DHCP relay agent

A router or host that passes DHCP messages between clients on one subnet and a DHCP server on another.

A client with no address yet starts with a broadcast, and RFC 2131 says relay agents remove the need for a DHCP server on every network segment. The relay also tells the server which subnet the client is on, which decides the pool its address comes from. Microsoft calls them IP Helpers.

When a relay forwards a client's request it writes the address of the interface the request arrived on into the giaddr field, if that field is still zero, and increments hops. RFC 1542 says the destination it relays to MUST be configurable, and that a request whose hops exceed 16 is silently discarded. The server picks the address from the subnet named in giaddr and sends its replies to the DHCP server port, 67, on the relay, which delivers them on the client's own link.

RFC 1542 calls giaddr rather poorly named: it is never a router for the client to use. A zero giaddr tells the server the client is on its own subnet, so a DHCPNAK is broadcast; otherwise the NAK goes to the relay, which forwards it to the client's hardware address even if the client has moved to a new network.

When one subnet gets no leases while others do, Microsoft's DHCP checklist asks whether the routers or VLAN switches are configured with relay agents, next to whether the scope for that subnet still has addresses. Both failures look the same from the client, because a DHCP server is not required to answer every message it receives.

Also known as: dhcp relay, relay agent, bootp relay agent, ip helper, giaddr

Sources

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