Most recent Linux distributions use PAMs (Pluggable Authentication Modules), which are the key to flexible authentication. A PAM is an ASP designed to modularize the process of authenticating a user to a service. It was developed by Sun Microsystems and can be found on recent versions of Sun Solaris, IBM AIX, Hewlett-Packard's HP-UX and Apple Mac OS X, though it is used most widely with the various free operating systems, including Linux (notably Red Hat, Debian and SuSE), FreeBSD and NetBSD.
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