path: root/relay/dhcrelay.c

/* dhcrelay.c

   DHCP/BOOTP Relay Agent. */

/*
 * Copyright (c) 2004-2006 by Internet Systems Consortium, Inc. ("ISC")
 * Copyright (c) 1997-2003 by Internet Software Consortium
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
 * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 *   Internet Systems Consortium, Inc.
 *   950 Charter Street
 *   Redwood City, CA 94063
 *   <info@isc.org>
 *   http://www.isc.org/
 *
 * This software has been written for Internet Systems Consortium
 * by Ted Lemon in cooperation with Vixie Enterprises and Nominum, Inc.
 * To learn more about Internet Systems Consortium, see
 * ``http://www.isc.org/''.  To learn more about Vixie Enterprises,
 * see ``http://www.vix.com''.   To learn more about Nominum, Inc., see
 * ``http://www.nominum.com''.
 */

#ifndef lint
static char ocopyright[] =
"$Id: dhcrelay.c,v 1.55 2006/02/27 23:56:13 dhankins Exp $ Copyright (c) 2004-2006 Internet Systems Consortium.  All rights reserved.\n";
#endif /* not lint */

#include "dhcpd.h"
#include "version.h"

static void usage PROTO ((void));

TIME default_lease_time = 43200; /* 12 hours... */
TIME max_lease_time = 86400; /* 24 hours... */
struct tree_cache *global_options [256];

/* Needed to prevent linking against conflex.c. */
int lexline;
int lexchar;
char *token_line;
char *tlname;

const char *path_dhcrelay_pid = _PATH_DHCRELAY_PID;

int bogus_agent_drops = 0;	/* Packets dropped because agent option
				   field was specified and we're not relaying
				   packets that already have an agent option
				   specified. */
int bogus_giaddr_drops = 0;	/* Packets sent to us to relay back to a
				   client, but with a bogus giaddr. */
int client_packets_relayed = 0;	/* Packets relayed from client to server. */
int server_packet_errors = 0;	/* Errors sending packets to servers. */
int server_packets_relayed = 0;	/* Packets relayed from server to client. */
int client_packet_errors = 0;	/* Errors sending packets to clients. */

int add_agent_options = 0;	/* If nonzero, add relay agent options. */
int drop_agent_mismatches = 0;	/* If nonzero, drop server replies that
				   don't have matching circuit-id's. */
int corrupt_agent_options = 0;	/* Number of packets dropped because
				   relay agent information option was bad. */
int missing_agent_option = 0;	/* Number of packets dropped because no
				   RAI option matching our ID was found. */
int bad_circuit_id = 0;		/* Circuit ID option in matching RAI option
				   did not match any known circuit ID. */
int missing_circuit_id = 0;	/* Circuit ID option in matching RAI option
				   was missing. */
int max_hop_count = 10;		/* Maximum hop count */


	/* Maximum size of a packet with agent options added. */
int dhcp_max_agent_option_packet_length = 576;

	/* What to do about packets we're asked to relay that
	   already have a relay option: */
enum { forward_and_append,	/* Forward and append our own relay option. */
       forward_and_replace,	/* Forward, but replace theirs with ours. */
       forward_untouched,	/* Forward without changes. */
       discard } agent_relay_mode = forward_and_replace;

u_int16_t local_port;
u_int16_t remote_port;

static char copyright [] = "Copyright 2004-2005 Internet Systems Consortium.";
static char arr [] = "All rights reserved.";
static char message [] = "Internet Systems Consortium DHCP Relay Agent";
static char url [] = "For info, please visit http://www.isc.org/sw/dhcp/";

int main (argc, argv, envp)
	int argc;
	char **argv, **envp;
{
	int i;
	struct servent *ent;
	struct server_list *sp = (struct server_list *)0;
	int no_daemon = 0;
	int quiet = 0;
	isc_result_t status;
	char *s;
	struct interface_info *tmp = (struct interface_info *)0;

