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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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This website comes in handy for that:

http://portquiz.net:1234/

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- portquiz.net:1234 
Port 1234 test successful!
Your IP: 198.252.206.16
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    portquiz.net is blocked by most corporate firewalls.
    – user379580
    Oct 14, 2014 at 17:12
  • Also, port 445 is not testable.
    – Max
    Nov 17, 2017 at 8:28
  • 2
    It's now 2019, and I have used nothing but corporate enterprise firewalls at all of the companies I have consulted for. None of them have blocked portquiz.net. If you're seeing it blocked, then most like the port you are testing is being blocked. Which answers your question of it being open or not...
    – John C
    Feb 1, 2019 at 16:26
  • What if the computer doesn't have internet access?
    – Geordie
    Jun 29, 2019 at 0:01

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