Linux Boot Process

Linux Boot Process

LINUX BOOT PROCESS

Boot sequence summary

•             BIOS

•             Master Boot Record (MBR)

•             LILO or GRUB

•             Kernel

•             init

•             Run Levels

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BIOS

Load boot sector from one of:

•             Floppy

•             CDROM

•             Hard drive

The boot order can be changed from within the BIOS. BIOS setup can be entered by pressing a key during bootup. The exact key depends varies, but is often one of Del, F1, F2, or F10.

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(DOS) Master Boot Record (MBR)

DOS in the context includes MS-DOS, Win95, and Win98.

•             BIOS loads and execute the first 512 bytes off the disk (/dev/hda)

•             Standard DOS MBR will:

o             look for a primary partition (/dev/hda1-4) marked bootable

o             load and execute first 512 bytes of this partition

•             can be restored with fdisk /mbr from DOS

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LILO

•             does not understand filesystems

•             code and kernel image to be loaded is stored as raw disk offsets

•             uses the BIOS routines to load

Loading sequence

•             load menu code, typically /boot/boot.b

•             prompt for (or timeout to default) partition or kernel

•             for "image=" (ie Linux) option load kernel image

•             for "other=" (ie DOS) option load first 512 bytes of the partition

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Reconfiguring LILO

One minute guide to installing a new kernel

•             copy kernel image (bzImage) and modules to /boot and /lib/modules

•             edit /etc/lilo.conf

o             duplicate image= section, eg:

o            

      image=/bzImage-2.4.14

o            

        label=14

o            

        read-only

o             man lilo.conf for details

•             run /sbin/lilo

•             reboot to test

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GRUB

•             Understands file systems

•             config lives in /boot/grub/menu.lst or /boot/boot/menu.lst

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Kernel

•             initialise devices

•             (optionally loads initrd, see below)

•             mounts root filesystem

o             specified by lilo or loadin with root= parameter

o             kernel prints: VFS: Mounted root (ext2 filesystem) readonly.

•             runs /sbin/init which is process number 1 (PID=1)

o             init prints: INIT: version 2.76 booting

o             can be changed with boot= parameter to lilo, eg boot=/bin/sh can be useful to rescue a system which is having trouble booting.

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initrd

Allows setup to be performed before root FS is mounted

•             lilo or loadlin loads ram disk image

•             kernel runs /linuxrc

o             load modules

o             initialise devices

o             /linuxrc exits

•             "real" root is mounted

•             kernel runs /sbin/init

Details in /usr/src/linux/Documentation/initrd.txt (part of the kernel source).

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/sbin/init

•             reads /etc/inittab (see man inittab which specifies the scripts below

o             Run boot scripts:

             debian: run /etc/init.d/rcS which runs:

             /etc/rcS.d/S* scripts

             /etc/rc.boot/* (depreciated)

             redhat: /etc/rc.d/rc.sysinit script which: loads modules, check root FS and mount RW, mount local FS, setup network, and mount remote FS

o             switches to default runlevel eg 3.

             run scripts /etc/rc3.d/S*

             run programs specified in /etc/inittab

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Run Levels

•             0 halt

•             1 single user

•             2-4 user defined

•             5 X11 only (0 or 1 text console)

•             6 Reboot

•             Default is defined in /etc/inittab, eg:

o             id:3:initdefault:

•             The current runlevel can be changed by running /sbin/telinit # where # is the new runlevel, eg typing telinit 6 will reboot.

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Run Level programs

•             Scripts in /etc/rc*.d/* are symlinks to /etc/init.d

o             Scripts prefixed with S will be started when the runlevel is entered, eg /etc/rc5.d/S99xdm

o             Scripts prefixed with K will be killed when the runlevel is entered, eg /etc/rc6.d/K20apache

o             X11 login screen is typically started by one of S99xdm, S99kdm, or S99gdm.

•             Run programs for specified run level

•             /etc/inittab lines:

o             1:2345:respawn:/sbin/getty 9600 tty1

             Always running in runlevels 2, 3, 4, or 5

             Displays login on console (tty1)

o             2:234:respawn:/sbin/getty 9600 tty2

             Always running in runlevels 2, 3, or 4

             Displays login on console (tty2)

o             l3:3:wait:/etc/init.d/rc 3

             Run once when switching to runlevel 3.

             Uses scripts stored in /etc/rc3.d/

o             ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

             Run when control-alt-delete is pressed

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Boot Summary

•             lilo

o             /etc/lilo.conf

•             debian runs

o             /etc/rcS.d/S* scripts

o             /etc/rc3.d/S* scripts

•             redhat runs

o             /etc/rc.d/rc.sysinit script

o             /etc/rc.d/rc3.d/S* scripts