path: root/FAQ

Frequently Asked Questions

Table of Contents

  o 1. chrony compared to other programs
      ? 1.1. How does chrony compare to ntpd?
  o 2. Configuration issues
      ? 2.1. What is the minimum recommended configuration for an NTP client?
      ? 2.2. How do I make an NTP server?
      ? 2.3. Should all computers on a LAN be clients of an external server?
      ? 2.4. Must I specify servers by IP address if DNS is not available on
        chronyd start?
      ? 2.5. How can I make chronyd more secure?
      ? 2.6. How can I improve the accuracy of the system clock with NTP
        sources?
      ? 2.7. Does chronyd have an ntpdate mode?
      ? 2.8. Can chronyd be configured to control the clock like ntpd?
      ? 2.9. Can NTP server be separated from NTP client?
      ? 2.10. Should be a leap smear enabled on NTP server?
      ? 2.11. Does chrony support PTP?
      ? 2.12. What happened to the commandkey and generatecommandkey
        directives?
  o 3. Computer is not synchronising
      ? 3.1. Behind a firewall?
      ? 3.2. Are NTP servers specified with the offline option?
      ? 3.3. Is chronyd allowed to step the system clock?
      ? 3.4. Using a Windows NTP server?
      ? 3.5. An unreachable source is selected?
      ? 3.6. Does selected source drop new measurements?
      ? 3.7. Using a PPS reference clock?
  o 4. Issues with chronyc
      ? 4.1. I keep getting the error 506 Cannot talk to daemon
      ? 4.2. I keep getting the error 501 Not authorised
      ? 4.3. What is the reference ID reported by the tracking command?
      ? 4.4. Is the chronyc / chronyd protocol documented anywhere?
  o 5. Real-time clock issues
      ? 5.1. What is the real-time clock (RTC)?
      ? 5.2. Does hwclock have to be disabled?
      ? 5.3. I just keep getting the 513 RTC driver not running message
      ? 5.4. I get Could not open /dev/rtc, Device or resource busy in my
        syslog file
      ? 5.5. What if my computer does not have an RTC or backup battery?
  o 6. NTP-specific issues
      ? 6.1. Can chronyd be driven from broadcast/multicast NTP servers?
      ? 6.2. Can chronyd transmit broadcast NTP packets?
      ? 6.3. Can chronyd keep the system clock a fixed offset away from real
        time?
      ? 6.4. What happens if the network connection is dropped without using
        chronyc's offline command first?
      ? 6.5. Why is an offset measured between two computers synchronised to
        each another?
  o 7. Operating systems
      ? 7.1. Does chrony support Windows?
      ? 7.2. Are there any plans to support Windows?

1. chrony compared to other programs

1.1. How does chrony compare to ntpd?

chrony and ntpd are two different implementations of the Network Time Protocol
(NTP).

chrony is a newer implementation, which was designed to work well in a wider
range of conditions. It can usually synchronise the system clock faster and
with better time accuracy. It has many features, but it does not implement some
of the less useful NTP modes like broadcast client or multicast server/client.

If your computer is connected to the Internet only for few minutes at a time,
the network connection is often congested, you turn your computer off or
suspend it frequently, the clock is not very stable (e.g. there are rapid
changes in the temperature or it is a virtual machine), or you want to use NTP
on an isolated network with no hardware reference clocks in sight, chrony will
probably work better for you.

For a more detailed comparison of features and performance, see the comparison
page on the chrony website.

2. Configuration issues

2.1. What is the minimum recommended configuration for an NTP client?

First, the client needs to know which NTP servers it should ask for the current
time. They are specified by the server or pool directive. The pool directive
can be used for names that resolve to multiple addresses. For good reliability
the client should have at least three servers. The iburst option speeds up the
initial synchronisation.

To stabilise the initial synchronisation on the next start, the estimated drift
of the system clock is saved to a file specified by the driftfile directive.

If the system clock can be far from the true time after boot for any reason,
chronyd should be allowed to correct it quickly by stepping instead of slewing,
which would take a very long time. The makestep directive does that.

In order to keep the real-time clock (RTC) close to the true time, so the
system time is reasonably close to the true time when it is initialised on the
next boot from the RTC, the rtcsync directive enables a mode in which the
system time is periodically copied to the RTC. It is supported on Linux and
macOS.

