When printing a list of link-layer types for -L, give the name of the

device for which they're being reported, so if no -i flag was given
people don't think it's something such as the union of of all link-layer
types supported by all devices, and, if we can determine whether the
device supports monitor mode (with libpcap 1.0.0 and later), indicate
whether the link-layer types are the ones supported when not in monitor
mode (if -I wasn't specified) or when in monitor mode (if -I was
specified), so people don't wonder why some types aren't available (as
some are available only in monitor mode, and others are available only
when not in monitor mode, on some platforms).

diff --git a/tcpdump.c b/tcpdump.c
index 25feb7a04457083ffa55abc51ee0c9eb0b1a80e4..745d367e63047b979352e5fc6df8e014ca6598e2 100644 (file)
--- a/tcpdump.c
+++ b/tcpdump.c
@@ -105,7 +105,7 @@ int32_t thiszone;           /* seconds offset from gmt to local time */
 static RETSIGTYPE cleanup(int);
 static RETSIGTYPE child_cleanup(int);
 static void usage(void) __attribute__((noreturn));
-static void show_dlts_and_exit(pcap_t *pd) __attribute__((noreturn));
+static void show_dlts_and_exit(const char *device, pcap_t *pd) __attribute__((noreturn));
 
 static void print_packet(u_char *, const struct pcap_pkthdr *, const u_char *);
 static void ndo_default_print(netdissect_options *, const u_char *, u_int);
@@ -303,6 +303,8 @@ lookup_printer(int type)
 
 static pcap_t *pd;
 
+static int supports_monitor_mode;
+
 extern int optind;
 extern int opterr;
 extern char *optarg;
@@ -319,7 +321,7 @@ struct dump_info {
 };
 
 static void
-show_dlts_and_exit(pcap_t *pd)
+show_dlts_and_exit(const char *device, pcap_t *pd)
 {
        int n_dlts;
        int *dlts = 0;
@@ -331,7 +333,21 @@ show_dlts_and_exit(pcap_t *pd)
        else if (n_dlts == 0 || !dlts)
                error("No data link types.");
 
-       (void) fprintf(stderr, "Data link types (use option -y to set):\n");
+       /*
+        * If the interface is known to support monitor mode, indicate
+        * whether these are the data link types available when not in
+        * monitor mode, if -I wasn't specified, or when in monitor mode,
+        * when -I was specified (the link-layer types available in
+        * monitor mode might be different from the ones available when
+        * not in monitor mode).
+        */
+       if (supports_monitor_mode)
+               (void) fprintf(stderr, "Data link types for %s %s (use option -y to set):\n",
+                   device,
+                   Iflag ? "when in monitor mode" : "when not in monitor mode");
+       else
+               (void) fprintf(stderr, "Data link types for %s (use option -y to set):\n",
+                   device);
 
        while (--n_dlts >= 0) {
                dlt_name = pcap_datalink_val_to_name(dlts[n_dlts]);
@@ -975,6 +991,13 @@ main(int argc, char **argv)
                pd = pcap_create(device, ebuf);
                if (pd == NULL)
                        error("%s", ebuf);
+               /*
+                * Is this an interface that supports monitor mode?
+                */
+               if (pcap_can_set_rfmon(pd) == 1)
+                       supports_monitor_mode = 1;
+               else
+                       supports_monitor_mode = 0;
                status = pcap_set_snaplen(pd, snaplen);
                if (status != 0)
                        error("%s: pcap_set_snaplen failed: %s",
@@ -1053,7 +1076,7 @@ main(int argc, char **argv)
                        }
 #endif /* !defined(HAVE_PCAP_CREATE) && defined(WIN32) */
                if (Lflag)
-                       show_dlts_and_exit(pd);
+                       show_dlts_and_exit(device, pd);
                if (gndo->ndo_dlt >= 0) {
 #ifdef HAVE_PCAP_SET_DATALINK
                        if (pcap_set_datalink(pd, gndo->ndo_dlt) < 0)