== The system initialization

// vim: set sts=2 expandtab:
// Use ":set nowrap" to edit table

It is wise for you as the system administrator to know roughly how the Debian system is started and configured.  Although the exact details are in the source files of the packages installed and their documentations, it is a bit overwhelming for most of us.

I did my best to provide a quick overview of the key points of the Debian system and their configuration for your reference, based on the current and previous knowledge of mine and others. Since the Debian system is a moving target, the situation over the system may have been changed.  Before making any changes to the system, you should refer to the latest documentation for each package.

WARNING: This chapter is getting outdated since this is based on Debian 7.0 (`Wheezy`) released in 2013.

=== An overview of the boot strap process

The computer system undergoes several phases of http://en.wikipedia.org/wiki/Booting[boot strap processes] from the power-on event until it offers the fully functional operating system (OS) to the user.

For simplicity, I limit discussion to the typical PC platform with the default installation.

The typical boot strap process is like a four-stage rocket.  Each stage rocket hands over the system control to the next stage one.

- <<_stage_1_the_bios>>
- <<_stage_2_the_boot_loader>>
- <<_stage_3_the_mini_debian_system>>
- <<_stage_4_the_normal_debian_system>>

Of course, these can be configured differently.  For example, if you compiled your own kernel, you may be skipping the step with the mini-Debian system.  So please do not assume this is the case for your system until you check it yourself.

NOTE: For non-legacy PC platform such as the SUN or the Macintosh system, the BIOS on ROM and the partition on the disk may be quite different (<<_disk_partition_configuration>>).  Please seek the platform specific documentations elsewhere for such a case.

==== Stage 1: the BIOS

The http://en.wikipedia.org/wiki/BIOS[BIOS] is the 1st stage of the boot process which is started by the power-on event. The http://en.wikipedia.org/wiki/BIOS[BIOS] residing on the http://en.wikipedia.org/wiki/Read-only_memory[read only memory (ROM)] is executed from the particular memory address to which the program counter of CPU is initialized by the power-on event.

This BIOS performs the basic initialization of the hardware (http://en.wikipedia.org/wiki/Power-on_self-test[POST: power on self test]) and hands the system control to the next step which you provide.  The BIOS is usually provided with the hardware.

The BIOS startup screen usually indicates what key(s) to press to enter the BIOS setup screen to configure the BIOS behavior. Popular keys used are F1, F2, F10, Esc, Ins, and Del. If your BIOS startup screen is hidden by a nice graphics screen, you may press some keys such as Esc to disable this.  These keys are highly dependent on the hardware.

The hardware location and the priority of the code started by the BIOS can be selected from the BIOS setup screen.  Typically, the first few sectors of the first found selected device (hard disk, floppy disk, CD-ROM, ...) are loaded to the memory and this initial code is executed.  This initial code can be any one of the following.

- The boot loader code
- The kernel code of the stepping stone OS such as http://www.freedos.org/[FreeDOS]
- The kernel code of the target OS if it fits in this small space

Typically, the system is booted from the specified partition of the primary hard disk partition. First 2 sectors of the hard disk on legacy PC contain the http://en.wikipedia.org/wiki/Master_boot_record[master boot record (MBR)].  The disk partition information including the boot selection is recorded at the end of this MBR. The first boot loader code executed from the BIOS occupies the rest of this MBR.

==== Stage 2: the boot loader

The http://en.wikipedia.org/wiki/Boot_loader[boot loader] is the 2nd stage of the boot process which is started by the BIOS.  It loads the system kernel image and the http://en.wikipedia.org/wiki/Initrd[initrd] image to the memory and hands control over to them.  This initrd image is the root filesystem image and its support depends on the bootloader used.

The Debian system normally uses the Linux kernel as the default system kernel. The initrd image for the current 2.6/3.x Linux kernel is technically the initramfs (initial RAM filesystem) image. The initramfs image is a gzipped cpio archive of files in the root filesystem.

WARNING: The above is no longer true with the new multi-segment initramfs. See https://bugs.debian.org/790100[Bug #790100].

The default install of the Debian system places first-stage GRUB boot loader code into the http://en.wikipedia.org/wiki/Master_boot_record[MBR] for the PC platform. There are many boot loaders and configuration options available.

