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 
56 int
57 main(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