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@@ -0,0 +1,1159 @@ +Frequently Asked Questions + +Table of Contents + + o 1. chrony compared to other programs + ? 1.1. How does chrony compare to ntpd? + ? 1.2. Should I prefer chrony over timesyncd if I do not need to run a + server? + 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 make the system clock more secure? + ? 2.7. How can I improve the accuracy of the system clock with NTP + sources? + ? 2.8. Does chronyd have an ntpdate mode? + ? 2.9. Can chronyd be configured to control the clock like ntpd? + ? 2.10. Can NTP server be separated from NTP client? + ? 2.11. How can chronyd be configured to minimise downtime during + restarts? + ? 2.12. Should be a leap smear enabled on NTP server? + ? 2.13. How should chronyd be configured with gpsd? + ? 2.14. Does chrony support PTP? + ? 2.15. How can I avoid using wrong PHC refclock? + ? 2.16. Why are client log records dropped before reaching + clientloglimit? + ? 2.17. 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 name resolution working correctly? + ? 3.4. Is chronyd allowed to step the system clock? + ? 3.5. Using NTS? + ? 3.6. Using a Windows NTP server? + ? 3.7. An unreachable source is selected? + ? 3.8. Does selected source drop new measurements? + ? 3.9. 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. When I start chronyd, the log says Could not enable RTC interrupt + : Invalid argument (or it may say disable) + ? 5.6. 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. Operation + ? 7.1. What clocks does chronyd use? + o 8. Operating systems + ? 8.1. Does chrony support Windows? + ? 8.2. 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. + +1.2. Should I prefer chrony over timesyncd if I do not need to run a server? + +Generally, yes. + +systemd-timesyncd is a very simple NTP client included in the systemd suite. It +lacks almost all features of chrony and other advanced client implementations +listed on the comparison page. One of its main limitations is that it cannot +poll multiple servers at the same time and detect servers having incorrect time +(falsetickers in the NTP terminology). It should be used only with trusted +reliable servers, ideally in local network. + +Using timesyncd with pool.ntp.org is problematic. The pool is very robust as a +whole, but the individual servers run by volunteers cannot be relied on. +Occasionally, servers drift away or make a step to distant past or future due +to misconfiguration, problematic implementation, and other bugs (e.g. in +firmware of a GPS receiver). The pool monitoring system detects such servers +and quickly removes them from the pool DNS, but clients like timesyncd cannot +recover from that. They follow the server as long as it claims to be +synchronised. They need to be restarted in order to get a new address from the +pool DNS. + +Note that the complexity of NTP and clock synchronisation is on the client +side. The amount of code in chrony specific to NTP server is very small and it +is disabled by default. If it was removed, it would not significantly reduce +the amount of memory or storage needed. + +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 is +used with names that resolve to multiple addresses of different servers. For +reliable operation, the client should have at least three servers. + +The iburst option enables a burst of requests to speed 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 wanted 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? + +By default, chronyd does not operate as an NTP server. You need to add an allow +directive to the chrony.conf file in order for chronyd to open the server NTP +port and respond to client requests. + +allow 192.168.1.0/24 + +An allow directive 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 make the system clock more secure? + +An NTP client synchronising the system clock to an NTP server is susceptible to +various attacks, which can break applications and network protocols relying on +accuracy of the clock (e.g. DNSSEC, Kerberos, TLS, WireGuard). + +Generally, a man-in-the-middle (MITM) attacker between the client and server +can + + o make fake responses, or modify real responses from the server, to create an + arbitrarily large time and frequency offset, make the server appear more + accurate, insert a leap second, etc. + + o delay the requests and/or responses to create a limited time offset and + temporarily also a limited frequency offset + + o drop the requests or responses to prevent updates of the clock with new + measurements + + o redirect the requests to a different server + +The attacks can be combined for a greater effect. The attacker can delay +packets to create a significant frequency offset first and then drop all +subsequent packets to let the clock quickly drift away from the