/reverse-ssh-tunnel

Directions for setting up a reverse ssh tunnel as a systemd service in Linux

We assume that we have full access to a server (from now SSH server) and we want to get SSH access to a computer behind a firewall (from now client).

We also assume that the operating system is a Linux system which runs systemd services

Steps:

  1. Generate an id_rsa key using the ssh-keygen command.
  2. Copy the generated id_rsa.pub file to the SSH server.
  3. Add the line from the file sshd_config_edit to the end of the sshd_config file of your SSH server.
  4. Edit the file rtunnel.service to contain the correct information and add it in the folder /etc/system/systemd/
  5. Clean ~/.ssh/known_hosts and /root/.ssh/known_hosts if necessary
  6. Open the port [REMOTE PORT] at the SSH server
  7. Run as root the command systemctl daemon-reload
  8. Start the service with the command sudo systemctl start rtunnel
  9. Enable on startup: sudo systemctl enable rtunnel
  10. Connect to the client by issuing the command ssh -p 2225 remote.host.com where 2225 is a port of your choice and remote.host.com is the SSH server.

Debuging

To debug run the command /usr/bin/autossh -M 0 -f -N -i /path/to/.ssh/id_rsa -R 2225:localhost:22 -l username remote.host.com from the rtunnel.service file without the -f flag.

Explanation of flags

  • -M 0 : do not monitor
  • -f : run in the background
  • -N : do not do anything after connecting
  • -l : username (should be the same as the username appearing in the id_rsa and id_rsa.pub file )
  • -i : path to authentication file
  • -R : port forwarding. Forwards the port 2225 to the port 22 in the localhost.

Explanation of parameters:

  • username: the username mentioned in the id_rsa files.
  • remote.host.com: the SSH server
  • 2225 the port you want to use to forward the 22 port of your client