Custom authentication method

Hello,

I'm interested in writing a custom authentication method for poptop. I 
wrote an auth plugin for OpenVPN which checks against a PostgreSQL 
database for username / password and I'm interested in reusing it so 
account information isn't duplicated & I don't have to run a script 
every 5s to update chap-secrets.

Any ideas where I should begin? I downloaded the latest release (1.3.4) 
but I can't seem to find the code that handles user authentication when 
someone connects.

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