Added a copy of my little program to find the IP address of the outgoing

interface for a particular remote host.

Requested by: Joerg Wunsch <j@uriah.heep.sax.de>

1 files changed, 111 insertions, 0 deletions

diff --git a/share/examples/find_interface/find_interface.c b/share/examples/find_interface/find_interface.c
new file mode 100644
index 0000000..50044d6
--- /dev/null
+++ b/share/examples/find_interface/find_interface.c
@@ -0,0 +1,111 @@
+/*
+ * 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.
+ *
+ * $Id$
+ */
+
+/*
+ * 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++;
+	} while(rv < 0 && errno == EADDRINUSE);
+
+	if (rv < 0)
+		err(EX_OSERR, "bind");
+
+	do {
+		rv = connect(s, (struct sockaddr *)&remote, sizeof remote);
+		remote.sin_port++;
+	} 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;
+}