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+.Dd February 4 2015
+.Dt NTPD @NTPD_MS@ User Commands
+.Os
+.\" EDIT THIS FILE WITH CAUTION (ntpd-opts.mdoc)
+.\"
+.\" It has been AutoGen-ed February 4, 2015 at 02:42:12 AM by AutoGen 5.18.5pre4
+.\" From the definitions ntpd-opts.def
+.\" and the template file agmdoc-cmd.tpl
+.Sh NAME
+.Nm ntpd
+.Nd NTP daemon program
+.Sh SYNOPSIS
+.Nm
+.\" Mixture of short (flag) options and long options
+.Op Fl flags
+.Op Fl flag Op Ar value
+.Op Fl \-option\-name Ns Oo Oo Ns "=| " Oc Ns Ar value Oc
+[ <server1> ... <serverN> ]
+.Pp
+.Sh DESCRIPTION
+The
+.Nm
+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.
+.Pp
+The
+.Nm
+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.
+.Pp
+Ordinarily,
+.Nm
+reads the
+.Xr 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.
+.Pp
+If NetInfo support is built into
+.Nm ,
+then
+.Nm
+will attempt to read its configuration from the
+NetInfo if the default
+.Xr ntp.conf 5
+file cannot be read and no file is
+specified by the
+.Fl c
+option.
+.Pp
+Various internal
+.Nm
+variables can be displayed and
+configuration options altered while the
+.Nm
+is running
+using the
+.Xr ntpq @NTPQ_MS@
+and
+.Xr ntpdc @NTPDC_MS@
+utility programs.
+.Pp
+When
+.Nm
+starts it looks at the value of
+.Xr umask 2 ,
+and if zero
+.Nm
+will set the
+.Xr umask 2
+to 022.
+.Sh "OPTIONS"
+.Bl -tag
+.It Fl 4 , Fl \-ipv4
+Force IPv4 DNS name resolution.
+This option must not appear in combination with any of the following options:
+ipv6.
+.sp
+Force DNS resolution of following host names on the command line
+to the IPv4 namespace.
+.It Fl 6 , Fl \-ipv6
+Force IPv6 DNS name resolution.
+This option must not appear in combination with any of the following options:
+ipv4.
+.sp
+Force DNS resolution of following host names on the command line
+to the IPv6 namespace.
+.It Fl a , Fl \-authreq
+Require crypto authentication.
+This option must not appear in combination with any of the following options:
+authnoreq.
+.sp
+Require cryptographic authentication for broadcast client,
+multicast client and symmetric passive associations.
+This is the default.
+.It Fl A , Fl \-authnoreq
+Do not require crypto authentication.
+This option must not appear in combination with any of the following options:
+authreq.
+.sp
+Do not require cryptographic authentication for broadcast client,
+multicast client and symmetric passive associations.
+This is almost never a good idea.
+.It Fl b , Fl \-bcastsync
+Allow us to sync to broadcast servers.
+.sp
+.It Fl c Ar string , Fl \-configfile Ns = Ns Ar string
+configuration file name.
+.sp
+The name and path of the configuration file,
+\fI/etc/ntp.conf\fP
+by default.
+.It Fl d , Fl \-debug\-level
+Increase debug verbosity level.
+This option may appear an unlimited number of times.
+.sp
+.It Fl D Ar number , Fl \-set\-debug\-level Ns = Ns Ar number
+Set the debug verbosity level.
+This option may appear an unlimited number of times.
+This option takes an integer number as its argument.
+.sp
+.It Fl f Ar string , Fl \-driftfile Ns = Ns Ar string
+frequency drift file name.
+.sp
+The name and path of the frequency file,
+\fI/etc/ntp.drift\fP
+by default.
+This is the same operation as the
+\fBdriftfile\fP \fIdriftfile\fP
+configuration specification in the
+\fI/etc/ntp.conf\fP
+file.
+.It Fl g , Fl \-panicgate
+Allow the first adjustment to be Big.
+This option may appear an unlimited number of times.
+.sp
+Normally,
+\fBntpd\fP
+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,
+\fBntpd\fP
+will exit with a message to the system log. This option can be used with the
+\fB\-q\fP
+and
+\fB\-x\fP
+options.
+See the
+\fBtinker\fP
+configuration file directive for other options.
+.It Fl i Ar string , Fl \-jaildir Ns = Ns Ar string
+Jail directory.
+.sp
+Chroot the server to the directory
+\fIjaildir\fP
+.
+This option also implies that the server attempts to drop root privileges at startup.
+You may need to also specify a
+\fB\-u\fP
+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
+\fB\-\-enable\-clockctl\fP) or Linux (configure with
+\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP).
+.It Fl I Ar iface , Fl \-interface Ns = Ns Ar iface
+Listen on an interface name or address.
