DHCP options

When adding a DHCP server, you can include DHCP codes and options. The DHCP options are BOOTP vendor information fields that provide additional vendor-independent configuration parameters to manage the DHCP server. For example, you might need to configure a FortiGate DHCP server that gives out a separate option as well as an IP address, such as an environment that needs to support PXE boot with Windows images.

The option numbers and codes are specific to the application. The documentation for the application indicates the values to use. Option codes are represented in an option value/HEX value pairs. The option is a value between 1 and 255.

You can add up to three DHCP code/option pairs per DHCP server.

For detailed information about DHCP options, see RFC 2132, DHCP Options and BOOTP Vendor Extensions.

To configure option 252 with value http://192.168.1.1/wpad.dat using the CLI:

config system dhcp server
    edit <server_entry_number>
        set option1 252 687474703a2f2f3139322e3136382e312e312f777061642e646174
    next
end

In the example above, 687474703a2f2f3139322e3136382e312e312f777061642e646174 is the hexadecimal equivalent of the ASCII text http://192.168.1.1/wpad.dat.

Option 82

The DHCP relay agent information option (option 82 in RFC 3046) helps protect the FortiGate against attacks such as spoofing (forging) of IP addresses and MAC addresses, and DHCP IP address starvation.

This option is disabled by default. However, when dhcp-relay-service is enabled, dhcp-relay-agent-option becomes enabled.

To configure the DHCP relay agent option using the CLI:

config system interface
    edit <interface>
        set vdom root
        set dhcp-relay-service enable
        set dhcp-relay-ip <ip>
        set dhcp-relay-agent-option enable
        set vlanid <id>
    next
end

See IP address assignment with relay agent information option for an example.

Option 42

This option specifies a list of the NTP servers available to the client by IP address.

config system dhcp server 
    edit 2
        set ntp-service {local | default | specify}
        set ntp-server1 <class_ip>
        set ntp-server2 <class_ip>
        set ntp-server3 <class_ip>
    next
end

The NTP service options include:

• local: The IP address of the interface that the DHCP server is added to becomes the client's NTP server IP address.
• default: Clients are assigned the FortiGate's configured NTP servers.
• specify: Specify up to three NTP servers in the DHCP server configuration.

DHCP server option fields

In place of specific fields, the DHCP server maintains a table for the potential options. The FortiOS DHCP server supports up to a maximum of 30 custom options. These optional fields are set in the CLI.

To get to the DHCP server:

config system dhcp server
    edit <integer - ID of the specific DHCP server>
    config options
        edit <integer>
            set code <option integer>
            set type {hex | string | ip | fqdn}
            set value <option content for DHCP option types hex and string>
        next
    end
end