Network Time Protocol daemon

Related articles
NTPSec
Chrony
Time synchronization
Network Time Protocol
is the most common method to synchronize the
software clock
of a GNU/Linux system with internet time servers. It is designed to mitigate the effects of variable network latency and can usually maintain time to within tens of milliseconds over the public Internet. The accuracy on local area networks is even better, up to one millisecond.
The NTP Project
provides a reference implementation of the protocol called simply NTP. This article further describes how to set up and run the NTP daemon, both as a client and as a server.
See
System time#Time synchronization
for other NTP implementations.
Installation
Install
the
ntp
package. By default,
ntpd
works in client mode without further configuration. You can skip to
#Usage
if you want to use the Arch Linux default configuration file for it. For server configuration, see
#NTP server mode
.
Configuration
The main daemon is
ntpd
, which is configured in
/etc/ntp.conf
. Refer to
ntp.conf(5)
for detail.
Connection to NTP servers
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 choose a server pool close to your location from the
pool.ntp.org
servers (
alternative link
).
Since
ntp
version 4.2.7.p465-2, Arch Linux uses its own default vendor pool of NTP servers provided by
the NTP Pool Project
(see
FS#41700
). Modify those to suit your needs, e.g. if you want to use your country's servers with an option:
/etc/ntp.conf
server 0.fr.pool.ntp.org iburst
server 1.fr.pool.ntp.org iburst
server 2.fr.pool.ntp.org iburst
server 3.fr.pool.ntp.org iburst
The
iburst
option is recommended, and sends a burst of packets only if it cannot obtain a connection with the first attempt. The
burst
option always does this, even on the first attempt, and should never be used without explicit permission and may result in blacklisting.
Leap seconds file
In order for the system to be able to provide the
International Atomic Time
to an application that requests it, the list of leap seconds must be loaded. The list is part of the
tzdata
package and can be loaded by adding the following line to the NTP configuration file:
leapfile /usr/share/zoneinfo/leap-seconds.list
NTP server mode
If setting up an NTP server, check that you have
orphan mode
enabled, so that, in case it loses internet access, it will continue serving time to the network; enable orphan mode using the
tos
configuration parameter (you can set up to
stratum 15
) so that it will never be used unless internet access is lost:
tos orphan 15
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
ntpd
(with
ntpq
or
ntpdc
), and
noquery
is important to
prevent
dumping status data from
ntpd
(also with
ntpq
or
ntpdc
).
You can also add other options:
restrict default kod limited nomodify notrap nopeer noquery
Note
This still allows other people to query your time server. You need to add
noserve
to stop serving time. It will also block time synchronization since it blocks all packets except
ntpq
and
ntpdc
queries.
If you want to change any of these, see the full docs for the "restrict" option in
ntp.conf(5)
, the detailed ntp
instructions
and
#Usage
.
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:
restrict 127.0.0.1
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 limited 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:
/etc/ntp.conf
server 0.arch.pool.ntp.org iburst
server 1.arch.pool.ntp.org iburst
server 2.arch.pool.ntp.org iburst
server 3.arch.pool.ntp.org iburst
tos orphan 15

restrict default kod limited nomodify notrap nopeer noquery
restrict -6 default kod limited nomodify notrap nopeer noquery

