16.5 BOOTP Through a Router

We said in Section 5.4 that one of the drawbacks of RARP is that it uses a link-layer broadcast which is normally not forwarded by a router. This required an RARP server on each physical network. BOOTP can be used through a router, if supported by the router. (Most major router vendors do support this feature.)

This is mainly intended for diskless routers, because if a multiuser system with a disk is used as a router, it can probably run a BOOTP server itself. Alternatively, the common Unix BOOTP server (Appendix F) supports this relay mode, but again, if you can run a BOOTP server on the physical network, there's normally no need to forward the requests to yet another server on another network.

What happens is that the router (also called the "BOOTP relay agent") listens for BOOTP requests on the server's well-known port (67). When a request is received, the relay agent places its IP address into the gateway IP address field in the BOOTP request, and sends the request to the real BOOTP server. (The address placed by the relay agent into the gateway field is the IP address of the interface on which the request was received.) The relay agent also increments the hops field by one. (This is to prevent infinite loops in case the request is reforwarded. RFC 951 mentions that the request should probably be thrown away if the hop count reaches 3.) Since the outgoing request is a unicast datagram (as opposed to the original client request that was broadcast), it can follow any route to the real BOOTP server, passing through other routers. The real server gets the request, forms the BOOTP reply, and sends it back to the relay agent, not the client. The real server knows that the request has been forwarded, since the gateway field in the request is nonzero. The relay agent receives the reply and sends it to the client.