Linux: The hole trick to bypass firewall restriction.

As long as remote is behaving itself, it will send back a “port unreachable” response via ICMP – however this is of no consequence. On the second attempt
remote# echo "hello" | nc -p 53 -u local-fw 14141
The netcat listener on console local/1 then coughs up a “hello” – the UDP packet from outside has passed through the firewall and arrived at the computer behind it.