Chapter 8. The Boot Loader / 8.3. Booting with GRUB | ||||
|---|---|---|---|---|
 | 8.2. Selecting a Boot Loader | 8.4. Configuring the Boot Loader with YaST |  | |
Chapter 8. The Boot Loader / 8.3. Booting with GRUB | ||||
|---|---|---|---|---|
| 8.2. Selecting a Boot Loader | 8.4. Configuring the Boot Loader with YaST | | |
GRUB (Grand Unified Bootloader) comprises two stages. stage1 consists of 512 bytes and is written to the MBR or the boot sector of a hard disk partition or floppy disk. Subsequently, stage2 is loaded. This stage contains the actual program code. The only task of the first stage is to load the second stage of the boot loader.
stage2 is able to access file systems. Currently, Ext2, Ext3, ReiserFS,
Minix, and the DOS FAT file system used by Windows are supported. To a
certain extent, JFS, XFS, and UFS and FFS used by BSD systems are also
supported. Since version 0.95, GRUB is also able to boot from a CD or DVD
containing an ISO 9660 standard file system pursuant to the “El
Torito” specification. Even before the system is booted, GRUB can
access file systems of supported BIOS disk devices (floppy disks or hard
disks, CD drives, and DVD drives detected by the BIOS). Therefore, changes
to the GRUB configuration file (menu.lst) do not
require a reinstallation of the boot manager. When the system is booted,
GRUB reloads the menu file together with the valid paths and partition
data of the kernel or the initial RAM disk (initrd) and
locates these files.
The actual configuration of GRUB is based on three files that are described below:
/boot/grub/menu.lstThis file contains all information about partitions or operating systems that can be booted with GRUB. Without this information, the system control cannot be passed to the operating system.
/boot/grub/device.mapThis file translates device names from the GRUB and BIOS notation to Linux device names.
/etc/grub.confThis file contains the parameters and options the GRUB shell needs for installing the boot loader correctly.
GRUB can be controlled in various ways. Boot entries from an existing
configuration can be selected from the graphical menu (splash screen). The
configuration is loaded from the file
menu.lst.
In GRUB, all boot parameters can be changed prior to booting. For example,
errors made when editing the menu file can be corrected in this way. Boot
commands can also be entered interactively over a kind of input prompt (see
Section 8.3.1.3, “Editing Menu Entries during the Boot Procedure”). GRUB offers the
possibility of determining the location of the kernel and the
initrd prior to booting. In this way, you can even boot
an installed operating system for which no entry exists in the boot loader
configuration.
The GRUB shell provides an emulation of GRUB in the installed system. It can be used to install GRUB or test new settings before applying them. See Section 8.3.4, “The GRUB Shell”.
The graphical splash screen with the boot menu is based on the GRUB
configuration file /boot/grub/menu.lst, which contains
all information about all partitions or operating systems that can be
booted by means of the menu.
Every time the system is booted, GRUB loads the menu file from the file system. For this reason, GRUB does not need to be reinstalled after every change to the file. Use the YaST boot loader to modify the GRUB configuration as described in Section 8.4, “Configuring the Boot Loader with YaST”.
The menu file contains commands. The syntax is very simple. Every line
contains a command followed by optional parameters separated by spaces like
in the shell. For historical reasons, some commands permit an
= in front of the first parameter. Comments are
introduced by a hash (#).
To identify the menu items in the menu overview, specify a
title for every entry. The text (including any spaces)
following the keyword title is displayed as a selectable
option in the menu. All commands up to the next title
are executed when this menu item is selected.
The simplest case is the redirection to boot loaders of other operating
systems. The command is chainloader and the argument is
usually the boot block of another partition in GRUB block notation. For
example:
chainloader (hd0,3)+1
The device names in GRUB are explained in Section 8.3.1.1, “Naming Conventions for Hard Disks and Partitions”. The above example specifies the first block of the fourth partition of the first hard disk.
Use the command kernel to specify a kernel image. The
first argument is the path to the kernel image in a partition. The other
arguments are passed to the kernel on the command line.
