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

What Is Forwarding?

Forwarding is a type of interaction with another network application, as shown in Figure 9-1. SSH intercepts a service request from some other program on one side of an SSH connection, sends it across the encrypted connection, and delivers it to the intended recipient on the other side. This process is mostly transparent to both sides of the connection: each believes it is talking directly to its partner and has no knowledge that forwarding is taking place. Even more powerfully, SSH forwarding can achieve certain types of communication that are impossible without it.

Figure 9-1. SSH forwarding

Forwarding isn’t a new concept. The basic operation of a terminal connection over a network (say, using telnet) is also a kind of forwarding. In a telnet connection, you sit on one end, your remote shell is on the other, and both sides operate as if directly connected by a serial cable. Nevertheless, sitting in the middle is a cooperating telnet client and server, forwarding bytes back and forth. SSH forwarding is much the same, except SSH plays fancy tricks with the data to add security.

We have also seen another type of SSH forwarding, agent forwarding . [6.3.5] This let us create SSH connections from one computer, through a second computer, and onto a third using public-key authentication, but without installing our private key on the second machine. To accomplish this, an ...