System Startup¶

In this chapter you will learn how the system start.

Objectives : In this chapter, future Linux administrators will learn:

The different stages of the booting process;
How Rocky Linux supports this boot via GRUB2 and systemd;
How to protect GRUB2 from an attack;
How to manage the services;
How to access to the logs from journald.

users

Knowledge:
Complexity:

阅读时间: 20 分钟

The boot process¶

It is important to understand the boot process of Linux in order to be able to solve problems that may occur.

The boot process includes:

The BIOS startup¶

The BIOS (Basic Input/Output System) performs the POST (power on self test) to detect, test and initialize the system hardware components.

It then loads the MBR (Master Boot Record).

The Master boot record (MBR)¶

The Master Boot Record is the first 512 bytes of the boot disk. The MBR discovers the boot device and loads the bootloader GRUB2 into memory and transfers control to it.

The next 64 bytes contain the partition table of the disk.

The GRUB2 bootloader¶

The default bootloader for the Rocky 8 distribution is GRUB2 (GRand Unified Bootloader). GRUB2 replaces the old GRUB bootloader (also called GRUB legacy).

The GRUB 2 configuration file is located under /boot/grub2/grub.cfg but this file should not be edited directly.

The GRUB2 menu configuration settings are located under /etc/default/grub and are used to generate the grub.cfg file.

# cat /etc/default/grub
GRUB_TIMEOUT=5
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root rhgb quiet net.ifnames=0"
GRUB_DISABLE_RECOVERY="true"

If changes are made to one or more of these parameters, the grub2-mkconfig command must be run to regenerate the /boot/grub2/grub.cfg file.

[root] # grub2-mkconfig –o /boot/grub2/grub.cfg

• GRUB2 looks for the compressed kernel image (the vmlinuz file) in the /boot directory.
• GRUB2 loads the kernel image into memory and extracts the contents of the initramfs image file into a temporary folder in memory using the tmpfs file system.

The kernel¶

The kernel starts the systemd process with PID 1.

root          1      0  0 02:10 ?        00:00:02 /usr/lib/systemd/systemd --switched-root --system --deserialize 23

systemd¶

Systemd is the parent of all system processes. It reads the target of the /etc/systemd/system/default.target link (e.g. /usr/lib/systemd/system/multi-user.target) to determine the default target of the system. The file defines the services to be started.

Systemd then places the system in the target-defined state by performing the following initialization tasks:

1. Set the machine name
2. Initialize the network
3. Initialize SELinux
4. Display the welcome banner
5. Initialize the hardware based on the arguments given to the kernel at boot time
6. Mount the file systems, including virtual file systems like /proc
7. Clean up directories in /var
8. Start the virtual memory (swap)

Protecting the GRUB2 bootloader¶

Why protect the bootloader with a password?

1. Prevent Single user mode access - If an attacker can boot into single user mode, he becomes the root user.
2. Prevent access to GRUB console - If an attacker manages to use GRUB console, he can change its configuration or collect information about the system by using the cat command.
3. Prevent access to insecure operating systems. If there is a dual boot on the system, an attacker can select an operating system like DOS at boot time that ignores access controls and file permissions.

To password protect the GRUB2 bootloader:

• Remove -unrestricted from the main CLASS= statement in the /etc/grub.d/10_linux file.

• If a user has not yet been configured, use the grub2-setpassword command to provide a password for the root user:

# grub2-setpassword

A /boot/grub2/user.cfg file will be created if it was not already present. It contains the hashed password of the GRUB2.

!!! Note This command only supports configurations with a single root user.

[root]# cat /boot/grub2/user.cfg
GRUB2_PASSWORD=grub.pbkdf2.sha512.10000.CC6F56....A21

• Recreate the configuration file with the grub2-mkconfig command:

[root]# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.0-327.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-327.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-f9725b0c842348ce9e0bc81968cf7181
Found initrd image: /boot/initramfs-0-rescue-f9725b0c842348ce9e0bc81968cf7181.img
done

• Restart the server and check.

All entries defined in the GRUB menu will now require a user and password to be entered at each boot. The system will not boot a kernel without direct user intervention from the console.

• When the user is requested, enter root;
• When a password is requested, enter the password provided at the grub2-setpassword command.

To protect only the editing of GRUB menu entries and access to the console, the execution of the grub2-setpassword command is sufficient. There may be cases where you have good reasons for doing only that. This might be particularly true in a remote data center where entering a password each time a server is rebooted is either difficult or impossible to do.

Systemd¶

Systemd is a service manager for the Linux operating systems.

It is developed to:

• remain compatible with older SysV initialization scripts,
• provide many features, such as parallel start of system services at system startup, on-demand activation of daemons, support for snapshots, or management of dependencies between services.

!!! Note Systemd is the default initialization system since RedHat/CentOS 7.

Systemd introduces the concept of systemd units.

!!! Note There are many types of units: Device unit, Mount unit, Path unit, Scope unit, Slice unit, Snapshot unit, Socket unit, Swap unit, Timer unit.

• Systemd supports system state snapshots and restore.

• Mount points can be configured as systemd targets.

• At startup, systemd creates listening sockets for all system services that support this type of activation and passes these sockets to these services as soon as they are started. This makes it possible to restart a service without losing a single message sent to it by the network during its unavailability. The corresponding socket remains accessible and all messages are queued.

• System services that use D-BUS for their inter-process communications can be started on demand the first time they are used by a client.

• Systemd stops or restarts only running services. Previous versions (before RHEL7) attempted to stop services directly without checking their current status.

• System services do not inherit any context (like HOME and PATH environment variables). Each service operates in its own execution context.