If you want to use public NTP servers from the pool.ntp.org project, the
minimal chrony.conf file could be:

pool pool.ntp.org iburst
driftfile /var/lib/chrony/drift
makestep 1 3
rtcsync

2.2. How do I make an NTP server?

You need to add an allow directive to the chrony.conf file in order to open the
NTP port and allow chronyd to reply to client requests. allow with no specified
subnet allows access from all IPv4 and IPv6 addresses.

2.3. Should all computers on a LAN be clients of an external server?

It depends on the requirements. Usually, the best configuration is to make one
computer the server, with the others as clients of it. Add a local directive to
the server's chrony.conf file. This configuration will be better because

  o the load on the external connection is less

  o the load on the external NTP server(s) is less

  o if your external connection goes down, the computers on the LAN will
    maintain a common time with each other.

2.4. Must I specify servers by IP address if DNS is not available on chronyd
start?

No, chronyd will keep trying to resolve the names specified by the server,
pool, and peer directives in an increasing interval until it succeeds. The
online command can be issued from chronyc to force chronyd to try to resolve
the names immediately.

2.5. How can I make chronyd more secure?

If you do not need to use chronyc, or you want to run chronyc only under the
root or chrony user (which can access chronyd through a Unix domain socket),
you can disable the IPv4 and IPv6 command sockets (by default listening on
localhost) by adding cmdport 0 to the configuration file.

You can specify an unprivileged user with the -u option, or the user directive
in the chrony.conf file, to which chronyd will switch after start in order to
drop root privileges. The configure script has a --with-user option, which sets
the default user. On Linux, chronyd needs to be compiled with support for the
libcap library. On other systems, chronyd forks into two processes. The child
process retains root privileges, but can only perform a very limited range of
privileged system calls on behalf of the parent.

Also, if chronyd is compiled with support for the Linux secure computing
(seccomp) facility, you can enable a system call filter with the -F option. It
will significantly reduce the kernel attack surface and possibly prevent kernel
exploits from the chronyd process if it is compromised. It is recommended to
enable the filter only when it is known to work on the version of the system
where chrony is installed as the filter needs to allow also system calls made
from libraries that chronyd is using (e.g. libc) and different versions or
implementations of the libraries might make different system calls. If the
filter is missing some system call, chronyd could be killed even in normal
operation.

2.6. How can I improve the accuracy of the system clock with NTP sources?

Select NTP servers that are well synchronised, stable and close to your
network. It is better to use more than one server, three or four is usually
recommended as the minimum, so chronyd can detect servers that serve false time
and combine measurements from multiple sources.

If you have a network card with hardware timestamping supported on Linux, it
can be enabled by the hwtimestamp directive in the chrony.conf file. It should
make local receive and transmit timestamps of NTP packets much more accurate.

There are also useful options which can be set in the server directive, they
are minpoll, maxpoll, polltarget, maxdelay, maxdelayratio, maxdelaydevratio,
and xleave.

The first three options set the minimum and maximum allowed polling interval,
and how should be the actual interval adjusted in the specified range. Their
default values are 6 (64 seconds) for minpoll, 10 (1024 seconds) for maxpoll
and 8 (samples) for polltarget. The default values should be used for general
servers on the Internet. With your own NTP servers, or if you have permission
to poll some servers more frequently, setting these options for shorter polling
intervals might significantly improve the accuracy of the system clock.

The optimal polling interval depends mainly on two factors, stability of the
network latency and stability of the system clock (which mainly depends on the
temperature sensitivity of the crystal oscillator and the maximum rate of the
temperature change).

Generally, if the sourcestats command usually reports a small number of samples
retained for a source (e.g. fewer than 16), a shorter polling interval should
be considered. If the number of samples is usually at the maximum of 64, a
longer polling interval might work better.

An example of the directive for an NTP server on the Internet that you are
allowed to poll frequently could be

server foo.example.net minpoll 4 maxpoll 6 polltarget 16

An example using shorter polling intervals with a server located in the same
LAN could be

server ntp.local minpoll 2 maxpoll 4 polltarget 30

The maxdelay options are useful to ignore measurements with an unusally large
delay (e.g. due to congestion in the network) and improve the stability of the
synchronisation. The maxdelaydevratio option could be added to the example with
local NTP server

server ntp.local minpoll 2 maxpoll 4 polltarget 30 maxdelaydevratio 2

If your server supports the interleaved mode (e.g. it is running chronyd), the
xleave option should be added to the server directive in order to allow the
server to send the client more accurate transmit timestamps (kernel or
preferably hardware). For example:

server ntp.local minpoll 2 maxpoll 4 xleave

When combined with local hardware timestamping, good network switches, and even
shorter polling intervals, a sub-microsecond accuracy and stability of a few
tens of nanoseconds might be possible. For example:

server ntp.local minpoll 0 maxpoll 0 xleave
hwtimestamp eth0

For best stability, the CPU should be running at a constant frequency (i.e.
disabled power saving and performance boosting). Energy-Efficient Ethernet
(EEE) should be disabled in the network. The switches should be configured to
prioritize NTP packets, especially if the network is expected to be heavily
loaded. The dscp directive can be used to set the Differentiated Services Code
Point in transmitted NTP packets if needed.

