Wireshark-dev: [Wireshark-dev] Adding support for a new PCAP-NG block
From: Paul Offord <[email protected]>
Date: Wed, 17 Jan 2018 12:47:48 +0000

I want to make a start on the plan below.  Last night I took a look at the relevant code.

 

I started by adding support for TSDBs into the function pcapng_open(…) in pcapng.c but I then stumbled across wtap_opttype_register_custom_block_type(…) in wtap_opttypes.c which seems to be a framework to add support for new block types.  I did a check on code that refers to this function and found that nothing uses it at the moment.

 

So my questions are:

 

  • Should I modify pcapng_open or use wtap_opttype_register_custom_block_type?
  • If the latter, is there any documentation, or even just an explanation of wtap_opttype_register_custom_block_type and how to use it?
  • I will need to save interpreted data from the new TSDBs somewhere that will later be accessible to dissectors.  Which structure should I use?

 

Thanks and regards…Paul

 

 

From: Paul Offord
Sent: 03 November 2017 16:41
To: Developer support list for Wireshark <[email protected]>
Subject: RE: [Wireshark-dev] Capture filename not available at plugin init time

 

So the plan would be:

 

  • Add support to read the TSDB and create the resulting structures
  • Add support to read Text Record Blocks (TRBs)
    • This is mostly stuff that Guy Harris described a while back
    • In my current code the data records are encapsulated in a dummy Ethernet frame
  • Add support to mergecap to correctly handle the TSDBs
    • Similar to adjusting IDBs when files are merged
  • Add the dumpcap code to read text files and produce pcap-ng

 

I think I’ll leave this idea to circulate for a few days before I start writing code.  Maybe I’ll pester the developers next week at SF EU.

 

I will also see if there is an easy way to get plugin_if_get_ws_info to work in the init routine as I still believe this will be useful to dissector developers.

 

Thanks and regards…Paul

 

From: Wireshark-dev [mailto:[email protected]] On Behalf Of Roland Knall
Sent: 03 November 2017 16:24
To: Developer support list for Wireshark <[email protected]>
Subject: Re: [Wireshark-dev] Capture filename not available at plugin init time

 

This is a different thing here. If TSDB is a common code block, I think the chances are really good.

 

But still it needs the basic read functionality in dumpcap

 

cheers

 

On Fri, Nov 3, 2017 at 5:18 PM, Paul Offord <[email protected]> wrote:

OK – I understand.

 

If I write the code to read the TSDB and make it available do you think it would be accepted into the main project?  I’m thinking about my syncro experience here.

 

Thanks and regards…Paul

 

From: Wireshark-dev [mailto:[email protected]] On Behalf Of Roland Knall
Sent: 03 November 2017 14:15


To: Developer support list for Wireshark <[email protected]>
Subject: Re: [Wireshark-dev] Capture filename not available at plugin init time

 

Hi Paul

 

You should never assume, that you will be able to read the file, while WS is reading it. If this is working right now, it might be out of pure coincidence, that said, the real thing here should be to get dumpcap to use pcapng as input format, which would give you the tsdb block where you need it to be, during dissection.

 

The support for any pcap-ng extension block is already in Wireshark. The issue still is, to get the block structure through.

 

cheers

Roland 

 

On Fri, Nov 3, 2017 at 3:07 PM, Paul Offord <[email protected]> wrote:

Thanks for responding Roland.

 

I’ve written a tool that reads a log file and converts it to a PCAP-NG with a matching dissector.  The pcap file carries a data descriptor block in a new PCAP-NG block type called as TSDB.  The TSDB carries the information needed to register the header fields.  To add support for the TSDB into core Wireshark is going to be a big job (which I will submit later).  As a quick solution, the dissector gets the information by directly opening and reading it from the PCAP-NG file – hence the need for the filename.

 

The above aside, and I can guess you are thinking I’m just trying to avoid a bigger coding job, it’s not unreasonable to expect plugin_if_get_ws_info to get the filename at init time, and init is called when the filename is known.

 

I have a really kludgie workaround which is to read the TSDB and register the hf structure on the first call to the dissector, but it’s not ideal.

 

Best regards…Paul

 

From: Wireshark-dev [mailto:[email protected]] On Behalf Of Roland Knall
Sent: 03 November 2017 13:53
To: Developer support list for Wireshark <[email protected]>
Subject: Re: [Wireshark-dev] Capture filename not available at plugin init time

 

Hi Paul

 

As far as I know, cf_open can still fail after calling the init-functions. In that case you would get the filename, but the capture is already closed.

 

My question is, why do you need the filename in the first place?

 

Also, you could set the filename at a later point. If you implement a tap-interface, you could set the filename in the first tap-print callback. Makes sense, 'cause you normally only have data at this point anyway.

 

You can raise this as an improvement (do not think it is a bug) if you want to, not really sure though, if it should be changed

 

cheers

 

 

On Fri, Nov 3, 2017 at 2:49 PM, Paul Offord <[email protected]> wrote:

I have a dissector that needs the capture file name at the time my dissector’s init function is called.  I attempt to get the name with plugin_if_get_ws_info(…), not an unreasonable request I think you’ll agree, but unfortunately the filename comes back as a NULL pointer.

 

I’ve traced through the code and this is what happens:

 

  • We pass through the MainWindow signal and slot stuff and eventually call cf_open(…) in file.c with the filename as one of the parameters
  • cf_open(…) opens the file to test the validity of the filename and then closes with cf_close(cf)
  • cf_close(cf) frees the memory holding the filename and NULLs the filename pointer in the cf structure
  • cf_open then creates a new epan session with ws_epan_new(cf)
  • ws_epan_new(cf) calls epan_new() which calls init_dissection() and this is where eventually my dissector’s init function gets called
  • My dissector calls plugin_if_get_ws_info(…) which attempts to get the filename info from the cf structure, which due to the above returns a NULL filename pointer
  • Eventually we return back to cf_open(…) and a little later we set up the file name in the cf structure – all too late for my dissector’s init function

 

So my questions are:

 

  • Can I raise this as a bug?
  • If not, would a solution that made the filename available to plugin_if_get_ws_info(…) at init time be accepted?
  • What would be an acceptable solution?

 

Thanks and regards…Paul


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