How to detect blocked UDP multicast port

**Peter Duniho** · Aug 16 '08, 04:55 PM

Re: How to detect blocked UDP multicast port

On Sat, 16 Aug 2008 03:30:07 -0700, Andy Bates <andy@ussdev.co mwrote:

Hi -
>
Can't see another newsgroup to post this in; so thought I'd post here.

There are in fact newsgroups dedicated to network programming, independent
of whatever language/platform being used. I don't know the names off the
top of my head, but for the best answers, that's where you'll find the
experts.

I have a C# application that relies on multicast UDP to detect how many
PCs
the application is executing on concurrently. This works okay providing
the
port is open but fails miserably if the port is blocked by a firewall.
All applications provide a server listening on the port for the ping but
for
some reason on the PC sending the ping irrespective of whether the port
is
open or blocked the request is still received. So I cannot even use this
to
detect that the port has been blocked.

It sounds like it's a NAT router blocking the datagram, not a firewall.
Which means that outbound traffic causes the router to set up a mapping
back to the sender, which allows responses to come back. A firewall would
just block the port without exception (other than that configured by the
user, of course).

The easiest solution is to just require the user to configure the NAT
router to forward traffic on your multicast port. A slightly more
complicated, but generally more reliable approach would be to implement
Universal Plug and Play support so that you can look for and configure a
NAT router explicitly.

Pete

How to detect blocked UDP multicast port

How to detect blocked UDP multicast port

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