chrony compared to other programs
1.1. How does
chrony compare to
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
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,
will probably work much better for you.
For a more detailed comparison of features and performance, see the
comparison page on the
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
pool directive. The
directive can be used for names that resolve to multiple addresses. For good
reliability the client should have at least three servers. The
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
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
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
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 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
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
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
directive or command, an NTP peer is configured, or the
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 127.0.0.1 bindcmdaddress ::1
If you don’t need to use
chronyc at all or you need to run
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
directive in the chrony.conf file, to which
chronyd will switch after start
in order to drop root privileges. The configure script has a
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.
chronyd is compiled with support for the Linux secure computing
(seccomp) facility, you can enable a system call filter with the
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
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
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
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).
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 very short polling intervals for 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 larger delay (e.g.
due to congestion in the network) and improve the stability of 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, the
xleave option should be
added to the
server directive in order to allow the server to send the
client more accurate hardware or kernel transmit timestamps. When combined with
local hardware timestamping, sub-microsecond accuracy may be possible. An
example could be
server ntp.local minpoll 2 maxpoll 2 xleave hwtimestamp eth0
chronyd have an ntpdate mode?
Yes. With the
chronyd will set the system clock once and exit.
-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. What happened to the
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?
Reach value printed by the
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
tcpdump to see
if you’re getting any responses from the server.
chronyd is receiving responses from the servers, the output of the
sources command issued few minutes after
chronyd start might look like
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
Check that you’re using
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
chronyd allowed to step the system clock?
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
tracking command is the remaining correction that needs to be
applied to the system clock.
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
maxdistance value needs to be increased in chrony.conf to enable
synchronisation to such a server. For example:
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
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
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
4.1. I keep getting the error
506 Cannot talk to daemon
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
directive. This isn’t necessary for localhost.
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
needs to run locally under the root or chrony user, which are allowed to
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 (
directive) and which key in the key file should be used for
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
option in the
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
option to disable resolving of IP addresses and read the second field (printed
in parentheses) on the
Reference ID line.
4.4. Is the
chronyd protocol documented anywhere?
Only by the source code. See cmdmon.c (
chronyd side) and client.c
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
monitor the rate at which the RTC gains or loses time. When
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 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
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
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
rtcfiledirective 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
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
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
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.
chronyd transmit broadcast NTP packets?
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).
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
option for all time sources in the chrony.conf file.
6.4. What happens if the network connection is dropped without using
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
chronyd would make new measurements immediately after issuing the
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
chrony support Windows?
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
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.