Linux Boot Process

Linux Boot Process (Startup Sequence)

Press the power button on your system, and after few moments you see the Linux login prompt.

Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?

The following are the 6 high level stages of a typical Linux boot process.

1. BIOS

• BIOS stands for Basic Input/Output System

• Performs some system integrity checks

• Searches, loads, and executes the boot loader program.

• It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F12 of F2, but it depends on your system) during the BIOS startup to change the boot sequence.

• Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.

So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

• MBR stands for Master Boot Record.

• It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda

• MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.

• It contains information about GRUB (or LILO in old systems).

So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

• GRUB stands for Grand Unified Bootloader.

• If you have multiple kernel images installed on your system, you can choose which one to be executed.

• GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

• GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).

• Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

#boot=/dev/sda

default=0

timeout=5

splashimage=(hd0,0)/boot/grub/splash.xpm.gz

hiddenmenu

title CentOS (2.6.18-194.el5PAE)

root (hd0,0)

kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/

initrd /boot/initrd-2.6.18-194.el5PAE.img

• As you notice from the above info, it contains kernel and initrd image.

So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

• Mounts the root file system as specified in the “root=” in grub.conf

• Kernel executes the /sbin/init program

• Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.

• initrd stands for Initial RAM Disk.

• initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

• Looks at the /etc/inittab file to decide the Linux run level.

• Following are the available run levels

0 – halt

1 – Single user mode

2 – Multiuser, without NFS

3 – Full multiuser mode

4 – unused

5 – X11

6 – reboot

• Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

• Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level

• If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.

• Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

• When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.

• Depending on your default init level setting, the system will execute the programs from one of the following directories.

Run level 0 – /etc/rc.d/rc0.d/

Run level 1 – /etc/rc.d/rc1.d/

Run level 2 – /etc/rc.d/rc2.d/

Run level 3 – /etc/rc.d/rc3.d/

Run level 4 – /etc/rc.d/rc4.d/

Run level 5 – /etc/rc.d/rc5.d/

Run level 6 – /etc/rc.d/rc6.d/

• Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.

• Under the /etc/rc.d/rc*.d/ directories, you would see programs that start with S and K.

• Programs starts with S are used during startup. S for startup.

• Programs starts with K are used during shutdown. K for kill.

• There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.

• For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

There you have it. That is what happens during the Linux boot process.

Kannel Start-Stop Daemon

Install kannel and configure with start-stop daemon

Step 1:

      Copy and extract the kannel in “/usr/local/” directory.

# cp gateway-1.4.1.tar.gz /usr/local

# tar -zxvf gateway-1.4.1.tar.gz

# mv gateway-1.4.1.tar.gz kannel

Step 2:

      Installing kannel with start-stop daemon.

# cd /usr/local/kannel/

# ./configure –enable-start-stop-daemon

# make

# make install

Step 3:

      Create start-stop daemon script in “/etc/init.d/” directory and set bearerbox, smsbox, start-stop-daemon and configuration path.

# vim /etc/init.d/kannel

#!/bin/sh

#

# gateway         This shell script takes care of starting and stopping

#                 the Kannel WAP gateway (bearer/wapbox)

#

# chkconfig: 2345 97 03

# description:  Start and stop the Kannel WAP gateway used to fetch \

#               some WML content from a Web server & compile it \

#               into WMLC mobile phone bytecode.

# probe: true

 

# Use start-stop-daemon

ver=1.4.1

BBOX=/usr/local/kannel/gw/bearerbox

SBOX=/usr/local/kannel/gw/smsbox

START="/usr/local/kannel/utils/start-stop-daemon --start --background --quiet --exec"

STOP="/usr/local/kannel/utils/start-stop-daemon --stop --quiet --oknodo --exec"

CONF=/usr/local/kannel/gw/smskannel.conf

LEVEL=-v\ 1

[ $# -eq 2 ] && ver=$2

 

# Source function library.

. /etc/rc.d/init.d/functions

 

# Source networking configuration.

. /etc/sysconfig/network

 

# Check that networking is up.

[ ${NETWORKING} = "no" ] && exit 0

[ -x $BBOX ] || exit 0

[ -x $SBOX ] || exit 0

[ -f $CONF ] || exit 0

 

RETVAL=0

# See how we were called.

case "$1" in

    start)

    # Start daemons.

            echo -n "Starting bearer service (gateway kannel $ver): "

            $START $BBOX -- $LEVEL $CONF

            RETVAL1=$?

            sleep 1 # Sleep for a while before we try to start smsbox

            echo

            echo -n "Starting smsbox service (gateway kannel $ver): "

            $START $SBOX -- $CONF

            RETVAL2=$?

            echo

            echo

            [ $RETVAL1 -eq 0 -a $RETVAL2 -eq 0 ] && touch /var/lock/subsys/gateway ||\

            RETVAL=1

            ;;

      stop)

            # Stop daemons.

            echo -n "Shutting down smsbox (kannel $ver): "

            $STOP $SBOX

            RETVAL2=$?

            echo

            echo -n "Shutting down bearerbox (kannel $ver): "

            $STOP $BBOX

            RETVAL1=$?

            echo

            [ $RETVAL1 -eq 0 -a $RETVAL2 -eq 0 ] && rm -f /var/lock/subsys/gateway

            echo ""

            ;;

      status)

            status bearerbox

            status smsbox

            exit $?

