options specific to the subnet. As many sets of subnet options can be created as are needed.

Global Lowest Applies to all DHCP clients. Only one set of global options can be created.

Option set type	Precedence	Description
Client	Highest	Generally used to specify a static IP address for an individual client. The DHCP server automatically assigns this address to the client rather than using one obtained from the address pool assigned to the subnet. You can also create a client entry without a static address, if you need to specify unique option values for the client but still want its IP address to be assigned dynamically. A DHCP client identifies itself to the server using its network hardware (MAC) address, or using an identifier if it is an opaque client. As many sets of client options can be created as are needed.
User class		Offered to any client that specifies the user class identifier in the ``discover'' message. As many sets of user class options can be created as are needed.
Vendor class		Offered to any client that specifies the vendor class identifier in the ``discover'' message. As many sets of vendor class options can be created as are needed.
Subnet		Includes the name of the address pool from which the DHCP server draws IP addresses to allocate to clients, the default and maximum leases for these addresses, and lease renewal and rebind times, along with a set of DHCP options specific to the subnet. As many sets of subnet options can be created as are needed.
Global	Lowest	Applies to all DHCP clients. Only one set of global options can be created.

The set of global DHCP options, for example, applies to all clients, whereas a set of subnet options applies only to the clients on a specified subnet. If you configure the same option in more than one set of options, the precedence of the option type decides which the DHCP server offers to the client.

For example, if the ``LPR Servers'' option is specified in both the subnet set of options and the global set of options, the DHCP server configures the client with the LPR Servers information defined for the subnet.

Mapping sets of DHCP options to a network

``Mapping sets of DHCP options to a network'' shows an example of how the option sets defined on the DHCP server can be considered as creating a map of the configuration parameters needed by DHCP clients:

In the figure, the set of global options is applicable to all the clients whether they are in subnet A or subnet B. This set could include information such as the domain name, and the IP addresses of DNS servers.
Each of subnets A and B requires its own specific set of subnet options. Each set of subnet options defines information specific to the subnet such as the name of the pool of IP addresses that are available for clients to use, a subdomain name, and the IP addresses of routers.
In subnet B, three machines are shown as belonging to a group which shares a vendor class set of options. For example, their architecture might require them to look in a different TFTP boot directory for configuration files or a bootstrap program than other machines on the same subnet. You could similarly group machines to use different pools of dynamically allocated IP addresses by specifying appropriate sets of user class options. You must take care not to configure pools which define overlapping ranges of available IP addresses.
Finally, in both subnet A and subnet B, a client is shown as having its own set of client options defined for it. This could be used to define a static IP address for the client. Clients which are themselves servers for other networking services and need to be always available at the same IP address should be configured in this way.

Option overload

Although the DHCP client can specify a larger response packet size, the practical limit for DHCP options is 274 bytes. (An additional 192 bytes can be provided if the option overload feature is enabled. Option overload is a server parameter.) Each option consists of a 2-byte header and the option data, such as an integer or an array of IP addresses. A single option cannot exceed 255 bytes of data.

If the server finds that the configured options exceed the allowable limit, it uses as many options as possible starting from the top of the configured options list and sends an error message to /usr/adm/syslog. Clients can send a configured options list in the ``discover'' message. In the situation mentioned above, this ``Parameters Request List'' takes precedence over any other configured options.

Default option values

Some DHCP options have default values. If you configure an option that has a default value and do not configure a value for the option, the DHCP server sends the default value.

DHCP options and defaults

Option name Default value

Broadcast address Network/subnet number with all 1's for the host

Router discovery 1 (enable)

IP forwarding 0 (disable)

Non-local source routing 0 (disallow)

TCP keepalive garbage 0 (disable)

Default IP time to live 64

Ethernet encapsulation 0 (RFC 894, Ethernet version 2)

Default TCP keepalive interval 0 (no keepalives)

Time offset Obtained from host

Boot file size Obtained from host

See RFC 2132 for descriptions of these options.

Option name	Default value
Broadcast address	Network/subnet number with all 1's for the host
Router discovery	1 (enable)
IP forwarding	0 (disable)
Non-local source routing	0 (disallow)
TCP keepalive garbage	0 (disable)
Default IP time to live	64
Ethernet encapsulation	0 (RFC 894, Ethernet version 2)
Default TCP keepalive interval	0 (no keepalives)
Time offset	Obtained from host
Boot file size	Obtained from host