--dhcp-relay=<local address>[,<server address>[#<server port>]][,<interface]
Configure dnsmasq to do DHCP relay. The local address is an address allocated to an interface on the host running dnsmasq. All DHCP requests arriving on that interface will be relayed to a remote DHCP server at the server address. It is possible to relay from a single local address to multiple remote servers by using multiple --dhcp-relay configs with the same local address and different server addresses. A server address must be an IP literal address, not a domain name. If the server address is omitted, the request will be forwarded by broadcast (IPv4) or multicast (IPv6). In this case the interface must be given and not be wildcard. The server address may specify a non-standard port to relay to. If this is used then --dhcp-proxy should likely also be set, otherwise parts of the DHCP conversation which do not pass through the relay will be delivered to the wrong port.

Access control for DHCP clients has the same rules as for the DHCP server, see --interface, --except-interface, etc. The optional interface name in the --dhcp-relay config has a different function: it controls on which interface DHCP replies from the server will be accepted. This is intended for configurations which have three interfaces: one being relayed from, a second connecting the DHCP server, and a third untrusted network, typically the wider internet. It avoids the possibility of spoof replies arriving via this third interface.

It is allowed to have dnsmasq act as a DHCP server on one set of interfaces and relay from a disjoint set of interfaces. Note that whilst it is quite possible to write configurations which appear to act as a server and a relay on the same interface, this is not supported: the relay function will take precedence.

Both DHCPv4 and DHCPv6 relay is supported. It's not possible to relay DHCPv4 to a DHCPv6 server or vice-versa.

The DHCP relay function for IPv6 includes the ability to snoop prefix-delegation from relayed DHCP transactions. See --dhcp-script for details.