Administration

Chapter 3: Booting And Shutting Down

Colorado State University
Computer Science Department

Original slides from Dr. James Walden at Northern Kentucky University.

Topics

Booting
Bootstrap loaders
Run levels
Startup scripts
Shutdown and reboot

Booting

ROM boot code (BIOS in PCs)
Master Boot Record (MBR)
Bootloader
Kernel
Hardware detection and configuration
Creation of system processes
Multiuser operation

BIOS

BIOS actions:
- Load CMOS settings.
- Initialize registers and power management.
- POST (Power On Self Test.)
- Display system settings (if key pressed.)
- Activate other BIOSes
  - Disk interfaces
  - Graphics cards
  - Network interfaces
- Find bootable device.
- Load MBR.
- Run MBR program.

MBR

Up to four primary partitions in MBR:
- Has CHS (Cyl/Head/Sector) addresses or partition
- LBA (Logical Block Addressing) for newer disk drives
One can be used as an extended partition
- Links to an Extended Boot Record (EBR) on first sector
- Each logical partition is described by its own EBR
- Includes link to the next EBR, to allow any # of partitions

Bootloader: GRUB

Stage 1 (stored in MBR or boot sector)
- Initialize the system briefly.
- Detect the geometry and access mode of the loading drive.
- Load the first sector of Stage 2.
- Jump to the starting address of the Stage 2.
Stage 1.5 (stored in sectors of MBR track)
- One version for each filesystem.
- Loads stage 2 from filesystem.
Stage 2 (stored in filesystem)
- Displays menu of OS choices.
- Often displays boot splash screen.
- Loads selected OS kernel.

GRUB Installation

At OS Install Time: Most distributions install GRUB by default.
Install via command-line: grub-install /dev/hda
Native install: Boot with CD and run grub.

    #>> root (hd0,0)
    #>> setup (hd0)
    #>> quit

GRUB Boot

Automatically boots default OS after timeout.
Key sequences bring up
- Menu of boot options (described in menu.lst)
- Edit kernel options (runlevel, other features)
- GRUB command shell
A boot prompt password may be required before edit privileges or shell access granted.
- Can bypass OS security by specifying kernel options or accessing files from GRUB shell.

Booting with GRUB

Direct Boot

Specify OS kernel image to load.

Chainloading

Specify another bootloader to load.
Chainload NTLDR to boot MS Windows.

Config: /boot/grub/menu.lst

    # timeout (sec) before booting default
    timeout 10
    # Ubuntu entry
    title Ubuntu, kernel 2.6.10-5-386
    root (hd1,0)
    kernel /boot/vmlinuz-2.6.10-5-386 root=/dev/hde1 ro quiet splash
    initrd /boot/initrd.img-2.6.10-5-386
    boot
    # Windows entry
    title Windows 2000
    root (hd0,0)
    makeactive
    chainloader +1

GRUB Naming Convention

GRUB uses BIOS hard disk numbers
- OS may not number disks identically to BIOS.
Examples
- (hd0,0) First partition of first hard disk
- (hd1,2) Third partition of second hard disk.

Kernel

GRUB loads kernel file into memory: /boot/vmlinuz-VERSION
Kernel uncompresses self and begins running.
Sets up interrupts and virtual memory.
Loads initial RAMdisk into memory: /boot/initrd.img-VERSION
Kernel uncompresses initrd and mounts as /sysroot.
Kernel loads drivers and modules from /sysroot.
- SCSI and RAID drivers, ext3 filesystem module, etc.
Switches from real mode to protected mode.
Creates kernel threads (ksoftirq, khelper, kblockd)
Starts init or systemd process.

Kernel Arguments

Numeric arguments

Specify runlevel.
single also specifies single user mode

Root device options

root= specifies which root device to use
ro, rw specify access type

Kernel Arguments (continued)

Console options

console=ttyS1,9600 will use serial console

Hardware options

Enable, disable specific hardware devices/features.

Run Levels

0 – Shutdown
1 – Single user
2 – Multi-user, no networking
3 – Multi-user, with networking
4 – Unused
5 – Multi-user, networking, and GUI
6 – Reboot

/etc/inittab

    # The default runlevel.
    id:2:initdefault:
    # Boot-time system config/initialization script.
    si::sysinit:/etc/init.d/rcS
    # What to do in single-user mode.
    ~~:S:wait:/sbin/sulogin
    # /etc/init.d executes the start and kill scripts
    l0:0:wait:/etc/init.d/rc 0
    l1:1:wait:/etc/init.d/rc 1
    l2:2:wait:/etc/init.d/rc 2
    l3:3:wait:/etc/init.d/rc 3
    l4:4:wait:/etc/init.d/rc 4
    l5:5:wait:/etc/init.d/rc 5
    l6:6:wait:/etc/init.d/rc 6
    # What to do when CTRL-ALT-DEL is pressed.
    ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

rc

Executed by init with runlevel as argument.
Starts and stops services for each runlevel.
/etc/rc#.d scripts
- Symlinks to actual scripts in /etc/init.d
- Kills services beginning with K
- Starts services beginning with S
- Executes in ASCII order.

Scripts in runlevel 2

    $ ls /etc/rc2.d
    S05vbesave         S13gdm          S20openntpd   S89cron
    S08iptables        S18hplip        S20postfix    S91apache2
    S10acpid           S18portmap      S20powernowd  S98usplash
    S10powernowd.early S19cupsys       S20rsync      S99acpi-support
    S10sysklogd        S20apmd         S20ssh        S99rc.local
    S10syslog-ng       S20argus-server S20xinetd     S99rmnologin
    S11klogd           S20hotkey-setup S21nfs-common S99stop-readahead
    S12dbus            S20makedev      S89anacron

Example init.d script

    #! /bin/sh
    case "$1" in
    'start')
        if [ -f /usr/local/sbin/sshd ]; then
            echo "starting SSHD daemon"
            /usr/local/sbin/sshd &
        fi
        ;;
    'stop')
        PID=`ps -e -u 0 | fgrep sshd | awk '{print $1}'`
        if [ ! -z "$PID" ] ; then
            kill ${PID} >/dev/null 2>&1
        fi
        ;;
    *) echo "usage: /etc/init.d/sshd {start|stop}";;
    esac

Turning Services On/Off

Add a new service
- Install startup script in /etc/init.d
- Create S symlinks in appropriate runlevels
- Create K symlinks in appropriate runlevels
Prevent a service from starting on boot
- Remove S links from /etc/rc?.d/*
- Remove K links from /etc/rc?.d/*

Red Hat Service Commands

Starting or stopping a service

    service NAME [start,restart,stop]

is equivalent to

    /etc/init.d/NAME [start,restart,stop]

Enabling or disabling a service at boot time

    chkconfig NAME on –level <runlevels>