NTP is a TCP/IP protocol for synchronizing time over a network. Basically a client requests the current time from a server, and uses it to set its own clock.
Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks, and tier two and three servers spreading the load of actually handling requests across the Internet. Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. But luckily all that complexity is hidden from you!
Ubuntu by default uses timedatectl / timesyncd to synchronize time and users can optionally use chrony to serve the Network Time Protocol.
Synchronizing your systems time
Since Ubuntu 16.04 timedatectl / timesyncd (which are part of systemd) replace most of ntpdate / ntp.
timesyncd is available by default and replaces not only ntpdate, but also the client portion of chrony (or formerly ntpd). So on top of the one-shot action that ntpdate provided on boot and network activation, now timesyncd by default regularly checks and keeps your local time in sync. It also stores time updates locally, so that after reboots monotonically advances if applicable.
If chrony is installed timedatectl steps back to let chrony do the time keeping. That shall ensure that no two time syncing services are fighting. While no more recommended to be used, this still also applies to ntpd being installed to retain any kind of old behavior/config that you had through an upgrade. But it also implies that on an upgrade from a former release ntp/ntpdate might still be installed and therefore renders the new systemd based services disabled.
ntpdate is considered deprecated in favor of timedatectl (or chrony) and thereby no more installed by default. timesyncd will generally do the right thing keeping your time in sync, and chrony will help with more complex cases. But if you had one of a few known special ntpdate use cases, consider the following:
If you require a one-shot sync use:
If you require a one-shot time check, without setting the time use:
Configuring timedatectl and timesyncd
The current status of time and time configuration via timedatectl and timesyncd can be checked with
$ timedatectl status Local time: Fr 2018-02-23 08:47:13 UTC Universal time: Fr 2018-02-23 08:47:13 UTC RTC time: Fr 2018-02-23 08:47:13 Time zone: Etc/UTC (UTC, +0000) System clock synchronized: yes systemd-timesyncd.service active: yes RTC in local TZ: no
If chrony is running it will automatically switch to:
[...] systemd-timesyncd.service active: no
Via timedatectl an admin can control the timezone, how the system clock should relate to the hwclock and if permanent synronization should be enabled or not. See
man timedatectl for more details.
timesyncd itself is still a normal service, so you can check its status also more in detail via.
$ systemctl status systemd-timesyncd systemd-timesyncd.service - Network Time Synchronization Loaded: loaded (/lib/systemd/system/systemd-timesyncd.service; enabled; vendor preset: enabled) Active: active (running) since Fri 2018-02-23 08:55:46 UTC; 10s ago Docs: man:systemd-timesyncd.service(8) Main PID: 3744 (systemd-timesyn) Status: "Synchronized to time server 188.8.131.52:123 (ntp.ubuntu.com)." Tasks: 2 (limit: 4915) CGroup: /system.slice/systemd-timesyncd.service |-3744 /lib/systemd/systemd-timesyncd Feb 23 08:55:46 bionic-test systemd: Starting Network Time Synchronization... Feb 23 08:55:46 bionic-test systemd: Started Network Time Synchronization. Feb 23 08:55:46 bionic-test systemd-timesyncd: Synchronized to time server 184.108.40.206:123 (ntp.ubuntu.com).
The nameserver to fetch time for timedatectl and timesyncd from can be specified in
/etc/systemd/timesyncd.conf and additional config files can be stored in
/etc/systemd/timesyncd.conf.d/. The entries for NTP= and FallbackNTP= are space separated lists. See
man timesyncd.conf for more.
Serve the Network Time Protocol
If in addition to synchronizing your system you also want to serve NTP information you need an NTP server. There are several options with chrony, ntpd and open-ntp. The recommended solution is chrony.
The NTP daemon chronyd calculates the drift and offset of your system clock and continuously adjusts it, so there are no large corrections that could lead to inconsistent logs for instance. The cost is a little processing power and memory, but for a modern server this is usually negligible.
