Re: capturing 802.11 frames with udp protocol..

[alex medvedev <alexm () pycckue org>]

hallo,

i do not think that a wireless card supplies ieee80211 header unless in
monitor mode (?) [which cisco can do, but in a receive only mode].
i think it just gives the ethernet header, so you don't have to skip the
ieee80211 header from eth1 when communicating normally.

-alexm
--
öÉÚÎØ.exe /?


On Wed, 16 Jul 2003, M. Onur ERGiN wrote:

> 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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