Red Hat Enterprise Linux 8.5 64bit

140 .C . H. .A . P. .T .E . R. . 8. . .G . E. .T .T . I.N . .G . .S . T. A. .R . T. .E . D. . W. . I.T . H. . T. .C . L. /. T. .K . .O . N. . .R .E . D. . H. .A . .T . E. .N . T. .E . R. P. .R . I.S . E. . L. .I N. .U . X. . 8. . . . . . . . . . . . . . . . . . . . . . . . .1 4. .1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. INTRODUCTION TO TCL/TK 141 8.2. NOTABLE CHANGES IN TCL/TK 8.6 141 8.3. MIGRATING TO TCL/TK 8.6 142 8.3.1. Migration path for developers of Tcl extensions 142 8.3.2. Migration path for users scripting their tasks with Tcl/Tk 142 .C . H. .A . P. .T .E . R. . 9. . .U . S. I. N. .G . .P . R. .E . F. I. X. .D . E. V. .N . .A . M. .E . .F . O. .R . .N . A. .M . .I N. . G. . O. .F . .E . T. .H . E. R. .N . .E .T . .N . E. .T . W. . O. .R . K. . I.N . T. .E . R. .F .A . C. .E . S. . . . . . . . . . . . . .1 .4 . 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. INTRODUCTION TO PREFIXDEVNAME 144 9.2. SETTING PREFIXDEVNAME 144 9.3. LIMITATIONS OF PREFIXDEVNAME 144 6Table of Contents 7Red Hat Enterprise Linux 8 Configuring basic system settings CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX The following sections provide an overview of the basic tasks that system administrators might need to perform just after Red Hat Enterprise Linux has been installed. NOTE Such tasks may include items that are usually done already during the installation proces, but they do not have to be done necessarily, such as the registration of the system. The sections dealing with such tasks provide a brief summary of how this can be achived during the installation and links to related documentation. For information on Red Hat Enterprise Linux installation, see Performing a standard RHEL installation. NOTE The following sections mention some commands to be performed. The commands that need to be entered by the root user have # in the prompt, while the commands that can be performed by a regular user, have $ in their prompt. Although all post-installation tasks can be achieved through the command line, you can also use the RHEL 8 web console to perform some of them. 1.1. WHAT THE RHEL 8 WEB CONSOLE IS AND WHICH TASKS IT CAN BE USED FOR The RHEL 8 web console is an interactive server administration interface. It interacts directly with the operating system from a real Linux session in a browser. The web console enables to perform these tasks: Monitoring basic system features, such as hardware information, time configuration, performance profiles, connection to the realm domain Inspecting system log files Managing network interfaces and configuring firewall Handling docker images Managing virtual machines Managing user accounts Monitoring and configuring system services Creating diagnostic reports Setting kernel dump configuration Managing packages 8CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX Configuring SELinux Updating software Managing system subscriptions Accessing the terminal For more information on installing and using the RHEL 8 web console, see Managing systems using the web console. 1.2. CONFIGURING SYSTEM SETTINGS IN THE WEB CONSOLE In this chapter, you will learn how to execute basic system settings in the web console and thus be able to: Restart or shutdown the system in the web console. Change a system host name. Join the system to a domain. Configure time and time zones. Change a performance profile. 1.2.1. Using the web console to restart the system The following procedure describes system restart executed in the web console. Prerequisites The web console must be installed and accessible. For details, see Installing the web console . Procedure 1. Log in to the RHEL 8 web console. For details, see Logging in to the web console . 2. Click System. 3. In the Power Options drop down list, select Restart. 9Red Hat Enterprise Linux 8 Configuring basic system settings 4. If there are users logged into the system, write a reason for the restart in the Restart dialog box. 5. In the Delay drop down list, select a time interval. 6. Click Restart. The system will be restarted according to your selection. 10CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX 1.2.2. Using the web console to shutdown the system The following procedure describes system shutdown executed in the web console. Prerequisites The web console must be installed and accessible. For details, see Installing the web console . Procedure 1. Log in to the RHEL 8 web console. For details, see Logging in to the web console . 2. Click System. 3. In the Power Options drop down list, select Shut Down. 4. If there are users logged into the system, write a reason for the shutdown in the Shut Down dialog box. 5. In the Delay drop down list, select a time interval. 6. Click Shut Down. The system will be turned off according to your selection. 1.2.3. Using the web console for setting a host name The host name identifies the system. By default, the host name is set to localhost, but you can change it. 11Red Hat Enterprise Linux 8 Configuring basic system settings Host names consists of two parts: Host name — It is a unique name which identifies a system. Domain — If you want to use the machine in the network and use names instead of just IP addresses, you need to add the domain as a suffix behind the host name. For example: mymachine.example.com You can configure also a pretty host name in the RHEL web console. The pretty host name allows you to enter a host name with capital letters, spaces, and so on. The pretty host name displays in the web console, but it does not have to correspond with the host name. Example: Pretty host name: My Machine Host name: mymachine Real host name (Fully qualified domain name): mymachine.idm.company.com Host names are stored in the /etc/hostname file, however, you can set or change the host name in the web console. Prerequisites The web console must be installed and accessible. For details, see Installing the web console . Procedure 1. Log in to the RHEL 8 web console. For details, see Logging in to the web console . 2. Click System. 3. Click the current host name. 4. In the Change Host Name dialog box, enter the host name in the Pretty Host Name field. 5. In the Real Host Name field, the pretty name will be compounded with a domain name. You can change the host name manually if it does not correspond with the pretty host name. 6. Click Change. 12CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX To verify that the host name is configured properly, try to log out from the web console and add to the browser the address with the new host name. 1.2.4. Joining the RHEL 8 system to the IdM domain using the web console The following procedure describes joining the RHEL 8 system to the IdM domain. Prerequisites IdM domain running and reachable from the client you want to join. IdM domain administrator credentials. Procedure 1. Log in to the RHEL web console. For details, see Logging in to the web console . 2. Open the System tab. 3. Click Join Domain. 13Red Hat Enterprise Linux 8 Configuring basic system settings 4. In the Join a Domain dialog box, enter the host name of the IdM server in the Domain Address field. 5. In the Authentication drop down list, select if you want to use password or one time password for authentication. 6. In the Domain Administrator Name field, enter the user name of the IdM administration account. 7. In the password field, add the password or one time password according to what you selected in the Authentication drop down list above. 8. Click Join. 14CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX If the RHEL 8 web console did not display an error, the system has been joined to the IdM domain and you can see the domain name in the System screen. WARNING  If you click to the joined domain in the System screen, the system will display a warning dialog with the information about leaving the domain. If you click Leave, the system will leave the domain. 15Red Hat Enterprise Linux 8 Configuring basic system settings 1.2.5. Using the web console for configuring time settings This section shows you how to set: The correct time zone Automatic time settings provided by an NTP server. A specific NTP server. Prerequisites The web console must be installed and accessible. For details, see Installing the web console . Procedure 1. Log in to the RHEL 8 web console. For details, see Logging in to the web console . 2. Click System. 3. Click the current system time. 4. In the Change System Time dialog box, change the time zone if necessary. 5. In the Set Time drop down menu, select: Manually Automatically using NTP server — This is a default option. If the time of the system is 16CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX Automatically using NTP server — This is a default option. If the time of the system is correct, leave it as it is. Automatically using specific NTP servers — Use this option only if you need to synchronize the system with a specific NTP server and add the DNS name or IP address of the server. 6. Click Change. The change is now available in the System tab. 1.2.6. Using the web console for selecting performance profiles Red Hat Enterprise Linux 8 includes performance profiles optimizing: Systems using Desktop Latency performance Network performance Low power consumption Virtual machines The following procedure describes setting up performance profiles in the web console. The RHEL 8 web console configures the tuned service. For details about the tuned service, see Monitoring and managing system status and performance . Prerequisites The web console must be installed and accessible. For details, see Installing the web console . Procedure 1. Log in to the RHEL 8 web console. For details, see Logging in to the web console . 17Red Hat Enterprise Linux 8 Configuring basic system settings 2. Click System. 3. In the Performance Profile field, click the current performance profile. 4. In the Change Performance Profile dialog box, change the profile if necessary. 5. Click Change. The change is now available in the System tab. 1.3. BASIC CONFIGURATION OF ENVIRONMENT Basic configuration of environment includes: Date and time System locales Keyboard layout Setting of these items is normally a part of the installation process. For more information, see Performing a standard RHEL installation . 1.3.1. Introduction to configuring the date and time 18CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX Accurate timekeeping is important for a number of reasons. In Red Hat Enterprise Linux, timekeeping is ensured by the NTP protocol, which is implemented by a daemon running in user space. The user space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources. Red Hat Enterprise Linux 8 uses the chronyd daemon to implement NTP. chronyd is available from the chrony package. For more information on configuring and using NTP with chronyd, see Chapter 5, Using the Chrony suite to configure NTP . Displaying the current date and time To display the current date and time, use one of the following commands: ~]$ date ~]$ timedatectl Note that the timedatectl command provides more verbose output, including universal time, currently used time zone, the status of the Network Time Protocol (NTP) configuration, and some additional information. For more information on configuring the date and time during the installation, see Performing a standard RHEL installation. 1.3.1.1. Managing time configurations in the RHEL 8 web console Time settings in the web console describes Using the web console for configuring time settings . 1.3.2. Introduction to configuring the system locale System-wide locale settings are stored in the /etc/locale.conf file, which is read at early boot by the systemd daemon. The locale settings configured in /etc/locale.conf are inherited by every service or user, unless individual programs or individual users override them. Basic tasks to handle the system locales: Listing available system locale settings: ~]$ localectl list-locales Displaying current status of the system locales settings: ~]$ localectl status Setting or changing the default system locale settings: ~]# localectl set-locale LANG=locale 1.3.3. Introduction to configuring the keyboard layout The keyboard layout settings control the layout used on the text console and graphical user interfaces. Basic tasks to handle the keyboard layout include: Listing available keymaps: 19Red Hat Enterprise Linux 8 Configuring basic system settings ~]$ localectl list-keymaps Displaying current status of keymap settings: ~]$ localectl status Setting or changing the default system keymap: ~]# localectl set-keymap 1.4. CONFIGURING AND MANAGING NETWORK ACCESS 1.4.1. Configuring network access during the installation process Ways to configure network access during the installation proces: The Network & Hostname menu at the Installation Summary screen in the graphical user interface of the Anaconda installation program The Network settings option in the text mode of the Anaconda installation program The Kickstart file When the system boots for the first time after the installation has finished, any network interfaces which you configured during the installation are automatically activated. For detailed information on configuration of network access during installation process, see Performing a standard RHEL installation. 1.4.2. Managing network connections after the installation process using nmcli Run the following commands to manage network connections using the nmcli utility. NOTE The nmcli utility has a powerful syntax completion feature when the Tab key is pressed twice. You need to have the bash-completion package installed to enable it. To create a new connection: ~]# nmcli con add type type of the connection con-name connection name ifname ifname ipv4.addresses ipv4 address ipv4.gateway gateway address Here, replace: type of the connection by the required type of the device connection name by the required connection name ifname by the required device name ipv4 address by the required IPv4 address/netmask 20CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX gateway address by the required gateway address Note that ipv4 address and gateway address are optional settings, while all remaining settings are required. You can also create a new connection in assisted mode. To do so, run this command, and follow the instructions that will prompt you for input of particular configuration settings of this connection: ~]# nmcli -a con add To modify the existing connection: ~]# nmcli con mod connection name setting.property newvalue Here, replace: connection name by the name of the connection that you want to modify setting.property by the configuration setting that you want to modify newvalue by the required value of this configuration setting For example, to set the method of the configuration of IPv4 address (ipv4.method) to auto for the connection named enp0, use the following command: ~]# nmcli con mod enp0 ipv4.method auto To edit a connection, run the following command: ~]# nmcli connection edit connection name Here, replace connection name by the name of the connection that you want to edit. To display all connections: ~]# nmcli con show To display active connections: ~]# nmcli con show --active To display all configuration settings of a particular connection: ~]# nmcli con show connection Here, replace connection by the identifier of the required connection. There are multiple ways to specify the connection, such as connection ID, UUID or path. For more information, see the nmcli man page. Then, follow the instructions that will prompt you for input of particular configuration settings. To display all possible options of any configuration setting, use the print command in the editor. 1.4.3. Managing network connections after the installation process using nmtui The NetworkManager text user interface (TUI) utility, nmtui, provides a text interface to configure 21Red Hat Enterprise Linux 8 Configuring basic system settings The NetworkManager text user interface (TUI) utility, nmtui, provides a text interface to configure networking by controlling NetworkManager. 1.4.4. Managing networking in the RHEL 8 web console in the web console, the Networking menu enables you: To display currently received and sent packets To display the most important characteristics of available network interfaces To display content of the networking logs. To add various types of network interfaces (bond, team, bridge, VLAN) For details, see Managing networking in the web console . Figure 1.1. Managing Networking in the RHEL 8 web console 1.5. REGISTERING THE SYSTEM AND MANAGING SUBSCRIPTIONS The products installed on Red Hat Enterprise Linux, including the operating system itself, are covered by subscriptions. A subscription to Red Hat Content Delivery Network is used to track: Registered systems Products installed on those system Subscriptions attached to those product 1.5.1. Registering the system after the installation Your subscription can be registered during the installation process. For more information, see Performing a standard RHEL installation . 22CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX If you have not registered your system during the installation process, you can do it afterwards by applying the following procedure. Note that all commands in this procedure need to be performed as the root user. Registering and subscribing your system 1. Register your system: ~]# subscription-manager register The command will prompt you to enter your Red Hat Customer Portal user name and password. 2. Determine the pool ID of a subscription that you require: ~]# subscription-manager list --available This command displays all available subscriptions for your Red Hat account. For every subscription, various characteristics are displayed, including the pool ID. 3. Attach the appropriate subscription to your system by replacing pool_id with the pool ID determined in the previous step: ~]# subscription-manager attach --pool=pool_id 1.5.2. Registering subscriptions with credentials in the web console The following describes subscribing the newly installed Red Hat Enterprise Linux using the RHEL 8 web console. Prerequisites Valid user account in the Red Hat Customer Portal. See the Create a Red Hat Login page. Active subscription for the RHEL system. Procedure 1. Type subscription in the search field and press the Enter key. Alternatively, you can log in to the RHEL 8 web console. For details, see Logging in to the web console. 2. In the polkit authentication dialog for privileged tasks, add the password belonging user name displayed in the dialog. 23Red Hat Enterprise Linux 8 Configuring basic system settings 3. Click Authenticate. 4. In the Subscriptions dialog box, click Register. 5. Enter your Customer Portal credentials. 24CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX 6. Enter the name of your organization. You need to add the organization name or organization ID, if you have more than one account in the Red Hat Customer Portal. To get the org ID, go to your Red Hat contact point. 7. Click the Register button. At this point, your RHEL 8 system has been successfully registered. 25Red Hat Enterprise Linux 8 Configuring basic system settings 1.6. INSTALLING SOFTWARE This section provides information to guide you through the basics of software installation. It mentions the prerequisites that you need to fulfil to be able to install software, provides the basic information on software packaging and software repositories, and references the ways to perform basic tasks related to software installation. 1.6.1. Prerequisites for software installation The Red Hat Content Delivery Network subscription service provides a mechanism to handle Red Hat software inventory and enables you to install additional software or update already installed packages. You can start installing software once you have registered your system and attached a subscription, as described in Section 1.5.1, “Registering the system after the installation” . 1.6.2. Introduction to the system of software packaging and software repositories All software on a Red Hat Enterprise Linux system is divided into RPM packages, which are stored in particular repositories. When a system is subscribed to the Red Hat Content Delivery Network, a repository file is created in the /etc/yum.repos.d/ directory. 26CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX Use the yum utility to manage package operations: Searching information about packages Installing packages Updating packages Removing packages Checking the list of currently available repositories Adding or removing a repository Enabling or disabling a repository For information on basic tasks related to the installation of software, see Section 1.6.3, “Managing basic software-installation tasks with subscription manager and yum”. 1.6.3. Managing basic software-installation tasks with subscription manager and yum The most basic software-installation tasks that you might need after the operating system has been installed include: Listing all available repositories: ~]# subscription-manager repos --list Listing all currently enabled repositories: ~]$ yum repolist Enabling or disabling a repository: ~]# subscription-manager repos --enable repository ~]# subscription-manager repos --disable repository Searching for packages matching a specific string: ~]$ yum search string Installing a package: ~]# yum install package_name Updating all packages and their dependencies: ~]# yum update Updating a package: 27Red Hat Enterprise Linux 8 Configuring basic system settings ~]# yum update package_name Uninstalling a package and any packages that depend on it: ~]# yum remove package_name Listing information on all installed and available packages: ~]$ yum list all Listing information on all installed packages: ~]$ yum list installed 1.7. MAKING SYSTEMD SERVICES START AT BOOT TIME Systemd is a system and service manager for Linux operating systems that introduces the concept of systemd units. This section provides the information on how to ensure that a service is enabled or disabled at boot time. It also explains how to manage the services through the web console. 1.7.1. Enabling or disabling the services You can determine services that are enabled or disabled at boot time already during the installation process, or you can enable or disable a service on an installed operating system. To create the list of services enabled or disabled at boot time during the installation process, use the services option in the Kickstart file: services [--disabled=list] [--enabled=list] NOTE The list of disabled services is processed before the list of enabled services. Therefore, if a service appears on both lists, it will be enabled. The list of the services should be given in the comma separated format. Do not include spaces in the list of services. To enable or disable a service on an already installed operating system: ~]# systemctl enable service_name ~]# systemctl disable service_name For further details on enabling and disabling services, see Section 3.2, “Managing system services” . 1.7.2. Managing services in the RHEL 8 web console in the web console, select Services to manage systemd targets, services, sockets, timers and paths. There you can check their status, start or stop them, enable or disable them. Figure 1.2. Managing services in the RHEL 8 web console 28CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX Figure 1.2. Managing services in the RHEL 8 web console 1.8. ENHANCING SYSTEM SECURITY WITH A FIREWALL, SELINUX AND SSH LOGINS Computer security is the protection of computer systems from the theft or damage to their hardware, software, or information, as well as from disruption or misdirection of the services they provide. Ensuring computer security is therefore an essential task not only in the enterprises processing sensitive data or handling some business transactions. Computer security includes a wide variety of features and tools. This section covers only the basic security features that you need to configure after you have installed the operating system. For detailed information on securing Red Hat Enterprise Linux, see the following documentation: Security hardening Securing networks Using SELinux 1.8.1. Ensuring the firewall is enabled and running 1.8.1.1. What a firewall is and how it enhances system security A firewall is a network security system that monitors and controls the incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted, secure internal network and another outside network. The firewall is provided by the firewalld service, which is automatically enabled during the installation. However, if you explicitly disabled the service, you can re-enable it, as described in Section 1.8.1.2, “Re- enabling the firewalld service”. 1.8.1.2. Re-enabling the firewalld service In case that the firewalld service is disabled after the installation, Red Hat recommends to consider re- enabling it. You can display the current status of firewalld even as a regular user: ~]$ systemctl status firewalld If firewalld is not enabled and running, switch to the root user, and change its status: 29Red Hat Enterprise Linux 8 Configuring basic system settings ~]# systemctl start firewalld ~]# systemctl enable firewalld For detailed information on configuring and using firewall, see Securing networks. 1.8.1.3. Managing firewall in the RHEL 8 web console In the web console, use the Firewall option under Networking to enable or disable the firewalld service. By default, the firewalld service in the web console is enabled. To disable it, set off as shown below. Additionally, you can choose the services that you want to allow through firewall. Figure 1.3. Managing firewall in the RHEL 8 web console 1.8.2. Ensuring the appropriate state of SELinux 1.8.2.1. What SELinux is and how it enhances system security Security Enhanced Linux (SELinux) is an additional layer of system security that determines which process can access which files, directories, and ports. SELinux states SELinux has two possible states: Enabled Disabled When SELinux is disabled, only Discretionary Access Control (DAC) rules are used. SELinux modes When SELinux is enabled, it can run in one of the following modes: Enforcing Permissive Enforcing mode means that SELinux policies are enforced. SELinux denies access based on SELinux policy rules, and enables only the interactions that are particularly allowed. Enforcing mode is the default mode after the installation and it is also the safest SELinux mode. Permissive mode means that SELinux policies are not enforced. SELinux does not deny access, but denials are logged for actions that would have been denied if running in enforcing mode. Permissive 30CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX mode is the default mode during the installation. Operating in permissive mode is also useful in some specific cases, for example if you require access to the Access Vector Cache (AVC) denials when troubleshooting problems. For further information on SELinux, see Using SELinux. 1.8.3. Ensuring the Required State of SELinux By default, SELinux operates in permissive mode during the installation and in enforcing mode when the installation has finished. However, in some specific scenarios, SELinux might be explicitly set to permissive mode or it might even be disabled on the installed operating system. This can be set for example in the kickstart configuration. IMPORTANT Red Hat recommends to keep your system in enforcing mode. To display the current SELinux mode, and to set the mode as needed: Ensuring the required state of SELinux 1. Display the current SELinux mode in effect: ~]$ getenforce 2. If needed, switch between the SELinux modes. The switch can be either temporary or permanent. A temporary switch is not persistent across reboots, while permanent switch is. To temporary switch to either enforcing or permissive mode: ~]# setenforce Enforcing ~]# setenforce Permissive To permanently set the SELinux mode, modify the SELINUX variable in the /etc/selinux/config configuration file. For example, to switch SELinux to enforcing mode: # This file controls the state of SELinux on the system. # SELINUX= can take one of these three values: # enforcing - SELinux security policy is enforced. # permissive - SELinux prints warnings instead of enforcing. # disabled - No SELinux policy is loaded. SELINUX=enforcing 1.8.3.1. Managing SELinux in the RHEL 8 web console in the web console, use the SELinux option to turn SELinux enforcing policy on or off. By default, SELinux enforcing policy in the web console is on, and SELinux operates in enforcing mode. 31Red Hat Enterprise Linux 8 Configuring basic system settings By default, SELinux enforcing policy in the web console is on, and SELinux operates in enforcing mode. By turning it off, you can switch SELinux into permissive mode. Note that such deviation from the default configuration in the /etc/sysconfig/selinux file is automatically reverted on the next boot. Figure 1.4. Managing SELinux in the RHEL 8 web console 1.8.4. Accessing system through SSH 1.8.4.1. What SSH-based access is and how it enhances system security If you want to secure your communication with another computer, you can use SSH-based authentication. Secure Shell (SSH) is a protocol which facilitates client-server communication and allows users to log in to any host system running SSH remotely. SSH secures the connection. The client transmits its authentication information to the server using encryption, and all data sent and received during a session are transferred under the encryption. SSH enables its users to authenticate without a password. To do so, SSH uses a private-public key scheme. For more information about SSH, see Securing networks. 1.8.4.2. Configuring key-based SSH access To be able to use SSH connection, create a pair of two keys consisting of a public and a private key. Creating the key files and Copying them to the Server 1. Generate a public and a private key: ~]$ ssh-keygen Both keys are stored in the ~/.ssh/ directory: ~/.ssh/id_rsa.pub - public key 32CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX ~/.ssh/id_rsa - private key The public key does not need to be secret. It is used to verify the private key. The private key is secret. You can choose to protect the private key with the passphrase that you specify during the key generation process. With the passphrase, authentication is even more secure, but is no longer password-less. You can avoid this using the ssh-agent command. In this case, you will enter the passphrase only once - at the beginning of a session. 2. Copy the most recently modified public key to a remote machine you want to log into: ~]# ssh-copy-id USER@hostname As a result, you are now able to enter the system in a secure way, but without entering a password. 