In another external file
In a named pipe through the pap-secrets @ feature
As an option to pppd, either on the command line or in a PPP configuration file
Through the +ua file
Specifying papcrypt and using passwords that are hashed by crypt(3C) in the pap-secrets file.
Specifying the login option to pppd and omitting the password from the pap-secrets file by placing double quotes ("") in the password column. In this instance, authentication is done through the UNIX passwd database or the pam(3pam) mechanism.