SSH, The Secure Shell: The Definitive Guide, 2nd Edition

Port Forwarding

SSH uses TCP/IP as its transport mechanism, usually TCP port 22 on the server machine, as it encrypts and decrypts the traffic passing over the connection. We now discuss a cool feature that encrypts and decrypts TCP/IP traffic belonging to other applications, on other TCP ports, using SSH. This process, called port forwarding, is largely transparent and quite powerful. Telnet, SMTP, NNTP, IMAP, and other insecure protocols running over TCP can be made secure by forwarding the connections through SSH. Port forwarding is sometimes called tunneling because the SSH connection provides a secure “tunnel” through which another TCP/IP connection may pass.

Suppose you have a home machine H that runs an IMAP-capable email reader, and you want to connect to an IMAP server on machine S to read and send mail. Normally, this connection is insecure, with your mail account password transmitted as plaintext between your mail program and the server. With SSH port forwarding, you can transparently reroute the IMAP connection (found on server S’s TCP port 143) to pass through SSH, securely encrypting the data over the connection.^[128] The IMAP server machine must be running an SSH server for port forwarding to provide real protection.

In short, with minimal configuration changes to your programs, SSH port forwarding protects arbitrary TCP/IP connections by redirecting them through an SSH session. Port forwarding can even pass a connection safely through a firewall if you configure things ...