1/*
2 * Copyright 1994, 1995 Massachusetts Institute of Technology
3 *
4 * Permission to use, copy, modify, and distribute this software and
5 * its documentation for any purpose and without fee is hereby
6 * granted, provided that both the above copyright notice and this
7 * permission notice appear in all copies, that both the above
8 * copyright notice and this permission notice appear in all
9 * supporting documentation, and that the name of M.I.T. not be used
10 * in advertising or publicity pertaining to distribution of the
11 * software without specific, written prior permission.  M.I.T. makes
12 * no representations about the suitability of this software for any
13 * purpose.  It is provided "as is" without express or implied
14 * warranty.
15 *
16 * THIS SOFTWARE IS PROVIDED BY M.I.T. ``AS IS''.  M.I.T. DISCLAIMS
17 * ALL EXPRESS OR IMPLIED WARRANTIES WITH REGARD TO THIS SOFTWARE,
18 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
19 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
20 * SHALL M.I.T. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
23 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
24 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
25 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
26 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 * SUCH DAMAGE.
28 */
29
30/*
31 * This is a simple program which demonstrates how to query the kernel
32 * routing mechanism using only a UDP socket.  Pass it a hostname on
33 * the command line (sorry, it doesn't parse dotted decimal) and it will
34 * print out an IP address which names the interface over which UDP
35 * packets intended for that destination would be sent.
36 * A more sophisticated program might use the list obtained from SIOCGIFCONF
37 * to match the address with an interface name, but applications programmers
38 * much more often need to know the address of the interface rather than
39 * the name.
40 */
41#include <sys/types.h>
42#include <sys/socket.h>
43#include <unistd.h>
44#include <netinet/in.h>
45#include <arpa/inet.h>
46#include <stdlib.h>
47#include <stdio.h>
48#include <netdb.h>
49#include <err.h>
50#include <errno.h>
51#include <string.h>
52#include <sysexits.h>
53
54int
55main(int argc, char **argv)
56{
57	struct sockaddr_in local, remote;
58	struct hostent *hp;
59	int s, rv, namelen;
60
61	argc--, argv++;
62
63	if (!*argv) {
64		errx(EX_USAGE, "must supply a hostname");
65	}
66
67	hp = gethostbyname(*argv);
68	if (!hp) {
69		errx(EX_NOHOST, "cannot resolve hostname: %s", *argv);
70	}
71
72	memcpy(&remote.sin_addr, hp->h_addr_list[0], sizeof remote.sin_addr);
73	remote.sin_port = htons(60000);
74	remote.sin_family = AF_INET;
75	remote.sin_len = sizeof remote;
76
77	local.sin_addr.s_addr = htonl(INADDR_ANY);
78	local.sin_port = htons(60000);
79	local.sin_family = AF_INET;
80	local.sin_len = sizeof local;
81
82	s = socket(PF_INET, SOCK_DGRAM, 0);
83	if (s < 0)
84		err(EX_OSERR, "socket");
85
86	do {
87		rv = bind(s, (struct sockaddr *)&local, sizeof local);
88		local.sin_port = htons(ntohs(local.sin_port) + 1);
89	} while(rv < 0 && errno == EADDRINUSE);
90
91	if (rv < 0)
92		err(EX_OSERR, "bind");
93
94	do {
95		rv = connect(s, (struct sockaddr *)&remote, sizeof remote);
96		remote.sin_port = htons(ntohs(remote.sin_port) + 1);
97	} while(rv < 0 && errno == EADDRINUSE);
98
99	if (rv < 0)
100		err(EX_OSERR, "connect");
101
102	namelen = sizeof local;
103	rv = getsockname(s, (struct sockaddr *)&local, &namelen);
104	if (rv < 0)
105		err(EX_OSERR, "getsockname");
106
107	printf("Route to %s is out %s\n", *argv, inet_ntoa(local.sin_addr));
108	return 0;
109}
110