If it is acceptable for NTP clients in the network to send requests at a high
rate, a sub-second polling interval can be specified. A median filter can be
enabled in order to update the clock at a reduced rate with more stable
measurements. For example:

server ntp.local minpoll -6 maxpoll -6 filter 15 xleave
hwtimestamp eth0 minpoll -6

2.7. Does chronyd have an ntpdate mode?

Yes. With the -q option chronyd will set the system clock once and exit. With
the -Q option it will print the measured offset without setting the clock. If
you do not want to use a configuration file, NTP servers can be specified on
the command line. For example:

# chronyd -q 'pool pool.ntp.org iburst'

The command above would normally take about 5 seconds if the servers were well
synchronised and responding to all requests. If not synchronised or responding,
it would take about 10 seconds for chronyd to give up and exit with a non-zero
status. A faster configuration is possible. A single server can be used instead
of four servers, the number of measurements can be reduced with the maxsamples
option to one (supported in chrony version 4.0), and a timeout can be specified
with the -t option. The following command would take only up to about one
second.

# chronyd -q -t 1 'server pool.ntp.org iburst maxsamples 1'

It is not recommended to run chronyd with the -q option periodically (e.g. from
a cron job) as a replacement for the daemon mode, because it performs
significantly worse (e.g. the clock is stepped and its frequency is not
corrected). If you must run it this way and you are using a public NTP server,
make sure chronyd does not always start around the first second of a minute,
e.g. by adding a random sleep before the chronyd command. Public servers
typically receive large bursts of requests around the first second as there is
a large number of NTP clients started from cron with no delay.

2.8. Can chronyd be configured to control the clock like ntpd?

It is not possible to perfectly emulate ntpd, but there are some options that
can configure chronyd to behave more like ntpd if there is a reason to prefer
that.

In the following example the minsamples directive slows down the response to
changes in the frequency and offset of the clock. The maxslewrate and
corrtimeratio directives reduce the maximum frequency error due to an offset
correction and the maxdrift directive reduces the maximum assumed frequency
error of the clock. The makestep directive enables a step threshold and the
maxchange directive enables a panic threshold. The maxclockerror directive
increases the minimum dispersion rate.

minsamples 32
maxslewrate 500
corrtimeratio 100
maxdrift 500
makestep 0.128 -1
maxchange 1000 1 1
maxclockerror 15

Note that increasing minsamples might cause the offsets in the tracking and
sourcestats reports/logs to be significantly smaller than the actual offsets
and be unsuitable for monitoring.

2.9. Can NTP server be separated from NTP client?

Yes, it is possible to run multiple instances of chronyd on the same computer.
One can be configured as an NTP client, and another as a server. They need to
use different pidfiles, NTP ports, command ports, and Unix domain command
sockets. The server instance should be started with the -x option to avoid
touching the clock. It can be configured to serve the system time with the
local directive, or synchronise its NTP clock to the client instance running on
localhost using a non-standard NTP port.

On Linux, starting with chrony version 4.0, it is also possible to run multiple
server instances sharing a port to utilise multiple cores of the CPU. Note that
the client/server interleaved mode requires that all packets from an address
are handled by the same server instance.

2.10. Should be a leap smear enabled on NTP server?

With the smoothtime and leapsecmode directives it is possible to enable a
server leap smear in order to hide leap seconds from clients and force them to
follow a slow server's adjustment instead.

This feature should be used only in local networks and only when necessary,
e.g. when the clients cannot be configured to handle the leap seconds as
needed, or their number is so large that configuring them all would be
impractical. The clients should use only one leap-smearing server, or multiple
identically configured leap-smearing servers. Note that some clients can get
leap seconds from external sources (e.g. with the leapsectz directive in
chrony) and they will not work correctly with a leap smearing server.

