Network Time Protocol
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This article describes the different possibilities of getting accurate system time and date for your system. First openntpd is explained, an easy to use solution. The original ntpd method is also explained for completeness' sake.
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OpenNTPD
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 brand new implementation of the ntp protocol. It is also much easier to configure and use than ntpd.
First, the OpenNTPD package must be installed. It is 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 sophisticated settings, the /etc/ntpd.conf 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.
pool.ntp.org
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 127.0.0.1
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 openntpd the DAEMONS variable in your /etc/rc.conf. If you are using openntpd just to set your local system time, you could add @openntpd instead of openntpd to decrease the boot time; adding the @ makes Arch run the other daemons following @openntpd without waiting for openntpd to finish.
If you want to see the status of your syncing, look at /var/log/daemon.log 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 "ntpd -s -d
" in the console. If ntpd is already running, you can simply restart it with "sudo /etc/rc.d/openntpd restart
", as the Arch openntpd package uses the "-s" flag by default. See "man ntpd
" for more info. You can also use "date --set=STRING
" to set the time as close to possible to the actual time and then let OpenNTPD fine tune the time.
Trouble shooting
Error adjusting time
If you find your time set incorrectly and in log you see
openntpd adjtime failed: Invalid argument
try "ntpd -s -d
" in console .
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 "openntpd" from the DAEMONS array in /etc/rc.conf and add the following line to /etc/rc.local:
(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 /etc/rc.d/openntpd
script as /etc/NetworkManager/dispatcher.d/openntpd-dispatch
#!/bin/bash CONF=/etc/conf.d/openntpd . /etc/rc.conf . /etc/rc.d/functions [ -f $CONF ] && . $CONF PID=`pidof -o %PPID /usr/sbin/ntpd` case "$2" in up) stat_busy "Starting OpenNTPD" [ -z "$PID" ] && /usr/sbin/ntpd $PARAMS if [ $? -gt 0 ]; then stat_fail else PID=`pidof -o %PPID /usr/sbin/ntpd` echo $PID >/var/run/openntpd.pid add_daemon openntpd stat_done fi ;; down) stat_busy "Stopping OpenNTPD" [ ! -z "$PID" ] && kill $PID &>/dev/null if [ $? -gt 0 ]; then stat_fail else rm_daemon openntpd stat_done fi ;; *) echo "usage: $0 {up|down}" esac exit 0
ntpd
# pacman -S ntp
/etc/ntp.conf
The very first line of your ntp.conf file should contain a line such as the following:
restrict default noquery notrust nomodify
This essentially restricts everyone from modifying anything. Following this, you need to let ntpd know what you want to let through into your NTP server. Here is where you would specify any other IP addresses you would like to synchronize on your NTP server. For example:
restrict 1.2.3.4 restrict 192.168.0.0 mask 255.255.255.0 nomodify
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 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.
server ntp1.cs.wisc.edu server ntp3.cs.wisc.edu server ntp3.sf-bay.org
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.
If we left it alone right now, we would never connect to a server because the response from any of the three servers listed above would never be allowed back into our server due to the fact that our default restrict statement would be in use (since we did not add the servers to our lesser restrictions (like we did with 127.0.0.1 and the subnet of 192.168.0.0).
To correct this, enter the following lines in ntp.conf:
restrict ntp1.cs.wisc.edu noquery nomodify restrict ntp3.cs.wisc.edu noquery nomodify restrict ntp3.sf-bay.org noquery nomodify
This will allow the response from the above servers into our system so our local clock can be synchronized. The noquery restriction will not allow any of the above three servers to query for information from our server. The nomodify restriction will not allow the three servers to modify anything (synchronization will still take place).