            ;;

      restart)

            $0 stop

            sleep 1

            $0 start

            ;;

      *)

            echo "Usage: named {start|stop|status|restart}"

            exit 1

esac

exit $RETVAL

      :wq

# chmod  +x /etc/init.d/kannel

# chkconfig - -list kannel

# chkconfig kannel on

# service kannel restart

# service kannel status

Change RDP Port in Windows

Change RDP (Remote Desktop) Port in Windows

Step 1:

Click on Start > Run > Type “regdit” and enter for registry editing.

Step 2:

Go to HKEY_LOCAL_MACHINE > SYSTEM > CurrentControlSet > Control > Terminal Server > WinStation > RDP_Tcp.

Step 3:

Look for “PortNumber” key-dword entry and double click on it. In the open windows chose “Decimal” and change “Value Data” field to desired port number, and click on Ok. The default RDP port number is 3389.

Step 4:

Make sure new port number is open in Firewall. Now the RDP port number is change in the system.

Repair Startup in Win 7

Repair startup issues in Windows 7

INTRODUCTION: We can use the Bootrec.exe tool in the Windows Recovery Environment (Windows RE) to troubleshoot and repair the following items in Windows Vista or Windows 7:

• A master boot record (MBR)

• A boot sector

• A Boot Configuration Data (BCD) store

Note: When we are troubleshooting startup issues by using the Windows RE, we should first try the Startup Repair option in the System Recovery Options dialog box. If the Startup Repair option does not resolve the issue, or if we must troubleshoot more steps manually, use the Bootrec.exe tool.

MORE INFORMATION: To run the Bootrec.exe tool, we must start Windows RE. To do this, follow these steps:

1.                Put the Windows Vista or Windows 7 installation disc in the disc drive, and then start the computer.

2.                Press a key when you are prompted.

3.                Select a language, a time, a currency, a keyboard or an input method, and then click Next.

4.                Click Repair your computer.

5.                Click the operating system that we want to repair, and then click Next.

6.                In the System Recovery Options dialog box, click Command Prompt.

7.                Type Bootrec.exe, and then press ENTER.

Note: To start the computer from the Windows Vista or Windows 7 DVD, the computer must be configured to start from the DVD drive. For more information about how to configure the computer to start from the DVD drive, see the documentation that is included with the computer or contact the computer manufacturer.

Bootrec.exe options: The Bootrec.exe tool supports the following options. Use the option that is appropriate for our situation.

Note: Use the Bootrec.exe tool to troubleshoot "Bootmgr Is Missing" issue. If rebuilding the BCD does not resolve the startup issue, we can export and delete the BCD, and then run this option again. By doing this, we make sure that the BCD is completely rebuilt.

To do this, type the following commands at the Windows RE command prompt:

• bcdedit /export C:\BCD_Backup

• c:

• cd boot

• attrib bcd -s -h -r

• ren c:\boot\bcd bcd.old

• bootrec /RebuildBcd

/FixMbr

The /FixMbr option writes a Windows 7 or Windows Vista-compatible MBR to the system partition. This option does not overwrite the existing partition table. Use this option when we must resolve MBR corruption issues, or when we have to remove non-standard code from the MBR.

/FixBoot

The /FixBoot option writes a new boot sector to the system partition by using a boot sector that is compatible with Windows Vista or Windows 7. Use this option if one of the following conditions is true:

• The boot sector has been replaced with a non-standard Windows Vista or Windows 7 boot sector.

• The boot sector is damaged.

• An earlier Windows operating system has been installed after Windows Vista or Windows 7 was installed. In this scenario, the computer starts by using Windows NT Loader (NTLDR) instead of Windows Boot Manager (Bootmgr.exe).

/ScanOs

The /ScanOs option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option displays the entries that are currently not in the BCD store. Use this option when there are Windows Vista or Windows 7 installations that the Boot Manager menu does not list.

/RebuildBcd

The /RebuildBcd option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option lets we select the installations that we want to add to the BCD store. Use this option when we must completely rebuild the BCD.