In order to create a custom kernel configuration file and build a custom kernel, the full FreeBSD source tree must first be installed.

If /usr/src/ does not exist or it is empty, source has not been installed. Source can be installed using Subversion and the instructions in Section A.3, “Using Subversion”.

Once source is installed, review the contents of /usr/src/sys. This directory contains a number of subdirectories, including those which represent the following supported architectures: amd64, i386, ia64, powerpc, and sparc64. Everything inside a particular architecture's directory deals with that architecture only and the rest of the code is machine independent code common to all platforms. Each supported architecture has a conf subdirectory which contains the GENERIC kernel configuration file for that architecture.

Do not make edits to GENERIC. Instead, copy the file to a different name and make edits to the copy. The convention is to use a name with all capital letters. When maintaining multiple FreeBSD machines with different hardware, it is a good idea to name it after the machine's hostname. This example creates a copy, named MYKERNEL, of the GENERIC configuration file for the amd64 architecture:

# cd /usr/src/sys/amd64/conf
# cp GENERIC MYKERNEL

MYKERNEL can now be customized with any ASCII text editor. The default editor is vi, though an easier editor for beginners, called ee, is also installed with FreeBSD.

The format of the kernel configuration file is simple. Each line contains a keyword that represents a device or subsystem, an argument, and a brief description. Any text after a # is considered a comment and ignored. To remove kernel support for a device or subsystem, put a # at the beginning of the line representing that device or subsystem. Do not add or remove a # for any line that you do not understand.

In addition to the brief descriptions provided in this file, additional descriptions are contained in NOTES, which can be found in the same directory as GENERIC for that architecture. For architecture independent options, refer to /usr/src/sys/conf/NOTES.

# cd /usr/src/sys/amd64/conf
# mkdir /root/kernels
# cp GENERIC /root/kernels/MYKERNEL
# ln -s /root/kernels/MYKERNEL

An include directive is available for use in configuration files. This allows another configuration file to be included in the current one, making it easy to maintain small changes relative to an existing file. If only a small number of additional options or drivers are required, this allows a delta to be maintained with respect to GENERIC, as seen in this example:

include GENERIC
ident MYKERNEL

options         IPFIREWALL
options         DUMMYNET
options         IPFIREWALL_DEFAULT_TO_ACCEPT
options         IPDIVERT

Using this method, the local configuration file expresses local differences from a GENERIC kernel. As upgrades are performed, new features added to GENERIC will also be added to the local kernel unless they are specifically prevented using nooptions or nodevice. A comprehensive list of configuration directives and their descriptions may be found in config(5).

# cd /usr/src/sys/arch/conf && make LINT