SSH

Remote Tunnels

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

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

client$ sshg3 -R [protocol/][listen-address:]listen-port:dst-host:dst-port server

Setting up remote tunneling allocates a listener port on the remote server. Whenever a connection is made to this listener, the connection is tunneled over Secure Shell to the local client and another connection is made from the client to a specified destination host and port. The connection from the client onwards will not be secure, it is a normal TCP connection.

For example, if you issue the following command, all traffic which comes to port 1234 on the server will be forwarded to port 23 on the client. See Figure 8.4.

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

The forwarding address in the command is resolved at the (local) end point of the tunnel. In this case localhost refers to the client host.

Remote tunnel

Figure 8.4. Remote tunnel

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

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

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

The server starts listeners according to the current address family settings. For example, if the server is configured for IPv4 only, the following command will start listener on port 2001, address 127.0.0.1:

client$ sshg3 -R 2001:localhost:2002 user@server

If the address family is any, two listeners will be started, on address ::1 and 127.0.0.1 on the same port 2001.

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