	/* Make sure we have stdin, stdout and stderr. */
	i = open ("/dev/null", O_RDWR);
	if (i == 0)
		i = open ("/dev/null", O_RDWR);
	if (i == 1) {
		i = open ("/dev/null", O_RDWR);
		log_perror = 0; /* No sense logging to /dev/null. */
	} else if (i != -1)
		close (i);

#ifdef SYSLOG_4_2
	openlog ("dhcrelay", LOG_NDELAY);
	log_priority = LOG_DAEMON;
#else
	openlog ("dhcrelay", LOG_NDELAY, LOG_DAEMON);
#endif

#if !(defined (DEBUG) || defined (SYSLOG_4_2))
	setlogmask (LOG_UPTO (LOG_INFO));
#endif	

	/* Set up the OMAPI. */
	status = omapi_init ();
	if (status != ISC_R_SUCCESS)
		log_fatal ("Can't initialize OMAPI: %s",
			   isc_result_totext (status));

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

	for (i = 1; i < argc; i++) {
		if (!strcmp (argv [i], "-p")) {
			if (++i == argc)
				usage ();
			local_port = htons (atoi (argv [i]));
			log_debug ("binding to user-specified port %d",
			       ntohs (local_port));
		} else if (!strcmp (argv [i], "-d")) {
			no_daemon = 1;
 		} else if (!strcmp (argv [i], "-i")) {
			if (tmp)
				interface_dereference (&tmp, MDL);
			status = interface_allocate (&tmp, MDL);
			if (status != ISC_R_SUCCESS)
				log_fatal ("%s: interface_allocate: %s",
					   argv [i],
					   isc_result_totext (status));
			if (++i == argc) {
				usage ();
			}
			strcpy (tmp -> name, argv [i]);
			interface_snorf (tmp, INTERFACE_REQUESTED);
 		} else if (!strcmp (argv [i], "-is")) {
			if (++i == argc)
				usage ();
			if (!tmp) {
				log_error ("-is must follow -i.");
				usage ();
			}
			new_relay_server (argv [i], &tmp -> servers);
		} else if (!strcmp (argv [i], "-q")) {
			quiet = 1;
			quiet_interface_discovery = 1;
		} else if (!strcmp (argv [i], "-a")) {
			add_agent_options = 1;
		} else if (!strcmp (argv [i], "-c")) {
			int hcount;
			if (++i == argc)
				usage ();
			hcount = atoi(argv[i]);
			if (hcount <= 255)
				max_hop_count= hcount;
			else
				usage ();
		} else if (!strcmp (argv [i], "-A")) {
			if (++i == argc)
				usage ();
			dhcp_max_agent_option_packet_length = atoi (argv [i]);
		} else if (!strcmp (argv [i], "-m")) {
			if (++i == argc)
				usage ();
			if (!strcasecmp (argv [i], "append")) {
				agent_relay_mode = forward_and_append;
			} else if (!strcasecmp (argv [i], "replace")) {
				agent_relay_mode = forward_and_replace;
			} else if (!strcasecmp (argv [i], "forward")) {
				agent_relay_mode = forward_untouched;
			} else if (!strcasecmp (argv [i], "discard")) {
				agent_relay_mode = discard;
			} else
				usage ();
		} else if (!strcmp (argv [i], "-D")) {
			drop_agent_mismatches = 1;
 		} else if (argv [i][0] == '-') {
 		    usage ();
		} else if (!strcmp (argv [i], "--version")) {
			log_info ("isc-dhcrelay-%s", DHCP_VERSION);
			exit (0);
 		} else {
			new_relay_server (argv [i], &servers);
 		}
	}

	limited_broadcast.s_addr = INADDR_BROADCAST;

	if (tmp)
		interface_dereference (&tmp, MDL);

	if ((s = getenv ("PATH_DHCRELAY_PID"))) {
		path_dhcrelay_pid = s;
	}

	if (!quiet) {
		log_info ("%s %s", message, DHCP_VERSION);
		log_info (copyright);
		log_info (arr);
		log_info (url);
	} else {
		quiet = 0;
		log_perror = 0;
	}

