Internet Core Protocols: The Definitive Guide

Firewalls Blocking UDP Messages

Many network managers block all UDP traffic, with the exception of a few critical ports (such as DNS). If you are experiencing problems connecting to remote UDP applications, then you should investigate whether or not a remote firewall is blocking UDP traffic. Note that the problem could also be reversed, with your firewall blocking the return UDP traffic. In this case, your UDP traffic may be leaving the network just fine, but the packets coming back from the remote system may be getting discarded by your own firewall.