Kernel-Space Modules Versus User-Space Applications

Some features such as network drivers, IP Security (IPSec), and IPv6 support are implemented in kernel space, meaning they are an integral part of the kernel sources or are available as dynamically loadable kernel modules. Other features such as httpd or vrrpd are implemented as user-space applications (processes or daemons) and run under special system accounts or (un)privileged users. Because applications can be more easily ported than kernel routines and a lot of stuff such as packet filtering that heavily interacts with the kernel needs to be placed in the kernel, knowing what is implemented in which space will help you as you go through this book.