ntpd -- Network Time Protocol (NTP) daemon _ _ 81215 _

ntpd -- Network Time Protocol daemon

ntpd [-aAbdgLmnPqx] [-c conffile] [-D level] [-f driftfile] [-k keyfile] [-l logfile] [-p pidfile] [-s statsdir] [-t key] [-v variable] [-V variable] [-N high] [-r broadcastdelay]

Maintains local time-of-day synchronism with time servers.

Reads ntp.conf at startup or the command line or -c conffile or NetInfo .
Use ntpq and ntpdc to query or change options on the fly.

-q quit after the first time the clock is set (as does ntpdate).
-g and -x can be used with this option.
This will not be sufficient to synchronize the clock.
-g disable sanity once. Normally, ntpd exits if the offset exceeds the sanity limit, which is 1000 seconds by default. If the sanity limit is set to zero, no sanity checking is performed and any offset is acceptable. After that, ntpd will exit if the limit is exceeded. used with the -q option.
-x slew time in all cases (prevents reducing the time). Normally, the time is slewed if the offset is less than the step threshold, (128 ms by default) and stepped if above the threshold.
If the step threshold is set to zero, all offsets are stepped, regardless of value and regardless of the -x option. Not a good idea, as it bypasses the clock state machine which is designed to cope with large time and frequency errors. Since the slew rate is limited to 0.5 ms/s, each second of adjustment requires an interval of 2,000 seconds ( 33 minutes). An adjustment of many seconds can take hours or days . This option can be used with the -q option.
-n Don't fork.
-a Enable authentication mode (default).
-A Disable authentication mode.
-c conffile name of the configuration file. (Disable netinfo?)
-d debugging mode. multiple times, greater detail of display.
-D level debugging level directly. D 9 sample
-f driftfile name of the drift file.
-l logfile name of the log file. The default is the system log facility syslog.
-s statsdir directory path for files created by the statistics facility.
-L Listen to virtual IPs.
-b
-m use broadcast messages.
use multicast messages on the IP multicast group address 224.0.1.1 (requires multicast kernel).
-p pidfile name to record the ntpd's process ID.
-N priority run the ntpdat a high priority.
-P Override the priority limit set by the operating system.
-r delay default propagation delay . necessary only if the delay cannot be computed automatically by the protocol.
-k keyfile file containing the NTP authentication keys.
-t key Add a key number to the trusted key list.
-v variable
-V variable Add a system variable listed by default.
The best choice of server is to use one of the server pools where the specific server changes periodically.

> cat ntp.conf server pool.ntp.org

A useful command is:

sudo ntpd -q -n -g -x -D 9 |more

which quits after one update, does NOT fork, disables the sanity check permiting large adjustments and slews to the new time.

How NTP Operates

By exchanging messages with servers at poll intervals, several several minutes can elapse before the clock is set. Adjustments to the clock are made in small steps so that the timescale is virtually continuous.

Hardware incorporates a time component (maintained by a battery) which runs when the power is off. When the machine is booted, this is used to initialize the operating system time. After synchronized to a NTP server, the operating system corrects the chip . If this component is more than 17 minutes from the server time, ntpd exits with a panic message to the system log.
-g overrides this check and the clock will be set to the server time regardless of the chip time. To protect against broken hardware, such as when the battery fails or the component becomes defective, once the clock has been set, an error greater than 1000secs will cause ntpd to exit anyway.

When ntpd is first started, the error may cause the clock to be set backwards. Where this is unacceptable -x causes slew corrections to be used.
The local clock can take a long time to converge (33 minutes) for each second the clock is outside the acceptable range. attempts to quickly correct the frequency and restore operation to the normal tracking mode.

Frequency Discipline

When the ntpd is started and the the frequency file does not exist, ntpd enters a special mode designed to quickly adapt. This takes approximately 15 minutes, then the time and frequency are set to nominal values and the ntpd enters normal mode. After one hour the frequency file is updated.

When the ntpd is started and the file exists, the ntpd frequency is initialized from the file and enters normal mode immediately.
After that the current frequency offset is written to the file at hourly intervals.

