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?


You have to make sure that the external server answers on every port, which is quite unusual. But if that is the case you are on the right track.

  • Thanks. I'm still having weird issues. My port scan returns the open port on the server(22), I can ping the server IP, yet the firewall is still blocking ssh connections. – Nick the magic man May 6 '14 at 15:12
  • Maybe some kind of content inspection. Try Corkscew through the web proxy and run your SSH on 443. Has been working the last time I tried it in a corporate lan. – ceving May 6 '14 at 15:47

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