Port forwarding encapsulates the TCP-based communication session between the SSH client and the SSH server programs. Any TCP-based application or service can use port forwarding to take advantage of all the benefits of SSH. Using port forwarding, SSH allows you to establish a "secure tunnel" between two hosts, through which the participating applications operate transparently. For example, when you forward a regular TELNET connection through SSH, all information, including your user name, password, and actual data, are automatically encrypted and checked for integrity.

X11 port forwarding encrypts the X protocol (for X Window Systems). Using SSH, you can invoke X programs on a remote machine and have them appear on your local display. In this case, all X-protocol data is secured. For more information, see Chapter 5.