cit 470: advanced network and system administration

CIT 470: Advanced Network and System Administration Slide #1

CIT 470: Advanced Network and System Administration

Booting and Runlevels


Topics

1. Booting

2. Bootstrap loaders

3. Run levels

4. Startup scripts

5. Shutdown and reboot


Booting

ROM boot code (BIOS)BootloaderKernelInit4

321


BIOS

1. 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.

1

Dell BIOS Screenshot


1


MBR contains Bootloader

Only 446 bytes!

MBR bootloader bootstraps itself by loading larger bootloader program from disk.

2

Bootloader: GRUB


2


GRUB Boot ProcessStage 1 (stored in MBR)

Detect the geometry and the accessing mode of the loading drive. Finds and runs stage 1.5.

Stage 1.5 (stored at beginning of partition)Understands filesystem; loads stage 2 from filesystem.

Stage 2 (stored in filesystem)Displays menu of OS choices.Often displays boot splashscreen.Loads selected OS kernel.

2


GRUB Installation

At OS Install TimeMost distributions install GRUB.

Linux Installgrub-install /dev/hda

Native installBoot with CD and run grub.

#>> root (hd0,0)

#>> setup (hd0)

#>> quit

2


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

options or accessing files from GRUB shell.

2


Booting with GRUB

Direct BootSpecify OS kernel image to load.

ChainloadingSpecify another bootloader to load.

Chainload NTLDR to boot MS Windows.

2


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

title Ubuntu, kernel 2.6.10-5-386root (hd1,0)kernel /boot/vmlinuz-2.6.10-5-386

root=/dev/hde1 ro quiet splashinitrd /boot/initrd.img-2.6.10-5-386boot

title Windows 2000root (hd0,0)makeactivechainloader +1

2


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.

2

Kernel Booting


3

Initial RAMDisk

Temporary filesystem used for booting.– Stored as /boot/initrd-VERSION– Loaded into memory by bootloader

Contains drivers needed to mount root fs– RAID, LVM, NFS or other device drivers– Filesystem drivers

Allows single kernel to be shipped by vendor– Drivers specific to your system on RAMdisk– Also supports hibernation for laptops


3


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

Console options– console=ttyS1,9600 will use serial console

Hardware options– Enable, disable specific hardware devices/features.

3


Run Levels

0 – Shutdown

1 – Single user mode

2 – Multi-user, no networking, no GUI

3 – Multi-user, with networking, no GUI

4 – Unused

5 – Multi-user, networking, and GUI

6 – Reboot

4

Single User Mode

Single user mode has– No GUI.

– No network access.

– Only one user (root) can use system.

Why use single user mode?– Run fsck to fix filesystem corruption.

– Backup files without filesystem active.

– Fix problems without user interference.

– May allow access to system without a password in case you have forgotten root’s password.


4

Booting in Single User Mode

1. At GRUB prompt, select ‘a’ to append args

2. Append this argument to list:

single

3. Hit return to complete argument changes

4. Select ‘b’ to boot the kernel

Single-user mode differs from run level 1 in that run level 1 runs the scripts for run level 1, then enters single-user mode.


4

What if you lost your root password?


1. At GRUB prompt, select ‘a’ to append arguments

2. Append this argument to list:

init=/bin/bash

3. Hit return to complete argument changes

4. Select ‘b’ to boot the kernel

5. At root prompt, remount /mount / -o rw,remount

6. Reset root passwordpasswd

7. Power cycle system

4


/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 change

l0:0:wait:/etc/init.d/rc 0







# What to do when CTRL-ALT-DEL is pressed.

ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now

4


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– Stops services beginning with K*– Starts services beginning with S*– Executes in ASCII order.

4


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

4


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

4


Turning Services On/Off

Add a new serviceInstall startup script in /etc/init.d

Create S symlinks in appropriate runlevels

Create 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/*

4


Red Hat Service Commands

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

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

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

4

Red Hat Service Configuration

RH Linuxes use files in /etc/sysconfigVARIABLE=value format

Documentation in RHEL manuals

Examplesdesktop: select GNOME, KDE, XFCE desktop

iptables: firewall configuration

network-scripts/ifcfg-eth0: eth0 cfg

4

/etc/sysconfig


4


Shutdown

shutdown –h now (or init 0)Offers time-delay option (-h time)

Runs server stop scripts.

Kill remaining processes.

Flushes writes to disk.

haltKills processes.

Flushes writes to disk (sync).

4


Reboot

reboot (or init 6)restarts system

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

4


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 authentication before nfs.

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

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

4


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.

4


Upstart

• Event-based init replacement.• Events

– Hardware device has been added.– Filesystem has been mounted.– Time (cron-replacement)– File has been modified.– Another job has begun or finished

running.

• Backwards compatible with /etc/init.d scripts.

• Respawns services that are down.

4

Boot Process Summary


Key PointsBoot Steps

1. BIOS

2. Bootloader (GRUB)

3. Kernel

4. init

init is PID 1, parent of all processes, started by kernel

– Uses rc command to run all scripts in /etc.rcN.d dir where N is runlevel

– RHEL: use chkconfig to configure, /etc/sysconfig for options

Run levels

1 and Single user mode (use to fix problems)

3 = multiuser + network (servers)

5 = multiuser + network + GUI (workstations)



References1. Aeleen Frisch, Essential System Administration, 3rd edition, O’Reilly, 2002.2. M. Tim Jones, “Inside the Linux boot process,”

http://www.ibm.com/developerworks/library/l-linuxboot/index.html, 2006.3. M. Tim Jones, “Parallelize applications for faster Linux booting,”

http://www.ibm.com/developerworks/linux/library/l-boot-faster/index.html, 2007.

4. Evi Nemeth et al, UNIX System Administration Handbook, 3rd edition, Prentice Hall, 2001.

5. Scott James Remnant, “Upstart in Universe,” http://www.netsplit.com/2006/08/26/upstart-in-universe/, 2006.

6. Starman, “Boot records revealed: MBR details,” http://mirror.href.com/thestarman/asm/mbr/index.html

7. “Extended boot record,” http://en.wikipedia.org/wiki/Extended_Boot_Record.8. GRUB manual, http://www.gnu.org/software/grub/manual/grub.html9. “How it works: Master Boot Record (MBR),”

http://www.ata-atapi.com/hiwmbr.htm10. “Replacement Init”, https://wiki.ubuntu.com/ReplacementInit

http://www.ibm.com/developerworks/library/l-linuxboot/index.html

http://www.ibm.com/developerworks/linux/library/l-boot-faster/index.html

http://www.netsplit.com/2006/08/26/upstart-in-universe/

http://en.wikipedia.org/wiki/Extended_Boot_Record

http://www.ata-atapi.com/hiwmbr.htm

cit 470: advanced network and system administration

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