Wireshark-dev: Re: [Wireshark-dev] Conversation tracking
From: Tobias Weiss <[email protected]>
Date: Fri, 11 May 2012 12:25:39 -0400

Thanks for your quick replies (Jeff & Lars).

I guess I have to explain my real problem in more detail. I want to implement a dissector for a quite old protocol that has 2 stages. The packets of the first stage have a fixed length (4 byte) and the packets of the second stage can have an arbitrary length but with a fixed header (8 bytes).

Unfortunately the content of a 4 byte frame can be the beginning of the 8 byte header. So I have to figure out where stage 1 ends and stage 2 starts. But as the payload of one TCP frame can contain the last stage 1 frame(s) and the first stage 2 frame, this is not straightforward. So my idea was to do this just once with the first packet and save the state in the conversation data and subsequently reuse that information. Because detecting the end of stage 1 is pretty easy if you know where you are in the stream.

And now I'm not sure if something like this could happen within the same conversation:

TCP connect -> stage 1 frames -> stage 2 frames -> TCP disconnect -> TCP connect -> stage 1 frames -> stage 2 frames

In this case I cannot just save the packet number where stage 1 ends if my dissector gets the same conversation for multiple connects/disconnects.

Tobi



"RUOFF, LARS (LARS)" <[email protected]>
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05/11/2012 11:33 AM

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Hi Tobi,
I don't understand your concern. (but that may be due to my weird understanding of English)
If those packets are part of the same stream, i.e. they are being identified as belonging to the same conversation (by whatever means, eg adress-port mapping etc.), then the whole point of conversation tracking is that you will get the *same* conversation object for all those packets.
So you can store things in there and retrieve them when dissecting another packet of that same conversation.

Regards,
Lars



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From: [email protected] [mailto:[email protected]] On Behalf Of Tobias Weiss
Sent: vendredi 11 mai 2012 16:30
To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] Conversation tracking



Right now I'm puzzled: I wanted to use conversation tracking in order to save information about the state of the communication across packet dissection.

The dissector is called completely out of order (which is reasonable) but I get always the same conversation, even when dissecting an older packet than the last one. So saving the state of the communication is completely pointless if the same conversation is used for dissecting random packets. Here is an example just in case you don't understand my possibly weird English ;-)

I have a stream of 10 packets and something interesting was send in packet number 3. Now I want to save this information in the conversations data in order to reuse it when dissecting future packets. But the dissector is called randomly (which is ok) but always with the same conversation (which is absolutely not ok in this case).

I simply cannot rely on the saved information. So how am I supposed to use conversation tracking in a sane way (as far as I can see I can't _track_ anything)???

Tobi




Tobias Weiss <[email protected]>
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05/10/2012 06:03 PM
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Section 2.2.3 (The example conversation code using se_alloc'd memory) from the trunk.

Tobi




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05/10/2012 05:59 PM

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---- On Thu, 10 May 2012 15:54:44 -0600 Tobias Weiss  wrote ----

>
>Thanks, this is working just fine.
>
>But in that case I suggest an update to README.developer as I copied the buggy source-code from there.

Which section(s) were you copying the source code from?

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