So for Bryant's question is the issue that his customer didn't capture the initial SYN/SYN-ACK handshake, and hence Wireshark didn't have opportunity to remember which was the initial destination port (and hence "server" port and the one the one he would be interested in dissecting for?
Maybe in this case we could have some other heuristic. One way would be to try both the source and destination port registered dissectors and see which one seems give a better result (maybe measured by the result in the Information field). Another way (and less reliable) might be based around the fact that clients are more likely to send zero-length ACK responses (but I might be wrong in that)
Regards, Martin
MartinVisser99@xxxxxxxxx
On Thu, May 28, 2009 at 8:10 AM, Guy Harris
<guy@xxxxxxxxxxxx> wrote:
On May 27, 2009, at 2:26 PM, Bryant Eastham wrote:
> Q: Does “dissector_add” differentiate between src and dst port? [I
> think not, the registration is by name and the dissector (TCP)
> chooses how to use it.]
No.
> Q: Does wireshark prioritize between built-in vs. plugin dissectors?
> [I think not.]
No.
> Q: Does wireshark prioritize between dissectors based on matches on
> src vs. dst port?
No.
It prioritizes based on the port number; lower port numbers are
preferred to higher port numbers, as they're more likely to be well-
known ports.
> My fundamental issue is that I would expect that priority be given
> to the dissector on the *server* (dst) port, as it is the more
> likely to be standardized vs. ephemeral.
The destination port is the server port only for requests. If the
capture includes the entire session, all the way back to the initial
SYN, and we remember, for each new TCP connection, the source and
destination ports for the initial SYN, we can determine, for packets
in that connection, which port is the "client" port and which the
"server" port.
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