|Linux PPP HOWTO|
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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.