If you consider yourself as Linux administrator this post might be useful for you. NSA published some nice papers with hardening Linux servers recommendations. NSA - National Security Agency - organization that is trusted. Latest version of 200 hundred pages guide in PDF for securing configuration of RHEL5. A very detailed manual, explaining general principles of secure system, including all necessary system changes, service configuration, network security, firewall, etc..

A must have thing for every admin. If you are Debian or Ubuntu admin it still worth reading, since principles stays the same and there is no much real difference in security between RHEL and Ubuntu.

Link to RHEL5 secure guide NSA site (local copy)

Quick one A4 page pamphlet (local copy)

Besides RHEL, there are many guides for other OSes as well


Debian and Ubuntu security guide (local copy)