Ubuntu Bionic: Using chrony to configure NTP

Joshua Powers

Joshua Powers

on 9 April 2018

This article originally appeared on Joshua Powers’ blog


Using chrony to configure NTP

Starting with Ubuntu Bionic, the choice for fast and accurate time synchronization is chrony. For the features available with chrony and how chrony compares to other time synchronization implementations check out the comparison page on the chrony site.

This post will demonstrate how to get started, a few helpful commands for end-users, and a couple features of chrony.

Getting Started

The package consists of two commands: chronyc the client and chronyd the daemon. If chrony is not already installed it is a simple apt install away:

sudo apt install chrony

To verify that chrony is successfully installed and to see the number of servers and peers that are connected run the activity command:

$ chronyc activity
200 OK
8 sources online
0 sources offline
0 sources doing burst (return to online)
0 sources doing burst (return to offline)
0 sources with unknown address


Configuration settings for chrony are kept in /etc/chrony/chrony.conf. For details on all the numerous possible arguments see the configuration file page on the chrony site.

By default, the Ubuntu package will come with the configuration file pointing at ntp.ubuntu.com and the ubuntu.pool.ntp.org as NTP pools to provide 6 dual-stack NTP sources and 2 additional IPv4-only sources.

The only other item to note is the default configuration is the makestep argument, which will step up the system clock if the adjustment is larger than 1 second, but only for the first three clock updates. All other adjustments will use clock skewing to gradually correct the time by speeding up or slowing down the clock.


Below are a few chronyc subcommands to aid in gathering the status of chrony. A user can run these commands with chronyc or use chronyc to pull up a chrony specific command prompt and run them there.


To see what server chrony is tracking with and performance metrics from that server execute the tracking command:

For additional details on each field view the tracking documentation

$ chronyc tracking
Reference ID    : AB42617E (srcf-ntp.stanford.edu)
Stratum         : 2
Ref time (UTC)  : Thu Apr 05 18:27:33 2018
System time     : 0.000669840 seconds slow of NTP time
Last offset     : -0.000506939 seconds
RMS offset      : 0.001261410 seconds
Frequency       : 28.552 ppm slow
Residual freq   : -0.000 ppm
Skew            : 88.846 ppm
Root delay      : 0.031207338 seconds
Root dispersion : 0.001206590 seconds
Update interval : 65.2 seconds
Leap status     : Normal


The sources command shows a list of servers available to the system, status, and offsets from the local clock and the source. Using the -v flag provides the meanings of each column. Users should pay attention for servers with a state of ‘?’, ‘x’, or ‘~‘.

For additional details on each field view the sources documentation

$ chronyc sources -v
210 Number of sources = 8

  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
 / .- Source state '*' = current synced, '+' = combined , '-' = not combined,
| /   '?' = unreachable, 'x' = time may be in error, '~' = time too variable.
||                                                 .- xxxx [ yyyy ] +/- zzzz
||      Reachability register (octal) -.           |  xxxx = adjusted offset,
||      Log2(Polling interval) --.      |          |  yyyy = measured offset,
||                                \     |          |  zzzz = estimated error.
||                                 |    |           \
MS Name/IP address         Stratum Poll Reach LastRx Last sample
^+ chilipepper.canonical.com     2   7   377    39  +1245us[+1245us] +/-  102ms
^+ pugot.canonical.com           2   6   377    37   -554us[ -554us] +/-   99ms
^+ golem.canonical.com           2   6   377    39  -1135us[-1135us] +/-   98ms
^+ alphyn.canonical.com          2   6   377    44    -87us[  +62us] +/-   74ms
^* clock.xmission.com            2   6   377    41  -4641ns[ +144us] +/-   41ms
^+ duffman.whackertech.com       2   6   377    40    -14ms[  -14ms] +/-   89ms
^+ palpatine.steven-mcdonal>     2   6   177    40  +2565us[+2565us] +/-   46ms
^+ paladin.latt.net              2   7   377   106   -746us[ -573us] +/-   41ms


The sourcestats command is used to show additional statistics for each server including the number of samples from each server and estimated offset from a source. Using the -v flag to get more details on each column is helpful initially.