+This option may appear an unlimited number of times.
+.sp
+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
+\fBinterface\fP command, which is more versatile.
+.It Fl k Ar string , Fl \-keyfile Ns = Ns Ar string
+path to symmetric keys.
+.sp
+Specify the name and path of the symmetric key file.
+\fI/etc/ntp.keys\fP
+is the default.
+This is the same operation as the
+\fBkeys\fP \fIkeyfile\fP
+configuration file directive.
+.It Fl l Ar string , Fl \-logfile Ns = Ns Ar string
+path to the log file.
+.sp
+Specify the name and path of the log file.
+The default is the system log file.
+This is the same operation as the
+\fBlogfile\fP \fIlogfile\fP
+configuration file directive.
+.It Fl L , Fl \-novirtualips
+Do not listen to virtual interfaces.
+.sp
+Do not listen to virtual interfaces, defined as those with
+names containing a colon. This option is deprecated. Please
+consider using the configuration file \fBinterface\fP command, which
+is more versatile.
+.It Fl M , Fl \-modifymmtimer
+Modify Multimedia Timer (Windows only).
+.sp
+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.
+.It Fl n , Fl \-nofork
+Do not fork.
+This option must not appear in combination with any of the following options:
+wait\-sync.
+.sp
+.It Fl N , Fl \-nice
+Run at high priority.
+.sp
+To the extent permitted by the operating system, run
+\fBntpd\fP
+at the highest priority.
+.It Fl p Ar string , Fl \-pidfile Ns = Ns Ar string
+path to the PID file.
+.sp
+Specify the name and path of the file used to record
+\fBntpd\fP's
+process ID.
+This is the same operation as the
+\fBpidfile\fP \fIpidfile\fP
+configuration file directive.
+.It Fl P Ar number , Fl \-priority Ns = Ns Ar number
+Process priority.
+This option takes an integer number as its argument.
+.sp
+To the extent permitted by the operating system, run
+\fBntpd\fP
+at the specified
+\fBsched_setscheduler(SCHED_FIFO)\fP
+priority.
+.It Fl q , Fl \-quit
+Set the time and quit.
+This option must not appear in combination with any of the following options:
+saveconfigquit, wait\-sync.
+.sp
+\fBntpd\fP
+will not daemonize and will exit after the clock is first
+synchronized. This behavior mimics that of the
+\fBntpdate\fP
+program, which will soon be replaced with a shell script.
+The
+\fB\-g\fP
+and
+\fB\-x\fP
+options can be used with this option.
+Note: The kernel time discipline is disabled with this option.
+.It Fl r Ar string , Fl \-propagationdelay Ns = Ns Ar string
+Broadcast/propagation delay.
+.sp
+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.
+.It Fl \-saveconfigquit Ns = Ns Ar string
+Save parsed configuration and quit.
+This option must not appear in combination with any of the following options:
+quit, wait\-sync.
+.sp
+Cause \fBntpd\fP to parse its startup configuration file and save an
+equivalent to the given filename and exit. This option was
+designed for automated testing.
+.It Fl s Ar string , Fl \-statsdir Ns = Ns Ar string
+Statistics file location.
+.sp
+Specify the directory path for files created by the statistics facility.
+This is the same operation as the
+\fBstatsdir\fP \fIstatsdir\fP
+configuration file directive.
+.It Fl t Ar tkey , Fl \-trustedkey Ns = Ns Ar tkey
+Trusted key number.
+This option may appear an unlimited number of times.
+.sp
+Add the specified key number to the trusted key list.
+.It Fl u Ar string , Fl \-user Ns = Ns Ar string
+Run as userid (or userid:groupid).
+.sp
+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
+\fB\-\-enable\-clockctl\fP) or Linux (configure with
+\fB\-\-enable\-linuxcaps\fP) or Solaris (configure with \fB\-\-enable\-solarisprivs\fP).
+.It Fl U Ar number , Fl \-updateinterval Ns = Ns Ar number
+interval in seconds between scans for new or dropped interfaces.
+This option takes an integer number as its argument.
+.sp
+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.
+.It Fl \-var Ns = Ns Ar nvar
+make ARG an ntp variable (RW).
+This option may appear an unlimited number of times.
+.sp
+.It Fl \-dvar Ns = Ns Ar ndvar
+make ARG an ntp variable (RW|DEF).
+This option may appear an unlimited number of times.
+.sp
+.It Fl w Ar number , Fl \-wait\-sync Ns = Ns Ar 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.
+.sp
+If greater than zero, alters \fBntpd\fP'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 \fBETIMEDOUT\fP.
+This provides the option for a script starting \fBntpd\fP to easily
+wait for the first set of the clock before proceeding.