1.8.4.3. Disabling SSH root login To increase system security, you can disable SSH access for the root user, which is enabled by default. Disabling SSH root login 1. Access the /etc/ssh/sshd_config file: ~]# vi /etc/ssh/sshd_config 2. Change the line that reads #PermitRootLogin yes to: PermitRootLogin no 3. Restart the sshd service: ~]# systemctl restart sshd NOTE When using PermitRootLogin no, the root user cannot login to system directly. Alternatively, the root user might be allowed to login, but using a password-less authentication method, typically the key-based authentication, described in Section 1.8.4.2, “Configuring key-based SSH access” . To ensure this, specify PermitRootLogin prohibit-password in the /etc/ssh/sshd_config file. 1.9. MANAGING USER ACCOUNTS Red Hat Enterprise Linux is a multi-user operating system, which enables multiple users on different computers to access a single system installed on one machine. Every user operates under its own account, and managing user accounts thus represents a core element of Red Hat Enterprise Linux system administration. 1.9.1. The basics of managing user accounts Normal and System Accounts Normal accounts are created for users of a particular system. Such accounts can be added, removed, and modified during normal system administration. 33Red Hat Enterprise Linux 8 Configuring basic system settings System accounts represent a particular applications identifier on a system. Such accounts are generally added or manipulated only at software installation time, and they are not modified later. WARNING  System accounts are presumed to be available locally on a system. If these accounts are configured and provided remotely, such as in the instance of an LDAP configuration, system breakage and service start failures can occur. For system accounts, user IDs below 1000 are reserved. For normal accounts, you can use IDs starting at 1000. However, the recommended practice is to assign IDs starting at 5000. See Reserved user and group IDs for more information. The guidelines for assigning IDs can be found in the /etc/login.defs file: # Min/max values for automatic uid selection in useradd # UID_MIN 1000 UID_MAX 60000 # System accounts SYS_UID_MIN 201 SYS_UID_MAX 999 What groups are and which purposes they can be used for A group in an entity which ties together multiple user accounts for a common purpose, such as granting access to particular files. 1.9.1.1. Basic command-line tools to manage user accounts and groups The most basic tasks to manage user accounts and groups, and the appropriate command-line tools, include: Displaying user and group IDs: ~]$ id Creating a new user account: ~]# useradd [options] user_name Assigning a new password to a user account belonging to username: ~]# passwd user_name Adding a user to a group: ~]# usermod -a -G group_name user_name For detailed information on managing users and groups, see Section 4.1, “Introduction to Users and Groups”. 34CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX 1.9.2. System user accounts managed in the web console With user accounts displayed in the RHEL 8 web console you can: Authenticate users when accessing the system. Set them access rights to the system. The RHEL 8 web console displays all user accounts located in the system. Therefore, you can see at least one user account just after the first login to the web console. Ones you are logged in to the RHEL 8 web console, you can: Create new users accounts. Change their parameters. Lock accounts. Terminate the user session. You can find the account management in the Accounts settings. 1.9.3. Adding new accounts in the web console The following describes adding system user accounts in the RHEL 8 web console and setting administration rights to the accounts. Procedure 1. Log in to the RHEL web console. 2. Click Accounts. 3. Click Create New Account. 4. In the Full Name field, enter the full name of the user. The RHEL web console automatically suggests a user name from the full name and fills it in the User Name field. If you do not want to use the original naming convention consisting of the first letter of the first name and the whole surname, update the suggestion. 5. In the Password/Confirm fields, enter the password and retype it for verification that your password is correct. The color bar placed below the fields shows you security level of the entered password, which does not allow you to create a user with a weak password. 35Red Hat Enterprise Linux 8 Configuring basic system settings 6. Click Create to save the settings and close the dialog box. 7. Select the newly created account. 8. Select Server Administrator in the Roles item. Now you can see the new account in the Accounts settings and you can use the credentials to connect to the system. 1.10. DUMPING THE CRASHED KERNEL USING THE KDUMP MECHANISM This section provides an introduction to the kernel crash dump mechanism, also called kdump, and briefly explains what kdump is used for in Section 1.10.1, “What kdump is and which tasks it can be used for”. Activation of the kdump service is a part of the installation process, as described in Installing Red Hat Enterprise Linux 8. This section summarizes how to manually enable the kdump service if it is disabled after the installation in Section 1.10.2, “Ensuring that kdump is installed and enabled after the installation process”. 36CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX You can also use the web console to configure kdump. See Section 1.10.3, “Configuring kdump in the RHEL 8 web console” for more information. 1.10.1. What kdump is and which tasks it can be used for In case of a system crash, you can use the kernel crash dump mechanism called kdump that enables you to save the content of the system’s memory for later analysis. The kdump mechanism relies on the kexec system call, which can be used to boot a Linux kernel from the context of another kernel, bypass BIOS, and preserve the contents of the first kernel’s memory that would otherwise be lost. When kernel crash occurs, kdump uses kexec to boot into a second kernel, a capture kernel, which resides in a reserved part of the system memory. The second kernel captures the contents of the crashed kernel’s memory, a crash dump, and saves it. For more detailed information about kdump, see Managing, monitoring and updating the kernel . 1.10.2. Ensuring that kdump is installed and enabled after the installation process To ensure that kdump is installed and to configure it: Checking whether kdump is Installed and Configuring kdump 1. To check whether kdump is installed on your system: ~]$ rpm -q kexec-tools 2. If not installed, to install kdump, enter as the root user: ~]# yum install kexec-tools 1. To configure kdump, use either the command line or the RHEL 8 web console. For more information on configuring kdump, see Managing, monitoring and updating the kernel . 1.10.3. Configuring kdump in the RHEL 8 web console In the web console, select Kernel dump configuration to verify: The kdump status The amount of memory reserved for kdump The location of the crash dump files Figure 1.5. Configuring kdump in the RHEL 8 web console 37Red Hat Enterprise Linux 8 Configuring basic system settings Figure 1.5. Configuring kdump in the RHEL 8 web console 1.11. PERFORMING SYSTEM RESCUE AND CREATING SYSTEM BACKUP WITH REAR When a software or hardware failure breaks the operating system, you need a mechanism to rescue the system. It is also useful to have the system backup saved. Red Hat recommends using the Relax-and- Recover (ReaR) tool to fulfil both these needs. 1.11.1. What ReaR is and which tasks it can be used for ReaR is a disaster recovery and system migration utility which enables you to create the complete rescue system. By default, this rescue system restores only the storage layout and the boot loader, but not the actual user and system files. Additionally, certain backup software enables you to integrate ReaR for disaster recovery. ReaR enables to perform the following tasks: Booting a rescue system on the new hardware Replicating the original storage layout Restoring user and system files 1.11.2. Quickstart to installation and configuration of ReaR To install ReaR, enter as the root user: ~]# yum install rear genisoimage syslinux Use the settings in the /etc/rear/local.conf file to configure ReaR. 1.11.3. Quickstart to creation of the rescue system with ReaR 38CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX To create the rescue system, perform the following command as the root user: ~]# rear mkrescue 1.11.4. Quickstart to configuration of ReaR with the backup software ReaR contains a fully-integrated built-in, or internal, backup method called NETFS. To make ReaR use its internal backup method, add these lines to the /etc/rear/local.conf file: BACKUP=NETFS BACKUP_URL=backup location You can also configure ReaR to keep the previous backup archives when the new ones are created by adding the following line to /etc/rear/local.conf: NETFS_KEEP_OLD_BACKUP_COPY=y To make the backups incremental, meaning that only the changed files are backed up on each run, add this line to /etc/rear/local.conf: BACKUP_TYPE=incremental 1.12. USING THE LOG FILES TO TROUBLESHOOT PROBLEMS When troubleshooting a problem, you may appreciate the log files that contain different information and messages about the operating system. The logging system in Red Hat Enterprise Linux is based on the built-in syslog protocol. Particular programs use this system to record events and organize them into log files, which are useful when auditing the operating system and troubleshooting various problems. 1.12.1. Services handling the syslog messages The syslog messages are handled by two services: The systemd-journald daemon The rsyslog service The systemd-journald daemon collects messages from various sources and forwards them to the rsyslog service for further processing. The sources from which the messages are collected are: Kernel Early stages of the boot process Standard output and error of daemons as they start up and run Syslog The rsyslog service sorts the syslog messages by type and priority, and writes them to the files in the /var/log directory, where the logs are persistently stored. 1.12.2. Subdirectories storing the syslog messages 39Red Hat Enterprise Linux 8 Configuring basic system settings The syslog messages are stored in various subdirectories under the /var/log directory according to what kind of messages and logs they contain: var/log/messages - all syslog messages except those mentioned below var/log/secure - security and authentication-related messages and errors var/log/maillog - mail server-related messages and errors var/log/cron - log files related to periodically executed tasks var/log/boot.log - log files related to system startup 1.12.2.1. Managing the log files in the RHEL 8 web console in the web console, use the Logs option if you want to inspect the log files. Figure 1.6. Inspecting the log files in the RHEL 8 web console 1.13. ACCESSING RED HAT SUPPORT To obtain support from Red Hat, use the Red Hat Customer Portal , which provides access to everything available with your subscription. This section describes: Obtaining Red Hat support, in Section 1.13.1, “Obtaining Red Hat Support through Red Hat Customer Portal” Using the SOS report to troubleshoot problems, in Section 1.13.2, “Using the SOS report to troubleshoot problems” 1.13.1. Obtaining Red Hat Support through Red Hat Customer Portal By using the Red Hat Customer Portal you can: Open a new support case Initiate a live chat with a Red Hat expert Contact a Red Hat expert by making a call or sending an email To access the Red Hat Customer Portal, go to https://access.redhat.com. 1.13.2. Using the SOS report to troubleshoot problems The SOS report collects configuration details, system information and diagnostic information from a 40CHAPTER 1. GETTING STARTED WITH SYSTEM ADMINISTRATION IN RED HAT ENTERPRISE LINUX The SOS report collects configuration details, system information and diagnostic information from a Red Hat Enterprise Linux system. Attach the report when you open a support case. Note that the SOS report is provided in the sos package, which is not installed with the default minimal installation of Red Hat Enterprise Linux. To install the sos package: ~]# yum install sos To generate an SOS report: ~]# sosreport To attach the sos report to your support case, see the Red Hat Knowledgebase article How can I attach a file to a Red Hat support case?. Note that you will be prompted to enter the number of the support case, when attaching the sos report. For more information on SOS report, see the Red Hat Knowledgebase article What is a sosreport and how to create one in Red Hat Enterprise Linux 4.6 and later?. 41Red Hat Enterprise Linux 8 Configuring basic system settings CHAPTER 2. INSTALLING SOFTWARE WITH YUM 2.1. INTRODUCTION TO INSTALLING SOFTWARE ON RED HAT ENTERPRISE LINUX 8 On Red Hat Enterprise Linux 8, installing software is ensured by the new version of the YUM tool, which is based on the DNF technology (YUM v4). YUM v4 has the following advantages over the previous YUM v3 used on Red Hat Enterprise Linux 7: Increased performance New features available, most significantly the support for managing the modular content Well-designed stable API for integration with tooling For detailed information about differences between the new YUM tool and the previous version YUM v3 from Red Hat Enterprise Linux 7, see Changes in DNF CLI compared to YUM . NOTE Note, that upstream calls this tool DNF. As a result, some output returned by the new YUM tool in Red Hat Enterprise Linux 8 mentions DNF, and upstream documentation identifies the technology as DNF. For installing software, you can use the yum command and its particular options in the same way as on Red Hat Enterprise Linux 7. Selected yum plug-ins and utilities have been ported to the new DNF back end, and can be installed under the same names as in Red Hat Enterprise Linux 7. They also provide compatibility symlinks, so the binaries, configuration files and directories can be found in usual locations. Note that the legacy Python API provided by YUM v3 is no longer available. Users are advised to migrate their plug-ins and scripts to the new API provided by YUM v4 (DNF Python API), which is stable and fully supported. The DNF Python API is available here. The Libdnf and Hawkey APIs (both C and Python) are unstable, and will likely change during RHEL 8 life cycle. 2.1.1. Application streams Red Hat Enterprise Linux 8.0 introduces the concept of Application Streams. Multiple versions of user space components are now delivered and updated more frequently than the core operating system packages. This provides greater flexibility to customize Red Hat Enterprise Linux without impacting the underlying stability of the platform or specific deployments. Components made available as Application Streams can be packaged as modules or RPM packages and are delivered through the AppStream repository in RHEL 8. Each Application Stream component has a given life cycle. For details, see Red Hat Enterprise Linux Life Cycle . Modules are collections of packages representing a logical unit: an application, a language stack, a database, or a set of tools. These packages are built, tested, and released together. Module streams represent versions of the Application Stream components. For example, two streams 42CHAPTER 2. INSTALLING SOFTWARE WITH YUM (versions) of the PostgreSQL database server are available in the postgresql module: PostgreSQL 10 (the default stream) and PostgreSQL 9.6. Only one module stream can be installed on the system. Different versions can be used in separate containers. Detailed module commands are described in the Installing, managing, and removing user space components document. For a list of modules available in AppStream, see the Package manifest. 2.2. INTRODUCTION TO YUM FUNCTIONALITY yum is the Red Hat package manager that is able to query for information about available packages, fetch packages from repositories, install and uninstall them, and update an entire system to the latest available version. Yum performs automatic dependency resolution when updating, installing, or removing packages, and thus is able to automatically determine, fetch, and install all available dependent packages. yum can be configured with new, additional repositories, or package sources , and also provides many plug-ins which enhance and extend its capabilities. Yum enables easy and simple package management. IMPORTANT yum provides secure package management by enabling Gnu Privacy Guard (GPG), also known as GnuPG, signature verification on GPG-signed packages to be turned on for all package repositories (package sources), or for individual repositories. You can also use yum to set up your own repositories with RPM packages for download and installation on other machines. When possible, yum uses parallel download of multiple packages and metadata to speed up downloading. NOTE You must have superuser privileges in order to use yum to install, update or remove packages on your system. All examples here assume that you have already obtained superuser privileges by using either the su or sudo command. 2.3. USING YUM FOR PARTICULAR TASKS This section describes how to use yum to achieve particular tasks. 2.3.1. Checking for updates and updating packages yum enables you to check if your system has any updates waiting to be applied. You can list packages that need to be updated and update them as a whole, or you can update a selected individual package. 2.3.1.1. Checking for updates To see which installed packages on your system have updates available, use the following command: yum check-update The command shows the list of packages and their dependencies that have an update available. The output for each package consists of: the name of the package 43Red Hat Enterprise Linux 8 Configuring basic system settings the CPU architecture the package was built for the version of the updated package to be installed the release of the updated package a build version, added as part of a z-stream update the repository in which the updated package is located. 2.3.1.1.1. Updating packages You can choose to update a single package, multiple packages, or all packages at once. If any dependencies of the package or packages you update have updates available themselves, then they are updated too. 2.3.1.1.1.1. Updating a single package To update a single package, run the following command as root: yum update package_name IMPORTANT yum always installs a new kernel regardless of whether you are using the yum update or yum install command to apply kernel updates. When using RPM, on the other hand, it is important to use the rpm -i kernel command which installs a new kernel instead of rpm -u kernel which replaces the current kernel. 2.3.1.1.1.2. Updating a package group To update a package group, type as root: yum group update group_name Here, replace group_name with a name of the package group you want to update. For more information on package groups, see Section 2.3.3, “Working with package groups”. 2.3.1.1.2. Updating all packages and their dependencies To update all packages and their dependencies, use the yum update command without any arguments: yum update 2.3.1.1.3. Updating security-related packages If packages have security updates available, you can update only these packages to their latest versions. Type as root: yum update --security You can also update packages only to versions containing the latest security updates. Type as root: 44CHAPTER 2. INSTALLING SOFTWARE WITH YUM yum update-minimal --security 2.3.2. Working with packages yum enables you to perform a complete set of operations with software packages, including searching for packages, viewing information about them, installing and removing. 2.3.2.1. Searching packages You can search all package names, descriptions and summaries by using the following command: yum search term... Replace term with a package name you want to search. The yum search command is useful for searching for packages you do not know the name of, but for which you know a related term. Note that by default, yum search returns matches in package name and summary, which makes the search faster. Use the yum search --all command for a more exhaustive but slower search, which also includes package descriptions. 2.3.2.1.1. Filtering the Results All of yum’s list commands allow you to filter the results by appending one or more glob expressions as arguments. Global expressions are normal strings of characters which contain one or more of the wildcard characters * (which expands to match any character subset) and ? (which expands to match any single character). Be careful to escape the global expressions when passing them as arguments to a yum command, otherwise the Bash shell will interpret these expressions as pathname expansions, and potentially pass all files in the current directory that match the global expressions to yum. To make sure the global expressions are passed to yum as intended, use one of the following methods: escape the wildcard characters by preceding them with a backslash character double-quote or single-quote the entire global expression. 2.3.2.2. Listing packages To list information on all installed and available packages type the following at a shell prompt: yum list --all To list installed and available packages that match inserted global expressions use the following command: yum list glob_expression... To list all packages installed on your system use the installed keyword. yum list --installed glob_expression... To list all packages in all enabled repositories that are available to install, use the command in the following form: 45Red Hat Enterprise Linux 8 Configuring basic system settings yum list --available glob_expression... 2.3.2.2.1. Listing repositories To list the repository ID, name, and number of packages for each enabled repository on your system, use the following command: yum repolist To list more information about these repositories, use the repoinfo command. With this command, information including the file name, overall size, date of the last update, and base URL are displayed for each listed repository. yum repoinfo To list both enabled and disabled repositories use the following command. A status column is added to the output list to show which of the repositories are enabled. yum repolist --all By passing disabled as a first argument, you can reduce the command output to disabled repositories. For further specification you can pass the ID or name of repositories or related glob_expressions as arguments. Note that if there is an exact match between the repository ID or name and the inserted argument, this repository is listed even if it does not pass the enabled or disabled filter. 2.3.2.2.2. Displaying package information To display information about one or more packages, use the following command (global expressions are valid here as well): yum info package_name... Replace package_name with the name of the package. 2.3.2.2.3. Installing packages To install a single package and all of its non-installed dependencies, enter a command in the following form as root: yum install package_name You can also install multiple packages simultaneously by appending their names as arguments. To do so, type as root: yum install package_name package_name... If you are installing packages on a multilib system, such as an AMD64 or Intel 64 machine, you can specify the architecture of the package (as long as it is available in an enabled repository) by appending .arch to the package name: yum install package_name.arch 46CHAPTER 2. INSTALLING SOFTWARE WITH YUM You can use global expressions to quickly install multiple similarly named packages. Execute as root: yum install glob_expression... In addition to package names and global expressions, you can also provide file names to yum install. If you know the name of the binary you want to install, but not its package name, you can give yum install the path name. As root, type: yum install /usr/sbin/named Yum then searches through its package lists, finds the package which provides /usr/sbin/named, if any, and prompts you as to whether you want to install it. As you can see in the above examples, the yum install command does not require strictly defined arguments. It can process various formats of package names and global expressions, which makes installation easier for users. On the other hand, it takes some time until yum parses the input correctly, especially if you specify a large number of packages. To optimize the package search, you can use the following commands to explicitly define how to parse the arguments: yum install-n name yum install-na name.architecture yum install-nevra name-epoch:version-release.architecture With install-n, yum interprets name as the exact name of the package. The install-na command tells yum that the subsequent argument contains the package name and architecture divided by the dot character. With install-nevra, yum will expect an argument in the form name-epoch:version- release.architecture. Similarly, you can use yum remove-n, yum remove-na, and yum remove-nevra when searching for packages to be removed. NOTE If you know you want to install the package that contains the named binary, but you do not know in which bin/ or sbin/ directory the file is installed, use the yum provides command with a global expression. yum provides "*/file_name" is a useful way to find the packages that contain file_name. To install a previously-downloaded package from a local directory on your system, use the following command: yum install path Replace path with the path to the package you want to install. Alternatively, you can use also the yum localinstall command to install a previously -nloaded package from a local directory. 2.3.2.2.4. Removing packages To uninstall a particular package, as well as any packages that depend on it, run the following command as root: 47Red Hat Enterprise Linux 8 Configuring basic system settings yum remove package_name... To remove multiple packages at once by adding more package names to the command. Similar to install, remove can take these arguments: package names global expressions file lists package provides WARNING  yum is not able to remove a package without also removing packages which depend on it. 2.3.3. Working with package groups A package group is a collection of packages that serve a common purpose, for instance System Tools or Sound and Video. Installing a package group pulls a set of dependent packages, saving time considerably. The yum groups command is a top-level command that covers all the operations that act on package groups in yum. 2.3.3.1. Listing package groups The summary option is used to view the number of: installed groups available groups available environment groups installed and available language groups yum groups summary To list all package groups from yum repositories add the list option. You can filter the command output by group names. yum group list glob_expression... Several optional arguments can be passed to this command, including --hidden to list also groups not marked as user visible, and -v to list group IDs. You can also add --installed, or --available options to reduce the command output to a specific group type. To list mandatory and optional packages contained in a particular group, use the following command: 48CHAPTER 2. INSTALLING SOFTWARE WITH YUM yum group info glob_expression... NOTE A package group can be marked with @. When using yum group list, info, install, or remove, pass @group_name to specify a package group or an environmental group. 2.3.3.2. Installing a package group You can install a package group by passing its full group name, without the groupid part, to the group install command. As root, type: yum group install group_name You can also install by groupid. As root, execute the following command: yum group install groupid You can pass the groupid or quoted group name to the install command if you prepend it with an @ symbol, which tells yum that you want to perform group install. As root, type: yum install @group Replace group with the groupid or quoted group name. The same logic appplies to environmental groups: yum install @group 2.3.3.3. Removing a package group You can remove a package group using syntax similar to the install syntax, with use of either name of the package group or its id. As root, type: yum group remove group_name yum group remove groupid Also, you can pass the groupid or quoted name to the remove command if you prepend it with an @ symbol, which tells yum that you want to perform group remove. As root, type: yum remove @group Replace group with the groupid or quoted group name. Similarly, you can replace an environmental group: yum remove @group 2.4. WORKING WITH TRANSACTION HISTORY The yum history command enables users to review information about a timeline of yum transactions, 49Red Hat Enterprise Linux 8 Configuring basic system settings the dates and times they occurred, the number of packages affected, whether these transactions succeeded or were aborted, and if the RPM database was changed between transactions. Additionally, this command can be used to undo or redo certain transactions. All history data is stored in the history DB in the /var/lib/yum/history/ directory. 2.4.1. Listing transactions To display a list of the twenty most recent transactions, as root, either run yum history with no additional arguments, or type the following at a shell prompt: yum history list To examine a particular transaction or transactions in more detail, run the following command as root: yum history info id... The id argument here stands for the ID of the transaction. This argument is optional and when you omit it, yum automatically uses the last transaction. 2.4.2. Reverting and repeating transactions Apart from reviewing the transaction history, the yum history command provides means to revert or repeat a selected transaction. To revert a transaction, type the following at a shell prompt as root: yum history undo id To repeat a particular transaction, as root, run the following command: yum history redo id Both commands also accept the last keyword to undo or repeat the latest transaction. Note that both yum history undo and yum history redo commands only revert or repeat the steps that were performed during a transaction. If the transaction installed a new package, the yum history undo command will uninstall it, and if the transaction uninstalled a package the command will again install it. This command also attempts to downgrade all updated packages to their previous version, if these older packages are still available. 