Bypassing Blocked Ports in (Public) Wifi Networks

Wifi networks at conferences or hotels often block certain ports, either for enhancing security  or simply by misconfiguration. Blocking ports can be used for preventing some applications from accessing the internet via their access points. Other usages can be to force users to use encrypted protocols, instead of their clear text alternatives. In order to make this process easier, in many cases the ports which should be allowed are whitelisted, whereas everything else is blocked. If your company mail server uses a rather exotic port for some reason, sending and receiving emails does not work. Eduroam is an example, where different policies and configurations are in place, where often the port 25 of mail servers is blocked.

You can use the following command to verify if a port is blocked or open:


telnet mail.example.org 25

If you receive the timeout after some time, the port is very likely to be blocked.


Trying 111.222.1.0 ...

telnet: Unable to connect to remote host: Connection timed out

Fortunately SSH allows tunneling connections, as long as its standard port 21 is open, which is often the case. All you need is a server with an SSH enabled account outside of your current network, which can connect to the mail server. The following command establishes such a SSH tunnel and connects your local port 25 via an SSH secure tunnel to your web server, where the tunnel exits and connects to your mail server.


sudo ssh -L 25:mail.example.org:25 user@web.server.org

You need to enter your password for your user account on the web server here. As soon as the tunnel is established, you can connect any service to your localhost and all traffic on port 25 gets tunneled via your server. If you now try the telnet example again, you should be able to connect.


telnet localhost 25

Trying 127.0.0.1...

Connected to localhost.

Escape character is '^]'.

220 Email Service

How to establish an SSH tunnel in Windows is described elsewhere.
You can now use this tunnel also in your email program such as Thunderbird, to send emails and bypass the blocked port by tunnelling.Note the localhost as the new server address. This is the only thing that you need to change.

Thunderbird SMTP via Tunnel

In some cases the tool will detect that the certificate does not match. So you need to accept the certificate warning or add the certificate by your mail server address manually in the dialogue.

 

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