I have a host that is NAT'ed and I have opened a single port in my router so the host is reachable by the external world. I tested if it was reachable by running a service and connecting to it externally.
I stopped the service, but the port is still open on the router. When I run nmap from an external server, it doesn't show that port and ends.
This is how I ran nmap from the external server:
sudo nmap -n -PN -sA x.x.x.x
and it returns:
Nmap done: 1 IP address (0 hosts up) scanned in 0.49 seconds
As far as I know my router blocks icmp requests. So how do I make nmap show unfiltered ports ?