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This chapter is from the book

Managing Systemd Targets

Systemd is the service in Red Hat Enterprise Linux 8 that is responsible for starting all kinds of things. Systemd goes way beyond starting services; other items are started from Systemd as well. In Chapter 11, “Working with Systemd,” you learned about the Systemd fundamentals; this chapter looks at how Systemd targets are used to boot your system into a specific state.

Understanding Systemd Targets

A Systemd target is basically just a group of units that belong together. Some targets are just that and nothing else, whereas other targets can be used to define the state a system is booting in, because these targets have one specific property that regular targets don’t have: they can be isolated. Isolatable targets contain everything a system needs to boot or change its current state. Four targets can be used while booting:

  • emergency.target: In this target only a minimal number of units are started, just enough to fix your system if something is seriously wrong. You’ll find that it is quite minimal, as some important units are not started.

  • rescue.target: This target starts all units that are required to get a fully operational Linux system. It doesn’t start nonessential services though.

  • multi-user.target: This target is often used as the default target a system starts in. It starts everything that is needed for full system functionality and is commonly used on servers.

  • graphical.target: This target also is commonly used. It starts all units that are needed for full functionality, as well as a graphical interface.

Working with Targets

Working with targets may seem complicated, but it is not. It drills down to three common tasks:

  • Adding units to be automatically started

  • Setting a default target

  • Running a nondefault target to enter troubleshooting mode

In Chapter 11 you learned how to use the systemctl enable and systemctl disable commands to add services to or remove services from targets. In this chapter you learn how to set a default target and how to run a nondefault target to enter troubleshooting mode. But first let’s take a closer look at the working of targets under the hood.

key_topic_icon.jpg

Understanding Target Units

Behind a target there is some configuration. This configuration consists of two parts:

  • The target unit file

  • The “wants” directory, which contains references to all unit files that need to be loaded when entering a specific target

Targets by themselves can have dependencies to other targets, which are defined in the target unit file. Example 17-1 shows the definition of the multi-user.target file, which defines the normal operational state of a RHEL server.

Example 17-1 The multi-user.target File

[root@localhost ~]# systemctl cat multi-user.target
# /usr/lib/systemd/system/multi-user.target
#  SPDX-License-Identifier: LGPL-2.1+
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as
     published by
#  the Free Software Foundation; either version 2.1 of the License,
     or
#  (at your option) any later version.
[Unit] Description=Multi-User System Documentation=man:systemd.special(7) Requires=basic.target Conflicts=rescue.service rescue.target After=basic.target rescue.service rescue.target AllowIsolate=yes

You can see that by itself the target unit does not contain much. It just defines what it requires and which services and targets it cannot coexist with. It also defines load ordering, by using the After statement in the [Unit] section. The target file does not contain any information about the units that should be included; that is in the individual unit files and the wants (explained in the upcoming section “Understanding Wants”).

Systemd targets look a bit like runlevels used in previous versions of RHEL, but targets are more than that. A target is a group of units, and there are multiple different targets. Some targets, such as the multi-user.target and the graphical.target, define a specific state that the system needs to enter. Other targets just bundle a group of units together, such as the nfs.target and the printer.target. These targets are included from other targets, such as multi-user.target or graphical.target.

Understanding Wants

Understanding the concept of a want simply requires understanding the verb want in the English language, as in “I want a cookie.” Wants in Systemd define which units Systemd wants when starting a specific target. Wants are created when Systemd units are enabled using systemctl enable, and this happens by creating a symbolic link in the /etc/systemd/system directory. In this directory, you’ll find a subdirectory for every target, containing wants as symbolic links to specific services that are to be started.

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Managing Systemd Targets

As an administrator, you need to make sure that the required services are started when your server boots. To do this, use the systemctl enable and systemctl disable commands. You do not have to think about the specific target a service has to be started in. Through the [Install] section in the service unit file, the services know for themselves in which targets they need to be started, and a want is created automatically in that target when the service is enabled. The following procedure walks you through the steps of enabling a service:

  1. Type yum install -y vsftpd, followed by systemctl status vsftpd. If the service has not yet been enabled, the Loaded line will show that it currently is disabled:

    [root@server202 ~]# systemctl status vsftpd
    vsftpd.service - Vsftpd ftp daemon
        Loaded: loaded (/usr/lib/systemd/system/vsftpd.service; disabled)
        Active: inactive (dead)
  2. Type ls /etc/systemd/system/multi-user.target.wants. You’ll see symbolic links that are taking care of starting the different services on your machine. You can also see that the vsftpd.service link does not exist.