.List of boot loaders
[grid="all"]
`--------------`-------------`------------`-------------`--------------------------------------------------------------`---------------------------------------------------------------------------------------------------------------------------------------
package        popcon        size         initrd        bootloader                                                     description
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
grub-legacy    @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/GNU_GRUB[GRUB Legacy]             This is smart enough to understand disk partitions and filesystems such as vfat, ext3, ....
grub-pc        @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/GNU_GRUB[GRUB 2]                  This is smart enough to understand disk partitions and filesystems such as vfat, ext4, .... (default)
grub-rescue-pc @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/GNU_GRUB[GRUB 2]                  This is GRUB 2 bootable rescue images (CD and floppy) (PC/BIOS version)
lilo           @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/LILO_(boot_loader)[Lilo]          This relies on the sector locations of data on the hard disk. (Old)
syslinux       @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/SYSLINUX[Isolinux]                This understands the ISO9660 filesystem.  This is used by the boot CD.
syslinux       @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/SYSLINUX[Syslinux]                This understands the http://en.wikipedia.org/wiki/File_Allocation_Table[MSDOS filesystem (FAT)].  This is used by the boot floppy.
loadlin        @-@popcon1@-@ @-@psize1@-@ Supported     http://en.wikipedia.org/wiki/Loadlin[Loadlin]                  New system is started from the FreeDOS/MSDOS system.
mbr            @-@popcon1@-@ @-@psize1@-@ Not supported http://www.chiark.greenend.org.uk/\~neilt/[MBR by Neil Turton] This is free software which substitutes MSDOS http://en.wikipedia.org/wiki/Master_boot_record[MBR].  This only understands disk partitions.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

WARNING: Do not play with boot loaders without having bootable rescue media (CD or floppy) created from images in the `grub-rescue-pc` package.  It makes you boot your system even without functioning bootloader on the hard disk.

For GRUB Legacy, the menu configuration file is located at "`/boot/grub/menu.lst`".  For example, it has entries as the following.

--------------------
title           Debian GNU/Linux
root            (hd0,2)
kernel          /vmlinuz root=/dev/hda3 ro
initrd          /initrd.img
--------------------

For GRUB 2, the menu configuration file is located at "`/boot/grub/grub.cfg`". It is automatically generated by "`/usr/sbin/update-grub`" using templates from "`/etc/grub.d/\*`" and settings from "`/etc/default/grub`".  For example, it has entries as the following.

--------------------
menuentry "Debian GNU/Linux" {
        set root=(hd0,3)
        linux /vmlinuz root=/dev/hda3
        initrd /initrd.img
}
--------------------

For these examples, these GRUB parameters mean the following.

.The meaning of GRUB parameters
[grid="all"]
`--------------`-------------------------------------------------------------------------------------------------------------
GRUB parameter meaning
-----------------------------------------------------------------------------------------------------------------------------
`root`         use 3rd partition on the primary disk by setting it as "`(hd0,2)`" in GRUB legacy or as "`(hd0,3)`" in GRUB 2
`kernel`       use kernel located at "`/vmlinuz`" with kernel parameter: "`root=/dev/hda3 ro`"
`initrd`       use http://en.wikipedia.org/wiki/Initrd[initrd/initramfs] image located at "`/initrd.img`"
-----------------------------------------------------------------------------------------------------------------------------

NOTE: The value of the partition number used by GRUB legacy program is one less than normal one used by Linux kernel and utility tools.  GRUB 2 program fixes this problem.

TIP: http://en.wikipedia.org/wiki/Universally_Unique_Identifier[UUID] (see <<_accessing_partition_using_uuid>>) may be used to identify a block special device instead of its file name such as "`/dev/hda3`", e.g."`root=UUID=81b289d5-4341-4003-9602-e254a17ac232 ro`".

TIP: If http://en.wikipedia.org/wiki/GNU_GRUB[GRUB] is used, the kernel boot parameter is set in `/boot/grub/grub.cfg`.  On Debian system, you should not edit `/boot/grub/grub.cfg` directly.  You should edit the `GRUB_CMDLINE_LINUX_DEFAULT` value in `/etc/default/grub` and run `update-grub`(8) to update `/boot/grub/grub.cfg`.

TIP: You can start a boot loader from another boot loader using techniques called http://en.wikipedia.org/wiki/Chain_loading[chain loading].

See "`info grub`" and `grub-install`(8).

