Today I learned a cool trick to test if a certain port in your router is open or not.

I’m trying to get Wireguard correctly configured, and for that, I needed to open UDP port 51820 on my router. But for the life of me I can’t figure out whether the problem is my Wireguard setup or the router.

The first thing I want to do before looking into the Wireguard setup is to check the router. My internet provider is notorious for locking down things on the router (at the same time they enforce really shitty admin passwords – go figure). To test it, first I shut down my Wireguard container:

docker compose stop wg-easy

And then launched GNU netcat utility to listen on port 51820:

nc -u -l -p 51820

Then, I tried to connect to it locally (i.e. using the server’s LAN IP):

nc -u $SERVER_IP 51820

To see if it works, I typed “Hi” and hit Enter. Presto! The message appeared on the server listener.