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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?

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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.

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  • 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
    Commented May 6, 2014 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
    Commented May 6, 2014 at 15:47

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