Network Time Protocol daemon

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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.

Installation

NTPd is available from [extra]:

Template:Cli

Configuration

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. Tipically, 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 should never be used without explicit permission and will likely result in blacklisting.

If you're setting up a ntp server, you need to add localhost as a server, so that, in case it loses internet access, it won't stop serving time to the network; add localhost as a stratum 10 server (using the fudge command) so that it will never be used unless internet access is lost:

server 127.127.1.0
fudge  127.127.1.0 stratum 10

The next thing you have to do is add 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

Now all that's left to do is 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 a ntp server:

restrict 127.0.0.1

Otherwise you can add more clients like in this example:

restrict 1.2.3.4 nomodify
restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap

This tells ntpd that 1.2.3.4 and all IP addresses from the 192.168.0.0 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 line in the Template:Filename).

In the end, for the most basic configuration, the complete file will look like this (all comments have been stripped out for clarity):

Template:File

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

Lastly, never forget man pages: Template:Codeline is likely to answer any doubts you could still have.

Template:Gentoo

Running as a daemon

Starting ntpd

Add ntpd to the DAEMONS array in Template:Filename to have it start at boot:

DAEMONS=(syslog-ng network @ntpd ...)

NetworkManager

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]:

Template:Cli

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:

Template:Cli

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

Template:Cli

Edit Template:Filename and change

NTPD_ARGS="-g"

to

NTPD_ARGS="-g -u ntp:ntp"

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 &
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)
  • The hardware clock is already automatically realigned with the system clock each time you halt the computer, through a command in Template:Filename
Warning:
  • 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 "ntpd -qg" 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.

Alternatives

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