Each runlevel uses a directory for its configuration and has specific meaning as the following.

You can change the runlevel from the console to, e.g., 4 by the following.

$ sudo telinit 4

	Caution
	The Debian system does not pre-assign any special meaning differences among the runlevels between 2 and 5. The system administrator on the Debian system may change this. (I.e., Debian is not Red Hat Linux nor Solaris by Sun Microsystems nor HP-UX by Hewlett Packard nor AIX by IBM nor …)

	Caution
	The Debian system does not populate directories for the runlevels between 7 and 9 when the package is installed. Traditional Unix variants don't use these runlevels.

When init(8) or telinit(8) commands goes into the runlevel to "<n>", the system basically executes the initialization scripts as follows.

For example, if you had the links "S10sysklogd" and "S20exim4" in a runlevel directory, "S10sysklogd" which is symlinked to "../init.d/sysklogd" would run before "S20exim4" which is symlinked to "../init.d/exim4".

This simple sequential initialization system is the classical System V style boot system and was used up to the Debian lenny system.

The recent Debian system is optimized to execute the initialization scripts concurrently, instead.

	Warning
	It is not advisable to make any changes to symlinks in "/etc/rcS.d/" unless you know better than the maintainer.

For example, let's set up runlevel system somewhat like Red Hat Linux as the following.

This can be done by using editor on the "/etc/inittab" file to change starting runlevel and using user friendly runlevel management tools such as sysv-rc-conf or bum to edit the runlevel. If you are to use command line only instead, here is how you do it (after the default installation of the gdm3 package and selecting it to be the choice of display manager).

# cd /etc/rc2.d ; mv S21gdm3 K21gdm3
# cd /etc ; perl -i -p -e 's/^id:.:/id:3:/' inittab

Please note the "/etc/X11/default-display-manager" file is checked when starting the display manager daemons: xdm, gdm3, kdm, and wdm.

	Note
	You can still start X from any console shell with the startx(1) command.

The default parameter for each init script in "/etc/init.d/" is given by the corresponding file in "/etc/default/" which contains environment variable assignments only. This choice of directory name is specific to the Debian system. It is roughly the equivalent of the "/etc/sysconfig" directory found in Red Hat Linux and other distributions. For example, "/etc/default/cron" can be used to control how "/etc/init.d/cron" works.

The "/etc/default/rcS" file can be used to customize boot-time defaults for motd(5), sulogin(8), etc.

If you cannot get the behavior you want by changing such variables then you may modify the init scripts themselves. These are configuration files editable by system administrators.

The kernel maintains the system hostname. The init script in runlevel S which is symlinked to "/etc/init.d/hostname.sh" sets the system hostname at boot time (using the hostname command) to the name stored in "/etc/hostname". This file should contain only the system hostname, not a fully qualified domain name.

To print out the current hostname run hostname(1) without an argument.

Although the root filesystem is mounted by the kernel when it is started, other filesystems are mounted in the runlevel S by the following init scripts.

The mount options of kernel filesystems are set in "/etc/default/rcS". See rcS(5).

The mount options of the filesystem are set in "/etc/fstab". See Section 9.3.6, “Optimization of filesystem by mount options”.

	Note
	The actual mounting of network filesystems waits for the start of the network interface.

	Warning
	After mounting all the filesystems, temporary files in "/tmp", "/var/lock", and "/var/run" are cleaned for each boot up.

Network interfaces are initialized in runlevel S by the init script symlinked to "/etc/init.d/ifupdown-clean" and "/etc/init.d/ifupdown". See Chapter 5, Network setup for how to configure them.

Many network services (see Chapter 6, Network applications) are started under multi-user mode directly as daemon processes at boot time by the init script, e.g., "/etc/rc2.d/S20exim4" (for RUNLEVEL=2) which is a symlink to "/etc/init.d/exim4".

Some network services can be started on demand using the super-server inetd (or its equivalents). The inetd is started at boot time by "/etc/rc2.d/S20inetd" (for RUNLEVEL=2) which is a symlink to "/etc/init.d/inetd". Essentially, inetd allows one running daemon to invoke several others, reducing load on the system.

