Wireshark · Ethereal-dev: [Ethereal-dev] Problems writing my first dissector for a special case of wireless LAN frames (802.11

["Klaus Woersdoerfer" <ib@xxxxxxxxxxxxxxxx>]

Hallo,

 

I'm developing my first dissector and have problems to instruct Ethereal to call my dissector routine.

 

I want to write a dissector to decode special WLAN 802.11 frames (customized by my company) not dissected by the standard wlan packet dissector in file packet-ieee80211.c.

 

My dissector should be called for all 802.11 frames which have the value 0x03 in the type bits of the frame control field (0x00 = management frames, 0x01 = control frames, 0x02 = data frames, 0x03 = reserved). This bit combination is not used by the standard (reserved) until now.

 

My problem is that I don't know how to say to Ethereal to give me these 802.11 reserved frames. We named our protocol "mopio" and I wrote this function

 

void
proto_reg_handoff_mopio(void)
{
dissector_handle_t mopio_handle;
mopio_handle = find_dissector("mopio");

dissector_add("wlan.fc.type" , 0x03, mopio_handle);
}

to tell Ethereal when to call my dissector, but the code doesn't work. The call of dissector_add() returns in line 601 in packet.c in the call of

    dissector_table_t sub_dissector = find_dissector_table(name);

 

with 0 and then generates an exception in the following assertion.

 

I don't know how to change the code to get it working and to describe the kind of 802.11 frames my dissectors wants to be called with.

 

Another possible solution could be, that my dissector receives all 802.11 frames, processes (filters) our special reserved frames and then calls the 802.11 dissector to dissect all other standard 802.11 frames. How could be it done?

 

Can anybody help me with this problem?

Thank you very much for any help.

 

Klaus W.