If you decide to not allow users to have shell access, then the best way to keep them from gaining it is to make their shell pppd. This is done in the /etc/passwd file. You can see /etc/passwd file that I did this for the last three users. The last field of the /etc/passwd file is the user's shell. You needn't do anything special to pppd in order to make it work. It gets executed as root when the user connects. This is certainly the simplest setup to be had, as well as the most secure, and ideal for large scale and corporate systems. I describe exactly what all needs to be done later in this document. You can Section 5.7 if you like.