Wireshark · Wireshark-dev: Re: [Wireshark-dev] Question on payload reassembly

[Jeff Morriss <jeff.morriss.ws@xxxxxxxxx>]

On Thu, Jul 28, 2016 at 8:35 AM, John Dunlop <jdunlop@xxxxxxxxxxxxx> wrote:

Hi,

 

Hope someone can help me with a question of payload reassembly.  

 

First up, I have been trawling the e-mail archives to find an equivalent answer and was wondering if there is a better way of searching the e-mail archives than opening up each individual month/year?

Personally I use Google with a search string like:

    what I'm interested in site://wireshark.org

 

Now my actual question is that I am dissecting  a packet payload which is split up into fragments with specific chunks as:

Begin

Middle (no begin/end flagged, so can be multiple)

End

 

I have a simple state machine that checks these transitions and keeps fragment counts so I can then call fragment_add_seq_check() with an appropriate unique id and an incrementing (from zero) frag_number.  I know the size of the individual fragments and there is a sequence number that increments on each packet, though a packet can have multiple fragments for the same or different channels .

 

This appears to ‘initially’ work ok from the various log prints I had added to check returns from process_reassembled_data() and the actual reassembled TVB size.

 

The problem I have, and this is probably my fundamental misunderstanding, is that it works on the initial pass through the packets but breaks horribly when I click on an individual packet as we are mid fragments. I also notice that wireshark parses the whole file once and then parses again the visible packets in the summary window, this also fails as the 1^st packet is parsed again after the last which could be in any state of fragmentation.

 

I suppose I am thinking if we have parsed the payload once for a given packet/fragment we should not parse and reassemble again but somehow look-up what reassembled payload it belongs to? Using something like fragment_get() ?

Hmm, the reassembly routines should take care of this for you.  See the first 'if' statement in `fragment_add_seq_check_work()` (in epan/reassemble.c): it checks if the current frame has already been dissected and, if so, it skips reassembly and just returns what was stored from the first pass.

It sounds like you are but are you *really* sure you're doing all the reassembly on the first pass (e.g., the reassembly calls aren't buried under an `if(tree)` for example)?

I suppose this won't answer your question but hopefully it might give you a direction to look in...