Acknowledging that system security is, on some level, futile is my way of admitting that this book isn't really about "Building Secure Servers." Clearly, the only way to make a computer absolutely secure is to disconnect it from the network, power it down, repeatedly degauss its hard drive and memory, and pulverize the whole thing into dust. This book contains very little information on degaussing or pulverizing. However, it contains a great deal of practical advice on the following:
 My original title was Attempting to Enhance Certain Elements of Linux System Security in the Face of Overwhelming Odds: Yo' Arms Too Short to Box with God, but this was vetoed by my editor (thanks, Andy!).
How to think about threats, risks, and appropriate responses to them
How to protect publicly accessible hosts via good network design
How to "harden" a fresh installation of Linux and keep it patched against newly discovered vulnerabilities with a minimum of ongoing effort
How to make effective use of the security features of some particularly popular and securable server applications
How to implement some powerful security applications, including Nessus and Snort
In particular, this book is about "bastionizing" Linux servers. The term bastion host can legitimately be used several ways, one of which is as a synonym for firewall. (This book is not about building Linux firewalls, though much of what I cover can/should be done on firewalls.) My definition of bastion host is a carefully configured, closely monitored host that provides restricted but publicly accessible services to nontrusted users and systems. Since the biggest, most important, and least trustworthy public network is the Internet, my focus is on creating Linux bastion hosts for Internet use.
I have several reasons for this seemingly-narrow focus. First, Linux has been particularly successful as a server platform: even in organizations that otherwise rely heavily on commercial operating systems such as Microsoft Windows, Linux is often deployed in "infrastructure" roles, such as SMTP gateway and DNS server, due to its reliability, low cost, and the outstanding quality of its server applications.
Second, Linux and TCP/IP, the lingua franca of the Internet, go together. Anything that can be done on a TCP/IP network can be done with Linux, and done extremely well, with very few exceptions. There are many, many different kinds of TCP/IP applications, of which I can only cover a subset if I want to do so in depth. Internet server applications are an important subset.
Third, this is my area of expertise. Since the mid-nineties my career has focused on network and system security: I've spent a lot of time building Internet-worthy Unix and Linux systems. By reading this book you will hopefully benefit from some of the experience I've gained along the way.