Wireshark · Wireshark-dev: Re: [Wireshark-dev] Extracting field values in a C post-dissector

[Paul Offord <Paul.Offord@xxxxxxxxxxxx>]

I thought I’d figured this out, then it turns out I hadn’t but now I think I have – are you still following?

 

The first thing I have realised is that the epan dissectors only generate displayable and filterable values (tcp.len, smb2.msg_id, etc.) if a tree is passed to them and the only place these values get saved is in the tree.  Normally Wireshark does not create a tree on the 1^st scan of a trace file.  However, if a tap is registered it can/is registered with a display filter and the Wireshark code ( specifically cf_read() ) checks for filtering taps and sets a gboolean ( create_proto_tree ) accordingly:

 

create_proto_tree =

    (dfcode != NULL || have_filtering_tap_listeners() || (tap_flags & TL_REQUIRES_PROTO_TREE));

 

Later this Boolean is used as a switch in:

 

epan_dissect_init(&edt, cf->epan, create_proto_tree, FALSE);

 

The edt then has an associated tree ( edt->tree ), and the dissectors are then called with the tree.

 

The reason MATE and LUA dissectors have access to the field values during the 1^st scan is because they both register filtering tap listeners.  So that’s what I’m going to try in TRANSUM.

 

 

From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of Paul Offord
Sent: 06 September 2016 22:47
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector

 

I’ve made some progress.  I traced MATE and looked at how it registers its post-dissector.  I now get a tree on the 1^st scan.  I’ll write up some notes on C post-dissectors when I get something that works.

 

Best regards…Paul

 

From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of Paul Offord
Sent: 05 September 2016 23:12
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector

 

I've hit a problem.  WS scans the trace file twice.  I need access to protocol fields (e.g. tcp.len and smb2.ses_id) during the first scan.

 

Unfortunately with the C postdissector the tree value passed during the first scan is NULL.  During the second scan I do get the tree.

 

I guess the LUA code uses the proto_tree_prime_hfid() outlined below.

 

Any suggestions how I move forward gratefully accepted.

 

Sent from Samsung Mobile on O2

 

-------- Original message --------

From: Guy Harris

Date:05/09/2016 03:59 (GMT+00:00)

To: Developer support list for Wireshark

Subject: Re: [Wireshark-dev] Extracting field values in a C post-dissector

 

On Aug 22, 2016, at 6:40 AM, Pascal Quantin <pascal.quantin@xxxxxxxxx> wrote:

> 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()

...which you have to do before the dissection starts.

Unfortunately, you can't do that in a post-dissector.

So...

> aOr 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.

...you might have to do it that way, instead.
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