Wireshark · Wireshark-dev: Re: [Wireshark-dev] heur_dissector_add()

["David Aggeler" <david_aggeler@xxxxxxxxxx>]

Hi Peter,

Thanks for the hint.  Ok. I just debugged and apparently the DICOM one as many others is DISSECTOR_TYPE_SIMPLE. Not sure how to change, but also not sure whether it is that relevant. However, I return 0 when it does not match (more like new style)

If the dissectors are combined like this, basically old and new need to follow the same interface, otherwise every parent of call_dissector_through_handle() would need a type selector, and  I've not seen that anywhere else. 

Also from this comment & code, I'd conclude that the retuned number is the number of processed bytes.
In that sense, for me, 0 maps to FALSE in the heuristic and >0 maps to TRUE. 

I've now coded according to the README.heuristic. To me that reflects 0/FALSE, >0 TRUE.

>> you can enable or disable heuristics dissectors it in the Enabled Protocols menu.

I'm blind. All I can see there is Ethernet PW (CW heuristic). That one?

Regards
David

-----Original Message-----
From: Wireshark-dev <wireshark-dev-bounces@xxxxxxxxxxxxx> On Behalf Of Peter Wu
Sent: Thursday, March 22, 2018 17:29
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-dev] heur_dissector_add()

Hi David,

On Thu, Mar 22, 2018 at 11:50:07AM +0100, David Aggeler wrote:
>  
> 
> I'm intending to re-enable the heuristic part in the DICOM dissector. 
> So I read though the updates readme and some other dissector, and to 
> my surprise, the return value of the heuristic still is supposed to be 
> boolean, where the static one returns int.
> 
>  
> 
> Implementation wise, by now I kind of only see 'return 
> tvb_captured_length(tvb)'. Wasn't this consumed bytes or needed bytes 
> at some point? I used to return the same int also in heuristic part 
> and never had an issue, but it looks wrong.

There are only two conventions for the integer return value, see the comment for call_dissector_through_handle in epan/packet.c:

    /* This function will return
     * old style dissector :
     *   length of the payload or 1 of the payload is empty
     * new dissector :
     *   >0  this protocol was successfully dissected and this was this protocol.
     *   0   this packet did not match this protocol.
     *
     * The only time this function will return 0 is if it is a new style dissector
     * and if the dissector rejected the packet.
     */

When I tried to change this such that the returned value is actually significant (https://www.wireshark.org/lists/wireshark-dev/201406/msg00221.html), I found that it would be quite risky to use the return value to signal reassembly.

> I did not understand that 8 years back, and I still don't. Does it 
> mean a heuristic can't re-assemble?

Heuristics can request reassembly, but then they must not reject the data (return true). But only do this when you are sure it is your protocol. To ensure that your normal (not heuristics) dissector is called in the future, you can use conversation_set_dissector.

> The other part that seems to have changed are the settings for this. 
> Is it not desired anymore, that the use can select at dissector level, 
> whether it shall do the heuristic math or not?

Rather than having a preference at every dissector, it is now taken of by in the core (see function "heur_dissector_add"). In the GUI, you can enable or disable heuristics dissectors it in the Enabled Protocols menu.
--
Kind regards,
Peter Wu
https://lekensteyn.nl
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