	/* Default to the DHCP/BOOTP port. */
	if (!local_port) {
		ent = getservbyname ("dhcps", "udp");
		if (!ent)
			local_port = htons (67);
		else
			local_port = ent -> s_port;
		endservent ();
	}
	remote_port = htons (ntohs (local_port) + 1);
  
	/* We need at least one server. */
	if (!servers) {
		usage ();
	}

	/* 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
	}

	if (interfaces) {
		interface_reference (&tmp, interfaces, MDL);
		do {
			struct interface_info *next = NULL;
			if (tmp -> next)
				interface_reference (&next, tmp -> next, MDL);
		
			for (sp = tmp -> 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
			}

			interface_dereference (&tmp, MDL);
			if (next) {
				interface_reference (&tmp, next, MDL);
				interface_dereference (&next, MDL);
			}
		} while (tmp);
	}

	/* 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... */
	if (!no_daemon) {
		int pid;
		FILE *pf;
		int pfdesc;

		log_perror = 0;

		if ((pid = fork()) < 0)
			log_fatal ("can't fork daemon: %m");
		else if (pid)
			exit (0);

		pfdesc = open (path_dhcrelay_pid,
			       O_CREAT | O_TRUNC | O_WRONLY, 0644);

		if (pfdesc < 0) {
			log_error ("Can't create %s: %m", path_dhcrelay_pid);
		} else {
			pf = fdopen (pfdesc, "w");
			if (!pf)
				log_error ("Can't fdopen %s: %m",
				      path_dhcrelay_pid);
			else {
				fprintf (pf, "%ld\n", (long)getpid ());
				fclose (pf);
			}	
		}

		close (0);
		close (1);
		close (2);
		pid = setsid ();
	}

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

	/*NOTREACHED*/
	return 0;
}

void new_relay_server (char *arg, struct server_list **servers)
{
	struct hostent *he;
	struct in_addr ia, *iap = (struct in_addr *)0;
	struct server_list *sp;

	if (inet_aton (arg, &ia)) {
		iap = &ia;
	} else {
		he = gethostbyname (arg);
		if (!he) {
			log_error ("%s: host unknown", arg);
		} else {
			iap = ((struct in_addr *)
			       he -> h_addr_list [0]);
		}
	}
	if (iap) {
		sp = ((struct server_list *)
		      dmalloc (sizeof *sp, MDL));
		if (!sp)
			log_fatal ("no memory for server.\n");
		memset(sp, 0, sizeof *sp);
		sp -> next = *servers;
		*servers = sp;
		memcpy (&sp -> to.sin_addr,
			iap, sizeof *iap);
	}
}

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;
	int i;

	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) {
			for (i = 0; i < out -> address_count; i++) {
				if (out -> addresses [i].s_addr ==
				    packet -> giaddr.s_addr)
					goto matched;
			}
		}
	} else {
		out = (struct interface_info *)0;
	}
      matched:

	/* 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.",
			      inet_ntoa (packet -> giaddr));
			++bogus_giaddr_drops;
			return;
		}

		if (out -> address_count < 1)
			log_fatal ("no IP address on interface %s!",
				   out -> name);
		if (send_packet (out,
				 (struct packet *)0,
				 packet, length, out -> addresses [0],
				 &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;

	if (ip -> address_count < 1)
		log_fatal ("no IP address on interface %s", ip -> name);

	/* Add relay agent options if indicated.   If something goes wrong,
	   drop the packet. */
	if (!(length = add_relay_agent_options (ip, packet, length,
						ip -> addresses [0])))
		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 -> addresses [0];
	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 = (ip -> servers
		   ? ip -> servers : servers); sp; sp = sp -> next) {
		if (send_packet ((fallback_interface
				  ? fallback_interface : interfaces),
				 (struct packet *)0,
				 packet, length, ip -> addresses [0],
				 &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;
		}
	}
}

static void usage ()
{
	log_fatal ("Usage: dhcrelay [-p <port>] [-d] [-D] [-i %s%s%s%s",
		"interface [-is server ... ]]\n                ",
		"[-c count] [-A length] ",
		"[-m append|replace|forward|discard]\n",
		"                [server1 [... serverN]]");
}

int write_lease (lease)
	struct lease *lease;
{
	return 1;
}

int write_host (host)
	struct host_decl *host;
{
	return 1;
}

int commit_leases ()
{
	return 1;
}

void bootp (packet)
	struct packet *packet;
{
}

void dhcp (packet)
	struct packet *packet;
{
}

int find_subnet (struct subnet **sp,
		 struct iaddr addr, const char *file, int line)
{
	return 0;
}

