The server can be specified by its hostname or IP address. If the hostname cannot be resolved on start, chronyd will try it again in increasing intervals, and also when the online command is issued in chronyc.

The DNS record can change over time. The used address will be replaced with a newly resolved address when the server becomes unreachable (i.e. no valid response to last 8 requests), unsynchronised, a falseticker (i.e. does not agree with a majority of other sources), or the root distance is too large (the limit can be configured by the maxdistance directive). The automatic replacement happens at most once per 30 minutes.

This directive can be used multiple times to specify multiple servers.

The directive supports the following options:

This directive can be used multiple times to specify multiple pools.

All options valid in the server directive can be used in this directive too. There is one option specific to the pool directive:

This directive can be used multiple times to specify multiple peers.

The following options of the server directive do not work in the peer directive: iburst, burst, nts, presend, copy.

When using the xleave option, both peers must support and have enabled the interleaved mode, otherwise the synchronisation will work in one direction only. When a key is specified by the key option to enable authentication, both peers must use the same key and the same key number.

Note that the symmetric mode is less secure than the client/server mode. A denial-of-service attack is possible on unauthenticated symmetric associations, i.e. when the peer was specified without the key option. An attacker who does not see network traffic between two hosts, but knows that they are peering with each other, can periodically send them unauthenticated packets with spoofed source addresses in order to disrupt their NTP state and prevent them from synchronising to each other. When the association is authenticated, an attacker who does see the network traffic, but cannot prevent the packets from reaching the other host, can still disrupt the state by replaying old packets. The attacker has effectively the same power as a man-in-the-middle attacker. A partial protection against this attack is implemented in chronyd, which can protect the peers if they are using the same polling interval and they never sent an authenticated packet with a timestamp from future, but it should not be relied on as it is difficult to ensure the conditions are met. If two hosts should be able to synchronise to each other in both directions, it is recommended to use two separate client/server associations (specified by the server directive on both hosts) instead.

The purpose of the initstepslew directive is to allow chronyd to make a rapid measurement of the system clock error at boot time, and to correct the system clock by stepping before normal operation begins. Since this would normally be performed only at an appropriate point in the system boot sequence, no other software should be adversely affected by the step.

If the correction required is less than a specified threshold, a slew is used instead. This makes it safer to restart chronyd whilst the system is in normal operation.

The initstepslew directive takes a threshold and a list of NTP servers as arguments. Each of the servers is rapidly polled several times, and a majority voting mechanism used to find the most likely range of system clock error that is present. A step or slew is applied to the system clock to correct this error. chronyd then enters its normal operating mode.

An example of the use of the directive is:

where 3 NTP servers are used to make the measurement. The 30 indicates that if the system’s error is found to be 30 seconds or less, a slew will be used to correct it; if the error is above 30 seconds, a step will be used.

The initstepslew directive can also be used in an isolated LAN environment, where the clocks are set manually. The most stable computer is chosen as the primary server and the other computers are its clients. If each of the clients is configured with the local directive, the server can be set up with an initstepslew directive which references some or all of the clients. Then, if the server machine has to be rebooted, the clients can be relied on to act analogously to a flywheel and preserve the time for a short period while the server completes its reboot.

The initstepslew directive is functionally similar to a combination of the makestep and server directives with the iburst option. The main difference is that the initstepslew servers are used only before normal operation begins and that the foreground chronyd process waits for initstepslew to finish before exiting. This prevent programs started in the boot sequence after chronyd from reading the clock before it has been stepped. With the makestep directive, the waitsync command of chronyc can be used instead.

This directive can be used multiple times to specify multiple reference clocks.

There are four drivers included in chronyd:

Note that the settime command can be enabled at run-time using the manual command in chronyc. (The idea of the two commands is that the manual command controls the manual clock driver’s behaviour, whereas the settime command allows samples of manually entered time to be provided.)

It can be set to the same port as is used by the NTP server (which can be configured with the port directive) to use only one socket for all NTP packets.

An example of the acquisitionport directive is:

This would change the source port used for client requests to UDP port 1123. You could then persuade the firewall administrator to open that port.

