IMHO, you don't have any FLOSS option to achieve your needs as of now. But in the very near future FreeIPA(http://www.freeipa.org/) can fulfill most of your requirements . The current version(1.2.1) implements full centralized authentication with LDAP backend. But does not have things like group policies and selinux support. Its proposed to be there in version 2 which is due in another 2-3 months. Development of the project is very fast and is a very stable software as of now itself. See the road map for version 2,
http://www.freeipa.org/page/Roadmap
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IMHO, you don't have any FLOSS option to achieve your needs as of now. But in the very near future FreeIPA(http://www.freeipa.org/) can fulfill most of your requirements . The current version(1.2.1) implements full centralized authentication with LDAP backend. But does not have things like group policies and selinux support. Its proposed to be there in version 2 which is due in another 2-3 months. Development of the project is very fast and is a very stable software as of now itself. See the road map for version 2, http://www.freeipa.org/page/Roadmap