Note by jsynacek:

This patch should be rewritten. There's no reason to be using a static variable

and returning its content from a function (in iterate_dev()). Also, some things

should be simplified (like iterate_dev()).

diff -up ./arpwatch.c.iselect ./arpwatch.c

--- ./arpwatch.c.iselect	2012-10-15 16:01:24.701335291 +0200

+++ ./arpwatch.c	2012-10-15 16:07:18.626322639 +0200

@@ -162,50 +162,52 @@ void dropprivileges(const char* user)

 }

 char *

-get_first_dev(pcap_t **pd, int *linktype, char *errbuf)

+try_dev(char *interface, pcap_t **pd, int *linktype, char *errbuf)

 {

-	static char interface[IF_NAMESIZE + 1];

 	register int snaplen, timeout;

-	pcap_if_t *alldevs;

-	pcap_if_t *dev;

-	char *ret = NULL;

 	snaplen = max(sizeof(struct ether_header),

 				  sizeof(struct fddi_header)) + sizeof(struct ether_arp);

 	timeout = 1000;

-	if (pcap_findalldevs(&alldevs, errbuf) == -1) {

-		(void)fprintf(stderr, "%s: lookup_device: %s\n",

-					  prog, errbuf);

-		exit(1);

+	*pd = pcap_open_live(interface, snaplen, 1, timeout, errbuf);

+	if (NULL == *pd) {

+		syslog(LOG_ERR, "pcap open %s: %s", interface,  errbuf);

+		return NULL;

 	}

+	*linktype = pcap_datalink(*pd);

+	/* Must be ethernet or fddi */

+	if (*linktype != DLT_EN10MB && *linktype != DLT_FDDI) {

+		syslog(LOG_ERR, "(%s) Link layer type %d not ethernet or fddi",

+			   interface, *linktype);

+		pcap_close(*pd);

+		return NULL;

+	}

+	return interface;

+}

-	for (dev = alldevs; dev; dev = dev->next) {

-		strncpy(interface, dev->name, strlen(dev->name)+1);

-

-		*pd = pcap_open_live(interface, snaplen, 1, timeout, errbuf);

-		if (*pd == NULL) {

-			syslog(LOG_ERR, "pcap open %s: %s, trying next...", interface, errbuf);

-			continue;

-			/* exit(1); */

-		}

+char *

+iterate_dev(char *arginterface, pcap_t **pd, int *linktype, char *errbuf)

+{

+	static char interface[64 + 1];

+	pcap_if_t *alldevs;

+	pcap_if_t *dev;

-		*linktype = pcap_datalink(*pd);

-		/* Must be ethernet or fddi */

-		if (*linktype != DLT_EN10MB && *linktype != DLT_FDDI) {

-			syslog(LOG_ERR, "(%s) Link layer type %d not ethernet or fddi, trying next...",

-				   interface, *linktype);

-			pcap_close(*pd);

+	if (NULL != arginterface) {

+		return try_dev(arginterface, pd, linktype, errbuf);

+	} else {

+		if (pcap_findalldevs(&alldevs, errbuf) == -1) {

+			(void)fprintf(stderr, "%s: lookup_device: %s\n",

+						  prog, errbuf);

+			exit(1);

 		}

-		else {

-			/* First match, use it */

-			ret = interface;

-			break;

+		for (dev = alldevs; dev && (arginterface == NULL); dev = dev->next) {

+			strncpy(interface, dev->name, strlen(dev->name)+1);

+			arginterface = try_dev(interface, pd, linktype, errbuf);

 		}

-

+		pcap_freealldevs(alldevs);

+		return arginterface;

 	}

-	pcap_freealldevs(alldevs);

-	return (ret);

 }

 int

@@ -315,8 +317,8 @@ main(int argc, char **argv)

 	} else {

 		/* Determine interface if not specified */

-		if (interface == NULL &&

-			(interface = get_first_dev(&pd, &linktype, errbuf)) == NULL) {

+		interface = iterate_dev(interface, &pd, &linktype, errbuf);

+		if (interface == NULL) {

 			(void)fprintf(stderr, "%s: lookup_device: no suitable interface found\n",

 						  prog);

 			exit(1);