I just spent several days helping to upgrade an organization's servers from Red Hat Enterprise 3 to the latest version, Red Hat Enterprise 5.2. One of the most important, and impressive, issues we dealt with during this upgrade was SELinux, or security-enhanced Linux. SELinux offers a great deal of functionality, and helps to protect Linux boxes from a variety of threats. It forces system administrators to learn a new vocabulary, as well as permissions, logfiles, and programs with which they were previously unfamiliar. Fortunately, there are many good tutorials for SELinux on the Web.