Each runlevel uses a directory for its configuration and has specific meaning:

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

$ 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 RedHat nor SOLARIS nor HP-UX 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.

The names of the symlinks in the runlevel directories have the form S<2-digit-number><original-name> or K<2-digit-number><original-name>. The 2-digit-number is used to determine the order in which to run the scripts. 'S' is for 'Start' and 'K' is for 'Kill'.

When init or telinit commands change the runlevel to <n>:

For example, if you had the links S10sysklogd and S20exim4 in a runlevel directory, S10sysklogd would run before S20exim4.

	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 Redhat system, i.e.:

The easy way is to use editor on the /etc/inittab file to change starting runlevel and use user friendly runlevel management tools such as sysv-rc-conf or bum to edit the runlevel. If you are to use command line only, here is how you do it (after the default installation of gdm package and selecting it to be the choice of display manager):

# cd /etc/rc2.d ; mv S21gdm K21gdm
# 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, gdm, kdm, and wdm.

	Note
	You can still start X from any console shell with the startx 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. The choice of directory name is specific to the Debian system. It is roughly the equivalent of the /etc/sysconfig directory found in Red Hat and other distributions.

For example, /etc/default/hotplug can be used to control how /etc/init.d/hotplug works. The /etc/default/rcS file can be used to customize boot-time defaults for motd, sulogin, etc.

If you cannot get the behavior you want by changing such variables then you may modify the init scripts themselves: they are all configuration files.

The kernel maintains the system hostname. The initscript /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 without an argument.

Network interfaces are initialized under single-user mode on boot by the initscript /etc/init.d/ifupdown-clean and /etc/init.d/ifupdown. See Chapter 6, Network setup for how to configure them.

Many network services (see Chapter 7, Network applications) are started directly as daemon processes at boot time, 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, 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 Sun-RPC based service in /etc/rpc.conf.

For system security, make sure to disable unused services in /etc/inetd.conf. Sun-RPC services need to be active for NFS and other RPC-based programs.

Sometimes, inetd does not start the intended server directly but starts the TCP wrapper, tcpd, 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.

If you have problems with remote access in a recent Debian system, comment out "ALL: PARANOID" in /etc/hosts.deny if it exists. (But you must be careful on security risks involved with this kind of action.)

For details, see inetd(8), inetd.conf(5), protocols(5), services(5), tcpd(8), hosts_access(5), and hosts_options(5).

For more information on Sun-RPC, see rpcinfo(8), portmap(8), and /usr/share/doc/portmap/portmapper.txt.gz.

The system message can be customized by /etc/syslog.conf for both the log file and on-screen display. See syslogd(8) and syslog.conf(5). See also Section 10.2.2, “Log analyzer”.

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

You may directly change the error message level by:

# dmesg -n3

Here:

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 2.2.12, “procfs and sysfs”), loads required kernel modules supporting it using the modprobe(8) program (see Section 4.5.11, “The kernel module initialization”), and creates corresponding device nodes.

The name of device nodes can be configured by files in /etc/udev/rules.d/ (see /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.

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 4.5.10, “The udev system”) automates its invocation to help the kernel module initialization.

Non-hardware modules and special hardware driver modules, such as:

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.

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 10.6.3, “The hardware identification”.

	Tip
	You may configure hardware at boot time to activate expected hardware features. See Section 10.6.4, “The hardware configuration”.

	Tip
	You can add support for your device by recompiling kernel. See Section 10.7, “The kernel”.