Theoretically I was thinking a firewall inspects the TCP header and looks at the destination port and discards/forwards based on its firewall rules.
So could I port scan an external server to the firewall and see what ports are returned?
I'm on a network where FTP, PPTP, and other stuff is blocked. I just want to know what ports are open on the firewall and available to me.
Am I on the right track or is there more to it?