SSH Tunnel Between Linux Servers - Poor Techies VPN

I recently had a need to create a permanent SSH tunnel between Linux servers. My need was to allow MYSQL replication .


Create a ‘tunnel’ user on SERVER 1:

[root@server1 ~]# useradd -d /home/tunnel tunnel
[root@server1 ~]# passwd tunnel       ## Set a strong password
[root@server1 ~]# su - tunnel           ## Become the user 'tunnel'
Now create a public/private key pair:

[tunnel@server1 ~]$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/tunnel/.ssh/id_rsa):    ## hit enter to accept the default
Enter passphrase (empty for no passphrase):                           ## don't use a  passphrase
Enter same passphrase again:
Your identification has been saved in /home/tunnel/.ssh/id_rsa.
Your public key has been saved in /home/tunnel/.ssh/id_rsa.pub.
The key fingerprint is:
6f:30:b8:e1:36:49:74:b9:32:68:6e:bf:3e:62:d3:c2 tunnel@server1
Now cat out the id_rsa.pub file which contains the public key that we will need to put on host b:

[tunnel@server1 ~]# cat /.ssh/id_rsa.pub
ssh-rsa blahAAAAB3NzaC1yc2EAAAABIwAAAQEA......6BEKKCxTIxgBqjLP tunnel@server1
Now create a ‘tunnel’ user on Host B and save the public key for tunnel@server1 in the authorized_keys file

[root@server2 ~]# useradd -d /home/tunnel tunnel
[root@server2 ~]# passwd tunnel       ## Set a strong password
[root@server2 ~]# su - tunnel
[tunnel@server2 ~]# mkdir .ssh
[tunnel@server2 ~]# vi .ssh/authorized_keys   ## Now paste in the public key for tunnel@server1
At this point you should be able to ssh from tunnel@server1 to tunnel@server2 without using a password. Depending on your configuration, you might need to allow the user ‘tunnel’ in /etc/ssh/sshd_config. You might also set some SSH options like the destination port in ~/.ssh/config.

Now, create this script as server1:/home/tunnel/check_ssh_tunnel.sh

createTunnel() {
    /usr/bin/ssh -f -N -L13306:server2:3306 -L19922:server2:22 tunnel@server2
    if [[ $? -eq 0 ]]; then
        echo Tunnel to server2 created successfully
    else
        echo An error occurred creating a tunnel to server2 RC was $?
    fi
}
## Run the 'ls' command remotely.  If it returns non-zero, then create a new connection
/usr/bin/ssh -p 19922 tunnel@localhost ls
if [[ $? -ne 0 ]]; then
    echo Creating new tunnel connection
    createTunnel
fi
Save that file and make it executable:

chmod 700 ~/check_ssh_tunnel.sh