Re: tcpdump: decode 802.11 country element

[Stefan Sperling]

On Fri, Jul 17, 2015 at 09:05:28PM +0300, Vadim Zhukov wrote:
> The tcpdump is hard. :(
 
indeed :-/

Index: print-802_11.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/print-802_11.c,v
retrieving revision 1.21
diff -u -p -r1.21 print-802_11.c
--- print-802_11.c      17 Jul 2015 17:41:41 -0000      1.21
+++ print-802_11.c      17 Jul 2015 18:57:14 -0000
@@ -29,6 +29,7 @@
 #include <net80211/ieee80211.h>
 #include <net80211/ieee80211_radiotap.h>
 
+#include <ctype.h>
 #include <pcap.h>
 #include <stdio.h>
 #include <string.h>
@@ -78,6 +79,7 @@ int    ieee80211_hdr(struct ieee80211_fram
 int     ieee80211_data(struct ieee80211_frame *, u_int);
 void    ieee80211_print_element(u_int8_t *, u_int);
 void    ieee80211_print_essid(u_int8_t *, u_int);
+void    ieee80211_print_country(u_int8_t *, u_int);
 void    ieee80211_print_htcaps(u_int8_t *, u_int);
 int     ieee80211_elements(struct ieee80211_frame *, u_int);
 int     ieee80211_frame(struct ieee80211_frame *, u_int);
@@ -233,6 +235,47 @@ ieee80211_print_essid(u_int8_t *essid, u
 
 /* Caller checks len */
 void
+ieee80211_print_country(u_int8_t *data, u_int len)
+{
+       u_int8_t first_chan, nchan, maxpower;
+
+       if (len < 6)
+               return;
+
+       /* country string */
+       printf((isprint(data[0]) ? " '%c" : " '\\%03o"), data[0]);
+       printf((isprint(data[1]) ? "%c" : "\\%03o"), data[1]);
+       printf((isprint(data[2]) ? "%c'" : "\\%03o'"), data[2]);
+
+       len -= 3;
+       data += 3;
+
+       /* channels and corresponding TX power limits */
+       while (len > 3) {
+               /* no pretty-printing for nonsensical zero values,
+                * nor for operating extension IDs (values >= 201) */
+               if (data[0] == 0 || data[1] == 0 ||
+                   data[0] >= 201 || data[1] >= 201) {
+                       printf(", %d %d %d", data[0], data[1], data[2]);
+                       continue;
+               }
+
+               first_chan = data[0];
+               nchan = data[1];
+               maxpower = data[2];
+
+               printf(", channel%s %d", nchan == 1 ? "" : "s", first_chan);
+               if (nchan > 1)
+                       printf("-%d", first_chan + nchan - 1);
+               printf(" limit %ddB", maxpower);
+
+               len -= 3;
+               data += 3;
+       }
+}
+
+/* Caller checks len */
+void
 ieee80211_print_htcaps(u_int8_t *data, u_int len)
 {
        u_int16_t htcaps;
@@ -392,8 +435,7 @@ ieee80211_elements(struct ieee80211_fram
                        break;
                case IEEE80211_ELEMID_COUNTRY:
                        printf(", country");
-                       for (i = len; i > 0; i--, data++)
-                               printf(" %u", data[0]);
+                       ieee80211_print_country(data, len);
                        break;
                case IEEE80211_ELEMID_CHALLENGE:
                        printf(", challenge");
@@ -437,6 +479,10 @@ ieee80211_elements(struct ieee80211_fram
                        printf(", htcaps");
                        if (vflag)
                                ieee80211_print_htcaps(data, len);
+                       break;
+               case IEEE80211_ELEMID_POWER_CONSTRAINT:
+                       ELEM_CHECK(1);
+                       printf(", power constraint %udB", data[0]);
                        break;
                case IEEE80211_ELEMID_QBSS_LOAD:
                        ELEM_CHECK(5);