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I'm behind fairly restrictive firewall regarding ports, that can be used for connecting outside. I'm also running service on my remote box, and I want to connect to it.

Problem is, that I'm unable to find out which ports I can access, so I don't know where I can bind the service. (The typical unrestricted ports like 80 and 443 are already used on given machine).

I tried to run nmap -sS behind firewall to scan all TCP ports and wireshark on the second machine to determine SYNs that I receive, but I can see false positives (when I listen with netcat on given port and try to connect, it fails).

Is there any tool that can serve my purpose?

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up vote 6 down vote accepted

This website comes in handy for that:

This example tests whether you are able to visit outbound port 1234. You simply change the port number to whatever you like. Also, the site gives some examples that could be used in a command line script:

$ wget -qO- 
Port 1234 test successful!
Your IP:
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Nice. Exactly what I needed, thanks! – Yossarian Aug 12 '13 at 20:10
1 is blocked by most corporate firewalls. – user379580 Oct 14 '14 at 17:12

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