Frequently Asked Questions

Table of Contents

1. Chrony compared to other programs

1.1. How does chrony compare to ntpd?

chrony can usually synchronise the system clock faster and with better time accuracy, but it doesn’t implement all NTP features, e.g. broadcast/multicast mode, or authentication based on public-key cryptography. For a more detailed comparison, see the comparison page on the chrony website and section Comparison with ntpd in the manual.

If your computer connects to the 'net only for few minutes at a time, you turn your Linux computer off or suspend it frequently, the clock is not very stable (e.g. it is a virtual machine), or you want to use NTP on an isolated network with no hardware clocks in sight, chrony will probably work much better for you.

The original reason chrony was written was that ntpd (called xntpd at the time) could not to do anything sensible on a PC which was connected to the 'net only for about 5 minutes once or twice a day, mainly to upload/download email and news. The requirements were

  • slew the time to correct it when going online and NTP servers become visible

  • determine the rate at which the computer gains or loses time and use this information to keep it reasonably correct between connects to the 'net. This has to be done using a method that does not care about the intermittent availability of the references or the fact the computer is turned off between groups of measurements.

  • maintain the time across reboots, by working out the error and drift rate of the computer’s real-time clock and using this information to set the system clock correctly at boot up.

Also, when working with isolated networks with no true time references at all ntpd was found to give no help with managing the local clock’s gain/loss rate on the NTP master node (which was set from watch). Some automated support was added to chrony to deal with this.

2. Configuration issues

2.1. 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 master, with the others as clients of it. Add a local directive to the master’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.

2.2. 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 hostnames specified in the server and peer directives in increasing intervals until it succeeds. The online command can be issued from chronyc to try to resolve them immediately.

2.3. 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, you can disable the command sockets by adding cmdport 0 to the configuration file.

On Linux, if chronyd is compiled with support for Linux capabilities (available in the libcap library), you can specify an unprivileged user with the -u option or user directive in the chrony.conf file to drop root privileges after start. The configure option --with-user can be used to drop the privileges by default.

2.4. 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 falsetickers and combine measurements from multiple sources.

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

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 6 (samples) for polltarget. The default values should be used for general servers on the internet. With your own NTP servers or if 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 on many factors, including the ratio between the wander of the clock and the network jitter (sometimes expressed in NTP documents as the Allan intercept), the temperature sensitivity of the crystal oscillator and the maximum rate of change of the temperature.

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

server 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 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

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?

If there is a firewall between you and the NTP server you’re trying to use, the packets may be blocked. Try using a tool like wireshark or tcpdump to see if you’re getting responses from the server. If you have an external modem, see if the receive light blinks straight after the transmit light (when the link is quiet apart from the NTP traffic.) Try adding log measurements to the chrony.conf file and look in the measurements.log file after chrony has been running for a short period. See if any measurements appear.

3.2. Are NTP servers specified with the offline option?

Check that you’re using chronyc's online and offline commands appropriately. Again, check in measurements.log to see if you’re getting any data back from the server.

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. 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 works with 1 second resolution. chronyd can monitor the rate at which the real-time clock gains or loses time, and compensate for it when you set the system time from it at the next reboot. See the documentation for details.

5.2. I want to use chronyd's real-time clock support. Must I disable hwclock?

The hwclock program is often set-up by default in the boot and shutdown scripts with many Linux installations. If you want to use chronyd's real-time clock support, the important thing is 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 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

  • a kernel that is supported (e.g. 2.2 onwards)

  • enhanced RTC support compiled into the kernel

  • an rtcfile directive in your chrony.conf file

6. Microsoft Windows

6.1. 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.

6.2. 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.

7. NTP-specific issues

7.1. Can chrony be driven from broadcast NTP servers?

No, this NTP mode is not implemented yet.

7.2. Can chronyd transmit broadcast NTP packets (e.g. to synchronise other computers on a private LAN)?

Yes. Starting from version 1.17, chrony has this capability.

7.3. Can chrony keep the system clock a fixed offset away from real time?

This is not possible as the program currently stands.

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

chronyd will keep trying to access the server(s) that it thinks are online. When the network is connected again, it will take some time (on average half of the current polling interval) before new measurements are made and the clock is corrected. If the servers were set to offline and the online command was issued when the network was connected, chronyd would make new measurements immediately.

The auto_offline option to the server entry in the chrony.conf file may be useful to switch the servers to the offline state automatically.

8. Linux-specific issues

8.1. 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.

9. Solaris-specific issues

9.1. I get an error message about not being able to open kvm to change dosynctodr

(The dosynctodr variable controls whether Solaris couples the equivalent of its BIOS clock into its system clock at regular intervals). The Solaris port of chrony was developed in the Solaris 2.5 era. Some aspect of the Solaris kernel has changed which prevents the same technique working. We no longer have root access to any Solaris machines to work on this, and we are reliant on somebody developing the patch and testing it.