For each of the IPv4 and IPv6 protocols, only one bindacqaddress directive can be specified.

An example of the directive is:

An example of the directive (setting the Expedited Forwarding class) is:

All supported systems, with the exception of macOS 10.12 and earlier, have operating system support for setting the rate of gain or loss to compensate for known errors. (On macOS 10.12 and earlier, chronyd must simulate such a capability by periodically slewing the system clock forwards or backwards by a suitable amount to compensate for the error built up since the previous slew.)

For such systems, it is possible to save the measurement history across restarts of chronyd (assuming no changes are made to the system clock behaviour whilst it is not running). The dumpdir directive defines the directory where the measurement histories are saved when chronyd exits, or the dump command in chronyc is issued.

If the directory does not exist, it will be created automatically.

The -r option of chronyd enables loading of the dump files on start. All dump files found in the directory will be removed after start, even if the -r option is not present.

An example of the directive is:

A source whose IP address is 1.2.3.4 would have its measurement history saved in the file /var/run/chrony/1.2.3.4.dat. History of reference clocks is saved to files named by their reference ID in form of refid:XXXXXXXX.dat.

As a special case, setting maxsamples to 1 disables frequency tracking in order to make the sources immediately selectable with only one sample. This can be useful when chronyd is started with the -q or -Q option.

Forcing chronyd to keep more samples than it would normally keep reduces noise in the estimated frequency and offset, but slows down the response to changes in the frequency and offset of the clock. The offsets in the tracking and sourcestats reports (and the tracking.log and statistics.log files) may be smaller than the actual offsets.

If the directory does not exist, it will be created automatically.

An example of the directive is:

This directory is used also by the NTS server to save keys.

The interval must be longer than polling intervals of all configured NTP sources using NTS, otherwise the source with a longer polling interval will refresh the keys on each poll and no NTP packets will be exchanged.

The optional set-ID argument is a number in the range 0 through 2^32-1, which selects the set of certificates where certificates from the specified file or directory are added. The default ID is 0, which is a set containing the system’s default trusted CAs (unless the nosystemcert directive is present). All other sets are empty by default. A set of certificates can be selected for verification of an NTS server by the certset option in the server or pool directive.

This directive can be used multiple times to specify one or more sets of trusted certificates, each containing certificates from one or more files and/or directories.

It is not necessary to restart chronyd in order to reload the certificates if they change (e.g. after a renewal).

An example is:

An example of the directive is:

This would disable the time checks until the clock is updated for the first time, assuming the first update corrects the clock and later checks can work with correct time.

The refresh command can be used to refresh all sources immediately.

When authentication is enabled for an NTP source, it is important to disable unauthenticated NTP sources which could be exploited in the attack, e.g. if they are not reachable only over a trusted network. Alternatively, the source selection can be configured with the require and trust options to synchronise to the unauthenticated sources only if they agree with the authenticated sources and might have a positive impact on the accuracy of the clock. Note that in this case the impact of the attack is higher. The attackers cannot cause an arbitrarily large step or slew, but they have more control over the frequency of the clock and can cause chronyd to report false information, e.g. a significantly smaller root delay and dispersion.

This directive determines the default selection options for authenticated and unauthenticated sources in order to simplify the configuration with the configuration file and chronyc commands. It sets a policy for authentication.

Sources specified with the noselect option are ignored (not counted as either authenticated or unauthenticated), and they always have only the selection options specified in the configuration.

There are four modes:

As an example, the following configuration using the default mix mode:

is equivalent to the following configuration using the ignore mode:

The combinelimit directive limits which sources are included in the combining algorithm. Their synchronisation distance has to be shorter than the distance of the selected source multiplied by the value of the limit. Also, their measured frequencies have to be close to the frequency of the selected source. If the selected source was specified with the prefer option, it can be combined only with other sources specified with this option.

By default, the limit is 3. Setting the limit to 0 effectively disables the source combining algorithm and only the selected source will be used to control the system clock.

By default, the maximum root distance is 3 seconds.

