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DNTPD(8) System Manager's Manual DNTPD(8)

dntpd — Network time protocol client daemon

dntpd [-46dnqstFSQ] [-f config_file] [-i insane_deviation] [-l log_level] [-T nominal_poll] [-L maximum_poll] [targets]

The dntpd daemon will synchronize the system clock to one or more external NTP time sources. By default an initial coarse offset correction will be made if time is off by greater than 2 minutes. Additional sliding offset corrections will be made if necessary. Once sufficient information is obtained, dntpd will also correct the clock frequency. Over the long haul the frequency can usually be corrected to within 2 ppm of the time source. Offset errors can typically be corrected to within 20 milliseconds, or within 1 millisecond of a low latency time source.

By default dntpd will load its configuration from /etc/dntpd.conf and run as a daemon (background itself). If you re-execute the binary it will automatically kill the currently running dntpd daemon. If you run dntpd with the -Q option any currently running daemon will be killed and no new daemon will be started.

The following command line options are available:

Forces dntpd to use only IPv4 addresses.
Forces dntpd to use only IPv6 addresses.
Run in debug mode. Implies -F, -l 99, and -f /dev/null and logs to stderr instead of syslog. The normal client code is run and time corrections will be made.
No-update mode. No actual update is made any time the client would otherwise normally update the system frequency or offset.
Quiet mode. Implies a logging level of 0.
Issue a coarse offset correction on startup. Normally a coarse offset correction is only made when the time differential is greater than 2 minutes. This option will cause the initial offset correction to be a coarse correction regardless. Note that the system will still not make a correction unless the offset error is greater than 4 times the standard deviation of the queries.
Test mode. Implies -F, -l 99, -n, and -f /dev/null and logs to stderr instead of syslog. A single linear regression is accumulated at the nominal polling rate and reported until terminated. No time corrections are made. This option is meant for testing only. Note that frequency corrections based on internet time sources typically require a long (10-30min) polling rate to be well correlated.
Run in the foreground. Unlike debug mode, this option will still log to syslog.
Do not set the time immediately on startup (default).
Terminate any running background daemon and exit.
config_file
Specify the configuration file. The default is /etc/dntpd.conf.
insane_deviation
Specify how much deviation is allowed in calculated offsets, in seconds. Fractions may be specified. A quorum of servers must agree with the one we select as being the best time source in order for us to select that source. The default deviation allowed is a fairly expansive 0.5 seconds. Note that offset errors due to internet packet latency can exceed 25ms (0.025).
log_level
Specify the log level. The default is 1. All serious errors are logged at log level 0. Major time corrections are logged at log level 1. All time corrections and state changes are logged at log level 2. Log levels 3 and 4 increase the amount of debugging information logged.
nominal_poll
Set the nominal polling interval, in seconds. This is the interval used while the client is in acquisition mode. The default is 300 seconds (5 minutes).
maximum_poll
Set the maximum polling interval, in seconds. This is the interval used while the client is in maintenance mode, after it believes it has stabilized the system's clock. The default is 1800 seconds (30 minutes).
targets
Specify targets in addition to the ones listed in the config file. Note that certain options (-d, -t) disable the config file, and you can specify a configuration file of /dev/null if you want to disable it otherwise.

dntpd runs two linear regressions for each target against the uncorrected system time. The two linear regressions are staggered so the second one is stable and can replace the first one once the first's sampling limit has been reached. The second linear regression is also capable of overriding the first if the target changes sufficiently to invalidate the first's correlation.

The linear regression is a line-fitting algorithm which allows us to calculate a running Y-intercept, slope, and correlation factor. The Y-intercept is currently not used but can be an indication of a shift in the time source. The slope basically gives us the drift rate which in turn allows us to correct the frequency. The correlation gives us a quality indication, with 0 being the worst and ± 1.0 being the best.

A standard deviation is calculated for offset corrections. A standard deviation gives us measure of the deviation from the mean of a set of samples. dntpd uses the sum(offset_error) and sum(offset_error^2) method to calculate a running standard deviation. The offset error relative to the frequency-corrected real time is calculated for each sample. Note that this differs from the uncorrected offset error that the linear regression uses to calculate the frequency correction.

