Tunneling is a way to forward otherwise unsecured application 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 tunnels, the ports under 1024 (the well-known service ports) are not allowed for ordinary 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. The different tunneling options are handled in the following sections.

All applications that attempt to connect using IPv6 addresses will be blocked if there is a rule that specifies a TCP_TUNNEL, FTP_TUNNEL or FTP-PROXY tunnel for that application. And in all other cases (when the application is defined for Block or Passthrough) it will behave the same way as with IPv4 addresses.

It is not possible to do any transparent tunnels specifying an IPv6 address.