true time. The +attacker might also be able to control the server's clock. + +Some attacks cannot be prevented. Monitoring is needed for detection, e.g. the +reachability register in the sources report shows missing packets. The extent +to which the attacker can control the client's clock depends on its +configuration. + +Enable authentication to prevent chronyd from accepting modified, fake, or +redirected packets. It can be enabled with a symmetric key specified by the key +option, or Network Time Security (NTS) by the nts option (supported since +chrony version 4.0). The server needs to support the selected authentication +mechanism. Symmetric keys have to be configured on both client and server, and +each client must have its own key (one per server). + +The maximum offset that the attacker can insert in an NTP measurement by +delaying packets can be limited by the maxdelay option. The default value is 3 +seconds. The measured delay is reported as the peer delay in the ntpdata report +and measurements log. Set the maxdelay option to a value larger than the +maximum value that is normally observed. Note that the delay can increase +significantly even when not under an attack, e.g. when the network is congested +or the routing has changed. + +The maximum accepted change in time offset between clock updates can be limited +by the maxchange directive. Larger changes in the offset will be ignored or +cause chronyd to exit. Note that the attacker can get around this limit by +splitting the offset into multiple smaller offsets and/or creating a large +frequency offset. When this directive is used, chronyd will have to be +restarted after a successful attack. It will not be able to recover on its own. +It must not be restarted automatically (e.g. by the service manager). + +The impact of a large accepted time offset can be reduced by disabling clock +steps, i.e. by not using the makestep and initstepslew directives. The offset +will be slowly corrected by speeding up or slowing down the clock at a rate +which can be limited by the maxslewrate directive. Disabling clock steps +completely is practical only if the clock cannot gain a larger error on its +own, e.g. when the computer is shut down or suspended, and the maxslewrate +limit is large enough to correct an expected error in an acceptable time. The +rtcfile directive with the -s option can be used to compensate for the RTC +drift. + +A more practical approach is to enable makestep for a limited number of clock +updates (the 2nd argument of the directive) and limit the offset change in all +updates by the maxchange directive. The attacker will be able to make only a +limited step and only if the attack starts in a short window after booting the +computer, or when chronyd is restarted without the -R option. + +The frequency offset can be limited by the maxdrift directive. The measured +frequency offset is reported in the drift file, tracking report, and tracking +log. Set maxdrift to a value larger than the maximum absolute value that is +normally observed. Note that the frequency of the clock can change due to aging +of the crystal, differences in calibration of the clock source between reboots, +migrated virtual machine, etc. A typical computer clock has a drift smaller +than 100 parts per million (ppm), but much larger drifts are possible (e.g. in +some virtual machines). + +Use only trusted servers, which you expect to be well configured and managed, +using authentication for their own servers, etc. Use multiple servers, ideally +in different locations. The attacker will have to deal with a majority of the +servers in order to pass the source selection and update the clock with a large +offset. Use the minsources directive to increase the required number of +selectable sources to make the selection more robust. + +Do not specify servers as peers. The symmetric mode is less secure than the +client/server mode. If not authenticated, it is vulnerable to off-path +denial-of-service attacks, and even when it is authenticated, it is still +susceptible to replay attacks. + +Mixing of authenticated and unauthenticated servers should generally be +avoided. If mixing is necessary (e.g. for a more accurate and stable +synchronisation to a closer server which does not support authentication), the +authenticated servers should be configured as trusted and required to not allow +the unauthenticated servers to override the authenticated servers in the source +selection. Since chrony version 4.0, the selection options are enabled in such +a case automatically. This behaviour can be disabled or modified by the +authselectmode directive. + +An example of a client configuration limiting the impact of the attacks could +be + +server ntp1.example.net iburst nts maxdelay 0.1 +server ntp2.example.net iburst nts maxdelay 0.2 +server ntp3.example.net iburst nts maxdelay 0.05 +server ntp4.example.net iburst nts maxdelay 0.1 +server ntp5.example.net iburst nts maxdelay 0.1 +minsources 3 +maxchange 100 0 0 +makestep 0.001 1 +maxdrift 100 +maxslewrate 100 +driftfile /var/lib/chrony/drift +ntsdumpdir /var/lib/chrony +rtcsync + +2.7. 