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https://zaiste.net/ssh_port_forwarding/#remote-port-forwarding

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.

Is destination_port port necessary on the machine that initiated the ssh command?

Isn't destination_port port on machine forward_to_host instead?

Isn't 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?

Thanks.

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Unfortunately all the examples in the documentation you link to forward to localhost, and it appears the description of -R is restricted to that. You’re correct in thinking that this isn’t an inherent limitation:

  • destination_port doesn’t have to be on the host where ssh is run;
  • destination_port is indeed on forward_to_host;
  • forward_to_host isn’t necessarily the host where ssh is run.

The whole point of -R is to allow a remote system to connect to any other system which is reachable from the initiating host. It’s the opposite of -L. The target host can be the initiating host itself, but it can also be useful to use this for other hosts — e.g. when a reverse tunnel is required for confidentiality, or when the initiating system is inside a network which the remote system can’t access directly.

  • It's maybe worth noting that source_port is only reachable on localhost on via_host, unless the server has GatewayPorts enabled. – Stephen Harris Feb 5 at 19:29

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