networking/udhcp/dhcprelay.c - codeaurora/busybox - Gitiles

 /* vi: set sw=4 ts=4: */
 /*
  * Port to Busybox Copyright (C) 2006 Jesse Dutton <jessedutton@gmail.com>
  *
  * Licensed under GPLv2, see file LICENSE in this source tree.
  *
  * DHCP Relay for 'DHCPv4 Configuration of IPSec Tunnel Mode' support
  * Copyright (C) 2002 Mario Strasser <mast@gmx.net>,
  *                   Zuercher Hochschule Winterthur,
  *                   Netbeat AG
  * Upstream has GPL v2 or later
  */
 //applet:IF_DHCPRELAY(APPLET(dhcprelay, BB_DIR_USR_SBIN, BB_SUID_DROP))

 //kbuild:lib-$(CONFIG_DHCPRELAY) += dhcprelay.o

 //usage:#define dhcprelay_trivial_usage
 //usage:       "CLIENT_IFACE[,CLIENT_IFACE2]... SERVER_IFACE [SERVER_IP]"
 //usage:#define dhcprelay_full_usage "\n\n"
 //usage:       "Relay DHCP requests between clients and server"

 #include "common.h"

 #define SERVER_PORT    67

 /* lifetime of an xid entry in sec. */
 #define MAX_LIFETIME   2*60
 /* select timeout in sec. */
 #define SELECT_TIMEOUT (MAX_LIFETIME / 8)

 /* This list holds information about clients. The xid_* functions manipulate this list. */
 struct xid_item {
 	unsigned timestamp;
 	int client;
 	uint32_t xid;
 	struct sockaddr_in ip;
 	struct xid_item *next;
 } FIX_ALIASING;

 #define dhcprelay_xid_list (*(struct xid_item*)bb_common_bufsiz1)
 #define INIT_G() do { setup_common_bufsiz(); } while (0)

 static struct xid_item *xid_add(uint32_t xid, struct sockaddr_in *ip, int client)
 {
 	struct xid_item *item;

 	/* create new xid entry */
 	item = xmalloc(sizeof(struct xid_item));

 	/* add xid entry */
 	item->ip = *ip;
 	item->xid = xid;
 	item->client = client;
 	item->timestamp = monotonic_sec();
 	item->next = dhcprelay_xid_list.next;
 	dhcprelay_xid_list.next = item;

 	return item;
 }

 static void xid_expire(void)
 {
 	struct xid_item *item = dhcprelay_xid_list.next;
 	struct xid_item *last = &dhcprelay_xid_list;
 	unsigned current_time = monotonic_sec();

 	while (item != NULL) {
 		if ((current_time - item->timestamp) > MAX_LIFETIME) {
 			last->next = item->next;
 			free(item);
 			item = last->next;
 		} else {
 			last = item;
 			item = item->next;
 		}
 	}
 }

 static struct xid_item *xid_find(uint32_t xid)
 {
 	struct xid_item *item = dhcprelay_xid_list.next;
 	while (item != NULL) {
 		if (item->xid == xid) {
 			break;
 		}
 		item = item->next;
 	}
 	return item;
 }

 static void xid_del(uint32_t xid)
 {
 	struct xid_item *item = dhcprelay_xid_list.next;
 	struct xid_item *last = &dhcprelay_xid_list;
 	while (item != NULL) {
 		if (item->xid == xid) {
 			last->next = item->next;
 			free(item);
 			item = last->next;
 		} else {
 			last = item;
 			item = item->next;
 		}
 	}
 }

 /**
  * get_dhcp_packet_type - gets the message type of a dhcp packet
  * p - pointer to the dhcp packet
  * returns the message type on success, -1 otherwise
  */
 static int get_dhcp_packet_type(struct dhcp_packet *p)
 {
 	uint8_t *op;

 	/* it must be either a BOOTREQUEST or a BOOTREPLY */
 	if (p->op != BOOTREQUEST && p->op != BOOTREPLY)
 		return -1;
 	/* get message type option */
 	op = udhcp_get_option(p, DHCP_MESSAGE_TYPE);
 	if (op != NULL)
 		return op[0];
 	return -1;
 }

 /**
  * make_iface_list - parses client/server interface names
  * returns array
  */
 static char **make_iface_list(char **client_and_server_ifaces, int *client_number)
 {
 	char *s, **iface_list;
 	int i, cn;