To install chrony, from a terminal prompt enter:
sudo apt install chrony
This will provide two binaries:
chronyd - the actual daemon to sync and serve via the NTP protocol
chronyc - command-line interface for chrony daemon
/etc/chrony/chrony.conf to add/remove server lines. By default these servers are configured:
# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board # on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for # more information. pool 0.ubuntu.pool.ntp.org iburst pool 1.ubuntu.pool.ntp.org iburst pool 2.ubuntu.pool.ntp.org iburst pool 3.ubuntu.pool.ntp.org iburst
man chrony.conf for more details on the configuration options. After changing the any of the config file you have to restart chrony:
sudo systemctl restart chrony.service
Of the pool
2.ubuntu.pool.ntp.org as well as
ntp.ubuntu.com also support ipv6 if needed. If one needs to force ipv6 there also is
ipv6.ntp.ubuntu.com which is not configured by default.
Serving the NTP Protocol
You can install chrony (above) and configure special Hardware (below) for a local synchronization
and as-installed that is the default to stay on the secure and conservative side. But if you want to serve NTP you need adapt your configuration.
To enable serving the NTP protocol you’ll need at least to set the
allow rule to which controls which clients/networks you want chrony to serve NTP to.
An example would be
See the section “NTP server” in the man page for more details on how you can control and restrict access to your NTP server.
Use chronyc to see query the status of the chrony daemon. For example to get an overview of the currently available and selected time sources.
chronyc sources MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== ^+ gamma.rueckgr.at 2 8 377 135 -1048us[-1048us] +/- 29ms ^- 2b.ncomputers.org 2 8 377 204 -1141us[-1124us] +/- 50ms ^+ www.kashra.com 2 8 377 139 +3483us[+3483us] +/- 18ms ^+ stratum2-4.NTP.TechFak.U> 2 8 377 143 -2090us[-2073us] +/- 19ms ^- zepto.mcl.gg 2 7 377 9 -774us[ -774us] +/- 29ms ^- mirrorhost.pw 2 7 377 78 -660us[ -660us] +/- 53ms ^- atto.mcl.gg 2 7 377 8 -823us[ -823us] +/- 50ms ^- static.220.127.116.11.cli> 2 8 377 9 -1503us[-1503us] +/- 45ms ^- 18.104.22.168 2 8 377 137 -11ms[ -11ms] +/- 117ms ^- 22.214.171.124 3 7 377 10 -3274us[-3274us] +/- 70ms ^- bagnikita.com 2 7 377 74 +3131us[+3131us] +/- 71ms ^- europa.ellipse.net 2 8 377 204 -790us[ -773us] +/- 97ms ^- tethys.hot-chilli.net 2 8 377 141 -797us[ -797us] +/- 59ms ^- 66-232-97-8.static.hvvc.> 2 7 377 206 +1669us[+1686us] +/- 133ms ^+ 126.96.36.199 1 8 377 205 +175us[ +192us] +/- 12ms ^* 46-243-26-34.tangos.nl 1 8 377 141 -123us[ -106us] +/- 10ms ^- pugot.canonical.com 2 8 377 21 -95us[ -95us] +/- 57ms ^- alphyn.canonical.com 2 6 377 23 -1569us[-1569us] +/- 79ms ^- golem.canonical.com 2 7 377 92 -1018us[-1018us] +/- 31ms ^- chilipepper.canonical.com 2 8 377 21 -1106us[-1106us] +/- 27ms chronyc sourcestats 210 Number of sources = 20 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev ============================================================================== gamma.rueckgr.at 25 15 32m -0.007 0.142 -878us 106us 2b.ncomputers.org 26 16 35m -0.132 0.283 -1169us 256us www.kashra.com 25 15 32m -0.092 0.259 +3426us 195us stratum2-4.NTP.TechFak.U> 25 14 32m -0.018 0.130 -2056us 96us zepto.mcl.gg 13 11 21m +0.148 0.196 -683us 66us mirrorhost.pw 6 5 645 +0.117 0.445 -591us 19us atto.mcl.gg 21 13 25m -0.069 0.199 -904us 103us static.188.8.131.52.cli> 25 18 34m -0.005 0.094 -1526us 78us 184.108.40.206 25 10 32m +0.412 0.110 -11ms 84us 220.127.116.11 24 12 30m -0.983 0.173 -3718us 122us bagnikita.com 17 7 31m -0.132 0.217 +3527us 139us europa.ellipse.net 26 15 35m +0.038 0.553 -473us 424us tethys.hot-chilli.net 25 11 32m -0.094 0.110 -864us 88us 66-232-97-8.static.hvvc.> 20 11 35m -0.116 0.165 +1561us 109us 18.104.22.168 26 11 35m -0.054 0.390 +129us 343us 46-243-26-34.tangos.nl 25 16 32m +0.129 0.297 -307us 198us pugot.canonical.com 25 14 34m -0.271 0.176 -143us 135us alphyn.canonical.com 17 11 1100 -0.087 0.360 -1749us 114us golem.canonical.com 23 12 30m +0.057 0.370 -988us 229us chilipepper.canonical.com 25 18 34m -0.084 0.224 -1116us 169us
