A local (outgoing) tunnel forwards traffic coming to a local port to a specified remote port.

With sshg3 on the command line, the syntax of the local tunneling command is as follows:

client$ sshg3 -L [protocol/][listen-address:]listen-port:dst-host:dst-port sshserver

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.

For example, when you use Tectia Client on the command line, and issue the following command, all traffic coming to port 1234 on the client will be forwarded to port 23 on the server. See Figure 8.1.

sshclient$ sshg3 -L 1234:localhost:23 username@sshserver

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 (sshserver).

Figure 8.1. Simple local tunnel

To use the tunnel, the application to be tunneled is set to connect to the local listener port instead of connecting to the server directly. Tectia Client forwards the connection securely to the remote server.

If you have three hosts, for example, sshclient, sshserver, and imapserver, and you forward the traffic coming to the sshclient port 143 to the imapserver port 143, only the connection between sshclient and sshserver will be secured. The command you use would be similar to the following one:

sshclient$ sshg3 -L 143:imapserver:143 username@sshserver

Figure 8.2 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 8.2. Local tunnel to an IMAP server

With transparent TCP tunneling active, there is no need to separately configure application software to use local ports to set up the tunnels. The applications to be tunneled are defined in the Connection Broker configuration (Filter Rules). The transparent TCP tunneling feature automatically captures the defined applications and the Connection Broker creates Secure Shell tunnels to the defined Tectia Server.

By default, local tunnels are allowed to all addresses for all users. The default setting equals the following in the ssh-server-config.xml file:

<services>
  <rule>
    <tunnel-local action="allow" />
    ...
  </rule>
</services>

The connections can be restricted by specifying allowed addresses with the src and dst elements. If any addresses are specified as allowed, local tunnels to all other addresses are implicitly denied. See Local Tunneling Rule Examples for usage examples.

Using the Tectia Server Configuration GUI, the tunneling settings are made under the Services page on the Local Tunnels tab. See Local Tunnels.