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2.4.3.2. Control Flag

All PAM modules generate a success or failure result when called. Control flags tell PAM what do with the result. Modules can be stacked in a particular order, and the control flags determine how important the success or failure of a particular module is to the overall goal of authenticating the user to the service.
There are four predefined control flags:

Important

The order in which required modules are called is not critical. Only the sufficient and requisite control flags cause order to become important.
A newer control flag syntax that allows for more precise control is now available for PAM.
The pam.d man page, and the PAM documentation, located in the /usr/share/doc/pam-<version-number>/ directory, where <version-number> is the version number for PAM on your system, describe this newer syntax in detail.