If the kernel does not have built-in drivers for access to the root
partition, initrd must be specified with a separate
GRUB command whose only argument is the path to the
initrd file. Because the loading address of the
initrd is written to the loaded kernel image, the
command initrd must follow immediately after the
kernel command.
The command root simplifies the specification of kernel
and initrd files. The only argument of root is a GRUB
device or a partition on a GRUB device. This device is used for all
kernel, initrd, or other file paths for which no
device is explicitly specified until the next root
command. This command is not used in the menu.lst file
generated during the installation. It merely facilitates manual editing.
The boot command is implied at the end of every menu
entry, so it does not need to be written into the menu file. However, if
you use GRUB interactively for booting, you must enter the
boot command at the end. The command itself has no
arguments. It merely boots the loaded kernel image or the specified chain
loader.
After writing all menu entries, define one of them as the
default entry. Otherwise, the first one (entry
0) is used. You can also specify a time-out in seconds
after which the default entry should boot. timeout and
default usually precede the menu entries. An example
file is described in Section 8.3.1.2, “An Example Menu File”.
The naming conventions GRUB uses for hard disks and partitions differ
from those used for normal Linux devices.
In GRUB, the numbering of the partitions starts with zero.
Thus, (hd0,0) is the first partition of the first
hard disk. On a common desktop machine with a hard disk connected as
primary master, the corresponding Linux device name is
/dev/hda1.
The four possible primary partitions are assigned the partition numbers
0 to 3. The logical partitions are
numbered from 4:
(hd0,0) first primary partition of the first hard disk
(hd0,1) second primary partition
(hd0,2) third primary partition
(hd0,3) fourth primary partition (usually an extended partition)
(hd0,4) first logical partition
(hd0,5) second logical partition
GRUB does not distinguish between IDE, SCSI, and RAID devices. All hard disks recognized by the BIOS or other controllers are numbered according to the boot sequence preset in the BIOS.
Unfortunately, GRUB is not able to map the Linux device names to BIOS
device names exactly. It generates this mapping with a help of an
algorithm and saves it to the file device.map, which
can be edited if necessary. Information about the file
device.map is available in Section 8.3.2, “The File device.map”.
A complete GRUB path consists of a device name written in parentheses and the path to the file in the file system in the specified partition. The path begins with a slash. For example, the bootable kernel could be specified as follows on a system with a single IDE hard disk containing Linux in its first partition:
(hd0,0)/boot/vmlinuz
The following example shows the structure of a GRUB menu file. The
example installation comprises a Linux boot partition under
/dev/hda5, a root partition under
/dev/hda7, and a Windows installation under
/dev/hda1.
gfxmenu (hd0,4)/message color white/blue black/light-gray default 0 timeout 8 title linux kernel (hd0,4)/vmlinuz root=/dev/hda7 vga=791 initrd (hd0,4)/initrd title windows chainloader(hd0,0)+1 title floppy chainloader(fd0)+1 title failsafe kernel (hd0,4)/vmlinuz.shipped root=/dev/hda7 ide=nodma \ apm=off acpi=off vga=normal nosmp maxcpus=0 3 initrd (hd0,4)/initrd.shipped
The first block defines the configuration of the splash screen:
The background image message is located in
/dev/hda5.
Color scheme: white (foreground), blue (background), black (selection), and light gray (background of the selection). The color scheme has no effect on the splash screen, only on the customizable GRUB menu that you can access by exiting the splash screen with Esc.
The first menu entry title linux is the one to boot
by default.
After eight seconds without any user input, GRUB automatically boots the default entry.
The second and largest block lists the various bootable operating systems.
The sections for the individual operating systems are introduced by
title.
The first entry (title linux) is responsible for
booting SUSE LINUX. The kernel (vmlinuz) is located
in the first logical partition (the boot partition) of the first hard
disk. Kernel parameters, such as the root partition and VGA mode, are
appended here. The root partition is specified according to the Linux
naming convention (/dev/hda7/), because this
information is read by the kernel and has nothing to do with GRUB. The
initrd is also located in the first logical partition
of the first hard disk.
The second entry is responsible for loading Windows. Windows is booted
from the first partition of the first hard disk
(hd0,0). The command chainloader
+1 causes GRUB to read and execute the first sector of the
specified partition.
