Section 10.5. Configuring the GRUB Bootloader

10.5. Configuring the GRUB Bootloader

GRUB is a powerful bootloader that can be used to boot Linux, Windows, DOS, and other operating systems as well as the Xen virtualization system. By mastering its configuration file and command-line options, you can configure GRUB to boot exactly the way you want.

10.5.1. How Do I Do That?

GRUB is configured through the file /boot/grub/grub.conf; typical contents of this file look like this:

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/main/root
#          initrd /initrd-version.img
#boot=/dev/hda
default=0
timeout=5
splashimage=(hd0,1)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.31-1.3420_fc6)
  root (hd0,1)
  kernel /vmlinuz-2.6.31-1.3420_fc6 ro root=/dev/main/root rhgb quiet
  initrd /initrd-2.6.31-1.3420_fc6.img
title Other
  rootnoverify (hd0,0)
  chainloader +1

This configuration file specifies two menu options, identified by the title keywords: Fedora Core and Windows (which Anaconda labels Other by default). Lines that start with a pound sign are comments. The first lines after the initial comments set up the appearance of the bootloader at startup time:

default= 0: Configures the first title enTRy as the default entry (they are numbered starting at 0)in this case, Fedora Core.
timeout= 5: Sets the delay in seconds before the default entry is booted.
splashimage=( hd0,1)/grub/splash.xpm.gz: Loads a graphical background for the boot display.
hiddenmenu: Does not display the boot menu unless the user presses a key during the timeout period, in which case all of the available operating system entries are shown.

The filename given in the splashimage line is in a special, GRUB-specific form: (hd0,1) specifies the first hard disk, second partition (/dev/hda2 in Linux terminology), and /grub/splash.xpm.gz identifies the pathname on that drive. Because /dev/hda1 is normally mounted on /boot, the full pathname within the Fedora system is /boot/grub/splash.xpm.gz.

GRUB numbers partitions starting at 0, while Linux numbers them starting at 1.

The remainder of this file configures the two menu options. The first one consists of these four lines:

title Fedora Core (2.6.31-1.3420_fc6 )
  root (hd0,1)
  kernel /vmlinuz-2.6.31-1.3420_fc6 ro root=/dev/main/root rhgb quiet
  initrd /initrd-2.6.31-1.3420_fc6.img

Each line provides specific information:

title Fedora Core (2.6.31-1.3420_fc6): The title displayed on the menu. The number in parentheses is the kernel version number; since it's standard practice to keep the second-most-recent kernel installed when the kernel is updated, just in case the new kernel does not boot properly, this information enables you to identify which kernel is newer.
root (hd0,1): The root filesystem for the boot process, written using GRUB notation. Note that this may not be the root directory of the Fedora Core installation; it's usually the filesystem mounted at /boot when the system is running.
kernel /vmlinuz-2.6.31-1.3420_fc6 ro root=/dev/main/root rhgb quiet: The kernel location within the root filesystem, plus boot options. These boot options specify that the root filesystem for Linux is /dev/main/root (logical volume root in volume group main), and the root filesystem will be mounted read-only (ro), that the Red Hat Graphical Boot (rhgb) display is enabled, and that noncritical kernel boot messages will be suppressed (quiet).
initrd /initrd-2.6.31-1.3420_fc6.img: The location of the initrd ramdisk file. This file contains a compressed filesystem image that contains all of the files other than the kernel necessary for the initial phases of the Fedora system startup, including device drivers, programs, and scripts.

The other title entry is simpler:

title Other
        rootnoverify (hd0,0)
        chainloader +1

The lines in this entry invoke the Windows Stage 2 bootloader, found at the start of the Windows partition:

rootnoverify (hd0,0): Similar to the root option in the Fedora Core entry, except that this partition will not be mounted, and therefore files cannot be accessed within the partition by GRUB.
chainloader +1: Specifies that the boot process should be turned over to the bootloader found in sector 1 of the partition.

10.5.1.1. Customizing the GRUB menu

You can directly edit the GRUB configuration file to change the appearance of the boot process.

To eliminate the boot menu entirely and directly boot the default entry, set the timeout value to zero:

timeout=0

This is a useful setting for end-user, single-boot systems with a stable kernel. On the other hand, if you have several operating systems installed, it may be convenient to remove the hiddenmenu line and use a longer timeout:

timeout=20

To turn the timeout off and wait indefinitely for the user to select the operating system, remove the timeout line from the file.

10.5.1.2. Using your own splash image

You can also customize or replace the boot image to include your company logo or a personalized message.

Use the GIMP graphics editor to create a 640x480 image. Reduce the number of colors by using the GIMP menu option ImageModeIndexed and selecting 14 colors.

When converting an existing image to 14 colors, the result may look better if you select the No Dithering option, especially if the original image contains large areas of solid color. If you are creating a new image, select the indexed mode before you start drawing.

