9.5 Building and Installing a Custom Kernel

Note: It is required to have the full FreeBSD source tree installed to build the kernel.

First, let us take a quick tour of the kernel build directory. All directories mentioned will be relative to the main /usr/src/sys directory, which is also accessible through the path name /sys. There are a number of subdirectories here representing different parts of the kernel, but the most important for our purposes are arch/conf, where you will edit your custom kernel configuration, and compile, which is the staging area where your kernel will be built. arch represents one of i386, amd64, ia64, powerpc, sparc64, or pc98 (an alternative development branch of PC hardware, popular in Japan). Everything inside a particular architecture's directory deals with that architecture only; the rest of the code is machine independent code common to all platforms to which FreeBSD could potentially be ported. Notice the logical organization of the directory structure, with each supported device, file system, and option in its own subdirectory.

The examples in this chapter assume that you are using the i386 architecture. If your system has a different architecture you need to change the path names accordingly.

Note: If the directory /usr/src/ does not exist on your system (or if it is empty), then the sources have not been installed. The easiest way to install the full source is to use csup(1) as described in Section 25.6. You should also create a symlink to /usr/src/sys/:

# ln -s /usr/src/sys /sys

Next, change to the arch/conf directory and copy the GENERIC configuration file to the name you want to give your kernel. For example:

# cd /usr/src/sys/i386/conf

Traditionally, this name is in all capital letters and, if you are maintaining multiple FreeBSD machines with different hardware, it is a good idea to name it after your machine's hostname. We will call it MYKERNEL for the purpose of this example.

Tip: Storing your kernel configuration file directly under /usr/src can be a bad idea. If you are experiencing problems it can be tempting to just delete /usr/src and start again. After doing this, it usually only takes a few seconds for you to realize that you have deleted your custom kernel configuration file. Also, do not edit GENERIC directly, as it may get overwritten the next time you update your source tree, and your kernel modifications will be lost.

You might want to keep your kernel configuration file elsewhere, and then create a symbolic link to the file in the i386 directory.

For example:

# cd /usr/src/sys/i386/conf
# mkdir /root/kernels
# cp GENERIC /root/kernels/MYKERNEL
# ln -s /root/kernels/MYKERNEL

Now, edit MYKERNEL with your favorite text editor. If you are just starting out, the only editor available will probably be vi, which is too complex to explain here, but is covered well in many books in the bibliography. However, FreeBSD does offer an easier editor called ee which, if you are a beginner, should be your editor of choice. Feel free to change the comment lines at the top to reflect your configuration or the changes you have made to differentiate it from GENERIC.

If you have built a kernel under SunOS™ or some other BSD operating system, much of this file will be very familiar to you. If you are coming from some other operating system such as DOS, on the other hand, the GENERIC configuration file might seem overwhelming to you, so follow the descriptions in the Configuration File section slowly and carefully.

Note: If you sync your source tree with the latest sources of the FreeBSD project, be sure to always check the file /usr/src/UPDATING before you perform any update steps. This file describes any important issues or areas requiring special attention within the updated source code. /usr/src/UPDATING always matches your version of the FreeBSD source, and is therefore more up to date with new information than this handbook.

You must now compile the source code for the kernel.

Building a Kernel

Note: It is required to have the full FreeBSD source tree installed to build the kernel.

  1. Change to the /usr/src directory:

    # cd /usr/src
  2. Compile the kernel:

    # make buildkernel KERNCONF=MYKERNEL
  3. Install the new kernel:

    # make installkernel KERNCONF=MYKERNEL

Tip: By default, when you build a custom kernel, all kernel modules will be rebuilt as well. If you want to update a kernel faster or to build only custom modules, you should edit /etc/make.conf before starting to build the kernel:

MODULES_OVERRIDE = linux acpi sound/sound sound/driver/ds1 ntfs

This variable sets up a list of modules to build instead of all of them.

WITHOUT_MODULES = linux acpi sound ntfs

This variable sets up a list of top level modules to exclude from the build process. For other variables which you may find useful in the process of building kernel, refer to make.conf(5) manual page.

The new kernel will be copied to the /boot/kernel directory as /boot/kernel/kernel and the old kernel will be moved to /boot/kernel.old/kernel. Now, shutdown the system and reboot to use your new kernel. If something goes wrong, there are some troubleshooting instructions at the end of this chapter that you may find useful. Be sure to read the section which explains how to recover in case your new kernel does not boot.

Note: Other files relating to the boot process, such as the boot loader(8) and configuration are stored in /boot. Third party or custom modules can be placed in /boot/kernel, although users should be aware that keeping modules in sync with the compiled kernel is very important. Modules not intended to run with the compiled kernel may result in instability or incorrectness.