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Optionally specify a host, subnet, or network from which to allow NTP connections to a machine acting as NTP server. The default is not to allow connections.

The UDP port number 123 needs to be open in the firewall in order to allow the client access:

~]#  firewall-cmd --zone=public --add-port=123/udp

If you want to open port 123 permanently, use the --permanent option:

~]#  firewall-cmd --permanent --zone=public --add-port=123/udp

This is similar to the allow directive (see section allow), except that it allows control access (rather than NTP client access) to a particular subnet or host. (By “control access” is meant that chronyc can be run on those hosts and successfully connect to chronyd on this computer.) The syntax is identical. There is also a cmddeny all directive with similar behavior to the cmdallow all directive.

Path to the directory to save the measurement history across restarts of chronyd (assuming no changes are made to the system clock behavior whilst it is not running). If this capability is to be used (via the dumponexit command in the configuration file, or the dump command in chronyc), the dumpdir command should be used to define the directory where the measurement histories are saved.

If this command is present, it indicates that chronyd should save the measurement history for each of its time sources recorded whenever the program exits. (See the dumpdir command above).

The hwtimestamp directive enables hardware timestamping for extremely accurate synchronization. For more details, see chrony.conf(5) manual page.

The local keyword is used to allow chronyd to appear synchronized to real time from the viewpoint of clients polling it, even if it has no current synchronization source. This option is normally used on the “master” computer in an isolated network, where several computers are required to synchronize to one another, and the “master” is kept in line with real time by manual input.

An example of the command is:

local stratum 10

A large value of 10 indicates that the clock is so many hops away from a reference clock that its time is unreliable. If the computer ever has access to another computer which is ultimately synchronized to a reference clock, it will almost certainly be at a stratum less than 10. Therefore, the choice of a high value like 10 for the local command prevents the machine’s own time from ever being confused with real time, were it ever to leak out to clients that have visibility of real servers.

The log command indicates that certain information is to be logged. It accepts the following options: The log files are written to the directory specified by the logdir command. An example of the command is:

log measurements statistics tracking

This directive allows the directory where log files are written to be specified. An example of the use of this directive is:

logdir /var/log/chrony

Normally chronyd will cause the system to gradually correct any time offset, by slowing down or speeding up the clock as required. In certain situations, the system clock may be so far adrift that this slewing process would take a very long time to correct the system clock. This directive forces chronyd to step 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 a specified limit (a negative value can be used to disable the limit). This is particularly useful when using reference clock, because the initstepslew directive only works with NTP sources.

An example of the use of this directive is:

makestep 1000 10

This would step the system clock if the adjustment is larger than 1000 seconds, but only in the first ten clock updates.

This directive sets the maximum allowed offset corrected on a clock update. The check is performed only after the specified number of updates to allow a large initial adjustment of the system clock. When an offset larger than the specified maximum occurs, it will be ignored for the specified number of times and then chronyd will give up and exit (a negative value can be used to never exit). In both cases a message is sent to syslog.

An example of the use of this directive is:

maxchange 1000 1 2

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.

One of chronyd's tasks is to work out how fast or slow the computer’s clock runs relative to its reference sources. In addition, it computes an estimate of the error bounds around the estimated value. If the range of error is too large, it indicates that the measurements have not settled down yet, and that the estimated gain or loss rate is not very reliable. The maxupdateskew parameter is the threshold for determining whether an estimate is too unreliable to be used. By default, the threshold is 1000 ppm. The format of the syntax is:

maxupdateskew skew-in-ppm

Typical values for skew-in-ppm might be 100 for a dial-up connection to servers over a telephone line, and 5 or 10 for a computer on a LAN. 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 master estimate. So if chronyd has an existing highly-reliable master estimate and a new estimate is generated which has large error bounds, the existing master estimate will dominate in the new master estimate.

The minsources directive sets the minimum number of sources that need to be considered as selectable in the source selection algorithm before the local clock is updated.

The format of the syntax is:

minsources number-of-sources

By default, number-of-sources is 1. Setting minsources to a larger number can be used to improve the reliability, because multiple sources will need to correspond with each other.

This directive, which takes no arguments, specifies that client accesses are not to be logged. Normally they are logged, allowing statistics to be reported using the clients command in chronyc.

When chronyd selects synchronization source from available sources, it will prefer the one with minimum synchronization distance. However, to avoid frequent reselecting when there are sources with similar distance, a fixed distance is added to the distance for sources that are currently not selected. This can be set with the reselectdist option. By default, the distance is 100 microseconds.

The format of the syntax is:

reselectdist dist-in-seconds

The stratumweight directive sets how much distance should be added per stratum to the synchronization distance when chronyd selects the synchronization source from available sources.

The format of the syntax is:

stratumweight dist-in-seconds

By default, dist-in-seconds is 1 millisecond. This means that sources with lower stratum are usually preferred to sources with higher stratum even when their distance is significantly worse. Setting stratumweight to 0 makes chronyd ignore stratum when selecting the source.

The rtcfile directive defines the name of the file in which chronyd can save parameters associated with tracking the accuracy of the system’s real-time clock (RTC). The format of the syntax is:

rtcfile /var/lib/chrony/rtc

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. Not all real-time clocks are supported as their code is system-specific. Note that if this directive is used then the real-time clock should not be manually adjusted as this would interfere with chrony's need to measure the rate at which the real-time clock drifts if it was adjusted at random intervals.

The rtcsync directive is present in the /etc/chrony.conf file by default. This will inform the kernel the system clock is kept synchronized and the kernel will update the real-time clock every 11 minutes.