// This is the body of the manual page for ntpd.
// It's included in two places: once for the docs/ HTML
// tree, and once to make an individual man page.

== SYNOPSIS
[verse]
ntpd
    [-46agGhLmnNqx] [assert] [-c 'conffile'] [-f 'driftfile']
    [-i 'jaildir'] [-k 'keyfile'] [-l 'logfile'] [-p 'pidfile']
    [-P 'priority'] [-s 'statsdir']  [-t  'key']
    [-u 'user'[:'group']] [-U 'interface_update_interval']
    [-v 'variable'] [-V 'variable'] [server...]

== 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 can synchronize time to a theoretical precision of
about 232 picoseconds. In practice, this limit is unattainable due to
quantum limits on the clock speed of ballistic-electron logic.

Ordinarily, +ntpd+ reads the {ntpdconfman} 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.

The +ntpd+ program normally operates continuously while adjusting the
system time and frequency, but in some cases this might not be
practical. With the +-q+ option +ntpd+ operates as in continuous mode,
but exits just after setting the clock for the first time. Most
applications will probably want to specify the +iburst+ option with
the +server+ command. With this option, a volley of messages is
exchanged to groom the data and set the clock in about ten
seconds. With -q, if nothing is heard after a few minutes, the daemon
times out and exits without setting the clock.

Various internal +ntpd+ variables can be displayed and configuration
options altered while the +ntpd+ is running using the
{ntpqman} utility program.  The state of +ntpd+ can be continuously
monitored using {ntpmonman}.

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+, +--assert+::
  REQUIRE(false) to test assert handler.
+
+-c+ _string_, +--configfile+=_string_::
  configuration file name.
+
The name and path of the configuration file, +{ntpconfpath}+ 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, e.g. +/etc/ntp.drift+.
This is the same operation as the _driftfile_
configuration specification in the +{ntpconfpath}+ 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+::
  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.

+-h+, +--help+::
  Print a usage message summarizing options and exit.

+-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 Linux, NetBSD, and Solaris.

+-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_::
  the path to symmetric keys.
+
Specify the name and path of the symmetric key file. +/etc/ntp.keys+
is a common location. This is the same operation as the _keys_
configuration file directive.

+-l+ _string_, +--logfile+=_string_::
  the 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_
configuration file directive.  See {ntpdconfman} for more info.

+-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+, +--mdns+::
  Register with mDNS as an NTP server.
+
Registers as an NTP server with the local mDNS server which allows the
server to be discovered via mDNS client lookup.

+-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_::
  the 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_
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 _pthread_setschedparam(SCHED_FIFO)_ priority.

+-q+, +--quit+::
  Set the time and quit. This option must not appear in combination with
  wait-sync.
+
+ntpd+ will not daemonize and will exit after the clock is first
synchronized. This behavior mimics that of the old _ntpdate_ program,
which has been 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.

+-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_
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.  The user and
group may be specified by name or numeric id.  If no group is specified,
then the default group for userid is used. This option is only available
if the OS supports adjusting the clock without full root privileges. This
option is supported under Linux, NetBSD, Solaris and other OS.

+-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.

+-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.
  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.

+-z+ _nvar_, +--var+=_nvar_::
  make ARG an ntp variable (RW). This option may appear an unlimited
  number of times.

+-Z+ _nvar_, +--dvar+=_ndvar_::
  make ARG an ntp variable (RW|DEF). This option may appear an unlimited
  number of times.

+-V, --version+::
  Output version of program and exit.

Any arguments given after options are interpreted as server addresses
or hostnames, with the _iburst_ option implied. Associations with
these are formed before any associations implied by the configuration
file.

== 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
{ntpdconfman}.

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 an 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 ms 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 at 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, 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
a separate utility to get the system clock close to correct before
starting {ntpdman}, but this was never more than a mediocre hack
and is no longer needed. If you are following the
link:#starting[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 {ntpdigman}.

