20.5.2. Port Forwarding

SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.

Port forwarding works by mapping a local port on the client to a remote port on the server. SSH can map any port from the server to any port on the client; port numbers do not need to match for this technique to work.

To create a TCP/IP port forwarding channel which listens for connections on the localhost, use the following command:

ssh -L local-port:remote-hostname:remote-port username@hostname

To check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:

ssh -L 1100:mail.example.com:110 mail.example.com

Once the port forwarding channel is in place between the client machine and the mail server, direct a POP3 mail client to use port 1100 on the localhost to check for new mail. Any requests sent to port 1100 on the client system are directed securely to the mail.example.com server.

If mail.example.com is not running an SSH server, but another machine on the same network is, SSH can still be used to secure part of the connection. However, a slightly different command is necessary:

ssh -L 1100:mail.example.com:110 other.example.com

In this example, POP3 requests from port 1100 on the client machine are forwarded through the SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com connects to port 110 on mail.example.com to check for new mail. Note, when using this technique only the connection between the client system and other.example.com SSH server is secure.

Port forwarding can also be used to get information securely through network firewalls. If the firewall is configured to allow SSH traffic via its standard port (22) but blocks access to other ports, a connection between two hosts using the blocked ports is still possible by redirecting their communication over an established SSH connection.