#if defined (DEBUG)
int check_collection (struct packet *p, struct lease *l,
		      struct collection *c)
{
	return 0;
}

void classify (struct packet *p, struct class *c)
{
}

isc_result_t find_class (struct class **class, const char *c1,
			 const char *c2, int i)
{
	return ISC_R_NOTFOUND;
}

int parse_allow_deny (struct option_cache **oc, struct parse *p, int i)
{
	return 0;
}

/* As a wise man once said in dhcpctl/omshell.c: */
/* Sigh */
isc_result_t dhcp_set_control_state (control_object_state_t oldstate,
				     control_object_state_t newstate)
{
	return ISC_R_SUCCESS;
}

#endif

/* Strip any Relay Agent Information options from the DHCP packet
   option buffer.   If there is a circuit ID suboption, look up the
   outgoing interface based upon it. */

int strip_relay_agent_options (in, out, packet, length)
	struct interface_info *in, **out;
	struct dhcp_packet *packet;
	unsigned length;
{
	int is_dhcp = 0;
	u_int8_t *op, *nextop, *sp, *max;
	int good_agent_option = 0;
	int status;

	/* If we're not adding agent options to packets, we're not taking
	   them out either. */
	if (!add_agent_options)
		return length;

	/* If there's no cookie, it's a bootp packet, so we should just
	   forward it unchanged. */
	if (memcmp (packet -> options, DHCP_OPTIONS_COOKIE, 4))
		return length;

	max = ((u_int8_t *)packet) + length;
	sp = op = &packet -> options [4];

	while (op < max) {
		switch (*op) {
			/* Skip padding... */
		      case DHO_PAD:
			if (sp != op)
				*sp = *op;
			++op;
			++sp;
			continue;

			/* If we see a message type, it's a DHCP packet. */
		      case DHO_DHCP_MESSAGE_TYPE:
			is_dhcp = 1;
			goto skip;
			break;

			/* Quit immediately if we hit an End option. */
		      case DHO_END:
			if (sp != op)
				*sp++ = *op++;
			goto out;

		      case DHO_DHCP_AGENT_OPTIONS:
			/* We shouldn't see a relay agent option in a
			   packet before we've seen the DHCP packet type,
			   but if we do, we have to leave it alone. */
			if (!is_dhcp)
				goto skip;

			/* Do not process an agent option if it exceeds the
			 * buffer.  Fail this packet.
			 */
			nextop = op + op[1] + 2;
			if (nextop > max)
				return 0;

			status = find_interface_by_agent_option (packet,
								 out, op + 2,
								 op [1]);
			if (status == -1 && drop_agent_mismatches)
				return 0;
			if (status)
				good_agent_option = 1;
			op = nextop;
			break;

		      skip:
			/* Skip over other options. */
		      default:
			/* Fail if processing this option will exceed the
			 * buffer (op[1] is malformed).
			 */
			nextop = op + op[1] + 2;
			if (nextop > max)
				return 0;

			if (sp != op) {
				memmove(sp, op, op[1] + 2);
				sp += op[1] + 2;
				op = nextop;
			} else
				op = sp = nextop;

			break;
		}
	}
      out:

	/* If it's not a DHCP packet, we're not supposed to touch it. */
	if (!is_dhcp)
		return length;

	/* If none of the agent options we found matched, or if we didn't
	   find any agent options, count this packet as not having any
	   matching agent options, and if we're relying on agent options
	   to determine the outgoing interface, drop the packet. */

	if (!good_agent_option) {
		++missing_agent_option;
		if (drop_agent_mismatches)
			return 0;
	}

	/* Adjust the length... */
	if (sp != op) {
		length = sp - ((u_int8_t *)packet);

		/* Make sure the packet isn't short (this is unlikely,
                   but WTH) */
		if (length < BOOTP_MIN_LEN) {
			memset (sp, DHO_PAD, BOOTP_MIN_LEN - length);
			length = BOOTP_MIN_LEN;
		}
	}
	return length;
}


/* Find an interface that matches the circuit ID specified in the
   Relay Agent Information option.   If one is found, store it through
   the pointer given; otherwise, leave the existing pointer alone.

