share/examples/find_interface/find_interface.c

   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 }