2.5. CONFIGURING YUM AND YUM REPOSITORIES The configuration information for yum and related utilities is located in the /etc/yum.conf file. This file contains one mandatory [main] section, which enables you to set yum options that have global effect, and can also contain one or more [repository] sections, which allow you to set repository-specific options. However, it is recommended to define individual repositories in new or existing .repo files in the /etc/yum.repos.d/ directory. The values you define in individual [repository] sections of the /etc/yum.conf file override values set in the [main] section. This section shows how to: set global yum options by editing the [main] section of the /etc/yum.conf configuration file; set options for individual repositories by editing the [repository] sections in /etc/yum.conf and .repo files in the /etc/yum.repos.d/ directory; 50CHAPTER 2. INSTALLING SOFTWARE WITH YUM add, enable, and disable yum repositories on the command line 2.5.1. Viewing the current configuration To display the current values of global yum options (that is, the options specified in the [main] section of the /etc/yum.conf file), execute the yum-config-manager command with the following command-line option: yum-config-manager --dump 2.5.2. Setting [main] options The /etc/yum.conf configuration file contains exactly one [main] section, and while some of the key- value pairs in this section affect how yum operates, others affect how yum treats repositories. You can add many additional options under the [main] section heading in /etc/yum.conf. For a complete list of available [main] options, see the [main] OPTIONS section of the yum.conf(5) manual page. 2.5.3. Setting [repository] options The [repository] sections, where repository is a unique repository ID such as my_personal_repo (spaces are not permitted), allow you to define individual yum repositories. To avoid conflicts, do not use names used by Red Hat repositories for custom repositories. For a complete list of available [repository] options, see the [repository] OPTIONS section of the yum.conf(5) manual page. 2.5.4. Adding, enabling, and disabling a yum repository Section 2.5.3, “Setting [repository] options” describes various options you can use to define a yum repository. This section explains how to add, enable, and disable a repository by using the yum-config- manager command. 2.5.4.1. Adding a yum repository To define a new repository, you can either add a [repository] section to the /etc/yum.conf file, or to a .repo file in the /etc/yum.repos.d/ directory. All files with the .repo file extension in this directory are read by yum, and it is recommended to define your repositories here instead of in /etc/yum.conf. WARNING  Obtaining and installing software packages from unverified or untrusted software sources other than Red Hat’s certificate-based Content Delivery Network (CDN) constitutes a potential security risk, and could lead to security, stability, compatibility, and maintainability issues. Yum repositories commonly provide their own .repo file. To add such a repository to your system and 51Red Hat Enterprise Linux 8 Configuring basic system settings Yum repositories commonly provide their own .repo file. To add such a repository to your system and enable it, run the following command as root: yum-config-manager --add-repo repository_url Here repository_url is a link to the .repo file. 2.5.4.2. Enabling a yum repository To enable a particular repository or repositories, type the following at a shell prompt as root: yum-config-manager --enable repository... Here repository is the unique repository ID (use yum repolist all to list available repository IDs). Disabling a yum repository To disable a yum repository, run the following command as root: yum-config-manager --disable repository... Here repository is the unique repository ID (use yum repolist all to list available repository IDs). 2.6. USING YUM PLUG-INS Yum provides plug-ins that extend and enhance its operations. Certain plug-ins are installed by default. Yum always informs you which plug-ins, if any, are loaded and active whenever you call any yum command. 2.6.1. Enabling, configuring, and disabling yum plug-ins To enable yum plug-ins, ensure that a line beginning with plugins= is present in the [main] section of /etc/yum.conf, and that its value is 1: plugins=1 You can disable all plug-ins by changing this line to plugins=0. IMPORTANT Disabling all plug-ins is not advised because certain plug-ins provide important yum services. In particular, the product-id and subscription-manager plug-ins provide support for the certificate-based Content Delivery Network (CDN). Disabling plug-ins globally is provided as a convenience option, and is generally only recommended when diagnosing a potential problem with yum. Every installed plug-in has its own configuration file in the /etc/dnf/plugins/ directory. You can set plug- in specific options in these files. Similar to the /etc/yum.conf file, the plug-in configuration files always contain a [main] section where the enabled= option controls whether the plug-in is enabled when you run yum commands. If this option is missing, you can add it manually to the file. If you disable all plug-ins by setting enabled=0 in /etc/yum.conf, then all plug-ins are disabled regardless 52CHAPTER 2. INSTALLING SOFTWARE WITH YUM If you disable all plug-ins by setting enabled=0 in /etc/yum.conf, then all plug-ins are disabled regardless of whether they are enabled in their individual configuration files. If you want to disable all yum plug-ins for a single yum command, use the --noplugins option. If you want to disable one or more yum plug-ins for a single yum command, add the -- disableplugin=plugin_name option to the command. 2.7. ADDITIONAL RESOURCES The following sources of information provide additional resources regarding YUM. 2.7.1. Installed Documentation yum(8) — The manual page for the yum command-line utility provides a complete list of supported options and commands. yum.conf(5) — The manual page named yum.conf documents available yum configuration options. 2.7.2. Online Documentation Red Hat Customer Portal Labs — The Red Hat Customer Portal Labs includes a "Yum Repository Configuration Helper". 53Red Hat Enterprise Linux 8 Configuring basic system settings CHAPTER 3. MANAGING SERVICES WITH SYSTEMD 3.1. INTRODUCTION TO SYSTEMD Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, or dependency-based service control logic. Starting with Red Hat Enterprise Linux 7, systemd replaced Upstart as the default init system. Systemd introduces the concept of systemd units. These units are represented by unit configuration files located in one of the directories listed in the following table. Table 3.1. Systemd unit files locations Directory Description /usr/lib/systemd/system/ Systemd unit files distributed with installed RPM packages. /run/systemd/system/ Systemd unit files created at run time. This directory takes precedence over the directory with installed service unit files. /etc/systemd/system/ Systemd unit files created by systemctl enable as well as unit files added for extending a service. This directory takes precedence over the directory with runtime unit files. The units encapsulate information about: System services Listening sockets Other objects that are relevant to the init system For a complete list of available systemd unit types, see the follwoing table. Table 3.2. Available systemd unit types Unit Type File Extension Description Service unit .service A system service. Target unit .target A group of systemd units. Automount unit .automount A file system automount point. Device unit .device A device file recognized by the kernel. 54CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Unit Type File Extension Description Mount unit .mount A file system mount point. Path unit .path A file or directory in a file system. Scope unit .scope An externally created process. Slice unit .slice A group of hierarchically organized units that manage system processes. Socket unit .socket An inter-process communication socket. Swap unit .swap A swap device or a swap file. Timer unit .timer A systemd timer. Overriding the default systemd configuration using system.conf The default configuration of systemd is defined during the compilation and it can be found in the systemd configuration file at /etc/systemd/system.conf. Use this file if you want to deviate from those defaults and override selected default values for systemd units globally. For example, to override the default value of the timeout limit, which is set to 90 seconds, use the DefaultTimeoutStartSec parameter to input the required value in seconds. DefaultTimeoutStartSec=required value For further information, see Example 3.20, “Changing the timeout limit” . 3.1.1. Main features The systemd system and service manager provides the following main features: Socket-based activation — At boot time, systemd creates listening sockets for all system services that support this type of activation, and passes the sockets to these services as soon as they are started. This not only allows systemd to start services in parallel, but also makes it possible to restart a service without losing any message sent to it while it is unavailable: the corresponding socket remains accessible and all messages are queued. Systemd uses socket units for socket-based activation. Bus-based activation — System services that use D-Bus for inter-process communication can be started on-demand the first time a client application attempts to communicate with them. Systemd uses D-Bus service files for bus-based activation. Device-based activation — System services that support device-based activation can be started on-demand when a particular type of hardware is plugged in or becomes available. Systemd uses device units for device-based activation. Path-based activation — System services that support path-based activation can be started 55Red Hat Enterprise Linux 8 Configuring basic system settings Path-based activation — System services that support path-based activation can be started on-demand when a particular file or directory changes its state. Systemd uses path units for path-based activation. Mount and automount point management — Systemd monitors and manages mount and automount points. Systemd uses mount units for mount points and automount units for automount points. Aggressive parallelization — Because of the use of socket-based activation, systemd can start system services in parallel as soon as all listening sockets are in place. In combination with system services that support on-demand activation, parallel activation significantly reduces the time required to boot the system. Transactional unit activation logic — Before activating or deactivating a unit, systemd calculates its dependencies, creates a temporary transaction, and verifies that this transaction is consistent. If a transaction is inconsistent, systemd automatically attempts to correct it and remove non-essential jobs from it before reporting an error. Backwards compatibility with SysV init — Systemd supports SysV init scripts as described in the Linux Standard Base Core Specification , which eases the upgrade path to systemd service units. 3.1.2. Compatibility changes The systemd system and service manager is designed to be mostly compatible with SysV init and Upstart. The following are the most notable compatibility changes with regards to Red Hat Enterprise Linux 6 system that used SysV init: Systemd has only limited support for runlevels. It provides a number of target units that can be directly mapped to these runlevels and for compatibility reasons, it is also distributed with the earlier runlevel command. Not all systemd targets can be directly mapped to runlevels, however, and as a consequence, this command might return N to indicate an unknown runlevel. It is recommended that you avoid using the runlevel command if possible. For more information about systemd targets and their comparison with runlevels, see Section 3.3, “Working with systemd targets” . The systemctl utility does not support custom commands. In addition to standard commands such as start, stop, and status, authors of SysV init scripts could implement support for any number of arbitrary commands in order to provide additional functionality. For example, the init script for iptables could be executed with the panic command, which immediately enabled panic mode and reconfigured the system to start dropping all incoming and outgoing packets. This is not supported in systemd and the systemctl only accepts documented commands. For more information about the systemctl utility and its comparison with the earlier service utility, see Table 3.3, “Comparison of the service utility with systemctl” . The systemctl utility does not communicate with services that have not been started by systemd. When systemd starts a system service, it stores the ID of its main process in order to keep track of it. The systemctl utility then uses this PID to query and manage the service. Consequently, if a user starts a particular daemon directly on the command line, systemctl is unable to determine its current status or stop it. Systemd stops only running services. Previously, when the shutdown sequence was initiated, Red Hat Enterprise Linux 6 and earlier releases of the system used symbolic links located in the /etc/rc0.d/ directory to stop all available system services regardless of their status. With systemd , only running services are stopped on shutdown. System services are unable to read from the standard input stream. When systemd starts a 56CHAPTER 3. MANAGING SERVICES WITH SYSTEMD System services are unable to read from the standard input stream. When systemd starts a service, it connects its standard input to /dev/null to prevent any interaction with the user. System services do not inherit any context (such as the HOME and PATH environment variables) from the invoking user and their session. Each service runs in a clean execution context. When loading a SysV init script, systemd reads dependency information encoded in the Linux Standard Base (LSB) header and interprets it at run time. All operations on service units are subject to a default timeout of 5 minutes to prevent a malfunctioning service from freezing the system. This value is hardcoded for services that are generated from initscripts and cannot be changed. However, individual configuration files can be used to specify a longer timeout value per service, see Example 3.20, “Changing the timeout limit”. For a detailed list of compatibility changes introduced with systemd, see the Migration Planning Guide for Red Hat Enterprise Linux 7. 3.2. MANAGING SYSTEM SERVICES Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, used init scripts located in the /etc/rc.d/init.d/ directory. These init scripts were typically written in Bash, and allowed the system administrator to control the state of services and daemons in their system. Starting with Red Hat Enterprise Linux 7, these init scripts have been replaced with service units. Service units end with the .service file extension and serve a similar purpose as init scripts. To view, start, stop, restart, enable, or disable system services, use the systemctl command as described in Comparison of the service utility with systemctl , Comparison of the chkconfig utility with systemctl , and further in this section. The service and chkconfig commands are still available in the system and work as expected, but are only included for compatibility reasons and should be avoided. Table 3.3. Comparison of the service utility with systemctl service systemctl Description service name start systemctl start name.service Starts a service. service name stop systemctl stop name.service Stops a service. service name restart systemctl restart Restarts a service. name.service service name condrestart systemctl try-restart Restarts a service only if it is name.service running. service name reload systemctl reload Reloads configuration. name.service 57Red Hat Enterprise Linux 8 Configuring basic system settings service systemctl Description service name status systemctl status Checks if a service is running. name.service systemctl is-active name.service service --status-all systemctl list-units --type Displays the status of all services. service --all Table 3.4. Comparison of the chkconfig utility with systemctl chkconfig systemctl Description chkconfig name on systemctl enable Enables a service. name.service chkconfig name off systemctl disable Disables a service. name.service chkconfig --list name systemctl status Checks if a service is enabled. name.service systemctl is-enabled name.service chkconfig --list systemctl list-unit-files -- Lists all services and checks if type service they are enabled. chkconfig --list systemctl list-dependencies Lists services that are ordered to --after start before the specified unit. chkconfig --list systemctl list-dependencies Lists services that are ordered to --before start after the specified unit. Specifying service units For clarity, all command examples in the rest of this section use full unit names with the .service file extension, for example: ~]# systemctl stop nfs-server.service However, the file extension can be omitted, in which case the systemctl utility assumes the argument is a service unit. The following command is equivalent to the one above: ~]# systemctl stop nfs-server Additionally, some units have alias names. Those names can have shorter names than units, which can be used instead of the actual unit names. To find all aliases that can be used for a particular unit, use: 58CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ~]# systemctl show nfs-server.service -p Names Behavior of systemctl in a chroot environment If you change the root directory using the chroot command, most systemctl commands refuse to perform any action. The reason for this is that the systemd process and the user that used the chroot command do not have the same view of the filesystem. This happens, for example, when systemctl is invoked from a kickstart file. The exception to this are unit file commands such as the systemctl enable and systemctl disable commands. These commands do not need a running system and do not affect running processes, but they do affect unit files. Therefore, you can run these commands even in chroot environment. For example, to enable the httpd service on a system under the /srv/website1/ directory: ~]# chroot /srv/website1 ~]# systemctl enable httpd.service Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service, pointing to /usr/lib/systemd/system/httpd.service. 3.2.1. Listing services To list all currently loaded service units, type the following at a shell prompt: systemctl list-units --type service For each service unit file, this command displays its full name (UNIT) followed by a note whether the unit file has been loaded (LOAD), its high-level ( ACTIVE) and low-level (SUB) unit file activation state, and a short description (DESCRIPTION). By default, the systemctl list-units command displays only active units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option: systemctl list-units --type service --all You can also list all available service units to see if they are enabled. To do so, type: systemctl list-unit-files --type service For each service unit, this command displays its full name (UNIT FILE) followed by information whether the service unit is enabled or not (STATE). For information on how to determine the status of individual service units, see Displaying service status. Example 3.1. Listing services To list all currently loaded service units, run the following command: ~]$ systemctl list-units --type service UNIT LOAD ACTIVE SUB DESCRIPTION abrt-ccpp.service loaded active exited Install ABRT coredump hook abrt-oops.service loaded active running ABRT kernel log watcher abrt-vmcore.service loaded active exited Harvest vmcores for ABRT abrt-xorg.service loaded active running ABRT Xorg log watcher abrtd.service loaded active running ABRT Automated Bug Reporting Tool … ​ 59Red Hat Enterprise Linux 8 Configuring basic system settings systemd-vconsole-setup.service loaded active exited Setup Virtual Console tog-pegasus.service loaded active running OpenPegasus CIM Server LOAD = Reflects whether the unit definition was properly loaded. ACTIVE = The high-level unit activation state, i.e. generalization of SUB. SUB = The low-level unit activation state, values depend on unit type. 46 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use ''systemctl list-unit-files'' To list all installed service unit files to determine if they are enabled, type: ~]$ systemctl list-unit-files --type service UNIT FILE STATE abrt-ccpp.service enabled abrt-oops.service enabled abrt-vmcore.service enabled abrt-xorg.service enabled abrtd.service enabled … ​ wpa_supplicant.service disabled ypbind.service disabled 208 unit files listed. 3.2.2. Displaying service status To display detailed information about a service unit that corresponds to a system service, type the following at a shell prompt: systemctl status name.service Replace name with the name of the service unit you want to inspect (for example, gdm). This command displays the name of the selected service unit followed by its short description, one or more fields described in Table 3.5, “Available service unit information” , and if it is executed by the root user, also the most recent log entries. Table 3.5. Available service unit information Field Description Loaded Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled. Active Information whether the service unit is running followed by a time stamp. Main PID The PID of the corresponding system service followed by its name. 60CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Field Description Status Additional information about the corresponding system service. Process Additional information about related processes. CGroup Additional information about related Control Groups (cgroups). To only verify that a particular service unit is running, run the following command: systemctl is-active name.service Similarly, to determine whether a particular service unit is enabled, type: systemctl is-enabled name.service Note that both systemctl is-active and systemctl is-enabled return an exit status of 0 if the specified service unit is running or enabled. For information on how to list all currently loaded service units, see Listing services. Example 3.2. Displaying service status The service unit for the GNOME Display Manager is named gdm.service. To determine the current status of this service unit, type the following at a shell prompt: ~]# systemctl status gdm.service gdm.service - GNOME Display Manager Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled) Active: active (running) since Thu 2013-10-17 17:31:23 CEST; 5min ago Main PID: 1029 (gdm) CGroup: /system.slice/gdm.service ├─1029 /usr/sbin/gdm ├─1037 /usr/libexec/gdm-simple-slave --display-id /org/gno… ​ └─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r… ​ Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager. Example 3.3. Displaying services ordered to start before a service To determine what services are ordered to start before the specified service, type the following at a shell prompt: ~]# systemctl list-dependencies --after gdm.service gdm.service ├─dbus.socket ├─getty@tty1.service ├─livesys.service ├─plymouth-quit.service 61Red Hat Enterprise Linux 8 Configuring basic system settings ├─system.slice ├─systemd-journald.socket ├─systemd-user-sessions.service └─basic.target [output truncated] Example 3.4. Displaying services ordered to start after a service To determine what services are ordered to start after the specified service, type the following at a shell prompt: ~]# systemctl list-dependencies --before gdm.service gdm.service ├─dracut-shutdown.service ├─graphical.target │ ├─systemd-readahead-done.service │ ├─systemd-readahead-done.timer │ └─systemd-update-utmp-runlevel.service └─shutdown.target ├─systemd-reboot.service └─final.target └─systemd-reboot.service 3.2.3. Starting a service To start a service unit that corresponds to a system service, type the following at a shell prompt as root: systemctl start name.service Replace name with the name of the service unit you want to start (for example, gdm). This command starts the selected service unit in the current session. For information on how to enable a service unit to be started at boot time, see Enabling a service. For information on how to determine the status of a certain service unit, see Displaying service status. Example 3.5. Starting a service The service unit for the Apache HTTP Server is named httpd.service. To activate this service unit and start the httpd daemon in the current session, run the following command as root: ~]# systemctl start httpd.service 3.2.4. Stopping a service To stop a service unit that corresponds to a system service, type the following at a shell prompt as root: systemctl stop name.service Replace name with the name of the service unit you want to stop (for example, bluetooth). This command stops the selected service unit in the current session. For information on how to disable a 62CHAPTER 3. MANAGING SERVICES WITH SYSTEMD service unit and prevent it from being started at boot time, see Disabling a service . For information on how to determine the status of a certain service unit, see Displaying service status. Example 3.6. Stopping a service The service unit for the bluetoothd daemon is named bluetooth.service. To deactivate this service unit and stop the bluetoothd daemon in the current session, run the following command as root: ~]# systemctl stop bluetooth.service 3.2.5. Restarting a service To restart a service unit that corresponds to a system service, type the following at a shell prompt as root: systemctl restart name.service Replace name with the name of the service unit you want to restart (for example, httpd). This command stops the selected service unit in the current session and immediately starts it again. Importantly, if the selected service unit is not running, this command starts it too. To tell systemd to restart a service unit only if the corresponding service is already running, run the following command as root: systemctl try-restart name.service Certain system services also allow you to reload their configuration without interrupting their execution. To do so, type as root: systemctl reload name.service Note that system services that do not support this feature ignore this command altogether. For convenience, the systemctl command also supports the reload-or-restart and reload-or-try-restart commands that restart such services instead. For information on how to determine the status of a certain service unit, see Displaying service status. Example 3.7. Restarting a service In order to prevent users from encountering unnecessary error messages or partially rendered web pages, the Apache HTTP Server allows you to edit and reload its configuration without the need to restart it and interrupt actively processed requests. To do so, type the following at a shell prompt as root: ~]# systemctl reload httpd.service 3.2.6. Enabling a service To configure a service unit that corresponds to a system service to be automatically started at boot time, type the following at a shell prompt as root: systemctl enable name.service 63Red Hat Enterprise Linux 8 Configuring basic system settings Replace name with the name of the service unit you want to enable (for example, httpd). This command reads the [Install] section of the selected service unit and creates appropriate symbolic links to the /usr/lib/systemd/system/name.service file in the /etc/systemd/system/ directory and its subdirectories. This command does not, however, rewrite links that already exist. If you want to ensure that the symbolic links are re-created, use the following command as root: systemctl reenable name.service This command disables the selected service unit and immediately enables it again. For information on how to determine whether a certain service unit is enabled to start at boot time, see Displaying service status. For information on how to start a service in the current session, see Starting a service . Example 3.8. Enabling a service To configure the Apache HTTP Server to start automatically at boot time, run the following command as root: ~]# systemctl enable httpd.service Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service. 3.2.7. Disabling a service To prevent a service unit that corresponds to a system service from being automatically started at boot time, type the following at a shell prompt as root: systemctl disable name.service Replace name with the name of the service unit you want to disable (for example, bluetooth). This command reads the [Install] section of the selected service unit and removes appropriate symbolic links to the /usr/lib/systemd/system/name.service file from the /etc/systemd/system/ directory and its subdirectories. In addition, you can mask any service unit to prevent it from being started manually or by another service. To do so, run the following command as root: systemctl mask name.service This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd. To revert this action and unmask a service unit, type as root: systemctl unmask name.service For information on how to determine whether a certain service unit is enabled to start at boot time, see Displaying service status. For information on how to stop a service in the current session, see Stopping a service. Example 3.9. Disabling a service Example 3.6, “Stopping a service” illustrates how to stop the bluetooth.service unit in the current session. To prevent this service unit from starting at boot time, type the following at a shell prompt as root: 64CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ~]# systemctl disable bluetooth.service Removed symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service. Removed symlink /etc/systemd/system/dbus-org.bluez.service. 