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Wireshark-dev: Re: [Wireshark-dev] RTCP Heuristic decoder

From: Bill Meier <wmeier@xxxxxxxxxxx>
Date: Fri, 11 Sep 2009 14:28:12 -0400
On Thu, Sep 10, 2009 at 4:59 PM, Guy Harris <guy@xxxxxxxxxxxx> wrote:

On Sep 10, 2009, at 4:30 PM, vinayak kamath wrote:

We have our own custom decoder for a tweaked version of RTCP protocol.
Heres the code snippet where in we say anything on this udp.port
send the message to custom decoder.
##########################################################
if(!TWS_PortList("FOO", configFile, (int*)portList, MAXPORTS))
      {
        while (count < MAXPORTS && portList[count]!=0)
        {
            dissector_add("udp.port", portList[count], foo_handle);
            count++;
        }
      }
##########################################################

But still wireshark decodes it as RTCP rather than FOO.
The dissector to which foo_handle refers is a non-heuristic dissector;
if RTCP's "Try to decode RTCP outside of conversations" preference is
set, the heuristic RTCP dissector will try to recognize RTCP packets.

If UDP's "Try heuristic sub-dissectors first" preference is set, the
UDP dissector will try heuristic dissectors before trying non-
heuristic dissectors; that means that if the RTCP dissector recognizes
packets for your tweaked protocol, it will dissect those packets and
your dissector will not be given a chance to dissect them.  If that
preference is set, try unsetting it.



vinayak kamath wrote:
> Thanks  .
> But in preferences i don't have "Try to decode RTCP outside of
> conversations" preference set. Even then it decodes as RTCP rather than FOO. > Is there any way for custom dissector taking precedence over heuristic RTCP
> dissector??
>
> Thanks
> vinayak
>

Have you also cleared the following as indicated in Guy's reply??

> If UDP's "Try heuristic sub-dissectors first" preference is set, the
> UDP dissector will try heuristic dissectors before trying non-
> heuristic dissectors
>

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