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

     ntpd -- Network Time Protocol (NTP) daemon

     ntpd [-aAbDdgLmnPqx] [-c conffile]	[-f driftfile] [-k keyfile]
	  [-l logfile] [-p pidfile] [-r	broadcastdelay]	[-s statsdir] [-t key]
	  [-v variable]	[-V variable]

     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-
     fined by RFC-1305,	and version 1 and 2, as	defined	by RFC-1059 and
     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 workstations
     and networks of today, it may be required with future gigahertz CPU
     clocks and	gigabit	LANs.

     Ordinarily, ntpd reads the	ntp.conf(5) configuration file at startup time
     in	order to determine the synchronization sources and operating modes.
     It	is also	possible to specify a working, although	limited, configuration
     entirely on the command line, obviating the need for a configuration
     file.  This may be	particularly useful when the local host	is to be con-
     figured as	a broadcast/multicast client, with all peers being determined
     by	listening to broadcasts	at run time.

     If	NetInfo	support	is built into ntpd, then ntpd will attempt to read its
     configuration from	the NetInfo if the default ntp.conf(5) file cannot be
     read and no file is specified by the -c option.

     Various internal ntpd variables can be displayed and configuration	op-
     tions altered while the ntpd is running using the ntpq(8) and ntpdc(8)
     utility programs.

     When ntpd starts it looks at the value of umask 2,	and if zero ntpd will
     set the umask 2 to	022.

     The following options are available:

     -a	     Require cryptographic authentication for broadcast	client,	multi-
	     cast client and symmetric passive associations.  This is the de-

     -A	     Do	not require cryptographic authentication for broadcast client,
	     multicast client and symmetric passive associations.  This	is al-
	     most never	a good idea.

     -b	     Enable the	client to synchronize to broadcast servers.

     -c	conffile
	     Specify the name and path of the configuration file, default

     -d	     Specify debugging mode.  This option may occur more than once,
	     with each occurrence indicating greater detail of display.	 You
	     need to compile ntpd with DEBUG in	order to use this.

     -D	level
	     Specify debugging level directly.

     -f	driftfile
	     Specify the name and path of the frequency	file, default
	     /etc/ntp.drift.  This is the same operation as the	driftfile
	     driftfile configuration command.

     -g	     Normally, ntpd exits with a message to the	system log if the off-
	     set exceeds the panic threshold, which is 1000 s by default.
	     This option allows	the time to be set to any value	without	re-
	     striction;	however, this can happen only once.  If	the threshold
	     is	exceeded after that, ntpd will exit with a message to the sys-
	     tem log.  This option can be used with the	-q and -x options.
	     See the tinker command for	other options.

     -k	keyfile
	     Specify the name and path of the symmetric	key file, default
	     /etc/ntp.keys.  This is the same operation	as the keys keyfile
	     configuration command.

     -l	logfile
	     Specify the name and path of the log file.	 The default is	the
	     system log	file.  This is the same	operation as the logfile
	     logfile configuration command.

     -L	     Do	not listen to virtual IPs.  The	default	is to listen.

     -m	     Enable the	client to synchronize to multicast servers at the IPv4
	     multicast group address

     -n	     Do	not fork.

     -N	     To	the extent permitted by	the operating system, run the ntpd at
	     the highest priority.

     -p	pidfile
	     Specify the name and path of the file used	to record the ntpd
	     process ID.  This is the same operation as	the pidfile pidfile
	     configuration command.

     -P	priority
	     To	the extent permitted by	the operating system, run the ntpd at
	     the specified priority.

     -q	     Exit the ntpd just	after the first	time the clock is set.	This
	     behavior mimics that of the ntpdate(8) program, which is to be
	     retired.  The -g and -x options can be used with this option.
	     Note: The kernel time discipline is disabled with this option.

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

     -s	statsdir
	     Specify the directory path	for files created by the statistics
	     facility.	This is	the same operation as the statsdir statsdir
	     configuration command.

     -t	key  Add a key number to the trusted key list.	This option can	occur
	     more than once.

     -v	variable

     -V	variable
	     Add a system variable listed by default.

     -x	     Normally, the time	is slewed if the offset	is less	than the step
	     threshold,	which is 128 ms	by default, and	stepped	if above the
	     threshold.	 This option sets the threshold	to 600 s, which	is
	     well within the accuracy window to	set the	clock manually.	 Note:
	     Since the slew rate of typical Unix kernels is limited to 0.5
	     ms/s, each	second of adjustment requires an amortization interval
	     of	2000 s.	 Thus, an adjustment as	much as	600 s will take	almost
	     14	days to	complete.  This	option can be used with	the -g and -q
	     options.  See the tinker command for other	options.  Note:	The
	     kernel time discipline is disabled	with this option.

   How NTP Operates
     The ntpd utility operates by exchanging messages with one or more config-
     ured servers at designated	poll intervals.	 When started, whether for the
     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
     to	protect	the network from bursts, the initial poll interval for each
     server is delayed an interval randomized over a few seconds.  At the de-
     fault initial poll	interval of 64s, several minutes can elapse before the
     clock is set.  The	initial	delay to set the clock can be reduced using
     the iburst	keyword	with the server	configuration command, as described in

     Most operating systems and	hardware of today incorporate a	time-of-year
     (TOY) chip	to maintain the	time during periods when the power is off.
     When the machine is booted, the chip is used to initialize	the operating
     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
     TOY chip or for some reason its time is more than 1000s from the server
     time, ntpd	assumes	something must be terribly wrong and the only reliable
     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.

