What is ntp server in linux

Network Time Protocol daemon

Network Time Protocol is the most common method to synchronize the software clock of a GNU/Linux system with internet time servers. It is designed to mitigate the effects of variable network latency and can usually maintain time to within tens of milliseconds over the public Internet. The accuracy on local area networks is even better, up to one millisecond.

The NTP Project provides a reference implementation of the protocol called simply NTP. This article further describes how to set up and run the NTP daemon, both as a client and as a server.

See System time#Time synchronization for other NTP implementations.

Contents

Installation

Install the ntp package. By default, ntpd works in client mode without further configuration. You can skip to #Usage, if you want to use the Arch Linux default configuration file for it. For server configuration, see #NTP server mode.

Configuration

The main daemon is ntpd, which is configured in /etc/ntp.conf . Refer to ntp.conf(5) for detail.

Connection to NTP servers

NTP servers are classified in a hierarchical system with many levels called strata: the devices which are considered independent time sources are classified as stratum 0 sources; the servers directly connected to stratum 0 devices are classified as stratum 1 sources; servers connected to stratum 1 sources are then classified as stratum 2 sources and so on.

It has to be understood that a server’s stratum cannot be taken as an indication of its accuracy or reliability. Typically, stratum 2 servers are used for general synchronization purposes: if you do not already know the servers you are going to connect to, you should choose a server pool close to your location from the pool.ntp.org servers (alternative link).

Since ntp version 4.2.7.p465-2, Arch Linux uses its own default vendor pool of NTP servers provided by the NTP Pool Project (see FS#41700). Modify those to suit your needs, e.g. if you want to use your country’s servers with an option:

The iburst option is recommended, and sends a burst of packets only if it cannot obtain a connection with the first attempt. The burst option always does this, even on the first attempt, and should never be used without explicit permission and may result in blacklisting.

NTP server mode

If setting up an NTP server, check that you have orphan mode enabled, so that, in case it loses internet access, it will continue serving time to the network; enable orphan mode using the tos configuration parameter (you can set up to stratum 15) so that it will never be used unless internet access is lost:

Next, define the rules that will allow clients to connect to your service (localhost is considered a client too) using the restrict command; you should already have a line like this in your file:

This restricts everyone from modifying anything and prevents everyone from querying the status of your time server: nomodify prevents reconfiguring ntpd (with ntpq or ntpdc), and noquery is important to prevent dumping status data from ntpd (also with ntpq or ntpdc).

You can also add other options:

If you want to change any of these, see the full docs for the «restrict» option in ntp.conf(5) , the detailed ntp instructions [dead link 2021-05-17 ⓘ] and #Usage.

Following this line, you need to tell ntpd what to allow through into your server; the following line is enough if you are not configuring an NTP server:

If you want to force DNS resolution to the IPv6 namespace, write -6 before the IP address or host name ( -4 forces IPv4 instead), for example:

Lastly, specify the drift file (which keeps track of your clock’s time deviation) and optionally the log file location:

A very basic configuration file will look like this:

Usage

The package has a default client-mode configuration and its own user and group to drop root privileges after starting. If you start it from the console, you should always do so with the -u option:

The -u option is employed by the two included systemd services. These services also use the -g option, which disables a threshold (so-called panic-gate). Hence, they will synchonize time even in case the ntp-server’s time exceeds the threshold deviation from the system clock.

Both services are tied to the system’s resolver, and will start synchronizing when an active network connection is detected.

Start ntpd at boot

Enable the daemon with ntpd.service . See also #Running in a chroot.

Use ntpq to see the list of configured peers and status of synchronization:

The delay, offset and jitter columns should be non-zero. The servers ntpd is synchronizing with are prefixed by an asterisk. It can take several minutes before ntpd selects a server to synchronize with; try checking after 17 minutes (1024 seconds).

Synchronize time once per boot

Alternatively, enable ntpdate.service to synchronize time once (option -q ) and non-forking (option -n ) per boot, instead of running the daemon in the background. This method is discouraged on servers, and in general on machines that run without rebooting for more than a few days.

If the synchronized time should be written to the hardware clock as well, configure the provided unit as described in systemd#Editing provided units before starting it:

Tips and tricks

Start ntpd on network connection

ntpd can be started by your network manager, so that the daemon only runs when the computer is online.

This article or section needs language, wiki syntax or style improvements. See Help:Style for reference.

Append the following lines to your netctl profile:

The ntpd daemon can be brought up/down along with a network connection through the use of NetworkManager’s dispatcher scripts. The networkmanager-dispatcher-ntpd AUR package installs one, pre-configured to start and stop the ntpd service with a connection.

For Wicd, create a start script in the postconnect directory and a stop script in the predisconnect directory. Remember to make them executable:

KDE can use NTP (ntp must be installed) by right clicking the clock and selecting Adjust date/time. However, this requires the ntp daemon to be disabled before configuring KDE to use NTP. [2]

Using ntpd with GPS

Most of the articles online about configuring ntpd to receive time from a GPS suggest to use the SHM (shared memory) method. However, at least since ntpd version 4.2.8 a much better method is available. It connects directly to gpsd, so gpsd needs to be installed.

Add these lines to your /etc/ntp.conf :

This will work as long as you have gpsd working. It connects to gpsd via the local socket and queries the «gpsd_json» object that is returned.

To test the setup, first ensure that gpsd is working by running:

Then wait a few minutes and run ntpq -p . This will show if ntpd is talking to gpsd:

Running in a chroot

Create a new directory /etc/systemd/system/ntpd.service.d/ if it does not exist and a file named customexec.conf inside with the following content:

Then, edit /etc/ntp.conf to change the driftfile path such that it is relative to the chroot directory, rather than to the real system root. Change:

Create a suitable chroot environment so that getaddrinfo() will work by creating pertinent directories and files (as root):

and by bind-mounting the aformentioned files:

Finally, restart ntpd daemon again. Once it restarted you can verify that the daemon process is chrooted by checking where /proc//root symlinks to:

should now link to /var/lib/ntp instead of / .

It is relatively difficult to be sure that your driftfile configuration is actually working without waiting a while, as ntpd does not read or write it very often. If you get it wrong, it will log an error; if you get it right, it will update the timestamp. If you do not see any errors about it after a full day of running, and the timestamp is updated, you should be confident of success.

Restrict listening sockets

You can limit sockets ntpd is listening to using the interface option:

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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.

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.

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.

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.

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.

Check Chronyd Source Stats

To check chrony tracking, run the following command.

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

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:

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

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:

Add a Pool of NTP Servers

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

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

View Chronyd Time Sources

To check chrony tracking status, run the following command.

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:

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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