 	/* get number of items */
 	cn = 2; /* 1 server iface + at least 1 client one */
 	s = client_and_server_ifaces[0]; /* list of client ifaces */
 	while (*s) {
 		if (*s == ',')
 			cn++;
 		s++;
 	}
 	*client_number = cn;

 	/* create vector of pointers */
 	iface_list = xzalloc(cn * sizeof(iface_list[0]));

 	iface_list[0] = client_and_server_ifaces[1]; /* server iface */

 	i = 1;
 	s = xstrdup(client_and_server_ifaces[0]); /* list of client ifaces */
 	goto store_client_iface_name;

 	while (i < cn) {
 		if (*s++ == ',') {
 			s[-1] = '\0';
  store_client_iface_name:
 			iface_list[i++] = s;
 		}
 	}

 	return iface_list;
 }

 /* Creates listen sockets (in fds) bound to client and server ifaces,
  * and returns numerically max fd.
  */
 static int init_sockets(char **iface_list, int num_clients, int *fds)
 {
 	int i, n;

 	n = 0;
 	for (i = 0; i < num_clients; i++) {
 		fds[i] = udhcp_listen_socket(/*INADDR_ANY,*/ SERVER_PORT, iface_list[i]);
 		if (n < fds[i])
 			n = fds[i];
 	}
 	return n;
 }

 static int sendto_ip4(int sock, const void *msg, int msg_len, struct sockaddr_in *to)
 {
 	int err;

 	errno = 0;
 	err = sendto(sock, msg, msg_len, 0, (struct sockaddr*) to, sizeof(*to));
 	err -= msg_len;
 	if (err)
 		bb_perror_msg("sendto");
 	return err;
 }

 /**
  * pass_to_server() - forwards dhcp packets from client to server
  * p - packet to send
  * client - number of the client
  */
 static void pass_to_server(struct dhcp_packet *p, int packet_len, int client, int *fds,
 			struct sockaddr_in *client_addr, struct sockaddr_in *server_addr)
 {
 	int type;

 	/* check packet_type */
 	type = get_dhcp_packet_type(p);
 	if (type != DHCPDISCOVER && type != DHCPREQUEST
 	 && type != DHCPDECLINE && type != DHCPRELEASE
 	 && type != DHCPINFORM
 	) {
 		return;
 	}

 	/* create new xid entry */
 	xid_add(p->xid, client_addr, client);

 	/* forward request to server */
 	/* note that we send from fds[0] which is bound to SERVER_PORT (67).
 	 * IOW: we send _from_ SERVER_PORT! Although this may look strange,
 	 * RFC 1542 not only allows, but prescribes this for BOOTP relays.
 	 */
 	sendto_ip4(fds[0], p, packet_len, server_addr);
 }

 /**
  * pass_to_client() - forwards dhcp packets from server to client
  * p - packet to send
  */
 static void pass_to_client(struct dhcp_packet *p, int packet_len, int *fds)
 {
 	int type;
 	struct xid_item *item;

 	/* check xid */
 	item = xid_find(p->xid);
 	if (!item) {
 		return;
 	}

 	/* check packet type */
 	type = get_dhcp_packet_type(p);
 	if (type != DHCPOFFER && type != DHCPACK && type != DHCPNAK) {
 		return;
 	}

 //TODO: also do it if (p->flags & htons(BROADCAST_FLAG)) is set!
 	if (item->ip.sin_addr.s_addr == htonl(INADDR_ANY))
 		item->ip.sin_addr.s_addr = htonl(INADDR_BROADCAST);

 	if (sendto_ip4(fds[item->client], p, packet_len, &item->ip) != 0) {
 		return; /* send error occurred */
 	}

 	/* remove xid entry */
 	xid_del(p->xid);
 }

 int dhcprelay_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int dhcprelay_main(int argc UNUSED_PARAM, char **argv)
 {
 	struct sockaddr_in server_addr;
 	char **iface_list;
 	int *fds;
 	int num_sockets, max_socket;
 	uint32_t our_nip;

 	INIT_G();

 	server_addr.sin_family = AF_INET;
 	server_addr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
 	server_addr.sin_port = htons(SERVER_PORT);

