30.2. A Detailed Look at the Boot Process
The beginning of the boot process varies depending on the hardware platform being used. However, once the kernel is found and loaded by the boot loader, the default boot process is identical across all architectures. This chapter focuses primarily on the x86 architecture.
When an x86 computer is booted, the processor looks at the end of system memory for the Basic Input/Output System or BIOS program and runs it. The BIOS controls not only the first step of the boot process, but also provides the lowest level interface to peripheral devices. For this reason it is written into read-only, permanent memory and is always available for use.
Other platforms use different programs to perform low-level tasks roughly equivalent to those of the BIOS on an x86 system. For instance, Itanium-based computers use the Extensible Firmware Interface (EFI) Shell.
Once loaded, the BIOS tests the system, looks for and checks peripherals, and then locates a valid device with which to boot the system. Usually, it checks any diskette drives and CD-ROM drives present for bootable media, then, failing that, looks to the system's hard drives. In most cases, the order of the drives searched while booting is controlled with a setting in the BIOS, and it looks on the master IDE device on the primary IDE bus. The BIOS then loads into memory whatever program is residing in the first sector of this device, called the Master Boot Record or MBR. The MBR is only 512 bytes in size and contains machine code instructions for booting the machine, called a boot loader, along with the partition table. Once the BIOS finds and loads the boot loader program into memory, it yields control of the boot process to it.
This section looks at the default boot loader for the x86 platform, GRUB. Depending on the system's architecture, the boot process may differ slightly. Refer to Section 184.108.40.206, “Boot Loaders for Other Architectures” for a brief overview of non-x86 boot loaders. For more information about configuring and using GRUB, see Chapter 9, The GRUB Boot Loader.
A boot loader for the x86 platform is broken into at least two stages. The first stage is a small machine code binary on the MBR. Its sole job is to locate the second stage boot loader and load the first part of it into memory.
GRUB has the advantage of being able to read ext2 and ext3  partitions and load its configuration file — /boot/grub/grub.conf — at boot time. Refer to Section 9.7, “GRUB Menu Configuration File” for information on how to edit this file.
If upgrading the kernel using the Red Hat Update Agent, the boot loader configuration file is updated automatically. More information on Red Hat Network can be found online at the following URL: https://rhn.redhat.com/.
Once the second stage boot loader is in memory, it presents the user with a graphical screen showing the different operating systems or kernels it has been configured to boot. On this screen a user can use the arrow keys to choose which operating system or kernel they wish to boot and press Enter. If no key is pressed, the boot loader loads the default selection after a configurable period of time has passed.
Once the second stage boot loader has determined which kernel to boot, it locates the corresponding kernel binary in the /boot/ directory. The kernel binary is named using the following format — /boot/vmlinuz-<kernel-version> file (where <kernel-version> corresponds to the kernel version specified in the boot loader's settings).
For instructions on using the boot loader to supply command line arguments to the kernel, refer to Chapter 9, The GRUB Boot Loader. For information on changing the runlevel at the boot loader prompt, refer Section 9.8, “Changing Runlevels at Boot Time”.
The boot loader then places one or more appropriate initramfs images into memory. Next, the kernel decompresses these images from memory to /boot/, a RAM-based virtual file system, via cpio. The initramfs is used by the kernel to load drivers and modules necessary to boot the system. This is particularly important if SCSI hard drives are present or if the systems use the ext3 file system.
Once the kernel and the initramfs image(s) are loaded into memory, the boot loader hands control of the boot process to the kernel.
For a more detailed overview of the GRUB boot loader, refer to Chapter 9, The GRUB Boot Loader.
220.127.116.11. Boot Loaders for Other Architectures
Once the kernel loads and hands off the boot process to the init command, the same sequence of events occurs on every architecture. So the main difference between each architecture's boot process is in the application used to find and load the kernel.
For example, the Itanium architecture uses the ELILO boot loader, the IBM eServer pSeries architecture uses yaboot, and the IBM System z systems use the z/IPL boot loader.
