Linux Booting Process - Essay Example

Running Header: Linux Booting Process Your name: Course name: Professors’ name: Date Executive summary In a nutshell, the Linux booting process start from the BIOS where the MBR is executed after integrity check is performed before the MBR loader is executed. The MBR has three main parts: the primary boot loader information, the partition table information and the MBR validation check. The MBR also plays a role in the execution of the next level of the booting process, the GRUB. The MBR executes the GRUB. The GRUB allows the user to input commands before loading the default kernel image indentified in the configuration file within the GRUB. The next level in the booting process is the Kernel. At this level the root file system is mounted and executed in the Kernel level. The Kernel has all the required drivers necessary in performing key functions such as acquiring hard drive partitions and other software. Init determines the Linux run level; there are a number of runlevels which are available at this stage. The next level and the final level is the runlevel booting process, at this stage one might see a number of these services getting initialized. Notably, there are two important initials at this stage, the S and the K which stand for Startup and Shutdown/Kill respectively. Table of Contents Executive summary 2 Table of Contents 3 Introduction 4 Linux Booting process 4 BIOS 5 MBR 6 GRUB 6 Kernel 7 Init 7 Runlevel 8 Conclusion 9 References 10 Introduction There are a number of steps which are sequentially followed during Linux system boot process, these steps perform different functions are different levels and also allow other programs which follow to be executed. The Linux system is an organized system with processes that perform different functions at different levels of the boot process. Essentially, Linux system is a command based system which employs, various commands are used to execute functions and acquire other file systems with defined functions within the Linux directory. In this report, sequential steps which happen during the Linux booting system are identified as follows, the BIOS level (Basic Input/Output Sysem), The MBR Master Boot Record, The GRUB, Grand Unified Bootloader, The Kernel, The init level and finally the Runlevel. Linux Booting process According to Ward (2004) when the system is booted for the first time, the processor executes code in a given location. In a Personal Computer for instance, the location os the BIOS which stores the flash memory (in the computer main board or motherboard), the CPU initiates reset vector which starts a program in flash/ROM. BIOS essentially determines the devices needed to commence the boot process. Upon finding the boot, the next stage is loading the boot loader into the RAM for execution. This is the common stages in the loading process in Linux. Fundamentally, there are six stages of Linux booting process as identified in the diagram below. (Natarajan, 2011) BIOS This stands for Basic Input/output System and it is the first stage of the booting process, the initial task at this stage is to perform system integrity checks. After the check is completed, the system then searchers boot loader program, loads and executes the program (Hagen, 2010). Notably, the boot loader program is normally stored in a CD-ROM, hard drive of the computer or an attached external hard drive or a floppy disk (old version) (Pogue, Altheide, & Haverkos, 2008). In most system, by pressing the F12 button or F2, one can change the boot sequence of the Linux Operating system. Once system detects boot loader program, it is then loaded into the memory through BIOS control. In summary, at this stage, the BIOS loads and then executes the MBR boot loader program (Siever, Figgins, Love, & Robbins, 2009). MBR The MBR which is essentially the Master Boot Record is normally located at the first sector (partition) of the bootable device, in Linux this is represented by dev/sda or dev/hda. The MBR is normally about 512 bytes or less, it has three main parts namely primary boot loader information, 1st 442 bytes, second, the partition table information in the following 64 bytes and thirdly, the mbr validation check in the 2 last bytes (Negus, 2006). The MBR also contains information about the next level known as the GRUB or LILO in traditional systems. In summary, the term MBR is responsible for loading and executing the GRUB boot loader (Smith, 2011). GRUB This term stands for Grand Unified Bootloader, in cases where there are more than one Kernel images which are installed in the system; one can choose the one to be executed in this boot process stage. According to Natarajan (2011) at this stage of the boor process, the GRUB shows a splash screen for about 5 seconds, at this level, it gives the user an option of inputting a command, if nothing is entered, it loads using a default kernel image which is specified in the GRUB configuration file. GRUB