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NTPD(8)			  BSD System Manager's Manual		       NTPD(8)

     ntpd -- Network Time Protocol (NTP) daemon


     The ntpd utility is an operating system daemon which sets and maintains
     the system	time of	day in synchronism with	Internet standard time
     servers.  It is a complete	implementation of the Network Time Protocol
     (NTP) version 4, but also retains compatibility with version 3, as	de-
     RFC-1119, respectively.

     The ntpd utility does most	computations in	64-bit floating	point arith-
     metic and does relatively clumsy 64-bit fixed point operations only when
     necessary to preserve the ultimate	precision, about 232 picoseconds.
     While the ultimate	precision, is not achievable with ordinary worksta-
     tions and networks	of today, it may be required with future gigahertz CPU
     clocks and	gigabit	LANs.

     in	order to determine the synchronization sources and operating modes.
     entirely on the command line, obviating the need for a configuration
     by	listening to broadcasts	at run time.

     read and no file is specified by the -c option.

     Various internal ntpd variables can be displayed and configuration	op-
     utility programs.

     set the umask(2) to 022.

     The following options are available:

     -a	     Enable authentication mode	(default).

     -A	     Disable authentication mode.

     -b	     Synchronize using NTP broadcast messages.

     -c	conffile
	     Specify the name and path of the configuration file.  (Disable

	     each occurrence indicating	greater	detail of display.

     -D	level
	     Specify debugging level directly.

     -f	driftfile
	     Specify the name and path of the drift file.

	     no	sanity checking	is performed and any offset is acceptable.
	     any value without restriction; however, this can happen only
	     option can	be used	with the -q option.

     -k	keyfile
	     tication keys.

     -l	logfile
	     system log	facility.

     -L	     Listen to virtual IPs.

     -m	     Synchronize using NTP multicast messages on the IP	multicast
	     group address (requires multicast kernel).

     -n	     Don't fork.

     -N	priority
	     a high priority.

     -p	pidfile
	     Specify the name and path to record the ntpd's process ID.

	     recommended for sissies.


     -r	broadcastdelay
	     Specify the default propagation delay from	the broadcast/multi-
	     cast server and this computer.  This is necessary only if the de-
	     lay cannot	be computed automatically by the protocol.

     -s	statsdir
	     Specify the directory path	for files created by the statistics


     -v	variable

     -V	variable
	     Add a system variable listed by default.

	     threshold.	 This option forces the	time to	be slewed in all
	     machine which is designed to cope with large time and frequency
	     second of adjustment requires an amortization interval of 2000 s.

   How NTP Operates
     The ntpd utility operates by exchanging messages with one or more config-
     first or subsequent times,	the program requires several exchanges from
     the majority of these servers so the signal processing and	mitigation al-
     gorithms can accumulate and groom the data	and set	the clock.  In order
     keyword with the server configuration command, as described in

     system time.  After the machine has synchronized to a NTP server, the op-
     erating system corrects the chip from time	to time.  In case there	is no
     action is for the operator	to intervene and set the clock by hand.	 This
     causes ntpd to exit with a	panic message to the system log.  The -g op-
     tion overrides this check and the clock will be set to the	server time
     regardless	of the chip time.  However, and	to protect against broken
     hardware, such as when the	CMOS battery fails or the clock	counter	be-
     comes defective, once the clock has been set, an error greater than 1000s
     will cause	ntpd to	exit anyway.

     the timescale is effectively continuous and without discontinuities.  Un-
     der conditions of extreme network congestion, the roundtrip delay jitter
     can exceed	three seconds and the synchronization distance,	which is equal
     large.  The ntpd algorithms discard sample	offsets	exceeding 128 ms, un-
     less the interval during which no sample offset is	less than 128 ms ex-
     ceeds 900s.  The first sample after that, no matter what the offset,

     congestion	and jitter.  Sometimes,	in particular when ntpd	is first

     The issues	should be carefully explored before deciding to	use the	-x op-
     tion.  The	maximum	slew rate possible is limited to 500 parts-per-million
     interval the local	clock will not be consistent with any other network
     clock and the system cannot be used for distributed applications that re-
     quire correctly synchronized network time.

     In	spite of the above precautions,	sometimes when large frequency errors
     correct the frequency and restore operation to the	normal tracking	mode.
     sional step/slew corrections and subsequent frequency corrections.	 It
     helps in these cases to use the burst keyword when	configuring the

   Frequency Discipline
     initialized from the file and enters normal mode immediately.  After that

   Operating Modes
     The ntpd utility can operate in any of several modes, including symmetric
     active/passive, client/server broadcast/multicast and manycast, as	de-
     scribed in	the "Association Management" page (available as	part of	the
     HTML documentation	provided in /usr/share/doc/ntp).  It normally operates
     continuously while	monitoring for small changes in	frequency and trimming
     the clock for the ultimate	precision.  However, 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	propa-
     gation 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.

     external servers is polled	at intervals determined	by an intricate	state
     machine.  The state machine measures the incidental roundtrip delay jit-
     ter and oscillator	frequency wander and determines	the best poll interval
     ments, the	state machine will start with 64s intervals and	eventually in-
     crease in steps to	1024s.	A small	amount of random variation is intro-
     server become unreachable for some	time, the poll interval	is increased
     in	steps to 1024s in order	to reduce network overhead.

     signal processing,	error checking and mitigation algorithms of ntpd.  The
     most applications will probably want to specify the iburst	keyword	with
     the server	configuration command.	With this keyword a volley of messages
     be	retired.

     When kernel support is available to discipline the	clock frequency, which
     continuous	mode with selected servers in order to measure and record the
     quency is read from the file and initializes the kernel frequency.

   Poll	Interval Control
     This version of NTP includes an intricate state machine to	reduce the
     network load while	maintaining a quality of synchronization consistent
     lor the operation in order	enhance	accuracy by reducing the interval or
     to	reduce network overhead	by increasing it.  However, the	user is	ad-
     vised to carefully	consider the consequences of changing the poll adjust-
     ment range	from the default minimum of 64 s to the	default	maximum	of
     1,024 s.  The default minimum can be changed with the tinker minpoll com-
     mand to a value not less than 16 s.  This value is	used for all config-
     ured associations,	unless overridden by the minpoll option	on the config-
     uration command.  Note 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 overrid-

     In	some cases involving dial up or	toll services, it may be useful	to in-
     crease the	minimum	interval to a few tens of minutes and maximum interval
     to	a day or so.  Under normal operation conditions, once the clock	disci-
     pline loop	has stabilized the interval will be increased in steps from
     the minimum to the	maximum.  However, 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
     able to initialize	the frequency on subsequent daemon restarts.

   The huff-n'-puff filter
     uploaded over telephone modems, timekeeping quality can be	seriously de-
     graded.  This occurs because the differential delays on the two direc-

     other traffic is present.	In common scenarios this occurs	during other
     than work hours.  The filter maintains a shift register that remembers
     the minimum delay over the	most recent interval measured usually in
     of	the offset.

     described in ntp.conf(5).

     /etc/ntp.conf   the default name of the configuration file
     /etc/ntp.drift  the default name of the drift file
     /etc/ntp.keys   the default name of the key file


     In	addition to the	manual pages provided, comprehensive documentation is
     available on the world wide web at  A	snapshot of




     The ntpd utility has gotten rather	fat.  While not	huge, it has gotten
     larger than might be desirable for	an elevated-priority ntpd running on a
     workstation, particularly since many of the fancy features	which consume
     the space were designed more with a busy primary server, rather than a
     high stratum workstation in mind.

BSD				August 2, 2001				   BSD


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