3.2.8. Starting a conflicting service In systemd, positive and negative dependencies between services exist. Starting particular service may require starting one or more other services (positive dependency) or stopping one or more services (negative dependency). When you attempt to start a new service, systemd resolves all dependencies automatically. Note that this is done without explicit notification to the user. If you are already running a service, and you attempt to start another service with a negative dependency, the first service is automatically stopped. For example, if you are running the postfix service, and you try to start the sendmail service, systemd first automatically stops postfix, because these two services are conflicting and cannot run on the same port. 3.3. WORKING WITH SYSTEMD TARGETS Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, implemented a predefined set of runlevels that represented specific modes of operation. These runlevels were numbered from 0 to 6 and were defined by a selection of system services to be run when a particular runlevel was enabled by the system administrator. Starting with Red Hat Enterprise Linux 7, the concept of runlevels has been replaced with systemd targets . Systemd targets are represented by target units. Target units end with the .target file extension and their only purpose is to group together other systemd units through 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 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. Red Hat Enterprise Linux 7 was distributed with a number of predefined targets that are more or less similar to the standard set of runlevels from the previous releases of this system. For compatibility reasons, it also provides aliases for these targets that directly map them to SysV runlevels. Table 3.6, “Comparison of SysV runlevels with systemd targets” provides a complete list of SysV runlevels and their corresponding systemd targets. Table 3.6. Comparison of SysV runlevels with systemd targets Runlevel Target Units Description 0 runlevel0.target, Shut down and power off the poweroff.target system. 1 runlevel1.target, Set up a rescue shell. rescue.target 2 runlevel2.target, multi- Set up a non-graphical multi-user user.target system. 65Red Hat Enterprise Linux 8 Configuring basic system settings Runlevel Target Units Description 3 runlevel3.target, multi- Set up a non-graphical multi-user user.target system. 4 runlevel4.target, multi- Set up a non-graphical multi-user user.target system. 5 runlevel5.target, Set up a graphical multi-user graphical.target system. 6 runlevel6.target, Shut down and reboot the system. reboot.target To view, change, or configure systemd targets, use the systemctl utility as described in Table 3.7, “Comparison of SysV init commands with systemctl” and in the sections below. The runlevel and telinit commands are still available in the system and work as expected, but are only included for compatibility reasons and should be avoided. Table 3.7. Comparison of SysV init commands with systemctl Old Command New Command Description runlevel systemctl list-units --type Lists currently loaded target units. target telinit runlevel systemctl isolate Changes the current target. name.target 3.3.1. Viewing the default target To determine which target unit is used by default, run the following command: systemctl get-default This command resolves the symbolic link located at /etc/systemd/system/default.target and displays the result. Example 3.10. Viewing the default target To display the default target unit, type: ~]$ systemctl get-default graphical.target 3.3.2. Viewing the current target To list all currently loaded target units, type the following command at a shell prompt: 66CHAPTER 3. MANAGING SERVICES WITH SYSTEMD systemctl list-units --type target For each target unit, this commands displays its full name (UNIT) followed by a note whether the unit has been loaded (LOAD), its high-level ( ACTIVE) and low-level (SUB) unit activation state, and a short description (DESCRIPTION). By default, the systemctl list-units command displays only active units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option: systemctl list-units --type target --all Example 3.11. Viewing the current target To list all currently loaded target units, run: ~]$ systemctl list-units --type target UNIT LOAD ACTIVE SUB DESCRIPTION basic.target loaded active active Basic System 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.target loaded active active Network paths.target loaded active active Paths remote-fs.target loaded active active Remote File Systems sockets.target loaded active active Sockets sound.target loaded active active Sound Card spice-vdagentd.target loaded active active Agent daemon for Spice guests swap.target loaded active active Swap sysinit.target loaded active active System Initialization time-sync.target loaded active active System Time Synchronized timers.target loaded active active Timers LOAD = Reflects whether the unit definition was properly loaded. ACTIVE = The high-level unit activation state, i.e. generalization of SUB. SUB = The low-level unit activation state, values depend on unit type. 17 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use ''systemctl list-unit-files''. 3.3.3. Changing the default target To configure the system to use a different target unit by default, type the following at a shell prompt as root: systemctl set-default name.target Replace name with the name of the target unit you want to use by default (for example, multi-user). This command replaces the /etc/systemd/system/default.target file with a symbolic link to /usr/lib/systemd/system/name.target, where name is the name of the target unit you want to use. 67Red Hat Enterprise Linux 8 Configuring basic system settings Example 3.12. Changing the default target To configure the system to use the multi-user.target unit by default, run the following command as root: ~]# 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'' 3.3.4. Changing the current target To change to a different target unit in the current session, type the following at a shell prompt as root: systemctl isolate name.target Replace name with the name of the target unit you want to use (for example, multi-user). This command starts the target unit named name and all dependent units, and immediately stops all others. Example 3.13. Changing the current target To turn off the graphical user interface and change to the multi-user.target unit in the current session, run the following command as root: ~]# systemctl isolate multi-user.target 3.3.5. Changing to rescue mode Rescue mode provides a convenient single-user environment and allows you to repair your system in situations when it is unable to complete a regular booting process. In rescue mode, the system attempts to mount all local file systems and start some important system services, but it does not activate network interfaces or allow more users to be logged into the system at the same time. Rescue mode requires the root password. To change the current target and enter rescue mode in the current session, type the following at a shell prompt as root: systemctl rescue This command is similar to systemctl isolate rescue.target, but it also sends an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the --no-wall command line option: systemctl --no-wall rescue For information on how to enter emergency mode, see Section 3.3.6, “Changing to emergency mode” . Example 3.14. Changing to rescue mode To enter rescue mode in the current session, run the following command as root: 68CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ~]# systemctl rescue Broadcast message from root@localhost on pts/0 (Fri 2013-10-25 18:23:15 CEST): The system is going down to rescue mode NOW! 3.3.6. Changing to emergency mode Emergency mode provides the most minimal environment possible and allows you to repair your system even in situations when the system is unable to enter rescue mode. In emergency mode, the system mounts the root file system only for reading, does not attempt to mount any other local file systems, does not activate network interfaces, and only starts a few essential services. Emergency mode requires the root password. To change the current target and enter emergency mode, type the following at a shell prompt as root: systemctl emergency This command is similar to systemctl isolate emergency.target, but it also sends an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the --no-wall command line option: systemctl --no-wall emergency For information on how to enter rescue mode, see Section 3.3.5, “Changing to rescue mode” . Example 3.15. Changing to emergency mode To enter emergency mode without sending a message to all users that are currently logged into the system, run the following command as root: ~]# systemctl --no-wall emergency 3.4. SHUTTING DOWN, SUSPENDING, AND HIBERNATING THE SYSTEM In Red Hat Enterprise Linux 7, the systemctl utility replaced a number of power management commands used in previous versions of Red Hat Enterprise Linux. The commands listed in Table 3.8, “Comparison of power management commands with systemctl” are still available in the system for compatibility reasons, but it is advised that you use systemctl when possible. Table 3.8. Comparison of power management commands with systemctl Old Command New Command Description halt systemctl halt Halts the system. poweroff systemctl poweroff Powers off the system. 69Red Hat Enterprise Linux 8 Configuring basic system settings Old Command New Command Description reboot systemctl reboot Restarts the system. pm-suspend systemctl suspend Suspends the system. pm-hibernate systemctl hibernate Hibernates the system. pm-suspend-hybrid systemctl hybrid-sleep Hibernates and suspends the system. 3.4.1. Shutting down the system The systemctl utility provides commands for shutting down the system, however the traditional shutdown command is also supported. Although the shutdown command will call the systemctl utility to perform the shutdown, it has an advantage in that it also supports a time argument. This is particularly useful for scheduled maintenance and to allow more time for users to react to the warning that a system shutdown has been scheduled. The option to cancel the shutdown can also be an advantage. Using systemctl commands To shut down the system and power off the machine, type the following at a shell prompt as root: systemctl poweroff To shut down and halt the system without powering off the machine, run the following command as root: systemctl halt By default, running either of these commands causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run the selected command with the --no-wall command line option, for example: systemctl --no-wall poweroff Using the shutdown command To shut down the system and power off the machine at a certain time, use a command in the following format as root: shutdown --poweroff hh:mm Where hh:mm is the time in 24 hour clock format. The /run/nologin file is created 5 minutes before system shutdown to prevent new logins. When a time argument is used, an optional message, the wall message, can be appended to the command. To shut down and halt the system after a delay, without powering off the machine, use a command in the following format as root: shutdown --halt +m 70CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Where +m is the delay time in minutes. The now keyword is an alias for +0. A pending shutdown can be canceled by the root user as follows: shutdown -c See the shutdown(8) manual page for further command options. 3.4.2. Restarting the system To restart the system, run the following command as root: systemctl reboot By default, this command causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the -- no-wall command line option: systemctl --no-wall reboot 3.4.3. Suspending the system To suspend the system, type the following at a shell prompt as root: systemctl suspend This command saves the system state in RAM and with the exception of the RAM module, powers off most of the devices in the machine. When you turn the machine back on, the system then restores its state from RAM without having to boot again. Because the system state is saved in RAM and not on the hard disk, restoring the system from suspend mode is significantly faster than restoring it from hibernation, but as a consequence, a suspended system state is also vulnerable to power outages. For information on how to hibernate the system, see Section 3.4.4, “Hibernating the system”. 3.4.4. Hibernating the system To hibernate the system, type the following at a shell prompt as root: systemctl hibernate This command saves the system state on the hard disk drive and powers off the machine. When you turn the machine back on, the system then restores its state from the saved data without having to boot again. Because the system state is saved on the hard disk and not in RAM, the machine does not have to maintain electrical power to the RAM module, but as a consequence, restoring the system from hibernation is significantly slower than restoring it from suspend mode. To hibernate and suspend the system, run the following command as root: systemctl hybrid-sleep For information on how to suspend the system, see Section 3.4.3, “Suspending the system”. 71Red Hat Enterprise Linux 8 Configuring basic system settings 3.5. WORKING WITH SYSTEMD UNIT FILES A unit file contains configuration directives that describe the unit and define its behavior. Several systemctl commands work with unit files in the background. To make finer adjustments, system administrator must edit or create unit files manually. Table 3.1, “Systemd unit files locations” lists three main directories where unit files are stored on the system, the /etc/systemd/system/ directory is reserved for unit files created or customized by the system administrator. Unit file names take the following form: unit_name.type_extension Here, unit_name stands for the name of the unit and type_extension identifies the unit type, see Table 3.2, “Available systemd unit types” for a complete list of unit types. For example, there usually is sshd.service as well as sshd.socket unit present on your system. Unit files can be supplemented with a directory for additional configuration files. For example, to add custom configuration options to sshd.service, create the sshd.service.d/custom.conf file and insert additional directives there. For more information on configuration directories, see Section 3.5.4, “Modifying existing unit files”. Also, the sshd.service.wants/ and sshd.service.requires/ directories can be created. These directories contain symbolic links to unit files that are dependencies of the sshd service. The symbolic links are automatically created either during installation according to [Install] unit file options or at runtime based on [Unit] options. It is also possible to create these directories and symbolic links manually. For more details on [Install] and [Unit] options, see the tables below. Many unit file options can be set using the so called unit specifiers – wildcard strings that are dynamically replaced with unit parameters when the unit file is loaded. This enables creation of generic unit files that serve as templates for generating instantiated units. See Section 3.5.5, “Working with instantiated units” for details. 3.5.1. Understanding the unit file structure Unit files typically consist of three sections: The [Unit] section — contains generic options that are not dependent on the type of the unit. These options provide unit description, specify the unit’s behavior, and set dependencies to other units. For a list of most frequently used [Unit] options, see Table 3.9, “Important [Unit] section options”. The [Unit type] section — if a unit has type-specific directives, these are grouped under a section named after the unit type. For example, service unit files contain the [Service] section. The [Install] section — contains information about unit installation used by systemctl enable and disable commands. For a list of options for the [Install] section, see Table 3.11, “Important [Install] section options”. Table 3.9. Important [Unit] section options Option[a] section, see the systemd.unit(5) Description manual page.] 72CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Option[a] section, see the systemd.unit(5) Description manual page.] Description A meaningful description of the unit. This text is displayed for example in the output of the systemctl status command. Documentation Provides a list of URIs referencing documentation for the unit. After[b] Defines the order in which units are started. The unit starts only after the units specified in After are active. Unlike Requires, After does not explicitly activate the specified units. The Before option has the opposite functionality to After. Requires Configures dependencies on other units. The units listed in Requires are activated together with the unit. If any of the required units fail to start, the unit is not activated. Wants Configures weaker dependencies than Requires. If any of the listed units does not start successfully, it has no impact on the unit activation. This is the recommended way to establish custom unit dependencies. Conflicts Configures negative dependencies, an opposite to Requires. [a] For a complete list of options configurable in the [Unit [b] In most cases, it is sufficient to set only the ordering dependencies with After and Before unit file options. If you also set a requirement dependency with Wants (recommended) or Requires, the ordering dependency still needs to be specified. That is because ordering and requirement dependencies work independently from each other. Table 3.10. Important [Service] section options Option[a] section, see the systemd.service(5) Description manual page.] 73Red Hat Enterprise Linux 8 Configuring basic system settings Option[a] section, see the systemd.service(5) Description manual page.] Type Configures the unit process startup type that affects the functionality of ExecStart and related options. One of: * simple – The default value. The process started with ExecStart is the main process of the service. * forking – The process started with ExecStart spawns a child process that becomes the main process of the service. The parent process exits when the startup is complete. * oneshot – This type is similar to simple, but the process exits before starting consequent units. * dbus – This type is similar to simple, but consequent units are started only after the main process gains a D-Bus name. * notify – This type is similar to simple, but consequent units are started only after a notification message is sent via the sd_notify() function. * idle – similar to simple, the actual execution of the service binary is delayed until all jobs are finished, which avoids mixing the status output with shell output of services. ExecStart Specifies commands or scripts to be executed when the unit is started. ExecStartPre and ExecStartPost specify custom commands to be executed before and after ExecStart. Type=oneshot enables specifying multiple custom commands that are then executed sequentially. ExecStop Specifies commands or scripts to be executed when the unit is stopped. ExecReload Specifies commands or scripts to be executed when the unit is reloaded. Restart With this option enabled, the service is restarted after its process exits, with the exception of a clean stop by the systemctl command. RemainAfterExit If set to True, the service is considered active even when all its processes exited. Default value is False. This option is especially useful if Type=oneshot is configured. 74CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Option[a] section, see the systemd.service(5) Description manual page.] [a] For a complete list of options configurable in the [Service Table 3.11. Important [Install] section options Option[a] section, see the systemd.unit(5) Description manual page.] Alias Provides a space-separated list of additional names for the unit. Most systemctl commands, excluding systemctl enable, can use aliases instead of the actual unit name. RequiredBy A list of units that depend on the unit. When this unit is enabled, the units listed in RequiredBy gain a Require dependency on the unit. WantedBy A list of units that weakly depend on the unit. When this unit is enabled, the units listed in WantedBy gain a Want dependency on the unit. Also Specifies a list of units to be installed or uninstalled along with the unit. DefaultInstance Limited to instantiated units, this option specifies the default instance for which the unit is enabled. See Section 3.5.5, “Working with instantiated units” [a] For a complete list of options configurable in the [Install A whole range of options that can be used to fine tune the unit configuration. The below example shows a service unit installed on the system. Moreover, unit file options can be defined in a way that enables dynamic creation of units as described in Working with instantiated units . Example 3.16. 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 75Red Hat Enterprise Linux 8 Configuring basic system settings 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 The [Unit] section describes the service, specifies the ordering dependencies, as well as conflicting units. In [Service], a sequence of custom scripts is specified to be executed during unit activation, on stop, and on reload. EnvironmentFile points to the location where environment variables for the service are defined, PIDFile specifies a stable PID for the main process of the service. Finally, the [Install] section lists units that depend on the service. 3.5.2. Creating custom unit files There are several use cases for creating unit files from scratch: you could run a custom daemon, create a second instance of some existing service (as in Creating a second instance of the sshd service ), or import a SysV init script (more in Converting SysV init scripts to unit files ). On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the instructions from Modifying existing unit files. The following procedure describes the general process of creating a custom service: 1. Prepare the executable file with the custom service. This can be a custom-created script, or an executable delivered by a software provider. If required, prepare a PID file to hold a constant PID for the main process of the custom service. It is also possible to include environment files to store shell variables for the service. Make sure the source script is executable (by executing the chmod a+x) and is not interactive. 2. Create a unit file in the /etc/systemd/system/ directory and make sure it has correct file permissions. Execute as root: touch /etc/systemd/system/name.service chmod 664 /etc/systemd/system/name.service Replace name with a name of the service to be created. Note that file does not need to be executable. 3. Open the name.service file created in the previous step, and add the service configuration options. There is a variety of options that can be used depending on the type of service you wish to create, see Section 3.5.1, “Understanding the unit file structure” . The following is an example unit configuration for a network-related service: [Unit] Description=service_description After=network.target [Service] ExecStart=path_to_executable Type=forking PIDFile=path_to_pidfile 76CHAPTER 3. MANAGING SERVICES WITH SYSTEMD [Install] WantedBy=default.target Where: service_description is an informative description that is displayed in journal log files and in the output of the systemctl status command. the After setting ensures that the service is started only after the network is running. Add a space-separated list of other relevant services or targets. path_to_executable stands for the path to the actual service executable. Type=forking is used for daemons that make the fork system call. The main process of the service is created with the PID specified in path_to_pidfile. Find other startup types in Table 3.10, “Important [Service] section options” . WantedBy states the target or targets that the service should be started under. Think of these targets as of a replacement of the older concept of runlevels, see Section 3.3, “Working with systemd targets” for details. 4. Notify systemd that a new name.service file exists by executing the following command as root: systemctl daemon-reload systemctl start name.service WARNING  Always run the systemctl daemon-reload command after creating new unit files or modifying existing unit files. Otherwise, the systemctl start or systemctl enable commands could fail due to a mismatch between states of systemd and actual service unit files on disk. Note, that on systems with a large number of units this can take a long time, as the state of each unit has to be serialized and subsequently deserialized during the reload. Example 3.17. Creating the emacs.service file When using the Emacs text editor, it is often faster and more convenient to have it running in the background instead of starting a new instance of the program whenever editing a file. The following steps show how to create a unit file for Emacs, so that it can be handled like a service. 1. Create a unit file in the /etc/systemd/system/ directory and make sure it has the correct file permissions. Execute as root: ~]# touch /etc/systemd/system/emacs.service ~]# chmod 664 /etc/systemd/system/emacs.service 77Red Hat Enterprise Linux 8 Configuring basic system settings 2. Add the following content to the file: [Unit] Description=Emacs: the extensible, self-documenting text editor [Service] Type=forking ExecStart=/usr/bin/emacs --daemon ExecStop=/usr/bin/emacsclient --eval "(kill-emacs)" Environment=SSH_AUTH_SOCK=%t/keyring/ssh Restart=always [Install] WantedBy=default.target With the above configuration, the /usr/bin/emacs executable is started in daemon mode on service start. The SSH_AUTH_SOCK environment variable is set using the "%t" unit specifier that stands for the runtime directory. The service also restarts the emacs process if it exits unexpectedly. 3. Execute the following commands to reload the configuration and start the custom service: ~]# systemctl daemon-reload ~]# systemctl start emacs.service As the editor is now registered as a systemd service, you can use all standard systemctl commands. For example, run systemctl status emacs to display the editor’s status or systemctl enable emacs to make the editor start automatically on system boot. Example 3.18. Creating a second instance of the sshd service System Administrators often need to configure and run multiple instances of a service. This is done by creating copies of the original service configuration files and modifying certain parameters to avoid conflicts with the primary instance of the service. The following procedure shows how to create a second instance of the sshd service: 1. Create a copy of the sshd_config file that will be used by the second daemon: ~]# cp /etc/ssh/sshd{,-second}_config 2. Edit the sshd-second_config file created in the previous step to assign a different port number and PID file to the second daemon: Port 22220 PidFile /var/run/sshd-second.pid See the sshd_config(5) manual page for more information on Port and PidFile options. Make sure the port you choose is not in use by any other service. The PID file does not have to exist before running the service, it is generated automatically on service start. 3. Create a copy of the systemd unit file for the sshd service: 78CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ~]# cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service 4. Alter the sshd-second.service created in the previous step as follows: a. Modify the Description option: Description=OpenSSH server second instance daemon b. Add sshd.service to services specified in the After option, so that the second instance starts only after the first one has already started: After=syslog.target network.target auditd.service sshd.service c. The first instance of sshd includes key generation, therefore remove the ExecStartPre=/usr/sbin/sshd-keygen line. d. Add the -f /etc/ssh/sshd-second_config parameter to the sshd command, so that the alternative configuration file is used: ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS e. After the above modifications, the sshd-second.service should look as follows: [Unit] Description=OpenSSH server second instance daemon After=syslog.target network.target auditd.service sshd.service [Service] EnvironmentFile=/etc/sysconfig/sshd ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS ExecReload=/bin/kill -HUP $MAINPID KillMode=process Restart=on-failure RestartSec=42s [Install] WantedBy=multi-user.target 5. If using SELinux, add the port for the second instance of sshd to SSH ports, otherwise the second instance of sshd will be rejected to bind to the port: ~]# semanage port -a -t ssh_port_t -p tcp 22220 6. Enable sshd-second.service, so that it starts automatically upon boot: ~]# systemctl enable sshd-second.service Verify if the sshd-second.service is running by using the systemctl status command. Also, verify if the port is enabled correctly by connecting to the service: ~]$ ssh -p 22220 user@server If the firewall is in use, make sure that it is configured appropriately in order to allow 79Red Hat Enterprise Linux 8 Configuring basic system settings If the firewall is in use, make sure that it is configured appropriately in order to allow connections to the second instance of sshd. To learn how to properly choose a target for ordering and dependencies of your custom unit files, see the following articles How to write a service unit file which enforces that particular services have to be started How to decide what dependencies a systemd service unit definition should have Additional information with some real-world examples of cases triggered by the ordering and dependencies in a unit file is available in Red Hat Knowledgebase article Is there any useful information about writing unit files? If you want to set limits for services started by systemd, see the Red Hat Knowledgebase article How to set limits for services in RHEL 7 and systemd. These limits need to be set in the service’s unit file. Note that systemd ignores limits set in the /etc/security/limits.conf and /etc/security/limits.d/*.conf configuration files. The limits defined in these files are set by PAM when starting a login session, but daemons started by systemd do not use PAM login sessions. 3.5.3. Converting SysV init scripts to unit files Before taking time to convert a SysV init script to a unit file, make sure that the conversion was not already done elsewhere. All core services installed on Red Hat Enterprise Linux come with default unit files, and the same applies for many third-party software packages. Converting an init script to a unit file requires analyzing the script and extracting the necessary information from it. Based on this data you can create a unit file. As init scripts can vary greatly depending on the type of the service, you might need to employ more configuration options for translation than outlined in this chapter. Note that some levels of customization that were available with init scripts are no longer supported by systemd units. The majority of information needed for conversion is provided in the script’s header. The following example shows the opening section of the init script used to start the postfix service on Red Hat Enterprise Linux 6: !