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. The option is a value between 1 and 255.

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

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.