For additional details on each field view the sourcestats documentation

$ chronyc sourcestats -v
210 Number of sources = 8
                             .- Number of sample points in measurement set.
                            /    .- Number of residual runs with same sign.
                           |    /    .- Length of measurement set (time).
                           |   |    /      .- Est. clock freq error (ppm).
                           |   |   |      /           .- Est. error in freq.
                           |   |   |     |           /         .- Est. offset.
                           |   |   |     |          |          |   On the -.
                           |   |   |     |          |          |   samples. \
                           |   |   |     |          |          |             |
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
chilipepper.canonical.com  19   9  1170     +1.015      2.938   +166us  1095us
pugot.canonical.com         7   4   456     -0.639     11.812  +1362us   710us
golem.canonical.com         9   4   519     -0.315      7.244  +1891us   693us
alphyn.canonical.com        9   4   518     -0.919      9.460  +1134us   880us
clock.xmission.com          6   3   390     -1.257     20.127  +1978us   823us
duffman.whackertech.com     6   3   323     +2.404     25.543    -13ms   680us
palpatine.steven-mcdonal>   7   3   390     -0.421     15.058  +1776us   742us
paladin.latt.net           21  14   20m     +1.002      2.846  +1085us  1373us


The ntpdata command by provides details on a specified address or for every server the system is currently using if no address is specified. The data includes information about the source and statistics from the last valid measurement. Unlike the other commands this one does require elevated privileges.

For additional details on each field view the ntpdata documentation.

$ sudo chronyc ntpdata

Remote address  : (5BBD5E04)
Remote port     : 123
Local address   : (C0A801CB)
Leap status     : Normal
Version         : 4
Mode            : Server
Stratum         : 2
Poll interval   : 9 (512 seconds)
Precision       : -23 (0.000000119 seconds)
Root delay      : 0.015991 seconds
Root dispersion : 0.023285 seconds
Reference ID    : 8CCBCC4D ()
Reference time  : Fri Apr 06 15:55:56 2018
Offset          : +0.004150821 seconds
Peer delay      : 0.144625142 seconds
Peer dispersion : 0.000000157 seconds
Response time   : 0.000040157 seconds
Jitter asymmetry: -0.50
NTP tests       : 111 111 1111
Interleaved     : No
Authenticated   : No
TX timestamping : Daemon
RX timestamping : Kernel
Total TX        : 38
Total RX        : 38
Total valid RX  : 38

Local Server

When having multiple systems on a network it is advisable to setup a single system to act as the server for all other local systems. This is similar to how cloud providers will run their own NTP pools for instances inside their own data centers. As stated on the chrony site, the benefits of this model include reduced load on external connections, reduced load on the remote NTP servers, and keeps local systems in sync with each other should the external connection or servers go down.

To enable a local server in the configuration file, specify the network and subnet to allow connections from. When developing the access list, access can be tested on the server by using the accheck <address> command. For example, the below lines would allow connections from and all of the subnet:

allow 192.168.2

Assuming the system is reachable on port 123 for incoming connections and the service is restarted the server is ready.

To setup the clients, update the chrony configuration to point at the new system and restart chrony. For example, my server is at I can update the configuration file to use it to synchronize by adding:


A user can then verify the connection to the specified server with the cronyc activity command and the status of the connection using the cronyc tracking command.

On the server, verify the list of clients using the clients command:

$ sudo chronyc clients
Hostname                      NTP   Drop Int IntL Last     Cmd   Drop Int  Last
viper.maas                    390      0   0   -     0       0      0   -     -
localhost                       0      0   -   -     -       1      0   -     2

Hardware Timestamping

To enable even more accurate time synchronization consider using hardware timestamping if hardware supports it. This features provides incoming and outgoing packets with precise timestamps using the network controller. If this feature is enabled, it is best to enable it on both the host and client.

Hardware Support

To determine hardware support of timestamping view ethtool output and first look for ALL of the following capabilities:


And then confirm that the NIC has at least one of the following capabilities:


For more details on these capabilities see the kernel timestamping documentation. Below is an example of a NIC that has all the necessary capabilities for both send and receive:

$ ethtool -T enp0s25
Time stamping parameters for enp0s25:
    hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
    software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
    hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
    software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
    software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
    hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
    off                   (HWTSTAMP_TX_OFF)
    on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
    none                  (HWTSTAMP_FILTER_NONE)
    all                   (HWTSTAMP_FILTER_ALL)
    ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
    ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
    ptpv2-l4-sync         (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
    ptpv2-l4-delay-req    (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
    ptpv2-l2-sync         (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
    ptpv2-l2-delay-req    (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
    ptpv2-event           (HWTSTAMP_FILTER_PTP_V2_EVENT)
    ptpv2-sync            (HWTSTAMP_FILTER_PTP_V2_SYNC)
    ptpv2-delay-req       (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)