Setting maxdistance to a larger value can be useful to allow synchronisation with a server that only has a very infrequent connection to its sources and can accumulate a large dispersion between updates of its clock.

By default, the maximum jitter is 1 second.

Setting this option to a larger number can be used to improve the reliability. More sources will have to agree with each other and the clock will not be updated when only one source (which could be serving incorrect time) is reachable.

By default, the weight is 0.001 seconds. This means that the stratum of the sources in the selection process matters only when the differences between the distances are in milliseconds.

The measured value works well in most cases. However, it generally overestimates the precision and it can be sensitive to the CPU speed, which can change over time to save power. In some cases with a high-precision clocksource (e.g. the Time Stamp Counter of the CPU) and hardware timestamping, setting the precision on the server to a smaller value can improve stability of clients' NTP measurements. The server’s precision is reported on clients by the ntpdata command.

An example setting the precision to 8 nanoseconds is:

The corrtimeratio directive sets the ratio between the duration in which the clock is slewed for an average correction according to the source history and the interval in which the corrections are done (usually the NTP polling interval). Corrections larger than the average take less time and smaller corrections take more time, the amount of the correction and the correction time are inversely proportional.

Increasing corrtimeratio improves the overall frequency error of the system clock, but increases the overall time error as the corrections take longer.

By default, the ratio is set to 3, the time accuracy of the clock is preferred over its frequency accuracy.

The maximum allowed slew rate can be set by the maxslewrate directive. The current remaining correction is shown in the tracking report as the System time value.

Whenever chronyd computes a new value of the gain or loss rate, it is desirable to record it somewhere. This allows chronyd to begin compensating the system clock at that rate whenever it is restarted, even before it has had a chance to obtain an equally good estimate of the rate during the new run. (This process can take many minutes, at least.)

The driftfile directive allows a file to be specified into which chronyd can store the rate information. Two parameters are recorded in the file. The first is the rate at which the system clock gains or loses time, expressed in parts per million, with gains positive. Therefore, a value of 100.0 indicates that when the system clock has advanced by a second, it has gained 100 microseconds in reality (so the true time has only advanced by 999900 microseconds). The second is an estimate of the error bound around the first value in which the true rate actually lies.

An example of the driftfile directive is:

The directive specifies the minimum and maximum interval since the last clock update to switch between fallback drifts. They are defined as a power of 2 (in seconds). The syntax is as follows:

In this example, the minimum interval is 16 (18 hours) and the maximum interval is 19 (6 days). The system clock frequency will be set to the first fallback 18 hours after last clock update, to the second after 36 hours, and so on. This might be a good setting to cover frequency changes due to daily and weekly temperature fluctuations. When the frequency is set to a fallback, the state of the clock will change to ‘Not synchronised’.

By default (or if the specified maximum or minimum is 0), no fallbacks are used and the clock frequency changes only with new measurements from NTP sources, reference clocks, or manual input.

For computer clocks that is a problem. The Unix time is defined as number of seconds since 00:00:00 UTC on 1 January 1970 without leap seconds. The system clock cannot have time 23:59:60, every minute has 60 seconds and every day has 86400 seconds by definition. The inserted leap second is skipped and the clock is suddenly ahead of UTC by one second. The leapsecmode directive selects how that error is corrected. There are four options:

When serving time to NTP clients that cannot be configured to correct their clocks for a leap second by slewing, or to clients that would correct at slightly different rates when it is necessary to keep them close together, the slew mode can be combined with the smoothtime directive to enable a server leap smear.

When smearing a leap second, the leap status is suppressed on the server and the served time is corrected slowly by slewing instead of stepping. The clients do not need any special configuration as they do not know there is any leap second and they follow the server time which eventually brings them back to UTC. Care must be taken to ensure they use only NTP servers which smear the leap second in exactly the same way for synchronisation.

This feature must be used carefully, because the server is intentionally not serving its best estimate of the true time.

