Let the platform decide how to check capturable interfaces.

(Git's annoying policy of expecting a short one-line description of every
change means that the first line isn't very explanatory.)

Make pcap_findalldevs_interfaces() take as an argument a function that's
used to check whether an interface can be captured on or not, rather
than doing the check by trying to open the device for capturing.

This lets pcap_findalldevs() find interfaces even if you don't have
permission to capture on them; that way, instead of users saying "why
isn't {tcpdump -D, tshark -D, dumpcap -D, Wireshark, etc.} showing me
any interfaces?", they'll say "why am I getting a 'you don't have
permissions' error when I try to capture on this interface?", which is a
better description of the underlying problem.

On some platforms, it also avoids a bunch of extra work when getting a
list of interfaces.

1 files changed, 41 insertions, 30 deletions

diff --git a/fad-getad.c b/fad-getad.c
index ec77f334..b67b5cdc 100644
--- a/fad-getad.c
+++ b/fad-getad.c
@@ -144,7 +144,8 @@ get_sa_len(struct sockaddr *addr)
  * could be opened.
  */
 int
-pcap_findalldevs_interfaces(pcap_if_t **alldevsp, char *errbuf)
+pcap_findalldevs_interfaces(pcap_if_t **alldevsp, char *errbuf,
+    int (*check_usable)(const char *))
 {
 	pcap_if_t *devlist = NULL;
 	struct ifaddrs *ifap, *ifa;
@@ -174,6 +175,45 @@ pcap_findalldevs_interfaces(pcap_if_t **alldevsp, char *errbuf)
 	}
 	for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next) {
 		/*
+		 * If this entry has a colon followed by a number at
+		 * the end, we assume it's a logical interface.  Those
+		 * are just the way you assign multiple IP addresses to
+		 * a real interface on Linux, so an entry for a logical
+		 * interface should be treated like the entry for the
+		 * real interface; we do that by stripping off the ":"
+		 * and the number.
+		 *
+		 * XXX - should we do this only on Linux?
+		 */
+		p = strchr(ifa->ifa_name, ':');
+		if (p != NULL) {
+			/*
+			 * We have a ":"; is it followed by a number?
+			 */
+			q = p + 1;
+			while (isdigit((unsigned char)*q))
+				q++;
+			if (*q == '\0') {
+				/*
+				 * All digits after the ":" until the end.
+				 * Strip off the ":" and everything after
+				 * it.
+				 */
+			       *p = '\0';
+			}
+		}
+
+		/*
+		 * Can we capture on this device?
+		 */
+		if (!(*check_usable)(ifa->ifa_name)) {
+			/*
+			 * No.
+			 */
+			continue;
+		}
+
+		/*
 		 * "ifa_addr" was apparently null on at least one
 		 * interface on some system.  Therefore, we supply
 		 * the address and netmask only if "ifa_addr" is
@@ -223,35 +263,6 @@ pcap_findalldevs_interfaces(pcap_if_t **alldevsp, char *errbuf)
 		}
 
 		/*
-		 * If this entry has a colon followed by a number at
-		 * the end, we assume it's a logical interface.  Those
-		 * are just the way you assign multiple IP addresses to
-		 * a real interface on Linux, so an entry for a logical
-		 * interface should be treated like the entry for the
-		 * real interface; we do that by stripping off the ":"
-		 * and the number.
-		 *
-		 * XXX - should we do this only on Linux?
-		 */
-		p = strchr(ifa->ifa_name, ':');
-		if (p != NULL) {
-			/*
-			 * We have a ":"; is it followed by a number?
-			 */
-			q = p + 1;
-			while (isdigit((unsigned char)*q))
-				q++;
-			if (*q == '\0') {
-				/*
-				 * All digits after the ":" until the end.
-				 * Strip off the ":" and everything after
-				 * it.
-				 */
-			       *p = '\0';
-			}
-		}
-
-		/*
 		 * Add information for this address to the list.
 		 */
 		if (add_addr_to_iflist(&devlist, ifa->ifa_name,