Dissector based on UDP Src or Dst port to find the direction of packets

Hi,

 

I'm new to Ethereal dissectors. I'm trying to write a dissector for a
VOIP proprietary protocol. It is based on UDP. Phone uses a specific
port number say X to communicate with the PBX. And the PBX uses port Y
to communicate with the phone. X and Y are fixed. My problem is, I need
to find the direction of the packet like who is talking to whom (Phone
to PBX Or PBX to Phone?). That can be done only by analyzing the UDP
Source or Destination port numbers. I tried using the following function
call ->

 

dissector_add("udp.srcport", PORT_NO, my_handle);

 

but that gave me a runtime fatal error. It's my understanding that I
can't use "udp.srcport"  for dissector_add(). If I use "udp.port",
things work fine.  But with "udp.port" I can't find the direction of the
packet.

 

Is there any other solution for this problem ?  All I need is to find
the direction of the packets.

 

Thanks in advance,

Vasanthi

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