Network Time Protocol daemon
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.
- 1 Installation
- 2 Configuration
- 3 Using without daemon
- 4 Running as a daemon
- 5 Alternatives
- 6 See also
- 7 External links
Configuring connection to NTP servers
The first thing you define in your
/etc/ntp.conf is the servers your machine will synchronize to. Some defaults are present already.
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 do not already know the servers you are 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.pool.ntp.org iburst server 1.pool.ntp.org iburst server 2.pool.ntp.org iburst server 3.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.
Configuring your own NTP server
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:
server 127.127.1.0 fudge 127.127.1.0 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 noquery
This restricts everyone from modifying anything and prevents everyone from querying the status of your time server:
nomodify prevents reconfiguring your ntpd (with ntpq or ntpdc), and
noquery prevents dumping status data from your ntpd (also with ntpq or ntpdc).
You can also add other options:
restrict default kod nomodify notrap nopeer noquery
noserveto stop serving time.
Full docs for the "restrict" option are in
man ntp_acc. See https://support.ntp.org/bin/view/Support/AccessRestrictions for detailed instructions.
Following this line, you need to tell ntpd what to allow through into your server; the following line is enough if you are not configuring an NTP server:
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 kod nomodify notrap nopeer noquery restrict -6 ::1 # ::1 is the IPv6 equivalent for 127.0.0.1
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):
server 0.pool.ntp.org iburst server 1.pool.ntp.org iburst server 2.pool.ntp.org iburst server 3.pool.ntp.org iburst restrict default kod nomodify notrap nopeer noquery restrict -6 default kod nomodify notrap nopeer noquery restrict 127.0.0.1 restrict -6 ::1 driftfile /var/lib/ntp/ntp.drift logfile /var/log/ntp.log
Other resources about NTP configuration
In conclusion, never forget man pages:
man ntp.conf is likely to answer any doubts you could still have (see also the related man pages:
Using without daemon
To synchronize your system clock just once, without starting the NTP daemon or changing the default configuration, run:
# ntpd -qg
You must also start the hwclock daemon:
# rc.d start hwclock
hwclock will just save the system clock to the hardware clock when you power off. Add
hwclock to your DAEMONS array to avoid the second step in the future.
-g option allows shifting the clock further than the panic threshold (15 min by default) without a warning. This behavior mimics that of the
ntpdate program, which is now deprecated. Note that such offset is abnormal and might indicate either wrong timezone setting, clock chip failure, or simply a very long period of neglect. If in these cases you would rather not set the clock and print an error to syslog, remove
# ntpd -q
For example, you could add a
ntpd -qg & line to your
/etc/rc.local to run at every boot. See Autostarting for alternative methods.
- 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.
ntpd -qgas 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.
- If something other already takes care of updating the hardware clock, for example another operating system in dual boot, you should avoid starting
rc.localis executed, the network connection has already been initialized (for example you should not background essential network-related daemons in
Running as a daemon
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 is running):
# rc.d stop hwclock
Start the ntpd daemon:
# rc.d start ntpd
Add ntpd to your DAEMONS array so it starts automatically on boot and make sure hwclock is disabled:
DAEMONS=(... !hwclock 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 the ntp directory to the ntp user/group:
# chown -R ntp:ntp /var/lib/ntp
/etc/conf.d/ntpd.conf and change
NTPD_ARGS="-g -u ntp:ntp"
Finally, restart the daemon:
# rc.d restart ntpd
Running in a chroot
/etc/conf.d/ntpd.conf and change
NTPD_ARGS="-g -u ntp:ntp"
NTPD_ARGS="-g -i /var/lib/ntp -u ntp:ntp"
/etc/ntp.conf to change the driftfile path such that it is relative to the chroot directory, rather than to the real system root. Change:
Create a suitable chroot environment so that getaddrinfo() will work by creating pertinent directories and files (as root):
# mkdir /var/lib/ntp/etc /var/lib/ntp/lib /var/lib/ntp/proc # touch /var/lib/ntp/etc/resolv.conf /var/lib/ntp/etc/services
and by bind-mounting the aformentioned files:
... #ntpd chroot mounts /etc/resolv.conf /var/lib/ntp/etc/resolv.conf none bind 0 0 /etc/services /var/lib/ntp/etc/services none bind 0 0 /lib /var/lib/ntp/lib none bind 0 0 /proc /var/lib/ntp/proc none bind 0 0
# mount -a
Finally, restart the daemon again:
# rc.d restart ntpd
It is relatively difficult to be sure that your driftfile configuration is actually working without waiting a while, as ntpd does not read or write it very often. If you get it wrong, it will log an error; if you get it right, it will update the timestamp. If you do not see any errors about it after a full day of running, and the timestamp is updated, you should be confident of success.
Available alternative to NTPd are Chrony, a dial-up friendly and specifically designed for systems that are not online all the time, and OpenNTPD, part of the OpenBSD project and currently not maintained for Linux.
- Time (for more information on computer timekeeping)