Recently, I was trying to connect to an Azure SQL database from a client’s Windows Server. The Windows admin was telling me he can connect to any website from server but not my SQL database. It was evident that only 80 and 443 outbound ports are allowed but I had to prove this to convince him to open a ticket to network team. I was looking for a public server that listens on a non regular port. Should I run nc on one of my machines? Wouldn’t it be nice if there was a server that listens on all ports?

As a matter of fact, there is: PortQuiz.net. It listens on all TCP ports, so you can test outbound connections with telnet, curl or nc.

$ telnet portquiz.net 666
Connected to portquiz.net.
Escape character is '^]'.

$ curl http://portquiz.net:666
Port 666 test successful!
Your IP:

$ nc -v portquiz.net 666
Ncat: Version 7.80 ( https://nmap.org/ncat )
Ncat: Connected to