[Guy Harris <gharris@xxxxxxxxx>]

Stefano Pettini wrote:

> My protocol (ALC, RFC 3450) is based on UDP, but the rfc doesn't define 
> a default port. Indeed, the protocol seems to use a set of consecutive 
> UDP ports, starting from a base one, which is not specified and is left 
> to the user discretion.

...so one possibility would be to make it a preference setting, as per 
one of Luis Ontanon's suggestions.  You'd have to pick a default port, 
though - although you could make a port number of 0 mean "don't register 
it", and make that the default.

> I've left proto_ref_handoff_xxx empty, but my dissector becomes 
> invisible (I can't choose it manually in the "decode as..." window).
>
> Maybe it's not possible to automatically decode, after a brief 
> inspection, an unknown UDP packet as an ALC one,

That'd be a heuristic dissector (as Luis mentioned) - a very quick look 
at RFC 3450 doesn't suggest a good heuristic.

> but do I have to call 
> dissect_add with a fake default UDP port, even if that default isn't 
> specified?

No - you could call "dissector_add_handle()" to add it to the list of 
dissectors that could be dissected based on the UDP port; it takes no 
port number, so it doesn't cause any particular port to be dissected 
with the dissector in question, but it does arrange that "Decode As" can 
be used with a UDP port number.