Network Time Protocol
This article describes two different methods to synchronize the Linux software clock (system clock) to internet time servers. For more information on computer timekeeping, see Time. Traditional ntpd is explained first and then OpenNTPD (part of the OpenBSD project) is explained. Both daemons can sync the local clock and act as a time server if needed. OpenNTPD is designed to be simple and secure, while ntpd has a larger set of features. Note that OpenNTPD is not currently maintained for Linux (see this thread).
- 1 ntpd
- 2 OpenNTPD
- 3 External links
ntp is available from [extra]:
# pacman -S ntp
The first line in your ntp.conf file should contain the following:
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 127.0.0.1 restrict 18.104.22.168 nomodify restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap
This tells ntpd that 22.214.171.124 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 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://support.ntp.org/bin/view/Servers/NTPPoolServers for a list of 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/.
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 /var/lib/ntp/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.
server 127.127.1.0 fudge 127.127.1.0 stratum 10
The complete file will look like this:
## default restrictions restrict default kod nomodify notrap nopeer noquery ## override the default restrictions here restrict 127.0.0.1 ## serve time to a local network #restrict 192.168.0.0 mask 255.255.255.0 nomodify notrap ## public NTP servers to sync with (all stratum 2) ## change these to servers near you or comment them out 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 server 127.127.1.0 fudge 127.127.1.0 stratum 10 ## NTP drift file - corrects for hardware clock time deviation driftfile /var/lib/ntp/ntp.drift ## NTP log file logfile /var/log/ntp.log
Running as a daemon
Add ntpd to the DAEMONS array in rc.conf to have it start at boot:
DAEMONS=(syslog-ng network @ntpd ...)
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]:
# pacman -S networkmanager-dispatcher-ntpd
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).
Create ntp group and ntp user:
# groupadd ntp # useradd -r -d /var/lib/ntp -g ntp -s /bin/false ntp
Change ownership of ntp directory to the ntp user/group:
# chown -R ntp:ntp /var/lib/ntp
Edit Template:Filename and change
NTPD_ARGS="-g -u ntp:ntp"
Syncing the clock without the daemon running
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 &
If you also want to update the hardware clock, use this line instead:
ntpd -qg && hwclock -w &
- 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.
OpenNTPD can be installed from [community]:
# pacman -S openntpd
The default configuration is actually usable if all you want is to sync the time of 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/@.
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.
listen on *
will listen on all interfaces, and
listen on 127.0.0.1
will only listen on the loopback interface.
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.
DAEMONS=(syslog-ng network openntpd ...)
If openntpd is being used to set local system time only, it may be safely backgrounded.
DAEMONS=(syslog-ng network @openntpd ...)
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.
Making openntpd dependent upon network access
If you have intermittent network access (you roam around on a laptop, you use dial-up, etc), it does not make sense to have Template:Codeline running as a system daemon on start up. Here are a few ways you can control Template:Codeline based on the presence of a network connection. These instructions should also work for Template:Codeline found further below.
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. You can install the needed script from [community]:
# pacman -S networkmanager-dispatcher-openntpd
These instructions require wicd 1.7.0 or later, which is available in the standard Arch repository. You will also need write access to Template:Filename.
Make one shell script inside Template:Filename with the following:
#!/bin/sh /etc/rc.d/openntpd start
Similarly, make another shell script inside Template:Filename with the following:
#!/bin/sh /etc/rc.d/openntpd stop
Using dhcpcd hooks
The following example uses Template:Filename to start and stop openntpd depending on dhcp status:
[ "$interface" != "eth0" ] && exit 0 if $if_up; then pgrep ntpd &> /dev/null || /etc/rc.d/openntpd start elif $if_down; then pgrep ntpd &> /dev/null && /etc/rc.d/openntpd stop fi
Error adjusting time
If you find your time set incorrectly and in log you see:
openntpd adjtime failed: Invalid argument
ntpd -s -d
This is also how you would manually sync your system.
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.
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.