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ChapterВ 5В Configuring Network Time

This chapter describes how to configure a system to use chrony as an implementation of the Network Time Protocol (NTP) feature, as a replacement for ntp . The chapter also describes the Precision Time Protocol (PTP) daemons that are used to set the system time.

5.1В About the chrony Suite

chrony is a feature that implements NTP to maintain accurate timekeeping on the network. In Oracle Linux 8, the chrony daemon service replaces ntpd for the management of NTP.

chrony has two components, which are provided in the chrony package:

chronyd service daemon

chornyc service utility

5.1.1В About the chronyd Service Daemon

The chronyd service daemon enables mobile systems and virtual machines to update their system clock after a period of suspension or disconnection from a network. The service can also be used to implement a simple NTP client or NTP server. As an NTP server, chronyd can synchronize with higher stratum NTP servers or act as a stratum 1 server using time signals that are received from the Global Positioning System (GPS) or radio broadcasts such as DCF77, MSF, or WWVB.

In an Oracle Linux 8 system, this service daemon is enabled by default

chronyd uses NTP version 3 (RFC 1305), with features that are compatible with NTP version 4 (RFC 5905). However, chronyd does not support several important features of NTP version 4, nor does it support the use of PTP.

For more information, see the chrony(1) manual page and files in the /usr/share/doc/chrony/ directory.

5.1.2В Using the chronyc Service Utility

The chronyc utility is a command that enables you to manage the chronyd service, display information about the service’s operation, or change the service’s configuration.

The command operates in two modes:

Non-interactive mode: In this mode, you use the following syntax:

Interactive mode: Typing the command by itself activates the interactive mode and displays the chronyc> prompt. From this prompt you can issue additional chronyc subcommands.

First you would type the sudo chronyc command to access the chronyc interactive UI, as indicated by the chronyc> prompt. Then you would issue the different chronyc commands as needed.

The chronyc sources command would provide the following information:

The chronyc sourcestats command would provide the following information:

To quit using the interactive mode, type exit .

Any changes you implement with the chronyc command are effective only until the next restart of the chronyd daemon. To make the changes permanent, you must enter these in the /etc/chrony.conf file. See Section 5.1.4, “Editing the chronyd Configuration File”.

For more information, see the chronyc(1) manual page and files in the /usr/share/doc/chrony/ directory.

5.1.3В Configuring the chronyd Service

To configure the chronyd service on a system:

Install the chrony package.

If remote access to the local NTP service is required, configure the system firewall to allow access to the NTP service in the appropriate zones, for example:

If necessary, start the chronyd service and configure it to start following a system reboot.

Note that by default, chrony is enabled after installation.

5.1.4В Editing the chronyd Configuration File

In the /etc/chrony.conf file, the default configuration assumes that the system has network access to public NTP servers with which it can synchronise.

The following example configuration for a system enables it to access three NTP servers:

To configure chronyd to act as an NTP server for a specified client or subnet, use the allow directive, as shown in bold in the following example:

To create keys for an authentication mechanism based on public key cryptography, use the chronyc keygen command.

Autokey in ntp is no longer supported in chrony .

If a system has only intermittent access to NTP servers, the following configuration might be appropriate:

If you specify the offline keyword, chronyd does not poll the NTP servers until it receives communication that network access is available. You can use the chronyc online and chronyc offline commands to inform chronyd of the state of network access.

For a more information about the configuration file and its directives, see the chrony.conf(5) manual page.

5.1.5В Converting From ntp to chrony

The following table shows file, command, and terminology equivalents between ntp and chrony .

Premier Support	Basic Support
24×7 telephone and online support	Yes	Yes
Around-the-clock access to enhancements, updates, and errata	Yes	Yes
Oracle Enterprise Manager for Linux Management	Yes	Yes
Oracle Linux Manager (formerly Spacewalk)	Yes	Yes
High availability with Oracle Clusterware	Yes	Yes
Comprehensive tracing with DTrace	Yes	Yes
Oracle Linux load balancer	Yes	Yes
Comprehensive indemnification	Yes	Yes
Oracle Container runtime for Docker	Yes	Yes
Oracle Linux Virtualization Manager	Yes	No
Zero-downtime patching with Ksplice	Yes	No
Oracle Linux Automation Manager/Engine	Yes	No
Oracle Cloud Native Environment:	Yes	No
— includes Kubernetes, Kata Containers, and more	Yes	No
Gluster Storage for Oracle Linux	Yes	No
Oracle Linux software collections	Yes	No
Oracle Linux high availability services support (Corosync and Pacemaker)	Yes	No
Premier backports	Yes	No
Lifetime sustaining support	Yes	No

ntp	chrony
/etc/ntp.conf	/etc/chrony.conf
/etc/ntp/keys	/etc/chrony.keys
ntpd	chronyd
ntpq command	chronyc command
ntpd.service	chronyd.service
ntp-wait.service	chrony-wait.service
ntpdate and sntp utilities	chronyd -q and chronyd -t commands

The ntpstat utility which is available in the ntpstat package, now supports chronyd . Thus, you can still use the utility in Oracle Linux 8. The command generates output that is similar to when it is used with ntp .

The /usr/share/doc/chrony/ntp2chrony.py script is available to help convert existing ntp configuration to chrony , for example:

The script supports the conversion of the most common directives in /etc/ntp.conf to chrony . In the example, the -b and -v options specify creating backup configuration files before converting and displaying verbose messages during the migration process, respectively.

To list the different options that you can use with the script, type the following command:

5.2В About PTP

PTP enables you to synchronise system clocks on a LAN to a higher accuracy than NTP. Provided that network drivers support either hardware or software time stamping, a PTP clock can use the time stamps in PTP messages to compensate for propagation delays across a network. Software time stamping allows PTP to synchronise systems to within a few tens of microseconds. With hardware time stamping, PTP can synchronise systems to within a few tenths of a microsecond. If you require high-precision time synchronization of systems, use hardware time stamping.

