This requires that you have mutual authentication
methods - that is you must allow for both your machine to authenticate
the remote server AND the remote server to authenticate your
machine.
So, if your machine is fred and the remote is barney, your
machine would set name fred remotename barney and the remote
machine would set name barney remotename fred in their respective
/etc/ppp/options.ttySx files.
The /etc/chap-secrets file for fred would look like
# Secrets for authentication using CHAP
# client server secret acceptable local IP addresses
fred barney flintstone
barney fred wilma |
and for barney
# Secrets for authentication using CHAP
# client server secret acceptable local IP addresses
barney fred flintstone
fred barney wilma |
Note in particular that both machines must have entries for
bidirectional authentication. This allows the local machine to
authenticate itself to the remote AND the remote machine to
authenticate itself to the local machine.
Take our 