restrict 127.0.0.1
restrict -6 ::1  

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log
Note
Defining the log file is not mandatory, but it is always a good idea to have feedback for
ntpd
operations.
Usage
The package has a default client-mode configuration and its own user and group to drop root privileges after starting. If you start it from the console, you should always do so with the
-u
option:
# ntpd -u ntp:ntp
The
-u
option is employed by the two included systemd services. These services also use the
-g
option, which disables a threshold (so-called
panic-gate
). Hence, they will synchronize time even in case the ntp-server's time exceeds the threshold deviation from the system clock.
Warning
One reason the panic-gate was introduced is that background jobs/services may be susceptible to time-jumps. If the system's clock was never synchronized before, consider stopping them before running
ntpd
for the first time.
Both services are tied to the system's resolver, and will start synchronizing when an active network connection is detected.
Start ntpd at boot
Enable
the daemon with
ntpd.service
. See also
#Running in a chroot
.
Note
The systemd command
timedatectl
can only be used to control
systemd-timesyncd
. Executing
timedatectl set-ntp 1
as root will inadvertedly stop a running
ntpd.service
.
[1]
Use
ntpq
to see the list of configured peers and status of synchronization:
$ ntpq -p
The delay, offset and jitter columns should be non-zero. The servers
ntpd
is synchronizing with are prefixed by an asterisk. It can take several minutes before
ntpd
selects a server to synchronize with; try checking after 17 minutes (1024 seconds).
Synchronize time once per boot
Alternatively,
enable
ntpdate.service
to synchronize time once (option
-q
) and non-forking (option
-n
) per boot, instead of running the daemon in the background. This method is discouraged on servers, and in general on machines that run without rebooting for more than a few days.
If the synchronized time should be written to the hardware clock as well, configure the provided unit as described in
systemd#Editing provided units
before starting it:
/etc/systemd/system/ntpdate.service.d/hwclock.conf
[Service]
ExecStart=/usr/bin/hwclock -w
Tips and tricks
Start ntpd on network connection
ntpd
can be started by your network manager, so that the daemon only runs when the computer is online.
Netctl
Append the following lines to your
netctl
profile:
ExecUpPost="systemctl start ntpd.service"
ExecDownPre="systemctl stop ntpd.service"
NetworkManager
The
ntpd
daemon can be brought up/down along with a network connection through the use of NetworkManager's
dispatcher
scripts. The
networkmanager-dispatcher-ntpd
AUR
package installs one, pre-configured to start and stop the
ntpd service
with a connection.
KDE
KDE can use NTP (ntp must be installed) by right clicking the clock and selecting
Adjust date/time
. However, this requires the ntp daemon to be
disabled
before configuring KDE to use NTP.
[2]
Using ntpd with GPS
Most of the articles online about configuring
ntpd
to receive time from a GPS suggest to use the SHM (shared memory) method. However, at least since
ntpd
version 4.2.8, a
much better
method is available. It connects directly to
gpsd
, so
gpsd
needs to be installed.
Add these lines to your
/etc/ntp.conf
:
/etc/ntp.conf
#=========================================================
#  GPSD native ntpd driver
#=========================================================
# This driver exists from at least ntp version 4.2.8
# Details at
#
https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver46.html
server 127.127.46.0 
fudge 127.127.46.0 time1 0.0 time2 0.0 refid GPS
This will work as long as you have
gpsd
working. It connects to
gpsd
via the local socket and queries the "gpsd_json" object that is returned.
To test the setup, first ensure that
gpsd
is working by running:
$ cgps -s
Then wait a few minutes and run
ntpq -p
. This will show if
ntpd
is talking to
gpsd
:
$ ntpq -p
remote           refid            st t when poll reach   delay   offset  jitter
 ==================================================================================
*GPSD_JSON(0)    .GPS.            0 l   55   64  377    0.000    2.556  14.109
Tip
If the
reach
column is 0, it means
ntpd
has not been able to talk to
gpsd
. Wait a few minutes and try again. Sometimes it takes
ntpd
a while.
The GPS device must have PPS capabilities. You can test if your device is capable running
ppscheck /dev/gps0
.
Note
ntpd
expects that your GPS device is called e.g.
/dev/gps0
. If your GPS device is connected via USB, it may appear as
/dev/ttyUSB0
instead, and you may have to create a symlink
ln -s /dev/ttyUSB0 /dev/gps0
and run
gpsd
on that linked
/dev/gps0
so that the
GPSD_JSON
line appears as expected.
gpsd
should be run with the
-n
flag on the
GPSD_OPTIONS
line and use
/dev/gps0
on the
DEVICES
line in the
/etc/default/gpsd
config file.
Running in a chroot
Note
ntpd
should be started as non-root (default in the Arch Linux package) before attempting to jail it in a chroot, since chroots are relatively useless at securing processes running as root.
Create a new directory
/etc/systemd/system/ntpd.service.d/
if it does not exist and a file named
customexec.conf
inside with the following content:
[Service]
ExecStart=
ExecStart=/usr/bin/ntpd -g -u ntp:ntp
-i /var/lib/ntp
Then, edit
/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:
driftfile       /var/lib/ntp/ntp.drift
to
driftfile       /ntp.drift
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
# mkdir /var/lib/ntp/usr /var/lib/ntp/usr/lib
# touch /var/lib/ntp/etc/resolv.conf /var/lib/ntp/etc/services
and by bind-mounting the aformentioned files:
/etc/fstab
...
#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
/usr/lib	  /var/lib/ntp/usr/lib none bind 0 0
/proc		  /var/lib/ntp/proc none bind 0 0
# mount -a
Finally, restart
ntpd
daemon again. Once it restarted you can verify that the daemon process is chrooted by checking where
/proc/{PID}/root
symlinks to:
# ps -C ntpd | awk '{print $1}' | sed 1d | while read -r PID; do ls -l /proc/$PID/root; done
should now link to
/var/lib/ntp
instead of
/
.
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.
Restrict listening sockets
You can limit sockets
ntpd
is listening to using the
interface
option:
interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard | name | address[/prefixlen]]
like
/etc/ntp.conf
interface listen lo
interface listen enp3s0
interface ignore enp5s0
See also
https://www.ntp.org/
https://support.ntp.org/
https://www.pool.ntp.org/
https://www.eecis.udel.edu/~mills/ntp/html/index.html
https://www.akadia.com/services/ntp_synchronize.html