CIT 500: IT Fundamentals

Startup

Slide #2

Topics

1. Booting2. Bootstrap loaders3. Run levels4. Startup scripts5. Shutdown and reboot

Slide #3

Booting

1. ROM boot code (BIOS in PCs)2. Master Boot Record (MBR)3. Bootloader4. Kernel5. Hardware detection and configuration6. Creation of system processes7. Multiuser operation

Slide #4

BIOS1. Load CMOS settings.2. Initialize registers and power management.3. POST (Power On Self Test.)4. Display system settings (if key pressed.)5. Activate other BIOSes

Disk interfacesGraphics cardsNetwork interfaces

6. Find bootable device.7. Load MBR.8. Run MBR program.

MBR4 primary partitions.

One can be used as an extended partition, which is a link to an Extended boot record on the 1st sector of that partition.Each logical partition is described by its own EBR, which links to the next EBR.

Slide #6

GRUB InstallationAt OS Install Time

Most distributions install GRUB.

Linux Installgrub-install /dev/hda

Native installRun grub command from live CD or hard disk.#>> root (hd0,0)

#>> setup (hd0)

#>> quit

Slide #7

GRUB Boot

• Auto 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/shell access granted.– Can bypass OS security by specifying kernel

options or accessing files from GRUB shell.

GRUB Menu

GRUB Edit Menu

Slide #10

/boot/grub/menu.lst# timeout (sec) before booting defaulttimeout 10

title CentOS (2.6.18-128.el5)root (hd1,0)kernel /boot/vmlinuz-2.6.18-128.el5

root=/dev/hdb1 ro rhgb quietinitrd /boot/initrd-2.6.18-128.el5.imgboot

title Windows 2008root (hd0,0)makeactivechainloader +1

Slide #11

Booting with GRUB

Direct BootSpecify OS kernel image to load.

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

GRUB Naming Convention

GRUB uses BIOS hard disk numbersOS may not number disks identically to BIOS.

Examples(hd0,0) First partition of first hard disk(hd1,2) Third partition of second hard disk.

Kernel1. GRUB loads kernel file into memory

/boot/vmlinuz-VERSION2. Kernel uncompresses self and begins running.3. Sets up interrupts and virtual memory.4. Loads initial RAMdisk into memory

/boot/initrd.img-VERSION5. Kernel uncompresses initrd and mounts as /sysroot.6. Kernel loads drivers and modules from /sysroot.

SCSI and RAID drivers, ext3 filesystem module, etc.7. Switches from real mode to protected mode.8. Creates kernel threads (ksoftirq, khelper, kblockd)9. Starts init process.

Kernel Arguments in GRUB

Slide #15

Kernel ArgumentsNumeric arguments

Specify runlevel.single also specifies single user mode

Root device optionsroot= specifies which root device to usero, rw specify access type

Console optionsconsole=ttyS1,9600 will use serial console

Hardware optionsEnable, disable specific hardware devices/features.

Slide #16

Run Levels

0 – Shutdown1 – Single user2 – Multi-user, no networking3 – Multi-user, with networking4 – Unused5 – Multi-user, networking, and GUI6 – Reboot

/etc/inittab# The default runlevel.id:2:initdefault:

# Boot-time system configuration/initialization script.si::sysinit:/etc/init.d/rcS

# What to do in single-user mode.~~:S:wait:/sbin/sulogin

# /etc/init.d executes the S and K scripts upon changel0:0:wait:/etc/init.d/rc 0l1:1:wait:/etc/init.d/rc 1l2:2:wait:/etc/init.d/rc 2l3:3:wait:/etc/init.d/rc 3l4:4:wait:/etc/init.d/rc 4l5:5:wait:/etc/init.d/rc 5l6:6: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• Run 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– Stops services beginning with K*– Starts services beginning with S*– Executes in ASCII order.