/bin/bash # postfix Postfix Mail Transfer Agent # chkconfig: 2345 80 30 # description: Postfix is a Mail Transport Agent, which is the program that moves mail from one machine to another. # processname: master # pidfile: /var/spool/postfix/pid/master.pid # config: /etc/postfix/main.cf # config: /etc/postfix/master.cf BEGIN INIT INFO # Provides: postfix MTA # Required-Start: $local_fs $network $remote_fs # Required-Stop: $local_fs $network $remote_fs # Default-Start: 2 3 4 5 # Default-Stop: 0 1 6 # Short-Description: start and stop postfix # Description: Postfix is a Mail Transport Agent, which is the program that moves mail from one machine to another. # END INIT INFO In the above example, only lines starting with # chkconfig and # description are mandatory, so you might not find the rest in different init files. The text enclosed between the BEGIN INIT INFO and END INIT INFO lines is called Linux Standard Base (LSB) header. If specified, LSB headers contain directives defining the service description, dependencies, and default runlevels. What follows is an overview of analytic tasks aiming to collect the data needed for a new unit file. The postfix init script is used as an example, see the resulting postfix unit file in Example 3.16, “postfix.service unit file” . Finding the service description Find descriptive information about the script on the line starting with #description. Use this description 80CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Find descriptive information about the script on the line starting with #description. Use this description together with the service name in the Description option in the [Unit] section of the unit file. The LSB header might contain similar data on the #Short-Description and #Description lines. Finding service dependencies The LSB header might contain several directives that form dependencies between services. Most of them are translatable to systemd unit options, see Table 3.12, “Dependency options from the LSB header” Table 3.12. Dependency options from the LSB header LSB Option Description Unit File Equivalent Provides Specifies the boot facility name – of the service, that can be referenced in other init scripts (with the "$" prefix). This is no longer needed as unit files refer to other units by their file names. Required-Start Contains boot facility names of After, Before required services. This is translated as an ordering dependency, boot facility names are replaced with unit file names of corresponding services or targets they belong to. For example, in case of postfix, the Required-Start dependency on $network was translated to the After dependency on network.target. Should-Start Constitutes weaker dependencies After, Before than Required-Start. Failed Should-Start dependencies do not affect the service startup. Required-Stop, Should-Stop Constitute negative Conflicts dependencies. Finding default targets of the service The line starting with #chkconfig contains three numerical values. The most important is the first number that represents the default runlevels in which the service is started. Map these runlevels to equivalent systemd targets. Then list these targets in the WantedBy option in the [Install] section of the unit file. For example, postfix was previously started in runlevels 2, 3, 4, and 5, which translates to multi- user.target and graphical.target. Note that the graphical.target depends on multiuser.target, therefore it is not necessary to specify both, as in Example 3.16, “postfix.service unit file” . You might find information on default and forbidden runlevels also at #Default-Start and #Default-Stop lines in the LSB header. The other two values specified on the #chkconfig line represent startup and shutdown priorities of the init script. These values are interpreted by systemd if it loads the init script, but there is no unit file equivalent. 81Red Hat Enterprise Linux 8 Configuring basic system settings Finding files used by the service Init scripts require loading a function library from a dedicated directory and allow importing configuration, environment, and PID files. Environment variables are specified on the line starting with #config in the init script header, which translates to the EnvironmentFile unit file option. The PID file specified on the #pidfile init script line is imported to the unit file with the PIDFile option. The key information that is not included in the init script header is the path to the service executable, and potentially some other files required by the service. In previous versions of Red Hat Enterprise Linux, init scripts used a Bash case statement to define the behavior of the service on default actions, such as start, stop, or restart, as well as custom-defined actions. The following excerpt from the postfix init script shows the block of code to be executed at service start. conf_check() { [ -x /usr/sbin/postfix ] || exit 5 [ -d /etc/postfix ] || exit 6 [ -d /var/spool/postfix ] || exit 5 } make_aliasesdb() { if [ "$(/usr/sbin/postconf -h alias_database)" == "hash:/etc/aliases" ] then # /etc/aliases.db might be used by other MTA, make sure nothing # has touched it since our last newaliases call [ /etc/aliases -nt /etc/aliases.db ] || [ "$ALIASESDB_STAMP" -nt /etc/aliases.db ] || [ "$ALIASESDB_STAMP" -ot /etc/aliases.db ] || return /usr/bin/newaliases touch -r /etc/aliases.db "$ALIASESDB_STAMP" else /usr/bin/newaliases fi } start() { [ "$EUID" != "0" ] && exit 4 # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 1 conf_check # Start daemons. echo -n $"Starting postfix: " make_aliasesdb >/dev/null 2>&1 [ -x $CHROOT_UPDATE ] && $CHROOT_UPDATE /usr/sbin/postfix start 2>/dev/null 1>&2 && success || failure $"$prog start" RETVAL=$? [ $RETVAL -eq 0 ] && touch $lockfile echo return $RETVAL } The extensibility of the init script allowed specifying two custom functions, conf_check() and make_aliasesdb(), that are called from the start() function block. On closer look, several external files and directories are mentioned in the above code: the main service executable /usr/sbin/postfix, the /etc/postfix/ and /var/spool/postfix/ configuration directories, as well as the /usr/sbin/postconf/ directory. Systemd supports only the predefined actions, but enables executing custom executables with 82CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ExecStart, ExecStartPre, ExecStartPost, ExecStop, and ExecReload options. The /usr/sbin/postfix together with supporting scripts are executed on service start. Consult the postfix unit file at Example 3.16, “postfix.service unit file” . Converting complex init scripts requires understanding the purpose of every statement in the script. Some of the statements are specific to the operating system version, therefore you do not need to translate them. On the other hand, some adjustments might be needed in the new environment, both in unit file as well as in the service executable and supporting files. 3.5.4. Modifying existing unit files Services installed on the system come with default unit files that are stored in the /usr/lib/systemd/system/ directory. System Administrators should not modify these files directly, therefore any customization must be confined to configuration files in the /etc/systemd/system/ directory. Depending on the extent of the required changes, pick one of the following approaches: Create a directory for supplementary configuration files at /etc/systemd/system/unit.d/. This method is recommended for most use cases. It enables extending the default configuration with additional functionality, while still referring to the original unit file. Changes to the default unit introduced with a package upgrade are therefore applied automatically. See Extending the default unit configuration for more information. Create a copy of the original unit file /usr/lib/systemd/system/ in /etc/systemd/system/ and make changes there. The copy overrides the original file, therefore changes introduced with the package update are not applied. This method is useful for making significant unit changes that should persist regardless of package updates. See Overriding the default unit configuration for details. In order to return to the default configuration of the unit, just delete custom-created configuration files in /etc/systemd/system/. To apply changes to unit files without rebooting the system, execute: systemctl daemon-reload The daemon-reload option reloads all unit files and recreates the entire dependency tree, which is needed to immediately apply any change to a unit file. As an alternative, you can achieve the same result with the following command, which must be executed under the root user: init q Also, if the modified unit file belongs to a running service, this service must be restarted to accept new settings: systemctl restart name.service IMPORTANT 83Red Hat Enterprise Linux 8 Configuring basic system settings IMPORTANT To modify properties, such as dependencies or timeouts, of a service that is handled by a SysV initscript, do not modify the initscript itself. Instead, create a systemd drop-in configuration file for the service as described in Extending the default unit configuration and Overriding the default unit configuration . Then manage this service in the same way as a normal systemd service. For example, to extend the configuration of the network service, do not modify the /etc/rc.d/init.d/network initscript file. Instead, create new directory /etc/systemd/system/network.service.d/ and a systemd drop-in file /etc/systemd/system/network.service.d/my_config.conf. Then, put the modified values into the drop-in file. Note: systemd knows the network service as network.service, which is why the created directory must be called network.service.d Extending the default unit configuration To extend the default unit file with additional configuration options, first create a configuration directory in /etc/systemd/system/. If extending a service unit, execute the following command as root: mkdir /etc/systemd/system/name.service.d/ Replace name with the name of the service you want to extend. The above syntax applies to all unit types. Create a configuration file in the directory made in the previous step. Note that the file name must end with the .conf suffix. Type: touch /etc/systemd/system/name.service.d/config_name.conf Replace config_name with the name of the configuration file. This file adheres to the normal unit file structure, therefore all directives must be specified under appropriate sections, see Section 3.5.1, “Understanding the unit file structure”. For example, to add a custom dependency, create a configuration file with the following content: [Unit] Requires=new_dependency After=new_dependency Where new_dependency stands for the unit to be marked as a dependency. Another example is a configuration file that restarts the service after its main process exited, with a delay of 30 seconds: [Service] Restart=always RestartSec=30 It is recommended to create small configuration files focused only on one task. Such files can be easily moved or linked to configuration directories of other services. To apply changes made to the unit, execute as root: systemctl daemon-reload systemctl restart name.service 84CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Example 3.19. Extending the httpd.service configuration To modify the httpd.service unit so that a custom shell script is automatically executed when starting the Apache service, perform the following steps. First, create a directory and a custom configuration file: ~]# mkdir /etc/systemd/system/httpd.service.d/ ~]# touch /etc/systemd/system/httpd.service.d/custom_script.conf Provided that the script you want to start automatically with Apache is located at /usr/local/bin/custom.sh, insert the following text to the custom_script.conf file: [Service] ExecStartPost=/usr/local/bin/custom.sh To apply the unit changes, execute: ~]# systemctl daemon-reload ~]# systemctl restart httpd.service NOTE The configuration files from configuration directories in /etc/systemd/system/ take precedence over unit files in /usr/lib/systemd/system/. Therefore, if the configuration files contain an option that can be specified only once, such as Description or ExecStart, the default value of this option is overridden. Note that in the output of the systemd- delta command, described in Monitoring overriden units , such units are always marked as [EXTENDED], even though in sum, certain options are actually overridden. Overriding the default unit configuration To make changes that will persist after updating the package that provides the unit file, first copy the file to the /etc/systemd/system/ directory. To do so, execute the following command as root: cp /usr/lib/systemd/system/name.service /etc/systemd/system/name.service Where name stands for the name of the service unit you wish to modify. The above syntax applies to all unit types. Open the copied file with a text editor, and make the desired changes. To apply the unit changes, execute as root: systemctl daemon-reload systemctl restart name.service Example 3.20. Changing the timeout limit You can specify a timeout value per service to prevent a malfunctioning service from freezing the 85Red Hat Enterprise Linux 8 Configuring basic system settings You can specify a timeout value per service to prevent a malfunctioning service from freezing the system. Otherwise, timeout is set by default to 90 seconds for normal services and to 300 seconds for SysV-compatible services. For example, to extend timeout limit for the httpd service: 1. Copy the httpd unit file to the /etc/systemd/system/ directory: cp /usr/lib/systemd/system/httpd.service /etc/systemd/system/httpd.service 2. Open file /etc/systemd/system/httpd.service and specify the TimeoutStartUSec value in the [Service] section: … ​ [Service] … ​ PrivateTmp=true TimeoutStartSec=10 [Install] WantedBy=multi-user.target … ​ 3. Reload the systemd daemon: systemctl daemon-reload 4. Optional. Verify the new timeout value: systemctl show httpd -p TimeoutStartUSec NOTE To change the timeout limit globally, input the DefaultTimeoutStartSec in the /etc/systemd/system.conf file. Monitoring overriden units To display an overview of overridden or modified unit files, use the following command: systemd-delta For example, the output of the above command can look as follows: [EQUIVALENT] /etc/systemd/system/default.target → /usr/lib/systemd/system/default.target [OVERRIDDEN] /etc/systemd/system/autofs.service → /usr/lib/systemd/system/autofs.service --- /usr/lib/systemd/system/autofs.service 2014-10-16 21:30:39.000000000 -0400 + /etc/systemd/system/autofs.service 2014-11-21 10:00:58.513568275 -0500 @@ -8,7 +8,8 @@ EnvironmentFile=-/etc/sysconfig/autofs ExecStart=/usr/sbin/automount $OPTIONS --pid-file /run/autofs.pid ExecReload=/usr/bin/kill -HUP $MAINPID 86CHAPTER 3. MANAGING SERVICES WITH SYSTEMD -TimeoutSec=180 +TimeoutSec=240 +Restart=Always [Install] WantedBy=multi-user.target [MASKED] /etc/systemd/system/cups.service → /usr/lib/systemd/system/cups.service [EXTENDED] /usr/lib/systemd/system/sssd.service → /etc/systemd/system/sssd.service.d/journal.conf 4 overridden configuration files found. Table 3.13, “systemd-delta difference types” lists override types that can appear in the output of systemd-delta. Note that if a file is overridden, systemd-delta by default displays a summary of changes similar to the output of the diff command. Table 3.13. systemd-delta difference types Type Description [MASKED] Masked unit files, see Section 3.2.7, “Disabling a service” for description of unit masking. [EQUIVALENT] Unmodified copies that override the original files but do not differ in content, typically symbolic links. [REDIRECTED] Files that are redirected to another file. [OVERRIDEN] Overridden and changed files. [EXTENDED] Files that are extended with .conf files in the /etc/systemd/system/unit.d/ directory. [UNCHANGED] Unmodified files are displayed only when the -- type=unchanged option is used. It is good practice to run systemd-delta after system update to check if there are any updates to the default units that are currently overridden by custom configuration. It is also possible to limit the output only to a certain difference type. For example, to view just the overridden units, execute: systemd-delta --type=overridden If you want to edit a unit file and automatically create a drop-in file with the submitted changes, use the following command: ~]# systemctl edit unit_name.type_extension To dump the unit configuration applying all overrides and drop-ins, use this command: ~]# systemctl cat unit_name.type_extension 87Red Hat Enterprise Linux 8 Configuring basic system settings Replace the unit_name.type_extension by the name of the required unit and its type, for example tuned.service. 3.5.5. Working with instantiated units It is possible to instantiate multiple units from a single template configuration file at runtime. The "@" character is used to mark the template and to associate units with it. Instantiated units can be started from another unit file (using Requires or Wants options), or with the systemctl start command. Instantiated service units are named the following way: template_name@instance_name.service Where template_name stands for the name of the template configuration file. Replace instance_name with the name for the unit instance. Several instances can point to the same template file with configuration options common for all instances of the unit. Template unit name has the form of: unit_name@.service For example, the following Wants setting in a unit file: Wants=getty@ttyA.service getty@ttyB.service first makes systemd search for given service units. If no such units are found, the part between "@" and the type suffix is ignored and systemd searches for the getty@.service file, reads the configuration from it, and starts the services. Wildcard characters, called unit specifiers, can be used in any unit configuration file. Unit specifiers substitute certain unit parameters and are interpreted at runtime. Table 3.14, “Important unit specifiers” lists unit specifiers that are particularly useful for template units. Table 3.14. Important unit specifiers Unit Specifier Meaning Description %n Full unit name Stands for the full unit name including the type suffix. %N has the same meaning but also replaces the forbidden characters with ASCII codes. %p Prefix name Stands for a unit name with type suffix removed. For instantiated units %p stands for the part of the unit name before the "@" character. %i Instance name Is the part of the instantiated unit name between the "@" character and the type suffix. %I has the same meaning but also replaces the forbidden characters for ASCII codes. 88CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Unit Specifier Meaning Description %H Host name Stands for the hostname of the running system at the point in time the unit configuration is loaded. %t Runtime directory Represents the runtime directory, which is either /run for the root user, or the value of the XDG_RUNTIME_DIR variable for unprivileged users. For a complete list of unit specifiers, see the systemd.unit(5) manual page. For example, the getty@.service template contains the following directives: [Unit] Description=Getty on %I … ​ [Service] ExecStart=-/sbin/agetty --noclear %I $TERM … ​ When the getty@ttyA.service and getty@ttyB.service are instantiated form the above template, Description= is resolved as Getty on ttyA and Getty on ttyB. 3.6. OPTIMIZING SYSTEMD TO SHORTEN THE BOOT TIME There is a list of systemd unit files that are enabled by default. System services that are defined by these unit files are automatically run at boot, which influences the boot time. This section describes: The tools to examine system boot performance. The purpose of systemd units enabled by default, and circumstances under which you can safely disable such systemd units in order to shorten the boot time. 3.6.1. Examining system boot performance To examine system boot performance, you can use the systemd-analyze command. This command has many options available. However, this section covers only the selected ones that may be important for systemd tuning in order to shorten the boot time. For a complete list and detailed description of all options, see the systemd-analyze man page. Prerequisites Before starting to examine systemd in order to tune the boot time, you may want to list all enabled services: ~]$ systemctl list-unit-files --state=enabled 89Red Hat Enterprise Linux 8 Configuring basic system settings Analyzing overall boot time Procedure For the overall information about the time that the last successful boot took, use: ~]$ systemd-analyze Analyzing unit initialization time Procedure For the information about the initialization time of each systemd unit, use: ~]$ systemd-analyze blame The output lists the units in descending order according to the time they took to initialize during the last successful boot. Identifying critical units Procedure To identify the units that took most time to initialize at the last successful boot, use: ~]$ systemd-analyze critical-chain The output highlights the units that critically slow down the boot with the red color. Figure 3.1. The output of the systemd-analyze critical-chain command 3.6.2. A guide to selecting services that can be safely disabled If you find the boot time of your system long, you can shorten it by disabling some of the services enabled on boot by default. To list such services, run: ~]$ systemctl list-unit-files --state=enabled To disable a service, run: 90CHAPTER 3. MANAGING SERVICES WITH SYSTEMD ~]# systemctl disable service_name However, certain services must stay enabled in order that your operating system is safe and functions in the way you need. You can use the table below as a guide to selecting the services that you can safely disable. The table lists all services enabled by default on a minimal installation of Red Hat Enterprise Linux 8, and for each service it states whether this service can be safely disabled. The table also provides more information about the circumstances under which the service can be disabled, or the reason why you should not disable the service. Table 3.15. Services enabled by default on a minimal installation of RHEL 8 Service Can it be disabled? More information name auditd.servic yes Disable auditd.service only if you do not need audit messages e from the kernel. Be aware that if you disable auditd.service, the /var/log/audit/audit.log file is not produced. Consequently, you are not able to retroactively review some commonly-reviewed actions or events, such as user logins, service starts or password changes. Also note that auditd has two parts: a kernel part, and a service itself. By using the systemctl disable auditd command, you only disable the service, but not the kernel part. To disable system auditing in its entirety, set audit=0 on kernel command line. autovt@.servi no This service runs only when it is really needed, so it does not need ce to be disabled. crond.service yes Be aware that no items from crontab will run if you disable crond.service. dbus- yes A symlink to firewalld.service org.fedorapr oject.Firewall D1.service dbus- yes A symlink to NetworkManager.service org.freedeskt op.NetworkM anager.servic e dbus- yes A symlink to NetworkManager-dispatcher.service org.freedeskt op.nm- dispatcher.se rvice firewalld.servi yes Disable firewalld.service only if you do not need firewall. ce 91Red Hat Enterprise Linux 8 Configuring basic system settings Service Can it be disabled? More information name getty@.servic no This service runs only when it is really needed, so it does not need e to be disabled. import- yes Disable import-state.service only if you do not need to boot state.service from a network storage. irqbalance.se yes Disable irqbalance.service only if you have just one CPU. Do not rvice disable irqbalance.service on systems with multiple CPUs. kdump.servic yes Disable kdump.service only if you do not need reports from e kernel crashes. loadmodules. yes This service is not started unless the /etc/rc.modules or service /etc/sysconfig/modules directory exists, which means that it is not started on a minimal RHEL 8 installation. lvm2- yes Disable lvm2-monitor.service only if you do not use Logical monitor.servi Volume Manager (LVM). ce microcode.se no Do not be disable the service because it provides updates of the rvice microcode software in CPU. NetworkMan yes Disable NetworkManager-dispatcher.service only if you do ager- not need notifications on network configuration changes (for dispatcher.se example in static networks). rvice NetworkMan yes Disable NetworkManager-wait-online.service only if you do ager-wait- not need working network connection available right after the online.service boot. If the service is enabled, the system does not finish the boot before the network connection is working. This may prolong the boot time significantly. NetworkMan yes Disable NetworkManager.service only if you do not need ager.service connection to a network. nis- yes Disable nis-domainname.service only if you do not use domainname. Network Information Service (NIS). service rhsmcertd.se no rvice 92CHAPTER 3. MANAGING SERVICES WITH SYSTEMD Service Can it be disabled? More information name rngd.service yes Disable rngd.service only if you do not need a lot of entropy on your system, or you do not have any sort of hardware generator. Note that the service is necessary in environments that require a lot of good entropy, such as systems used for generation of X.509 certificates (for example the FreeIPA server). rsyslog.servic yes Disable rsyslog.service only if you do not need persistent logs, e or you set systemd-journald to persistent mode. selinux- yes Disable selinux-autorelabel-mark.service only if you do not autorelabel- use SELinux. mark.service sshd.service yes Disable sshd.service only if you do not need remote logins by OpenSSH server. sssd.service yes Disable sssd.service only if there are no users who log in the system over the network (for example by using LDAP or Kerberos). Red Hat recommends to disable all sssd-* units if you disable sssd.service. syslog.servic yes An alias for rsyslog.service e tuned.service yes Disable tuned.service only if you do need to use performance tuning. lvm2- yes Disable lvm2-lvmpolld.socket only if you do not use Logical lvmpolld.sock Volume Manager (LVM). et dnf- yes Disable dnf-makecache.timer only if you do not need your makecache.ti package metadata to be updated automatically. mer unbound- yes Disable unbound-anchor.timer only if you do not need daily anchor.timer update of the root trust anchor for DNS Security Extensions (DNSSEC). This root trust anchor is used by Unbound resolver and resolver library for DNSSEC validation. To find more information about a service, you can run one of the following commands: ~]$ systemctl cat ~]$ systemctl help The systemctl cat command provides the content of the service file located under 93Red Hat Enterprise Linux 8 Configuring basic system settings The systemctl cat command provides the content of the service file located under /usr/lib/systemd/system/, as well as all applicable overrides. The applicable overrides include unit file overrides from the /etc/systemd/system/ file or drop-in files from a corresponding unit.type.d directory. For more information on drop-in files, see the systemd.unit man page. The systemctl help command shows the man page of the particular service. 3.7. ADDITIONAL RESOURCES For more information on systemd and its usage on Red Hat Enterprise Linux, see the resources listed below. 3.7.1. Installed Documentation systemctl(1) — The manual page for the systemctl command line utility provides a complete list of supported options and commands. systemd(1) — The manual page for the systemd system and service manager provides more information about its concepts and documents available command line options and environment variables, supported configuration files and directories, recognized signals, and available kernel options. systemd-delta(1) — The manual page for the systemd-delta utility that allows to find extended and overridden configuration files. systemd.directives(7) — The manual page named systemd.directives provides detailed information about systemd directives. systemd.unit(5) — The manual page named systemd.unit provides detailed information about systemd unit files and documents all available configuration options. systemd.service(5) — The manual page named systemd.service documents the format of service unit files. systemd.target(5) — The manual page named systemd.target documents the format of target unit files. systemd.kill(5) — The manual page named systemd.kill documents the configuration of the process killing procedure. 3.7.2. Online Documentation systemd Home Page — The project home page provides more information about systemd. 94CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES The control of users and groups is a core element of Red Hat Enterprise Linux system administration. This section explains how to add, manage, and delete users and groups in the graphical user interface and on the command line, and covers advanced topics, such as creating group directories. 4.1. INTRODUCTION TO USERS AND GROUPS While users can be either people (meaning accounts tied to physical users) or accounts that exist for specific applications to use, groups are logical expressions of organization, tying users together for a common purpose. Users within a group share the same permissions to read, write, or execute files owned by that group. Each user is associated with a unique numerical identification number called a user ID (UID). Likewise, each group is associated with a group ID (GID). A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file owner can be changed only by root, and access permissions can be changed by both the root user and file owner. 4.2. RESERVED USER AND GROUP IDS Red Hat Enterprise Linux reserves user and group IDs below 1000 for system users and groups. By default, the User Manager does not display the system users. Reserved user and group IDs are documented in the setup package. To view the documentation, use this command: cat /usr/share/doc/setup*/uidgid The recommended practice is to assign IDs starting at 5,000 that were not already reserved, as the reserved range can increase in the future. To make the IDs assigned to new users by default start at 5,000, change the UID_MIN and GID_MIN directives in the /etc/login.defs file: [file contents truncated] UID_MIN 5000 [file contents truncated] GID_MIN 5000 [file contents truncated] NOTE For users created before you changed UID_MIN and GID_MIN directives, UIDs will still start at the default 1000. Even with new user and group IDs beginning with 5,000, it is recommended not to raise IDs reserved by the system above 1000 to avoid conflict with systems that retain the 1000 limit. 4.3. USER PRIVATE GROUPS Red Hat Enterprise Linux uses a user private group (UPG) scheme, which makes UNIX groups easier to manage. A user private group is created whenever a new user is added to the system. It has the same name as the user for which it was created and that user is the only member of the user private group. 