+.It Fl x , Fl \-slew
+Slew up to 600 seconds.
+.sp
+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
+\fB\-g\fP
+and
+\fB\-q\fP
+options.
+See the
+\fBtinker\fP
+configuration file directive for other options.
+Note: The kernel time discipline is disabled with this option.
+.It Fl \-usepcc
+Use CPU cycle counter (Windows only).
+.sp
+Attempt to substitute the CPU counter for \fBQueryPerformanceCounter\fP.
+The CPU counter and \fBQueryPerformanceCounter\fP 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.
+.It Fl \-pccfreq Ns = Ns Ar string
+Force CPU cycle counter use (Windows only).
+.sp
+Force substitution the CPU counter for \fBQueryPerformanceCounter\fP.
+The CPU counter (RDTSC on x86) is used unconditionally with the
+given frequency (in Hz).
+.It Fl m , Fl \-mdns
+Register with mDNS as a NTP server.
+.sp
+Registers as an NTP server with the local mDNS server which allows
+the server to be discovered via mDNS client lookup.
+.It Fl \&? , Fl \-help
+Display usage information and exit.
+.It Fl \&! , Fl \-more\-help
+Pass the extended usage information through a pager.
+.It Fl \-version Op Brq Ar 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.
+.El
+.Sh "OPTION PRESETS"
+Any option that is not marked as \fInot presettable\fP may be preset
+by loading values from environment variables named:
+.nf
+ \fBNTPD_<option\-name>\fP or \fBNTPD\fP
+.fi
+.ad
+.Sh USAGE
+.Ss "How NTP Operates"
+The
+.Nm
+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
+.Cm iburst
+keyword with the
+.Ic server
+configuration
+command, as described in
+.Xr ntp.conf 5 .
+.Pp
+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
+.Nm
+detects that the time on the host
+is more than 1000s from the server time,
+.Nm
+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
+.Nm
+to exit with a panic message to
+the system log.
+The
+.Fl 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 \(em
+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
+.Nm
+to exit anyway.
+.Pp
+Under ordinary conditions,
+.Nm
+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
+.Nm
+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.
+.Pp
+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
+.Nm
+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
+.Fl 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
+.Fl 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.
+.Pp
+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,
+.Nm
+enters the same state as when the
+.Pa 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
+.Cm 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
+.Cm burst
+keyword when
+configuring the server, but
+ONLY
+when you have permission to do so from the owner of the target host.
+.Pp
+Finally,
+in the past many startup scripts would run
+.Xr ntpdate @NTPDATE_MS@
+to get the system clock close to correct before starting
+.Xr ntpd @NTPD_MS@ ,
+but this was never more than a mediocre hack and is no longer needed.
+If you are following the instructions in
+.Sx "Starting NTP (Best Current Practice)"
+and you still need to set the system time before starting
+.Nm ,
+please open a bug report and document what is going on,
+and then look at using
+.Xr sntp @SNTP_MS@ .
+.Pp
+There is a way to start
+.Xr ntpd @NTPD_MS@
+that often addresses all of the problems mentioned above.
+.Ss "Starting NTP (Best Current Practice)"
+First, use the
+.Cm iburst
+option on your
+.Cm server
+entries.
+.Pp
+If you can also keep a good
+.Pa ntp.drift
+file then
+.Xr ntpd @NTPD_MS@
+will effectively "warm\-start" and your system's clock will
+be stable in under 11 seconds' time.
+.Pp
+As soon as possible in the startup sequence, start
+.Xr ntpd @NTPD_MS@
+with at least the
+.Fl g
+and perhaps the
+.Fl N
+options.
+Then,
+start the rest of your "normal" processes.
+This will give
+.Xr ntpd @NTPD_MS@
+as much time as possible to get the system's clock synchronized and stable.
+.Pp
+Finally,
+if you have processes like
+.Cm dovecot
+or database servers
+that require
+monotonically\-increasing time,
+run
+.Xr ntp\-wait 1ntp\-waitmdoc
+as late as possible in the boot sequence
+(perhaps with the
+.Fl v
+flag)
+and after
+.Xr ntp\-wait 1ntp\-waitmdoc
+exits successfully
+it is as safe as it will ever be to start any process that require
+stable time.
+.Ss "Frequency Discipline"
+The
+.Nm
+behavior at startup depends on whether the
+frequency file, usually
+.Pa ntp.drift ,
+exists.
+This file
+contains the latest estimate of clock frequency error.
+When the
+.Nm
+is started and the file does not exist, the
+.Nm
+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
+.Nm
+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
+.Nm
+is started and the file does exist, the
+.Nm
+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.
+.Ss "Operating Modes"
+The
+.Nm
+utility can operate in any of several modes, including
+symmetric active/passive, client/server broadcast/multicast and
+manycast, as described in the
+.Qq Association Management
+page
+(available as part of the HTML documentation
+provided in
+.Pa /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.