Enable Hardware Timestamping

To enable hardware timestamping in the configuration add the hwtimestamp option and either specify a single interface or use the wildcard character (*) to enable hardware timestamping on all interfaces that support it:

hwtimestamp enp0s25
hwtimestamp *

Once the service is restarted a user can verify if hardware timestamping is used by a particular interface through messages in syslog:

Apr  5 21:09:31 nexus chronyd[4104]: Enabled HW timestamping on enp0s25

To verify that the connection to a server is using hardware timestamping view the output of the ntpdata command:

$ sudo chronyc ntpdata | grep timestamping
TX timestamping : Hardware
RX timestamping : Hardware

Additional Performance

It is worth consulting the chrony guide on improving accuracy as depending on the environment additional tuning to server or pool parameters may result in gains in performance.

As an example, when connecting to a local system the guide suggested changing the polling interval and to use the interleave mode for additional performance gains:

server minpoll 2 maxpoll 2 xleave


To secure chornyc commands and NTP packets a user can enable authentication.

Keys are stored in the /etc/chrony/chrony.keys file (as setup by chrony.conf) and the file can take keys in a large variety of hash functions.


The built in keygen command is useful to generate random keys. It takes the following optional arguments:

  1. The key ID (default: 1)
  2. The type of has function to use (default: SHA1 or MD5 if SHA1 is not available)
  3. The number of bits to use between 80 and 4096 (default: 160)

Here are a few examples:

# generate key id #1 with a 160-bit SHA1 key
$ chronyc keygen
1 SHA1 HEX:57545218761536EE5FCBCEF67D9F720DE462FB4B

# generate key id #3 with a 160-bit SHA1 key
$ chronyc keygen 3 SHA1

# generate key id #16 with a 128-bit SHA256key
$ chronyc keygen 16 SHA256 128
16 SHA256 HEX:9001F203D6333523E320864C04259B20

# generate key id #27 with a 512-bit SHA512
$ chronyc keygen 27 SHA512 512
27 SHA512 HEX:B08BAAB8DED064D2C2351ED8C9EE5AABE784C80482809C5329187A2BE9D80A0B1E6E18C4164946F6D8E36F1C4A2A966B3B754B1FDE89A0E66FE92CC1E65364E5

Example Authentication

First, the key files of the server and the client need the same key ID entry that will be used, otherwise no relationship between the computers will be possible. Both the client and server service need to be restarted to pick up the keys once there.

For example, if the client and server key files had the following entry:

27 SHA1 HEX:57545218761536EE5FCBCEF67D9F720DE462FB4B

On the client, in order to tell Chrony to use key #27 for a particular server add the key option to the corresponding server entry:

add server key 27

Restart the service and see if the server now shows as authenticated:

$ sudo chronyc ntpdata | grep Authenticated
Authenticated   : Yes



To determine the synchronizing status, check the values from the tracking command. If the values are all zeros and the reference time is wrong, then the system is not synchronizing correctly. For example if a user sees the following then no valid NTP data was received:

$ chronyc tracking
Reference ID    : 00000000 ()
Stratum         : 0
Ref time (UTC)  : Thu Jan 01 00:00:00 1970
System time     : 0.000000000 seconds fast of NTP time
Last offset     : +0.000000000 seconds
RMS offset      : 0.000000000 seconds
Frequency       : 0.888 ppm fast
Residual freq   : +0.000 ppm
Skew            : 0.000 ppm
Root delay      : 1.000000000 seconds
Root dispersion : 1.000000000 seconds
Update interval : 0.0 seconds
Leap status     : Not synchronised


By default logs for chrony go to syslog, therefore start by searching through /var/log/syslog for details on the chrony service.

Additional Logging

If additional logging is required to aid in debugging or tracking the performance of the NTP service, it is possible by commenting out or adding the line below to the chrony configuration file. In doing so enables additional data and statistics to be collected on an on-going basis:

# Uncomment the following line to turn additional logging on
log measurements statistics tracking
# or for even more logs add additional items
log measurements statistics tracking rtc refclocks tempcomp

These logs will appear under the default directory of /var/log/chrony

$ ls /var/log/chrony/
measurements.log  statistics.log  tracking.log


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