95Red Hat Enterprise Linux 8 Configuring basic system settings User private groups make it safe to set default permissions for a newly created file or directory, allowing both the user and the group of that user to make modifications to the file or directory. The setting which determines what permissions are applied to a newly created file or directory is called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX-based systems, the umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator’s group, are not allowed to make any modifications. However, under the UPG scheme, this "group protection" is not necessary since every user has their own private group. A list of all groups is stored in the /etc/group configuration file. 4.4. SHADOW PASSWORDS In environments with multiple users, it is very important to use shadow passwords provided by the shadow-utils package to enhance the security of system authentication files. For this reason, the installation program enables shadow passwords by default. The advantages of shadow passwords over the traditional way of storing passwords on UNIX-based systems are: Shadow passwords improve system security by moving encrypted password hashes from the world-readable /etc/passwd file to /etc/shadow, which is readable only by the root user. Shadow passwords store information about password aging. Shadow passwords allow to enforce some of the security policies set in the /etc/login.defs file. Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /etc/shadow file, some utilities and commands do not work without first enabling shadow passwords: The chage utility for setting password aging parameters. The gpasswd utility for administrating the /etc/group file. The usermod command with the -e, --expiredate or -f, --inactive option. The useradd command with the -e, --expiredate or -f, --inactive option. 4.5. MANAGING USERS IN A GRAPHICAL ENVIRONMENT The Users utility allows you to view, modify, add, and delete local users in the graphical user interface. 4.5.1. Using the Users Settings tool Press the Super key to enter the Activities Overview, type Users and then press Enter. The Users settings tool appears. The Super key appears in a variety of options, depending on the keyboard and other hardware, but often as either the Windows or Command key, and typically to the left of the Space bar. Alternatively, you can open the Users utility from the Settings menu after clicking your user name in the top right corner of the screen. To make changes to the user accounts, first select the Unlock button and authenticate yourself as indicated by the dialog box that appears. Note that unless you have superuser privileges, the application will prompt you to authenticate as root. To add and remove users, select the + and - button respectively. 96CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES To add a user to the administrative group wheel, change the Account Type from Standard to Administrator. To edit a user’s language setting, select the language and a drop-down menu appears. Figure 4.1. The Users Settings Tool When a new user is created, the account is disabled until a password is set. The Password drop-down menu, shown in Figure 4.2, “The Password Menu” , contains the options to set a password by the administrator immediately, choose a password by the user at the first login, or create a guest account with no password required to log in. You can also disable or enable an account from this menu. Figure 4.2. The Password Menu 97Red Hat Enterprise Linux 8 Configuring basic system settings Figure 4.2. The Password Menu 4.6. MANAGING USERS USING COMMAND-LINE TOOLS Apart from the Users settings tool described in Section 4.5.1, “Using the Users Settings tool” , which is designed for basic managing of users, you can use command line tools for managing users and groups that are listed in the table below. Table 4.1. Command line utilities for managing users and groups Utilities Description id Displays user and group IDs. useradd, usermod, userdel Standard utilities for adding, modifying, and deleting user accounts. groupadd, groupmod, groupdel Standard utilities for adding, modifying, and deleting groups. gpasswd Utility primarily used for modification of group password in the /etc/gshadow file which is used by the newgrp command. pwck, grpck Utilities that can be used for verification of the password, group, and associated shadow files. 98CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES Utilities Description pwconv, pwunconv Utilities that can be used for the conversion of passwords to shadow passwords, or back from shadow passwords to standard passwords. grpconv, grpunconv Similar to the previous, these utilities can be used for conversion of shadowed information for group accounts. 4.6.1. Adding a new user To add a new user to the system, type the following at a shell prompt as root: useradd options username Here options are command-line options as described in Table 4.2, “Common useradd command-line options”. WARNING  With RHEL 8, you cannot use all-numeric user names. The reason for not allowing such names is that this can confuse tools that work with user names and user ids, which are numbers. By default, the useradd command creates a locked user account. To unlock the account, run the following command as root to assign a password: passwd username Table 4.2. Common useradd command-line options Option -c ''comment'' comment can be replaced with any string. This option is generally used to specify the full name of a user. -d home_directory Home directory to be used instead of default /home/username/. -e date Date for the account to be disabled in the format YYYY-MM-DD. 99Red Hat Enterprise Linux 8 Configuring basic system settings Option -f days Number of days after the password expires until the account is disabled. If 0 is specified, the account is disabled immediately after the password expires. If - 1 is specified, the account is not disabled after the password expires. -g group_name Group name or group number for the user’s default (primary) group. The group must exist prior to being specified here. -G group_list List of additional (supplementary, other than default) group names or group numbers, separated by commas, of which the user is a member. The groups must exist prior to being specified here. -m Create the home directory if it does not exist. -M Do not create the home directory. -N Do not create a user private group for the user. -p password The password encrypted with crypt. -r Create a system account with a UID less than 1000 and without a home directory. -s User’s login shell, which defaults to /bin/bash. -u uid User ID for the user, which must be unique and greater than 999. IMPORTANT The default range of IDs for system and normal users has been changed in Red Hat Enterprise Linux 7 from earlier releases. Before Red Hat Enterprise Linux 7, UID 1-499 was used for system users and values above for normal users. The default range for system users is now 1-999. This change might cause problems when migrating to Red Hat Enterprise Linux 8 with existing users having UIDs and GIDs between 500 and 999. The default ranges of UID and GID can be changed in the /etc/login.defs file. The following steps illustrate what happens if the command useradd juan is issued on a system that has shadow passwords enabled: 1. A new line for juan is created in /etc/passwd: juan:x:1001:1001::/home/juan:/bin/bash The line has the following characteristics: 100CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES It begins with the user name juan. There is an x for the password field indicating that the system is using shadow passwords. A UID greater than 999 is created. Under Red Hat Enterprise Linux 7, UIDs below 1000 are reserved for system use and should not be assigned to users. A GID greater than 999 is created. Under Red Hat Enterprise Linux 7, GIDs below 1000 are reserved for system use and should not be assigned to users. The optional GECOS information is left blank. The GECOS field can be used to provide additional information about the user, such as their full name or phone number. The home directory for juan is set to /home/juan/. The default shell is set to /bin/bash. 2. A new line for juan is created in /etc/shadow: juan:!!:14798:0:99999:7::: The line has the following characteristics: It begins with the user name juan. Two exclamation marks (!!) appear in the password field of the /etc/shadow file, which locks the account. NOTE If an encrypted password is passed using the -p flag, it is placed in the /etc/shadow file on the new line for the user. The password is set to never expire. 3. A new line for a group named juan is created in /etc/group: juan:x:1001: A group with the same name as a user is called a user private group . For more information on user private groups, see Section 4.3, “User private groups”. The line created in /etc/group has the following characteristics: It begins with the group name juan. An x appears in the password field indicating that the system is using shadow group passwords. The GID matches the one listed for juan''s primary group in /etc/passwd. 4. A new line for a group named juan is created in /etc/gshadow: juan:!:: The line has the following characteristics: 101Red Hat Enterprise Linux 8 Configuring basic system settings It begins with the group name juan. An exclamation mark (!) appears in the password field of the /etc/gshadow file, which locks the group. All other fields are blank. 5. A directory for user juan is created in the /home directory: ~]# ls -ld /home/juan drwx------. 4 juan juan 4096 Mar 3 18:23 /home/juan This directory is owned by user juan and group juan. It has read, write, and execute privileges only for the user juan. All other permissions are denied. 6. The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory: ~]# ls -la /home/juan total 28 drwx------. 4 juan juan 4096 Mar 3 18:23 . drwxr-xr-x. 5 root root 4096 Mar 3 18:23 .. -rw-r— ​r--. 1 juan juan 18 Jun 22 2010 .bash_logout -rw-r— ​r--. 1 juan juan 176 Jun 22 2010 .bash_profile -rw-r— ​r--. 1 juan juan 124 Jun 22 2010 .bashrc drwxr-xr-x. 4 juan juan 4096 Nov 23 15:09 .mozilla At this point, a locked account called juan exists on the system. To activate it, the administrator must next assign a password to the account using the passwd command and, optionally, set password aging guidelines. 4.6.2. Adding a new group To add a new group to the system, type the following at a shell prompt as root: groupadd options group_name Here options are command-line options as described in Table 4.3, “Common groupadd command-line options”. WARNING  With RHEL 8, you cannot use all-numeric group names. The reason for not allowing such names is that this can confuse tools that work with group names and group ids, which are numbers. Table 4.3. Common groupadd command-line options 102CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES Option Description -f, --force When used with -g gid and gid already exists, groupadd will choose another unique gid for the group. -g gid Group ID for the group, which must be unique and greater than 999. -K, --key key=value Override /etc/login.defs defaults. -o, --non-unique Allows creating groups with duplicate GID. -p, --password password Use this encrypted password for the new group. -r Create a system group with a GID less than 1000. 4.6.3. Adding an existing user to an existing group Use the usermod utility to add an already existing user to an already existing group. Various options of usermod have different impact on user’s primary group and on his or her supplementary groups. To override user’s primary group, run the following command as root: usermod -g group_name user_name To override user’s supplementary groups, run the following command as root: usermod -G group_name1,group_name2,… ​ user_name Note that in this case all previous supplementary groups of the user are replaced by the new group or several new groups. To add one or more groups to user’s supplementary groups, run one of the following commands as root: usermod -aG group_name1,group_name2,… ​ user_name usermod --append -G group_name1,group_name2,… ​ user_name Note that in this case the new group is added to user’s current supplementary groups. 4.6.4. Creating group directories System administrators usually like to create a group for each major project and assign people to the group when they need to access that project’s files. With this traditional scheme, file management is difficult; when someone creates a file, it is associated with the primary group to which they belong. When a single person works on multiple projects, it becomes difficult to associate the right files with the right group. However, with the UPG scheme, groups are automatically assigned to files created within a 103Red Hat Enterprise Linux 8 Configuring basic system settings directory with the setgid bit set. The setgid bit makes managing group projects that share a common directory very simple because any files a user creates within the directory are owned by the group that owns the directory. For example, a group of people need to work on files in the /opt/myproject/ directory. Some people are trusted to modify the contents of this directory, but not everyone. 1. As root, create the /opt/myproject/ directory by typing the following at a shell prompt: mkdir /opt/myproject 2. Add the myproject group to the system: groupadd myproject 3. Associate the contents of the /opt/myproject/ directory with the myproject group: chown root:myproject /opt/myproject 4. Allow users in the group to create files within the directory and set the setgid bit: chmod 2775 /opt/myproject At this point, all members of the myproject group can create and edit files in the /opt/myproject/ directory without the administrator having to change file permissions every time users write new files. To verify that the permissions have been set correctly, run the following command: ls -ld /opt/myproject drwxrwsr-x. 3 root myproject 4096 Mar 3 18:31 /opt/myproject 5. Add users to the myproject group: usermod -aG myproject username 4.6.5. Setting default permissions for new files using umask When a process creates a file, the file has certain default permissions, for example, -rw-rw-r--. These initial permissions are partially defined by the file mode creation mask , also called file permission mask or umask. Every process has its own umask, for example, bash has umask 0022 by default. Process umask can be changed. 4.6.5.1. What umask consists of A umask consists of bits corresponding to standard file permissions. For example, for umask 0137, the digits mean that: 0 = no meaning, it is always 0 (umask does not affect special bits) 1 = for owner permissions, the execute bit is set 3 = for group permissions, the execute and write bits are set 104CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES 7 = for others permissions, the execute, write, and read bits are set Umasks can be represented in binary, octal, or symbolic notation. For example, the octal representation 0137 equals symbolic representation u=rw-,g=r--,o=---. Symbolic notation specification is the reverse of the octal notation specification: it shows the allowed permissions, not the prohibited permissions. 4.6.5.2. How umask works Umask prohibits permissions from being set for a file: When a bit is set in umask, it is unset in the file. When a bit is not set in umask, it can be set in the file, depending on other factors. The following figure shows how umask 0137 affects creating a new file. Figure 4.3. Applying umask when creating a file IMPORTANT For security reasons, a regular file cannot have execute permissions by default. Therefore, even if umask is 0000, which does not prohibit any permissions, a new regular file still does not have execute permissions. However, directories can be created with execute permissions: [john@server tmp]$ umask 0000 [john@server tmp]$ touch file [john@server tmp]$ mkdir directory [john@server tmp]$ ls -lh . total 0 drwxrwxrwx. 2 john john 40 Nov 2 13:17 directory -rw-rw-rw-. 1 john john 0 Nov 2 13:17 file 105Red Hat Enterprise Linux 8 Configuring basic system settings 4.6.5.3. Managing umask in Shells For popular shells, such as bash, ksh, zsh and tcsh, umask is managed using the umask shell builtin. Processes started from shell inherit its umask. 4.6.5.3.1. Displaying the current mask To show the current umask in octal notation: ~]$ umask 0022 To show the current umask in symbolic notation: umask -S u=rwx,g=rx,o=rx 4.6.5.3.2. Setting mask in shell using umask To set umask for the current shell session using octal notation run: umask octal_mask Substitute octal_mask with four or less digits from 0 to 7. When three or less digits are provided, permissions are set as if the command contained leading zeros. For example, umask 7 translates to 0007. Example 4.1. Setting umask Using Octal Notation To prohibit new files from having write and execute permissions for owner and group, and from having any permissions for others: umask 0337 Or: umask 337 To set umask for the current shell session using symbolic notation: umask -S symbolic_mask Example 4.2. Setting umask Using Symbolic Notation To set umask 0337 using symbolic notation: umask -S u=r,g=r,o= 4.6.5.3.3. Working with the default shell umask 106CHAPTER 4. MANAGING USER AND GROUP ACCOUNTS AND SETTING PERMISSIONS ON FILES Shells usually have a configuration file where their default umask is set. For bash, it is /etc/bashrc. To show the default bash umask: grep -i -B 1 umask /etc/bashrc The output shows if umask is set, either using the umask command or the UMASK variable. In the following example, umask is set to 022 using the umask command: grep -i -B 1 umask /etc/bashrc # By default, we want umask to get set. This sets it for non-login shell. —  if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then umask 002 else umask 022 To change the default umask for bash, change the umask command call or the UMASK variable assignment in /etc/bashrc. This example changes the default umask to 0227: if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then umask 002 else umask 227 4.6.5.3.4. Working with the default shell umask of a specific user By default, bash umask of a new user defaults to the one defined in /etc/bashrc. To change bash umaskfor a particular user, add a call to the umask command in $HOME/.bashrc file of that user. For example, to change bash umask of user john to 0227: john@server ~]$ echo ''umask 227'' [] /home/john/.bashrc 4.6.5.3.5. Setting default permissions for newly created home directories To change permissions with which user home directories are created, change the UMASK variable in the /etc/login.defs file: # The permission mask is initialized to this value. If not specified, # the permission mask will be initialized to 022. UMASK 077 4.7. ADDITIONAL RESOURCES For more information on how to manage users and groups on Red Hat Enterprise Linux, see the resources listed below. 4.7.1. Installed Documentation For information about various utilities for managing users and groups, see the following manual pages: useradd(8) — The manual page for the useradd command documents how to use it to create new users. 107Red Hat Enterprise Linux 8 Configuring basic system settings userdel(8) — The manual page for the userdel command documents how to use it to delete users. usermod(8) — The manual page for the usermod command documents how to use it to modify users. groupadd(8) — The manual page for the groupadd command documents how to use it to create new groups. groupdel(8) — The manual page for the groupdel command documents how to use it to delete groups. groupmod(8) — The manual page for the groupmod command documents how to use it to modify group membership. gpasswd(1) — The manual page for the gpasswd command documents how to manage the /etc/group file. grpck(8) — The manual page for the grpck command documents how to use it to verify the integrity of the /etc/group file. pwck(8) — The manual page for the pwck command documents how to use it to verify the integrity of the /etc/passwd and /etc/shadow files. pwconv(8) — The manual page for the pwconv, pwunconv, grpconv, and grpunconv commands documents how to convert shadowed information for passwords and groups. id(1) — The manual page for the id command documents how to display user and group IDs. For information about related configuration files, see: group(5) — The manual page for the /etc/group file documents how to use this file to define system groups. passwd(5) — The manual page for the /etc/passwd file documents how to use this file to define user information. shadow(5) — The manual page for the /etc/shadow file documents how to use this file to set passwords and account expiration information for the system. 108CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP 5.1. INTRODUCTION TO CONFIGURING NTP WITH CHRONY Accurate timekeeping is important for a number of reasons in IT. In networking for example, accurate time stamps in packets and logs are required. In Linux systems, the NTP protocol is implemented by a daemon running in user space. The user space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources. Usually, the Time Stamp Counter (TSC) is used. The TSC is a CPU register which counts the number of cycles since it was last reset. It is very fast, has a high resolution, and there are no interruptions. In Red Hat Enterprise Linux 8, the NTP protocol is implemented by the chronyd daemon, available from the repositories in the chrony package. These sections describe the use of the chrony suite. 5.2. INTRODUCTION TO CHRONY SUITE chrony is an implementation of the Network Time Protocol (NTP). You can use chrony: To synchronize the system clock with NTP servers To synchronize the system clock with a reference clock, for example a GPS receiver To synchronize the system clock with a manual time input As an NTPv4(RFC 5905) server or peer to provide a time service to other computers in the network chrony performs well in a wide range of conditions, including intermittent network connections, heavily congested networks, changing temperatures (ordinary computer clocks are sensitive to temperature), and systems that do not run continuously, or run on a virtual machine. Typical accuracy between two machines synchronized over the Internet is within a few milliseconds, and for machines on a LAN within tens of microseconds. Hardware timestamping or a hardware reference clock may improve accuracy between two machines synchronized to a sub-microsecond level. chrony consists of chronyd, a daemon that runs in user space, and chronyc, a command line program which can be used to monitor the performance of chronyd and to change various operating parameters when it is running. The chrony daemon, chronyd, can be monitored and controlled by the command line utility chronyc. This utility provides a command prompt which allows entering a number of commands to query the current state of chronyd and make changes to its configuration. By default, chronyd accepts only commands from a local instance of chronyc, but it can be configured to accept monitoring commands also from remote hosts. The remote access should be restricted. 5.2.1. Using chronyc to control chronyd To make changes to the local instance of chronyd using the command line utility chronyc in interactive mode, enter the following command as root: ~]# chronyc 109Red Hat Enterprise Linux 8 Configuring basic system settings chronyc must run as root if some of the restricted commands are to be used. The chronyc command prompt will be displayed as follows: chronyc> You can type help to list all of the commands. The utility can also be invoked in non-interactive command mode if called together with a command as follows: chronyc command NOTE Changes made using chronyc are not permanent, they will be lost after a chronyd restart. For permanent changes, modify /etc/chrony.conf. 5.3. DIFFERENCES BETWEEN CHRONY AND NTP Network Time Protocol (NTP) has two different implementations with similar basic functionality - ntp and chrony. Both ntp and chrony can operate as an NTP client in order to synchronize the system clock with NTP servers and they can operate as an NTP server for other computers in the network. Each implementation has some unique features. For comparison of ntp and chrony, see Comparison of NTP implementations. Configuration specific to an NTP client is identical in most cases. NTP servers are specified with the server directive. A pool of servers can be specified with the pool directive. Configuration specific to an NTP server differs in how the client access is controlled. By default, ntpd responds to client requests from any address. The access can be restricted with the restrict directive, but it is not possible to disable the access completely if ntpd uses any servers as a client. chronyd allows no access by default and operates as an NTP client only. To make chrony operate as an NTP server, you need to specify some addresses within the allow directive. ntpd and chronyd differ also in the default behavior with respect to corrections of the system clock. ntpd corrects the clock by step when the offset is larger than 128 milliseconds. If the offset is larger than 1000 seconds, ntpd exits unless it is the first correction of the clock and ntpd is started with the -g option. chronyd does not step the clock by default, but the default chrony.conf file provided in the chrony package allows steps in the first three updates of the clock. After that, all corrections are made slowly by speeding up or slowing down the clock. The chronyc makestep command can be issued to force chronyd to step the clock at any time. 5.4. MIGRATING TO CHRONY In Red Hat Enterprise Linux 7, users could choose between ntp and chrony to ensure accurate timekeeping. For differences between ntp and chrony, ntpd and chronyd, see Differences between ntpd and chronyd. In Red Hat Enterprise Linux 8, ntp is no longer supported. chrony is enabled by default. For this reason, you might need to migrate from ntp to chrony. 110CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP Migrating from ntp to chrony is straightforward in most cases. The corresponding names of the programs, configuration files and services are: Table 5.1. Corresponding names of the programs, configuration files and services when migrating from ntp to chrony ntp name chrony name /etc/ntp.conf /etc/chrony.conf /etc/ntp/keys /etc/chrony.keys ntpd chronyd ntpq chronyc ntpd.service chronyd.service ntp-wait.service chrony-wait.service The ntpdate and sntp utilities, which are included in the ntp distribution, can be replaced with chronyd using the -q option or the -t option. The configuration can be specified on the command line to avoid reading /etc/chrony.conf. For example, instead of running ntpdate ntp.example.com, chronyd could be started as: # chronyd -q ''server ntp.example.com iburst'' 2018-05-18T12:37:43Z chronyd version 3.3 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SIGND +ASYNCDNS +SECHASH +IPV6 +DEBUG) 2018-05-18T12:37:43Z Initial frequency -2.630 ppm 2018-05-18T12:37:48Z System clock wrong by 0.003159 seconds (step) 2018-05-18T12:37:48Z chronyd exiting The ntpstat utility, which was previously included in the ntp package and supported only ntpd, now supports both ntpd and chronyd. It is available in the ntpstat package. 5.4.1. Migration script A Python script called ntp2chrony.py is included in the documentation of the chrony package (/usr/share/doc/chrony). The script automatically converts an existing ntp configuration to chrony. It supports the most common directives and options in the ntp.conf file. Any lines that are ignored in the conversion are included as comments in the generated chrony.conf file for review. Keys that are specified in the ntp key file, but are not marked as trusted keys in ntp.conf are included in the generated chrony.keys file as comments. By default, the script does not overwrite any files. If /etc/chrony.conf or /etc/chrony.keys already exist, the -b option can be used to rename the file as a backup. The script supports other options. The --help option prints all supported options. An example of an invocation of the script with the default ntp.conf provided in the ntp package is: # python3 /usr/share/doc/chrony/ntp2chrony.py -b -v Reading /etc/ntp.conf 111Red Hat Enterprise Linux 8 Configuring basic system settings Reading /etc/ntp/crypto/pw Reading /etc/ntp/keys Writing /etc/chrony.conf Writing /etc/chrony.keys The only directive ignored in this case is disable monitor, which has a chrony equivalent in the noclientlog directive, but it was included in the default ntp.conf only to mitigate an amplification attack. The generated chrony.conf file typically includes a number of allow directives corresponding to the restrict lines in ntp.conf. If you do not want to run chronyd as an NTP server, remove all allow directives from chrony.conf. 5.5. CONFIGURING CHRONY 5.5.1. Adding a server address To specify the address of a server running an NTP service, use this procedure: Procedure Include the following line in the /etc/chrony.conf file: server address Here, address is the address or a DNS resolvable name of the server. 5.5.2. Adding a pool of servers To specify a pool of servers running an NTP service, use this procedure: Procedure Include the following line in the /etc/chrony.conf file: pool name where name is a DNS name that resolves to multiple server addresses. By default, /etc/chrony.conf file uses the pool.ntp.org pool. 5.5.3. Improving the time for initial synchronization with iburst To improve the time taken for initial synchronization, use this procedure: Procedure Add to the end of the server or pool command: iburst With this setting, a burst of four packets is sent on start. The iburst option is the default option in chrony configuration file. 112CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP 5.5.4. Configuring the poll interval To change the default poll interval, use this procedure: Procedure Add to the end of the server or pool command: minpoll value and maxpoll value The interval in seconds is expressed in log2 seconds. The default minpoll value is 6, which equates to 64 s. The default value for maxpoll is 10, which equates to 1024 s. Setting a shorter minpoll and/or maxpoll may improve clock accuracy. 