Check if AutoSSH-tunnel is available
The -p and -i flags are here just for extra clarity, in case the do differ from the default.
ssh -p 22 remote-user@remote-machine netstat -lpnt 2>&1 | grep -q '127.0.0.1:9000' || autossh -f -i ~/.ssh/id_rsa -p 22 remote-user@remote-machine -N -R 9000:localhost:22
You can then reverse-SSH with the following command: ssh root@localhost -p 9000 (on remote-server).
Another option for running SSH in background:
ssh -N -R 9000:localhost:22 -p 22 -i /root/.ssh/id_rsa remote-user@remote-server