Maintaining a secure Linux system requires that you understand a great deal about how Linux (or UNIX generally ) operates. You must be able to identify and shut down unnecessary servers, create user accounts as securely as possible, update software to fix security bugs , and keep your eyes open for successful and attempted intrusions. Because the security landscape is constantly changing, you must maintain your knowledge by monitoring security- related Web sites, mailing lists, and newsgroups. Although this sounds like a daunting task, once you're familiar with Linux you can keep yourself up to date by spending just a few minutes a day on the task. The time you take to improve your system's security will likely pay off at some point when a crack attempt is thwarted.
Chapters 23, 25 and 26 continue discussion of security themes by covering some more specific security-related topics.