title: beacon frame captured by libpcap is something strange
body: <p>I captured beacon frame with library libpcap(Ubuntu, c)</p>
  
  <p>I changed wlan mode to monitor and captured by following function calls</p>
  
  <pre><code>// 3000 is large enough number for test
  pcd = pcap_open_live(dev,3000,PROMISCUOUS,-1,errbuf)  
  // filter with "wlan type mgt subtype beacon"    
  pcap_compile(pcd,&amp;bpg,"wlan type mgt subtype beacon",1,PCAP_NETMASK_UNKNOWN)
  pcap_setfilter(pcd, &amp;bpg)
  </code></pre>
  
  <p>and following is packet_view</p>
  
  <pre><code>void packet_view(
      unsigned char *user,
      const struct pcap_pkthdr *h,
      const unsigned char *p
  ){
      int len;
      len = 0;
  
      printf("PACKET\n");
      while(len &lt; h-&gt;len) {
          printf("%02x ", *(p++));
          if(!(++len % 16))
              printf("\n");
      }
      printf("\n");
      return ;
  }
  </code></pre>
  
  <p>and my got following result (just most significant 32bits)</p>
  
  <pre><code>00 00 12 00 2e 48 00 00 00 02 6c 09 a0 00 bb 01 
  00 00 80 00 00 00 ff ff ff ff ff ff 00 08 9f bf 
  </code></pre>
  
  <p>but... actually beacon frame must start with bit "08"
  because beacon frame's protocol version = 00 type = 00 and subtype = 1000 => 00001000(08)</p>
  
  <p>what is the reason that i got packet start with 00? althougt I filtered with "wlan type mgt subtype beacon" ??</p>