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.