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Mitesh Shah

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System Initialization

20 minutes to read (2042 words) ()

Overview

System Initialization

Checking Your System State

[mitesh@Matrix ~]$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.0 (Santiago)

# Identifying Kernel
[mitesh@Matrix ~]$ uname -r
2.6.32-71.el6.x86_64

[mitesh@Matrix ~]$ yum list installed kernel\*
[mitesh@Matrix ~]$ rpm -qa kernel\*
kernel-firmware-2.6.32-71.el6.noarch
kernel-2.6.32-71.el6.x86_64

Runlevels

  • The Runlevel Determines Which Services Are Started Automatically On Your Linux System.

  • The Runlevel Is Selected By

    • Passing An Argument From The Bootloader
    • The Default Runlevel Is Stored In /etc/inittab File id:5:initdefault:
    • Using The Command Line: init New_Runlevel
Linux Default Runlevels
/-----------------------------------------------------------------------------------------------\
|												|
|	0		|	Halt - Shutdown To A Poweroff State				|
|			|	shutdown -h now							|
|												|
|	1,S		|	Single User Mode						|
|	s,single	|	Run rc.sysinit							|
|												|
|	emergency	|	Bypass rc.sysinit, sulogin - prompts for root password		|
|												|
|	2		|	Multiuser Text Mode Without NFS					|
|												|
|	3		|	Full Multiuser Text Mode With NFS				|
|												|
|	4		|	Unused								|
|												|
|	5		|	X11								|
|												|
|	6		|	Reboot - Shutdown & Soft Reboot					|
|			|	shutdown -r now							|
|												|
\-----------------------------------------------------------------------------------------------/

# Identifying Runlevel
[mitesh@Matrix ~]$ who -r
[mitesh@Matrix ~]$ /sbin/runlevel

# Default Runlevel
[mitesh@Matrix ~]$ grep initdefault: /etc/inittab

Controlling Services

  • The Job Of The System V Initialization Scripts Is To Start Services At Boot Time.
  • Most Of These Services Are Run As Daemons Such As cups, crond and sendmail.
The Service Command

  • The service command is used to status start stop restart A Standalone Service Immediately

    • service --status-all
    • service servicename status | start | stop | restart | reload | condrestart

NOTE!: The Effect Of service command Is Temporarily The Effect Of service command Is Lost After The Reboot.

The ntsysv Command
  • The ntsysv command Is Used To Configuring Runlevel Services
  • By Default The ntsysv command Configures The Current Runlevel
  • By Using ntsysv --level Option You Can Configure Other Runlevels

NOTE!: After Reboot Is Take Effect & Permanent For Specific Runlevel

The chkconfig Command
  • chkconfig --list Display The List Of Services
  • Whether They Are Started Stopped In Each Runlevels 
# Start | Stop Service In Runlevel 2,3,4,5
[mitesh@Matrix ~]$ chkconfig servicename on | off

# Start | Stop Service In Runlevel 2 And 4
[mitesh@Matrix ~]$ chkconfig --level 24 servicename on | off

NOTE!: After Reboot Is Take Effect & Permanent

The system-config-services
  • System -> Administration -> Services
  • The system-config-services command Opens A X Client in GUI

  • The system-config-services command Display The List Of Services

  • By Using GUI Of The system-config-services command
  • You Can Start Stop Restart The Services In Particular Or All The Runlevels

Boot Sequence

  1. BIOS Initialization
  2. GRUB Bootloader
  3. Kernel Initialization
  4. Init Initialization

1. BIOS Initialization

| Reset SMPS |--------CMOS-------| CPU |------------------| BIOS |
  • POST - Power On Self Test
  • Initialize All The Hardwares
  • Checks For The Bootable Devices FDD, HDD, USB, CD-ROM, DVD
  • Check For The Boot Sector MBR - Master Boot Records
/-----------------------------------------------------------------------\
|				512 Bytes				|
|----------------------------------------------------------------------
|									|
|	  446 Bytes	|	64 Bytes	|	2 Bytes		|
|			|			|			|
|	   NTLDR	|	Partition	|	0xAA55		|
|	    IPL		|	Table		|			|
|	(GRUB Stage 1)	|	Information	|	OS Signatures	|
|									|
\-----------------------------------------------------------------------/