Scripts in runlevel 2$ ls /etc/rc2.dS05vbesave S13gdm S20openntpd S89cronS08iptables S18hplip S20postfix S91apache2S10acpid S18portmap S20powernowd S98usplashS10powernowd.early S19cupsys S20rsync S99acpi-supportS10sysklogd S20apmd S20ssh S99rc.localS10syslog-ng S20argus-server S20xinetd S99rmnologinS11klogd S20hotkey-setup S21nfs-common S99stop-readaheadS12dbus S20makedev S89anacron

Slide #20

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=`/usr/bin/ps -e -u 0 | /usr/bin/fgrep sshd |

/usr/bin/awk '{print $1}'` if [ ! -z "$PID" ] ; then

/usr/bin/kill ${PID} >/dev/null 2>&1 fi ;;

*) echo "usage: /etc/init.d/sshd {start|stop}“;;

esac

Slide #21

Enabling or Disabling Services

Add a new serviceInstall startup script in /etc/init.dCreate S symlinks in appropriate runlevelsCreate K symlinks in appropriate runlevels

Prevent a service from starting on bootRemove S links from /etc/rc?.d/*Remove K links from /etc/rc?.d/*

Slide #22

Red Hat service Command

Starting or stopping a serviceservice NAME [start,restart,stop]

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

Service command options:start

restart

stop

status

Use command without any options to get help message with all available options.

Slide #23

Red Hat chkconfig Command

List configured serviceschkconfig

Enabling or disabling a service at boot timechkconfig --level # NAME

is equivalent toln –s /etc/init.d/NAME /etc/init.d/rc.d/rc#.d/S??name

Adding or removing a service entirelychkconfig --level

Red Hat Service Configuration

Red Hat Linuxes use files in /etc/sysconfigVARIABLE=value formatDocumentation in RHEL manuals

Examplesdesktop: select GNOME, KDE, XFCE desktopiptables: firewall configurationnetwork: hostname, gateway routernetwork-scripts/ifcfg-eth0: eth0 config

Slide #25

Single User Mode

Kernel boots (with arg 1 or “single”)System asks for root passwordRoot shell starts

No system processesNo networking

Only root filesystem mounted (read-only).Remount for RW: mount –o remount,rw /

Why?Fix w/o user interference: disks, backups, security.

Slide #26

What if single user mode doesn’t work?

Bypass init by booting into shellPass init=/bin/bash argument to kernel.Probably want to specify rw option as well.No need for root password.No housekeeping performed.

Boot from CD RedHat CD with linux rescue Knoppix CD

Slide #27

Shutdown

shutdown (or init 0)Offers time-delay option.Runs server stop scripts.Kill remaining processes.Flushes writes to disk.

haltKills processes.Flushes writes to disk (sync).

Slide #28

Reboot

init 6shutdown

restarts system

Ctrl-Alt-DelDefined in /etc/inittabca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

reboothalt

restarts system

Slide #29

What's wrong with init?Performance

– Starts services sequentially.– Shell scripts spawn lots of processes.

Manual configuration– Order to start services determined manually.– i.e. network before auth before nfs.

Reliability– Init doesn’t monitor or restart services after boot.

Hotplug hardware– Drives mounted at boot.– What about USB or network drives?

Slide #30

Init Alternatives

iniNG– Service config specifies dependencies.– need = system/initial net/all; – Starts services in parallel once dependencies met.

launchd– Mac replacement for init, rc, cron, atd, inetd– Monitors services, starts on demand.

Service Management Facility– Sun replacement for init with service monitoring.

Upstart

• Event-based init replacement.• Supports wider range of events than launchd.– Hardware device has been added.– Filesystem has been mounted.– Time-based (can replace cron as well as init.)– File has been modified.– Another job has begun or finished running.

• Backwards compatible with /etc/init.d scripts.• Respawns services that are down.• Used in Ubuntu >= 6.10, Fedora >= 9

References

1. Red Hat, RHEL Installation Guide, http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Installation_Guide/index.html, 2009.

2. Syed Mansoor Sarwar, Robert Koretsky, Syed Ageel Sarwar, UNIX: The Textbook, 2nd edition, Addison-Wesley, 2004.

3. Nicholas Wells, The Complete Guide to Linux System Administration, Thomson Course Technology, 2005.