The next entry enables booting from floppy disk without modifying the BIOS settings.
The boot option failsafe starts Linux with a
selection of kernel parameters that enables Linux to boot even on
problematic systems.
The menu file can be changed whenever necessary. GRUB then uses the modified settings during the next boot. Edit the file permanently using YaST or an editor of your choice. Alternatively, make temporary changes interactively using the edit function of GRUB. See Section 8.3.1.3, “Editing Menu Entries during the Boot Procedure”.
In the graphical GRUB boot menu, select the operating system to boot with the arrow keys. If you select a Linux system, you can enter additional boot parameters at the boot prompt. To edit individual menu entries directly, press Esc to exit the splash screen and press E. Changes made in this way only apply to the current boot procedure and are not adopted permanently.
![[Important]](admon/important.png) | Keyboard Layout during the Boot Procedure |
|---|---|
The US keyboard layout is the only one available when booting. | |
After activating the editing mode, use the arrow keys to select the menu entry of which to edit the configuration. To make the configuration editable, press E once more. In this way, edit incorrect partition or path specifications before they have a negative effect on the boot process. Press Enter to exit the editing mode and return to the menu. Then press B to boot this entry. Further possible actions are displayed in the help text at the bottom.
To enter changed boot options permanently and pass them to the kernel,
open the file menu.lst as the user
root and append the respective kernel parameters to the
existing line, separated by spaces:
title linux kernel (hd0,0)/vmlinuz root=/dev/hda3 additional parameter initrd (hd0,0)/initrd
GRUB automatically adopts the new parameters the next time the system is booted. Alternatively, this change can also be made with the YaST boot loader module. Append the new parameters to the existing line, separated by spaces.
Especially when developing or using custom kernels, you
need either to change the entries in
menu.lst or edit the command line to reflect
the current kernel and initrd filenames. To simplify this
procedure, use wild cards to
update the kernel list of GRUB dynamically. All kernel images
that match a specific pattern are then automatically added to
the list of bootable images. Note that there is no support
for this feature.
Activate the wild card option by entering an additional menu entry
in menu.lst. To be useful, all kernel and
initrd images must have a common base name and an identifier that
matches the kernel with its associated initrd. Consider the
following setup:
initrd-default initrd-test vmlinuz-default vmlinuz-test
In this case, you may add both boot images in one GRUB
configuration. To get the menu entries
linux-default and
linux-test, the following entry in
menu.lst would be needed:
title linux-* wildcard (hd0,4)/vmlinuz-* kernel (hd0,4)/vmlinuz-* root=/dev/hda7 vga=791 initrd (hd0,4)/initrd-*
In this example, GRUB searches the partition (hd0,4) for entries
matching the wild card. These entries are used to
generate new GRUB menu entries. In the previous example,
GRUB would behave as if the following entries existed in
menu.lst:
title linux-default
wildcard (hd0,4)/vmlinuz-default
kernel (hd0,4)/vmlinuz-default root=/dev/hda7 vga=791
initrd (hd0,4)/initrd-default
title linux-test
wildcard (hd0,4)/vmlinuz-test
kernel (hd0,4)/vmlinuz-test root=/dev/hda7 vga=791
initrd (hd0,4)/initrd-test
Problems with this configuration can be expected if filenames are not used consistently or if one of the expanded files, such as an initrd image, is missing.
The file device.map maps GRUB device names to Linux
device names. In a mixed system containing IDE and SCSI hard disks, GRUB
must try to determine the boot sequence by means of a special procedure,
because GRUB does not have access to the BIOS information on the boot
sequence. GRUB saves the result of this analysis in the file
/boot/grub/device.map. For a system on which the boot
sequence in the BIOS is set to IDE before SCSI, the file
device.map could appear as follows:
(fd0) /dev/fd0 (hd0) /dev/hda (hd1) /dev/sda
Because the order of IDE, SCSI, and other hard disks depends on various
factors and Linux is not able to identify the mapping, the sequence in the
file device.map can be set manually. If you encounter
problems when booting, check if the sequence in this file corresponds to
the sequence in the BIOS and use the GRUB shell, described in
Section 8.3.4, “The GRUB Shell”, to modify it temporarily
if necessary. Once the
Linux system has booted, the file
device.map can be edited permanently with the YaST
boot loader module or an editor of your choice.
