DHCP Relay实现分析

int main (argc, argv, envp)
	int argc;
	char **argv, **envp;
{
	/* parameters definition */
	/* Make sure we have stdin, stdout and stderr. */
	/* Set up the OMAPI. */
	status = omapi_init ();

	/* Set up the OMAPI wrappers for the interface object. */
	interface_setup ();
	/* usage like shell */

	/* Default to the DHCP/BOOTP port. */

	/* Set up the server sockaddrs. */
	for (sp = servers; sp; sp = sp -> next) {
		sp -> to.sin_port = local_port;
		sp -> to.sin_family = AF_INET;
#ifdef HAVE_SA_LEN
		sp -> to.sin_len = sizeof sp -> to;
#endif
	}

	/* Get the current time... */
	GET_TIME (&cur_time);

	/* Discover all the network interfaces. */
	discover_interfaces (DISCOVER_RELAY);

	/* Set up the bootp packet handler... */
	bootp_packet_handler = relay;

	/* Become a daemon... */
	
	/* Start dispatching packets and timeouts... */
	dispatch ();

	/*NOTREACHED*/
	return 0;
}

void relay (ip, packet, length, from_port, from, hfrom)
	struct interface_info *ip;
	struct dhcp_packet *packet;
	unsigned length;
	unsigned int from_port;
	struct iaddr from;
	struct hardware *hfrom;
{
	struct server_list *sp;
	struct sockaddr_in to;
	struct interface_info *out;
	struct hardware hto, *htop;

	if (packet -> hlen > sizeof packet -> chaddr) {
		log_info ("Discarding packet with invalid hlen.");
		return;
	}

	/* Find the interface that corresponds to the giaddr
	   in the packet. */
	if (packet -> giaddr.s_addr) {
		for (out = interfaces; out; out = out -> next) {
			if (!memcmp (&out -> primary_address,
				     &packet -> giaddr,
				     sizeof packet -> giaddr))
				break;
		}
	} else {
		out = (struct interface_info *)0;
	}

	/* If it's a bootreply, forward it to the client. */
	if (packet -> op == BOOTREPLY) {
		if (!(packet -> flags & htons (BOOTP_BROADCAST)) &&
			can_unicast_without_arp (out)) {
			to.sin_addr = packet -> yiaddr;
			to.sin_port = remote_port;

			/* and hardware address is not broadcast */
			htop = &hto;
		} else {
			to.sin_addr.s_addr = htonl (INADDR_BROADCAST);
			to.sin_port = remote_port;

			/* hardware address is broadcast */
			htop = NULL;
		}
		to.sin_family = AF_INET;
#ifdef HAVE_SA_LEN
		to.sin_len = sizeof to;
#endif

		memcpy (&hto.hbuf [1], packet -> chaddr, packet -> hlen);
		hto.hbuf [0] = packet -> htype;
		hto.hlen = packet -> hlen + 1;

		/* Wipe out the agent relay options and, if possible, figure
		   out which interface to use based on the contents of the
		   option that we put on the request to which the server is
		   replying. */
		if (!(length =
		      strip_relay_agent_options (ip, &out, packet, length)))
			return;

		if (!out) {
			log_error ("packet to bogus giaddr %s.n",
			      inet_ntoa (packet -> giaddr));
			++bogus_giaddr_drops;
			return;
		}

		if (send_packet (out,
				 (struct packet *)0,
				 packet, length, out -> primary_address,
				 &to, htop) < 0) {
			++server_packet_errors;
		} else {
			log_debug ("forwarded BOOTREPLY for %s to %s",
			       print_hw_addr (packet -> htype, packet -> hlen,
					      packet -> chaddr),
			       inet_ntoa (to.sin_addr));

			++server_packets_relayed;
		}
		return;
	}

	/* If giaddr matches one of our addresses, ignore the packet -
	   we just sent it. */
	if (out)
		return;

	/* Add relay agent options if indicated.   If something goes wrong,
	   drop the packet. */
	if (!(length = add_relay_agent_options (ip, packet, length,
						ip -> primary_address)))
		return;

	/* If giaddr is not already set, Set it so the server can
	   figure out what net it's from and so that we can later
	   forward the response to the correct net.    If it's already
	   set, the response will be sent directly to the relay agent
	   that set giaddr, so we won't see it. */
	if (!packet -> giaddr.s_addr)
		packet -> giaddr = ip -> primary_address;
	if (packet -> hops < max_hop_count)
		packet -> hops = packet -> hops + 1;
	else
		return;

	/* Otherwise, it's a BOOTREQUEST, so forward it to all the
	   servers. */
	for (sp = servers; sp; sp = sp -> next) {
		if (send_packet ((fallback_interface
				  ? fallback_interface : interfaces),
				 (struct packet *)0,
				 packet, length, ip -> primary_address,
				 &sp -> to, (struct hardware *)0) < 0) {
			++client_packet_errors;
		} else {
			log_debug ("forwarded BOOTREQUEST for %s to %s",
			       print_hw_addr (packet -> htype, packet -> hlen,
					      packet -> chaddr),
			       inet_ntoa (sp -> to.sin_addr));
			++client_packets_relayed;
		}
	}
				 
}