Check what ports on a local firewall are open to the outside

My computer is on a local network and the network is controlled by a firewall. I am not the administrator of the firewall and do not have access to it. Is there a way to scan open ports on the firewall from my local computer? I need to see what ports are open to outside.

1 Answer

The best solution is probably to ask the system administrator which is the actual port, or in case you can log into the machine somehow (i.e. locally) check with 'netstat -tulpen' or something like that, to identify the ssh port.

If you really want to scan the ports, it is as simple as:

(*Warning: make sure your system/network administrator is OK with you using nmap)

nmap IP_Of_SERVER

Or adding some options...

sudo nmap -P0 -sV -A -T4 -p1-5000 IP_Of_SERVER

This will show you the status of the ports from 1 to 5000.

You can use another upper limit (default is 1 to 1024). But if they changed the default port, it might probably be in somewhere far enough to make it more difficult to find.

The -sV option makes checks on open ports to determine the actual service running on the port.

The -P0 is to make sure you get to the machine even if ping is filtered, and -A will show you (among other things) the service and version running at each port. The -T4 makes it faster (but also the network admins angrier...).