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I have successfully connected to a mac using VNC on a LAN, but I can't connect when trying through the internet. I have forwarded the port in the router. How can I know if the ISPs are blocking VNC? How can I get undetected if this is happening? VPN? How can I know if the port forwarding is working?

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I tried to connect via SSH. The same thing happens, I'm able to connect using the local IP but I get a connection refused using extern IP. I fowarded port 22 to the machine. About the NAT reflection problem, I didn't understand, but trying from different network I get a connection timeout error. –  Roberto Jun 17 '11 at 2:32
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You could try port forwarding the SSH and then run your VNC over an SSH tunnel. While it does not let you know if VNC is being blocked, I've always found SSH easier to troubleshoot.

http://www.cyberciti.biz/tips/tunneling-vnc-connections-over-ssh-howto.html

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This is a good use for NMAP, if there is a server running on an open port, it will return open to NMAP, if the port is blocked, it will return closed (or if the server isn't running). Basically, if something is accepting incoming connections on a port and the port is open to the internet, NMAP will see it and report it back.

You should also note that if you are trying to hit a server that is behind NAT your own public IP from computer on the same NAT network, if the router doesn't support NAT reflection, the attempt will fail. You might get different results from another computer at a different public IP.

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