Boot sequence summary
* BIOS
* Master Boot Record (MBR)
* LILO or GRUB
* Kernel
* init
* Run Levels
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.
(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
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
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:
image=/bzImage-2.4.14
label=14
read-only
o man lilo.conf for details
* run /sbin/lilo
* reboot to test
GRUB
* Understands file systems
* config lives in /boot/grub/menu.lst or /boot/boot/menu.lst
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.
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).
/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
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.
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
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
No comments:
Post a Comment