Difference between revisions of "Network Time Protocol"

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{{Note| HARDWARECLOCK controls the setting of an hourly cron-job that adjust the time, causing ntp to go out of sync. You may want to disable the cron-job too (adjtime in cron.hourly).}}
{{Note| HARDWARECLOCK controls the setting of an hourly cron-job that adjusts the time for drift, causing ntp to go out of sync. You may want to disable the cron-job too (adjtime in cron.hourly).}}
Add ntpd to the DAEMONS array in rc.conf to have it start on boot:
Add ntpd to the DAEMONS array in rc.conf to have it start on boot:

Revision as of 19:31, 11 February 2010

Template:I18n links start Template:I18n entry Template:I18n entry Template:I18n links end This article describes two different possibilities to synchronize the Linux software clock (system clock) to Internet time servers. Openntpd is explained, then the original ntpd method is explained. For more information on computer timekeeping, see Time.


OpenNTPD is a free, easy to use implementation of the Network Time Protocol. It provides the ability to sync the local clock to remote NTP servers and can act as NTP server itself, redistributing the local clock. OpenNTPD is primarily developed by Henning Brauer as part of the OpenBSD Project, and is a new implementation of the NTP protocol. It is lighter and easier to configure than ntpd, but it is not as versatile.

Note: OpenNTPD is not currently maintained for linux (see this thread).

First, the OpenNTPD package must be installed (available in the Arch Linux community repository):

# pacman -S openntpd

The default configuration is actually usable if all you want is to sync the local computer. For more detailed settings, the Template:Filename file must be edited:

To sync to a particular server, uncomment and edit the "server" directive. You can find the server's URL in your area at www.pool.ntp.org/zone/@.

server ntp.example.org

The "servers" directive works the same as the "server" directive, however, if the DNS name resolves to multiple IP address, ALL of them will be synced to. The default, "pool.ntp.org" is working and should be acceptable in most cases.


Any number of "server" or "servers" directives may be used.

If you want the computer you run OpenNTPD on to also be a time server, simply uncomment and edit the "listen" directive.

For example:

listen on *

will listen on all interfaces, and

listen on

will only listen on the loopback interface.

However, your time server will only begin to serve time after it has synchronized itself to a high resolution. This may take hours, or days, depending on the accuracy of your system.

If you would like to run OpenNTPD at boot, add Template:Codeline the DAEMONS variable in your Template:Filename following your network daemon. If oenntpd is being used to set local system time only, it may be safely backgrounded like so:


To see the status of NTP syncing, visit Template:Filename and look for entries with "ntpd".

OpenNTPD adjusts the clock by small amounts at a time. It is designed this way to prevent sudden, large time fluctuations in your system, which could adversely affect system services (e.g., cron jobs). Thus, it can take some time to correct the time.

If your clock is off by more than 180 seconds you can try "Template:Codeline" in the console. If ntpd is already running, you can simply restart it with Template:Codeline, as the Arch openntpd package uses the "-s" flag by default. See Template:Codeline for more info. You can also set the system clock to as close to possible to the actual time and then let OpenNTPD fine tune the time.


Error adjusting time

If you find your time set incorrectly and in log you see:

openntpd adjtime failed: Invalid argument


ntpd -s -d

Increasing time shift

Starting openntpd in the background could lead to synchronization errors between the actual time and the time stored on your computer. If you recognize an increasing time difference between your desktop clock and the actual time, try to start the openntpd daemon normal and not in the background.

Initialization Failure

Openntpd may fail to initialize properly if it is started before the network is fully configured. In some cases you may want to remove Template:Codeline from the DAEMONS array in Template:Filename and add the following line to Template:Filename:

(sleep 300 && /etc/rc.d/openntpd start) &

This will wait 5 minutes before starting openntpd, which should give the system sufficient time to set up the network properly. If your network settings change often, you may also consider restarting the daemon regularly with cron.

If you are using netcfg, you can also start/stop openntpd as a POST_UP/PRE_DOWN command in your network profile:

POST_UP="/etc/rc.d/openntpd start || true"
PRE_DOWN="/etc/rc.d/openntpd stop || true"

Of course, you will have to specify this manually for each network profile.

