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 1st 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
1st scan is because they both register filtering tap listeners. So that’s what
I’m going to try in TRANSUM.
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul Offord
Sent: 06 September 2016 22:47
To: Developer support list for Wireshark <[email protected]>
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 1st scan. I’ll write up some notes on
C post-dissectors when I get something that works.
Best regards…Paul
From:
[email protected]<mailto:[email protected]>
[mailto:[email protected]] On Behalf Of Paul Offord
Sent: 05 September 2016 23:12
To: Developer support list for Wireshark
<[email protected]<mailto:[email protected]>>
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
<[email protected]<mailto:[email protected]>> 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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