DHCPrelay

DHCP Relay (often implemented as a relay agent) is a network function that forwards Dynamic Host Configuration Protocol (DHCP) messages between clients and servers across different IP subnets. It enables centralized DHCP servers to serve address assignments and configuration options to clients that are not on the same broadcast domain as the server, avoiding the need for a server instance on every subnet.

When a client broadcasts a DHCPDISCOVER, a relay agent on the local router or switch intercepts the broadcast, records the client's originating subnet in the gateway IP address field (giaddr), and forwards the request as a unicast to a configured DHCP server. The server uses giaddr to determine the appropriate address pool and replies to the relay agent, which then forwards the response back to the client. Many implementations support the DHCP Relay Agent Information option (Option 82) to include additional identifying information such as circuit or remote identifiers.

DHCP Relay is commonly configured on Layer 3 devices (routers, multilayer switches) and is supported in server and client software such as ISC dhcrelay and enterprise network equipment. For DHCPv6, relay behavior is similar but uses DHCPv6-specific relay message formats and link-address fields.

Operational considerations include securing relays to prevent spoofing, using access controls or DHCP snooping, and ensuring correct server configuration for pools and options. Alternatives include deploying local DHCP servers per subnet or using virtualization/containerization techniques for distributed addressing.