Using NetworkManager dispatcher

OpenNTPD can be brought up/down along with a network connection through the use of NetworkManager's dispatcher scripts. Place the following slightly modified Template:Filename script as Template:Filename



. /etc/rc.conf
. /etc/rc.d/functions

[ -f $CONF ] && . $CONF

PID=`pidof -o %PPID /usr/sbin/ntpd`
case "$2" in
    stat_busy "Starting OpenNTPD"
    [ -z "$PID" ] && /usr/sbin/ntpd $PARAMS
    if [ $? -gt 0 ]; then
      PID=`pidof -o %PPID /usr/sbin/ntpd`
      echo $PID >/var/run/openntpd.pid
      add_daemon openntpd
    stat_busy "Stopping OpenNTPD"
    [ ! -z "$PID" ]  && kill $PID &>/dev/null
    if [ $? -gt 0 ]; then
      rm_daemon openntpd
    echo "usage: $0 {up|down}"  
exit 0

Cron Job instead of Daemon

Running OpenNTPD as a daemon may not always be desirable if you have a special way you configure your network, or if you have a bad hardware clock. You can add a cron job that will update the system clock hourly, daily..., set the hardware clock, and remove the time drift file. Open your root crontab by typing "crontab -e" into terminal and (for example) to have your time set at 12:03am daily:

03  00  *   *   *       ntpd -s && hwclock -w && rm /var/lib/hwclock/adjtime


# pacman -S ntp


The first line in your ntp.conf file should contain the following line:

restrict default kod nomodify notrap nopeer noquery

This restricts everyone from modifying anything and prevents everyone from querying your time server. In the past "notrust" was used here too, but the function of this option has changed to mean authentication with a key is required.

Following this line, you need to tell ntpd what to allow through into your server.

restrict nomodify
restrict mask nomodify

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 line in the ntp.conf).

Now, is where the stratum 2 servers that our server will synchronize with come into play. The lines in ntp.conf will be used to tell ntpd what servers we would like to use for synchronizing (these are just examples; use ntp servers that are closest to your location). Please see http://ntp.isc.org/bin/view/Servers/NTPPoolServers for a list a closer servers. 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.

server ntp1.cs.wisc.edu iburst
server ntp3.cs.wisc.edu iburst
server ntp3.sf-bay.org iburst

Unless you have a good reason not to, it is advisable to use the pool.ntp.org servers: http://www.pool.ntp.org/. Alternatively, a list of ntp servers is available at http://www.eecis.udel.edu/~mills/ntp/clock2a.html. Please pay attention to the Access Policies.

The only thing left to do is add the drift file (which keeps track of yours clocks time deviation) and the log file location:

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

In case our server loses internet access, we need to add localhost as a server or it will stop serving time to the network. We add localhost as a "stratum 10" server so that it will never be used unless internet access is lost.

fudge stratum 10

The complete file will look like this:

# default restrictions
restrict default kod nomodify notrap nopeer noquery

# override the default restrictions here
#restrict mask nomodify ## uncomment this to serve time to a local network

# public NTP servers to sync with (all stratum 2)
server ntp1.cs.wisc.edu iburst
server ntp3.cs.wisc.edu iburst
server ntp3.sf-bay.org iburst

# ntp pool servers
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

## local server
fudge stratum 10

# NTP drift file - corrects for hardware clock time deviation
driftfile /etc/ntp.drift

# NTP log file
logfile /var/log/ntp.log


Disable HARDWARECLOCK to avoid interfering with ntpd. It is done by setting it to something else than "UTC" or "localtime":

Note: HARDWARECLOCK controls the setting of an hourly cron-job that adjusts the time for drift, causing ntp to go out of sync. You may want to disable the cron-job too (adjtime in cron.hourly).

Add ntpd to the DAEMONS array in rc.conf to have it start on boot:

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

Note that before starting ntpd for the first time, you should set the time to a sane starting point (within a few minutes) manually, or using ntpdate. ntpd will not modify the time if it is too far off in order to prevent causing problems. If your clock tends to drift too far during shutdown, you should add ntpdate before ntpd in the DAEMONS array.

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

Do not background ntpdate, because it uses the same port as ntpd and will prevent it from starting.


This is for /etc/rc.d/ntpdate configuration. Edit NTP_CLIENT_SERVER for your ntp server.

# change this to a server closer to your location

# client options

# timeout for the ntp-client

# arguments passed to ntpd when started

External links