Boot sequence summary

• BIOS
• Master Boot Record (MBR)
• Kernel
• init

BIOS

Load boot sector from one of:

• Floppy
• CDROM
• SCSI drive
• IDE drive

Master Boot Record

• MBR (loaded from /dev/hda or /dev/sda) contains:
  • lilo
    • load kernel (image=), or
    • load partition boot sector (other=)
  • DOS
    • load "bootable" partition boot sector (set with fdisk)
• partition boot sector (eg /dev/hda2) contains:
  • DOS
    • loadlin
  • lilo
    • kernel

LILO

One minute guide to installing a new kernel

• edit /etc/lilo.conf
  • duplicate image= section, eg:

          image=/bzImage-2.2.12
                    
            label=12
                    
            read-only
                    

  • man lilo.conf for details
• run /sbin/lilo
• (copy modules)
• reboot to test

Kernel

• initialise devices
• (optionally loads initrd, see below)
• mount root FS
  • specified by lilo or loadin
  • kernel prints:
    • VFS: Mounted root (ext2 filesystem) readonly.
• run /sbin/init, PID 1
  • can be changed with boot=
  • init prints:
    • INIT: version 2.76 booting

initrd

Allows setup to be performed before root FS is mounted

• lilo or loadlin loads ram disk image
• kernel runs /linuxrc
  • load modules
  • initialise devices
  • /linuxrc exits
• "real" root is mounted
• kernel runs /sbin/init

Details in /usr/src/linux/Documentation/initrd.txt

/sbin/init

• reads /etc/inittab
• runs script defined by this line:
  • si::sysinit:/etc/init.d/rcS
• switches to runlevel defined by
  • id:3:initdefault:

sysinit

• debian: /etc/init.d/rcS which runs
  • /etc/rcS.d/S* scripts
    • symlinks to /etc/init.d/*
  • /etc/rc.boot/* (depreciated)
• redhat: /etc/rc.d/rc.sysinit script which
  • load modules
  • check root FS and mount RW
  • mount local FS
  • setup network
  • mount remote FS

Example Debian /etc/rcS.d/ directory

README
S05keymaps-lct.sh -> ../init.d/keymaps-lct.sh
S10checkroot.sh -> ../init.d/checkroot.sh
S20modutils -> ../init.d/modutils
S30checkfs.sh -> ../init.d/checkfs.sh
S35devpts.sh -> ../init.d/devpts.sh
S35mountall.sh -> ../init.d/mountall.sh
S35umsdos -> ../init.d/umsdos
S40hostname.sh -> ../init.d/hostname.sh
S40network -> ../init.d/network
S41ipmasq -> ../init.d/ipmasq
S45mountnfs.sh -> ../init.d/mountnfs.sh
S48console-screen.sh -> ../init.d/console-screen.sh
S50hwclock.sh -> ../init.d/hwclock.sh
S55bootmisc.sh -> ../init.d/bootmisc.sh
S55urandom -> ../init.d/urandom

Run Levels

• 0 halt
• 1 single user
• 2-4 user defined
• 5 X11
• 6 Reboot
• Default in /etc/inittab, eg
  • id:3:initdefault:
• Change using /sbin/telinit

Run Level programs

• Run programs for specified run level
• /etc/inittab lines:
  • 1:2345:respawn:/sbin/getty 9600 tty1
    • Always running in runlevels 2, 3, 4, or 5
    • Displays login on console (tty1)
  • 2:234:respawn:/sbin/getty 9600 tty2
    • Always running in runlevels 2, 3, or 4
    • Displays login on console (tty2)
  • l3:3:wait:/etc/init.d/rc 3
    • Run once when switching to runlevel 3.
    • Uses scripts stored in /etc/rc3.d/
  • ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
    • Run when control-alt-delete is pressed

Typical /etc/rc3.d/ directory

• Kills K##scripts
• Starts S##scripts

K25nfs-server -< ../init.d/nfs-server
K99xdm -< ../init.d/xdm
S10sysklogd -< ../init.d/sysklogd
S12kerneld -< ../init.d/kerneld
S15netstd_init -< ../init.d/netstd_init
S18netbase -< ../init.d/netbase
S20acct -< ../init.d/acct
S20anacron -< ../init.d/anacron
S20gpm -< ../init.d/gpm
S20postfix -< ../init.d/postfix
S20ppp -< ../init.d/ppp
S20ssh -< ../init.d/ssh
S20xfs -< ../init.d/xfs
S20xfstt -< ../init.d/xfstt
S20xntp3 -< ../init.d/xntp3
S89atd -< ../init.d/atd
S89cron -< ../init.d/cron
S99rmnologin -< ../init.d/rmnologin

Boot Summary

• lilo
  • /etc/lilo.conf
• debian runs
  • /etc/rcS.d/S* and /etc/rc.boot/
  • /etc/rc3.d/S* scripts
• redhat runs
  • /etc/rc.d/rc.sysinit
  • /etc/rc.d/rc3.d/S* scripts