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 * $FreeBSD$
30 */
31
32/*
33 * This is a simple program which demonstrates how to query the kernel
34 * routing mechanism using only a UDP socket.  Pass it a hostname on
35 * the command line (sorry, it doesn't parse dotted decimal) and it will
36 * print out an IP address which names the interface over which UDP
37 * packets intended for that destination would be sent.
38 * A more sophisticated program might use the list obtained from SIOCGIFCONF
39 * to match the address with an interface name, but applications programmers
40 * much more often need to know the address of the interface rather than
41 * the name.
42 */
43#include <sys/types.h>
44#include <sys/socket.h>
45#include <unistd.h>
46#include <netinet/in.h>
47#include <arpa/inet.h>
48#include <stdlib.h>
49#include <stdio.h>
50#include <netdb.h>
51#include <err.h>
52#include <errno.h>
53#include <string.h>
54#include <sysexits.h>
55
56int
57main(int argc, char **argv)
58{
59	struct sockaddr_in local, remote;
60	struct hostent *hp;
61	int s, rv, namelen;
62
63	argc--, argv++;
64
65	if (!*argv) {
66		errx(EX_USAGE, "must supply a hostname");
67	}
68
69	hp = gethostbyname(*argv);
70	if (!hp) {
71		errx(EX_NOHOST, "cannot resolve hostname: %s", *argv);
72	}
73
74	memcpy(&remote.sin_addr, hp->h_addr_list[0], sizeof remote.sin_addr);
75	remote.sin_port = htons(60000);
76	remote.sin_family = AF_INET;
77	remote.sin_len = sizeof remote;
78
79	local.sin_addr.s_addr = htonl(INADDR_ANY);
80	local.sin_port = htons(60000);
81	local.sin_family = AF_INET;
82	local.sin_len = sizeof local;
83
84	s = socket(PF_INET, SOCK_DGRAM, 0);
85	if (s < 0)
86		err(EX_OSERR, "socket");
87
88	do {
89		rv = bind(s, (struct sockaddr *)&local, sizeof local);
90		local.sin_port = htons(ntohs(local.sin_port) + 1);
91	} while(rv < 0 && errno == EADDRINUSE);
92
93	if (rv < 0)
94		err(EX_OSERR, "bind");
95
96	do {
97		rv = connect(s, (struct sockaddr *)&remote, sizeof remote);
98		remote.sin_port = htons(ntohs(remote.sin_port) + 1);
99	} while(rv < 0 && errno == EADDRINUSE);
100
101	if (rv < 0)
102		err(EX_OSERR, "connect");
103
104	namelen = sizeof local;
105	rv = getsockname(s, (struct sockaddr *)&local, &namelen);
106	if (rv < 0)
107		err(EX_OSERR, "getsockname");
108
109	printf("Route to %s is out %s\n", *argv, inet_ntoa(local.sin_addr));
110	return 0;
111}
112