PAM modules are processed as stacks. Different types of modules have different purposes, for example, one module checks the password, another one verifies the location from which the system is accessed, and yet another one reads user-specific settings. PAM knows about four different types of modules:
auth
The purpose of this type of module is to check the user's authenticity. This is traditionally done by querying a password, but it can also be achieved with the help of a chip card or through biometrics (fingerprints or iris scan).
account
Modules of this type check whether the user has general permission to use the requested service. As an example, such a check should be performed to ensure that no one can log in under the username of an expired account.
password
The purpose of this type of module is to enable the change of an authentication token. In most cases, this is a password.
session
Modules of this type are responsible for managing and configuring user sessions. They are started before and after authentication to register login attempts in system logs and configure the user's specific environment (mail accounts, home directory, system limits, etc.).
The second column contains control flags to influence the behavior of the modules started:
required
A module with this flag must be successfully processed before the authentication may proceed. After the failure of a module with the required flag, all other modules with the same flag are processed before the user receives a message about the failure of the authentication attempt.
requisite
Modules having this flag must also be processed successfully, in much the same way as a module with the required flag. However, in case of failure a module with this flag gives immediate feedback to the user and no further modules are processed. In case of success, other modules are subsequently processed, just like any modules with the required flag. The requisite flag can be used as a basic filter checking for the existence of certain conditions that are essential for a correct authentication.
sufficient
After a module with this flag has been successfully processed, the calling application receives an immediate message about the success and no further modules are processed, provided there was no preceding failure of a module with the required flag. The failure of a module with the sufficient flag has no direct consequences, in the sense that any subsequent modules are processed in their respective order.
optional
The failure or success of a module with this flag does not have any direct consequences. This can be useful for modules that are only intended to display a message (for example, to tell the user that mail has arrived) without taking any further action.
include
If this flag is given, the file specified as argument is inserted at this place.
The module path does not need to be specified explicitly, as long as the module is located in the default directory /lib/security. The fourth column may contain an option for the given module, such as debug (enables debugging) or nullok (allows the use of empty passwords).
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source:http://linuxpoison.blogspot.com/2008/09/13578175716227.html