A recommended configuration to enable a server leap smear is:

The first directive is necessary to disable the clock step which would reset the smoothing process. The second directive limits the slewing rate of the local clock to 1000 ppm, which improves the stability of the smoothing process when the local correction starts and ends. The third directive enables the server time smoothing process. It will start when the clock gets to 00:00:00 UTC and it will take 62500 seconds (about 17.36 hours) to finish. The frequency offset will be changing by 0.001024 ppm per second and will reach a maximum of 32 ppm in 31250 seconds. The leaponly option makes the duration of the leap smear constant and allows the clients to safely synchronise with multiple identically configured leap smearing servers.

The duration of the leap smear can be calculated from the specified wander as

When a leap second is announced, the timezone needs to be updated at least 12 hours before the leap second. It is not necessary to restart chronyd.

This directive is useful with reference clocks and other time sources which do not announce leap seconds, or announce them too late for an NTP server to forward them to its own clients. Clients of leap smearing servers must not use this directive.

It is also useful when the system clock is required to have correct TAI-UTC offset. Note that the offset is set only when leap seconds are handled by the kernel, i.e. leapsecmode is set to system.

The specified timezone is not used as an exclusive source of information about leap seconds. If a majority of time sources announce on the last day of June or December that a leap second should be inserted or deleted, it will be accepted even if it is not included in the timezone.

An example of the directive is:

The following shell command verifies that the timezone contains leap seconds and can be used with this directive:

This directive forces chronyd to step the system clock if the adjustment is larger than a threshold value, but only if there were no more clock updates since chronyd was started than the specified limit. A negative value disables the limit.

On most systems it is desirable to step the system clock only on boot, before starting programs that rely on time advancing monotonically forwards.

An example of the use of this directive is:

This would step the system clock if the adjustment is larger than 0.1 seconds, but only in the first three clock updates.

The check is enabled after the specified number of clock updates to allow a large initial offset to be corrected on start. Offsets larger than the specified maximum will be ignored for the specified number of times. Another large offset will cause chronyd to give up and exit. A negative value can be used to disable the limit to ignore all large offsets. A syslog message will be generated when an offset is ignored or it causes the exit.

An example of the use of this directive is:

After the first clock update, chronyd will check the offset on every clock update, it will ignore two adjustments larger than 1000 seconds and exit on another one.

By default, the maximum error is 1 ppm.

Typical values for error-in-ppm might be 10 for a low quality clock and 0.1 for a high quality clock using a temperature compensated crystal oscillator.

By default, the maximum assumed drift is 500000 ppm, i.e. the adjustment is limited by the system driver rather than this directive.

If the range of error is too large, it probably indicates that the measurements have not settled down yet, and that the estimated gain or loss rate is not very reliable.

The maxupdateskew directive sets the threshold for determining whether an estimate might be so unreliable that it should not be used. By default, the threshold is 1000 ppm.

Typical values for skew-in-ppm might be 100 for NTP sources polled over a wireless network, and 10 or smaller for sources on a local wired network.

It should be noted that this is not the only means of protection against using unreliable estimates. At all times, chronyd keeps track of both the estimated gain or loss rate, and the error bound on the estimate. When a new estimate is generated following another measurement from one of the sources, a weighted combination algorithm is used to update the existing estimate. If it has significantly smaller error bounds than the new estimate, the existing estimate will dominate in the new combined value.

For each system there is a maximum frequency offset of the clock that can be set by the driver. On Linux it is 100000 ppm, on FreeBSD, NetBSD and macOS 10.13+ it is 5000 ppm, and on illumos it is 32500 ppm. Also, due to a kernel limitation, setting maxslewrate on FreeBSD, NetBSD, macOS 10.13+ to a value between 500 ppm and 5000 ppm will effectively set it to 500 ppm.

By default, the maximum slew rate is set to 83333.333 ppm (one twelfth).

If there are temperature measurements available from a sensor close to the oscillator, the tempcomp directive can be used to compensate for the changes in the temperature and improve the stability and accuracy of the clock.

The result depends on many factors, including the resolution of the sensor, the amount of noise in the measurements, the polling interval of the time source, the compensation update interval, how well the compensation is specified, and how close the sensor is to the oscillator. When it is working well, the frequency reported in the tracking.log file is more stable and the maximum reached offset is smaller.

