Firewall rules in Google Compute Engine not working

Question

In my GCE Console, I have added firewall rules to allow TCP connections to ports 81 and 82. But when I try to telnet into them, they do not work. Another port (83), which I opened in the same firewall rule, works fine and I am able to connect to it. What could be the reason for the first two ports remaining blocked?

The concerned firewall rule looks like this:

Source Ranges: 0.0.0.0/0

Allowed Protocols or Ports:

tcp:81
tcp:82
tcp:83

Answer 1

Do you have anything listening on ports 81 and 82?

The firewall will send RST packets for blocked TCP traffic, but your OS will do the same if there is nothing listening on that port. I would use tcpdump on the target machine to see if you see the SYN packets from your telnet connection.