==== Stage 3: the mini-Debian system

// The choice of "mini-Debian" over "mini-Linux" is intentional.  Think about kFreeBSD or any alternative kernel
// which may support similar on memory system.  (Maybe NFS mounted ...)
// I thought about "initramfs system" too but it may confuse people.  It is Linux specific too.
// I know I only talk about Linux+initramfs here.
// Avoided to repeat the stage explanation.

The mini-Debian system is the 3rd stage of the boot process which is started by the boot loader.  It runs the system kernel with its root filesystem on the memory. This is an optional preparatory stage of the boot process.

NOTE: The term "the mini-Debian system" is coined by the author to describe this 3rd stage boot process for this document.  This system is commonly referred as the http://en.wikipedia.org/wiki/Initrd[initrd] or initramfs system. Similar system on the memory is used by http://www.debian.org/devel/debian-installer/[the Debian Installer].

The "`/init`" script is executed as the first program in this root filesystem on the memory.  It is a shell script program which initializes the kernel in user space and hands control over to the next stage. This mini-Debian system offers flexibility to the boot process such as adding kernel modules before the main boot process or mounting the root filesystem as an encrypted one.

You can interrupt this part of the boot process to gain root shell by providing "`break=init`" etc. to the kernel boot parameter. See the "`/init`" script for more break conditions.  This shell environment is sophisticated enough to make a good inspection of your machine@@@sq@@@s hardware.

Commands available in this mini-Debian system are stripped down ones and mainly provided by a GNU tool called `busybox`(1).

CAUTION: You need to use "`-n`" option for `mount` command when you are on the readonly root filesystem.

==== Stage 4: the normal Debian system

The normal Debian system is the 4th stage of the boot process which is started by the mini-Debian system.  The system kernel for the mini-Debian system continues to run in this environment. The root filesystem is switched from the one on the memory to the one on the real hard disk filesystem.

The http://en.wikipedia.org/wiki/Init[init] program is executed as the first program with PID=1 to perform the main boot process of starting many programs.  The default file path for the init program is "`/sbin/init`" but it can be changed by the kernel boot parameter as "`init=/path/to/init_program`".

The default init program has been changing:

- Debian before `squeeze` uses the simple http://en.wikipedia.org/wiki/UNIX_System_V[SysV]-style init.
- Debian `wheezy` improves the SysV-style init by ordering the boot sequence with LSB header and starting boot scripts in parallel.
- Debian `jessie` switches its default init to the http://en.wikipedia.org/wiki/Systemd[systemd] for the event-driven and parallel initialization.

TIP:  All boot mechanisms are compatible through "`/etc/init.d/rc`", "`/etc/init.d/rcS`", "`/usr/sbin/update-rc.d`", and "`/usr/sbin/invoke-rc.d`" scripts.

TIP: The actual init command on your system can be verified by the "`ps --pid 1 -f`" command.

.List of boot utilities for the Debian system
[grid="all"]
`-------------------`-------------`------------`-----------------------------------------------------------------------------
package             popcon        size         description
---------------------------------------------------------------------------------------------------------------------------
`initscripts`       @-@popcon1@-@ @-@psize1@-@ scripts for initializing and shutting down the system
`sysvinit-core`     @-@popcon1@-@ @-@psize1@-@ System-V-like `init`(8) utilities
`sysv-rc`           @-@popcon1@-@ @-@psize1@-@ System-V-like runlevel change mechanism
`sysvinit-utils`    @-@popcon1@-@ @-@psize1@-@ System-V-like utilities (`startpar`(8), `bootlogd`(8), ...)
`lsb-base`          @-@popcon1@-@ @-@psize1@-@ http://en.wikipedia.org/wiki/Linux_Standard_Base[Linux Standard Base] 3.2 init script functionality
`insserv`           @-@popcon1@-@ @-@psize1@-@ tool to organize boot sequence using LSB init.d script dependencies
`systemd`           @-@popcon1@-@ @-@psize1@-@ event-based `init`(8) daemon for concurrency (alternative to `sysvinit`)
`uswsusp`           @-@popcon1@-@ @-@psize1@-@ tools to use userspace software suspend provided by Linux
`kexec-tools`       @-@popcon1@-@ @-@psize1@-@ kexec tool for `kexec`(8) reboots (warm reboot)
`bootchart2`        @-@popcon1@-@ @-@psize1@-@ boot process performance analyser
`pybootchartgui`    @-@popcon1@-@ @-@psize1@-@ boot process performance analyser (visualisation)
`mingetty`          @-@popcon1@-@ @-@psize1@-@ console-only `getty`(8)
`mgetty`            @-@popcon1@-@ @-@psize1@-@ smart modem `getty`(8) replacement
---------------------------------------------------------------------------------------------------------------------------

TIP: See https://wiki.debian.org/BootProcessSpeedup[Debian wiki: BootProcessSpeedup] for the latest tips to speed up the boot process.