When the kernel is loaded, it immediately initializes and configures the computer's memory and configures the various hardware attached to the system, including all processors, I/O subsystems, and storage devices. It then looks for the compressed initramfs image(s) in a predetermined location in memory, decompresses it directly to /sysroot/, and loads all necessary drivers. Next, it initializes virtual devices related to the file system, such as LVM or software RAID, before completing the initramfs processes and freeing up all the memory the disk image once occupied.
The kernel then creates a root device, mounts the root partition read-only, and frees any unused memory.
At this point, the kernel is loaded into memory and operational. However, since there are no user applications that allow meaningful input to the system, not much can be done with the system.
To set up the user environment, the kernel executes the /sbin/init program.
30.2.4. The /sbin/init Program
The /sbin/init program (also called init) coordinates the rest of the boot process and configures the environment for the user.
When the init command starts, it becomes the parent or grandparent of all of the processes that start up automatically on the system. First, it runs the /etc/rc.d/rc.sysinit script, which sets the environment path, starts swap, checks the file systems, and executes all other steps required for system initialization. For example, most systems use a clock, so rc.sysinit reads the /etc/sysconfig/clock configuration file to initialize the hardware clock. Another example is if there are special serial port processes which must be initialized, rc.sysinit executes the /etc/rc.serial file.
The init command then runs the /etc/inittab script, which describes how the system should be set up in each SysV init runlevel. Runlevels are a state, or mode, defined by the services listed in the SysV /etc/rc.d/rc<x>.d/ directory, where <x> is the number of the runlevel. For more information on SysV init runlevels, refer to Section 30.4, “SysV Init Runlevels”.
Next, the init command sets the source function library, /etc/rc.d/init.d/functions, for the system, which configures how to start, kill, and determine the PID of a program.
The init program starts all of the background processes by looking in the appropriate rc directory for the runlevel specified as the default in /etc/inittab. The rc directories are numbered to correspond to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel 5.
When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to determine which processes to start and stop.
Below is an example listing of the /etc/rc.d/rc5.d/ directory:
K05innd -> ../init.d/innd K05saslauthd -> ../init.d/saslauthd K10dc_server -> ../init.d/dc_server K10psacct -> ../init.d/psacct K10radiusd -> ../init.d/radiusd K12dc_client -> ../init.d/dc_client K12FreeWnn -> ../init.d/FreeWnn K12mailman -> ../init.d/mailman K12mysqld -> ../init.d/mysqld K15httpd -> ../init.d/httpd K20netdump-server -> ../init.d/netdump-server K20rstatd -> ../init.d/rstatd K20rusersd -> ../init.d/rusersd K20rwhod -> ../init.d/rwhod K24irda -> ../init.d/irda K25squid -> ../init.d/squid K28amd -> ../init.d/amd K30spamassassin -> ../init.d/spamassassin K34dhcrelay -> ../init.d/dhcrelay K34yppasswdd -> ../init.d/yppasswdd K35dhcpd -> ../init.d/dhcpd K35smb -> ../init.d/smb K35vncserver -> ../init.d/vncserver K36lisa -> ../init.d/lisa K45arpwatch -> ../init.d/arpwatch K45named -> ../init.d/named K46radvd -> ../init.d/radvd K50netdump -> ../init.d/netdump K50snmpd -> ../init.d/snmpd K50snmptrapd -> ../init.d/snmptrapd K50tux -> ../init.d/tux K50vsftpd -> ../init.d/vsftpd K54dovecot -> ../init.d/dovecot K61ldap -> ../init.d/ldap K65kadmin -> ../init.d/kadmin K65kprop -> ../init.d/kprop K65krb524 -> ../init.d/krb524 K65krb5kdc -> ../init.d/krb5kdc K70aep1000 -> ../init.d/aep1000 K70bcm5820 -> ../init.d/bcm5820 K74ypserv -> ../init.d/ypserv K74ypxfrd -> ../init.d/ypxfrd K85mdmpd -> ../init.d/mdmpd K89netplugd -> ../init.d/netplugd K99microcode_ctl -> ../init.d/microcode_ctl S04readahead_early -> ../init.d/readahead_early S05kudzu -> ../init.d/kudzu S06cpuspeed -> ../init.d/cpuspeed S08ip6tables -> ../init.d/ip6tables S08iptables -> ../init.d/iptables S09isdn -> ../init.d/isdn S10network -> ../init.d/network S12syslog -> ../init.d/syslog S13irqbalance -> ../init.d/irqbalance S13portmap -> ../init.d/portmap S15mdmonitor -> ../init.d/mdmonitor S15zebra -> ../init.d/zebra S16bgpd -> ../init.d/bgpd S16ospf6d -> ../init.d/ospf6d S16ospfd -> ../init.d/ospfd S16ripd -> ../init.d/ripd S16ripngd -> ../init.d/ripngd S20random -> ../init.d/random S24pcmcia -> ../init.d/pcmcia S25netfs -> ../init.d/netfs S26apmd -> ../init.d/apmd S27ypbind -> ../init.d/ypbind S28autofs -> ../init.d/autofs S40smartd -> ../init.d/smartd S44acpid -> ../init.d/acpid S54hpoj -> ../init.d/hpoj S55cups -> ../init.d/cups S55sshd -> ../init.d/sshd S56rawdevices -> ../init.d/rawdevices S56xinetd -> ../init.d/xinetd S58ntpd -> ../init.d/ntpd S75postgresql -> ../init.d/postgresql S80sendmail -> ../init.d/sendmail S85gpm -> ../init.d/gpm S87iiim -> ../init.d/iiim S90canna -> ../init.d/canna S90crond -> ../init.d/crond S90xfs -> ../init.d/xfs S95atd -> ../init.d/atd S96readahead -> ../init.d/readahead S97messagebus -> ../init.d/messagebus S97rhnsd -> ../init.d/rhnsd S99local -> ../rc.local
As illustrated in this listing, none of the scripts that actually start and stop the services are located in the /etc/rc.d/rc5.d/ directory. Rather, all of the files in /etc/rc.d/rc5.d/ are symbolic links pointing to scripts located in the /etc/rc.d/init.d/ directory. Symbolic links are used in each of the rc directories so that the runlevels can be reconfigured by creating, modifying, and deleting the symbolic links without affecting the actual scripts they reference.
The name of each symbolic link begins with either a K or an S. The K links are processes that are killed on that runlevel, while those beginning with an S are started.
The init command first stops all of the K symbolic links in the directory by issuing the /etc/rc.d/init.d/<command> stop command, where <command> is the process to be killed. It then starts all of the S symbolic links by issuing /etc/rc.d/init.d/<command> start.
After the system is finished booting, it is possible to log in as root and execute these same scripts to start and stop services. For instance, the command /etc/rc.d/init.d/httpd stop stops the Apache HTTP Server.
Each of the symbolic links are numbered to dictate start order. The order in which the services are started or stopped can be altered by changing this number. The lower the number, the earlier it is started. Symbolic links with the same number are started alphabetically.
After the init command has progressed through the appropriate rc directory for the runlevel, the /etc/inittab script forks an /sbin/mingetty process for each virtual console (login prompt) allocated to the runlevel. Runlevels 2 through 5 have all six virtual consoles, while runlevel 1 (single user mode) has one, and runlevels 0 and 6 have none. The /sbin/mingetty process opens communication pathways to tty devices, sets their modes, prints the login prompt, accepts the user's username and password, and initiates the login process.
In runlevel 5, the /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script executes the preferred X display manager — gdm, kdm, or xdm, depending on the contents of the /etc/sysconfig/desktop file.
Once finished, the system operates on runlevel 5 and displays a login screen.