     Under ordinary conditions,	ntpd adjusts the clock in small	steps so that
     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
     to	one-half the roundtrip delay plus error	budget terms, can become very
     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,
     steps the clock to	the indicated time.  In	practice this reduces the
     false alarm rate where the	clock is stepped in error to a vanishingly low

     As	the result of this behavior, once the clock has	been set, it very
     rarely strays more	than 128 ms, even under	extreme	cases of network path
     congestion	and jitter.  Sometimes,	in particular when ntpd	is first
     started, the error	might exceed 128 ms.  This may on occasion cause the
     clock to be set backwards if the local clock time is more than 128	s in
     the future	relative to the	server.	 In some applications, this behavior
     may be unacceptable.  If the -x option is included	on the command line,
     the clock will never be stepped and only slew corrections will be used.

     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
     (PPM) as a	consequence of the correctness principles on which the NTP
     protocol and algorithm design are based.  As a result, the	local clock
     can take a	long time to converge to an acceptable offset, about 2,000 s
     for each second the clock is outside the acceptable range.	 During	this
     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
     are present the resulting time offsets stray outside the 128-ms range and
     an	eventual step or slew time correction is required.  If following such
     a correction the frequency	error is so large that the first sample	is
     outside the acceptable range, ntpd	enters the same	state as when the
     ntp.drift file is not present.  The intent	of this	behavior is to quickly
     correct the frequency and restore operation to the	normal tracking	mode.
     In	the most extreme cases ( comes to mind), there may be oc-
     casional step/slew	corrections and	subsequent frequency corrections.  It
     helps in these cases to use the burst keyword when	configuring the

   Frequency Discipline
     The ntpd behavior at startup depends on whether the frequency file, usu-
     ally ntp.drift, exists.  This file	contains the latest estimate of	clock
     frequency error.  When the	ntpd is	started	and the	file does not exist,
     the ntpd enters a special mode designed to	quickly	adapt to the particu-
     lar system	clock oscillator time and frequency error.  This takes approx-
     imately 15	minutes, after which the time and frequency are	set to nominal
     values and	the ntpd enters	normal mode, where the time and	frequency are
     continuously tracked relative to the server.  After one hour the fre-
     quency file is created and	the current frequency offset written to	it.
     When the ntpd is started and the file does	exist, 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
     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.

     By	default, ntpd runs in continuous mode where each of possibly several
     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
     using a heuristic algorithm.  Ordinarily, and in most operating environ-
     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-
     duced in order to avoid bunching at the servers.  In addition, should a
     server become unreachable for some	time, the poll interval	is increased
     in	steps to 1024s in order	to reduce network overhead.

     In	some cases it may not be practical for ntpd to run continuously.  A
     common workaround has been	to run the ntpdate(8) program from a cron(8)
     job at designated times.  However,	this program does not have the crafted
     signal processing,	error checking and mitigation algorithms of ntpd.  The
     -q	option is intended for this purpose.  Setting this option will cause
     ntpd to exit just after setting the clock for the first time.  The	proce-
     dure for initially	setting	the clock is the same as in continuous mode;
     most applications will probably want to specify the iburst	keyword	with
     the server	configuration command.	With this keyword a volley of messages
     are exchanged to groom the	data and the clock is set in about 10 s.  If
     nothing is	heard after a couple of	minutes, the daemon times out and ex-
     its.  After a suitable period of mourning,	the ntpdate(8) program may be

     When kernel support is available to discipline the	clock frequency, which
     is	the case for stock Solaris, Tru64, Linux and FreeBSD, a	useful feature
     is	available to discipline	the clock frequency.  First, 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 fre-
     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
     with the observed jitter and wander.  There are a number of ways to tai-
     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
     greater than this,	the drift file 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	avail-
     able to initialize	the frequency on subsequent daemon restarts.

   The huff-n'-puff Filter
     In	scenarios where	a considerable amount of data are to be	downloaded or
     uploaded over telephone modems, timekeeping quality can be	seriously de-
     graded.  This occurs because the differential delays on the two direc-
     tions of transmission can be quite	large.	In many	cases the apparent
     time errors are so	large as to exceed the step threshold and a step cor-
     rection can occur during and after	the data transfer is in	progress.

     The huff-n'-puff filter is	designed to correct the	apparent time offset
     in	these cases.  It depends on knowledge of the propagation delay when no
     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
     hours.  Under conditions of severe	delay, the filter corrects the appar-
     ent offset	using the sign of the offset and the difference	between	the
     apparent delay and	minimum	delay.	The name of the	filter reflects	the
     negative (huff) and positive (puff) correction, which depends on the sign
     of	the offset.

     The filter	is activated by	the tinker command and huffpuff	keyword, as
     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

     ntp.conf(5), ntpdate(8), ntpdc(8),	ntpq(8)

     In	addition to the	manual pages provided, comprehensive documentation is
     available on the world wide web at  A	snapshot of
     this documentation	is available in	HTML format in /usr/share/doc/ntp.

     David L. Mills, Network Time Protocol (Version 1),	RFC1059.

     David L. Mills, Network Time Protocol (Version 2),	RFC1119.

     David L. Mills, Network Time Protocol (Version 3),	RFC1305.

     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				 May 18, 2010				   BSD


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