Listen for connections on a port

On the server run netcat to listen for any connections and receive messages on a port:

netcat -v -v -l -n -p 443

This is for TCP connections. If you want to test for UDP protocol, you will need to add a -u to the command:

netcat -v -v -l -n -u -p 17000

Connect to server

Next connect to the server, to check, whether a firewall blocks the connection:

netcat domain_or_ip_address 443 test

Then test should appear as text output on the server.