+.Pp
+By default,
+.Nm
+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.
+.Pp
+In some cases it may not be practical for
+.Nm
+to run continuously.
+A common workaround has been to run the
+.Xr ntpdate @NTPDATE_MS@
+or
+.Xr sntp @SNTP_MS@
+programs from a
+.Xr cron 8
+job at designated
+times.
+However, these programs do not have the crafted signal
+processing, error checking or mitigation algorithms of
+.Nm .
+The
+.Fl q
+option is intended for this purpose.
+Setting this option will cause
+.Nm
+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
+.Cm iburst
+keyword with the
+.Ic 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
+.Xr ntpdate @NTPDATE_MS@
+program will be
+retired.
+.Pp
+When kernel support is available to discipline the clock
+frequency, which is the case for stock Solaris, Tru64, Linux and
+.Fx ,
+a useful feature is available to discipline the clock
+frequency.
+First,
+.Nm
+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
+.Nm
+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.
+.Ss "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
+.Ic tinker
+.Cm minpoll
+command to a value not less than 16 s.
+This value is used for all
+configured associations, unless overridden by the
+.Cm 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.
+.Pp
+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
+.Pa 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.
+.Ss "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.
+.Pp
+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.
+.Pp
+The filter is activated by the
+.Ic tinker
+command and
+.Cm huffpuff
+keyword, as described in
+.Xr ntp.conf 5 .
+.Sh "ENVIRONMENT"
+See \fBOPTION PRESETS\fP for configuration environment variables.
+.Sh FILES
+.Bl -tag -width /etc/ntp.drift -compact
+.It Pa /etc/ntp.conf
+the default name of the configuration file
+.It Pa /etc/ntp.drift
+the default name of the drift file
+.It Pa /etc/ntp.keys
+the default name of the key file
+.El
+.Sh "EXIT STATUS"
+One of the following exit values will be returned:
+.Bl -tag
+.It 0 " (EXIT_SUCCESS)"
+Successful program execution.
+.It 1 " (EXIT_FAILURE)"
+The operation failed or the command syntax was not valid.
+.It 70 " (EX_SOFTWARE)"
+libopts had an internal operational error. Please report
+it to autogen\-users@lists.sourceforge.net. Thank you.
+.El
+.Sh "SEE ALSO"
+.Xr ntp.conf 5 ,
+.Xr ntpdate @NTPDATE_MS@ ,
+.Xr ntpdc @NTPDC_MS@ ,
+.Xr ntpq @NTPQ_MS@ ,
+.Xr sntp @SNTP_MS@
+.Pp
+In addition to the manual pages provided,
+comprehensive documentation is available on the world wide web
+at
+.Li http://www.ntp.org/ .
+A snapshot of this documentation is available in HTML format in
+.Pa /usr/share/doc/ntp .
+.Rs
+.%A David L. Mills
+.%T Network Time Protocol (Version 1)
+.%O RFC1059
+.Re
+.Rs
+.%A David L. Mills
+.%T Network Time Protocol (Version 2)
+.%O RFC1119
+.Re
+.Rs
+.%A David L. Mills
+.%T Network Time Protocol (Version 3)
+.%O RFC1305
+.Re
+.Rs
+.%A David L. Mills
+.%A J. Martin, Ed.
+.%A J. Burbank
+.%A W. Kasch
+.%T Network Time Protocol Version 4: Protocol and Algorithms Specification
+.%O RFC5905
+.Re
+.Rs
+.%A David L. Mills
+.%A B. Haberman, Ed.
+.%T Network Time Protocol Version 4: Autokey Specification
+.%O RFC5906
+.Re
+.Rs
+.%A H. Gerstung
+.%A C. Elliott
+.%A B. Haberman, Ed.
+.%T Definitions of Managed Objects for Network Time Protocol Version 4: (NTPv4)
+.%O RFC5907
+.Re
+.Rs
+.%A R. Gayraud
+.%A B. Lourdelet
+.%T Network Time Protocol (NTP) Server Option for DHCPv6
+.%O RFC5908
+.Re
+.Sh "AUTHORS"
+The University of Delaware and Network Time Foundation
+.Sh "COPYRIGHT"
+Copyright (C) 1992\-2015 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>.
+.Sh BUGS
+The
+.Nm
+utility has gotten rather fat.
+While not huge, it has gotten
+larger than might be desirable for an elevated\-priority
+.Nm
+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.
+.Pp
+Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org
+.Sh NOTES
+Portions of this document came from FreeBSD.
+.Pp
+This manual page was \fIAutoGen\fP\-erated from the \fBntpd\fP
+option definitions.
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