dhcrelay

dhcrelay is a DHCP relay agent used in networks to forward Dynamic Host Configuration Protocol messages between clients on one subnet and DHCP servers on another. It is commonly provided by the ISC DHCP software suite as a dedicated daemon (often named dhcrelay or dhcrelay6 for IPv6) and is deployed on routers or layer-3 devices that interconnect subnets. The relay enables centralized DHCP services across multiple subnets without placing a server on every subnet.

How it works: when a client broadcasts a DHCPDISCOVER on its local subnet, the relay agent on that subnet intercepts the broadcast and forwards the message to the configured DHCP server(s) using unicast UDP to the server port. Before forwarding, the relay sets the giaddr (gateway IP address) field to the relay’s interface address on the client’s subnet. This giaddr allows the DHCP server to determine which subnet’s address pool to use for the client. The server replies to the relay, which then forwards the response back to the client. This mechanism enables address assignment and configuration information to be supplied across routed networks.

Configuration and deployment: dhcrelay is typically configured with one or more DHCP server addresses and the interfaces on which to listen for client traffic. In Linux, the daemon is invoked with options specifying interfaces and server addresses; on many routers, similar functionality is provided by commands like ip helper-address. Some environments use separate relays for IPv4 (dhcrelay) and IPv6 (dhcrelay6) to handle DHCPv4 and DHCPv6 traffic respectively.

DHCP relay is fundamental for scalable network design, enabling centralized DHCP management while maintaining subnet isolation.