Remote port forwarding is created with -R parameter.
ssh -R source_port:forward_to_host:destination_port via_host
This command connects to via_host. via_host runs a SSH server. It then forwards all connection attempts to source_port on the remote via_host machine to destination_port port on the local machine (a machine that initiated the ssh command) . forward_to_host machine must be reachable from the the local machine machine. Forwarding can be also done through Unix sockets.
destination_port port necessary on the machine that
initiated the ssh command?
destination_port port on machine
forward_to_host not necessarily the machine that initiated the ssh command?
Isn't the only requirement on
forward_to_host is that it must be any machine reachable from the machine that initiated the ssh command?