DHCP/BOOTP Relay Agent
 

1. Do Linux implementations of BOOTP/DHCP relay agent use UDP/Datagram Berkely sockets ?
2. On what port DHCP/Bootp server sends unicast BOOTPREPLY/DHCPREPLY UDP replies back to BOOTP/DHCP relay agent ?
3. RFC 1542 says: "Relay agent MUST silently discard any received UDP messages whose UDP destination port number is BOOTPC (68). " - so what the router (where Relay agent is resident) should do with such unicast packets whose UDP destination port number is BOOTPC (68), when they arrive on its interfaces ? Examine if the destination port is matching one of the router's own routing interfaces and drop such, if not matching - route those to the destination ?

Have you tried http://www.isc.org ? the DHCP/BOOTP server that runs on almost all linux systems is made by them.

in dhcrelay.c (Internet Software Consortium) DHCP/BOOTP relay code is using Berkeley Packet Filter ( bpf). Based on the description, the bpf is essentially putting the interface into promiscuous mode, and allowing arbitrary processing on the frames received on the interface (e.g. custom filters).