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Editing: 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. * * $FreeBSD$ */ /* * 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; }
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