Whenever a request for service arrives at super-server inetd , its protocol and service are identified by looking them up in the databases in "/etc/protocols" and "/etc/services". inetd then looks up a normal Internet service in the "/etc/inetd.conf" database, or a Open Network Computing Remote Procedure Call (ONC RPC)/Sun RPC based service in "/etc/rpc.conf".

Sometimes, inetd does not start the intended server directly but starts the TCP wrapper program, tcpd(8), with the intended server name as its argument in "/etc/inetd.conf". In this case, tcpd runs the appropriate server program after logging the request and doing some additional checks using "/etc/hosts.deny" and "/etc/hosts.allow".

For system security, disable as much network service programs as possible. See Section 4.6.4, “Restricting access to some server services”.

See inetd(8), inetd.conf(5), protocols(5), services(5), tcpd(8), hosts_access(5), hosts_options(5), rpcinfo(8), portmap(8), and "/usr/share/doc/portmap/portmapper.txt.gz".

The system message can be customized by "/etc/default/rsyslog" and "/etc/rsyslog.conf" for both the log file and on-screen display. See rsyslogd(8) and rsyslog.conf(5). See also Section 9.2.2, “Log analyzer”.

The kernel message can be customized by "/etc/default/klogd" for both the log file and on-screen display. Set "KLOGD='-c 3'" in this file and run "/etc/init.d/klogd restart". See klogd(8).

You may directly change the error message level by the following.

# dmesg -n3

For Linux kernel 2.6, the udev system provides mechanism for the automatic hardware discovery and initialization (see udev(7)). Upon discovery of each device by the kernel, the udev system starts a user process which uses information from the sysfs filesystem (see Section 1.2.12, “procfs and sysfs”), loads required kernel modules supporting it using the modprobe(8) program (see Section 3.5.12, “The kernel module initialization”), and creates corresponding device nodes.

	Tip
	If "/lib/modules/<kernel-version>/modules.dep" was not generated properly by depmod(8) for some reason, modules may not be loaded as expected by the udev system. Execute "depmod -a" to fix it.

The name of device nodes can be configured by udev rule files in "/etc/udev/rules.d/". Current default rules tend to create dynamically generated names resulting non-static device names except for cd and network devices. By adding your custom rules similar to what cd and network devices do, you can generate static device names for other devices such as USB memory sticks, too. See "Writing udev rules" or "/usr/share/doc/udev/writing_udev_rules/index.html".

Since the udev system is somewhat a moving target, I leave details to other documentations and describe the minimum information here.

	Tip
	For mounting rules in "/etc/fstab", device nodes do not need to be static ones. You can use UUID to mount devices instead of device names such as "/dev/sda". See Section 9.3.3, “Accessing partition using UUID”.

The modprobe(8) program enables us to configure running Linux kernel from user process by adding and removing kernel modules. The udev system (see Section 3.5.11, “The udev system”) automates its invocation to help the kernel module initialization.

There are non-hardware modules and special hardware driver modules as the following which need to be pre-loaded by listing them in the "/etc/modules" file (see modules(5)).

The configuration files for the modprobe(8) program are located under the "/etc/modprobes.d/" directory as explained in modprobe.conf(5). (If you want to avoid some kernel modules to be auto-loaded, consider to blacklist them in the "/etc/modprobes.d/blacklist" file.)

The "/lib/modules/<version>/modules.dep" file generated by the depmod(8) program describes module dependencies used by the modprobe(8) program.

	Note
	If you experience module loading issues with boot time module loading or with modprobe(8), "depmod -a" may resolve these issues by reconstructing "modules.dep".

The modinfo(8) program shows information about a Linux kernel module.

The lsmod(8) program nicely formats the contents of the "/proc/modules", showing what kernel modules are currently loaded.

	Tip
	You can identify exact hardware on your system. See Section 9.6.3, “Hardware identification”.

	Tip
	You may configure hardware at boot time to activate expected hardware features. See Section 9.6.4, “Hardware configuration”.

	Tip
	You can add support for your device by recompiling kernel. See Section 9.7, “The kernel”.