UCS 2.4 - Univention

10.8 GRUB boot manager
The GRUB boot manager (see Chapter 10.8) loads the kernel and the initrd into the system memory, where
the initrd archive is extracted and mounted as a temporary root file system. The real root file system can
then be mounted onto this, before the temporary archive is removed and the system start implemented.
The Univention Configuration Registry variable initramfs/modules can then be used to configure
which modules should be integrated into the initrd. If it is set to most, all framebuffer, ACPI, file system
and hard drive modules are integrated; netboot only adds basic and network drivers and dep performs
automatic recognition of the modules which are to be integrated.
If the Univention Configuration Registry variable initramfs/busybox is set to yes, Busybox, a compact
implementation of the standard Unix tools, is added, which is useful in failure analysis amongst other
things.
The drivers to be used are recognised automatically during system start and loaded via the udev device
manager. At this point, the necessary device links are also created under /dev. If drivers are not recog-
nised (which can occur if no respective hardware IDs are registered or hardware is employed which cannot
be recognised automatically, e.g., ISA boards), kernel modules to be loaded automatically can be added
via Univention Configuration Registry variable kernel/modules. If more than one kernel module is to be
added, these must be separated by a semicolon.
10.8 GRUB boot manager
In Univention Corporate Server GNU GRUB is used as the boot manager. GRUB provides a menu which
allows the selection of a Linux kernel or another operating system to be booted. In contrast to the boot
loader LILO, which was used in earlier UCS versions, GRUB can also access file systems directly and can
thus, for example, load another kernel in case of an error.
The selection of kernels to be started in the boot menu is stored in the file /boot/grub/menu.lst.
There is a five second waiting period during which a selection can be made. This value can be changed
via the Univention Configuration Registry variable grub/timeout.
In the basic setting, a screen of 1024x768 pixels size and 16 Bit colour depth is pre-set. A different value
can be set via the Univention Configuration Registry variable grub/vga. A list of available modes can be
found at http://en.wikipedia.org/wiki/VESA_BIOS_Extensions.
Setting the Univention Configuration Registry variable grub/memtest86 to true means a memory test
program is included in the boot selection. For this purpose, the package memtest86+ has to be installed
on the corresponding system.
GRUB loads the operating system in a two-step procedure; in the Master Boot Record of the hard drive,
the Stage 1 loader is written which refers to the data of Stage 2, which in turn manage the rest of the boot
procedure.
The kernel which is about to be booted, can be provided with configuration options by setting the Univention
Configuration Registry variable grub/append.
GRUB uses its own notation for the partitions of a hard drive, which is independent from the device names
under Linux. The format of this notation is (hdX,Y), wherein X means the drive within the system, and Y
the partition on the drive (counting begins with 0 in each case). So (hd0,0) means the first partition on the
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