There are two forms of the directive. The first one has six parameters: a path to the file containing the current temperature from the sensor (in text format), the compensation update interval (in seconds), and temperature coefficients T0, k0, k1, k2.

The frequency compensation is calculated (in ppm) as

The result has to be between -10 ppm and 10 ppm, otherwise the measurement is considered invalid and will be ignored. The k0 coefficient can be adjusted to keep the compensation in that range.

An example of the use is:

The measured temperature will be read from the file in the Linux sysfs filesystem every 30 seconds. When the temperature is 26000 (26 degrees Celsius), the frequency correction will be zero. When it is 27000 (27 degrees Celsius), the clock will be set to run faster by 0.183 ppm, etc.

The second form has three parameters: the path to the sensor file, the update interval, and a path to a file containing a list of (temperature, compensation) points, from which the compensation is linearly interpolated or extrapolated.

An example is:

where the /etc/chrony.tempcomp file could have

Valid measurements with corresponding compensations are logged to the tempcomp.log file if enabled by the log tempcomp directive.

The default is that no clients are allowed access, i.e. chronyd operates purely as an NTP client. If the allow directive is used, chronyd will be both a client of its servers, and a server to other clients.

This directive can be used multiple times.

Examples of the use of the directive are as follows:

The first directive allows access from an IPv4 address. The second directive allows access from all computers in an IPv4 subnet specified in the CIDR notation. The third directive specifies the same subnet using a simpler notation where the prefix length is determined by the number of dots. The fourth directive specifies an IPv6 subnet. The fifth and sixth directives allow access from all IPv4 and IPv6 addresses respectively. The seventh directive allows access from all addresses (both IPv4 or IPv6).

A second form of the directive, allow all, has a greater effect, depending on the ordering of directives in the configuration file. To illustrate the effect, consider the two examples:

and

In the first example, the effect is the same regardless of what order the three directives are given in. So the 1.2.0.0/16 subnet is allowed access, except for the 1.2.3.0/24 subnet, which is denied access, however the host 1.2.3.4 is allowed access.

In the second example, the allow all 1.2.0.0/16 directive overrides the effect of any previous directive relating to a subnet within the specified subnet. Within a configuration file this capability is probably rather moot; however, it is of greater use for reconfiguration at run-time via chronyc with the allow all command.

The rules are internally represented as a tree of tables with one level per four bits of the IPv4 or IPv6 address. The order of the allow and deny directives matters if they modify the same records of one table, i.e. if one subnet is included in the other subnet and their prefix lengths are at the same level. For example, 1.2.3.0/28 and 1.2.3.0/29 are in different tables, but 1.2.3.0/25 and 1.2.3.0/28 are in the same table. The configuration can be verified for individual addresses with the accheck command in chronyc.

A hostname can be specified in the directives instead of the IP address, but the name must be resolvable when chronyd is started, i.e. the network is already up and DNS is working. If the hostname resolves to multiple addresses, only the first address (in the order returned by the system resolver) will be allowed or denied.

Note, if the initstepslew directive is used in the configuration file, each of the computers listed in that directive must allow client access by this computer for it to work.

The syntax is identical and the directive can be used multiple times too.

There is also a deny all directive with similar behaviour to the allow all directive.

An example of the use of the directive is:

Currently, for each of the IPv4 and IPv6 protocols, only one bindaddress directive can be specified. Therefore, it is not useful on computers which should serve NTP on multiple network interfaces.

An example of the directive is:

This directive can be used multiple times to specify multiple addresses.

The syntax is as follows:

In the first example, the destination port defaults to UDP port 123 (the normal NTP port). In the second example, the destination port is specified as 12123. The first parameter in each case (32 or 64 respectively) is the interval in seconds between broadcast packets being sent. The second parameter in each case is the broadcast address to send the packet to. This should correspond to the broadcast address of one of the network interfaces on the computer where chronyd is running.

You can have more than 1 broadcast directive if you have more than 1 network interface onto which you want to send NTP broadcast packets.

chronyd itself cannot act as a broadcast client; it must always be configured as a point-to-point client by defining specific NTP servers and peers. This broadcast server feature is intended for providing a time source to other NTP implementations.

