capturing 802.11 frames with udp protocol..

["M. Onur ERGiN" <monurergin () yahoo com>]

Hi;
I am quite new in these and implementing an adhoc
routing algorithm in user-space of linux.
I can capture some packets which are send from cisco
340 wireless clients. the packets simply include data
of a simple udp program.
I do the following:
/***********************************
  int size_MAC; //size of the mac layer header (802.11
or ethernet)
        if(strcmp(dev,"eth1")==0)
                size_MAC = sizeof(struct ieee_802_11_header)+2;
        else size_MAC = sizeof(struct ether_header);
  int size_ip = sizeof(struct ip);
  //int size_tcp = sizeof(struct tcphdr);
  int size_udp = sizeof(struct udphdr);
  struct scanpac sp;
  static int i =0;

  printf("DEVICE: %s\n",dev);
  printf("FONK!%d\n",i++);

  if(strcmp(dev,"eth1")!=0)
          eptr = (struct ether_header *)(data);
        else ieee_802_11_hdr = (struct ieee_802_11_header
*)(data+2);

  ip = (struct ip *)(data + size_MAC);
  udp = (struct udphdr *)(data + size_MAC + size_ip);
  payload = (u_char *)(data + size_MAC + size_ip +
size_udp);
**********************************/
in most of the times, I get the headers correctly (at
least they seem so :) ) but can never get the payload.

first, I cast the complete packet to a 802.11 header.
then I cast the rest to an ip header and the rest to a
udp header.
but when I try to get the very last one (which is
supposed  to be the payload) I see nothing on the
screen using printf("%s...);

any ideas?

regards,

Onur.

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