Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.


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

NAME
     ntpd - NTP daemon program

SYNOPSIS
     ntpd [-flags] [-flag [value]] [--option-name[[=| ]value]] [ <server1> ...
          <serverN> ]

DESCRIPTION
     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, as defined by RFC-5905, but also retains compatibility
     with version 3, as defined by RFC-1305, and versions 1 and 2, as defined
     by RFC-1059 and RFC-1119, respectively.

     The ntpd utility does most computations in 64-bit floating point
     arithmetic 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
     configured 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
     options altered while the ntpd is running using the ntpq(1ntpqmdoc) and
     ntpdc(1ntpdcmdoc) utility programs.

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

OPTIONS
     -4, --ipv4
             Force IPv4 DNS name resolution.  This option must not appear in
             combination with any of the following options: ipv6.

             Force DNS resolution of following host names on the command line
             to the IPv4 namespace.

     -6, --ipv6
             Force IPv6 DNS name resolution.  This option must not appear in
             combination with any of the following options: ipv4.

             Force DNS resolution of following host names on the command line
             to the IPv6 namespace.

     -a, --authreq
             Require crypto authentication.  This option must not appear in
             combination with any of the following options: authnoreq.

             Require cryptographic authentication for broadcast client,
             multicast client and symmetric passive associations.  This is the
             default.

     -A, --authnoreq
             Do not require crypto authentication.  This option must not
             appear in combination with any of the following options: authreq.

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

     -b, --bcastsync
             Allow us to sync to broadcast servers.


     -c string, --configfile=string
             configuration file name.

             The name and path of the configuration file, /etc/ntp.conf by
             default.

     -d, --debug-level
             Increase debug verbosity level.  This option may appear an
             unlimited number of times.


     -D number, --set-debug-level=number
             Set the debug verbosity level.  This option may appear an
             unlimited number of times.  This option takes an integer number
             as its argument.


     -f string, --driftfile=string
             frequency drift file name.

             The name and path of the frequency file, /etc/ntp.drift by
             default.  This is the same operation as the driftfile driftfile
             configuration specification in the /etc/ntp.conf file.

     -g, --panicgate
             Allow the first adjustment to be Big.  This option may appear an
             unlimited number of times.

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

     -G, --force-step-once
             Step any initial offset correction..

             Normally, ntpd steps the time if the time offset exceeds the step
             threshold, which is 128 ms by default, and otherwise slews the
             time.  This option forces the initial offset correction to be
             stepped, so the highest time accuracy can be achieved quickly.
             However, this may also cause the time to be stepped back so this
             option must not be used if applications requiring monotonic time
             are running.  See the tinker configuration file directive for
             other options.

     -i string, --jaildir=string
             Jail directory.

             Chroot the server to the directory jaildir This option also
             implies that the server attempts to drop root privileges at
             startup.  You may need to also specify a -u option.  This option
             is only available if the OS supports adjusting the clock without
             full root privileges.  This option is supported under NetBSD
             (configure with --enable-clockctl) or Linux (configure with
             --enable-linuxcaps) or Solaris (configure with
             --enable-solarisprivs).

     -I iface, --interface=iface
             Listen on an interface name or address.  This option may appear
             an unlimited number of times.

             Open the network address given, or all the addresses associated
             with the given interface name.  This option may appear multiple
             times.  This option also implies not opening other addresses,
             except wildcard and localhost.  This option is deprecated. Please
             consider using the configuration file interface command, which is
             more versatile.

     -k string, --keyfile=string
             path to symmetric keys.

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

     -l string, --logfile=string
             path to the log file.

             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 file directive.

     -L, --novirtualips
             Do not listen to virtual interfaces.

             Do not listen to virtual interfaces, defined as those with names
             containing a colon.  This option is deprecated.  Please consider
             using the configuration file interface command, which is more
             versatile.

     -M, --modifymmtimer
             Modify Multimedia Timer (Windows only).

             Set the Windows Multimedia Timer to highest resolution.  This
             ensures the resolution does not change while ntpd is running,
             avoiding timekeeping glitches associated with changes.

     -n, --nofork
             Do not fork.  This option must not appear in combination with any
             of the following options: wait-sync.


     -N, --nice
             Run at high priority.

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

     -p string, --pidfile=string
             path to the PID file.

             Specify the name and path of the file used to record ntpd's
             process ID.  This is the same operation as the pidfile pidfile
             configuration file directive.

     -P number, --priority=number
             Process priority.  This option takes an integer number as its
             argument.

             To the extent permitted by the operating system, run ntpd at the
             specified sched_setscheduler(SCHED_FIFO) priority.

