In computing, cron is a time-based scheduling service in Unix-like computer operating systems. The name is derived from Greek chronos (χρόνος), meaning time.
Pluggable authentication modules or PAM are a mechanism to integrate multiple low-level authentication schemes into a high-level API, which allows for programs that rely on authentication to be written independently of the underlying authentication scheme. PAM was first proposed by Sun Microsystems in an Open Software Foundation RFC dated October, 1995. It was adopted at the authentication framework of the Common Desktop Environment. As a stand-alone infrastructure, however, PAM first appeared from an open-source, Linux-PAM, development in Red Hat Linux 3.0.4 in August of 1996. PAM is currently supported in AIX, FreeBSD, HP-UX, Linux, Mac OS X, NetBSD and Solaris. PAM was later standardized as part of the X/Open UNIX standardization process, resulting in the X/Open Single Sign-on (XSSO) standard.
The pluggable nature of PAM is one reason for using dynamic linking of system binaries. However, this necessitates the availability of a recovery mechanism should a problem develop in the linker or shared libraries; for example both NetBSD and FreeBSD supply a /rescue directory containing statically linked versions of important system binaries.
As the XSSO standard differs from both the original RFC, Linux and Sun APIs, and also from most other implementations, PAM implementations do not all operate in the same manner. For this and other reasons, OpenBSD has chosen to adopt BSD Authentication, an alternative authentication framework which originated from BSD/OS.
http://www.kernel.org/pub/linux/libs/pam/
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