At the time of this writing, two major Apache branches are widely used. The Apache 1.x branch is the well-known, and well-tested, web server that led Apache to dominate the web server market. The 2.0.x branch is the next-generation web server, but one that has suffered from the success of the previous branch. Apache 1 is so good that many of its users do not intend to upgrade in the near future. A third branch, 2.2.x will eventually become publicly available. Although no one can officially retire an older version, the new 2.2.x branch is a likely candidate for a version to replace Apache 1.3.x. The Apache branches have few configuration differences. If you are not a programmer (meaning you do not develop modules to extend Apache), a change from an older branch to a newer branch should be straightforward.
This book covers both current Apache branches. Wherever there are differences in the configuration for the two branches, such differences are explained. The 2.2.x branch is configured in practically the same way as the 2.0.x branch, so when the new branch goes officially public, the book will apply to it equally well.
Many web security issues are directly related to the operating system Apache runs on. For most of this book, your operating system is irrelevant. The advice I give applies no matter whether you are running some Unix flavor, Windows, or some other operating system. However, in most cases I will assume you are running Apache on a Unix platform. Though Apache runs well on Windows, Unix platforms offer another layer of configuration options and security features that make them a better choice for security-conscious deployments. Where examples related to the operating system are given, they are typically shown for Linux. But such examples are in general very easy to translate to other Unix platforms and, if you are running a different Unix platform, I trust you will have no problems with translation.