5.5.5. Configuring server preference To specify that a particular server is preferred above others of similar statistical quality, use this procedure: Procedure Add the following option to the end of the server or pool command: prefer 5.5.6. Other chronyd directives This section describes selected chronyd configuration options: Comments Comments should be preceded by #, %, ; or ! allow Optionally specify a host, subnet, or network from which to allow NTP connections to a machine acting as NTP server. The default is not to allow connections. Examples: allow 192.0.2.0/24 Use this command to grant access to a specific network. allow 2001:0db8:85a3::8a2e:0370:7334 Use this this command to grant access to an IPv6. The UDP port number 123 needs to be open in the firewall in order to allow the client access: ~]# firewall-cmd --zone=public --add-port=123/udp If you want to open port 123 permanently, use the --permanent option: ~]# firewall-cmd --permanent --zone=public --add-port=123/udp 113Red Hat Enterprise Linux 8 Configuring basic system settings cmdallow This is similar to the allow directive (see section allow), except that it allows control access (rather than NTP client access) to a particular subnet or host. (By "control access" is meant that chronyc can be run on those hosts and successfully connect to chronyd on this computer.) The syntax is identical. There is also a cmddeny all directive with similar behavior to the cmdallow all directive. dumpdir Path to the directory to save the measurement history across restarts of chronyd (assuming no changes are made to the system clock behavior whilst it is not running). If this capability is to be used (via the dumponexit command in the configuration file, or the dump command in chronyc), the dumpdir command should be used to define the directory where the measurement histories are saved. dumponexit If this command is present, it indicates that chronyd should save the measurement history for each of its time sources recorded whenever the program exits. (See the dumpdir command above). hwtimestamp The hwtimestamp directive enables hardware timestamping for extremely accurate synchronization. For more details, see the chrony.conf(5) manual page. local The local keyword is used to allow chronyd to appear synchronized to real time from the viewpoint of clients polling it, even if it has no current synchronization source. This option is normally used on the "master" computer in an isolated network, where several computers are required to synchronize to one another, and the "master" is kept in line with real time by manual input. An example of the command is: local stratum 10 A large value of 10 indicates that the clock is so many hops away from a reference clock that its time is unreliable. If the computer ever has access to another computer which is ultimately synchronized to a reference clock, it will almost certainly be at a stratum less than 10. Therefore, the choice of a high value like 10 for the local command prevents the machine’s own time from ever being confused with real time, were it ever to leak out to clients that have visibility of real servers. log The log command indicates that certain information is to be logged. It accepts the following options: measurements This option logs the raw NTP measurements and related information to a file called measurements.log. statistics This option logs information about the regression processing to a file called statistics.log. tracking This option logs changes to the estimate of the system’s gain or loss rate, and any slews made, to a file called tracking.log. rtc This option logs information about the system’s real-time clock. refclocks This option logs the raw and filtered reference clock measurements to a file called refclocks.log. tempcomp This option logs the temperature measurements and system rate compensations to a file called 114CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP This option logs the temperature measurements and system rate compensations to a file called tempcomp.log. The log files are written to the directory specified by the logdir command. An example of the command is: log measurements statistics tracking logdir This directive allows the directory where log files are written to be specified. An example of the use of this directive is: logdir /var/log/chrony makestep Normally chronyd will cause the system to gradually correct any time offset, by slowing down or speeding up the clock as required. In certain situations, the system clock may be so far adrift that this slewing process would take a very long time to correct the system clock. This directive forces chronyd to step system clock if the adjustment is larger than a threshold value, but only if there were no more clock updates since chronyd was started than a specified limit (a negative value can be used to disable the limit). This is particularly useful when using reference clock, because the initstepslew directive only works with NTP sources. An example of the use of this directive is: makestep 1000 10 This would step the system clock if the adjustment is larger than 1000 seconds, but only in the first ten clock updates. maxchange This directive sets the maximum allowed offset corrected on a clock update. The check is performed only after the specified number of updates to allow a large initial adjustment of the system clock. When an offset larger than the specified maximum occurs, it will be ignored for the specified number of times and then chronyd will give up and exit (a negative value can be used to never exit). In both cases a message is sent to syslog. An example of the use of this directive is: maxchange 1000 1 2 After the first clock update, chronyd will check the offset on every clock update, it will ignore two adjustments larger than 1000 seconds and exit on another one. maxupdateskew One of chronyd''s tasks is to work out how fast or slow the computer’s clock runs relative to its reference sources. In addition, it computes an estimate of the error bounds around the estimated value. If the range of error is too large, it indicates that the measurements have not settled down yet, and that the estimated gain or loss rate is not very reliable. The maxupdateskew parameter is the threshold for determining whether an estimate is too unreliable to be used. By default, the threshold is 1000 ppm. 115Red Hat Enterprise Linux 8 Configuring basic system settings The format of the syntax is: maxupdateskew skew-in-ppm Typical values for skew-in-ppm might be 100 for a dial-up connection to servers over a telephone line, and 5 or 10 for a computer on a LAN. It should be noted that this is not the only means of protection against using unreliable estimates. At all times, chronyd keeps track of both the estimated gain or loss rate, and the error bound on the estimate. When a new estimate is generated following another measurement from one of the sources, a weighted combination algorithm is used to update the master estimate. So if chronyd has an existing highly-reliable master estimate and a new estimate is generated which has large error bounds, the existing master estimate will dominate in the new master estimate. minsources The minsources directive sets the minimum number of sources that need to be considered as selectable in the source selection algorithm before the local clock is updated. The format of the syntax is: minsources number-of-sources By default, number-of-sources is 1. Setting minsources to a larger number can be used to improve the reliability, because multiple sources will need to correspond with each other. noclientlog This directive, which takes no arguments, specifies that client accesses are not to be logged. Normally they are logged, allowing statistics to be reported using the clients command in chronyc and enabling the clients to use interleaved mode with the xleave option in the server directive. reselectdist When chronyd selects synchronization source from available sources, it will prefer the one with minimum synchronization distance. However, to avoid frequent reselecting when there are sources with similar distance, a fixed distance is added to the distance for sources that are currently not selected. This can be set with the reselectdist option. By default, the distance is 100 microseconds. The format of the syntax is: reselectdist dist-in-seconds stratumweight The stratumweight directive sets how much distance should be added per stratum to the synchronization distance when chronyd selects the synchronization source from available sources. The format of the syntax is: stratumweight dist-in-seconds By default, dist-in-seconds is 1 millisecond. This means that sources with lower stratum are usually preferred to sources with higher stratum even when their distance is significantly worse. Setting stratumweight to 0 makes chronyd ignore stratum when selecting the source. rtcfile The rtcfile directive defines the name of the file in which chronyd can save parameters associated with tracking the accuracy of the system’s real-time clock (RTC). 116CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP The format of the syntax is: rtcfile /var/lib/chrony/rtc chronyd saves information in this file when it exits and when the writertc command is issued in chronyc. The information saved is the RTC’s error at some epoch, that epoch (in seconds since January 1 1970), and the rate at which the RTC gains or loses time. Not all real-time clocks are supported as their code is system-specific. Note that if this directive is used then the real-time clock should not be manually adjusted as this would interfere with chrony''s need to measure the rate at which the real-time clock drifts if it was adjusted at random intervals. rtcsync The rtcsync directive is present in the /etc/chrony.conf file by default. This will inform the kernel the system clock is kept synchronized and the kernel will update the real-time clock every 11 minutes. 5.5.7. Configuring chrony for security chronyc can access chronyd in two ways: Internet Protocol, IPv4 or IPv6. Unix domain socket, which is accessible locally by the root or chrony user. By default, chronyc connects to the Unix domain socket. The default path is /var/run/chrony/chronyd.sock. If this connection fails, which can happen for example when chronyc is running under a non-root user, chronyc tries to connect to 127.0.0.1 and then ::1. Only the following monitoring commands, which do not affect the behavior of chronyd, are allowed from the network: activity manual list rtcdata smoothing sources sourcestats tracking waitsync The set of hosts from which chronyd accepts these commands can be configured with the cmdallow directive in the configuration file of chronyd, or the cmdallow command in chronyc. By default, the commands are accepted only from localhost (127.0.0.1 or ::1). All other commands are allowed only through the Unix domain socket. When sent over the network, chronyd responds with a Not authorised error, even if it is from localhost. Accessing chronyd remotely with chronyc 1. Allow access from both IPv4 and IPv6 addresses by adding the following to the 117Red Hat Enterprise Linux 8 Configuring basic system settings 1. Allow access from both IPv4 and IPv6 addresses by adding the following to the /etc/chrony.conf file: bindcmdaddress 0.0.0.0 or bindcmdaddress : 2. Allow commands from the remote IP address, network, or subnet by using the cmdallow directive. Add the following content to the /etc/chrony.conf file: cmdallow 192.168.1.0/24 3. Open port 323 in the firewall to connect from a remote system. ~]# firewall-cmd --zone=public --add-port=323/udp If you want to open port 323 permanently, use the --permanent. ~]# firewall-cmd --permanent --zone=public --add-port=323/udp Note that the allow directive is for NTP access whereas the cmdallow directive is to enable receiving of remote commands. It is possible to make these changes temporarily using chronyc running locally. Edit the configuration file to make permanent changes. 5.6. USING CHRONY 5.6.1. Installing chrony The chrony suite is installed by default on Red Hat Enterprise Linux. To ensure that it is, run the following command as root: ~]# yum install chrony The default location for the chrony daemon is /usr/sbin/chronyd. The command line utility will be installed to /usr/bin/chronyc. 5.6.2. Checking the status of chronyd To check the status of chronyd, issue the following command: ~]$ systemctl status chronyd chronyd.service - NTP client/server Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled) Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago 5.6.3. Starting chronyd To start chronyd, issue the following command as root: 118CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP ~]# systemctl start chronyd To ensure chronyd starts automatically at system start, issue the following command as root: ~]# systemctl enable chronyd 5.6.4. Stopping chronyd To stop chronyd, issue the following command as root: ~]# systemctl stop chronyd To prevent chronyd from starting automatically at system start, issue the following command as root: ~]# systemctl disable chronyd 5.6.5. Checking if chrony is synchronized To check if chrony is synchronized, make use of the tracking, sources, and sourcestats commands. 5.6.5.1. Checking chrony tracking To check chrony tracking, issue the following command: ~]$ chronyc tracking Reference ID : CB00710F (foo.example.net) Stratum : 3 Ref time (UTC) : Fri Jan 27 09:49:17 2017 System time : 0.000006523 seconds slow of NTP time Last offset : -0.000006747 seconds RMS offset : 0.000035822 seconds Frequency : 3.225 ppm slow Residual freq : 0.000 ppm Skew : 0.129 ppm Root delay : 0.013639022 seconds Root dispersion : 0.001100737 seconds Update interval : 64.2 seconds Leap status : Normal The fields are as follows: Reference ID This is the reference ID and name (or IP address) if available, of the server to which the computer is currently synchronized. Reference ID is a hexadecimal number to avoid confusion with IPv4 addresses. Stratum The stratum indicates how many hops away from a computer with an attached reference clock we are. Such a computer is a stratum-1 computer, so the computer in the example is two hops away (that is to say, a.b.c is a stratum-2 and is synchronized from a stratum-1). Ref time This is the time (UTC) at which the last measurement from the reference source was processed. 119Red Hat Enterprise Linux 8 Configuring basic system settings System time In normal operation, chronyd never steps the system clock, because any jump in the timescale can have adverse consequences for certain application programs. Instead, any error in the system clock is corrected by slightly speeding up or slowing down the system clock until the error has been removed, and then returning to the system clock’s normal speed. A consequence of this is that there will be a period when the system clock (as read by other programs using the gettimeofday() system call, or by the date command in the shell) will be different from chronyd''s estimate of the current true time (which it reports to NTP clients when it is operating in server mode). The value reported on this line is the difference due to this effect. Last offset This is the estimated local offset on the last clock update. RMS offset This is a long-term average of the offset value. Frequency The "frequency" is the rate by which the system’s clock would be wrong if chronyd was not correcting it. It is expressed in ppm (parts per million). For example, a value of 1 ppm would mean that when the system’s clock thinks it has advanced 1 second, it has actually advanced by 1.000001 seconds relative to true time. Residual freq This shows the "residual frequency" for the currently selected reference source. This reflects any difference between what the measurements from the reference source indicate the frequency should be and the frequency currently being used. The reason this is not always zero is that a smoothing procedure is applied to the frequency. Each time a measurement from the reference source is obtained and a new residual frequency computed, the estimated accuracy of this residual is compared with the estimated accuracy (see skew) of the existing frequency value. A weighted average is computed for the new frequency, with weights depending on these accuracies. If the measurements from the reference source follow a consistent trend, the residual will be driven to zero over time. Skew This is the estimated error bound on the frequency. Root delay This is the total of the network path delays to the stratum-1 computer from which the computer is ultimately synchronized. Root delay values are printed in nanosecond resolution. In certain extreme situations, this value can be negative. (This can arise in a symmetric peer arrangement where the computers’ frequencies are not tracking each other and the network delay is very short relative to the turn-around time at each computer.) Root dispersion This is the total dispersion accumulated through all the computers back to the stratum-1 computer from which the computer is ultimately synchronized. Dispersion is due to system clock resolution, statistical measurement variations etc. Root dispersion values are printed in nanosecond resolution. Leap status This is the leap status, which can be Normal, Insert second, Delete second or Not synchronized. 5.6.5.2. Checking chrony sources The sources command displays information about the current time sources that chronyd is accessing. The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns. 120CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP ~]$ chronyc sources 210 Number of sources = 3 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #* GPS0 0 4 377 11 -479ns[ -621ns] /- 134ns ^? a.b.c 2 6 377 23 -923us[ -924us] +/- 43ms ^ d.e.f 1 6 377 21 -2629us[-2619us] +/- 86ms The columns are as follows: M This indicates the mode of the source. ^ means a server, = means a peer and # indicates a locally connected reference clock. S This column indicates the state of the sources. "*" indicates the source to which chronyd is currently synchronized. "+" indicates acceptable sources which are combined with the selected source. "-" indicates acceptable sources which are excluded by the combining algorithm. "?" indicates sources to which connectivity has been lost or whose packets do not pass all tests. "x" indicates a clock which chronyd thinks is a falseticker (its time is inconsistent with a majority of other sources). "~" indicates a source whose time appears to have too much variability. The "?" condition is also shown at start- up, until at least 3 samples have been gathered from it. Name/IP address This shows the name or the IP address of the source, or reference ID for reference clock. Stratum This shows the stratum of the source, as reported in its most recently received sample. Stratum 1 indicates a computer with a locally attached reference clock. A computer that is synchronized to a stratum 1 computer is at stratum 2. A computer that is synchronized to a stratum 2 computer is at stratum 3, and so on. Poll This shows the rate at which the source is being polled, as a base-2 logarithm of the interval in seconds. Thus, a value of 6 would indicate that a measurement is being made every 64 seconds. chronyd automatically varies the polling rate in response to prevailing conditions. Reach This shows the source’s reach register printed as an octal number. The register has 8 bits and is updated on every received or missed packet from the source. A value of 377 indicates that a valid reply was received for all of the last eight transmissions. LastRx This column shows how long ago the last sample was received from the source. This is normally in seconds. The letters m, h, d or y indicate minutes, hours, days or years. A value of 10 years indicates there were no samples received from this source yet. Last sample This column shows the offset between the local clock and the source at the last measurement. The number in the square brackets shows the actual measured offset. This may be suffixed by ns (indicating nanoseconds), us (indicating microseconds), ms (indicating milliseconds), or s (indicating seconds). The number to the left of the square brackets shows the original measurement, adjusted to allow for any slews applied to the local clock since. The number following the +/- indicator shows the margin of error in the measurement. Positive offsets indicate that the local clock is ahead of the source. 121Red Hat Enterprise Linux 8 Configuring basic system settings 5.6.5.3. Checking chrony source statistics The sourcestats command displays information about the drift rate and offset estimation process for each of the sources currently being examined by chronyd. The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns. ~]$ chronyc sourcestats 210 Number of sources = 1 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev =============================================================================== abc.def.ghi 11 5 46m -0.001 0.045 1us 25us The columns are as follows: Name/IP address This is the name or IP address of the NTP server (or peer) or reference ID of the reference clock to which the rest of the line relates. NP This is the number of sample points currently being retained for the server. The drift rate and current offset are estimated by performing a linear regression through these points. NR This is the number of runs of residuals having the same sign following the last regression. If this number starts to become too small relative to the number of samples, it indicates that a straight line is no longer a good fit to the data. If the number of runs is too low, chronyd discards older samples and re-runs the regression until the number of runs becomes acceptable. Span This is the interval between the oldest and newest samples. If no unit is shown the value is in seconds. In the example, the interval is 46 minutes. Frequency This is the estimated residual frequency for the server, in parts per million. In this case, the computer’s clock is estimated to be running 1 part in 109 slow relative to the server. Freq Skew This is the estimated error bounds on Freq (again in parts per million). Offset This is the estimated offset of the source. Std Dev This is the estimated sample standard deviation. 5.6.6. Manually Adjusting the System Clock To step the system clock immediately, bypassing any adjustments in progress by slewing, issue the following command as root: ~]# chronyc makestep If the rtcfile directive is used, the real-time clock should not be manually adjusted. Random adjustments would interfere with chrony''s need to measure the rate at which the real-time clock drifts. 122CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP 5.7. SETTING UP CHRONY FOR DIFFERENT ENVIRONMENTS 5.7.1. Setting up chrony for a system in an isolated network For a network that is never connected to the Internet, one computer is selected to be the master timeserver. The other computers are either direct clients of the master, or clients of clients. On the master, the drift file must be manually set with the average rate of drift of the system clock. If the master is rebooted, it will obtain the time from surrounding systems and calculate an average to set its system clock. Thereafter it resumes applying adjustments based on the drift file. The drift file will be updated automatically when the settime command is used. On the system selected to be the master, using a text editor running as root, edit /etc/chrony.conf as follows: driftfile /var/lib/chrony/drift commandkey 1 keyfile /etc/chrony.keys initstepslew 10 client1 client3 client6 local stratum 8 manual allow 192.0.2.0 Where 192.0.2.0 is the network or subnet address from which the clients are allowed to connect. On the systems selected to be direct clients of the master, using a text editor running as root, edit the /etc/chrony.conf as follows: server master driftfile /var/lib/chrony/drift logdir /var/log/chrony log measurements statistics tracking keyfile /etc/chrony.keys commandkey 24 local stratum 10 initstepslew 20 master allow 192.0.2.123 Where 192.0.2.123 is the address of the master, and master is the host name of the master. Clients with this configuration will resynchronize the master if it restarts. On the client systems which are not to be direct clients of the master, the /etc/chrony.conf file should be the same except that the local and allow directives should be omitted. In an isolated network, you can also use the local directive that enables a local reference mode, which allows chronyd operating as an NTP server to appear synchronized to real time, even when it was never synchronized or the last update of the clock happened a long time ago. To allow multiple servers in the network to use the same local configuration and to be synchronized to one another, without confusing clients that poll more than one server, use the orphan option of the local directive which enables the orphan mode. Each server needs to be configured to poll all other servers with local. This ensures that only the server with the smallest reference ID has the local reference active and other servers are synchronized to it. When the server fails, another one will take over. 123Red Hat Enterprise Linux 8 Configuring basic system settings 5.8. CHRONY WITH HW TIMESTAMPING 5.8.1. Understanding hardware timestamping Hardware timestamping is a feature supported in some Network Interface Controller (NICs) which provides accurate timestamping of incoming and outgoing packets. NTP timestamps are usually created by the kernel and chronyd with the use of the system clock. However, when HW timestamping is enabled, the NIC uses its own clock to generate the timestamps when packets are entering or leaving the link layer or the physical layer. When used with NTP, hardware timestamping can significantly improve the accuracy of synchronization. For best accuracy, both NTP servers and NTP clients need to use hardware timestamping. Under ideal conditions, a sub-microsecond accuracy may be possible. Another protocol for time synchronization that uses hardware timestamping is PTP. Unlike NTP, PTP relies on assistance in network switches and routers. If you want to reach the best accuracy of synchronization, use PTP on networks that have switches and routers with PTP support, and prefer NTP on networks that do not have such switches and routers. 5.8.2. Verifying support for hardware timestamping To verify that hardware timestamping with NTP is supported by an interface, use the ethtool -T command. An interface can be used for hardware timestamping with NTP if ethtool lists the SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE capabilities and also the HWTSTAMP_FILTER_ALL filter mode. Example 5.1. Verifying support for hardware timestamping on a specific interface ~]# ethtool -T eth0 Output: Timestamping parameters for eth0: Capabilities: hardware-transmit (SOF_TIMESTAMPING_TX_HARDWARE) software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE) hardware-receive (SOF_TIMESTAMPING_RX_HARDWARE) software-receive (SOF_TIMESTAMPING_RX_SOFTWARE) software-system-clock (SOF_TIMESTAMPING_SOFTWARE) hardware-raw-clock (SOF_TIMESTAMPING_RAW_HARDWARE) PTP Hardware Clock: 0 Hardware Transmit Timestamp Modes: off (HWTSTAMP_TX_OFF) on (HWTSTAMP_TX_ON) Hardware Receive Filter Modes: none (HWTSTAMP_FILTER_NONE) all (HWTSTAMP_FILTER_ALL) ptpv1-l4-sync (HWTSTAMP_FILTER_PTP_V1_L4_SYNC) ptpv1-l4-delay-req (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ) ptpv2-l4-sync (HWTSTAMP_FILTER_PTP_V2_L4_SYNC) ptpv2-l4-delay-req (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ) ptpv2-l2-sync (HWTSTAMP_FILTER_PTP_V2_L2_SYNC) ptpv2-l2-delay-req (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ) 124CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP ptpv2-event (HWTSTAMP_FILTER_PTP_V2_EVENT) ptpv2-sync (HWTSTAMP_FILTER_PTP_V2_SYNC) ptpv2-delay-req (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ) 5.8.3. Enabling hardware timestamping To enable hardware timestamping, use the hwtimestamp directive in the /etc/chrony.conf file. The directive can either specify a single interface, or a wildcard character can be used to enable hardware timestamping on all interfaces that support it. Use the wildcard specification in case that no other application, like ptp4l from the linuxptp package, is using hardware timestamping on an interface. Multiple hwtimestamp directives are allowed in the chrony configuration file. Example 5.2. Enabling hardware timestamping by using the hwtimestamp directive hwtimestamp eth0 hwtimestamp eth1 hwtimestamp * 5.8.4. Configuring client polling interval The default range of a polling interval (64-1024 seconds) is recommended for servers on the Internet. For local servers and hardware timestamping, a shorter polling interval needs to be configured in order to minimize offset of the system clock. The following directive in /etc/chrony.conf specifies a local NTP server using one second polling interval: server ntp.local minpoll 0 maxpoll 0 5.8.5. Enabling interleaved mode NTP servers that are not hardware NTP appliances, but rather general purpose computers running a software NTP implementation, like chrony, will get a hardware transmit timestamp only after sending a packet. This behavior prevents the server from saving the timestamp in the packet to which it corresponds. In order to enable NTP clients receiving transmit timestamps that were generated after the transmission, configure the clients to use the NTP interleaved mode by adding the xleave option to the server directive in /etc/chrony.conf: server ntp.local minpoll 0 maxpoll 0 xleave 5.8.6. Configuring server for large number of clients The default server configuration allows a few thousands of clients at most to use the interleaved mode concurrently. To configure the server for a larger number of clients, increase the clientloglimit directive in /etc/chrony.conf. This directive specifies the maximum size of memory allocated for logging of clients'' access on the server: clientloglimit 100000000 125Red Hat Enterprise Linux 8 Configuring basic system settings 5.8.7. Verifying hardware timestamping To verify that the interface has successfully enabled hardware timestamping, check the system log. The log should contain a message from chronyd for each interface with successfully enabled hardware timestamping. Example 5.3. Log messages for interfaces with enabled hardware timestamping chronyd[4081]: Enabled HW timestamping on eth0 chronyd[4081]: Enabled HW timestamping on eth1 When chronyd is configured as an NTP client or peer, you can have the transmit and receive timestamping modes and the interleaved mode reported for each NTP source by the chronyc ntpdata command: Example 5.4. Reporting the transmit, receive timestamping and interleaved mode for each NTP source ~]# chronyc ntpdata Output: Remote address : 203.0.113.15 (CB00710F) Remote port : 123 Local address : 203.0.113.74 (CB00714A) Leap status : Normal Version : 4 Mode : Server Stratum : 1 Poll interval : 0 (1 seconds) Precision : -24 (0.000000060 seconds) Root delay : 0.000015 seconds Root dispersion : 0.000015 seconds Reference ID : 47505300 (GPS) Reference time : Wed May 03 13:47:45 2017 Offset : -0.000000134 seconds Peer delay : 0.000005396 seconds Peer dispersion : 0.000002329 seconds Response time : 0.000152073 seconds Jitter asymmetry: +0.00 NTP tests : 111 111 1111 Interleaved : Yes Authenticated : No TX timestamping : Hardware RX timestamping : Hardware Total TX : 27 Total RX : 27 Total valid RX : 27 Example 5.5. Reporting the stability of NTP measurements 126CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP # chronyc sourcestats With hardware timestamping enabled, stability of NTP measurements should be in tens or hundreds of nanoseconds, under normal load. This stability is reported in the Std Dev column of the output of the chronyc sourcestats command: Output: 210 Number of sources = 1 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev ntp.local 12 7 11 +0.000 0.019 +0ns 49ns 5.8.8. Configuring PTP-NTP bridge If a highly accurate Precision Time Protocol (PTP) grandmaster is available in a network that does not have switches or routers with PTP support, a computer may be dedicated to operate as a PTP slave and a stratum-1 NTP server. Such a computer needs to have two or more network interfaces, and be close to the grandmaster or have a direct connection to it. This will ensure highly accurate synchronization in the network. Configure the ptp4l and phc2sys programs from the linuxptp packages to use one interface to synchronize the system clock using PTP. Configure chronyd to provide the system time using the other interface: Example 5.6. Configuring chronyd to provide the system time using the other interface bindaddress 203.0.113.74 hwtimestamp eth1 local stratum 1 5.9. ACHIEVING SOME SETTINGS PREVIOUSLY SUPPORTED BY NTP IN CHRONY Some settings that were in previous major version of Red Hat Enterprise Linux supported by ntp, are not supported by chrony. This section lists such settings, and describes ways to achieve them on a system with chrony. 