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Hi, I've written a post-dissector to add some additional info into the tree for TCP packets. I'd like to do something like the http.response_in field but for TCP, or put another way, provide the inverse of the tcp.analysis.acks_frame - rather than saying this is an ACK to the earlier segment in frame x say this segment is ACKed later in frame y.

It seems like this would require saving state and then updating an already dissected packet. Is this even possible with LUA, and if so could someone provide a few hints or point to similar code?

thanks a bundle,


asked 02 Feb '17, 02:07

gaddman's gravatar image

accept rate: 0%

edited 08 Feb '17, 13:42

I faced the same issue when I wrote the TRANSUM LUA. Guy corrected me the other day regarding Wireshark scanning so my following description is a bit loose, and I'm doing this from memory.

Your dissector will be called at least twice for each packet; once on an initial scan with pinfo.visited false and a second (and further times) with pinfo.visited true. You can generate the new values on the first scan and add the data to the protocol tree on the second (and later) scans.

You might want to take a look at the TRANSUM LUA code which you can download from

Best regards...Paul

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answered 04 Feb '17, 03:12

PaulOfford's gravatar image

accept rate: 11%

Paul, that's even better than what I'd hoped for. I've had a look through your TRANSUM code and realised it was actually a small change to my code. Updated my dissector here:

(08 Feb '17, 13:35) gaddman

I doubt this can be done in a post dissector, let alone LUA. I think this has to be done in the TCP dissector itself. The concept however is possible. Other dissector do have the feature where 'requests' and 'responses' are matched both ways. There is a README on this (request response tracking) in the doc directory of the source tree to give you some insight in how this is achieved.

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answered 02 Feb '17, 02:39

Jaap's gravatar image

Jaap ♦
accept rate: 14%

Thanks for the pointer, I read the source docs and there's a good outline of how to write a dissector in C:;a=blob_plain;f=doc/README.request_response_tracking;hb=HEAD

(08 Feb '17, 13:36) gaddman
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question asked: 02 Feb '17, 02:07

question was seen: 1,433 times

last updated: 08 Feb '17, 13:47

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