=== SysV-style init

CAUTION: The current default Debian system doesn't use SysV-style init.  Please read other resources for the modern systemd based init.  See https://www.debian.org/doc/manuals/debian-handbook/[The Debian Administrator's Handbook]

This section describes how the good old SysV-style init used to boot the system.  Your Debian system does not function exactly as described here but it is quite educational to know this basics since the newer init system tends to offer equivalent functionalities.

The SysV-style boot process essentially goes through the following.

1. The Debian system goes into **runlevel N** (none) to initialize the system by following the "`/etc/inittab`" description.
2. The Debian system goes into **runlevel S** to initialize the system under the single-user mode to complete hardware initialization etc.
3. The Debian system goes into one of the specified **multi-user runlevels (2 to 5)** to start the system services.

The initial runlevel used for multi-user mode is specified with the "`init=`" kernel boot parameter or in the "initdefault" line of the "`/etc/inittab`". The Debian system as installed starts at the **runlevel 2**.

All actual script files executed by the init system are located in the directory "`/etc/init.d/`".

See `init`(8), `inittab`(5), and "`/usr/share/doc/sysv-rc/README.runlevels.gz`" for the exact explanation.

==== The meaning of the runlevel

Each http://en.wikipedia.org/wiki/Runlevel[runlevel] uses a directory for its configuration and has specific meaning as the following.

.List of runlevels and description of their usage
[grid="all"]
`--------`-------------`--------------------------------------------------------------------
runlevel directory     description of runlevel usage
--------------------------------------------------------------------------------------------
`N`      none          system bootup (NONE) level (no "`/etc/rcN.d/`" directory)
`0`      `/etc/rc0.d/` halt the system
`S`      `/etc/rcS.d/` single-user mode on boot (alias: "`s`")
`1`      `/etc/rc1.d/` single-user mode switched from multi-user mode
`2`      `/etc/rc2.d/` multi-user mode
`3`      `/etc/rc3.d/` ,,
`4`      `/etc/rc4.d/` ,,
`5`      `/etc/rc5.d/` ,,
`6`      `/etc/rc6.d/` reboot the system
`7`      `/etc/rc7.d/` valid multi-user mode but not normally used
`8`      `/etc/rc8.d/` ,,
`9`      `/etc/rc9.d/` ,,
--------------------------------------------------------------------------------------------

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 http://en.wikipedia.org/wiki/Runlevel[runlevels] between 2 and 5.** The system administrator on the Debian system may change this.  (I.e., Debian is not http://en.wikipedia.org/wiki/Red_Hat_Linux[Red Hat Linux] nor http://en.wikipedia.org/wiki/Solaris_(operating_system)[Solaris by Sun Microsystems] nor http://en.wikipedia.org/wiki/HP-UX[HP-UX by Hewlett Packard] nor http://en.wikipedia.org/wiki/IBM_AIX[AIX by IBM] nor ...)

CAUTION: The Debian system does not populate directories for the http://en.wikipedia.org/wiki/Runlevel[runlevels] between 7 and 9 during installation.  Traditional http://en.wikipedia.org/wiki/Unix-like[Unix variants] don't use these http://en.wikipedia.org/wiki/Runlevel[runlevels].

==== The configuration of the runlevel

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

1. The script names starting with a "`K`" in "`/etc/rc<n>.d/`" are executed in alphabetical order with the single argument "`stop`". (killing services)
2. The script names starting with an "`S`" in "`/etc/rc<n>.d/`" are executed in alphabetical order with the single argument "`start`". (starting services)

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 http://en.wikipedia.org/wiki/UNIX_System_V[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.

- The `insserv`(8) command uses the LSB package dependency information and calculates the dependencies between all scripts.
 * See "`/usr/share/doc/insserv/README.Debian`".
- The LSB package dependency information defined in the header of init scripts ensures availabilities of required resources.
 * See http://refspecs.linuxbase.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/tocsysinit.html[Linux Standard Base Core Specification 3.1, VII. System Initialization].
- http://lists.debian.org/debian-devel-announce/2010/05/msg00009.html[Announcement of "Parallel booting enabled by default"]

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

// (In other word, if the maintainer did something stupid to unstable package, you may need to touch this after lots of thinking.)

