Chapter 3. The system initialization

The BIOS is the 1st stage of the boot process which is started by the power-on event. The BIOS residing on the 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 (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.

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 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.

The boot loader is the 2nd stage of the boot process which is started by the BIOS. It loads the system kernel image and the 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 basic initrd image is a compressed cpio archive of files in the root filesystem. The kernel can update microcode very early during boot before loading this basic initrd image. This is facilitated by the combined initrd image which is microcode binary blob in uncompressed cpio format followed by the basic initrd image.

The default install of the Debian system places first-stage GRUB boot loader code into the MBR for the PC platform. There are many boot loaders and configuration options available.

	Warning
	Do not play with boot loaders without having bootable rescue media (USB memory stick, 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.

	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
	UUID (see Section 9.5.3, “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 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 chain loading.

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

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 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:

	Tip
	See Debian wiki: BootProcessSpeedup for the latest tips to speed up the boot process.

The mount options of normal disk and network filesystems are set in "/etc/fstab". See fstab(5) and Section 9.5.7, “Optimization of filesystem by mount options”.

The configuration of the encrypted filesystem is set in "/etc/crypttab". See crypttab(5)

The configuration of software RAID with mdadm(8) is set in "/etc/mdadm/mdadm.conf". See mdadm.conf(5).

Network interfaces are typically initialized in "networking.service" for the lo interface and "NetworkManager.service" for other interfaces on modern Debian desktop system under systemd.

See Chapter 5, Network setup for how to configure them.

The kernel error message displayed to the console can be configured by setting its threshold level.

# dmesg -n3

Under systemd, both kernel and system messages are logged by the journal service systemd-journald.service (a.k.a journald) either into a persistent binary data below "/var/log/journal" or into a volatile binary data below "/run/log/journal/". These binary log data are accessed by the journalctl(1) command.

Under systemd, the system logging utility rsyslogd(8) changes its behavior to read the volatile binary log data (instead of pre-systemd default "/dev/log") and to create traditional permanent ASCII system log data.

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 systemd offers not only init system but also generic system management functionalities such as journal logging, login management, time management, network management. etc..

The systemd(1) is managed by several commands:

• the systemctl(1) command controls the systemd system and service manager (CLI),

• the systemsdm(1) command controls the systemd system and service manager (GUI),

• the journalctl(1) command queries the systemd journal,

• the loginctl(1) command controls the systemd login manager, and

• the systemd-analyze(1) analyzes system boot-up performance.

Here are a list of typical systemd management command snippets. For the exact meanings, please read the pertinent manpages.

Here, "$unit" in the above examples may be a single unit name (suffix such as .service and .target are optional) or, in many cases, multiple unit specifications (shell-style globs "*", "?", "[]" using fnmatch(3) which will be matched against the primary names of all units currently in memory).

System state changing commands in the above examples are typically preceded by the "sudo" to attain the required administrative privilege.

The output of the "systemctl status $unit|$PID|$device" uses color of the dot ("●") to summarize the unit state at a glance.

• White "●" indicates an "inactive" or "deactivating" state.

• Red "●" indicates a "failed" or "error" state.

• Green "●" indicates an "active", "reloading" or "activating" state.

With default installation, many network services (see Chapter 6, Network applications) are started as daemon processes after network.target at boot time by systemd. The "sshd" is no exception. Let's change this to on-demand start of "sshd" as a customization example.

First, disable system installed service unit.

 $ sudo systemctl stop sshd.service
 $ sudo systemctl mask sshd.service

The on-demand socket activation system of the classic Unix services was through the indetd superserver. Under systemd, the equivalent can be enabled by adding *.socket and *.service unit configuration files.

sshd.socket for specifying a socket to listen on

[Unit]
Description=SSH Socket for Per-Connection Servers

[Socket]
ListenStream=22
Accept=yes

[Install]
WantedBy=sockets.target

sshd@.service as the matching service file of sshd.socket

[Unit]
Description=SSH Per-Connection Server

[Service]
ExecStart=-/usr/sbin/sshd -i
StandardInput=socket

Then reload.

 $ sudo systemctl daemon-reload

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 3.3, “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.

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.