Turning on a computer and starting the operating system poses an interesting dilemma. By definition, the computer does not know how to do anything until the operating system is started. This includes running programs from the disk. If the computer can not run a program from the disk without the operating system, and the operating system programs are on the disk, how is the operating system started?
This problem parallels one in the book The Adventures of Baron Munchausen. A character had fallen part way down a manhole, and pulled himself out by grabbing his bootstraps and lifting. In the early days of computing, the term bootstrap was applied to the mechanism used to load the operating system. It has since become shortened to “booting”.
On x86 hardware, the Basic Input/Output System (BIOS) is responsible for loading the operating system. The BIOS looks on the hard disk for the Master Boot Record (MBR), which must be located in a specific place on the disk. The BIOS has enough knowledge to load and run the MBR, and assumes that the MBR can then carry out the rest of the tasks involved in loading the operating system, possibly with the help of the BIOS.
FreeBSD provides for booting from both the older MBR standard, and the newer GUID Partition Table (GPT). GPT partitioning is often found on computers with the Unified Extensible Firmware Interface (UEFI). However, FreeBSD can boot from GPT partitions even on machines with only a legacy BIOS with gptboot(8). Work is under way to provide direct UEFI booting.
The code within the MBR is typically referred to as a boot manager, especially when it interacts with the user. The boot manager usually has more code in the first track of the disk or within the file system. Examples of boot managers include the standard FreeBSD boot manager boot0, also called Boot Easy, and Grub, which is used by many Linux® distributions.
If only one operating system is installed, the MBR searches for the first bootable (active) slice on the disk, and then runs the code on that slice to load the remainder of the operating system. When multiple operating systems are present, a different boot manager can be installed to display a list of operating systems so the user can select one to boot.
The remainder of the FreeBSD bootstrap system is divided into three stages. The first stage knows just enough to get the computer into a specific state and run the second stage. The second stage can do a little bit more, before running the third stage. The third stage finishes the task of loading the operating system. The work is split into three stages because the MBR puts limits on the size of the programs that can be run at stages one and two. Chaining the tasks together allows FreeBSD to provide a more flexible loader.
The kernel is then started and begins to probe for devices and initialize them for use. Once the kernel boot process is finished, the kernel passes control to the user process init(8), which makes sure the disks are in a usable state, starts the user-level resource configuration which mounts file systems, sets up network cards to communicate on the network, and starts the processes which have been configured to run at startup.
This section describes these stages in more detail and demonstrates how to interact with the FreeBSD boot process.
The boot manager code in the MBR is sometimes referred to as stage zero of the boot process. By default, FreeBSD uses the boot0 boot manager.
The MBR installed by the FreeBSD installer
is based on /boot/boot0. The size and
capability of boot0 is restricted
to 446 bytes due to the slice table and
0x55AA identifier at the end of the
MBR. If boot0
and multiple operating systems are installed, a message
similar to this example will be displayed at boot time:
Other operating systems will overwrite an existing MBR if they are installed after FreeBSD. If this happens, or to replace the existing MBR with the FreeBSD MBR, use the following command:
#fdisk -B -b /boot/boot0device
where device is the boot disk,
such as ad0 for the first
IDE disk, ad2 for the
first IDE disk on a second
IDE controller, or da0
for the first SCSI disk. To create a
custom configuration of the MBR, refer to
boot0cfg(8).
Conceptually, the first and second stages are part of the
same program on the same area of the disk. Because of space
constraints, they have been split into two, but are always
installed together. They are copied from the combined
/boot/boot by the FreeBSD installer or
bsdlabel.
These two stages are located outside file systems, in the first track of the boot slice, starting with the first sector. This is where boot0, or any other boot manager, expects to find a program to run which will continue the boot process.
The first stage, boot1, is very
simple, since it can only be 512 bytes in size. It knows just
enough about the FreeBSD bsdlabel, which
stores information about the slice, to find and execute
boot2.
Stage two, boot2, is slightly more
sophisticated, and understands the FreeBSD file system enough to
find files. It can provide a simple interface to choose the
kernel or loader to run. It runs
loader, which is much more
sophisticated and provides a boot configuration file. If the
boot process is interrupted at stage two, the following
interactive screen is displayed:
To replace the installed boot1 and
boot2, use bsdlabel,
where diskslice is the disk and
slice to boot from, such as ad0s1 for the
first slice on the first IDE disk:
#bsdlabel -Bdiskslice
If just the disk name is used, such as
ad0, bsdlabel will
create the disk in “dangerously dedicated
mode”, without slices. This is probably not the
desired action, so double check the
diskslice before pressing
Return.
The loader is the final stage
of the three-stage bootstrap process. It is located on the
file system, usually as
/boot/loader.
The loader is intended as an interactive method for configuration, using a built-in command set, backed up by a more powerful interpreter which has a more complex command set.
During initialization, loader will probe for a console and for disks, and figure out which disk it is booting from. It will set variables accordingly, and an interpreter is started where user commands can be passed from a script or interactively.
The loader will then read
/boot/loader.rc, which by default reads
in /boot/defaults/loader.conf which sets
reasonable defaults for variables and reads
/boot/loader.conf for local changes to
those variables. loader.rc then acts on
these variables, loading whichever modules and kernel are
selected.
