Boot sequence summary

You may find that your server isn't actually booting to runlevel 3, maybe it's going to 5 (with graphical login)? who -r or runlevel should tell you the current runlevel, and grep initdefault /etc/inittab the boot-time default.


Boot sequence summary
1BIOS
2Master Boot Record (MBR)
3Kernel
4init

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BIOS
Load boot sector from one of:

•Floppy
•CDROM
•SCSI drive
•IDE drive

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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

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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

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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

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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
◦load modules
◦initialise devices
◦/linuxrc exits
•"real" root is mounted
•kernel runs /sbin/init
Details in /usr/src/linux/Documentation/initrd.txt


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/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:

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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

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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

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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

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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

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Typical /etc/rc3.d/ directory
When changing runlevels /etc/init.d/rc 3:
•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

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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