Difference between revisions of "Network Time Protocol daemon"

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{{Package Official|ntp}} is available from the extra repository:
{{Package Official|ntp}} is available from the extra repository:
# pacman -S ntp
# pacman -S ntp

Revision as of 22:40, 8 July 2011

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This article describes how to set up and run NTPd (Network Time Protocol daemon), the most common method to synchronize the software clock of a GNU/Linux system with internet time servers using the Network Time Protocol; if set up correctly, NTPd can make your computer act as a time server itself.


Template:Package Official is available from the extra repository:

# pacman -S ntp


The first thing you define in your Template:Filename is the servers your machine will synchronize to. 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 don't already know the servers you're going to connect to, you should use the pool.ntp.org servers (alternate link) and choose the server pool that is closest to your location.

The following lines are just an example:

server 0.it.pool.ntp.org iburst
server 1.it.pool.ntp.org iburst
server 2.it.pool.ntp.org iburst
server 3.it.pool.ntp.org iburst

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

If setting up an NTP server, you need to add local clock as a server, so that, in case it loses internet access, it will continue serving time to the network; add local clock as a stratum 10 server (using the fudge command) so that it will never be used unless internet access is lost:

fudge stratum 10

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:

restrict default nomodify nopeer

This restricts everyone from modifying anything and prevents everyone from querying your time server.

You can also add other options:

restrict default kod nomodify notrap nopeer noquery

In the past, notrust option was used too, but its function has changed to mean that authentication with a key is required.

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


Otherwise you can add more clients like in this example:

restrict nomodify
restrict mask nomodify notrap

This tells ntpd that and all IP addresses from the range will be allowed to synchronize on this server, but they will not be allowed to modify anything. All other IP addresses in the world will still obey the default restrictions (the first restrict line in the Template:Filename).

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:

restrict -6 default nomodify nopeer
restrict -6 ::1    # ::1 is the IPv6 equivalent for

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

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log

A very basic configuration file will look like this (all comments have been stripped out for clarity):


Actually, defining the log file is not mandatory, but it is always a good idea to have feedback for ntpd operations.

In conclusion, never forget man pages: Template:Codeline is likely to answer any doubts you could still have (see also the related man pages: Template:Codeline).


Running as a daemon

Starting ntpd

ntpd sets 11 minute mode, which syncs the system clock to hardware every 11 minutes. The hwclock daemon measures hardware clock drift and syncs it, which conflicts with ntpd.

Stop the hwclock daemon (if it's running):


Start the ntpd daemon: Template:Cli

Add ntpd to your DAEMONS array so it starts automatically on boot and make sure hwclock is disabled: Template:File


Note: ntpd should still be running when the network is down if the hwclock daemon is disabled, so you shouldn't use this.

ntpd can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. You can install the needed script from [community]:


Running as non-root user

When compiled with --enable-linux-caps, ntp can be run as a non-root user for increased security (the vanilla Arch Linux package has this enabled).

Note: Before attempting this, make sure ntp has already created Template:Filename.

Create ntp group and ntp user:


Change ownership of the ntp directory to the ntp user/group:


Edit Template:Filename and change



NTPD_ARGS="-g -u ntp:ntp"

Finally, restart the daemon:


Syncing the clock without running the daemon

If what you want is just synchronize your system clock at boot time without running ntpd as a daemon, you can add this line to your Template:Filename:

ntpd -qg &

This behavior mimics that of the ntpdate program, which is now deprecated.

Note: In order for this method to work you have to make sure that, when Template:Filename is executed, the network connection has already been initialized (for example you shouldn't background essential network-related daemons in Template:Filename)
  • Using this method is highly discouraged on servers and in general on machines that need to run continuously for more than 2 or 3 days, as the system clock will be updated only once at boot time.
  • Running "Template:Codeline" as a cron event is to be completely avoided, unless you are perfectly aware of how your running applications would react to instantaneous system time changes.

Check that the DAEMONS array in Template:Filename includes hwclock, to ensure the hardware clock is periodically updated:


Warning: If something other already takes care of updating the hardware clock, for example another operating system in dual boot, you should avoid starting hwclock.


An available alternative to NTPd is OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.

See also

  • Time (for more information on computer timekeeping)

External links