2.11. Does chrony support PTP?

No, the Precision Time Protocol (PTP) is not supported and there are no plans
to support it. It is a complex protocol, which shares some issues with the NTP
broadcast mode. One of the main differences between NTP and PTP is that PTP was
designed to be easily supported in hardware (e.g. network switches and routers)
in order to make more stable and accurate measurements. PTP relies on the
hardware support. NTP does not rely on any support in the hardware, but if it
had the same support as PTP, it could perform equally well.

On Linux, chrony supports hardware clocks that some NICs have for PTP. They are
called PTP hardware clocks (PHC). They can be used as reference clocks
(specified by the refclock directive) and for hardware timestamping of NTP
packets (enabled by the hwtimestamp directive) if the NIC can timestamp other
packets than PTP, which is usually the case at least for transmitted packets.
The ethtool -T command can be used to verify the timestamping support.

2.12. What happened to the commandkey and generatecommandkey directives?

They were removed in version 2.2. Authentication is no longer supported in the
command protocol. Commands that required authentication are now allowed only
through a Unix domain socket, which is accessible only by the root and chrony
users. If you need to configure chronyd remotely or locally without the root
password, please consider using ssh and/or sudo to run chronyc under the root
or chrony user on the host where chronyd is running.

3. Computer is not synchronising

This is the most common problem. There are a number of reasons, see the
following questions.

3.1. Behind a firewall?

Check the Reach value printed by the chronyc's sources command. If it is zero,
it means chronyd did not get any valid responses from the NTP server you are
trying to use. If there is a firewall between you and the server, the packets
might be blocked. Try using a tool like wireshark or tcpdump to see if you are
getting any responses from the server.

When chronyd is receiving responses from the servers, the output of the sources
command issued few minutes after chronyd start might look like this:

210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* foo.example.net               2   6   377    34   +484us[ -157us] +/-   30ms
^- bar.example.net               2   6   377    34    +33ms[  +32ms] +/-   47ms
^+ baz.example.net               3   6   377    35  -1397us[-2033us] +/-   60ms

3.2. Are NTP servers specified with the offline option?

Check that the chronyc's online and offline commands are used appropriately
(e.g. in the system networking scripts). The activity command prints the number
of sources that are currently online and offline. For example:

200 OK
3 sources online
0 sources offline
0 sources doing burst (return to online)
0 sources doing burst (return to offline)
0 sources with unknown address

3.3. Is chronyd allowed to step the system clock?

By default, chronyd adjusts the clock gradually by slowing it down or speeding
it up. If the clock is too far from the true time, it will take a long time to
correct the error. The System time value printed by the chronyc's tracking
command is the remaining correction that needs to be applied to the system
clock.

The makestep directive can be used to allow chronyd to step the clock. For
example, if chrony.conf had

makestep 1 3

the clock would be stepped in the first three updates if its offset was larger
than one second. Normally, it is recommended to allow the step only in the
first few updates, but in some cases (e.g. a computer without an RTC or virtual
machine which can be suspended and resumed with an incorrect time) it might be
necessary to allow the step on any clock update. The example above would change
to

makestep 1 -1

3.4. Using a Windows NTP server?

A common issue with Windows NTP servers is that they report a very large root
dispersion (e.g. three seconds or more), which causes chronyd to ignore the
server for being too inaccurate. The sources command might show a valid
measurement, but the server is not selected for synchronisation. You can check
the root dispersion of the server with the chronyc's ntpdata command.

The maxdistance value needs to be increased in chrony.conf to enable
synchronisation to such a server. For example:

maxdistance 16.0

3.5. An unreachable source is selected?

When chronyd is configured with multiple time sources, it tries to select the
most accurate and stable sources for synchronisation of the system clock. They
are marked with the * or + symbol in the report printed by the sources command.

When the best source (marked with the * symbol) becomes unreachable (e.g. NTP
server stops responding), chronyd will not immediately switch to the second
best source in an attempt to minimise the error of the clock. It will let the
clock run free for as long as its estimated error (in terms of root distance)
based on previous measurements is smaller than the estimated error of the
second source, and there is still an interval which contains some measurements
from both sources.

If the first source was significantly better than the second source, it can
take many hours before the second source is selected, depending on its polling
interval. You can force a faster reselection by increasing the clock error rate
(maxclockerror directive), shortening the polling interval (maxpoll option), or
reducing the number of samples (maxsamples option).