Save the image in the /boot/grub directory, using the file extension .xpm.gz.

Another way to generate a splash image is to convert an existing landscape-oriented digital photo or a desktop wallpaper file using the ImageMagick convert program:

# convert -resize 640x480 -colors 14 photo.jpg /boot/grub/new_splash.xpm.gz

Finally, edit the splashimage line to point to your new creation:

splashimage=(hd0,1)/grub/new_splash.xpm.gz

An example of a modified splash image is shown in Figure 10-22.

Figure 10-22. Modified splash image

10.5.1.3. Creating additional boot entries

Creating additional boot entries is simply a matter of entering additional lines with the options that you want.

For example, you could create two separate entries for Fedora Coreone for runlevel 5 (GUI) and one for runlevel 3 (text mode):

title Fedora Core 
                     - Graphical Login  (2.6.31-1.3420_fc6 )
  root (hd0,1)
  kernel /vmlinuz-2.6.31-1.3420_fc6  ro root=/dev/main/root rhgb quiet
  initrd /initrd-2.6.31-1.3420_fc6 .img
title Fedora Core  - Text Login  (2.6.31-1.3420_fc6 )
  root (hd0,1)
  kernel /vmlinuz- 2.6.31-1.3420_fc6  ro root=/dev/main/root rhgb quiet  3 
  initrd /initrd-2.6.31-1.3420_fc6 .img

These two options are identical except for the descriptions on the title lines and the addition of the number 3 to the end of the kernel line for text-mode entry.

Installing a new kernel RPM will add an additional boot option and make it the default. If you are using yum to perform updating, a maximum of two versions of the kernel will be installed at once (configurable in /etc/yum/pluginconf.d/installonlyn.conf), so old kernel versions and their corresponding GRUB entries may be removed from the menu by yum when updating.

10.5.1.4. Installing GRUB's boot record from Fedora

Anaconda normally installs the GRUB boot record on the first disk drive automatically.

There are two situations where it may be necessary to manually install GRUB on an existing system:

When you're using a RAID 1 /boot partition, Anaconda will install the Grub boot record only on the first disk drive. Having a mirrored copy of /boot won't help if the first disk drive fails and you can't boot from the second drivea situation easily remedied by installing the GRUB boot record on the second drive as well.
When your boot record is corrupted or overwritten by another program, such as an installer for another operating system, you may need to manually install the GRUB boot record again.

The easiest way to install GRUB is to use the grub-install script:

# grub-install --root-directory=/boot /dev/hda
Installation finished. No error reported.
This is the contents of the device map /boot/boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script \Qgrub-install'.

(fd0)   /dev/fd0
(hd0)   /dev/hda
(hd1)   /dev/hdb
(hd2)   /dev/hdc
(hd3)   /dev/sdb

The --root-directory argument specifies the root directory for the boot files and should be used only if /boot is a mount point for a separate boot partition. The drive argument at the end of the line (/dev/hda) specifies the hard drive that GRUB will be installed on.

grub-install uses Linux disk names, such as /dev/hdc, instead of Grub disk names such as (hd2).

10.5.1.5. Installing GRUB's boot record from a GRUB DVD or floppy disk

Sometimes the GRUB boot record gets damaged, making it impossible to boot the system normally. It may be necessary to boot from a GRUB DVD or floppy disk to fix this type of problem.

To create a GRUB DVD on a Fedora system (obviously not the one that won't boot!), enter these commands:

# cd /usr/share/grub/
# growisofs -Z /dev/cdrom -R -b stage2_eltorito -no-emul-boot      -boot-load-size 4 -boot-info-table i386-redhat

Type the entire growisofs command on one continuous line.

To create a bootable floppy instead of a DVD:

# cd /usr/share/grub/i386-redhat
# cat stage1 stage2 >/dev/fd0

It's worthwhile keeping a GRUB DVD or floppy with your system manuals just in case you ever find that you can't boot your system due to bootloader problems.

Boot your system with this disc or floppy. A GRUB command prompt will appear, as shown in Figure 10-23.

Figure 10-23. GRUB command prompt from a CD/DVD boot

At this prompt, search for your stage1 file:

grub> find /boot/grub/stage1

Error 15: File not found

grub> find /grub/stage1
 (hd0,0)

If your boot files are in your root filesystem, GRUB will find /boot/grub/stage1, but if you have a separate /boot partition, GRUB will find /grub/stage1. In the previous example, the partition (hd0,0) contains the stage1 file.

