How to Access SSH Servers Behind Firewall with SSH Reverse Tunneling

Created: Jul, 2015 Last modified: Jul, 2015

Think of a case like this: Alice runs a server with public IP, and she wants to access Bob’s server behind a firewall:

Bob ([email protected], SSH port 22) <–> Firewall <–> Alice ([email protected], SSH port 2222).

Although direct SSH won’t work, she can still use SSH reverse tunneling.

• Step 1: Bob initiates an SSH session to Alice

ssh -p2222 -R 12345:localhost:22 [email protected]

with the password to Alice’s server. This will create an SSH tunnel between port 12345 (facing Alice) on Alice’s server and port 22 on Bob’s server.

• Step 2: On Alice’s server:

ssh -p12345 bob@localhost

with password to Bob’s server.

• Step 3: No further step. Done!

Here is another case: what if two sides (Bob and Alice) are both behind firewalls? Then a third server (Jim) with public IP should be involved. Bob initiates a SSH tunnel to Jim’s server, and Alice can log in Jim’s server to access Bob’s server, which is very similar to the above.

One can also use autossh for this task like:

autossh -M 0 -N -p2222 -R 12345:localhost:22 [email protected]

Autossh will monitor the SSH link state, and take actions when exceptions happen (e.g., reconnect if the link is lost, etc.).