3.6. Does selected source drop new measurements?

chronyd can drop a large number of successive NTP measurements if they are not
passing some of the NTP tests. The sources command can report for a selected
source the fully-reachable value of 377 in the Reach column and at the same
time a LastRx value that is much larger than the current polling interval. If
the source is online, this indicates that a number of measurements was dropped.
You can use the ntpdata command to check the NTP tests for the last
measurement. Usually, it is the test C which fails.

This can be an issue when there is a long-lasting increase in the measured
delay, e.g. due to a routing change in the network. Unfortunately, chronyd does
not know for how long it should wait for the delay to come back to the original
values, or whether it is a permanent increase and it should start from scratch.

The test C is an adaptive filter. It can take many hours before it accepts a
measurement with the larger delay, and even much longer before it drops all
measurements with smaller delay, which determine an expected delay used by the
test. You can use the reset sources command to drop all measurements
immediately (available in chrony 4.0 and later). If this issue happens
frequently, you can effectively disable the test by setting the
maxdelaydevratio option to a very large value (e.g. 1000000), or speed up the
recovery by increasing the clock error rate with the maxclockerror directive.

3.7. Using a PPS reference clock?

A pulse-per-second (PPS) reference clock requires a non-PPS time source to
determine which second of UTC corresponds to each pulse. If it is another
reference clock specified with the lock option in the refclock directive, the
offset between the two reference clocks must be smaller than 0.2 seconds in
order for the PPS reference clock to work. With NMEA reference clocks it is
common to have a larger offset. It needs to be corrected with the offset
option.

One approach to find out a good value of the offset option is to configure the
reference clocks with the noselect option and compare them to an NTP server.
For example, if the sourcestats command showed

Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
PPS0                        0   0     0     +0.000   2000.000     +0ns  4000ms
NMEA                       58  30   231    -96.494     38.406   +504ms  6080us
foo.example.net             7   3   200     -2.991     16.141   -107us   492us

the offset of the NMEA source would need to be increased by about 0.504
seconds. It does not have to be very accurate. As long as the offset of the
NMEA reference clock stays below 0.2 seconds, the PPS reference clock should be
able to determine the seconds corresponding to the pulses and allow the samples
to be used for synchronisation.

4. Issues with chronyc

4.1. I keep getting the error 506 Cannot talk to daemon

When accessing chronyd remotely, make sure that the chrony.conf file (on the
computer where chronyd is running) has a cmdallow entry for the computer you
are running chronyc on and an appropriate bindcmdaddress directive. This is not
necessary for localhost.

Perhaps chronyd is not running. Try using the ps command (e.g. on Linux, ps
-auxw) to see if it is running. Or try netstat -a and see if the UDP port 323
is listening. If chronyd is not running, you might have a problem with the way
you are trying to start it (e.g. at boot time).

Perhaps you have a firewall set up in a way that blocks packets on the UDP port
323. You need to amend the firewall configuration in this case.

4.2. I keep getting the error 501 Not authorised

This error indicates that chronyc sent the command to chronyd using a UDP
socket instead of the Unix domain socket (e.g. /var/run/chrony/chronyd.sock),
which is required for some commands. For security reasons, only the root and
chrony users are allowed to access the socket.

It is also possible that the socket does not exist. chronyd will not create the
socket if the directory has a wrong owner or permissions. In this case there
should be an error message from chronyd in the system log.

4.3. What is the reference ID reported by the tracking command?

The reference ID is a 32-bit value used in NTP to prevent synchronisation
loops.

In chrony versions before 3.0 it was printed in the quad-dotted notation, even
if the reference source did not actually have an IPv4 address. For IPv4
addresses, the reference ID is equal to the address, but for IPv6 addresses it
is the first 32 bits of the MD5 sum of the address. For reference clocks, the
reference ID is the value specified with the refid option in the refclock
directive.

Since version 3.0, the reference ID is printed as a hexadecimal number to avoid
confusion with IPv4 addresses.

If you need to get the IP address of the current reference source, use the -n
option to disable resolving of IP addresses and read the second field (printed
in parentheses) on the Reference ID line.

4.4. Is the chronyc / chronyd protocol documented anywhere?

Only by the source code. See cmdmon.c (chronyd side) and client.c (chronyc
side).

5. Real-time clock issues

5.1. What is the real-time clock (RTC)?

This is the clock which keeps the time even when your computer is turned off.
It is used to initialise the system clock on boot. It normally does not drift
more than few seconds per day.

There are two approaches how chronyd can work with it. One is to use the
rtcsync directive, which tells chronyd to enable a kernel mode which sets the
RTC from the system clock every 11 minutes. chronyd itself won't touch the RTC.
If the computer is not turned off for a long time, the RTC should still be
close to the true time when the system clock will be initialised from it on the
next boot.

