Wireshark-dev: Re: [Wireshark-dev] Extracting field values in a C post-dissector
From: Pascal Quantin <[email protected]>
Date: Mon, 22 Aug 2016 15:40:25 +0200
Hi Paul,

2016-08-22 11:57 GMT+02:00 Paul Offord <[email protected]>:

I’m struggling a bit here.  Can someone give me a pointer to the code that shows me how LUA extracts dissected protocol values?

By having a quick look at the code, I *think* you will want first to retrieve the hfindex of a given field by using proto_registrar_get_id_byname(), then mark it as "interesting" with proto_tree_prime_hfid() and then once dissection is done call proto_get_finfo_ptr_array() to retrieve an array of the values for a given tree (you can go back to the root tree with proto_tree_get_root()). Or a cll to proto_find_finfo() should work also without the need to prime the field, but should be slower according to the comments in proto.h.

Worth testing it and giving your findings as I have never tried them myself. I hope I gave you valid hints :) Fingers crossed.



Thanks and regards…Paul


From: Paul Offord
Sent: 06 June 2016 10:42
To: 'Developer support list for Wireshark' <[email protected]>
Subject: RE: [Wireshark-dev] Extracting field values in a C post-dissector


No problem.  I’ll take a look at the code as you suggest.


From: [email protected]wireshark.org [mailto:[email protected]wireshark.org] On Behalf Of Graham Bloice
Sent: 03 June 2016 12:34

To: Developer support list for Wireshark <[email protected]>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector




On 3 June 2016 at 12:04, Paul Offord <[email protected]> wrote:

Hi Graham,


My post-dissector should run after all other dissectors have completed.  The reference to tcp.len is just an example.  I need access to the full stack e.g. everything from ethertype to SMB msg id.  I purposely don’t want to do any protocol dissection myself.


I guess that what I am effectively asking for is access to everything in the dissector tree.


Thanks for the pointer to README.dissector – I have written a C dissector before, goodness knows why I had forgotten this doc L  I’ve scanned through it but it doesn’t seem to cover my issue.


Best regards…Paul


Ah, sorry Paul I skipped over the phrase "post-dissector".  Those are a bit of an odd fish, I suggest you have a look at how the Lua field extractor interface is defined in the C code.



From: [email protected]wireshark.org [mailto:[email protected]wireshark.org] On Behalf Of Graham Bloice
Sent: 03 June 2016 09:16
To: Developer support list for Wireshark <[email protected]>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector




On 3 June 2016 at 08:47, Paul Offord <[email protected]> wrote:

I want to rewrite a LUA post-dissector in C.  How can my code get dissected data values, e.g. tcp.len, into a C variable?  In LUA you define a field extractor.  Is there an equivalent in C?


I’ve looked at the wiki and I’ve looked at the MATE code but I haven’t found how to do it.


Thanks and regards…Paul



doc\README.dissector is your guide for C-based dissectors.


Are you aiming to get values from dissectors called before yours, e.g. your protocol runs on tcp, so tcp dissection is done first, then the data from the tcp segment is handed to your dissector?


If so, then the pinfo structure passed to your dissector has a lot of info about previous dissection, see epan\packet_info.h for the structure members.


Is your reference to tcp.len just a general example or an actual value you *think* you need?  Generally in tcp based dissectors, the length of any individual tcp segment is immaterial, you just process the data handed to your dissector in the *tvb.  This is especially the case when a protocol PDU can either be spread over multiple tcp segments, or multiple PDU's in a single segment.  This is where tcp reassembly/desegmentation is used.




Graham Bloice




Graham Bloice


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