If ntpd is used as the broadcast client, it will try to measure the round-trip delay between the server and client with normal client mode packets. Thus, the broadcast subnet should also be the subject of an allow directive.

An example of the use of this directive is:

This directive is normally used in an isolated network, where computers are required to be synchronised to one another, but not necessarily to real time. The server can be kept vaguely in line with real time by manual input.

The local directive has the following options:

An example of the directive is:

Note that MS-SNTP requests are not authenticated and any client that is allowed to access the server by the allow directive, or the allow command in chronyc, can get an MS-SNTP response signed with a trust account’s password and try to crack the password in a brute-force attack. Access to the server should be carefully controlled.

An example of the directive is:

The port will be open only when a certificate and key is specified by the ntsservercert and ntsserverkey directives.

This directive can be used multiple times to specify multiple certificates for different names of the server.

The files are loaded only once. chronyd needs to be restarted in order to load a renewed certificate. The ntsdumpdir and dumpdir directives with the -r option of chronyd are recommended for a near-seamless server operation.

This directive can be used multiple times to specify multiple keys. The number of keys must be the same as the number of certificates and the corresponding files must be specified in the same order.

An example of the directive is:

This directory is used also by the NTS client to save NTS cookies.

The automatic rotation of the keys can be disabled by setting ntsrotate to 0. In this case the keys are assumed to be managed externally. chronyd will not save the keys to the ntskeys file and will reload the keys from the file when the rekey command is issued in chronyc. The file can be periodically copied from another server running chronyd (which does not have ntsrotate set to 0) in order to have one or more servers dedicated to NTS-KE. The file includes the subsequent key to which the NTS-KE server will switch on the next rotation, i.e. the process copying and reloading the file does not need to be timed precisely (it can be delayed by up to one rotation interval). The NTS-KE servers need to be configured with the ntsntpserver directive to point the clients to the right NTP server.

An example of the directive is:

The default value is 123, the standard NTP port. If set to 0, chronyd will never open the server port and will operate strictly in a client-only mode. The source port used in NTP client requests can be set by the acquisitionport directive.

The ratelimit directive supports a number of options (which can be defined in any order):

An example use of the directive is:

This would reduce the response rate for IP addresses sending packets on average more than once per 2 seconds, or sending packets in bursts of more than 16 packets, by up to 75% (with default leak of 2).

An example of the use of the directive is:

BE WARNED: The server is intentionally not serving its best estimate of the true time. If a large offset has been accumulated, it can take a very long time to smooth it out. This directive should be used only when the clients are not configured to also poll another NTP server, because they could reject this server as a falseticker or fail to select a source completely.

The smoothing process is implemented with a quadratic spline function with two or three pieces. It is independent from any slewing applied to the local system clock, but the accumulated offset and frequency will be reset when the clock is corrected by stepping, e.g. by the makestep directive or the makestep command in chronyc. The process can be reset without stepping the clock by the smoothtime reset command.

The first two arguments of the directive are the maximum frequency offset of the smoothed time to the tracked NTP time (in ppm) and the maximum rate at which the frequency offset is allowed to change (in ppm per second). leaponly is an optional third argument which enables a mode where only leap seconds are smoothed out and normal offset and frequency changes are ignored. The leaponly option is useful in a combination with the leapsecmode slew directive to allow the clients to use multiple time smoothing servers safely.

The smoothing process is activated automatically when 1/10000 of the estimated skew of the local clock falls below the maximum rate of frequency change. It can be also activated manually by the smoothtime activate command, which is particularly useful when the clock is synchronised only with manual input and the skew is always larger than the threshold. The smoothing command can be used to monitor the process.

An example suitable for clients using ntpd and 1024 second polling interval could be:

An example suitable for clients using chronyd on Linux could be:

This directive can also change the path of the Unix domain command socket, which is used by chronyc to send configuration commands. The socket must be in a directory that is accessible only by the root or chrony user. The directory will be created on start if it does not exist. The compiled-in default path of the socket is /var/run/chrony/chronyd.sock. The socket can be disabled by setting the path to /.

