Re: Extracting field values in a C post-dissector

[Paul Offord <Paul.Offord () advance7 com>]

Hi Pascal,

The use of proto_registrar_get_id_byname() looked good but there’s a problem.  It’s declared in proto.h as:

  extern int proto_registrar_get_id_byname(const char *field_name);

I think this means that it’s not exported for use by plugin DLLs; it would need to be defined like this:

  WS_DLL_PUBLIC int proto_registrar_get_id_byname(const char *field_name);

Or am I missing something?

Best regards…Paul

From: wireshark-dev-bounces () wireshark org [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Pascal Quantin
Sent: 22 August 2016 14:40
To: Developer support list for Wireshark <wireshark-dev () wireshark org>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector

Hi Paul,

2016-08-22 11:57 GMT+02:00 Paul Offord <Paul.Offord () advance7 com<mailto:Paul.Offord () advance7 com>>:
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.
Pascal.

Thanks and regards…Paul

From: Paul Offord
Sent: 06 June 2016 10:42
To: 'Developer support list for Wireshark' <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
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: wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org> 
[mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Graham Bloice
Sent: 03 June 2016 12:34

To: Developer support list for Wireshark <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector



On 3 June 2016 at 12:04, Paul Offord <Paul.Offord () advance7 com<mailto:Paul.Offord () advance7 com>> 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 ☹  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: wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org> 
[mailto:wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org>] On Behalf Of Graham 
Bloice
Sent: 03 June 2016 09:16
To: Developer support list for Wireshark <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector



On 3 June 2016 at 08:47, Paul Offord <Paul.Offord () advance7 com<mailto:Paul.Offord () advance7 com>> 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

______________________________________________________________________

This message contains confidential information and is intended only for the individual named. If you are not the named 
addressee you should not disseminate, distribute or copy this e-mail. Please notify the sender immediately by e-mail if 
you have received this e-mail by mistake and delete this e-mail from your system.

Any views or opinions expressed are solely those of the author and do not necessarily represent those of Advance Seven 
Ltd. E-mail transmission cannot be guaranteed to be secure or error-free as information could be intercepted, 
corrupted, lost, destroyed, arrive late or incomplete, or contain viruses. The sender therefore does not accept 
liability for any errors or omissions in the contents of this message, which arise as a result of e-mail transmission.

Advance Seven Ltd. Registered in England & Wales numbered 2373877 at Endeavour House, Coopers End Lane, Stansted, Essex 
CM24 1SJ

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please visit http://www.symanteccloud.com
______________________________________________________________________

___________________________________________________________________________
Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Archives:    https://www.wireshark.org/lists/wireshark-dev
Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev
             mailto:wireshark-dev-request () wireshark org<mailto:wireshark-dev-request () wireshark 
org>?subject=unsubscribe


______________________________________________________________________

This message contains confidential information and is intended only for the individual named. If you are not the named 
addressee you should not disseminate, distribute or copy this e-mail. Please notify the sender immediately by e-mail if 
you have received this e-mail by mistake and delete this e-mail from your system.

Any views or opinions expressed are solely those of the author and do not necessarily represent those of Advance Seven 
Ltd. E-mail transmission cannot be guaranteed to be secure or error-free as information could be intercepted, 
corrupted, lost, destroyed, arrive late or incomplete, or contain viruses. The sender therefore does not accept 
liability for any errors or omissions in the contents of this message, which arise as a result of e-mail transmission.

Advance Seven Ltd. Registered in England & Wales numbered 2373877 at Endeavour House, Coopers End Lane, Stansted, Essex 
CM24 1SJ

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please visit http://www.symanteccloud.com
______________________________________________________________________

___________________________________________________________________________
Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org>
Archives:    https://www.wireshark.org/lists/wireshark-dev
Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev
             mailto:wireshark-dev-request () wireshark org?subject=unsubscribe