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
The complete file will look like this:
# default restrictions restrict default noquery notrust nomodify # override the default restrictions here restrict 10.1.1.0 mask 255.255.255.0 nomodify # public NTP servers to sync with (all stratum 2) server ntp1.cs.wisc.edu server ntp3.cs.wisc.edu server ntp3.sf-bay.org restrict ntp1.cs.wisc.edu noquery nomodify restrict ntp3.cs.wisc.edu noquery nomodify restrict ntp3.sf-bay.org noquery nomodify # NTP drift file - used to keep track of your system clocks # time deviation driftfile /etc/ntp.drift # NTP log file logfile /var/log/ntp.log
Take note that this is for a client and a server ntp.conf configuration. If you just want to synchronize with a stratum server and are not concerned with other PCs synchronizing with your ntp server, then you can do something like the following (note that only 127.0.0.1 is allowed to be synchronized):
# default restrictions restrict default noquery notrust nomodify # Permit all access over the loopback interface restrict 127.0.0.1 # public NTP servers to sync with (all stratum 2) server ntp1.cs.wisc.edu server ntp3.cs.wisc.edu server ntp3.sf-bay.org restrict ntp1.cs.wisc.edu noquery nomodify restrict ntp3.cs.wisc.edu noquery nomodify restrict ntp3.sf-bay.org noquery nomodify # NTP drift file - used to keep track of your system clocks # time deviation driftfile /etc/ntp.drift # NTP log file logfile /var/log/ntp.log
... or if you do not care about restrictions at all, something like this (note there are no restrictions, thus no need to reduce restrictions for 127.0.0.1 to allow your local clock to synchronize):
# public NTP servers to sync with (all stratum 2) server ntp1.cs.wisc.edu server ntp3.cs.wisc.edu server ntp3.sf-bay.org # NTP drift file - used to keep track of your system clocks # time deviation driftfile /etc/ntp.drift # NTP log file logfile /var/log/ntp.log
The reason for the 'restrict' lines is security: If you do not want a secure NTP server, do not add any restrict lines to your ntp.conf file. If you want a secure NTP server, start out by adding a default restrict that does not allow anything to contact your server, then add more (less restrictive) restrict lines - allowing certain addresses various access privileges.
Making ntpd keep the servers in the list while network is still down
During startup ntpd may start before you have a usable network connection. In this case ntpd will try to resolve the names of the servers and it will fail and remove all servers from the list of usable servers. To avoid this you may want to add the addresses of the ntp servers you will be using to the /etc/hosts file. What you need to add to the /etc/hosts file looks like:
128.105.39.11 ntp1.cs.wisc.edu 128.105.37.11 ntp3.cs.wisc.edu 206.55.70.42 ntp3.sf-bay.org
You can find the IP address of the servers you want to use with:
nslookup ntp1.cs.wisc.edu
/etc/rc.d/network
One more thing that you may want to do. In some cases, your /etc/ntp.conf file may be overwritten by dhcp. To avoid this, edit the /etc/conf.d/dhcpcd file and add -N
to the line that starts with 'dhcpcd -t 10
'.
On the system, /etc/conf.d/dhcpcd contains a single line:
DHCPCD_ARGS="-t 30 -h $HOSTNAME"
Change it to:
DHCPCD_ARGS="-N -t 30 -h $HOSTNAME"
Some have suggested adding -R
to preserve /etc/resolv.conf as well.
/etc/conf.d/ntp-client.conf
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 NTP_CLIENT_SERVER="pool.ntp.org" # client options NTP_CLIENT_OPTION="-b -u" # timeout for the ntp-client NTPCLIENT_TIMEOUT=10 # arguments passed to ntpd when started NTPD_ARGS="-g"
/etc/rc.conf
Add ntpdate and ntpd to the DAEMONS array:
DAEMONS=(syslog-ng network ntpdate ntpd ...)
Updating immediately using ntpdate
It is recommended to add a line like the following to your /etc/rc.local file so when you boot your system, your time will be correct (use an NTP server close to your location).
/usr/bin/ntpdate ntp1.cs.wisc.edu
Running ntpdate when you boot up is a good idea because ntpd may take a long time to synchronize your local clock depending on how far off the time is. If your clock is synchronized when ntpd starts, then its sole purpose is to keep it synchronized. To run ntpd at startup, add ntpd to the daemons section of the /etc/rc.conf file.
ntpd will work well if you have a connection to the internet all the time. If you are using dialup, you may just want to stick with using ntpdate via the command line.
Using NetworkManager dispatcher
With NetworkManager dispatcher scripts it is possible to update the time right after it has connected successfully. For this you need to place the following script to /etc/NetworkManager/dispatcher.d/time-sync
and make it executable (e.g. chmod +x time-sync
).
#!/bin/sh case "$2" in up) ntpdate pool.ntp.org &>/dev/null ;; *) echo "usage: $0 {up}" esac exit 0
Querying a NTP server using ntpq
There is a default restrict statement for the localhost that includes an ignore flag. Without overriding it (adding the line restrict 127.0.0.1) you will not be able to query your NTP server. If that is not a concern to you, then leave out the restrict line for your localhost. You will still be able to synchronize with your stratum 2 servers.