how to find if port is blocked by router over ssh

Question

I have several servers deployed behind consumer grade NAT (home routers). My client connects periodically to these servers on port 3000, I have a hole punched in the firewall for port 3000 and port 22 for ssh debugging.

Occasionally the client can't connect to one or more server. How can I debug this situation?. I want to be able to tell if the failure was caused by the firewall blocking my client or if it was a bug in the server/client code.

Solution

You should use netcat, it is a Linux command line utility that will allow to create a server or client on the fly on the remote machine and create the corresponding server or client on your machine, then you send messages from one end and if it is received on the other end then the port is open. https://www.digitalocean.com/community/tutorials/how-to-use-netcat-to-establish-and-test-tcp-and-udp-connections-on-a-vps