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I have two machines A and B which are in different subnets, both behind separate firewalls. Machine A can see B, but B cannot see A. I have a user account (non-root) on both machines, I can SSH B from A, and I would like to be able to SSH A from B instead, but that cannot be done directly.

I have used tunnelling to hop through intermediate servers with SSH, but what I am asking is different here, and I don't know what it would be called. Is there a way to open a connection from A to B, which could in turn be used "in reverse" from machine B to run commands on A?

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    This looks like what you're after: unix.stackexchange.com/questions/46235 . Basically you want the -R option of forwarding (exposing a port on the dialling machine to the remote host)
    – zefixlluja
    Jan 27, 2020 at 14:33
  • Reading through the answers now, but looks like it does :) Thanks a lot @zefixlluja! Feel free to vote to close as duplicate. I will support the vote once I finish reading if that answers my question, or clarify otherwise.
    – Jonathan H
    Jan 27, 2020 at 14:38

1 Answer 1

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The short answer is yes you can, the how is:

machine-A$ ssh -R 127.0.0.1:2222:127.0.0.1:22 [ip__or_name_of_B]

Then on B you can ssh to A with:

machine-B$ ssh -p2222 127.0.0.1

This says the following:

  • On A create a tunnel on the remote side (-R), such that any traffic that goes to localhost (127.0.0.1) on port 2222 should come back through the tunnel and be sent to localhost (127.0.0.1 now on the local side) port 22

  • The B command simply says, ssh to localhost port 2222 which is the entrance to the tunnel.

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  • thanks to you i understand with a simple command
    – jonathan
    May 28, 2020 at 20:37

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