1. chrony compared to other programs

1.1. How does chrony compare to ntpd?

chronyd was designed to work well in a wide range of conditions and it can usually synchronise the system clock faster and with better time accuracy. It doesn’t 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’s a virtual machine), or you want to use NTP on an isolated network with no hardware reference clocks in sight, chrony will probably work much better for you.

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

2. Configuration issues

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’s 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

2.2. How do I make an NTP server from an NTP client?

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. I have several computers on a LAN. Should be all clients of an external server?

The best configuration is usually 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

  • the load on the external connection is less

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

  • 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. Starting from version 1.25, 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 don’t need to serve time to NTP clients or peers, you can add port 0 to the chrony.conf file to completely disable the NTP server functionality and prevent NTP requests from reaching chronyd. Starting from version 2.0, the NTP server port is open only when client access is allowed by the allow directive or command, an NTP peer is configured, or the broadcast directive is used.

If you don’t need to use chronyc remotely, you can add the following directives to the configuration file to bind the command sockets to the loopback interface. This is done by default since version 2.0.

bindcmdaddress ::1

If you don’t need to use chronyc at all or you need to run chronyc only under the root or chrony user (which can access chronyd through a Unix domain socket since version 2.2), you can disable the internet command sockets completely 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’s compromised. It’s recommended to enable the filter only when it’s 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 may 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’s 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 may 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 may 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 may 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.

If it is acceptable for NTP clients in the network to send requests at an excessive rate, a sub-second polling interval may 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 don’t want to use a configuration file, NTP servers can be specified on the command line. For example:

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

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.

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 may 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. 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’s 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 may be blocked. Try using a tool like wireshark or tcpdump to see if you’re 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 you’re using chronyc's online and offline commands appropriately. 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’s 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 may 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 may 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. 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 isn’t necessary for localhost.

Perhaps chronyd is not running. Try using the ps command (e.g. on Linux, ps -auxw) to see if it’s running. Or try netstat -a and see if the ports 123/udp and 323/udp are listening. If chronyd is not running, you may 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 port 323/udp. You need to amend the firewall configuration in this case.

4.2. I keep getting the error 501 Not authorised

Since version 2.2, the password command doesn’t do anything and chronyc needs to run locally under the root or chrony user, which are allowed to access the chronyd's Unix domain command socket.

With older versions, you need to authenticate with the password command first or use the -a option to authenticate automatically on start. The configuration file needs to specify a file which contains keys (keyfile directive) and which key in the key file should be used for chronyc authentication (commandkey directive).

4.3. Why does chronyc tracking always print an IPv4 address as reference ID?

The reference ID is a 32-bit value and in versions before 3.0 it was printed in 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 doesn’t 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’s 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. I want to use chronyd's RTC support. Must I disable hwclock?

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’s important to disable hwclock in the shutdown procedure. If you don’t, 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

  • an RTC in your computer

  • a Linux kernel with enabled RTC support

  • 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 may 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 the mode may be useful to 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’s not supported in chronyd, 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 may be acceptable to keep all sources online all the time.

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.

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