Wireshark-dev: Re: [Wireshark-dev] Is it possible to write a dissector for an asymmetrical prot
On Fri, Mar 30, 2007 at 04:52:10PM -0400, Bob Doolittle wrote:
> 1 Although encapsulated in UDP, it doesn't use a
> well-known port. I'm OK with telling wireshark
> to decode using the protocol specifically (right-click
> on packet and select "Decode As"), and that's
> working for me at a gross level. Now I want to
> decode the details...
> 2 The protocol is asymmetrical. Packets sent by client
> and server have different formats, only distinguished
> by whether the sender is client or server. There is
> a common header that I'll want to share decoding for,
> but after that the packet structures differ.
>
> Is it possible to write a dissector for such a protocol?
Yes, but the problem with using "Decode As" is that it assigns that port
to your protocol bi-directionally. The dissector then can't tell the
difference in the sender and receiver. Also the "Decode As" settings
aren't saved between Wireshark sessions.
> Say I select a particular packet and choose "Decode As", and require
> that the packet chosen be one sent by the server, for instance. Now I
> know what UDP port and IP address identify the server and ditto for
> the client. Can I now build separate proto trees depending on which
> is sending the packet?
You need a way to tell which is the sender and which is the receiver
port. Do the common headers specify in any way if it's coming from the
client or server? Is the sender or receiver port number always higher
than the other? etc. One method you can use is to set up a preference
pane for your protocol and have fields to enter both direction's ports
source ports you're expecting the protocol to be using. These can be
saved between sessions. The doc/README.developer talks about setting up
preferences. You would then go through your common header dissection
like normal and then when you reach the direction specific fields, put
an if statement similar to the following:
if(pinfo->srcport == <variable set by preferences>) {
/* Decode traffic from port number 1 */
} else {
/* Decode traffic from port number 2 */
}
Let us know if you need more help.
Steve
