Get pty-redir-0.1-bart.tar.gz (slight modification to the original: ftp://ftp.vein.hu/ssa/contrib/mag/pty-redir-0.1.tar.gz) compile it and install it in /usr/local/bin/pty-redir on the master.
Make sure you have installed ssh on both machines and sudo on the slave.
Login in as root on the master and run ssh-keygen (if you don't have ~root/.ssh/identity.pub).
Make an account for user vpn on the slave computer (or whatever you want this user to be called). Login as this slave and execute ssh-keygen to initialize the ssh engine for this user. Now copy over the ~root/.ssh/identity.pub from the master to ~vpn/.ssh/authorized_keys on the slave). Test the ssh keys by running ssh vpn@slave ls on the master.
On the slave computer give the vpn account access to route and pppd. The eazy way of doing it is with sudo. Sudo grants super user access to normal users if given permissions listed in the /etc/pseudoers file. The proper way to edit this file is with visudo. So add these lines:
Cmnd_Alias VPN=/usr/sbin/pppd,/sbin/route vpn ALL=NOPASSWD: VPNNOTE: This is not a very good solution since anybody that gains access to the account vpn can mess up your routes.
Copy my vpn init.d script (vpn) to /etc/rc.d/init.d/vpn on your master and edit the top portion to indicate your network configuration.
Now execute the script /etc/rc.d/init.d/vpn start ... if you configured everything properly it should work. If not email me.