By default, chronyd binds the UDP sockets to the addresses 127.0.0.1 and ::1 (i.e. the loopback interface). This blocks all access except from localhost. To listen for command packets on all interfaces, you can add the lines:

to the configuration file.

For each of the IPv4, IPv6, and Unix domain protocols, only one bindcmdaddress directive can be specified.

An example that sets the path of the Unix domain command socket is:

An example of the directive is:

The syntax is identical to the allow directive.

There is also a cmdallow all directive with similar behaviour to the allow all directive (but applying to monitoring access in this case, of course).

Note that chronyd has to be configured with the bindcmdaddress directive to not listen only on the loopback interface to actually allow remote access.

The syntax is identical.

There is also a cmddeny all directive with similar behaviour to the cmdallow all directive.

An example shows the syntax:

This would make chronyd use UDP 257 as its command port. (chronyc would need to be run with the -p 257 switch to inter-operate correctly.)

An example of the use of the directive is:

The compiled-in default value is ''.

An example of the directive is:

This directive is effective only with the rtcfile directive.

An example of the use of this directive is:

This would set the threshold error to 30 seconds.

An example of the directive is:

chronyd saves information in this file when it exits and when the writertc command is issued in chronyc. The information saved is the RTC’s error at some epoch, that epoch (in seconds since January 1 1970), and the rate at which the RTC gains or loses time.

So far, the support for real-time clocks is limited; their code is even more system-specific than the rest of the software. You can only use the RTC facilities (the rtcfile directive and the -s command-line option to chronyd) if the following three conditions apply:

If you keep the RTC on local time and your computer is off when daylight saving (summer time) starts or ends, the computer’s system time will be one hour in error when you next boot and start chronyd.

An alternative is for the RTC to keep Universal Coordinated Time (UTC). This does not suffer from the 1 hour problem when daylight saving starts or ends.

If the rtconutc directive appears, it means the RTC is required to keep UTC. The directive takes no arguments. It is equivalent to specifying the -u switch to the Linux hwclock program.

Note that this setting is overridden by the hwclockfile file and is not relevant for the rtcsync directive.

On Linux, the RTC copy is performed by the kernel every 11 minutes.

On macOS, chronyd will perform the RTC copy every 60 minutes when the system clock is in a synchronised state.

On other systems this directive does nothing.

The logbanner directive specifies after how many entries in the log file should be the banner written. The default is 32, and 0 can be used to disable it entirely.

By default, the threshold is 1 second.

An example of the use is:

which would cause a syslog message to be generated if a system clock error of over 0.1 seconds starts to be compensated.

An example of the use of this directive is:

An example of the use of this directive is:

This would send a mail message to root if a change of more than 0.5 seconds were applied to the system clock.

This directive cannot be used when a system call filter is enabled by the -F option as the chronyd process will not be allowed to fork and execute the sendmail binary.

Multiple directories (up to 10) can be specified with a single confdir directive. In this case, if multiple directories contain a file with the same name, only the first file in the order of the specified directories will be included. This enables a fragmented configuration where existing fragments can be replaced by adding files to a different directory.

This directive can be used multiple times.

An example of the directive is:

This directive can be used multiple times.

An example of the directive is:

This directive can be used multiple times.

An example of the directive is:

This directive is supported on Linux 3.19 and newer. The NIC must support HW timestamping, which can be verified with the ethtool -T command. The list of capabilities should include hardware-raw-clock, hardware-transmit, and hardware-receive. The receive filter all, or ntp, is necessary for timestamping of received NTP packets. Timestamping of packets received on bridged and bonded interfaces is supported on Linux 4.13 and newer. If HW timestamping does not work for received packets, chronyd will use kernel receive timestamps instead. Transmit-only HW timestamping can still be useful to improve stability of the synchronisation.

chronyd does not synchronise the NIC clock. It assumes the clock is running free. Multiple instances of chronyd can use the same interface with enabled HW timestamping. Applications which need HW timestamping with a synchronised clock (e.g. a PTP daemon) should use a virtual clock running on top of the physical clock created by writing to /sys/class/ptp/ptpX/n_vclocks. This feature is available on Linux 5.14 and newer.