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. It should make local receive and +transmit timestamps of NTP packets much more stable and accurate. + +The server directive has some useful options: minpoll, maxpoll, polltarget, +maxdelay, maxdelayratio, maxdelaydevratio, xleave, filter. + +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 ntp.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 unusually 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 to enable the server to +provide the client with 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 + +Since chrony version 4.3, the minimum minpoll is -7 and a filter using a +long-term estimate of a delay quantile can be enabled by the maxdelayquant +option to replace the default maxdelaydevratio filter, which is sensitive to +outliers corrupting the minimum delay. For example: + +server ntp.local minpoll -7 maxpoll -7 filter 31 maxdelayquant 0.3 xleave + +Since version 4.2, chronyd supports an NTPv4 extension field containing an +additional timestamp to enable frequency transfer and significantly improve +stability of synchronisation. It can be enabled by the extfield F323 option. +For example: + +server ntp.local minpoll 0 maxpoll 0 xleave extfield F323 + +Since version 4.5, chronyd can apply corrections from PTP one-step end-to-end +transparent clocks (e.g. network switches) to significantly improve accuracy of +synchronisation in local networks. It requires the PTP transport to be enabled +by the ptpport directive, HW timestamping, and the extfield F324 option. For +example: + +server ntp.local minpoll -4 maxpoll -4 xleave extfield F323 extfield F324 port 319 +ptpport 319 +hwtimestamp eth0 minpoll -4 + +2.8. 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 since 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. + +2.9. 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. + +2.10. Can NTP server be separated from NTP client? + +Yes, it is possible to run multiple instances of chronyd on a computer at the +same time. One can operate primarily as an NTP client to synchronise the system +clock and another as a server for other computers. If they use the same +filesystem, they need to be configured with different pidfiles, Unix domain +command sockets, and any other file or directory specified in the configuration +file. If they run in the same network namespace, they need to use different NTP +and command ports, or bind the ports to different addresses or interfaces. + +The server instance should be started with the -x option to prevent it from +adjusting the system clock and interfering with the client instance. It can be +configured as a client to synchronise its NTP clock to other servers, or the +client instance running on the same computer. In the latter case, the copy +option (added in chrony version 4.1) can be used to assume the reference ID and +stratum of the client instance, which enables detection of synchronisation +loops with its own clients. + +On Linux, starting with chrony version 4.0, it is possible to run multiple +server instances sharing a port to better utilise multiple cores of the CPU. +Note that for rate limiting and client/server interleaved mode to work well it +is necessary that all packets received from the same address are handled by the +same server instance. + +An example configuration of the client instance could be + +pool pool.ntp.org iburst +allow 127.0.0.1 +port 11123 +driftfile /var/lib/chrony/drift +makestep 1 3 +rtcsync + +and configuration of the first server instance could be + +server 127.0.0.1 port 11123 minpoll 0 maxpoll 0 copy +allow +cmdport 11323 +bindcmdaddress /var/run/chrony/chronyd-server1.sock +pidfile /var/run/chronyd-server1.pid +driftfile /var/lib/chrony/drift-server1 + +2.11. How can chronyd be configured to minimise downtime during restarts? + +The dumpdir directive in chrony.conf provides chronyd a location to save a +measurement history of the sources it uses when the service exits. The -r +option then enables chronyd to load state from the dump files, reducing the +synchronisation time after a restart. + +Similarly, the ntsdumpdir directive provides a location for chronyd to save NTS +cookies received from the server to avoid making a NTS-KE request when chronyd +is started. When operating as an NTS server, chronyd also saves cookies keys to +this directory to allow clients to continue to use the old keys after a server +restart for a more seamless experience. + +On Linux systems, systemd socket activation provides a mechanism to reuse +server sockets across chronyd restarts, so that client requests will be +buffered until the service is again able to handle the requests. This allows +for zero-downtime service restarts, simplified dependency logic at boot, and +on-demand service spawning (for instance, for separated server chronyd +instances run with the -x flag). + +Socket activation is supported since chrony version 4.5. The service manager +(systemd) creates sockets and passes file descriptors to them to the process +via the LISTEN_FDS environment variable. Before opening new sockets, chronyd +first checks for and attempts to reuse matching sockets passed from the service +manager. For instance, if an IPv4 datagram socket bound on bindaddress and port +is available, it will be used by the NTP server to accept incoming IPv4 +requests. + +An example systemd socket unit is below, where chronyd is configured with +bindaddress 0.0.0.0, bindaddress ::, port 123, and ntsport 4460. + +[Unit] +Description=chronyd server sockets + +[Socket] +Service=chronyd.service +# IPv4 NTP server +ListenDatagram=0.0.0.0:123 +# IPv6 NTP server +ListenDatagram=[::]:123 +# IPv4 NTS-KE server +ListenStream=0.0.0.0:4460 +# IPv6 NTS-KE server +ListenStream=[::]:4460 +BindIPv6Only=ipv6-only + +[Install] +WantedBy=sockets.target + +2.12. 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 other sources (e.g. with the leapsectz directive in chrony) +and they will not work correctly with a leap smearing server. + +2.13. How should chronyd be configured with gpsd? + +A GPS or other GNSS receiver can be used as a reference clock with gpsd. It can +work as one or two separate time sources for each connected receiver. The first +time source is based on timestamping of messages sent by the receiver. +Typically, it is accurate to milliseconds. The other source is much more +accurate. It is timestamping a pulse-per-second (PPS) signal, usually connected +to a serial port (e.g. DCD pin) or GPIO pin. + +If the PPS signal is connected to the serial port which is receiving messages +from the GPS/GNSS receiver, gpsd should detect and use it automatically. If it +is connected to a GPIO pin, or another serial port, the PPS device needs to be +specified on the command line as an additional data source. On Linux, the +ldattach utility can be used to create a PPS device for a serial device. + +The PPS-based time source provided by gpsd is available as a SHM 1 refclock, or +other odd number if gpsd is configured with multiple receivers, and also as +SOCK /var/run/chrony.DEV.sock where DEV is the name of the serial device (e.g. +ttyS0). + +The message-based time source is available as a SHM 0 refclock (or other even +number) and since gpsd version 3.25 also as SOCK /var/run/chrony.clk.DEV.sock +where DEV is the name of the serial device. + +The SOCK refclocks should be preferred over SHM for better security (the shared +memory segment needs to be created by chronyd or gpsd with an expected owner +and permissions before an untrusted application or user has a chance to create +its own in order to feed chronyd with false measurements). gpsd needs to be +started after chronyd in order to connect to the socket. + +With chronyd and gpsd both supporting PPS, there are two different recommended +configurations: + +# First option +refclock SOCK /var/run/chrony.ttyS0.sock refid GPS + +# Second option +refclock PPS /dev/pps0 lock NMEA refid GPS +refclock SOCK /var/run/chrony.clk.ttyS0.sock offset 0.5 delay 0.1 refid NMEA noselect + +They both have some advantages: + + o SOCK can be more accurate than PPS if gpsd corrects for the sawtooth error + provided by the receiver in serial data + + o PPS can be used with higher PPS rates (specified by the rate option), but + it requires a second refclock or another time source to pair pulses with + seconds, and the SOCK offset needs to be specified correctly to compensate + for the message delay, while gpsd can apply HW-specific information + +If the PPS signal is not available, or cannot be used for some reason, the only +option is the message-based timing + +refclock SOCK /var/run/chrony.clk.ttyS0.sock offset 0.5 delay 0.1 refid GPS + +or the SHM equivalent if using gpsd version before 3.25 + +refclock SHM 0 offset 0.5 delay 0.1 refid GPS + +2.14. Does chrony support PTP? + +No, the Precision Time Protocol (PTP) is not supported as a protocol for +synchronisation of clocks 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. + +As an experimental feature added in version 4.2, chrony can use PTP as a +transport for NTP messages (NTP over PTP) to enable hardware timestamping on +hardware which can timestamp PTP packets only. It can be enabled by the ptpport +directive. Since version 4.5, chrony can also apply corrections provided by PTP +one-step end-to-end transparent clocks to reach the accuracy of ordinary PTP +clocks. The application of PTP corrections can be enabled by the extfield F324 +option. + +2.15. How can I avoid using wrong PHC refclock? + +If your system has multiple PHC devices, normally named by udev as /dev/ptp0, / +dev/ptp1, and so on, their order can change randomly across reboots depending +on the order of initialisation of their drivers. If a PHC refclock is specified +by this name, chronyd could be using a wrong refclock after reboot. To prevent +that, you can configure udev to create a stable symlink for chronyd with a rule +like this (e.g. written to /etc/udev/rules.d/80-phc.rules): + +KERNEL=="ptp[0-9]*", DEVPATH=="/devices/pci0000:00/0000:00:01.2/0000:02:00.0/ptp/*", SYMLINK+="ptp-i350-1" + +You can get the full DEVPATH of an existing PHC device with the udevadm info +command. You will need to execute the udevadm trigger command, or reboot the +system, for these changes to take effect. + +2.16. Why are client log records dropped before reaching clientloglimit? + +The number of dropped client log records reported by the serverstats command +can be increasing before the number of clients reported by the clients command +reaches the maximum value corresponding to the memory limit set by the +clientloglimit directive. + +This is due to the design of the data structure keeping the client records. It +is a hash table which can store only up to 16 colliding addresses per slot. If +a slot has more collisions and the table already has the maximum size, the +oldest record will be dropped and replaced by the new client. + +Note that the size of the table is always a power of two and it can only grow. +The limit set by the clientloglimit directive takes into account that two +copies of the table exist when it is being resized. This means the actual +memory usage reported by top and other utilities can be significantly smaller +than the limit even when the maximum number of records is used. + +The absolute maximum number of client records kept at the same time is +16777216. + +2.17. 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 requests +sent to the UDP port 123 of the server or responses sent back from the port +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: + +MS Name/IP address Stratum Poll Reach LastRx Last sample +=============================================================================== +^* ntp1.example.net 2 6 377 34 +484us[ -157us] +/- 30ms +^- ntp2.example.net 2 6 377 34 +33ms[ +32ms] +/- 47ms +^+ ntp3.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 name resolution working correctly? + +NTP servers specified by their hostname (instead of an IP address) have to have +their names resolved before chronyd can send any requests to them. If the +activity command prints a non-zero number of sources with unknown address, +there is an issue with the resolution. Typically, a DNS server is specified in +/etc/resolv.conf. Make sure it is working correctly. + +Since chrony version 4.0, you can run chronyc -N sources -a command to print +all sources, even those that do not have a known address yet, with their names +as they were specified in the configuration. This can be useful to verify that +the names specified in the configuration are used as expected. + +3.4. 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 + +3.5. Using NTS? + +The Network Time Security (NTS) mechanism uses Transport Layer Security (TLS) +to establish the keys needed for authentication of NTP packets. + +Run the authdata command to check whether the key establishment was successful: + +# chronyc -N authdata +Name/IP address Mode KeyID Type KLen Last Atmp NAK Cook CLen +========================================================================= +ntp1.example.net NTS 1 15 256 33m 0 0 8 100 +ntp2.example.net NTS 1 15 256 33m 0 0 8 100 +ntp3.example.net NTS 1 15 256 33m 0 0 8 100 + +The KeyID, Type, and KLen columns should have non-zero values. If they are +zero, check the system log for error messages from chronyd. One possible cause +of failure is a firewall blocking the client's connection to the server's TCP +port 4460. + +Another possible cause of failure is a certificate that is failing to verify +because the client's clock is wrong. This is a chicken-and-egg problem with +NTS. You might need to manually correct the date, or temporarily disable NTS, +in order to get NTS working. If your computer has an RTC and it is backed up by +a good battery, this operation should be needed only once, assuming the RTC +will be set periodically with the rtcsync directive, or compensated with the +rtcfile directive and the -s option. + +If the computer does not have an RTC or battery, you can use the -s option +without rtcfile directive to restore time of the last shutdown or reboot from +the drift file. The clock will start behind the true time, but if the computer +was not