This book shows in detail how to build enterprise-level secure, redundant, and highly scalable services from scratch on top of the open source Linux operating system, suitable for small companies as well as big universities. The core architecture presented is based on Kerberos, LDAP, AFS, and Samba. It is shown how to integrate web, message related, data base and other services with this backbone. This architecture provides a Single-Sign-On solution for different client platforms and can also be employed for clustering. Although it is implemented with Debian GNU/Linux, the content can be applied to other UNIX flavors.