Finally, by default, loader issues a 10 second wait for key presses, and boots the kernel if it is not interrupted. If interrupted, the user is presented with a prompt which understands the command set, where the user may adjust variables, unload all modules, load modules, and then finally boot or reboot. Table 13.1, “Loader Built-In Commands” lists the most commonly used loader commands. For a complete discussion of all available commands, refer to loader(8).
| Variable | Description |
|---|---|
autoboot
seconds | Proceeds to boot the kernel if not interrupted within the time span given, in seconds. It displays a countdown, and the default time span is 10 seconds. |
boot
[-options]
[kernelname] | Immediately proceeds to boot the kernel, with
any specified options or kernel name. Providing a
kernel name on the command-line is only applicable
after an unload has been issued.
Otherwise, the previously-loaded kernel will be
used. If kernelname is not
qualified it will be searched under
/boot/kernel and
/boot/modules. |
| boot-conf | Goes through the same automatic configuration of
modules based on specified variables, most commonly
kernel. This only makes sense if
unload is used first, before
changing some variables. |
help
[topic] | Shows help messages read from
/boot/loader.help. If the topic
given is index, the list of
available topics is displayed. |
include filename
… | Reads the specified file and interprets it line
by line. An error immediately stops the
include. |
load [-t
type]
filename | Loads the kernel, kernel module, or file of the
type given, with the specified filename. Any
arguments after filename
are passed to the file. If
filename is not qualified it
will be searched under
/boot/kernel
and /boot/modules. |
ls [-l]
[path] | Displays a listing of files in the given path, or
the root directory, if the path is not specified. If
-l is specified, file sizes will
also be shown. |
| lsdev [-v] | Lists all of the devices from which it may be
possible to load modules. If -v is
specified, more details are printed. |
| lsmod [-v] | Displays loaded modules. If -v
is specified, more details are shown. |
more filename | Displays the files specified, with a pause at
each LINES displayed. |
| reboot | Immediately reboots the system. |
set variable, set
variable=value | Sets the specified environment variables. |
| unload | Removes all loaded modules. |
Here are some practical examples of loader usage. To boot the usual kernel in single-user mode :
boot -sTo unload the usual kernel and modules and then load the previous or another, specified kernel:
unloadloadkernel.old
Use kernel.GENERIC to refer to the
default kernel that comes with an installation, or
kernel.old, to refer to the previously
installed kernel before a system upgrade or before configuring
a custom kernel.
Use the following to load the usual modules with another kernel:
unloadset kernel="kernel.old"boot-conf
To load an automated kernel configuration script:
load -t userconfig_script /boot/kernel.confOnce the kernel is loaded by either loader or by boot2, which bypasses loader, it examines any boot flags and adjusts its behavior as necessary. Table 13.2, “Kernel Interaction During Boot” lists the commonly used boot flags. Refer to boot(8) for more information on the other boot flags.
| Option | Description |
|---|---|
-a | During kernel initialization, ask for the device to mount as the root file system. |
-C | Boot the root file system from a CDROM. |
-s | Boot into single-user mode. |
-v | Be more verbose during kernel startup. |
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. This is the last stage of the boot
process.
The boot sequence makes sure that the file systems
available on the system are consistent. If a
UFS file system is not, and
fsck cannot fix the inconsistencies,
init drops the system into
single-user mode so that the system administrator can resolve
the problem directly. Otherwise, the system boots into
multi-user mode.
A user can specify this mode by booting with
-s or by setting the
boot_single variable in
loader. It can also be reached
by running shutdown now from multi-user
mode. Single-user mode begins with this message:
Enter full pathname of shell or RETURN for /bin/sh:
If the user presses Enter, the system will enter the default Bourne shell. To specify a different shell, input the full path to the shell.
Single-user mode is usually used to repair a system that
will not boot due to an inconsistent file system or an error
in a boot configuration file. It can also be used to reset
the root password
when it is unknown. These actions are possible as the
single-user mode prompt gives full, local access to the
system and its configuration files. There is no networking
in this mode.
While single-user mode is useful for repairing a system, it poses a security risk unless the system is in a physically secure location. By default, any user who can gain physical access to a system will have full control of that system after booting into single-user mode.
If the system console is changed to
insecure in
/etc/ttys, the system will first prompt
for the root
password before initiating single-user mode. This adds a
measure of security while removing the ability to reset the
root password when
it is unknown.
/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 insecureAn insecure console means that
physical security to the console is considered to be
insecure, so only someone who knows the root password may use
single-user mode.
If init finds the file
systems to be in order, or once the user has finished their
commands in single-user mode and has typed
exit to leave 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. It then proceeds to
mount the system file systems listed in
/etc/fstab. It starts up networking
services, miscellaneous system daemons, then the startup
scripts of locally installed packages.
To learn more about the resource configuration system,
refer to rc(8) and examine the scripts located in
/etc/rc.d.
All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/
Questions that are not answered by the
documentation may be
sent to <[email protected]>.
Send questions about this document to <[email protected]>.