shut down for too long and the server's certificate was not renewed too +close to its expiration, it should be sufficient for the time checks to +succeed. + +If you run your own server, you can use a self-signed certificate covering all +dates where the client can start (e.g. years 1970-2100). The certificate needs +to be installed on the client and specified with the ntstrustedcerts directive. +The server can have multiple names and certificates. To avoid trusting a +certificate for too long, a new certificate can be added to the server +periodically (e.g. once per year) and the client can have the server name and +trusted certificate updated automatically (e.g. using a package repository, or +a cron script downloading the files directly from the server over HTTPS). A +client that was shut down for years will still be able to synchronise its clock +and perform the update as long as the server keeps the old certificate. + +As a last resort, you can disable the time checks by the nocerttimecheck +directive. This has some important security implications. To reduce the +security risk, you can use the nosystemcert and ntstrustedcerts directives to +disable the system's default trusted certificate authorities and trust only a +minimal set of selected authorities needed to validate the certificates of used +NTP servers. + +3.6. 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 + +3.7. 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). + +3.8. 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. + +3.9. 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 0.4 seconds (0.2 +seconds with chrony versions before 4.1) 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 +ntp1.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 the limit, 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). + +Note that this protocol is not compatible with the mode 6 or mode 7 protocol +supported by ntpd, i.e. the ntpq or ntpdc utility cannot be used to monitor +chronyd, and chronyc cannot be used to monitor ntpd. + +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 will not 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 run by default in the boot and/or shutdown scripts in +some 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 do that, it will overwrite 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. When I start chronyd, the log says Could not enable RTC interrupt : +Invalid argument (or it may say disable) + +Your real-time clock hardware might not support the required ioctl requests: + + o RTC_UIE_ON + + o RTC_UIE_OFF + +A possible solution could be to build the Linux kernel with support for +software emulation instead; try enabling the following configuration option +when building the Linux kernel: + + o CONFIG_RTC_INTF_DEV_UIE_EMUL + +5.6. 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 is enabled by +the xleave option. Otherwise, chronyd will use different transmit timestamps +(e.g. daemon timestamp vs kernel timestamp) for serving time and +synchronisation of its own clock, which will cause the other computer to +measure a significant offset. + +7. Operation + +7.1. What clocks does chronyd use? + +There are several different clocks used by chronyd: + + o System clock: software clock maintained by the kernel. It is the main clock + used by applications running on the computer. It is synchronised by chronyd + to its NTP clock, unless started with the -x option. + + o NTP clock: software clock (virtual) based on the system clock and internal + to chronyd. It keeps the best estimate of the true time according to the + configured time sources, which is served to NTP clients unless time + smoothing is enabled by the smoothtime directive. The System time value in + the tracking report is the current offset between the system and NTP clock. + + o Real-time clock (RTC): hardware clock keeping time even when the computer + is turned off. It is used by the kernel to initialise the system clock on + boot and also by chronyd to compensate for its measured drift if configured + with the rtcfile directive and started with the -s option. The clock can be + kept accurate only by stepping enabled by the rtcsync or rtcautotrim + directive. + + o Reference clock: hardware clock used as a time source. It is specified by + the refclock directive. + + o NIC clock (also known as PTP hardware clock): hardware clock timestamping + packets received and transmitted by a network device specified by the + hwtimestamp directive. The clock is expected to be running free. It is not + synchronised by chronyd. Its offset is tracked relative to the NTP clock in + order to convert the hardware timestamps. + +8. Operating systems + +8.1. 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. + +8.2. 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 2023-12-05 14:22:10 +0100 |