In order to make a frequency correction a minimum of 8 samples and a correlation ≥ 0.99, or 16 samples and a correlation ≥ 0.96 is required. Once these requirements are met a frequency correction will typically be made each sampling period. Frequency corrections do not 'jump' the system time or otherwise cause fine-time computations to be inaccurate and thus can pretty much be made at will.

In order to make an offset correction a minimum of 4 samples is required and the standard deviation must be less than ¼ the current calculated offset error. The system typically applies offset corrections slowly over time. The algorithm will make an offset correction whenever these standards are met but the fact that the offset error must be greater than 4 times the standard deviation generally results in very few offset corrections being made once time has been frequency-corrected. dntpd will not attempt to make a followup offset correction until the system has completed applying the previous offset correction, as doing so would cause a serious overshoot or undershoot. It is possible to use a more sophisticated algorithm to take running offset corrections into account but we do not do that (yet).

dntpd maintains an operations mode for each target. An initial 6 samples are taken at 5 second intervals, after which samples are taken at 5 minute intervals. If the time source is deemed to be good enough (using fairly relaxed correlation and standard deviation comparisons) the polling interval is increased to 30 minutes. Note that long intervals are required to get good correlations from internet time sources.

If a target stops responding to NTP requests the operations mode goes into a failed state which polls the target at the nominal polling rate (e.g., 5 minutes). Once re-acquired dntpd will either go back to the 5-second startup mode or to the 5-minute acquisition mode depending on how long the target was in the failed state.

If the system clock is naturally off-frequency dntpd will be forced to make several offset corrections before it gets enough data to make a frequency correction. Once the frequency has been corrected dntpd can typically keep the time synchronized to within 1-20 milliseconds depending on the source and both the number of offset corrections and the size of the offset corrections should be significantly reduced.

It will take up to 30 seconds for dntpd to make the initial coarse offset correction. It can take anywhere from 5 minutes to 3 hours for dntpd to make the initial frequency correction, depending on the time source. Internet time sources require long delays between samples to get a high quality correlation in order to issue a frequency correction.

It is difficult to calculate the packet latency for an internet time source and in some cases this can result in time sources which disagree as much as 20ms with each other. If you specify multiple targets and run in debug or a high-logging mode you may observe this issue.

Multiple servers may be specified in the configuration file. Pool domains are supported and the same domain name may be specified several times to connect to several different targets within the pool. Your DNS server must rotate IPs for this to work properly (all UNIX name servers will rotate IPs). dntpd will automatically weed out any duplicate IPs.

When two or more time sources are configured, dntpd will do a quorum-based sanity check on its best pick and fail the server if its offset deviates significantly from other servers.

If a server fails, dntpd will relookup its domain name and attempt to reconnect to it. To avoid overloading servers due to packet routing snafus, reconnections can take upwards of an hour to cycle.

The /etc/dntpd.conf file contains a list of servers in the 'server <servername>' format, one per line. Any information after a '#' is assumed to be a comment. Any number of servers may be specified but it is usually wasteful to have more than four.

The system will start dntpd at boot if you add the line:

dntpd_enable="YES"

to /etc/rc.conf. dntpd will periodically re-resolve failed DNS queries and failed servers and may be enabled at boot time even if the network is not yet operational.

/var/run/dntpd.pid
When started as a daemon, dntpd stores its pid in this file. When terminating a running dntpd this file is used to obtain the pid.

/etc/dntpd.conf
The default configuration file.

The dntpd command first appeared in DragonFly 1.3.

This program was written by Matthew Dillon.

An algorithm is needed to deal with time sources with packet-latency-based offset errors.

The offset correction needs to be able to operate while a prior offset correction is still in-progress.

We need to record the frequency correction in a file which is then read on startup, to avoid having to recorrect the frequency from scratch every time the system is rebooted.

January 6, 2009 DragonFly-5.6.1