There is a way to start {ntpdman} that often addresses all of
the problems mentioned above.

[[starting]]
=== Starting NTP (Best Current Practice)

First, use the _iburst_ option on your _server_ and _pool_ entries.

If you can also keep a good +ntp.drift+ file then {ntpdman} 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
{ntpdman} with at least the +-g+ and perhaps the +-N+ options.
Then, start the rest of your "normal" processes. This will give
{ntpdman} 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 {ntpwaitman} as
late as possible in the boot sequence (perhaps with the +-v+ flag) and
after {ntpwaitman} exits successfully it is as safe as it
will ever be to start any processes 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

+ntpd+ 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.

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.
The +-q+ option is provided to support running +ntpd+ periodically
from a cron(8) job. 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 sec. If
nothing is heard after a couple of minutes, the daemon times out and
exits.

When kernel support is available to discipline the clock frequency,
which is the case for stock Solaris, 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 the 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 {ntpdconfman}.

== FILES

[options="header"]
|===================================================================
|File                    |Default           |Option      |Option
|configuration file      |+{ntpconfpath}+      |+-c+        |+conffile+
|configuration directory |+/etc/ntp.d+      |+-c+        |+conffile+
|frequency file          |none              |+-f+        |+driftfile+
|leapseconds file        |none              |            |+leapfile+
|process ID file         |none              |+-p+        |+pidfile+
|log file                |system log        |+-l+        |+logfile+
|include file            |none              |none        |+includefile+
|statistics path         |+/var/NTP+        |+-s+        |+statsdir+
|keys file               |none              |+-k+        |+keys+
|===================================================================

Configuration files are parsed according to the following rules:

. The plain config file (normally +{ntpconfpath}+ but the path can be
  overridden by the -c option) is read first if it exists.

. Then the configuration directory, if it exists, is scanned. Normally
  this directory is /etc/ntp.d, but if the -c option is specified the
  /etc will be specified by the directory name of the -c argument.

. Each file beneath the configuration directory with the extension
  ".conf" is interpreted.  Files are interpreted in ASCII sort order
  of their pathnames.  Files with other extensions or no extensions
  are ignored.

== SIGNALS

SIGQUIT, SIGINT, and SIGTERM will cause ntpd to clean up and exit.

SIGHUP checks various things that would otherwise
require restarting ntpd.

It will reopen the log file if it has changed and
check for a new leapseconds file if one was specified.

If the NTS server is enabled, it will reload the
certificate file if it has changed.  (It doesn't check
for a new key file, but reloads it when it reloads
the certificate file.)

It will also retry any pending DNS or NTS lookups.

On most systems, you can send SIGHUP to +ntpd+ with
-----
  # killall -HUP ntpd
-----

If built with debugging enabled (waf configured with +--enable-debug+)
SIGUSR1 will increase the debug level by 1 and
SIGUSR2 will decrease it by 1.  This may be helpful if you are
running with +-n+, either just to see the logging on your screen
or with gdb.

== BUGS

The '-V' option is not backward-compatible with its use (as the
equivalent of -Z) in older versions.

== STANDARDS

RFC 1059::
  David L. Mills, _Network Time Protocol (Version 1)_, RFC 1059

RFC 1119::
  David L. Mills, _Network Time Protocol (Version 2)_, RFC 1119

RFC 1305::
  David L. Mills, _Network Time Protocol (Version 3)_, RFC 1305

RFC 5905::
  David L. Mills and J. Martin, Ed. and J. Burbank and W. Kasch, _Network
  Time Protocol Version 4: Protocol and Algorithms Specification_, RFC 5905

RFC 5907::
  H&#x2e; Gerstung and C. Elliott and B. Haberman, Ed., _Definitions of Managed
  Objects for Network Time Protocol Version 4: (NTPv4)_, RFC 5907

RFC 5908::
  R&#x2e; Gayraud and B. Lourdelet, _Network Time Protocol (NTP) Server Option
  for DHCPv6_, RFC 5908

//end