All service unit operations are subject to a default timeout of 5 minutes to prevent a malfunctioning service from freezing the system.

Managing system services¶

Service units end with the .service file extension and have a similar purpose to init scripts. The systemctl command is used to display, start, stop, restart a system service:

The systemctl command is also used for the enable or disable of system a service and displaying associated services:

Examples:

systemctl stop nfs-server.service
# or
systemctl stop nfs-server

To list all units currently loaded:

systemctl list-units --type service

To list all units to check if they are activated:

systemctl list-unit-files --type service

systemctl enable httpd.service
systemctl disable bluetooth.service

Example of a .service file for the postfix service¶

postfix.service Unit File
What follows is the content of the /usr/lib/systemd/system/postfix.service unit file as currently provided by the postfix package:

[Unit]
Description=Postfix Mail Transport Agent
After=syslog.target network.target
Conflicts=sendmail.service exim.service

[Service]
Type=forking
PIDFile=/var/spool/postfix/pid/master.pid
EnvironmentFile=-/etc/sysconfig/network
ExecStartPre=-/usr/libexec/postfix/aliasesdb
ExecStartPre=-/usr/libexec/postfix/chroot-update
ExecStart=/usr/sbin/postfix start
ExecReload=/usr/sbin/postfix reload
ExecStop=/usr/sbin/postfix stop

[Install]
WantedBy=multi-user.target

Using system targets¶

On Rocky8/RHEL8, the concept of run levels has been replaced by Systemd targets.

Systemd targets are represented by target units. Target units end with the .target file extension and their sole purpose is to group other Systemd units into a chain of dependencies.

For example, the graphical.target unit, which is used to start a graphical session, starts system services such as the GNOME display manager (gdm.service) or the accounts service (accounts-daemon.service) and also activates the multi-user.target unit.

Similarly, the multi-user.target unit starts other essential system services, such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and activates another target unit named basic.target.

The default target¶

To determine which target is used by default:

systemctl get-default

This command searches for the target of the symbolic link located at /etc/systemd/system/default.target and displays the result.

$ systemctl get-default
graphical.target

The systemctl command can also provide a list of available targets:

systemctl list-units --type target
UNIT                   LOAD   ACTIVE SUB    DESCRIPTION
basic.target           loaded active active Basic System
bluetooth.target       loaded active active Bluetooth
cryptsetup.target      loaded active active Encrypted Volumes
getty.target           loaded active active Login Prompts
graphical.target       loaded active active Graphical Interface
local-fs-pre.target    loaded active active Local File Systems (Pre)
local-fs.target        loaded active active Local File Systems
multi-user.target      loaded active active Multi-User System
network-online.target  loaded active active Network is Online
network.target         loaded active active Network
nss-user-lookup.target loaded active active User and Group Name Lookups
paths.target           loaded active active Paths
remote-fs.target       loaded active active Remote File Systems
slices.target          loaded active active Slices
sockets.target         loaded active active Sockets
sound.target           loaded active active Sound Card
swap.target            loaded active active Swap
sysinit.target         loaded active active System Initialization
timers.target          loaded active active Timers

To configure the system to use a different default target:

systemctl set-default name.target

Example:

# systemctl set-default multi-user.target
rm '/etc/systemd/system/default.target'
ln -s '/usr/lib/systemd/system/multi-user.target' '/etc/systemd/system/default.target'

To switch to a different target unit in the current session:

systemctl isolate name.target

The Rescue mode provides a simple environment to repair your system in cases where it is impossible to perform a normal boot process.

In rescue mode, the system attempts to mount all local file systems and start several important system services, but does not enable a network interface or allow other users to connect to the system at the same time.

On Rocky 8, the rescue mode is equivalent to the old single user mode and requires the root password.

To change the current target and enter rescue mode in the current session:

systemctl rescue

Emergency mode provides the most minimalist environment possible and allows the system to be repaired even in situations where the system is unable to enter rescue mode. In the emergency mode, the system mounts the root file system only for reading. It will not attempt to mount any other local file system, will not activate any network interface, and will start some essential services.

To change the current target and enter emergency mode in the current session:

systemctl emergency

Shutdown, suspension and hibernation¶

The systemctl command replaces a number of power management commands used in previous versions:

The journald process¶

Log files can, in addition to rsyslogd, also be managed by the journald daemon which is a component of systemd.

The journald daemon captures Syslog messages, kernel log messages, messages from the initial RAM disk and from the start of boot, as well as messages written to the standard output and the standard error output of all services, then indexes them and makes them available to the user.

The format of the native log file, which is a structured and indexed binary file, improves searches and allows for faster operation, it also stores metadata information, such as timestamps or user IDs.

journalctl command¶

The journalctl command displays the log files.

journalctl

The command lists all log files generated on the system. The structure of this output is similar to that used in /var/log/messages/ but it offers some improvements:

• the priority of entries is marked visually;
• timestamps are converted to the local time zone of your system;
• all logged data is displayed, including rotating logs;
• the beginning of a start is marked with a special line.

Using continuous display¶

With continuous display, log messages are displayed in real time.

journalctl -f

This command returns a list of the ten most recent log lines. The journalctl utility then continues to run and waits for new changes to occur before displaying them immediately.

Filtering messages¶

It is possible to use different filtering methods to extract information that fits different needs. Log messages are often used to track erroneous behavior on the system. To view entries with a selected or higher priority:

journalctl -p priority

You must replace priority with one of the following keywords (or a number):

• debug (7),
• info (6),
• notice (5),
• warning (4),
• err (3),
• crit (2),
• alert (1),
• and emerg (0).