Certain chronyc commands are privileged and can not be run via the network without explicitly allowing them. See section Command and monitoring access in
man chrony.conf for more details. A local admin can use sudo as usually as this will grant him access to the local admin socket
Chrony supports various PPS types natively. It can use kernel PPS API as well as PTP hardware clock. Most general GPS receivers can be leveraged via GPSD. The latter (and potentially more) can be accessed via SHM or via a socket (recommended). All of the above can be used to augment chrony with additional high quality time sources for better accuracy, jitter, drift, longer-or-short term accuracy (Usually each kind of clock type is good at one of those, but non-perfect at the others). For more details on configuration see some of the external PPS/GPSD resource listed below.
Note: at the release of 20.04 there was a bug which until fixed you might want to add this content to your
Example configuration for GPSD to feed Chrony
For the setup you need
$ sudo apt install gpsd chrony
But you will want to test/debug your setup and especially the GPS reception, therefore also install
$ sudo apt install pps-tools gpsd-clients
GPS devices usually will communicate via serial interfaces, yet the most common type these days are USB GPS devices which have a serial converter behind USB. If you want to use this for PPS please be aware that the majority does not signal PPS via USB. Check the GPSD hardware list for details. The examples below were run with a Navisys GR701-W.
When plugging in such a device (or at boot time)
dmesg should report serial connection of some sorts, example:
[ 52.442199] usb 1-1.1: new full-speed USB device number 3 using xhci_hcd [ 52.546639] usb 1-1.1: New USB device found, idVendor=067b, idProduct=2303, bcdDevice= 4.00 [ 52.546654] usb 1-1.1: New USB device strings: Mfr=1, Product=2, SerialNumber=0 [ 52.546665] usb 1-1.1: Product: USB-Serial Controller D [ 52.546675] usb 1-1.1: Manufacturer: Prolific Technology Inc. [ 52.602103] usbcore: registered new interface driver usbserial_generic [ 52.602244] usbserial: USB Serial support registered for generic [ 52.609471] usbcore: registered new interface driver pl2303 [ 52.609503] usbserial: USB Serial support registered for pl2303 [ 52.609564] pl2303 1-1.1:1.0: pl2303 converter detected [ 52.618366] usb 1-1.1: pl2303 converter now attached to ttyUSB0
We see above that it appeared as
ttyUSB0. To later on have
chrony accept being feeded time information by that we have to set it up in
/etc/chrony/chrony.conf (Please replace
USB0 to whatever applies to your setup):
refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2 refclock SOCK /var/run/chrony.ttyUSB0.sock refid PPS
chrony to make the socket available and waiting.
sudo systemctl restart chrony
Next one needs to tell
gpsd which device to manager, therefore in
/etc/default/gpsd we set:
Furthermore the default use-case of
gpsd is, well for gps position tracking. Therefore it will normally not consume any cpu since it is not running the service but waiting on a socket for clients. Furthermore the client will then tell
gpsd what it requests and
gpsd will only do that.
