Who needs to secure their Linux systems? Arguably, anybody who has one connected to a network. This book should therefore be useful both for the Linux hobbyist with a web server in the basement and for the consultant who audits large companies' enterprise systems.
Obviously, the stakes and the scale differ greatly between those two types of users, but the problems, risks, and threats they need to consider have more in common than not. The same buffer-overflow that can be used to "root" a host running "Foo-daemon Version X.Y.Z" is just as much of a threat to a 1,000-host network with 50 Foo-daemon servers as it is to a 5-host network with one.
This book is addressed, therefore, to all Linux system administrators ? whether they administer 1 or 100 networked Linux servers, and whether they run Linux for love or for money.