Ethereal-users: Re: [Ethereal-users] Capture Filter LDP packets

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From: Guy Harris <guy@xxxxxxxxxx>
Date: Tue, 11 Dec 2001 11:55:58 -0800 (PST)
> I am new to ethereal.  Is it possible to write a capture filter which 
> captures only LDP (Label Distribution Protocol) packets?

Yes, *IF* you know the port number that will be used for LDP, and *IF*
no other service used on the network uses that port number.

If it's port N for both TCP and UDP, then

	port N

will work.  If it's port N for TCP, and port M for UDP, then

	tcp port N or udp port M

will work.

The port specified by RFC 3036 is port 646, so

	port 646 will work.

*However*, there is no guarantee that no other service used on the
network will use that port number; a machine that doesn't receive LDP
packets may well use port 646 for other reasons.  You'd have to add "not
dst host XXX" clauses to the filter to filter out non-LDP packets sent
to port 646 on those hosts.