find_interface.c

/*
 * Copyright 1994, 1995 Massachusetts Institute of Technology
 *
 * Permission to use, copy, modify, and distribute this software and
 * its documentation for any purpose and without fee is hereby
 * granted, provided that both the above copyright notice and this
 * permission notice appear in all copies, that both the above
 * copyright notice and this permission notice appear in all
 * supporting documentation, and that the name of M.I.T. not be used
 * in advertising or publicity pertaining to distribution of the
 * software without specific, written prior permission.  M.I.T. makes
 * no representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied
 * warranty.
 * 
 * THIS SOFTWARE IS PROVIDED BY M.I.T. ``AS IS''.  M.I.T. DISCLAIMS
 * ALL EXPRESS OR IMPLIED WARRANTIES WITH REGARD TO THIS SOFTWARE,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
 * SHALL M.I.T. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * This is a simple program which demonstrates how to query the kernel
 * routing mechanism using only a UDP socket.  Pass it a hostname on
 * the command line (sorry, it doesn't parse dotted decimal) and it will
 * print out an IP address which names the interface over which UDP
 * packets intended for that destination would be sent.
 * A more sophisticated program might use the list obtained from SIOCGIFCONF
 * to match the address with an interface name, but applications programmers
 * much more often need to know the address of the interface rather than
 * the name.
 */
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <stdio.h>
#include <netdb.h>
#include <err.h>
#include <errno.h>
#include <string.h>
#include <sysexits.h>

int
main(int argc, char **argv)
{
	struct sockaddr_in local, remote;
	struct hostent *hp;
	int s, rv, namelen;

	argc--, argv++;

	if (!*argv) {
		errx(EX_USAGE, "must supply a hostname");
	}

	hp = gethostbyname(*argv);
	if (!hp) {
		errx(EX_NOHOST, "cannot resolve hostname: %s", *argv);
	}

	memcpy(&remote.sin_addr, hp->h_addr_list[0], sizeof remote.sin_addr);
	remote.sin_port = htons(60000);
	remote.sin_family = AF_INET;
	remote.sin_len = sizeof remote;

	local.sin_addr.s_addr = htonl(INADDR_ANY);
	local.sin_port = htons(60000);
	local.sin_family = AF_INET;
	local.sin_len = sizeof local;

	s = socket(PF_INET, SOCK_DGRAM, 0);
	if (s < 0)
		err(EX_OSERR, "socket");

	do {
		rv = bind(s, (struct sockaddr *)&local, sizeof local);
		local.sin_port = htons(ntohs(local.sin_port) + 1);
	} while(rv < 0 && errno == EADDRINUSE);

	if (rv < 0)
		err(EX_OSERR, "bind");

	do {
		rv = connect(s, (struct sockaddr *)&remote, sizeof remote);
		remote.sin_port = htons(ntohs(remote.sin_port) + 1);
	} while(rv < 0 && errno == EADDRINUSE);

	if (rv < 0)
		err(EX_OSERR, "connect");

	namelen = sizeof local;
	rv = getsockname(s, (struct sockaddr *)&local, &namelen);
	if (rv < 0)
		err(EX_OSERR, "getsockname");

	printf("Route to %s is out %s\n", *argv, inet_ntoa(local.sin_addr));
	return 0;
}