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I have machines A and B that can access a Relay machine via ssh, but not vice versa. Can I access machine B from A via the Relay machine without opening ports except 22 as that port is the only one accessible?

For example, a possible solution would be to ssh from B into Relay and opening files for reading stdin and writing stdout. Connecting from A to relay allows reading and writing into these files for communication to B.

Is there a way with command line tools to achieve this?

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Well, there is obviously already a port open on the relay, otherwise ssh wouldn't be possible.

The only way I see is for B to ssh to the relay and along with that, open a reverse tunnel which A can then use. You're looking for the "-R" option.

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Here is one way:

  1. ssh A to relay and forward a local port (for e.g. ssh -L 6022:127.0.0.1:6022) This command forwards local port 6022 on A to port 6022 on relay over SSH.
  2. ssh B to relay and forward a remote port to local ( ssh -R 6022:127.0.0.1:22) This command forwards remote port 6022 on relay to local port 22 on B
  3. Now on A, connect to localhost port 6022 using ssh and should be able to login to B.

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