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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.
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. Agent forwarding is a special cases of a remote tunnel.