NOTE!: NTLDR = Windows Boot Loader
IPL = Initial Program Loader

Partition Table Information
/-----------------------------------------------------------------------\
|				64 Bytes				|
-----------------------------------------------------------------------
|	16 Bytes	|	1	|	Primary Partition	|
|			|---------------|				|
|	16 Bytes	|	2 	|	Primary Partition	|
|			|---------------|				|
|	16 Bytes	|	3	|	Primary Partition	|
|			|---------------|				|
|	16 Bytes	|	|-------|				|
|			|   4   |-------|	Extended Partition	|
|			|	|-------|				|
\-----------------------------------------------------------------------/

2. GRand Unified Bootloader - GRUB

  • The Bootloader Is Responsible For Loading And Starting The Linux OS.
GRUB Stages

1st Stage:

  • Small, Added To MBR Or Boot Sector During The Installation
  • Loads The Grub 2nd Stage Into Memory
  • Repair GRUB:
# Method 1
[root@Matrix ~]# /sbin/grub-install /dev/sda

# Method 2
[root@Matrix ~]# grub
grub> root (hd0,0)
grub> setup (hd0)
grub> quit

2nd Stage:

  • Loaded From Filesystem Containing /boot
  • Configuration: /boot/grub/grub.conf
The GRUB Boot Screen
  • When Grub Starts Press The Esc Key

  • Then Grub Splash Screen Has Been Display With The List Of Menu Entries (Boot Images)

  • Select The Menu Entry With The Help Of Up & Down Arrow Keys

  • After That Press Enter For Boot The System

  • If Grub Password Is Set Then Press The Password(p) Key

  • Then Provide Your Grub Password To Unlock The Grub Menu

  • You Can Modify The Grub Menu Entry With The Help Of Edit(e) Key

  • After Modify The Menu Entry Press Boot(b) Key For Boot The System

  • To Access The Grub Command Line Then Press The Command(c) Key
Argument Passing

  • Change An Existing Boot Stanza In The Menu Editing Mode Such As Change The Root Filesystem, Kernel And Runlevel

  • In Grub CLI

    • Experiments With GRUB
    • Perform Diagnostic Tests
    • View The Contents Of The File
Password Protection
  • Block Menu Editing Mode
  • Block Particular Menu Entry (Boot Image) Selection
# Generate Grub Password:
[root@Matrix ~]# grub-md5-crypt
Password:
Retype password:
$1$3SoEu8O7$AmXQgjKBvRKPL5jxC5oBj1

[root@Matrix ~]# vim /boot/grub/grub.conf
password --md5 $1$3SoEu8O7$AmXQgjKBvRKPL5jxC5oBj1

[root@Matrix ~]# cat /boot/grub/grub.conf
default=0
timeout=2
splashimage=(hd0,0)/grub/splash.xpm.gz

hiddenmenu
password --md5 $1$3SoEu8O7$AmXQgjKBvRKPL5jxC5oBj1

title Red Hat Enterprise Linux (2.6.32-71.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-71.el6.x86_64 ro root=LABEL=/ rhgb quiet i8042.reset
initrd /initramfs-2.6.32-71.el6.x86_64.img

NOTE! i8042.reset is extra argument which will fix Lenovo Y500 Laptop mouse problem

3. Kernel Initialization

Kernel Boot Time Function
  • Device Detection
  • Device Driver Initialization
  • Mount The Root Filesystem - Read Only

  • Load The Initial Process - /sbin/init With PID 1

  • Kernel Boot Messages: /var/log/dmesg
Device Detection
  • The Device Drivers compiled into the Kernel are called And attempt to locate their corresponding Devices
Device Driver Initialization
  • The Essential Drivers has been compiled as Modules
  • The Modules Loaded from initramfs-*.img
  • The Kernel Temporarily Mount These Modules On RAM Disk
Mount Root Filesystem
  • After All The Essential Drivers Are Loaded, The Kernel Will Mount The Root Filesystems As Read Only
Load The Initial Process
  • Then The First Process - /sbin/init Is Loaded With PID 1
  • Then The Kernel Pass The Control To The /sbin/init Process
  • The Init Is The Parent Of All Processes & You Can Verify That With pstree command