 	/* dhcprelay CLIENT_IFACE1[,CLIENT_IFACE2...] SERVER_IFACE [SERVER_IP] */
 	if (!argv[1] || !argv[2])
 		bb_show_usage();
 	if (argv[3]) {
 		if (!inet_aton(argv[3], &server_addr.sin_addr))
 			bb_perror_msg_and_die("bad server IP");
 	}

 	iface_list = make_iface_list(argv + 1, &num_sockets);

 	fds = xmalloc(num_sockets * sizeof(fds[0]));

 	/* Create sockets and bind one to every iface */
 	max_socket = init_sockets(iface_list, num_sockets, fds);

 	/* Get our IP on server_iface */
 	if (udhcp_read_interface(argv[2], NULL, &our_nip, NULL))
 		return 1;

 	/* Main loop */
 	while (1) {
 // reinit stuff from time to time? go back to make_iface_list
 // every N minutes?
 		fd_set rfds;
 		struct timeval tv;
 		int i;

 		FD_ZERO(&rfds);
 		for (i = 0; i < num_sockets; i++)
 			FD_SET(fds[i], &rfds);
 		tv.tv_sec = SELECT_TIMEOUT;
 		tv.tv_usec = 0;
 		if (select(max_socket + 1, &rfds, NULL, NULL, &tv) > 0) {
 			int packlen;
 			struct dhcp_packet dhcp_msg;

 			/* server */
 			if (FD_ISSET(fds[0], &rfds)) {
 				packlen = udhcp_recv_kernel_packet(&dhcp_msg, fds[0]);
 				if (packlen > 0) {
 					pass_to_client(&dhcp_msg, packlen, fds);
 				}
 			}

 			/* clients */
 			for (i = 1; i < num_sockets; i++) {
 				struct sockaddr_in client_addr;
 				socklen_t addr_size;

 				if (!FD_ISSET(fds[i], &rfds))
 					continue;

 				addr_size = sizeof(client_addr);
 				packlen = recvfrom(fds[i], &dhcp_msg, sizeof(dhcp_msg), 0,
 						(struct sockaddr *)(&client_addr), &addr_size);
 				if (packlen <= 0)
 					continue;

 				/* Get our IP on corresponding client_iface */
 // RFC 1542
 // 4.1 General BOOTP Processing for Relay Agents
 // 4.1.1 BOOTREQUEST Messages
 //   If the relay agent does decide to relay the request, it MUST examine
 //   the 'giaddr' ("gateway" IP address) field.  If this field is zero,
 //   the relay agent MUST fill this field with the IP address of the
 //   interface on which the request was received.  If the interface has
 //   more than one IP address logically associated with it, the relay
 //   agent SHOULD choose one IP address associated with that interface and
 //   use it consistently for all BOOTP messages it relays.  If the
 //   'giaddr' field contains some non-zero value, the 'giaddr' field MUST
 //   NOT be modified.  The relay agent MUST NOT, under any circumstances,
 //   fill the 'giaddr' field with a broadcast address as is suggested in
 //   [1] (Section 8, sixth paragraph).

 // but why? what if server can't route such IP? Client ifaces may be, say, NATed!

 // 4.1.2 BOOTREPLY Messages
 //   BOOTP relay agents relay BOOTREPLY messages only to BOOTP clients.
 //   It is the responsibility of BOOTP servers to send BOOTREPLY messages
 //   directly to the relay agent identified in the 'giaddr' field.
 // (yeah right, unless it is impossible... see comment above)
 //   Therefore, a relay agent may assume that all BOOTREPLY messages it
 //   receives are intended for BOOTP clients on its directly-connected
 //   networks.
 //
 //   When a relay agent receives a BOOTREPLY message, it should examine
 //   the BOOTP 'giaddr', 'yiaddr', 'chaddr', 'htype', and 'hlen' fields.
 //   These fields should provide adequate information for the relay agent
 //   to deliver the BOOTREPLY message to the client.
 //
 //   The 'giaddr' field can be used to identify the logical interface from
 //   which the reply must be sent (i.e., the host or router interface
 //   connected to the same network as the BOOTP client).  If the content
 //   of the 'giaddr' field does not match one of the relay agent's
 //   directly-connected logical interfaces, the BOOTREPLY message MUST be
 //   silently discarded.
 				if (udhcp_read_interface(iface_list[i], NULL, &dhcp_msg.gateway_nip, NULL)) {
 					/* Fall back to our IP on server iface */
 // this makes more sense!
 					dhcp_msg.gateway_nip = our_nip;
 				}
 // maybe dhcp_msg.hops++? drop packets with too many hops (RFC 1542 says 4 or 16)?
 				pass_to_server(&dhcp_msg, packlen, i, fds, &client_addr, &server_addr);
 			}
 		}
 		xid_expire();
 	} /* while (1) */

