Tunneling is a way to forward otherwise unsecured TCP traffic through Secure Shell in encrypted format. Tunneling can provide secure application connectivity, for example, to POP3-, SMTP-, and HTTP-based applications that would otherwise be unsecured.

The Secure Shell v2 connection protocol provides channels that can be used for a wide range of purposes. All of these channels are multiplexed into a single encrypted tunnel and can be used for tunneling (forwarding) arbitrary TCP/IP ports and X11 connections.

The client-server applications using the tunnel will carry out their own authentication procedures, the same way they would without the encrypted tunnel.

The protocol/application might only be able to connect to a fixed port number (e.g. IMAP 143). Otherwise any available port can be chosen for tunneling. For remote (incoming) tunnels, the ports under 1024 (the well-known service ports) are not allowed for the regular users, but are available only for system administrators (root privileges).

There are two basic kinds of tunnels: local (outgoing) and remote (incoming). X11 forwarding and agent forwarding are special cases of a remote tunnel.

SSH Tectia Client and SSH Tectia ConnectSecure both provide basic tunneling functionalities and transparent TCP tunneling. SSH Tectia ConnectSecure has also additional more advanced tunneling features, see SSH Tectia ConnectSecure Administrator Manual.

This chapter gives an example of the SSH Tectia Server settings for a transparent TCP tunneling case and describes the different tunneling options available together with SSH Tectia Client and SSH Tectia ConnectSecure.