Linux-PAM (Pluggable Authentication Modules for Linux) is a library that enables the local system administrator to choose how individual applications authenticate users. For an overview of the Linux-PAM library see the Linux-PAM System Administrators' Guide.
A Linux-PAM module is a single
executable binary file that can be loaded by the
Linux-PAM interface library.
This PAM library is configured locally with a system file,
/etc/pam.conf, to authenticate a user
request via the locally available authentication modules. The
modules themselves will usually be located in the directory
/lib/security (or
/lib64/security, depending on the architecture)
and take the form of dynamically loadable object files (see
dlopen(3). Alternatively, the modules can be statically
linked into the Linux-PAM library;
this is mostly to allow Linux-PAM to
be used on platforms without dynamic linking available, but this is
a deprecated functionality. It is the
Linux-PAM interface that is called
by an application and it is the responsibility of the library to
locate, load and call the appropriate functions in a
Linux-PAM-module.
Except for the immediate purpose of interacting with the user (entering a password etc..) the module should never call the application directly. This exception requires a "conversation mechanism" which is documented below.