 	/* return 0; - not reached */
 }
	/* vi: set sw=4 ts=4: */
	/*
	* Port to Busybox Copyright (C) 2006 Jesse Dutton <jessedutton@gmail.com>
	*
	* Licensed under GPLv2, see file LICENSE in this source tree.
	*
	* DHCP Relay for 'DHCPv4 Configuration of IPSec Tunnel Mode' support
	* Copyright (C) 2002 Mario Strasser <mast@gmx.net>,
	* Zuercher Hochschule Winterthur,
	* Netbeat AG
	* Upstream has GPL v2 or later
	*/
	//applet:IF_DHCPRELAY(APPLET(dhcprelay, BB_DIR_USR_SBIN, BB_SUID_DROP))

	//kbuild:lib-$(CONFIG_DHCPRELAY) += dhcprelay.o

	//usage:#define dhcprelay_trivial_usage
	//usage: "CLIENT_IFACE[,CLIENT_IFACE2]... SERVER_IFACE [SERVER_IP]"
	//usage:#define dhcprelay_full_usage "\n\n"
	//usage: "Relay DHCP requests between clients and server"

	#include "common.h"

	#define SERVER_PORT 67

	/* lifetime of an xid entry in sec. */
	#define MAX_LIFETIME 2*60
	/* select timeout in sec. */
	#define SELECT_TIMEOUT (MAX_LIFETIME / 8)

	/* This list holds information about clients. The xid_* functions manipulate this list. */
	struct xid_item {
	unsigned timestamp;
	int client;
	uint32_t xid;
	struct sockaddr_in ip;
	struct xid_item *next;
	} FIX_ALIASING;

	#define dhcprelay_xid_list ((struct xid_item)bb_common_bufsiz1)
	#define INIT_G() do { setup_common_bufsiz(); } while (0)

	static struct xid_item xid_add(uint32_t xid, struct sockaddr_in ip, int client)
	{
	struct xid_item *item;

	/* create new xid entry */
	item = xmalloc(sizeof(struct xid_item));

	/* add xid entry */
	item->ip = *ip;
	item->xid = xid;
	item->client = client;
	item->timestamp = monotonic_sec();
	item->next = dhcprelay_xid_list.next;
	dhcprelay_xid_list.next = item;

	return item;
	}

	static void xid_expire(void)
	{
	struct xid_item *item = dhcprelay_xid_list.next;
	struct xid_item *last = &dhcprelay_xid_list;
	unsigned current_time = monotonic_sec();

	while (item != NULL) {
	if ((current_time - item->timestamp) > MAX_LIFETIME) {
	last->next = item->next;
	free(item);
	item = last->next;
	} else {
	last = item;
	item = item->next;
	}
	}
	}

	static struct xid_item *xid_find(uint32_t xid)
	{
	struct xid_item *item = dhcprelay_xid_list.next;
	while (item != NULL) {
	if (item->xid == xid) {
	break;
	}
	item = item->next;
	}
	return item;
	}

	static void xid_del(uint32_t xid)
	{
	struct xid_item *item = dhcprelay_xid_list.next;
	struct xid_item *last = &dhcprelay_xid_list;
	while (item != NULL) {
	if (item->xid == xid) {
	last->next = item->next;
	free(item);
	item = last->next;
	} else {
	last = item;
	item = item->next;
	}
	}
	}

	/**
	* get_dhcp_packet_type - gets the message type of a dhcp packet
	* p - pointer to the dhcp packet
	* returns the message type on success, -1 otherwise
	*/
	static int get_dhcp_packet_type(struct dhcp_packet *p)
	{
	uint8_t *op;

	/* it must be either a BOOTREQUEST or a BOOTREPLY */
	if (p->op != BOOTREQUEST && p->op != BOOTREPLY)
	return -1;
	/* get message type option */
	op = udhcp_get_option(p, DHCP_MESSAGE_TYPE);
	if (op != NULL)
	return op[0];
	return -1;
	}

	/**
	* make_iface_list - parses client/server interface names
	* returns array
	*/
	static char make_iface_list(char client_and_server_ifaces, int *client_number)
	{
	char s, *iface_list;
	int i, cn;