5.9.1. Monitoring by ntpq and ntpdc chronyd cannot be monitored by the ntpq and ntpdc utilities from the ntp distribution, because chrony does not support the NTP modes 6 and 7. It supports a different protocol and chronyc is the client implementation. For more information, see the chronyc(1) man page. To monitor the status of the system clock sychronized by chronyd, you can: Use the tracking command Use the ntpstat utility, which supports chrony and provides a similar output as it used to with ntpd 127Red Hat Enterprise Linux 8 Configuring basic system settings Example 5.7. Using the tracking command $ chronyc -n tracking Reference ID : 0A051B0A (10.5.27.10) Stratum : 2 Ref time (UTC) : Thu Mar 08 15:46:20 2018 System time : 0.000000338 seconds slow of NTP time Last offset : +0.000339408 seconds RMS offset : 0.000339408 seconds Frequency : 2.968 ppm slow Residual freq : +0.001 ppm Skew : 3.336 ppm Root delay : 0.157559142 seconds Root dispersion : 0.001339232 seconds Update interval : 64.5 seconds Leap status : Normal Example 5.8. Using the ntpstat utility $ ntpstat synchronised to NTP server (10.5.27.10) at stratum 2 time correct to within 80 ms polling server every 64 s 5.9.2. Using authentication mechanism based on public key cryptography In Red Hat Enterprise Linux 7, ntp supported Autokey, which is an authentication mechanism based on public key cryptography. Autokey is not supported in chronyd. On a Red Hat Enterprise Linux 8 system, it is recommended to use symmetric keys instead. Generate the keys with the chronyc keygen command. A client and server need to share a key specified in /etc/chrony.keys. The client can enable authentication using the key option in the server, pool, or peer directive. 5.9.3. Using ephemeral symmetric associations In Red Hat Enterprise Linux 7, ntpd supported ephemeral symmetric associations, which can be mobilized by packets from peers which are not specified in the ntp.conf configuration file. In Red Hat Enterprise Linux 8, chronyd needs all peers to be specified in chrony.conf. Ephemeral symmetric associations are not supported. Note that using the client/server mode enabled by the server or pool directive is more secure compared to the symmetric mode enabled by the peer directive. 5.9.4. multicast/broadcast client Red Hat Enterprise Linux 7 supported the broadcast/multicast NTP mode, which simplifies configuration of clients. With this mode, clients can be configured to just listen for packets sent to a multicast/broadcast address instead of listening for specific names or addresses of individual servers, which may change over time. In Red Hat Enterprise Linux 8, chronyd does not support the broadcast/multicast mode. The main 128CHAPTER 5. USING THE CHRONY SUITE TO CONFIGURE NTP In Red Hat Enterprise Linux 8, chronyd does not support the broadcast/multicast mode. The main reason is that it is less accurate and less secure than the ordinary client/server and symmetric modes. There are several options of migration from an NTP broadcast/multicast setup: Configure DNS to translate a single name, such as ntp.example.com, to multiple addresses of different servers Clients can have a static configuration using only a single pool directive to synchronize with multiple servers. If a server from the pool becomes unreacheable, or otherwise unsuitable for synchronization, the clients automatically replace it with another server from the pool. Distribute the list of NTP servers over DHCP When NetworkManager gets a list of NTP servers from the DHCP server, chronyd is automatically configured to use them. This feature can be disabled by adding PEERNTP=no to the /etc/sysconfig/network file. Use the Precision Time Protocol (PTP) This option is suitable mainly for environments where servers change frequently, or if a larger group of clients needs to be able to synchronize to each other without having a designated server. PTP was designed for multicast messaging and works similarly to the NTP broadcast mode. A PTP implementation is available in the linuxptp package. PTP normally requires hardware timestamping and support in network switches to perform well. However, PTP is expected to work better than NTP in the broadcast mode even with software timestamping and no support in network switches. In networks with very large number of PTP slaves in one communication path, it is recommended to configure the PTP slaves with the hybrid_e2e option in order to reduce the amount of network traffic generated by the slaves. You can configure a computer running chronyd as an NTP client, and possibly NTP server, to operate also as a PTP grandmaster to distribute synchronized time to a large number of computers using multicast messaging. 5.10. ADDITIONAL RESOURCES The following sources of information provide additional resources regarding chrony. 5.10.1. Installed Documentation chronyc(1) man page — Describes the chronyc command-line interface tool including commands and command options. chronyd(8) man page — Describes the chronyd daemon including commands and command options. chrony.conf(5) man page — Describes the chrony configuration file. 5.10.2. Online Documentation https://chrony.tuxfamily.org/doc/3.3/chronyc.html https://chrony.tuxfamily.org/doc/3.3/chronyd.html https://chrony.tuxfamily.org/doc/3.3/chrony.conf.html 129Red Hat Enterprise Linux 8 Configuring basic system settings For answers to FAQs, see https://chrony.tuxfamily.org/faq.html 130CHAPTER 6. USING PYTHON IN RED HAT ENTERPRISE LINUX 8 CHAPTER 6. USING PYTHON IN RED HAT ENTERPRISE LINUX 8 6.1. INTRODUCTION TO PYTHON Python is a high-level programming language that supports multiple programming paradigms, such as object-oriented, imperative, functional, and procedural. Python has dynamic semantics and can be used for general-purpose programming. With Red Hat Enterprise Linux, many packages that are installed on the system, such as packages providing system tools, tools for data analysis or web applications are written in Python. To be able to use these packages, you need to have the python packages installed. 6.1.1. Python versions Two incompatible versions of Python are widely used, Python 2.x and Python 3.x. Red Hat Enterprise Linux 8 uses Python 3.6 by default. However, Python 2.7 is also provided to support existing software. WARNING  Neither the default python package nor the unversioned /usr/bin/python executable is distributed with Red Hat Enterprise Linux 8. IMPORTANT Always specify the major version of Python when installing it, invoking it, or otherwise interacting with it. For example, use python3, instead of python, in package and command names. All Python-related commands should also include the version, for example, pip3 or pip2. Alternatively, configure the system default version by using the alternatives command as described in Configuring the unversioned Python. As a system administrator, you are recommended to use preferably Python 3 for the following reasons: Python 3 represents the main development direction of the Python project. Support for Python 2 in the upstream community ends in 2020. Popular Python libraries are dropping Python 2 support in upstream. Python 2 in Red Hat Enterprise Linux 8 will have a shorter life cycle and its aim is to facilitate smoother transition to Python 3 for customers. For developers, Python 3 has the following advantages over Python 2: Python 3 allows writing expressive, maintainable, and correct code more easily. 131Red Hat Enterprise Linux 8 Configuring basic system settings Code written in Python 3 will have greater longevity. Python 3 has new features, including asyncio, f-strings, advanced unpacking, keyword only arguments, chained exceptions and more. However, existing software tends to require /usr/bin/python to be Python 2. For this reason, no default python package is distributed with Red Hat Enterprise Linux 8, and you can choose between using Python 2 and 3 as /usr/bin/python, as described in Section 6.2.3, “Configuring the unversioned Python” . 6.1.2. The internal platform-python package System tools in Red Hat Enterprise Linux 8 use a Python version 3.6 provided by the internal platform- python package. Red Hat advises customers to use the python36 package instead. 6.2. INSTALLING AND USING PYTHON WARNING  Using the unversioned python command to install or run Python does not work by default due to ambiguity. Always specify the major version of Python as described in Installing and using Python 3 and Installing and using Python 2 , or configure the system default version by using the alternatives command as described in Section 6.2.3, “Configuring the unversioned Python” . 6.2.1. Installing and using Python 3 In Red Hat Enterprise Linux 8, Python 3 is distributed as the python36 module in the AppStream repository. For details regarding modules, see Installing, managing, and removing user space components . 6.2.1.1. Installing Python 3 To install Python 3, use the following command as root: yum install python3 This command installs Python 3.6 from the python36 module in AppStream. 6.2.1.2. Using Python 3 To run Python 3, use the python3 command. Use the version in all other related commands, such as pip3. 6.2.1.3. Naming conventions for Python 3 packages Packages with add-on modules for Python 3 generally use the python3- prefix. For example, to install the Requests module that is used for writing HTTP clients, run: 132CHAPTER 6. USING PYTHON IN RED HAT ENTERPRISE LINUX 8 yum install python3-requests 6.2.2. Installing and using Python 2 Some software has not yet been fully ported to Python 3, and needs Python 2 to operate. Red Hat Enterprise Linux 8 allows parallel installation of Python 3 and Python 2. If you need the Python 2 functionality, install the python27 module, which is available in the AppStream repository. For details regarding modules, see Installing, managing, and removing user space components . WARNING  Note that Python 3 is the main development direction of the Python project. The support for Python 2 is being phased out. The python27 module has a shorter support period than Red Hat Enterprise Linux 8. 6.2.2.1. Installing Python 2 To install Python 2, run as root: yum install python2 This command installs Python 2.7 from the python27 module in AppStream. 6.2.2.2. Using Python 2 To run Python 2, use the python2 command. Use the version in all other related commands, such as pip2. 6.2.2.2.1. Naming conventions for Python 2 packages Packages with add-on modules for Python 2 generally use the python2- prefix. For example, to install the Requests module that is used for writing HTTP clients, run: yum install python2-requests 6.2.3. Configuring the unversioned Python System administrators can configure the unversioned python command on the system using the alternatives command. Note that the required package, either python3 or python2, needs to be installed before configuring the unversioned command to the respective version. To configure the unversioned python command to Python 3 directly, run: alternatives --set python /usr/bin/python3 Use an analogous command if you choose Python 2. 133Red Hat Enterprise Linux 8 Configuring basic system settings Alternatively, you can configure the unversioned python command interactively: 1. Run the following command: alternatives --config python 2. Select the required version from the provided list. To reset this configuration and remove the unversioned python command, run: alternatives --auto python WARNING  Additional Python-related commands, such as pip3, do not have configurable unversioned variants. 6.3. MIGRATING FROM PYTHON 2 TO PYTHON 3 As a developer, you may want to migrate your former code that is written in Python 2 to Python 3. For more information on how to migrate large code bases to Python 3, see The Conservative Python 3 Porting Guide. Note that after this migration, the original Python 2 code becomes interpretable by the Python 3 interpreter and stays interpretable for the Python 2 interpreter as well. 6.4. PACKAGING OF PYTHON 3 RPMS Most Python projects use Setuptools for packaging, and define package information in the setup.py file. For more information on Setuptools packaging, see Setuptools documentation. You can also package your Python project into an RPM package, which provides the following advantages compared to Setuptools packaging: Specification of dependencies of a package on other RPMs (even non-Python) Cryptographic signing With cryptographic signing, content of RPM packages can be verified, integrated, and tested with the rest of the operating system. For more information on RPM packaging, see Packaging and distributing software . 6.4.1. Typical SPEC file description for a Python RPM package A SPEC file contains instructions that the rpmbuild utility uses to build an RPM. The instructions are included in a series of sections. A SPEC file has two main parts in which the sections are defined: Preamble (contains a series of metadata items that are used in the Body) 134CHAPTER 6. USING PYTHON IN RED HAT ENTERPRISE LINUX 8 Body (contains the main part of the instructions) For further information about SPEC files, see Packaging and distributing software . An RPM SPEC file for Python projects has some specifics compared to non-Python RPM SPEC files. Most notably, a name of any RPM package of a Python library must always include the python3 prefix. Other specifics are shown in the following SPEC file example for the python3-detox package. For description of such specifics, see the notes below the example. %global modname detox 1 Name: python3-detox 2 Version: 0.12 Release: 4%{?dist} Summary: Distributing activities of the tox tool License: MIT URL: https://pypi.io/project/detox Source0: https://pypi.io/packages/source/d/%{modname}/%{modname}-%{version}.tar.gz BuildArch: noarch BuildRequires: python36-devel 3 BuildRequires: python3-setuptools BuildRequires: python36-rpm-macros BuildRequires: python3-six BuildRequires: python3-tox BuildRequires: python3-py BuildRequires: python3-eventlet %?python_enable_dependency_generator 4 %description Detox is the distributed version of the tox python testing tool. It makes efficient use of multiple CPUs by running all possible activities in parallel. Detox has the same options and configuration that tox has, so after installation you can run it in the same way and with the same options that you use for tox. $ detox %prep %autosetup -n %{modname}-%{version} %build %py3_build 5 %install %py3_install %check %{__python3} setup.py test 6 %files -n python3-%{modname} %doc CHANGELOG 135Red Hat Enterprise Linux 8 Configuring basic system settings %license LICENSE %{_bindir}/detox %{python3_sitelib}/%{modname}/ %{python3_sitelib}/%{modname}-%{version}* %changelog ... 1 The modname macro contains the name of the Python project. In this example it is detox. 2 When packaging a Python project into RPM, the python3 prefix always needs to be added to the original name of the project. The original name here is detox and the name of the RPM is python3-detox. 3 BuildRequires specifies what packages are required to build and test this package. In BuildRequires, always include items providing tools necessary for building Python packages: python36-devel and python3-setuptools. The python36-rpm-macros package is required so that files with /usr/bin/python3 shebangs are automatically changed to /usr/bin/python3.6. For more information, see Section 6.4.4, “Handling hashbangs in Python scripts” . 4 Every Python package requires some other packages to work correctly. Such packages need to be specified in the SPEC file as well. To specify the dependencies, you can use the %python_enable_dependency_generator macro to automatically use dependencies defined in the setup.py file. If a package has dependencies that are not specified using Setuptools, specify them within additional Requires directives. 5 The %py3_build and %py3_install macros run the setup.py build and setup.py install commands, respectively, with additional arguments to specify installation locations, the interpreter to use, and other details. 6 The check section provides a macro that runs the correct version of Python. The %{__python3} macro contains a path for the Python 3 interpreter, for example /usr/bin/python3. We recommend to always use the macro rather than a literal path. 6.4.2. Common macros for Python 3 RPM packages In a SPEC file, always use the macros below rather than hardcoding their values. In macro names, always use python3 or python2 instead of unversioned python. Macro Normal Definition Description %{__python3} /usr/bin/python3 Python 3 interpreter %{python3_version} 3.6 The full version of the Python 3 interpreter. %{python3_sitelib} /usr/lib/python3.6/site-packages Where pure-Python modules are installed. %{python3_sitearch} /usr/lib64/python3.6/site- Where modules containing packages architecture-specific extensions are installed. 136CHAPTER 6. USING PYTHON IN RED HAT ENTERPRISE LINUX 8 Macro Normal Definition Description %py3_build Runs the setup.py build command with arguments suitable for a system package. %py3_install Runs the setup.py install command with arguments suitable for a system package. 6.4.3. Automatic provides for Python RPM packages When packaging a Python project, make sure that, if present, the following directories are included in the resulting RPM: .dist-info .egg-info .egg-link From these directories, the RPM build process automatically generates virtual pythonX.Ydist provides, for example python3.6dist(detox). These virtual provides are used by packages that are specified by the %python_enable_dependency_generator macro. 6.4.4. Handling hashbangs in Python scripts In Red Hat Enterprise Linux 8, executable Python scripts are expected to use hashbangs (shebangs) specifying explicitly at least the major Python version. The /usr/lib/rpm/redhat/brp-mangle-shebangs buildroot policy (BRP) script is run automatically when building any RPM package, and attempts to correct hashbangs in all executable files. The BRP script will generate errors when encountering a Python script with an ambiguous hashbang, such as: #! /usr/bin/python or #! /usr/bin/env python To modify hashbangs in the Python scripts causing these build errors at RPM build time, use the pathfix.py script from the platform-python-devel package: pathfix.py -pn -i %{__python3} PATH … ​ Multiple PATHs can be specified. If a PATH is a directory, pathfix.py recursively scans for any Python 137Red Hat Enterprise Linux 8 Configuring basic system settings Multiple PATHs can be specified. If a PATH is a directory, pathfix.py recursively scans for any Python scripts matching the pattern ^[a-zA-Z0-9_]+\.py$, not only those with an ambiguous hashbang. Add this command to the %prep section or at the end of the %install section. Alternatively, modify the packaged Python scripts so that they conform to the expected format. For this purpose, pathfix.py can be used outside the RPM build process, too. When running pathfix.py outside a RPM build, replace __python3 from the example above with a path for the hashbang, such as /usr/bin/python3. If the packaged Python scripts require Python version 2, replace the number 3 with 2 in the commands above. Additionally, hashbangs in the form /usr/bin/python3 are by default replaced with hashbangs pointing to Python from the platform-python package used for system tools with Red Hat Enterprise Linux. To change the /usr/bin/python3 hashbangs in their custom packages to point to a version of Python installed from Application Stream, in the form /usr/bin/python3.6, add the python36-rpm-macros package into the BuildRequires section of the SPEC file: BuildRequires: python36-rpm-macros NOTE To prevent hashbang check and modification by the BRP script, use the following RPM directive: %undefine %brp_mangle_shebangs 138CHAPTER 7. INSTALLING AND USING LANGPACKS CHAPTER 7. INSTALLING AND USING LANGPACKS 7.1. INTRODUCTION TO LANGPACKS Langpacks are meta-packages which install extra add-on packages containing translations, dictionaries and locales for every package installed on the system. On a Red Hat Enterprise Linux 8 system, langpacks installation is based on the langpacks- language meta-packages and RPM weak dependencies (Supplements tag). There are two prerequisites to be able to use langpacks for a selected language: 1. The langpacks- language meta-package for the selected language has been installed on the system. On Red Hat Enterprise Linux 8, the langpacks meta packages are installed automatically with the initial installation of the operating system using the Anaconda installer, because these packages are available in the in Application Stream repository. To find which languages provide langpacks, execute the following command: ~]# yum list langpacks-* 2. The base package, for which you want to search the local packages, has already been installed on the system. If these prerequisites are fulfilled, the language meta-packages pull their langpack for the selected language automatically in the transaction set. 7.2. WORKING WITH RPM WEAK DEPENDENCY-BASED LANGPACKS 7.2.1. Querying RPM weak dependency-based langpacks To list the already installed language support, run the following command: ~]# yum list installed langpacks* To check if any language support is available for another language, run the following command: ~]# yum list available langpacks* To list what packages get installed for any language, run the following command: ~]# yum repoquery --whatsupplements langpacks- 7.2.2. Installing language support To add new a language support, run the following command as the root user: ~]# yum install langpacks- 139Red Hat Enterprise Linux 8 Configuring basic system settings 7.2.3. Removing language support To remove any installed language support, run the following command as the root user: ~]# yum remove langpacks- 7.3. SAVING DISK SPACE BY USING GLIBC-LANGPACK- Currently, all locales are stored in the /usr/lib/locale/locale-archive file, which requires a lot of disk space. On systems where disk space is a critical issue, such as containers and cloud images, or only a few locales are needed, you can use the glibc locale langpack packages (glibc-langpack-). By using the following command instead of that described in Section 7.2.2, “Installing language support” , you can install locales individually and thus gain a smaller package installation footprint: ~]# yum install glibc-langpack- When installing the operating system with Anaconda, glibc-langpack- is installed for the language you used during the installation and also for the languages you selected as additional languages. Note that glibc-all-langpacks, which contains all locales, is installed by default, so some locales are duplicated. If you installed glibc-langpack- for one or more selected languages, you can delete glibc-all-langpacks after the installation to save the disk space. Note that installing only selected glibc-langpack- packages instead of glibc-all- langpacks has impact on run time performance. NOTE If disk space is not an issue, keep all locales installed by using the glibc-all-langpacks package. 140CHAPTER 8. GETTING STARTED WITH TCL/TK ON RED HAT ENTERPRISE LINUX 8 CHAPTER 8. GETTING STARTED WITH TCL/TK ON RED HAT ENTERPRISE LINUX 8 8.1. INTRODUCTION TO TCL/TK Tool command language (Tcl) is a dynamic programming language. The interpreter for this language, together with the C library, is provided by the tcl package. Using Tcl paired with Tk (Tcl/Tk) enables creating cross-platform GUI applications. Tk is provided by the tk package. Note that Tk can refer to any of the the following: A programming toolkit for multiple languages A Tk C library bindings available for multiple languages, such as C, Ruby, Perl and Python A wish interpreter that instantiates a Tk console A Tk extension that adds a number of new commands to a particular Tcl interpreter For more information about Tcl/Tk, see the Tcl/Tk manual or Tcl/Tk documentation web page . 8.2. NOTABLE CHANGES IN TCL/TK 8.6 Major changes in Tcl/Tk 8.6 compared to Tcl/Tk 8.5 are: Object-oriented programming support Stackless evaluation implementation Enhanced exceptions handling Collection of third-party packages built and installed with Tcl Multi-thread operations enabled SQL database-powered scripts support IPv6 networking support Built-in Zlib compression List processing Two new commands, lmap and dict map are available, which allow the expression of transformations over Tcl containers. Stacked channels by script Two new commands, chan push and chan pop are available, which allow to add or remove transformations to or from I/O channels. Major changes in Tk include: Built-in PNG image support 141Red Hat Enterprise Linux 8 Configuring basic system settings Busy windows A new command, tk busy is available, which disables user interaction for a window or a widget and shows the busy cursor. New font selection dialog interface Angled text support Moving things on a canvas support For the detailed list of changes between Tcl 8.5 and Tcl 8.6, see Changes in Tcl/Tk 8.6 . 8.3. MIGRATING TO TCL/TK 8.6 Red Hat Enterprise Linux 7 used Tcl/Tk 8.5. With Red Hat Enterprise Linux 8, Tcl/Tk version 8.6 is provided in the Base OS repository. This section describes migration path to Tcl/Tk 8.6 for: developers writing Tcl extensions or embedding Tcl interpreter into their applications (see Section 8.3.1, “Migration path for developers of Tcl extensions” ) users scripting tasks with Tcl/Tk (see Section 8.3.2, “Migration path for users scripting their tasks with Tcl/Tk”) 8.3.1. Migration path for developers of Tcl extensions Tcl 8.6 limits direct access to members of the interp structure. For example, if your code reads interp→errorLine, rewrite the code to use the Tcl_GetErrorLine(interp) function. To make your code compatible with both Tcl 8.5 and Tcl 8.6, use the following code snippet in a header file of your C or C++ application or extension that includes the Tcl library: # include # if !defined(Tcl_GetErrorLine) # define Tcl_GetErrorLine(interp) (interp→errorLine) # endif 8.3.2. Migration path for users scripting their tasks with Tcl/Tk In Tcl 8.6, most scripts work the same way as with the previous version of Tcl. When writing a portable code, make sure to not use the commands that are no longer supported in Tk 8.6: tkIconList_Arrange tkIconList_AutoScan tkIconList_Btn1 tkIconList_Config tkIconList_Create tkIconList_CtrlBtn1 142CHAPTER 8. GETTING STARTED WITH TCL/TK ON RED HAT ENTERPRISE LINUX 8 tkIconList_Curselection tkIconList_DeleteAll tkIconList_Double1 tkIconList_DrawSelection tkIconList_FocusIn tkIconList_FocusOut tkIconList_Get tkIconList_Goto tkIconList_Index tkIconList_Invoke tkIconList_KeyPress tkIconList_Leave1 tkIconList_LeftRight tkIconList_Motion1 tkIconList_Reset tkIconList_ReturnKey tkIconList_See tkIconList_Select tkIconList_Selection tkIconList_ShiftBtn1 tkIconList_UpDown You can check the list of unsupported commands also in the /usr/share/tk8.6/unsupported.tcl file. 143Red Hat Enterprise Linux 8 Configuring basic system settings CHAPTER 9. USING PREFIXDEVNAME FOR NAMING OF ETHERNET NETWORK INTERFACES This documentation describes how to set the prefixes for consistent naming ot Ethernet network interfaces in case that you do not want to use the default naming scheme of such interfaces. However, Red Hat recommends to use the default naming scheme, which is the same as in Red Hat Enterprise Linux 7. For more details about this scheme, see Consistent Network Device Naming . 9.1. INTRODUCTION TO PREFIXDEVNAME The prefixdevname tool is a udev helper utility that enables you to define your own prefix used for naming of the Ethernet network interfaces. 9.2. SETTING PREFIXDEVNAME The setting of the prefix with prefixdevname is done during system installation. To set and activate the required prefix for your Ethernet network interfaces, add the following on the kernel command line: net.ifnames.prefix= WARNING  Red Hat does not support the use of prefixdevname on already deployed systems. After the prefix was once set, and the operating system was rebooted, the prefix is effective every time when a new network interface appears. The new device is assigned a name in the form of . For example, if your selected prefix is net, and the interfaces with net0 and net1 prefixes already exist on the system, the new interface is named net2. The prefixdevname utility then generates the new .link file in the /etc/systemd/network directory that applies the name to the interface with the MAC address that just appeared. The configuration is persistent across reboots. 9.3. LIMITATIONS OF PREFIXDEVNAME There are certain limitations for prefixes of Ethernet network interfaces. The prefix that you choose must meet the following requirements: be ASCII string be alphanumeric string be shorter than 16 characters 144CHAPTER 9. USING PREFIXDEVNAME FOR NAMING OF ETHERNET NETWORK INTERFACES WARNING  The prefix cannot conflict with any other well-known prefix used for network interface naming on Linux. Specifically, you cannot use these prefixes: eth, eno, ens, em. 145">