Operating Modes

symmetric active/passive, client/server broadcast/multicast and manycast, "Association Management" It normally operates continuously while monitoring for small changes in frequency and trimming the clock for the ultimate precision. It can operate in a one-time mode where the time is set from an external server and frequency is set from a previously recorded frequency file. A broadcast/multicast or manycast client can discover remote servers, compute server-client propagation delay correction factors and configure itself automatically. This makes it possible to deploy a fleet of workstations without specifying configuration details specific to the local environment. By default, ntpd runs in continuous mode where each of possibly several external servers is polled It may not be practical for ntpd to run continuously. -q is intended for this purpose. ntpd to exit just after setting the clock for the first time. ntpd is run in continuous mode with selected servers in order to measure and record the intrinsic clock frequency offset in the frequency file. It may take some hours for the frequency and offset to settle down. Then the ntpd is stopped and run in one-time mode as required. At each startup, the frequency is read from the file and initializes the kernel frequency.

Poll Interval Control

An intricate state machine to reduce the network load while maintaining a quality of synchronization consistent with the observed jitter and wander. There are a number of ways to tailor the operation in order enhance accuracy by reducing the interval or to reduce network overhead by increasing it. carefully consider the consequences of changing the poll adjustment range from the default minimum of 64 s to the default maximum of 1,024 seconds(33 minutes). The default minimum can be changed with the tinker minpoll command to a value not less than 16 secs. This value is used for all configured associations, unless overridden by the minpoll option on the configuration command. that most device drivers will not operate properly if the poll interval is less than 64 s and that the broadcast server and manycast client associations will also use the default, unless overridden.

To increase the minimum interval to a few tens of minutes and maximum interval to a day, once the clock discipline loop has stabilized the interval will be increased in steps from the minimum to the maximum. This assumes the intrinsic clock frequency error is small enough for the discipline loop correct it. The capture range of the loop is 500 PPM at an interval of 64s decreasing by a factor of two for each doubling of interval. at a minimum of 1,024 s, for example, the capture range is only 31 PPM. If the intrinsic error is greater than this ntp.drift will have to be specially tailored to reduce the residual error below this limit. Once this is done, the drift file is automatically updated once per hour and is available to initialize the frequency on subsequent daemon restarts. If the system is conserving energy by spinning down the disk or sleeping when idle then the update is deferred until ntpd terminates.

If umask for ntpd is zero, it is set to 022 so files are created with rwxr-xr-x.

FILES /etc/ntp.conf , /etc/ntp.drift , /etc/ntp.keys

SEE ALSO ntp.conf, ntpdate, ntpdc, ntpq(8), sntp(client)

Local documentation is at file:///usr/share/doc/ntp/index.html
NTP.org is a great reference and discusses choosing a time server to sync to.

-q	quit after the first time the clock is set (as does `ntpdate`). `-g` and `-x` can be used with this option. This will not be sufficient to synchronize the clock.
-g	disable sanity once. Normally, ntpd exits if the offset exceeds the sanity limit, which is 1000 seconds by default. If the sanity limit is set to zero, no sanity checking is performed and any offset is acceptable. After that, ntpd will exit if the limit is exceeded. used with the -q option.
-x	slew time in all cases (prevents reducing the time). Normally, the time is slewed if the offset is less than the step threshold, (128 ms by default) and stepped if above the threshold. If the step threshold is set to zero, all offsets are stepped, regardless of value and regardless of the `-x` option. Not a good idea, as it bypasses the clock state machine which is designed to cope with large time and frequency errors. Since the slew rate is limited to 0.5 ms/s, each second of adjustment requires an interval of 2,000 seconds ( 33 minutes). An adjustment of many seconds can take hours or days . This option can be used with the `-q` option.
-n	Don't fork.
-a	Enable authentication mode (default).
-A	Disable authentication mode.
-c conffile	name of the configuration file. (Disable netinfo?)
-d	debugging mode. multiple times, greater detail of display.
-D level	debugging level directly. D 9 sample
-f driftfile	name of the drift file.
-l logfile	name of the log file. The default is the system log facility syslog.
-s statsdir	directory path for files created by the statistics facility.
-L	Listen to virtual IPs.
-b -m	use broadcast messages. use multicast messages on the IP multicast group address 224.0.1.1 (requires multicast kernel).
-p pidfile	name to record the ntpd's process ID.
-N priority	run the ntpdat a high priority.
-P	Override the priority limit set by the operating system.
-r delay	default propagation delay . necessary only if the delay cannot be computed automatically by the protocol.
-k keyfile	file containing the NTP authentication keys.
-t key	Add a key number to the trusted key list.
-v variable -V variable	Add a system variable listed by default.

How NTP Operates

Frequency Discipline

Operating Modes

Poll Interval Control

Sample output from -D 9

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