	/* get number of items */
	cn = 2; /* 1 server iface + at least 1 client one */
	s = client_and_server_ifaces[0]; /* list of client ifaces */
	while (*s) {
	if (*s == ',')
	cn++;
	s++;
	}
	*client_number = cn;

	/* create vector of pointers */
	iface_list = xzalloc(cn * sizeof(iface_list[0]));

	iface_list[0] = client_and_server_ifaces[1]; /* server iface */

	i = 1;
	s = xstrdup(client_and_server_ifaces[0]); /* list of client ifaces */
	goto store_client_iface_name;

	while (i < cn) {
	if (*s++ == ',') {
	s[-1] = '\0';
	store_client_iface_name:
	iface_list[i++] = s;
	}
	}

	return iface_list;
	}

	/* Creates listen sockets (in fds) bound to client and server ifaces,
	* and returns numerically max fd.
	*/
	static int init_sockets(char *iface_list, int num_clients, int fds)
	{
	int i, n;

	n = 0;
	for (i = 0; i < num_clients; i++) {
	fds[i] = udhcp_listen_socket(/INADDR_ANY,/ SERVER_PORT, iface_list[i]);
	if (n < fds[i])
	n = fds[i];
	}
	return n;
	}

	static int sendto_ip4(int sock, const void msg, int msg_len, struct sockaddr_in to)
	{
	int err;

	errno = 0;
	err = sendto(sock, msg, msg_len, 0, (struct sockaddr) to, sizeof(to));
	err -= msg_len;
	if (err)
	bb_perror_msg("sendto");
	return err;
	}

	/**
	* pass_to_server() - forwards dhcp packets from client to server
	* p - packet to send
	* client - number of the client
	*/
	static void pass_to_server(struct dhcp_packet p, int packet_len, int client, int fds,
	struct sockaddr_in client_addr, struct sockaddr_in server_addr)
	{
	int type;

	/* check packet_type */
	type = get_dhcp_packet_type(p);
	if (type != DHCPDISCOVER && type != DHCPREQUEST
	&& type != DHCPDECLINE && type != DHCPRELEASE
	&& type != DHCPINFORM
	) {
	return;
	}

	/* create new xid entry */
	xid_add(p->xid, client_addr, client);

	/* forward request to server */
	/* note that we send from fds[0] which is bound to SERVER_PORT (67).
	* IOW: we send _from_ SERVER_PORT! Although this may look strange,
	* RFC 1542 not only allows, but prescribes this for BOOTP relays.
	*/
	sendto_ip4(fds[0], p, packet_len, server_addr);
	}

	/**
	* pass_to_client() - forwards dhcp packets from server to client
	* p - packet to send
	*/
	static void pass_to_client(struct dhcp_packet p, int packet_len, int fds)
	{
	int type;
	struct xid_item *item;

	/* check xid */
	item = xid_find(p->xid);
	if (!item) {
	return;
	}

	/* check packet type */
	type = get_dhcp_packet_type(p);
	if (type != DHCPOFFER && type != DHCPACK && type != DHCPNAK) {
	return;
	}

	//TODO: also do it if (p->flags & htons(BROADCAST_FLAG)) is set!
	if (item->ip.sin_addr.s_addr == htonl(INADDR_ANY))
	item->ip.sin_addr.s_addr = htonl(INADDR_BROADCAST);

	if (sendto_ip4(fds[item->client], p, packet_len, &item->ip) != 0) {
	return; /* send error occurred */
	}

	/* remove xid entry */
	xid_del(p->xid);
	}

	int dhcprelay_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
	int dhcprelay_main(int argc UNUSED_PARAM, char **argv)
	{
	struct sockaddr_in server_addr;
	char **iface_list;
	int *fds;
	int num_sockets, max_socket;
	uint32_t our_nip;

	INIT_G();

	server_addr.sin_family = AF_INET;
	server_addr.sin_addr.s_addr = htonl(INADDR_BROADCAST);
	server_addr.sin_port = htons(SERVER_PORT);

	/* dhcprelay CLIENT_IFACE1[,CLIENT_IFACE2...] SERVER_IFACE [SERVER_IP] */
	if (!argv[1] \|\| !argv[2])
	bb_show_usage();
	if (argv[3]) {
	if (!inet_aton(argv[3], &server_addr.sin_addr))
	bb_perror_msg_and_die("bad server IP");
	}

	iface_list = make_iface_list(argv + 1, &num_sockets);

	fds = xmalloc(num_sockets * sizeof(fds[0]));

