Pluggable authentication modules or PAM are a mechanism to integrate multiple low-level authentication schemes into a high-level Application Programming Interface, which allows for programs that rely on authentication to be written independently of the underlying authentication scheme. PAM were first developed by Sun Microsystems, and are currently supported in AIX, HP-UX, Solaris Operating Environment, Linux, FreeBSD and NetBSD.

The pluggable nature of PAM is one reason for using dynamic linking of system binaries. However, there needs to be a recovery mechanism in case a problem appears with the linker or shared libraries; for example both NetBSD and FreeBSD supply a [http://www.freebsd.org/cgi/man.cgiquery=rescue /rescue] directory of statically linked versions of important system binaries.

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