essentially has adequate knowledge of the file system which was initially not there in the old form (LILO). The configuration file of GRUB is /boot/grub/grub.conf. An example at this stage is as follows. #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 can be identified, the above information reveals that this stage contains both the initrd image and the kernel. This means that GRUB Kernel At this level, the root file system is mounted as identified in the grub.conf above; the /sbin/init program is executed in the kernel. It is important to note that since init was the initial program executed within the kernel, it contains the process ID (Stanfield, & Smith, 2006)). The next level in the employment of initrid which is utilized by the kernel to store temporary root file system during the booting process, it is here where the root file is actually mounted. Intrid means initial RAM Disk and it has all the booting drivers within it which helps it to acquire hard drive partitions and other necessary hardware(Hagen, 2010). Init Determines the Linux run level by looking at /etc/inittab, the run levels available are as follows: halt, reboot, single user mode, full multiuser mode, unused and x11. At this level, the init determines the initlevel from /etc/inittab, normally the default run level. The user can therefore set the default run level to full multiuser mode or X11. By setting it to halt or reboot will definitely lead to trouble (Jang, 2006). Runlevel When the booting process is proceeding, one might see the various services getting initialized, for instance something like starting send mail...ok may be seen in the screen. These are examples of run level programs which are basically executed from the run level directory defined when setting up the run level program (Hagen, 2010). As per the set default init level, the system will execute 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/ Within the  /etc/rc.d/rc*.d/ directories, programs that commence with S and K can be seen. In essence, programs which begin with S are used during the startup process while those that start with K are used for shutting down the system, K initialized Kill in Linux while S initialized Startup (Altheide & Carvey, 2011).. Normally there are numbers just next to the S and K, these numbers initializes the sequence in which the program should be started or shut down (Ward, 2004). As an example, S12sylog means starting up syslog deamon, its sequence number is 12.S80sendmail is to initialize sendmail daemon, sequence number 80. With this respect the S12sylog will be initialized before the 12.S80sendmail (Altheide & Carvey, 2011). Conclusion As identified in this brief report, the Linux booting process contains a number of stages, the BIOS system which is responsible for integrity check and executing the MBR. The MBR is plays an important role executing the next level, the GRUB. The GRUB allows user to input command before loading a default kernel image. The kernel stores drivers’ necessary in performing the run level program, the Init essentially determines the Linux run level since there are a number of them at this level. The user can determine the run level to implement at this stage. The run level is thus the last level in the Linux booting process. In Linux booting process, it is important for the user to note that some commands provided can affect the success of the booting process, notably, there are two essential commands, S meaning startup and K meaning Kill. In general, Linux is a secure system which mainly finds its important role in Linux based servers. References Altheide, C., & Carvey, H.(2011). Digital Forensics with Open Source Tools: Using Open Source Platform Tools for Performing Computer Forensics on TargetSystems: Windows, Mac, Linux, Unix, etc. New York, NY: Elsevier. Hagen, W.(2010). Ubuntu Linux Bible: Featuring Ubuntu 10.04 LTS. New York, NY: John Wiley & Sons. Hollabaugh, C.(2002). Embedded Linux[: Hardware, Software, and Interfacing. New York, NY: Addison-Wesley Professional. Jang, M.(2006). Mastering Red Hat Enterprise Linux 3. New Jersey, NJ: John Wiley & Sons. Natarajan, R . (2011). 6 Stages of Linux Boot Process (Startup Sequence), Accessed from http://www.thegeekstuff.com/2011/02/linux-boot-process/ Negus, C. (2006). Live Linux CDs: Building And Customizing Bootables. New York, NY: Prentice Hall Professional. Pogue, C., Altheide, C., & Haverkos, T.(2008). UNIX and Linux Forensic Analysis DVD Toolkit. New York, NY: Syngress. Siever, E., Figgins, S.,Love, R.,& Robbins, A .(2009). Linux in a Nutshell. New Jersey, NJ: O'Reilly Media, Inc.,  Smith, R.W.(2011). LPIC-1: Linux Professional Institute Certification Study Guide: (Exams 101 and 102). New Jersey, NJ: John Wiley & Sons. Stanfield, V., & Smith, R.W.(2006). Linux System Administration. New York, NY: John Wiley & Sons. Ward, B. (2004). How Linux Works: What Every SuperUser Should Know. 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