A typical PTP configuration on an enterprise local area network consists of:

One or more grandmaster clock systems.

A grandmaster clock is typically implemented as specialized hardware that can use high-accuracy GPS signals or lower-accuracy code division multiple access (CDMA) signals, radio clock signals, or NTP as a time reference source. If several grandmaster clocks are available, the best master clock (BMC) algorithm selects the grandmaster clock based on the settings of their priority1 , clockClass , clockAccuracy , offsetScaledLogVariance , and priority2 parameters and their unique identifier, in that order.

Several boundary clock systems.

Each boundary clock is backed up to a grandmaster clock on one subnetwork and relays PTP messages to one or more additional subnetworks. A boundary clock is usually implemented as a function of a network switch.

Multiple slave clock systems.

Each slave clock on a subnetwork is backed up to a boundary clock, which acts as the master clock for that slave clock.

For a simpler configuration, set up a single grandmaster clock and multiple slave clocks on the same network segment and thus eliminates any need for an intermediate layer of boundary clocks.

Grandmaster and slave clock systems that use only one network interface for PTP are termed ordinary clocks .

Boundary clocks require at least two network interfaces for PTP: one interface acts a slave to a grandmaster clock or a higher-level boundary clock; the other interfaces act as masters to slave clocks or lower-level boundary clocks.

Synchronization of boundary and slave clock systems is achieved by sending time stamps in PTP messages. By default, PTP messages are sent in UDPv4 datagrams. It is also possible to configure PTP to use UDPv6 datagrams or Ethernet frames as its transport mechanism.

To use PTP on a system, the driver for at least one of the system’s network interfaces must support either software or hardware time stamping. To find out whether the driver for a network interface supports time stamping, use the ethtool command:

The following output is displayed:

The output in the example shows that the en1 interface supports both hardware and software time stamping capabilities.

With software time stamping, ptp4l synchronises the system clock to an external grandmaster clock.

If hardware time stamping is available, ptp4l can synchronise the PTP hardware clock to an external grandmaster clock. In this case, you use the phc2sys daemon to synchronise the system clock with the PTP hardware clock.

5.2.1В Configuring the PTP Service

To configure the PTP service on a system:

Install the linuxptp package.

Edit /etc/sysconfig/ptp4l and define the start-up options for the ptp4l daemon.

Grandmaster clocks and slave clocks require that you define only one interface.

For example, to use hardware time stamping with interface en1 on a slave clock:

To use software time stamping instead of hardware time stamping, specify the -S option:

The -s option specifies that the clock operates only as a slave ( slaveOnly mode). Do not specify this option for a grandmaster clock or a boundary clock.

For a grandmaster clock, omit the -s option, for example:

A boundary clock requires that you define at least two interfaces, for example:

You might need to edit the /etc/ptp4l.conf file to make further adjustments to the configuration of ptp4l , for example:

For a grandmaster clock, set the value of the priority1 parameter to a value between 0 and 127, where lower values have higher priority when the BMC algorithm selects the grandmaster clock. For a configuration that has a single grandmaster clock, a value of 127 is suggested.

If you set the value of summary_interval to an integer value N instead of 0, ptp4l writes summary clock statistics to /var/log/messages every 2 N seconds instead of every second (2 0 = 1). For example, a value of 10 would correspond to an interval of 2 10 or 1024 seconds.

The logging_level parameter controls the amount of logging information that ptp4l records. The default value of logging_level is 6 , which corresponds to LOG_INFO . To turn off logging completely, set the value of logging_level to 0 . Alternatively, specify the -q option to ptp4l .

See the ptp4l(8) manual page.

Configure the system firewall to allow access by PTP event and general messages to UDP ports 319 and 320 in the appropriate zone, for example:

Start the ptp4l service and configure it to start following a system reboot.

To configure phc2sys on a clock system that uses hardware time stamping:

Edit the /etc/sysconfig/phc2sys file and define the start-up options for the phc2sys daemon.

On a boundary clock or slave clock, synchronise the system clock with the PTP hardware clock that is associated with the slave network interface, for example:

The slave network interface on a boundary clock is the one that it uses to communicate with the grandmaster clock.

The -w option specifies that phc2sys waits until ptp4l has synchronised the PTP hardware clock before attempting to synchronise the system clock.

On a grandmaster clock, which derives its system time from a reference time source such as GPS, CDMA, NTP, or a radio time signal, synchronise the network interface’s PTP hardware clock from the system clock, for example:

See the phc2sys(8) manual page.

Start the phc2sys service and configure it to start following a system reboot.

You can use the pmc command to query the status of ptp4l operation. The following example shows the results of running pmc on a slave clock system that is directly connected to the grandmaster clock system without any intermediate boundary clocks:

The following output is displayed:

The following output is displayed:

This output examples include the following useful information:

The unique identifier of the grandmaster clock, which is based on the MAC address of its network interface.

Whether an external grandmaster clock is available. This value is displayed as false on the grandmaster clock itself.

An estimate of how many nanoseconds by which synchronization messages are delayed.

The most recent measurement of the time difference in nanoseconds relative to the grandmaster clock.

The number of network steps between this system and the grandmaster clock.

For more information, see the phc2sys(8) , pmc(8) , and ptp4l(8) manual pages, and IEEE 1588.

5.2.2В Using PTP as a Time Source for NTP

To make the PTP-adjusted system time on an NTP server available to NTP clients, include the following entries in the /etc/chrony.conf file on the NTP server:

These entries define the local system clock as the time reference.

Do not configure any additional server lines in the file.

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