Andrew Hudson - Fedora™ Unleashed, 2008 edition

Although it is not intended that you modify the rc.sysinitscript, knowledge of the contents of the file might aid you in diagnosing a problem if the boot process fails during this step. Look at /etc/rc.d/rc.sysinit, and you will discover that it's just a text file filled with shell script language.

After the rc.sysinitscript has run, the basic system is configured and the kernel is in control of the system. If the boot process were halted at this point, the system would just sit idle and the screen would be blank. To make the system useful for users, you need to start the system services. Those services are some of the applications that enable you to interact with the system.

After finishing with rc.sysinitscript during the bootloading process, the initcommand uses the Linux system initialization table found in /etc/inittabto boot Fedora to a specific system state. The state of the system is commonly referred to as its runlevel.

Several different ways of starting and stopping system services exist, and Fedora uses a method derived from System V Unix. The System V (pronounced "System Five") method uses runlevels and different combinations of services to define different states of operation. Runlevels determine which of the many available system services are started, as well as in which order they start. A special runlevel is used to stop the system, and a special runlevel is used for system maintenance. As you will see, there are other runlevels for special purposes.

The System V method makes extensive use of symbolic links, which are ways to reference a file in another location and make it appear as if it were in two or more places at once. The benefit is that you need to edit only one file to change them all. In addition, any reorganization to be done means that only links need to be changed, not the files themselves.

You use runlevels to manage the system services running on your computer. All these special files and scripts are set up during your installation of Fedora Linux, and they receive their initial values based on your choices during the installation — as described in Chapter 1, "Installing Fedora," You can change and control them manually, as you learn later in this chapter, using tools of varying sophistication.

The Fedora runlevels are defined for the Fedora system in /etc/inittab.

Not all Linux distributions use the same runlevel configurations or runlevel definitions! For example, although Fedora uses runlevel 3 for a full, console-based multiuser mode, pre-7.1 versions of SUSE Linux defined this system state as runlevel 2. Red Hat, Fedora, and SUSE now use the same runlevels to conform to the Linux Standards Base, or LSB. As a system administrator, you should be aware of this issue, especially if you have devised any administrative scripts or tools that deal with system states.

Each runlevel tells the initcommand what services to start or stop. Although runlevels might all have custom definitions, Fedora has adopted some standards for runlevels:

► Runlevel 0— Known as "halt," this runlevel is used to shut down the system.

► Runlevel 1— This is a special runlevel, defined as "single," which boots Fedora to a root access shell prompt where only the root user may log in. Networking, X, and multiuser access are turned off. This is the maintenance or rescue mode. It allows the system administrator to perform work on the system, make backups, or repair configuration or other files.

► Runlevel 2— This runlevel dictates that Fedora be booted to a console, or text-based mode, with multiuser access.

► Runlevel 3— This runlevel is identical to runlevel 2, except that it also starts any networking services.

► Runlevel 4— This runlevel is undefined, and it can readily be configured to boot Fedora to a custom system state.

► Runlevel 5— This runlevel boots Fedora to a networking, multiuser state with an active X session. This is the most common runlevel for home users who want a graphical interface.

► Runlevel 6— This runlevel is used to reboot the system.

Runlevel 1 (also known as single-user mode or maintenance mode ) is most commonly used to repair file systems and change the root password on a system when the password has been forgotten. Trespassers with physical access to the machine can also use runlevel 1 to access your system.

Never forget that uncontrolled physical access is virtually a guarantee of access to your data by an intruder.

Entries in /etc/inittabuse a field-based notation that determines the runlevel — when to execute the process, whether or not the process is executed when booting, whether or not to wait for the process to complete, and when to execute the process during booting. The default choices are adequate and need be changed only in unique circumstances that the average user is not likely to encounter.

The value of the default entry, or the initdefaultline in /etc/inittab, determines the particular system state in which Fedora is when the login prompt is finally presented. For example,

id:5:initdefault:

In this example, Fedora boots to runlevel 5, a network-enabled, multiuser mode with an active X session and a graphical login. The value 5is forwarded to the script named rcunder the /etc/rc.ddirectory. This script is used when booting or changing runlevels; it also acts as an interpreter when you boot Fedora in "Interactive" mode when you press i during the boot.

After /etc/rc.d/rc.sysinithas finished, inituses the corresponding /etc/inittabentry that matches the designated default runlevel. Using the previous example, the line in /etc/inittabwould then be:

l5:5:wait:/etc/rc.d/rc 5

Under the /etc/rc.ddirectory is a series of directories that correspond to each runlevel:

# ls /etc/rc.d

init.d rc0.d rc2.d rc4.d rc6.d rc.sysinit

rc rc1.d rc3.d rc5.d rc.local

Assuming that the value is 5, the rc script executes all the scripts under the /etc/rc.d/rc.5directory and then launches the graphical login.

If Fedora is booted to runlevel 5, it executes scripts from the /etc/rc.d/rc5.ddirectory. Scripts beginning with the letter K are executed first, followed by scripts beginning with the letter S:

# ls /etc/rc.d/rc5.d/

K01yum K20bootparamd K28amd K45named K61ldap

K74ypxfrd S05kudzu S24pcmcia S85gpm K05innd

K20iscsi K30sendmail K46radvd K65identd K84bgpd

S08ip6tables S25netfs S90crond K05saslauthd K20netdump-server

K34dhcrelay K50netdump K65kadmin K84ospf6d S08ipchains

S26apmd S90FreeWnn K10psacct K20nfs K34yppasswdd

K50snmpd K65kprop K84ospfd S08iptables S28autofs

S90xfs K10radiusd K20rstatd K35atalk K50snmptrapd

K65krb524 K84ripd S10network S40smartd S92lisa

K12canna K20rusersd K35dhcpd K50tux K65krb5kdc

K84ripngd S12syslog S44acpid S95anacron K12mailman

K20rwalld K35smb K50vsftpd K70aep1000 K85zebra

S13irqbalance S55cups S95atd K12mysqld K20rwhod