For the use case of gpsd as PPS-providing-daemon you want to set the option to:
- immediately start even without a client connected, this can be set in
- enable the service itself and not wait for a client to reach the socket in the future:
sudo systemctl enable /lib/systemd/system/gpsd.service
gpsd will now initialize the PPS from GPS and in
dmesg you will see
pps_ldisc: PPS line discipline registered pps pps0: new PPS source usbserial0 pps pps0: source "/dev/ttyUSB0" added
In case you have multiple PPS the tool
ppsfind might be useful to identify which PPS belongs to which GPS. You could check that with:
$ sudo ppsfind /dev/ttyUSB0 pps0: name=usbserial0 path=/dev/ttyUSB0
To get any further you need your GPS to get a lock.
gpsmon need to report a 3D Fix for the data being any good.
Then you’d want to check to really have PPS data reported on that with
$ cgps ... │ Status: 3D FIX (7 secs) ...
Next one might want to ensure that the pps device really submits PPS data, to do so run
$ sudo ppstest /dev/pps0 trying PPS source "/dev/pps0" found PPS source "/dev/pps0" ok, found 1 source(s), now start fetching data... source 0 - assert 1588140739.099526246, sequence: 69 - clear 1588140739.999663721, sequence: 70 source 0 - assert 1588140740.099661485, sequence: 70 - clear 1588140739.999663721, sequence: 70 source 0 - assert 1588140740.099661485, sequence: 70 - clear 1588140740.999786664, sequence: 71 source 0 - assert 1588140741.099792447, sequence: 71 - clear 1588140740.999786664, sequence: 71
gpsd is now running, the GPS reception has found a fix and this is fed into
Now lets check that from the point of view of
Initially (e.g. before
gpsd has started or before it has a lock) those will be new and “untrusted” sources marked with an “?”. If your devices stay in the “?” state and won’t leave it even after quite some time then
gpsd seems not to feed any data to
chrony and you’d need to debug why.
chronyc> sources 210 Number of sources = 10 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #? GPS 0 4 0 - +0ns[ +0ns] +/- 0ns #? PPS 0 4 0 - +0ns[ +0ns] +/- 0ns
Over time chrony will classify them as good or bad.
In the example case the raw GPS had too much deviation but PPS is good.
chronyc> sources 210 Number of sources = 10 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #x GPS 0 4 177 24 -876ms[ -876ms] +/- 200ms #- PPS 0 4 177 21 +916us[ +916us] +/- 63us ^- chilipepper.canonical.com 2 6 37 53 +33us[ +33us] +/- 33ms
And finally after a while it used the hardware PPS input as it was better:
chronyc> sources 210 Number of sources = 10 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #x GPS 0 4 377 20 -884ms[ -884ms] +/- 200ms #* PPS 0 4 377 18 +6677ns[ +52us] +/- 58us ^- alphyn.canonical.com 2 6 377 20 -1303us[-1258us] +/- 114ms
The PPS might also be ok but used in a combined way with e.g. the selected server.
man chronyc for more details about how all these combinations will look like.
chronyc> sources 210 Number of sources = 11 MS Name/IP address Stratum Poll Reach LastRx Last sample =============================================================================== #? GPS 0 4 0 - +0ns[ +0ns] +/- 0ns #+ PPS 0 4 377 22 +154us[ +154us] +/- 8561us ^* chilipepper.canonical.com 2 6 377 50 -353us[ -300us] +/- 44ms
And if you wonder if your shm based GPS data is any good, you can check for that as well.
