user-fn

Specifies the name of the user-provided function that will be used to perform authentication in place of the built-in authentication. If authentication succeeds, the function should return REQ-PROCEED and if authentication fails, it should return REQ-NOACTION. 

userdb 

Specifies the full path and base filename of the user database in the server’s native format. The native format is a system DBM file, which is a hashed file format allowing instantaneous access to billions of users.