manpagez: man(ual) pages & more
man ntpd(8)
Home | html | info | man
ntpd(8)                   BSD System Manager's Manual                  ntpd(8)


     ntpd -- Network Time Protocol (NTP) daemon


     ntpd [-aAbdgLmnPqx] [-c conffile] [-D level] [-f driftfile] [-k keyfile]
          [-l logfile] [-N high] [-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
     defined 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 worksta-
     tions 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.

     Various internal ntpd variables can be displayed and configuration
     options 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      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

     -d      Specify debugging mode.  This flag may occur multiple times, with
             each occurrence indicating greater detail of display.

     -D level
             Specify debugging level directly.

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

     -g      Normally, ntpd exits if the offset exceeds the sanity limit,
             which is 1000 s by default.  If the sanity limit is set to zero,
             no sanity checking is performed and any offset is acceptable.
             This option overrides the limit and allows the time to be set to
             any value without restriction; however, this can happen only
             once.  After that, ntpd will exit if the limit is exceeded.  This
             option can be used with the -q option.

     -k keyfile
             Specify the name and path of the file containing the NTP authen-
             tication keys.

     -l logfile
             Specify the name and path of the log file.  The default is the
             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
             To the extent permitted by the operating system, run the ntpd at
             a high priority.

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

     -P      Override the priority limit set by the operating system.  Not
             recommended for sissies.

     -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.

     -r broadcastdelay
             Specify the default propagation delay from the broadcast/multi-
             cast server and this computer.  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

     -t key  Add a key number to the trusted key list.

     -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 forces the time to be slewed in all
             cases.  If the step threshold is set to zero, all offsets are
             stepped, regardless of value and regardless of the -x option.  In
             general, this is not a good idea, as it bypasses the clock state
             machine which is designed to cope with large time and frequency
             errors Note: Since the slew rate is limited to 0.5 ms/s, each
             second of adjustment requires an amortization interval of 2000 s.
             Thus, an adjustment of many seconds can take hours or days to
             amortize.  This option can be used with the -q 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
     algorithms 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 0-16s.  At the default ini-
     tial 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
     operating 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 option 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
     becomes 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.
     Under conditions of extreme network congestion, the roundtrip delay jit-
     ter 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,
     unless the interval during which no sample offset is less than 128 ms
     exceeds 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
     option.  The maximum slew rate possible is limited to 500 parts-per-mil-
     lion (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.  Dur-
     ing this interval the local clock will not be consistent with any other
     network clock and the system cannot be used for distributed applications
     that require 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 occa-
     sional 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
     described 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
     increase 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 a minute.
     If nothing is heard after a couple of minutes, the daemon times out and
     exits.  After a suitable period of mourning, the ntpdate(8) program may
     be retired.

     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
     advised to carefully consider the consequences of changing the poll
     adjustment 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
     command to a value not less than 16 s.  This value is used for all con-
     figured associations, unless overridden by the minpoll option on the con-
     figuration command.  Note that most device drivers will not operate prop-
     erly if the poll interval is less than 64 s and that the broadcast server
     and manycast client associations will also use the default, unless over-

     In some cases involving dial up or toll services, it may be useful to
     increase the minimum interval to a few tens of minutes and maximum inter-
     val to a day or so.  Under normal operation conditions, once the clock
     discipline 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. If the
     system is conserving energy by spinning down the disk or sleeping when
     idle then the update is deferred until ntpd terminates.

   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
     degraded.  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                             August 2, 2001                             BSD

Mac OS X 10.6 - Generated Thu Sep 17 20:26:09 CDT 2009
© 2000-2017
Individual documents may contain additional copyright information.