[Tarjei Knapstad <tarjei.knapstad@xxxxxxxxx>]

On 23 July 2010 13:49, Alan Bowman <alan.michael.bowman@xxxxxxxxx> wrote:
> I have written two dissectors for some protocols I am using, one of
> which is wrapped inside the other.  Protocol B is used if a particular
> byte (A6, below) in protocol A is a particular value.  I have
> registered A using register_dissector_table().  I can create a subset
> tvb and use dissector_try_port() to get protocol B dissector to be
> called and to have the data (B0..BN) passed to it.  However, B's
> dissection also depends on knowing whether A was sent as a broadcast
> packet or as a direct response to a request, which is stored in A's
> header (A2).  (Not my design!).  Can anyone suggest how I should pass
> this information to B?  Should I tack it onto the start of the tvb as
> a composite buffer, or is there some way of attaching metadata to it?
> Is there a similar protocol I should look at?
>
> A0 A1 A2 A3 A4 A5 A6 B0 B1 B2 ... BN
>
> Thanks in advance
>

Alan,

I think this is usually achieved by passing the data along in the
packet_info->private_data field. In your case I guess it would suffice
to just pass along a boolean. Hopefully someone will correct me if I'm
wrong here, but I think the standard procedure is along these lines:

  void* saved_private_data = pinfo->private_data;
  gboolean isBroadcastPacket = check_if_broadcast_packet(/*...*/);
  pinfo->private_data = &isBroadcastPacket;
  dissector_try_port(/*...*/);
  pinfo->private_data = saved_private_data;

Regards,
Tarjei