Once the kernel has finished booting, it passes control to the user process init(8), which is located at /sbin/init, or the program path specified in the init_path variable in loader.
The automatic reboot sequence makes sure that the file systems available on the system are consistent. If they are not, and fsck(8) cannot fix the inconsistencies, init(8) drops the system into single-user mode for the system administrator to take care of the problems directly.
This mode can be reached through the automatic reboot sequence, or by the user
booting with the -s option or setting the boot_single variable in loader.
It can also be reached by calling shutdown(8) without
the reboot (-r) or halt (-h)
options, from multi-user mode.
If the system console is set to insecure in /etc/ttys, then the system prompts for the root password before initiating single-user mode.
Example 13-3. An Insecure Console in /etc/ttys
# name getty type status comments # # If console is marked "insecure", then init will ask for the root password # when going to single-user mode. console none unknown off insecure
Note: An insecure console means that you consider your physical security to the console to be insecure, and want to make sure only someone who knows the root password may use single-user mode, and it does not mean that you want to run your console insecurely. Thus, if you want security, choose insecure, not secure.
If init(8) finds your file systems to be in order, or once the user has finished in single-user mode, the system enters multi-user mode, in which it starts the resource configuration of the system.
The resource configuration system reads in configuration defaults from /etc/defaults/rc.conf, and system-specific details from /etc/rc.conf, and then proceeds to mount the system file systems mentioned in /etc/fstab, start up networking services, start up miscellaneous system daemons, and finally runs the startup scripts of locally installed packages.
The rc(8) manual page is a good reference to the resource configuration system, as is examining the scripts themselves.