This patch against arpwatch-2.1a11 enables arpwatch to listen on an interface
that has no IPv4 assigned address


diff -Naur arpwatch-2.1a11/arpwatch.8 arpwatch-2.1a11.oden/arpwatch.8
--- arpwatch-2.1a11/arpwatch.8	2000-10-08 22:31:28.000000000 +0200
+++ arpwatch-2.1a11.oden/arpwatch.8	2004-06-01 02:06:34.396094208 +0200
@@ -27,7 +27,7 @@
 .na
 .B arpwatch
 [
-.B -dN
+.B -dNw
 ] [
 .B -f
 .I datafile
@@ -83,6 +83,10 @@
 flag disables reporting any bogons.
 .LP
 The
+.B -w
+flag is used to specify an interface without a valid IPv4 address.
+.LP
+The
 .B -r
 flag is used to specify a savefile
 (perhaps created by
diff -Naur arpwatch-2.1a11/arpwatch.c arpwatch-2.1a11.oden/arpwatch.c
--- arpwatch-2.1a11/arpwatch.c	2000-10-14 04:07:35.000000000 +0200
+++ arpwatch-2.1a11.oden/arpwatch.c	2004-06-01 02:06:34.397094056 +0200
@@ -145,7 +145,7 @@
 main(int argc, char **argv)
 {
 	register char *cp;
-	register int op, pid, snaplen, timeout, linktype, status;
+	register int op, pid, snaplen, timeout, linktype, status, noip = 0;
 #ifdef TIOCNOTTY
 	register int fd;
 #endif
@@ -170,7 +170,7 @@
 	interface = NULL;
 	rfilename = NULL;
 	pd = NULL;
-	while ((op = getopt(argc, argv, "df:i:n:Nr:")) != EOF)
+	while ((op = getopt(argc, argv, "df:i:n:wNr:")) != EOF)
 		switch (op) {
 
 		case 'd':
@@ -194,6 +194,10 @@
 				usage();
 			break;
 
+		case 'w':
+			++noip;
+			break;
+
 		case 'N':
 			++nobogons;
 			break;
@@ -221,12 +225,19 @@
 			exit(1);
 		}
 
+		if(noip){
+		/* Be able to sniff on interfaces without address */
+		net = 0;
+		netmask = 0;
+		} else {
 		/* Determine network and netmask */
-		if (pcap_lookupnet(interface, &net, &netmask, errbuf) < 0) {
-			(void)fprintf(stderr, "%s: bad interface %s: %s\n",
-			    prog, interface, errbuf);
-			exit(1);
+			if (pcap_lookupnet(interface, &net, &netmask, errbuf) < 0) {
+				(void)fprintf(stderr, "%s: bad interface %s: %s\n",
+				    prog, interface, errbuf);
+				exit(1);
+			}
 		}
+		
 
 		/* Drop into the background if not debugging */
 		if (!debug) {
@@ -750,7 +761,7 @@
 	extern char version[];
 
 	(void)fprintf(stderr, "Version %s\n", version);
-	(void)fprintf(stderr, "usage: %s [-dN] [-f datafile] [-i interface]"
+	(void)fprintf(stderr, "usage: %s [-dNw] [-f datafile] [-i interface]"
 	    " [-n net[/width]] [-r file]\n", prog);
 	exit(1);
 }