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OpenSSH has the ability to create a tunnel to encapsulate another protocol in an encrypted session.

The following command tells ssh to create a tunnel for telnet:

% ssh -2 -N -f -L 5023:localhost:23 user@foo.example.com

The ssh command is used with the following options:

-2

Forces ssh to use version 2 of the protocol. (Do not use if you are working with older SSH servers)

-N

Indicates no command, or tunnel only. If omitted, ssh would initiate a normal session.

-f

Forces ssh to run in the background.

-L

Indicates a local tunnel in localport:remotehost:remoteport fashion.

user@foo.example.com

The remote SSH server.

An SSH tunnel works by creating a listen socket on localhost on the specified port. It then forwards any connection received on the local host/port via the SSH connection to the specified remote host and port.

In the example, port 5023 on localhost is being forwarded to port 23 on localhost of the remote machine. Since 23 is telnet, this would create a secure telnet session through an SSH tunnel.

This can be used to wrap any number of insecure TCP protocols such as SMTP, POP3, FTP, etc.

Example 1: Using SSH to Create a Secure Tunnel for SMTP

% ssh -2 -N -f -L 5025:localhost:25 user@mailserver.example.com
user@mailserver.example.com's password: *****
% telnet localhost 5025
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailserver.example.com ESMTP

This can be used in conjunction with an ssh-keygen and additional user accounts to create a more seamless/hassle-free SSH tunneling environment. Keys can be used in place of typing a password, and the tunnels can be run as a separate user.

Example 2: Secure Access of a POP3 Server

At work, there is an SSH server that accepts connections from the outside. On the same office network resides a mail server running a POP3 server. The network, or network path between your home and office may or may not be completely trustable. Because of this, you need to check your e-mail in a secure manner. The solution is to create an SSH connection to your office's SSH server, and tunnel through to the mail server.

% ssh -2 -N -f -L 2110:mail.example.com:110 user@ssh-server.example.com
user@ssh-server.example.com's password: ******

When the tunnel is up and running, you can point your mail client to send POP3 requests to localhost port 2110. A connection here will be forwarded securely across the tunnel to mail.example.com.

Example 3: Bypassing Firewall

Some network administrators impose extreme firewall rules, filtering not only incoming connections, but outgoing connections. You may be only given access to contact remote machines on ports 22 and 80 for SSH and web surfing.

You may wish to access another (perhaps non-work related) service, such as an Ogg Vorbis server to stream music. If this Ogg Vorbis server is streaming on some other port than 22 or 80, you will not be able to access it.

The solution is to create an SSH connection to a machine outside of your network's firewall, and use it to tunnel to the Ogg Vorbis server.

% ssh -2 -N -f -L 8888:music.example.com:8000 user@unfirewalled-system.example.org
user@unfirewalled-system.example.org's password: *******

Your streaming client can now be pointed to localhost port 8888, which will be forwarded over to music.example.com port 8000, successfully evading the firewall.

Similarly you can view websites secure and anonymously:

% ssh -2 -N -f -L 8080:unblocked-proxy.web-server.com:80 user@ssh-server.example.org

In your browser setting, configure it to use proxy via 127.0.0.1 and 8080 as the port number.

When the tunnel is up, you can now browse the blocked websites secure and anonymously over your proxy server.

There are more interesting articles at Hacking Linux Exposed.

Reference:

OpenSSH.com