Tunneling is a way to forward otherwise unsecured TCP traffic through Secure Shell. 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, if any, 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 and remote. They are also called outgoing and incoming tunnels, respectively. X11 forwarding and agent forwarding are special cases of a remote tunnel.

SSH Tectia Client and all versions of SSH Tectia Server provide the basic tunneling functionality. SSH Tectia Connector and SSH Tectia Server with Tunneling Expansion Pack used together provide dynamic secure application tunneling that is transparent to the end user (secure application connectivity).

This chapter gives an example of SSH Tectia Server with Tunneling Expansion Pack settings for an SSH Tectia Connector tunneling user and describes the different tunneling options available with SSH Tectia Client and SSH Tectia Server.