how does freebsd know which kernel to boot ?

Polytropon freebsd at
Sun Jul 25 15:07:06 UTC 2010

On Sun, 25 Jul 2010 16:21:38 +0200, claudiu vasadi <claudiu.vasadi at> wrote:
> hello ppl,
> I was wondering about the following:
> After I do "make build|install kernel" and reboot, the new kernel is
> automatically booted but the old one is still on the disk. Therefore, how
> does FreeBSD know which one to boot ?
> I'm guessing it's writing this info somewhere but I do not know where or
> how.
> Can anyone give me some short description ?

The kernel is booted by the kernel loader. Its configuration is
in /boot/loader.conf and /boot/default/loader.conf. It will default
to "kernel" (the name of the kernel to boot) by default, but you
can specify another name, too. You can also interrupt the boot and
request a different than the default kernel; using the "nextboot"
can achieve similar results.

Upon kernel installation by "make installkernel", the existing
kernel will be moved to kernel.old; the newly installed kernel
will then be kernel, and the loader will load exactly this name
as before.

Interesting man pages are:
	% man loader
	% man loader.conf
	% man nextboot
	% man boot
	% less /boot/defaults/loader.conf

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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