	/* Create sockets and bind one to every iface */
	max_socket = init_sockets(iface_list, num_sockets, fds);

	/* Get our IP on server_iface */
	if (udhcp_read_interface(argv[2], NULL, &our_nip, NULL))
	return 1;

	/* Main loop */
	while (1) {
	// reinit stuff from time to time? go back to make_iface_list
	// every N minutes?
	fd_set rfds;
	struct timeval tv;
	int i;

	FD_ZERO(&rfds);
	for (i = 0; i < num_sockets; i++)
	FD_SET(fds[i], &rfds);
	tv.tv_sec = SELECT_TIMEOUT;
	tv.tv_usec = 0;
	if (select(max_socket + 1, &rfds, NULL, NULL, &tv) > 0) {
	int packlen;
	struct dhcp_packet dhcp_msg;

	/* server */
	if (FD_ISSET(fds[0], &rfds)) {
	packlen = udhcp_recv_kernel_packet(&dhcp_msg, fds[0]);
	if (packlen > 0) {
	pass_to_client(&dhcp_msg, packlen, fds);
	}
	}

	/* clients */
	for (i = 1; i < num_sockets; i++) {
	struct sockaddr_in client_addr;
	socklen_t addr_size;

	if (!FD_ISSET(fds[i], &rfds))
	continue;

	addr_size = sizeof(client_addr);
	packlen = recvfrom(fds[i], &dhcp_msg, sizeof(dhcp_msg), 0,
	(struct sockaddr *)(&client_addr), &addr_size);
	if (packlen <= 0)
	continue;

	/* Get our IP on corresponding client_iface */
	// RFC 1542
	// 4.1 General BOOTP Processing for Relay Agents
	// 4.1.1 BOOTREQUEST Messages
	// If the relay agent does decide to relay the request, it MUST examine
	// the 'giaddr' ("gateway" IP address) field. If this field is zero,
	// the relay agent MUST fill this field with the IP address of the
	// interface on which the request was received. If the interface has
	// more than one IP address logically associated with it, the relay
	// agent SHOULD choose one IP address associated with that interface and
	// use it consistently for all BOOTP messages it relays. If the
	// 'giaddr' field contains some non-zero value, the 'giaddr' field MUST
	// NOT be modified. The relay agent MUST NOT, under any circumstances,
	// fill the 'giaddr' field with a broadcast address as is suggested in
	// [1] (Section 8, sixth paragraph).

	// but why? what if server can't route such IP? Client ifaces may be, say, NATed!

	// 4.1.2 BOOTREPLY Messages
	// BOOTP relay agents relay BOOTREPLY messages only to BOOTP clients.
	// It is the responsibility of BOOTP servers to send BOOTREPLY messages
	// directly to the relay agent identified in the 'giaddr' field.
	// (yeah right, unless it is impossible... see comment above)
	// Therefore, a relay agent may assume that all BOOTREPLY messages it
	// receives are intended for BOOTP clients on its directly-connected
	// networks.
	//
	// When a relay agent receives a BOOTREPLY message, it should examine
	// the BOOTP 'giaddr', 'yiaddr', 'chaddr', 'htype', and 'hlen' fields.
	// These fields should provide adequate information for the relay agent
	// to deliver the BOOTREPLY message to the client.
	//
	// The 'giaddr' field can be used to identify the logical interface from
	// which the reply must be sent (i.e., the host or router interface
	// connected to the same network as the BOOTP client). If the content
	// of the 'giaddr' field does not match one of the relay agent's
	// directly-connected logical interfaces, the BOOTREPLY message MUST be
	// silently discarded.
	if (udhcp_read_interface(iface_list[i], NULL, &dhcp_msg.gateway_nip, NULL)) {
	/* Fall back to our IP on server iface */
	// this makes more sense!
	dhcp_msg.gateway_nip = our_nip;
	}
	// maybe dhcp_msg.hops++? drop packets with too many hops (RFC 1542 says 4 or 16)?
	pass_to_server(&dhcp_msg, packlen, i, fds, &client_addr, &server_addr);
	}
	}
	xid_expire();
	} /* while (1) */

	/* return 0; - not reached */
	}