4. Init Initialization

  • In The Red Hat Enterprise Linux 6, The Upstart Init(8) Is The Event Based Daemon

  • The Processes Managed By The Init Daemon Are Known As Jobs

  • On Startup, Upstart Init(8) Daemon Reads Its Jobs Configuration From The /etc/init/ Directory

    • Initialize Runlevel
    • System Initialization
    • Standalone Service Initialization
    • Runlevel Specific Script Directories
    • Spawn Gettys On Virtual Consoles
    • Initialize X Server In Runlevel 5
Initialize Runlevel
  • The System First Check The /proc/cmdline File

  • The /proc/cmdline File Contains The Kernel Arguments Passed During System Bootup Time Via Grub Menu Editing Mode

  • If The Runlevel Is Specified In The Kernel Arguments Then The System Initialize The Specified Runlevel

  • If The Runlevel Is Not Specified In The Kernel Arguments Then The System Initialize The Initdefault Runlevel

  • Default Runlevel: /etc/inittab
System Initialization

  • /etc/rc.d/rc.sysinit

  • Activate udev
  • Activate selinux

  • Sets The Kernel Parameters- /etc/sysctl.conf

  • Sets Hostname
  • Activate RAID Devices

  • Activate LVM Devices

  • Checking Filesystems
  • Mounting The Local Filesystems
  • Enabling The Local Filesystems Quotas
  • Enabling The /etc/fstab Swaps Partition

  • Remount The Root Filesystem Read-Write Mode

  • Cleans Up State Locks And PID Files
Standalone Service Initialization

  • /etc/rc.d/rc

  • Initialize /etc/inittab Initdefault Runlevel

  • Start/Stop Service Whenever Runlevel Changed

  • Findout Previous & Current Runlevel
  • First Kill Services In Runlevel Specific Scripts Directories
  • Now Starts Services In Runlevel Specific Scripts Directories
Runlevel Specific Scripts Directories

/etc/rc.d/rc[0-6].d/

  • K* Symbolic Link Called With A Stop Arguments

  • S* Symbolic Link Called With A Start Arguments

  • The System V Init Scripts Resides In: /etc/rc.d/init.d/
  • Behaviour Configured With /etc/sysconfig/

Non-Service Startup

  • /etc/rc.d/rc.local

  • Common Place For Custome Modification
  • Runs At The End Of Runlevel Specific Scripts Directories (S99local)

Better Practice

  • Create A System V Init Script
  • The /etc/rc.d/init.d/ Scripts Can Be Used As A Starting Point
  • The /etc/rc.d/init.d/ Scripts Are Simple Case Statements, That Handle The 1st Arguments Of status star stop restart

Three Addition Things To Do Within The Scripts

i) Modify The Comment Line To Support The chkconfig
 
     
  • #chkconfig: <runlevel> <SS> <KK>
    •  
  • #chkconfig: 2345 55 25
    •  
 
ii) Manage A File Named /var/lock/subsys/servicename
 
     
  • Whose Existence Declares That The Service Is Running
    •  
  • And Used By The /etc/rc.d/rc Script To Determine Whether To Run The Symbolic Linked Scripts
    •  
 
iii) Manage A File Named /var/run/servicename.pid
 
     
  • Store The PID Number Of The Service
    •  
 
Transient Services
 
     
  •  
    The xinetd Daemon Manages On Demand Services
     
    •  
  • Service IP Redirection
    •  
  • Host Based Authentication
    •  
  • Service Statistics & Logging
    •  
  • Less Frequently Needed Services
    •  
  •  
    Requiring Additional Resource Managements
     
    •  
  • The xinetd Daemon Uses /etc/services For Port To Service Managements
    •  
 
Configuration Files
 
     
  • /etc/xinetd.conf - Global Configuration
    •  
  • /etc/xinet.d/ - Override Global Configuration
    •  
 
NEED MORE HELP LOOK AT HERE
 
     
  • man 5 xinetd.conf
    •  
  • /usr/share/doc/xinetd-*/
    •  
 
Spawn Gettys On Virtual Consoles
 
     
  • Configuration File: /etc/init/start-ttys.conf
    •  
 
Initialize X Server In Runlevel 5
 
     
  • Configuration File: /etc/init/prefdm.conf
    •  
    
 


   
 
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