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. Table 7-1, ``Option sets and precedence'' shows the possible types of option sets and their precedence.

Table 7-1 Option sets and precedence

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 Option set type   Precedence   Description
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 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.

Figure 7-2 Mapping sets of DHCP options to a network

Figure 7-2, ``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.

Table 7-2 DHCP options and defaults

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 Option name                      Default value
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 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