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Chapter 30. Using Chrony

The following sections describe how to install, start, and stop chronyd, and how to check if chrony is synchronized. Sections also describe how to manually adjust System Clock.

30.1. Managing chrony

The following procedure describes how to install, start, stop, and check the status of chronyd.


  1. 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.

  2. 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
  3. To start chronyd, issue the following command as root:

    # systemctl start chronyd

    To ensure chronyd starts automatically at system start, issue the following command as root:

    # systemctl enable chronyd
  4. 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

30.2. Checking if chrony is synchronized

The following procedure describes how to check if chrony is synchronized with the use of the tracking, sources, and sourcestats commands.


  1. To check chrony tracking, issue the following command:

    chronyc tracking
    Reference ID    : CB00710F (
    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
  2. The sources command displays information about the current time sources that chronyd is accessing. To check chrony sources, issue the following command:

    $ 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 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.

  3. The sourcestats command displays information about the drift rate and offset estimation process for each of the sources currently being examined by chronyd. To check chrony source statistics, issue the following command:

    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 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.

Additional resources

  • chronyc(1) man page

30.3. Manually adjusting the System Clock

The following procedure describes how to manually adjust the System Clock.


  1. 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.

30.4. 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.

The following procedure describes how to set up chrony for asystem in an isolated network.


  1. 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

    Where is the network or subnet address from which the clients are allowed to connect.

  2. 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

    Where 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.

30.5. Configuring remote monitoring access

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 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 ( 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.

The following procedure describes how to access chronyd remotely with chronyc.


  1. Allow access from both IPv4 and IPv6 addresses by adding the following to the /etc/chrony.conf file:



    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:

  3. Open port 323 in the firewall to connect from a remote system:

    #  firewall-cmd --zone=public --add-port=323/udp

    Optionally, you can open port 323 permanently using the --permanent option:

    #  firewall-cmd --permanent --zone=public --add-port=323/udp
  4. If you opened port 323 permanently, reload the firewall configuration:

    firewall-cmd --reload

Additional resources

  • chrony.conf(5) man page

30.6. Managing time synchronization using RHEL System Roles

You can manage time synchronization on multiple target machines using the timesync role. The timesync role installs and configures an NTP or PTP implementation to operate as an NTP client or PTP slave in order to synchronize the system clock with NTP servers or grandmasters in PTP domains.

Note that using the timesync role also facilitates migration to chrony, because you can use the same playbook on all versions of Red Hat Enterprise Linux starting with RHEL 6 regardless of whether the system uses ntp or chrony to implement the NTP protocol.


The timesync role replaces the configuration of the given or detected provider service on the managed host. Previous settings are lost, even if they are not specified in the role variables. The only preserved setting is the choice of provider if the timesync_ntp_provider variable is not defined.

The following example shows how to apply the timesync role in a situation with just one pool of servers.

Example 30.1. An example playbook applying the timesync role for a single pool of servers

- hosts: timesync-test
      - hostname:
        pool: yes
        iburst: yes
    - rhel-system-roles.timesync

For a detailed reference on timesync role variables, install the rhel-system-roles package, and see the or README.html files in the /usr/share/doc/rhel-system-roles/timesync directory.

30.7. Additional resources