Depending on your systems administration experience and background, the kernel is a subject of great mystery and speculation. It might be something you reconfigure when the whim strikes you, or something you know to leave alone.

Most commercial operating systems provide only a few basic hooks for configuring the kernel. Many open source operating systems tell you to rebuild the kernel from source code whenever you change anything.

OpenBSD falls somewhere in the middle.

The standard OpenBSD kernel is intended to be perfectly usable without modifications, but you have the tools to perform any tweaks or adjustments necessary for your environment. Additionally, you have the complete source code and kernel-building tools in case you decide to perform wholesale kernel surgery.

OpenBSD lets you adjust kernel behavior even as the system is running, via sysctl(8). Some hardware or protocols require special OpenBSD kernel tweaks to run in specific environments. This chapter will cover both kinds of changes, but first, let’s talk about the kernel in general.