The other option is to use the rtcfile directive, which tells chronyd to
monitor the rate at which the RTC gains or loses time. When chronyd is started
with the -s option on the next boot, it will set the system time from the RTC
and also compensate for the drift it has measured previously. The rtcautotrim
directive can be used to keep the RTC close to the true time, but it is not
strictly necessary if its only purpose is to set the system clock when chronyd
is started on boot. See the documentation for details.

5.2. Does hwclock have to be disabled?

The hwclock program is often set-up by default in the boot and shutdown scripts
with many Linux installations. With the kernel RTC synchronisation (rtcsync
directive), the RTC will be set also every 11 minutes as long as the system
clock is synchronised. If you want to use chronyd's RTC monitoring (rtcfile
directive), it is important to disable hwclock in the shutdown procedure. If
you do not that, it will over-write the RTC with a new value, unknown to
chronyd. At the next reboot, chronyd started with the -s option will compensate
this (wrong) time with its estimate of how far the RTC has drifted whilst the
power was off, giving a meaningless initial system time.

There is no need to remove hwclock from the boot process, as long as chronyd is
started after it has run.

5.3. I just keep getting the 513 RTC driver not running message

For the real-time clock support to work, you need the following three things

  o an RTC in your computer

  o a Linux kernel with enabled RTC support

  o an rtcfile directive in your chrony.conf file

5.4. I get Could not open /dev/rtc, Device or resource busy in my syslog file

Some other program running on the system might be using the device.

5.5. What if my computer does not have an RTC or backup battery?

In this case you can still use the -s option to set the system clock to the
last modification time of the drift file, which should correspond to the system
time when chronyd was previously stopped. The initial system time will be
increasing across reboots and applications started after chronyd will not
observe backward steps.

6. NTP-specific issues

6.1. Can chronyd be driven from broadcast/multicast NTP servers?

No, the broadcast/multicast client mode is not supported and there is currently
no plan to implement it. While this mode can simplify configuration of clients
in large networks, it is inherently less accurate and less secure (even with
authentication) than the ordinary client/server mode.

When configuring a large number of clients in a network, it is recommended to
use the pool directive with a DNS name which resolves to addresses of multiple
NTP servers. The clients will automatically replace the servers when they
become unreachable, or otherwise unsuitable for synchronisation, with new
servers from the pool.

Even with very modest hardware, an NTP server can serve time to hundreds of
thousands of clients using the ordinary client/server mode.

6.2. Can chronyd transmit broadcast NTP packets?

Yes, the broadcast directive can be used to enable the broadcast server mode to
serve time to clients in the network which support the broadcast client mode
(it is not supported in chronyd). Note that this mode should generally be
avoided. See the previous question.

6.3. Can chronyd keep the system clock a fixed offset away from real time?

Yes. Starting from version 3.0, an offset can be specified by the offset option
for all time sources in the chrony.conf file.

6.4. What happens if the network connection is dropped without using chronyc's
offline command first?

chronyd will keep trying to access the sources that it thinks are online, and
it will take longer before new measurements are actually made and the clock is
corrected when the network is connected again. If the sources were set to
offline, chronyd would make new measurements immediately after issuing the
online command.

Unless the network connection lasts only few minutes (less than the maximum
polling interval), the delay is usually not a problem, and it might be
acceptable to keep all sources online all the time.

6.5. Why is an offset measured between two computers synchronised to each
another?

When two computers are synchronised to each other using the client/server or
symmetric NTP mode, there is an expectation that NTP measurements between the
two computers made on both ends show an average offset close to zero.

With chronyd that can be expected only when the interleaved mode (xleave
option) is enabled. Otherwise, chronyd will use different transmit timestamps
(e.g. daemon timestamp vs kernel timestamp) for serving time and
synchronisation of its own clock, which creates an asymmetry in the
timestamping and causes the other end to measure a significant offset.

7. Operating systems

7.1. Does chrony support Windows?

No. The chronyc program (the command-line client used for configuring chronyd
while it is running) has been successfully built and run under Cygwin in the
past. chronyd is not portable, because part of it is very system-dependent. It
needs adapting to work with Windows' equivalent of the adjtimex() call, and it
needs to be made to work as a service.

7.2. Are there any plans to support Windows?

We have no plans to do this. Anyone is welcome to pick this work up and
contribute it back to the project.

Last updated 2020-10-07 17:27:34 +0200