Accessing resources blocked by campus IT with SSH reverse tunnels

Problem: Campus IT decides to shut down external SSH access to your lab workstations. Solution: Reverse SSH tunnels!

Background

We have several high-powered workstations that various members of the lab log into to run simulations and analyses. We used to have SSH access to these machines from off campus via dedicated IP addresses. Recently, the campus IT folk decided to “harden” security by turning off all access to externally available IPs, so now we are in a pickle. We still want terminal access to these boxes, but how to make it happen? Reverse SSH tunnels to the rescue!

Reverse SSH tunnel magic

The logic of reverse SSH tunnels is that you bounce traffic to the firewalled machines off a box that you do have control of somewhere else on the internet. The mechanics involve routing SSH traffic through an available port on the external server to the SSH process on the firewalled workstation.

Here’s the gist:

  • Sit down at (or log into) the firewalled workstation.
  • Open a reverse ssh connection from the firewalled box to the external machine.
  • Establish an SSH connection to the external box to the new port and the traffic is forwarded to the previously unavailable workstation.
  • BAM! You have a terminal on the workstation inside the firewall.

Configuring the reverse SSH tunnel

For the purposes of this tutorial let’s say the firewalled workstation is called workstation1 and the local user account on the firewalled workstation you’d like to log in to is “local_user”. Assume that the externally available computer lives at remote_box.com, has SSH enabled, and that the user you’d like to log into on this box is remote_user. Here’s how to establish a reverse SSH tunnel to give you remote access to this machine:

  • Sit down at workstation1
  • Type the following command ssh -o ServerAliveInterval=60 -fN -R \*:9666:localhost:22 remote_user@remote_box.com

    NB: This will establish a reverse SSH tunnel from the remote_box port port 9666, to the firewalled box port 22.

  • Enter the password for the remote_user on the remote machine. This establishes the reverse SSH tunnel.
  • Test the setup: From any computer, anywhere on the internet type: ssh local_user@remote_box.com -p 9666

    NB:: This will bounce traffic off the publically available remote_box port 9666 to the firewalled workstation1 default SSH port (22).

MAGIC!!!!!!

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