     -q, --quit
             Set the time and quit.  This option must not appear in
             combination with any of the following options: saveconfigquit,
             wait-sync.

             ntpd will not daemonize and will exit after the clock is first
             synchronized.  This behavior mimics that of the ntpdate program,
             which will soon be replaced with a shell script.  The -g and -x
             options can be used with this option.  Note: The kernel time
             discipline is disabled with this option.

     -r string, --propagationdelay=string
             Broadcast/propagation delay.

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

     --saveconfigquit=string
             Save parsed configuration and quit.  This option must not appear
             in combination with any of the following options: quit,
             wait-sync.

             Cause ntpd to parse its startup configuration file and save an
             equivalent to the given filename and exit.  This option was
             designed for automated testing.

     -s string, --statsdir=string
             Statistics file location.

             Specify the directory path for files created by the statistics
             facility.  This is the same operation as the statsdir statsdir
             configuration file directive.

     -t tkey, --trustedkey=tkey
             Trusted key number.  This option may appear an unlimited number
             of times.

             Add the specified key number to the trusted key list.

     -u string, --user=string
             Run as userid (or userid:groupid).

             Specify a user, and optionally a group, to switch to.  This
             option is only available if the OS supports adjusting the clock
             without full root privileges.  This option is supported under
             NetBSD (configure with --enable-clockctl) or Linux (configure
             with --enable-linuxcaps) or Solaris (configure with
             --enable-solarisprivs).

     -U number, --updateinterval=number
             interval in seconds between scans for new or dropped interfaces.
             This option takes an integer number as its argument.

             Give the time in seconds between two scans for new or dropped
             interfaces.  For systems with routing socket support the scans
             will be performed shortly after the interface change has been
             detected by the system.  Use 0 to disable scanning. 60 seconds is
             the minimum time between scans.

     --var=nvar
             make ARG an ntp variable (RW).  This option may appear an
             unlimited number of times.


     --dvar=ndvar
             make ARG an ntp variable (RW|DEF).  This option may appear an
             unlimited number of times.


     -w number, --wait-sync=number
             Seconds to wait for first clock sync.  This option must not
             appear in combination with any of the following options: nofork,
             quit, saveconfigquit.  This option takes an integer number as its
             argument.

             If greater than zero, alters ntpd's behavior when forking to
             daemonize.  Instead of exiting with status 0 immediately after
             the fork, the parent waits up to the specified number of seconds
             for the child to first synchronize the clock.  The exit status is
             zero (success) if the clock was synchronized, otherwise it is
             ETIMEDOUT.  This provides the option for a script starting ntpd
             to easily wait for the first set of the clock before proceeding.

     -x, --slew
             Slew up to 600 seconds.

             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 configuration file directive for other
             options.  Note: The kernel time discipline is disabled with this
             option.

     --usepcc
             Use CPU cycle counter (Windows only).

             Attempt to substitute the CPU counter for
             QueryPerformanceCounter.  The CPU counter and
             QueryPerformanceCounter are compared, and if they have the same
             frequency, the CPU counter (RDTSC on x86) is used directly,
             saving the overhead of a system call.

     --pccfreq=string
             Force CPU cycle counter use (Windows only).

             Force substitution the CPU counter for QueryPerformanceCounter.
             The CPU counter (RDTSC on x86) is used unconditionally with the
             given frequency (in Hz).

     -m, --mdns
             Register with mDNS as a NTP server.

             Registers as an NTP server with the local mDNS server which
             allows the server to be discovered via mDNS client lookup.

     -?, --help
             Display usage information and exit.

     -!, --more-help
             Pass the extended usage information through a pager.

     --version [{v|c|n}]
             Output version of program and exit.  The default mode is `v', a
             simple version.  The `c' mode will print copyright information
             and `n' will print the full copyright notice.

OPTION PRESETS
     Any option that is not marked as not presettable may be preset by loading
     values from environment variables named:
       NTPD_<option-name> or NTPD

USAGE
   How NTP Operates
     The ntpd utility operates by exchanging messages with one or more
     configured servers over a range of 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 a few seconds.  At the default initial poll interval of 64s, several
     minutes can elapse before the clock is set.  This initial delay to set
     the clock can be safely and dramatically reduced using the iburst keyword
     with the server configuration command, as described in ntp.conf(5).

     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 the default
     case, if ntpd detects that the time on the host 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.  (Reasons for this include there is no TOY chip, or its battery
     is dead, or that the TOY chip is just of poor quality.)  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 (up to 68 years in the past or future -- this is a limitation
     of the NTPv4 protocol).  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
     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, 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 incidence.