If the kernel supports software timestamping, it will be enabled for all interfaces automatically.

The source of timestamps (i.e. hardware, kernel, or daemon) is indicated on the client side in the measurements.log file (if enabled by the log directive) and the ntpdata report. On the server side, the number of served timestamps from each source is provided in the serverstats report.

This directive can be used multiple times to enable HW timestamping on multiple interfaces. If the specified interface is *, chronyd will try to enable HW timestamping on all available interfaces.

The hwtimestamp directive has the following options:

Examples of the directive are:

The default value is 0.001 seconds, which should be sufficient with most hardware. If you frequently see kernel transmit timestamps in the measurements.log file or ntpdata report, and it is not a server handling a high rate of requests in the interleaved mode on the same interface (which would compete with timestamping of the server’s own requests), increasing the timeout to 0.01 or possibly even longer might help. Note that the maximum timeout is limited by the NTP polling interval.

The format of the directive is shown in the example below:

The argument is simply the name of the file containing the ID-key pairs. The format of the file is shown below:

Each line consists of an ID, optional type, and key.

The ID can be any positive integer in the range 1 through 2^32-1.

The type is a name of a cryptographic hash function or cipher which is used to generate and verify the MAC. The default type is MD5, which is always supported. If chronyd was built with enabled support for hashing using a crypto library (Nettle, GnuTLS, NSS, or LibTomCrypt), the following functions are available: MD5, SHA1, SHA256, SHA384, SHA512. Depending on which library and version is chronyd using, some of the following hash functions and ciphers may also be available: SHA3-224, SHA3-256, SHA3-384, SHA3-512, TIGER, WHIRLPOOL, AES128, AES256.

The key can be specified as a string of ASCII characters not containing white space with an optional ASCII: prefix, or as a hexadecimal number with the HEX: prefix. The maximum length of the line is 2047 characters. If the type is a cipher, the length of the key must match the cipher (i.e. 128 bits for AES128 and 256 bits for AES256).

It is recommended to use randomly generated keys, specified in the hexadecimal format, which are at least 128 bits long (i.e. they have at least 32 characters after the HEX: prefix). chronyd will log a warning to syslog on start if a source is specified in the configuration file with a key shorter than 80 bits.

The recommended key types are AES ciphers and SHA3 hash functions. MD5 should be avoided unless no other type is supported on the server and client, or peers.

The keygen command of chronyc can be used to generate random keys for the key file. By default, it generates 160-bit MD5 or SHA1 keys.

For security reasons, the file should be readable only by root and the user under which chronyd is normally running (to allow chronyd to re-read the file when the rekey command is issued by chronyc).

The NTP-over-PTP support is experimental. The protocol and configuration can change in future. It should be used only in local networks and expected to work only between servers and clients running the same version of chronyd.

The PTP port will be open even if chronyd is not configured to operate as a server or client. The directive does not change the default protocol of specified NTP sources. Each NTP source that should use NTP-over-PTP needs to be specified with the port option set to the PTP port. To actually enable hardware timestamping on NICs which can timestamp PTP packets only, the rxfilter option of the hwtimestamp directive needs to be set to ptp.

An example of client configuration is:

On systems other than macOS, this directive uses the pthread_setschedparam() system call to instruct the kernel to use the SCHED_FIFO first-in, first-out real-time scheduling policy for chronyd with the specified priority. This means that whenever chronyd is ready to run it will run, interrupting whatever else is running unless it is a higher priority real-time process. This should not impact performance as chronyd resource requirements are modest, but it should result in lower and more consistent latency since chronyd will not need to wait for the scheduler to get around to running it. You should not use this unless you really need it. The pthread_setschedparam(3) man page has more details.

On macOS, this directive uses the thread_policy_set() kernel call to specify real-time scheduling. As noted above, you should not use this directive unless you really need it.

On Linux, chronyd needs to be compiled with support for the libcap library. On macOS, FreeBSD, NetBSD and illumos 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.

The compiled-in default value is root.