==== The runlevel management example

For example, let@@@sq@@@s set up runlevel system somewhat like http://en.wikipedia.org/wiki/Red_Hat_Linux[Red Hat Linux] as the following.

- `init` starts the system in runlevel=3 as the default.
- `init` does not start `gdm3`(1) in runlevel=(0,1,2,6).
- `init` starts `gdm3`(1) in runlevel=(3,4,5).

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`, `sddm`, and `wdm`.

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

==== The default parameter for each init script

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 http://en.wikipedia.org/wiki/Red_Hat_Linux[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 http://www.debian.org/doc/debian-policy/ch-opersys#s9.3.2[modify the init scripts] themselves.  These are configuration files editable by system administrators.

==== The hostname

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.

==== The filesystem

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.

- "`/etc/init.d/mountkernfs.sh`" for kernel filesystems in "`/proc`", "`/sys`", etc.
- "`/etc/init.d/mountdevsubfs.sh`" for virtual filesystems in "`/dev`"
- "`/etc/init.d/mountall.sh`" for normal filesystems using "`/etc/fstab`"
- "`/etc/init.d/mountnfs.sh`" for network filesystems using"`/etc/fstab`"

The mount options of special kernel filesystems (procfs, sysfs, and tmpfs for `/proc`, `/sys`, `/tmp`, `/run`, etc.) are set in "`/etc/default/rcS`".  See `rcS`(5).

The mount options of normal disk and network filesystems are set in "`/etc/fstab`".  See <<_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 interface initialization

Network interfaces are initialized in runlevel S by the init script symlinked to "`/etc/init.d/ifupdown-clean`" and "`/etc/init.d/ifupdown`".  See <<_network_setup>> for how to configure them.

==== Network service initialization

Many network services (see <<_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 http://en.wikipedia.org/wiki/Super-server[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 http://en.wikipedia.org/wiki/Super-server[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 http://en.wikipedia.org/wiki/Open_Network_Computing_Remote_Procedure_Call[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 http://en.wikipedia.org/wiki/TCP_Wrapper[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  <<_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

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 <<_log_analyzer>>.

==== The kernel message

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
--------------------

.List of kernel error levels
[grid="all"]
`-----------------`----------------`--------------------------------
error level value error level name meaning
--------------------------------------------------------------------
0                 KERN_EMERG       system is unusable
1                 KERN_ALERT       action must be taken immediately
2                 KERN_CRIT        critical conditions
3                 KERN_ERR         error conditions
4                 KERN_WARNING     warning conditions
5                 KERN_NOTICE      normal but significant condition
6                 KERN_INFO        informational
7                 KERN_DEBUG       debug-level messages
--------------------------------------------------------------------

=== The udev system

For Linux kernel 2.6 and newer, http://en.wikipedia.org/wiki/Udev[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 http://en.wikipedia.org/wiki/Sysfs[sysfs] filesystem (see <<_procfs_and_sysfs>>), loads required kernel modules supporting it using the `modprobe`(8) program (see <<_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 "http://www.reactivated.net/writing_udev_rules.html[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 http://en.wikipedia.org/wiki/Universally_Unique_Identifier[UUID] to mount devices instead of device names such as "`/dev/sda`".  See <<_accessing_partition_using_uuid>>.

==== The kernel module initialization

The `modprobe`(8) program enables us to configure running Linux kernel from user process by adding and removing kernel modules.  The udev system (see <<_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)).

- http://en.wikipedia.org/wiki/TUN/TAP[TUN/TAP] modules providing virtual Point-to-Point network device (TUN) and virtual Ethernet network device (TAP),
- http://en.wikipedia.org/wiki/Netfilter[netfilter] modules providing netfilter firewall capabilities (`iptables`(8), <<_netfilter_infrastructure>>), and
- http://en.wikipedia.org/wiki/Watchdog_timer[watchdog timer] driver modules.

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 <<_hardware_identification>>.

TIP: You may configure hardware at boot time to activate expected hardware features. See <<_hardware_configuration>>.

TIP: You can probably add support for your special device by recompiling the kernel.  See <<_the_kernel>>.