First of all chrony will not only tell you if it classified it as good or bad. Via
sourcestats you can also check the details
chronyc> sourcestats 210 Number of sources = 10 Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev ============================================================================== GPS 20 9 302 +1.993 11.501 -868ms 1208us PPS 6 3 78 +0.324 5.009 +3365ns 41us golem.canonical.com 15 10 783 +0.859 0.509 -750us 108us
You can also track the raw data that gpsd or other ntpd compliant refclocks are sending via shared memory by using
$ sudo ntpshmmon -o ntpshmmon: version 3.20 # Name Offset Clock Real L Prc sample NTP1 0.000223854 1588265805.000223854 1588265805.000000000 0 -10 sample NTP0 0.125691783 1588265805.125999851 1588265805.000308068 0 -20 sample NTP1 0.000349341 1588265806.000349341 1588265806.000000000 0 -10 sample NTP0 0.130326636 1588265806.130634945 1588265806.000308309 0 -20 sample NTP1 0.000485216 1588265807.000485216 1588265807.000000000 0 -10
In Chrony 4.0 (first appeared in Ubuntu 21.04 Hirsute) support for Network Time Security “NTS” as added.
To set up your server with NTS you’ll need certificates so that the server can authenticate itself and based on that allow to encrypt and verify the NTP traffic.
In addition to the
allow statement that any chrony working as NTP server needs there are two mandatory config entries that will be needed. Those for the certificates like
Example entries would look like:
ntsservercert /etc/chrony/fullchain.pem ntsserverkey /etc/chrony/privkey.pem
It is important to note that for isolation reasons chrony by default runs as user and group
_chrony. Therefore you need to grant access to the certificates for that user.
$ sudo chown _chrony:_chrony /etc/chrony/*.pem
Then restart chrony
systemctl restart chrony and it will be ready to provide NTS based time service.
A running chrony server has various statistics, one also accounts the number of NTS connections that were established (or dropped):
$ sudo chronyc -N serverstats NTP packets received : 213 NTP packets dropped : 0 Command packets received : 117 Command packets dropped : 0 Client log records dropped : 0 NTS-KE connections accepted: 2 NTS-KE connections dropped : 0 Authenticated NTP packets : 197
And there is also a per-client statistic which can be enabled by the
-p option of the
$ sudo chronyc -N clients -k Hostname NTP Drop Int IntL Last NTS-KE Drop Int Last =============================================================================== 10.172.196.173 197 0 10 - 595 2 0 5 48h ...
For more complex scenarios there are more advanced options to configure NTS documented in the man page.
A Note about certificate placement:
Chrony by default is isolated via apparmor and in addition uses all kind of
protect*features of systemd. Due to that there are not many paths chrony can access for the certificates. But /etc/chrony/* is allowed as read-only and that is enough.
/etc/apparmor.d/usr.sbin.chronydif you want other paths or allow custom paths in
The client needs to specify
server as usual (
pool directives do not work with NTS). As usual after the server adress options can be listed and there
nts has to be added.
server <server-fqdn-or-IP> iburst nts
One can check the authdata of the connections the client established like
$ sudo chronyc -N authdata Name/IP address Mode KeyID Type KLen Last Atmp NAK Cook CLen ========================================================================= <server-fqdn-or-ip> NTS 1 15 256 48h 0 0 8 100
Again there are more advanced options documented in the man page. Common use cases are specifying an explicit trusted certificate.
Bad Clocks and secure time syncing - “A Chicken and Egg” Problem:
There is one problem with systems that have very bad clocks. NTS is based on TLS and that needs a a somewhat correct clock. Due to that an NTS based sync might fail. On Hardware affected by this one can consider using the
nocerttimecheckoption which allows to set a number of times time can be synced without checking validation and expiration.
See the Ubuntu Time wiki page for more information.