A local (outgoing) tunnel forwards traffic coming to a local port to a specified remote port.
ssh2 on the command line, the syntax of the local tunneling command is the following:
$ ssh2 -L [protocol/][listen-address:]listen-port:dst-host:dst-port server
Setting up local tunneling allocates a listener port on the local client. Whenever a connection is made to this listener, the connection is tunneled over Secure Shell to the remote server and another connection is made from the server to a specified destination host and port. The connection from the server onwards will not be secure, it is a normal TCP connection.
Figure Local tunneling terminology shows the different hosts and ports involved in local port forwarding.
Figure : Local tunneling terminology
For example, when you issue the following command, all traffic coming to port 1234 on the client will be forwarded to port 23 on the server. See Figure Local tunnel.
$ ssh2 -L 1234:localhost:23 username@sshserver
Figure : Simple local (outgoing) tunnel
The forwarding address in the command is resolved at the (remote) end point of the tunnel. In this case
localhost refers to the server host (
If you have three hosts, for example,
imapserver, and you forward the traffic coming to the
sshclient port 143 to the
imapserver port 143, only the connection between
sshserver will be secured. The command you use would be similar to the following:
$ ssh2 -L 143:imapserver:143 username@sshserver
Figure Tunnel to an IMAP server shows an example where the Secure Shell server resides in the DMZ network. The connection is encrypted from the Secure Shell client to the Secure Shell server and continues unencrypted in the corporate network to the IMAP server.
Figure : Local (outgoing) tunnel to an IMAP server
By default, the server allows local tunnels to all addresses for all users. To restrict tunneling for all or for specified users, see Section Securing Tunneling.