  3. Type systemctl enable vsftpd. The command shows you that it is creating a symbolic link for the file /usr/lib/systemd/system/vsftpd.service to the directory /etc/systemd/system/multi-user.target.wants. So basically, when you enable a Systemd unit file, in the background a symbolic link is created.

Isolating Targets

As already discussed, on Systemd machines there are several targets. You also know that a target is a collection of units. Some of those targets have a special role because they can be isolated. These are also the targets that you can set as the targets to get into after system start.

By isolating a target, you start that target with all of its dependencies. Only targets that have the isolate option enabled can be isolated. We’ll explore the systemctl isolate command later in this section. Before doing that, let’s take a look at the default targets on your computer.

To get a list of all targets currently loaded, type systemctl --type=target. You’ll see a list of all the targets currently active. If your server is running a graphical environment, this will include all the dependencies required to install the graphical.target also. However, this list shows only the active targets, not all the targets. Type systemctl --type=target --all for an overview of all targets that exist on your computer. You’ll now see inactive targets also (see Example 17-2).

Example 17-2 Showing System Targets

root@localhost ~]# systemctl --type=target --all
  UNIT                     LOAD      ACTIVE   SUB    DESCRIPTION                  
  basic.target             loaded    active   active Basic System                 
  bluetooth.target         loaded    active   active Bluetooth                    
  cryptsetup.target        loaded    active   active Local Encrypted
                                                       Volumes      
  dbus.target              not-found inactive dead   dbus.target
  emergency.target         loaded    inactive dead   Emergency Mode
  getty-pre.target         loaded    active   active Login Prompts
                                                       (Pre)        
  getty.target             loaded    active   active Login Prompts

 graphical.target          loaded    active   active Graphical
                                                       Interface
  initrd-fs.target         loaded    inactive dead   Initrd File
                                                       Systems
  initrd-root-device.targetloaded    inactive dead   Initrd Root
                                                       Device
  initrd-root-fs.target    loaded    inactive dead   Initrd Root File
                                                       System      
  initrd-switch-root.targetloaded    inactive dead   Switch Root
  initrd.target            loaded    inactive dead   Initrd Default
                                                       Target        
  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-pre.target       loaded    active   active Network (Pre)
  network.target           loaded    active   active Network
  nfs-client.target        loaded    active   active NFS client
                                                       services          
  nss-lookup.target        loaded  inactive dead     Host and Network
                                                       Name Lookups
  nss-user-lookup.target   loaded    active   active User and Group
                                                       Name Lookups  
  paths.target             loaded    active   active Paths
  remote-fs-pre.target     loaded    active   active Remote File
                                                       Systems (Pre)
  remote-fs.target         loaded    active   active Remote File
                                                       Systems
  rescue.target            loaded    inactive dead   Rescue Mode
  rpc_pipefs.target        loaded    active   active rpc_pipefs.
                                                       target
  rpcbind.target           loaded    active   active RPC Port Mapper
  shutdown.target          loaded    inactive dead   Shutdown
  slices.target            loaded    active   active Slices
  sockets.target           loaded    active   active Sockets
  sound.target             loaded    active   active Sound Card
  sshd-keygen.target       loaded    active   active sshd-keygen.
                                                       target
  swap.target              loaded    active   active Swap
  sysinit.target           loaded    active   active System
                                                       Initialization

Of the targets on your system, a few have an important role because they can be started (isolated) to determine the state your server starts in. These are also the targets that can be set as the default targets. These targets also roughly correspond to runlevels used on earlier versions of RHEL. These are the following targets:

poweroff.target

runlevel 0

rescue.target

runlevel 1

multi-user.target

runlevel 3

graphical.target

runlevel 5

reboot.target

runlevel 6

If you look at the contents of each of these targets, you’ll also see that they contain the AllowIsolate=yes line. That means that you can switch the current state of your computer to either one of these targets using the systemctl isolate command. Exercise 17-1 shows you how to do this.

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Setting the Default Target

Setting the default target is an easy procedure that can be accomplished from the command line. Type systemctl get-default to see the current default target and use systemctl set-default to set the desired default target.

To set the graphical.target as the default target, you need to make sure that the required packages are installed. If this is not the case, you can use the yum group list command to show a list of all RPM package groups. The “server with gui” and “GNOME Desktop” package groups both apply. Use yum group install "server with gui" to install all GUI packages on a server where they have not been installed yet.

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