After manually changing device.map, execute
the following command to reinstall GRUB. This command causes the file
device.map to be reloaded and the commands listed in
grub.conf to be executed:
grub --batch < /etc/grub.conf
The third important GRUB configuration file apart from
menu.lst and device.map is
/etc/grub.conf. This file contains the parameters and
options the command grub needs for installing the boot
loader correctly:
root (hd0,4) install /grub/stage1 d (hd0) /grub/stage2 0x8000 (hd0,4)/grub/menu.lst quit
Meaning of the individual entries:
This command tells GRUB to apply the following commands to the first logical partition of the first hard disk (the location of the boot files).
parameter
The command grub should be run with the parameter
install. stage1 of the boot
loader should be installed in the MBR of the first hard disk
(/grub/stage1 d (hd0)). stage2
should be loaded to the memory address 0x8000 (/grub/stage2
0x8000). The last entry
((hd0,4)/grub/menu.lst) tells GRUB where to look
for the menu file.
GRUB actually exists in two versions: as a boot loader and as a normal
Linux program in /usr/sbin/grub. This program is
referred to as the GRUB shell. The functionality to
install GRUB as boot loader on a hard disk or floppy disk is integrated
in GRUB in the form of the commands install and
setup. This is available in the GRUB shell when Linux
is loaded.
However, the commands setup and
install are also available during the boot procedure
before Linux is started. This facilitates the repair of a defective system
that can no longer be booted, because the faulty configuration file of the
boot loader can be circumvented by manually entering
parameters. Manually entering parameters during the boot procedure
is also useful for
testing new settings without impairing the native system. Simply enter the
experimental configuration file with a syntax similar to that in
menu.lst. Then test the functionality of this entry
without changing the existing configuration file. For example, to test a
new kernel, enter the command kernel and the path to the
new kernel. If the boot procedure fails, you can continue using the intact
menu.lst the next time you boot. Similarly, the
command line interface can also be used to boot a system despite a faulty
menu.lst file by entering the corrected parameters.
In the running system, the correct parameters can be entered in
menu.lst to make the system permanently bootable.
The mapping of GRUB devices to Linux device names is only relevant when
running the GRUB shell as a Linux program (by entering
grub as described in Section 8.3.2, “The File device.map”). For this purpose, the program reads the
file
device.map. For more information, see
Section 8.3.2, “The File device.map”.
Even before the operating system is booted, GRUB enables access to file systems. Users without root permissions can access files in your Linux system to which they have no access once the system is booted. To block this kind of access or prevent users from booting certain operating systems, set a boot password.
As the user root, proceed as follows to set a boot
password:
At the root prompt, enter grub.
Encrypt the password in the GRUB shell:
grub> md5crypt Password: **** Encrypted: $1$lS2dv/$JOYcdxIn7CJk9xShzzJVw/
Paste the encrypted string into the global section of the file
menu.lst:
gfxmenu (hd0,4)/message color white/blue black/light-gray default 0 timeout 8 password --md5 $1$lS2dv/$JOYcdxIn7CJk9xShzzJVw/
Now GRUB commands can only be executed at the boot prompt after pressing P and entering the password. However, users can still boot all operating systems from the boot menu.
To prevent one or several operating systems from being booted from the
boot menu, add the entry lock to every section in
menu.lst that should not be bootable without
entering a password. For example:
title linux kernel (hd0,4)/vmlinuz root=/dev/hda7 vga=791 initrd (hd0,4)/initrd lock
After rebooting the system and selecting the Linux entry from the boot menu, the following error message is displayed:
Error 32: Must be authenticated
Press Enter to enter the menu. Then press P to get a password prompt. After entering the password and pressing Enter, the selected operating system (Linux in this case) should boot.
| Boot Password and Splash Screen |
|---|---|
If you use a boot password for GRUB, the usual splash screen is not displayed. | |
