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Essential System Administration, 3rd Edition by Æleen Frisch

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User Authentication with PAM

Traditionally, with very few exceptions, user authentication on Unix systems occurs at login time. In recent years, however, a new scheme has emerged that allows the authentication process to be performed and customized for a variety of system contexts. This functionality is provided by the PAM facility.

PAM stands for Pluggable Authentication Modules. PAM is a general user authentication facility available under and provided by current versions of FreeBSD, HP-UX, Linux, and Solaris. PAM’s goal is to provide a flexible and administrator-configurable mechanism for authenticating users, independent of the various programs and facilities which require authentication services. In this way, programs can be developed independently of any specific user-authentication scheme instead of having one explicitly or implicitly embedded within them. When using this approach, utilities call various authentication modules at runtime to perform the actual user-validation process, and the utilities then act appropriately depending on the results the modules return to them.

There are several components to the PAM facility:

  • PAM-aware versions of traditional Unix authentication programs (for example, login and passwd). Such programs are referred to as services.

  • Modules to perform various specific authentication tasks. These are implemented as shared libraries (.so files), stored in /lib/security under Linux, /usr/lib/security under Solaris and HP-UX, and in /usr/lib under FreeBSD. ...

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