Make this partition your root partition:

grub> root (hd0,0)
 Filesystem type is ext2fs, partition type 0x83

Now instruct GRUB to set up the boot record on that drive:

grub> setup (hd0)
 Checking if "/boot/grub/stage1" exists... no
 Checking if "/grub/stage1" exists... yes
 Checking if "/grub/stage2" exists... yes
 Checking if "/grub/e2fs_stage1_5" exists... yes
 Running "embed /grub/e2fs_stage1_5 (hd0)"... 15 sectors are embedded.
succeeded
 Running "install /grub/stage1 (hd0) (hd0)1+15 p (hd0,0)/grub/stage2 /grub/grub
.conf"... succeeded.
Done

Note that the setup command was given the drive (hd0) instead of the partition (hd0,0) to install the boot record at the start of the drive instead of the start of the boot partition.

You can now remove the GRUB disc/floppy and boot directly from the hard drive.

10.5.1.6. Editing boot options

To temporarily override a GRUB menu option, select a menu option on the boot menu using the up/down arrow keys, and then press E (for edit). The screen shown in Figure 10-24 will be displayed.

If you have a bootloader password configured, you will be prompted for it at this point.

Figure 10-24. Selecting a menu-entry line to edit

Use the arrow keys to select the line you wish to edit, and then press E again. You can now move across the line using the arrow keys, as shown in Figure 10-25. Type new text to insert it into the line, or use the Backspace/Delete keys to remove text. Press Enter when done.

Figure 10-25. Edit a line in a menu entry

Press Enter to accept your changes or Esc to undo them. In either case you will return to the menu-entry display shown in Figure 10-24; press B to boot, or press Esc to return to the boot menu.

As a shortcut, if you are adding boot options only to an existing menu entry, select the entry using the up/down arrow keys, then press A (for append). You can then type the additional option(s), such as a runlevel. Press Enter to proceed with booting or Esc to cancel and return to the boot menu.

10.5.1.7. Installing or changing a GRUB password

To protect against the unauthorized use of runlevel S or other boot options, it's a good idea to add a password entry to the boot menu. If you didn't do this during the installation, you can add the password at any time by following these steps:

Generate an encrypted password with the grub-md5-crypt command:

$ grub-md5-crypt
Password: 
                     bigsecret
Retype password: 
                     bigsecret
$1$f1z061$j/UEYyBn0e0996w0gjq4k/

The line in bold at the bottom of the listing is the encrypted (scrambled) version of the password.

Next, edit the /boot/grub/grub.conf file and add this line at the top, substituting the password you just generated:

password --md5 $1$f1z061$j/UEYyBn0e0996w0gjq4k/

When you boot the system, you will still be able to select a boot menu entry, but to perform any advanced operations (such as appending runlevel information to a boot entry), you will need to enter the password.

10.5.2. How Does It Work?

GRUB actually consists of four pieces of software, plus some utilities:

stage 1: The boot record. This tiny piece of code is less than 512 bytes long.
stage 1.5: Additional drivers for filesystems, such as ext2, to enable GRUB to find the stage 2 files.
stage 2: The standalone GRUB command shell and menu program.
/sbin/grub: A version of the GRUB command shell that can be executed inside a running Fedora system.

During boot, the system BIOS loads stage 1 as the boot record and executes it, which then loads stage 1.5 (if necessary) and finds stage 2. stage 2 then seeks out the GRUB configuration file menu.lst (a symbolic link to grub.conf, which the Fedora developers apparently consider to be a better name).

The GRUB command shell supports over three dozen commands; most of these are never used except by experts and developers. Instead, most users interact with the GRUB menu.

The grub.conf file permits a set of boot options to be presented to the user as a menu entry, removing most of the complexity from the user's view.

A typical Linux entry in grub.conf sets the root filesystem, which is mounted by GRUB to enable access to the kernel and other boot drives. The entry also specifies the name of the kernel and initrd ramdisk to be loaded into memory, and also indicates any configuration options that are to be passed to the kernel.

10.5.3. What About...

10.5.3.1. ...an archive of GRUB splash images?

The author of the splash-image code maintains a small gallery of tested splash images at http://ruslug.rutgers.edu/~mcgrof/grub-images/images/ and an archive at http://ruslug.rutgers.edu/~mcgrof/grub-images/images/working-splashimages/.

10.5.3.2. ...dual-booting between different Linux distributions?

This works well. Simply add the entries for the other Linux distributions to the active /boot/grub/grub.conf (or /boot/grub/menu.lst) file. This can be done by specifying that the second Linux distribution install GRUB at the start of that distribution's root filesystem partition instead of placing it in the master boot record for the drive. Then copy the /boot/grub/grub.conf entries from the second Linux distribution to the first one.

10.5.4. Where Can I Learn More?

The Grub manual: http://www.gnu.org/software/grub/manual/ (also installed as an info document in Fedora Core)
The Grub web site: http://www.gnu.org/software/grub/
The sample configuration file /usr/share/doc/grub-0.95/menu.lst
The manpage for convert