     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 without a valid drift file on a system with a large intrinsic
     drift the error might grow to exceed 128 ms, which would 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.  There are several solutions, however.  If the -x option
     is included on the command line, the clock will never be stepped and only
     slew corrections will be used.  But this choice comes with a cost that
     should be carefully explored before deciding to use the -x option.  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 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 (the host time.ien.it comes to mind), there may
     be occasional step/slew corrections and subsequent frequency corrections.
     It helps in these cases to use the burst keyword when configuring the
     server, but ONLY when you have permission to do so from the owner of the
     target host.

     Finally, in the past many startup scripts would run ntpdate(1ntpdatemdoc)
     or sntp(1sntpmdoc) to get the system clock close to correct before
     starting ntpd(8), but this was never more than a mediocre hack and is no
     longer needed.  If you are following the instructions in Starting NTP
     (Best Current Practice) and you still need to set the system time before
     starting ntpd, please open a bug report and document what is going on,
     and then look at using sntp(1sntpmdoc) if you really need to set the
     clock before starting ntpd.

     There is a way to start ntpd(8) that often addresses all of the problems
     mentioned above.

   Starting NTP (Best Current Practice)
     First, use the iburst option on your server entries.

     If you can also keep a good ntp.drift file then ntpd(8) will effectively
     "warm-start" and your system's clock will be stable in under 11 seconds'
     time.

     As soon as possible in the startup sequence, start ntpd(8) with at least
     the -g and perhaps the -N options.  Then, start the rest of your "normal"
     processes.  This will give ntpd(8) as much time as possible to get the
     system's clock synchronized and stable.

     Finally, if you have processes like dovecot or database servers that
     require monotonically-increasing time, run ntp-wait(1ntp-waitmdoc) as
     late as possible in the boot sequence (perhaps with the -v flag) and
     after ntp-wait(1ntp-waitmdoc) exits successfully it is as safe as it will
     ever be to start any process that require stable time.

   Frequency Discipline
     The ntpd behavior at startup depends on whether the frequency file,
     usually 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
     particular system clock oscillator time and frequency error.  This takes
     approximately 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 frequency 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
     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 at intervals determined by an intricate state
     machine.  The state machine measures the incidental roundtrip delay
     jitter and oscillator frequency wander and determines the best poll
     interval using a heuristic algorithm.  Ordinarily, and in most operating
     environments, the state machine will start with 64s intervals and
     eventually increase in steps to 1024s.  A small amount of random
     variation is introduced 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(1ntpdatemdoc) or
     sntp(1sntpmdoc) programs from a cron(8) job at designated times.
     However, these programs do not have the crafted signal processing, error
     checking or 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 procedure 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
     exits.  After a suitable period of mourning, the ntpdate(1ntpdatemdoc)
     program will 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
     frequency 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
     tailor 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
     configured associations, unless overridden by the minpoll option on the
     configuration 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
     overridden.

     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
     interval 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 available 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
     degraded.  This occurs because the differential delays on the two
     directions 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 correction 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
     apparent 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).

ENVIRONMENT
     See OPTION PRESETS for configuration environment variables.

FILES
     /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

EXIT STATUS
     One of the following exit values will be returned:

     0  (EXIT_SUCCESS)
             Successful program execution.

     1  (EXIT_FAILURE)
             The operation failed or the command syntax was not valid.

     70  (EX_SOFTWARE)
             libopts had an internal operational error.  Please report it to
             autogen-users@lists.sourceforge.net.  Thank you.

SEE ALSO
     ntp.conf(5), ntpdate(1ntpdatemdoc), ntpdc(1ntpdcmdoc), ntpq(1ntpqmdoc),
     sntp(1sntpmdoc)

     In addition to the manual pages provided, comprehensive documentation is
     available on the world wide web at http://www.ntp.org/.  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.

     David L. Mills, J. Martin, Ed., J. Burbank, and W. Kasch, Network Time
     Protocol Version 4: Protocol and Algorithms Specification, RFC5905.

     David L. Mills and B. Haberman, Ed., Network Time Protocol Version 4:
     Autokey Specification, RFC5906.

     H. Gerstung, C. Elliott, and B. Haberman, Ed., Definitions of Managed
     Objects for Network Time Protocol Version 4: (NTPv4), RFC5907.

     R. Gayraud and B. Lourdelet, Network Time Protocol (NTP) Server Option
     for DHCPv6, RFC5908.

AUTHORS
     The University of Delaware and Network Time Foundation

COPYRIGHT
     Copyright (C) 1992-2020 The University of Delaware and Network Time
     Foundation all rights reserved.  This program is released under the terms
     of the NTP license, <http://ntp.org/license>.

BUGS
     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.

     Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org

NOTES
     Portions of this document came from FreeBSD.

     This manual page was AutoGen-erated from the ntpd option definitions.

NetBSD 10.99                     June 23 2020                     NetBSD 10.99