Port forwarding through ssh

There are two kinds of port forwarding: local and remote forwarding. They are also called outgoing and incoming tunnels, respectively. Local port forwarding forwards traffic coming to a local port to a specified remote port.

For example, if you issue the command

ssh2 -L 1234:localhost:23 username@host

all traffic coming to port 1234 on the client will be forwarded to port 23 on the server (host). Note that localhost will be resolved by the sshdserver after the connection is established. In this case localhost therefore refers to the server (host) itself.

Remote port forwarding does the opposite: it forwards traffic coming to a remote port to a specified local port.

For example, if you issue the command

ssh2 -R 1234:localhost:23 username@host

all traffic which comes to port 1234 on the server (host) will be forwarded to port 23 on the client (localhost).

It is important to realize that if you have three hosts, client, sshdserver, and appserver, and you forward the traffic coming to the client's port x to the appserver's port y, only the connection between the client and sshdserver will be secured. See Figure Forwarding to a third host. The command you use would be something like the following:

ssh2 -L x:appserver:y username@sshdserver