OpenSSH Concepts

Internal

• OpenSSH

sshd

The OpenSSH server.

Tunneling

Local Forwarding

Local forwarding configures ssh tunneling to listen on a local port and forward the data sent into that port to a remote server, on the specified "remote" port.

-L [bind_address:]port:host:hostport
-L [bind_address:]port:remote_socket
-L local_socket:host:hostport
-L local_socket:remote_socket

This works by allocating a socket to listen to either a TCP "port" on the local side, optionally bound to the specified "bind_address", or to a Unix socket. Whenever a connection is made to the local port or socket, the connection is forwarded over the secure channel, and a connection is made to either "host" port "hostport", or the Unix socket "remote_socket", from the remote machine.

Remote Forwarding

Remote forwarding configures ssh tunneling to listen on a port on the remote side of the tunnel and forward requests to a local port.

-R [bind_address:]port:host:hostport
-R [bind_address:]port:local_socket
-R remote_socket:host:hostport
-R remote_socket:local_socket
-R [bind_address:]port

This works by allocating a socket to listen to either a TCP port or to a Unix socket on the remote side. Whenever a connection is made to that port or Unix socket, the connection is forwarded over the secure channel, and a connection is made from the local machine to either an explicit destination specified by "host" port "hostport", or "local_socket", or, if no explicit destination was specified, ssh will act as a SOCKS 4/5 proxy and forward connections to the destinations requested by the remote SOCKS client.

Tunneling Operations

Setup a SSH Tunnel

ssh Agent

Also see ssh Agent Forwarding