   We actually deviate somewhat from the current specification here:
   if the option buffer is corrupt, we suggest that the caller not
   respond to this packet.  If the circuit ID doesn't match any known
   interface, we suggest that the caller to drop the packet.  Only if
   we find a circuit ID that matches an existing interface do we tell
   the caller to go ahead and process the packet. */

int find_interface_by_agent_option (packet, out, buf, len)
	struct dhcp_packet *packet;
	struct interface_info **out;
	u_int8_t *buf;
	int len;
{
	int i = 0;
	u_int8_t *circuit_id = 0;
	unsigned circuit_id_len = 0;
	struct interface_info *ip;

	while (i < len) {
		/* If the next agent option overflows the end of the
		   packet, the agent option buffer is corrupt. */
		if (i + 1 == len ||
		    i + buf [i + 1] + 2 > len) {
			++corrupt_agent_options;
			return -1;
		}
		switch (buf [i]) {
			/* Remember where the circuit ID is... */
		      case RAI_CIRCUIT_ID:
			circuit_id = &buf [i + 2];
			circuit_id_len = buf [i + 1];
			i += circuit_id_len + 2;
			continue;

		      default:
			i += buf [i + 1] + 2;
			break;
		}
	}

	/* If there's no circuit ID, it's not really ours, tell the caller
	   it's no good. */
	if (!circuit_id) {
		++missing_circuit_id;
		return -1;
	}

	/* Scan the interface list looking for an interface whose
	   name matches the one specified in circuit_id. */

	for (ip = interfaces; ip; ip = ip -> next) {
		if (ip -> circuit_id &&
		    ip -> circuit_id_len == circuit_id_len &&
		    !memcmp (ip -> circuit_id, circuit_id, circuit_id_len))
			break;
	}

	/* If we got a match, use it. */
	if (ip) {
		*out = ip;
		return 1;
	}

	/* If we didn't get a match, the circuit ID was bogus. */
	++bad_circuit_id;
	return -1;
}

/* Examine a packet to see if it's a candidate to have a Relay
   Agent Information option tacked onto its tail.   If it is, tack
   the option on.  */

int add_relay_agent_options (ip, packet, length, giaddr)
	struct interface_info *ip;
	struct dhcp_packet *packet;
	unsigned length;
	struct in_addr giaddr;
{
	int is_dhcp = 0, agent_options_present = 0;
	unsigned optlen;
	u_int8_t *op, *nextop, *sp, *max, *end_pad = NULL;

	/* If we're not adding agent options to packets, we can skip
	   this. */
	if (!add_agent_options)
		return length;

	/* If there's no cookie, it's a bootp packet, so we should just
	   forward it unchanged. */
	if (memcmp(packet->options, DHCP_OPTIONS_COOKIE, 4))
		return length;

	max = ((u_int8_t *)packet) + length;

	/* Commence processing after the cookie. */
	sp = op = &packet->options[4];

	while (op < max) {
		switch (*op) {
			/* Skip padding... */
		      case DHO_PAD:
			/* Remember the first pad byte so we can commandeer
			 * padded space.
			 *
			 * XXX: Is this really a good idea?  Sure, we can
			 * seemingly reduce the packet while we're looking,
			 * but if the packet was signed by the client then
			 * this padding is part of the checksum (RFC3118),
			 * and its nonpresence would break authentication.
			 */
			if (end_pad == NULL)
				end_pad = sp;

			if (sp != op)
				*sp++ = *op++;
			else
				sp = ++op;

			continue;

