I believe these errors happen if a remote host or an intermediate router sends back an ICMP unreachable message for that UDP ip adress / port number.

Because it's all a bit asynchronous, it might not come back immediately, you could get one later.

The remote host should send a maximum of one unreachable per UDP datagram received, I would have thought. Moreover, it should apply rate-limiting in case it gets bombarded with invalid UDP packets (Although I think Windows does not implement this).

So unless you're continuing to bombard the host with udp even after it's stopped listening, you shouldn't keep getting these messages back.

Of course any messages that were in transit or queued, may cause responses for that host. So you might get a few. But they should stop after you stop sending to that host.

Are you sure you're processing them on behalf of the right host?

Have you tried it on a lan with several real hosts and with a packet logger watching to see what's happening?

Mark

Thanks for getting back Mark. I'm in the middle or more testing.
I've never used a packet logger before, any good free ones out there you could name for me?

Ethereal is pretty much what you want.

Mark