How to Synchronize Time with NTP in Linux

The Network Time Protocol (NTP) is a protocol used to synchronize computer system clock automatically over a networks. The machine can have the system clock use Coordinated Universal Time (UTC) rather than local time.

Maintaining accurate time on Linux systems especially servers is a important task for many reasons. For example, in a networked environment, accurate timekeeping is required for accurate timestamps in packets and system logs for root-cause analysis, determining when problems occurred, and finding correlations.

Chrony is now the default NTP implementation package on the latest versions of Linux operating systems such as CentOS, RHEL, Fedora and Ubuntu/Debian among others and comes pre-installed by default. The package consists of chronyd, a daemon that runs in userspace, and chronyc a command-line program for monitoring and controlling chronyd.

Chrony is a versatile NTP implementation and performs well in a wide range of conditions (check out the comparison of chrony suite to other NTP implementations). It can be used to synchronize the system clock with NTP servers (act as a client), with a reference clock (e.g a GPS receiver), or with a manual time input. It can also be employed as an NTPv4 (RFC 5905) server or peer to provide a time service to other computers in the network.

In this article, you will learn how to synchronize server time with NTP in Linux using chrony.

Installing Chrony in Linux Server

In most Linux systems, the chrony command is not installed by default. To install it, execute the below command.

$ sudo apt-get install chrony    [On Debian/Ubuntu]
$ sudo yum  install chrony       [On CentOS/RHEL]
$ sudo dnf install chrony        [On Fedora 22+]

Once the installation is complete, start the chrony service and enable it to automatically start at system boot, then check if it is up and running.

# systemctl enable --now chronyd
# systemctl status chronyd
Check Chrony Status
Check Chrony Status

To cross-check if chrony is now up and running fine and to see the number of servers and peers that are connected to it, run the following chronyc command.

# chronyc activity
Check Chrony Activity
Check Chrony Activity

Checking Chrony Synchronization

To display information (list of servers available, status, and offsets from the local clock and the source) about the current time sources that chronyd is accessing, run the following command with the -v flag shows the description for each column.

# chronyc sources
OR
# chronyc sources -v
Check Chronyd Time Sources
Check Chronyd Time Sources

Concerning the previous command, to display other useful information for each of the sources currently being examined by chronyd (such as the drift rate and offset estimation process), use the sourcestats command.

# chronyc sourcestats
OR
# chronyc sourcestats -v
Check Chronyd Source Stats
Check Chronyd Source Stats

To check chrony tracking, run the following command.

# chronyc tracking

In the output of this command, the reference ID specifies the name (or IP address) if available, of the server to which the computer is currently synchronized, out of all the available servers.

Display Chrony Tracking
Display Chrony Tracking

Configuring Chrony Time Sources

The main chrony configuration file is located at /etc/chrony.conf (CentOS/RHEL/Fedora) or /etc/chrony/chrony.conf (Ubuntu/Debian).

When installing a Linux OS in the cloud, your system should have some default servers or a pool of servers added during the installation process. To add or change the default servers, open the configuratioon file for editing:

# vim /etc/chrony.conf
OR
# vim /etc/chrony/chrony.conf

You can either add several servers using the server directive as shown.

server 0.europe.pool.ntp.org iburst
server 1.europe.pool.ntp.org iburst
server 2.europe.pool.ntp.org ibusrt
server 3.europe.pool.ntp.org ibusrt
Add NTP Servers
Add NTP Servers

or in most cases, it’s best to use ntppool.org to find an NTP server. This allows the system to try to find the closest available servers for you. To add a pool, use the pool directive:

pool 0.pool.ntp.org burst
Add a Pool of NTP Servers
Add a Pool of NTP Servers

There are many other options you can configure in the file. After making changes, restart the chrony service.

$ sudo systemctl restart chrony		
OR
# systemctl restart chronyd

To show information about the current time sources that chronyd is querying, run the following command once more.

# chronyc sources
View Chronyd Time Sources
View Chronyd Time Sources

To check chrony tracking status, run the following command.

# chronyc tracking
Check Chrony Synchronizing Status
Check Chrony Synchronizing Status

To display the current time on your system, check whether system clock is synchronized and whether NTP is indeed active, run the timedatectl command:

# timedatectl
Check Current Server Time
Check Current Server Time

That brings us to the end of this guide. If you have any questions, reach us via the comment section below. For more information, check out: using the chrony suite to configure NTP from the RHEL 8 documentation or using chrony to configure NTP from the Ubuntu official blog.

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4 thoughts on “How to Synchronize Time with NTP in Linux”

  1. I have a dual boot W10/Ubuntu PC. When I boot to Ubuntu from W10, the displayed time is correct before the Ubuntu home page comes up, then decreases by 4 hours when the Ubuntu home page is displayed.

    When booting back to W10, the time stays in error by 4 hours.

    How do I get correct time without having to synchronize after every boot ?

    Reply
  2. Use ntpdate on machine where you don’t want to run a NTP server. A simple Cron task can automate time synchronization with a upstream NTP server.

    Reply

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