			/* If we see a message type, it's a DHCP packet. */
		      case DHO_DHCP_MESSAGE_TYPE:
			is_dhcp = 1;
			goto skip;
			break;

			/* Quit immediately if we hit an End option. */
		      case DHO_END:
			goto out;

		      case DHO_DHCP_AGENT_OPTIONS:
			/* We shouldn't see a relay agent option in a
			   packet before we've seen the DHCP packet type,
			   but if we do, we have to leave it alone. */
			if (!is_dhcp)
				goto skip;

			end_pad = NULL;

			/* There's already a Relay Agent Information option
			   in this packet.   How embarrassing.   Decide what
			   to do based on the mode the user specified. */

			switch (agent_relay_mode) {
			      case forward_and_append:
				goto skip;
			      case forward_untouched:
				return length;
			      case discard:
				return 0;
			      case forward_and_replace:
			      default:
				break;
			}

			/* Skip over the agent option and start copying
			   if we aren't copying already. */
			op += op[1] + 2;
			break;

		      skip:
			/* Skip over other options. */
		      default:
			/* Fail if processing this option will exceed the
			 * buffer (op[1] is malformed).
			 */
			nextop = op + op[1] + 2;
			if (nextop > max)
				return 0;

			end_pad = NULL;

			if (sp != op) {
				memmove(sp, op, op[1] + 2);
				sp += op[1] + 2;
				op = nextop;
			} else
				op = sp = nextop;

			break;
		}
	}
      out:

	/* If it's not a DHCP packet, we're not supposed to touch it. */
	if (!is_dhcp)
		return length;

	/* If the packet was padded out, we can store the agent option
	   at the beginning of the padding. */

	if (end_pad != NULL)
		sp = end_pad;

	/* Remember where the end of the packet was after parsing
	   it. */
	op = sp;

	/* Sanity check.  Had better not ever happen. */
	if ((ip->circuit_id_len > 255) || (ip->circuit_id_len < 1))
		log_fatal("circuit id length %d out of range [1-255] on "
			  "%s\n", ip->circuit_id_len, ip->name);
	optlen = ip->circuit_id_len + 2;            /* RAI_CIRCUIT_ID + len */

	if (ip->remote_id) {
		if (ip->remote_id_len > 255 || ip->remote_id_len < 1)
			log_fatal("remote id length %d out of range [1-255] "
				  "on %s\n", ip->circuit_id_len, ip->name);
		optlen += ip->remote_id_len + 2;    /* RAI_REMOTE_ID + len */
	}

	/* We do not support relay option fragmenting (multiple options to
	 * support an option data exceeding 255 bytes).
	 */
	if ((optlen < 3) || (optlen > 255))
		log_fatal ("total agent option length (%u) out of range "
			   "[3 - 255] on %s\n", optlen, ip->name);

	/* Is there room for the option, its code+len, and DHO_END? */
	if ((sp > max) || (max - sp < optlen + 3))
		return 0;

	/* Okay, cons up *our* Relay Agent Information option. */
	*sp++ = DHO_DHCP_AGENT_OPTIONS;
	*sp++ = optlen;

	/* Copy in the circuit id... */
	*sp++ = RAI_CIRCUIT_ID;
	*sp++ = ip->circuit_id_len;
	memcpy(sp, ip->circuit_id, ip->circuit_id_len);
	sp += ip->circuit_id_len;

	/* Copy in remote ID... */
	if (ip->remote_id) {
		*sp++ = RAI_REMOTE_ID;
		*sp++ = ip->remote_id_len;
		memcpy(sp, ip->remote_id, ip->remote_id_len);
		sp += ip->remote_id_len;
	}

	/* Deposit an END option. */
	*sp++ = DHO_END;

	/* Recalculate total packet length. */
	length = sp - ((u_int8_t *)packet);

	/* Make sure the packet isn't short (this is unlikely, but WTH) */
	if (length < BOOTP_MIN_LEN) {
		memset(sp, DHO_PAD, BOOTP_MIN_LEN - length);
		return BOOTP_MIN_LEN;
	}

	return length;
}