NTP daemon program
<server1> ... <serverN> ]
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.
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.
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
will attempt to read its configuration
from the NetInfo if the default
file cannot be read and no file is specified by the
variables can be
displayed and configuration options altered while the
is running using the
starts it looks at the value of
and if zero
will set the
- 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
- 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
- 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.
- 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
- Allow us to sync to broadcast servers.
- configuration file name.
The name and path of the configuration file, /etc/ntp.conf by
- Increase debug verbosity level. This option may appear an unlimited number
- Set the debug verbosity level. This option may appear an unlimited number
of times. This option takes an integer number as its argument.
- 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.
- 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.
- 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.
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- Do not fork. This option must not appear in combination with any of the
following options: wait-sync.
- Run at high priority.
To the extent permitted by the operating system, run ntpd at 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 pidfile
configuration file directive.
- 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.
- 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.
- 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.
- 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
- 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.
- Trusted key number. This option may appear an unlimited number of times.
Add the specified key number to the trusted key list.
- 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
- 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.
- make ARG an ntp variable (RW). This option may appear an unlimited number
- make ARG an ntp variable (RW|DEF). This option may appear an unlimited
number of times.
- 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.
- 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.
- 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.
- 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).
- 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.
- Display usage information and exit.
- Pass the extended usage information through a pager.
- 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.
Any option that is not marked as not presettable
may be preset by loading
values from environment variables named:
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
keyword with the
configuration command, as described
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
detects that the time on the host is
more than 1000s from the server time,
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
with a panic message to the system log. The
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
to exit anyway.
Under ordinary conditions,
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
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
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
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
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
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,
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
comes to mind), there may
be occasional step/slew corrections and subsequent frequency corrections. It
helps in these cases to use the
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
to get the system clock close to correct before starting
but this was never more than a mediocre hack and is no longer needed. If you
are following the instructions in
NTP (Best Current Practice)
and you still need to set the system time
, please open a bug
report and document what is going on, and then look at using
if you really need to set the clock before starting
There is a way to start
that often addresses all of the problems mentioned above.
First, use the
option on your
If you can also keep a good ntp.drift
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
with at least the
and perhaps the
options. Then, start the rest of your
"normal" processes. This will give
as much time as possible to get the system's clock synchronized and stable.
Finally, if you have processes like
or database servers that require monotonically-increasing time, run
as late as possible in the boot sequence (perhaps with the
flag) and after
exits successfully it is as safe as it will ever be to start any process that
require stable time.
behavior at startup depends on
whether the frequency file, usually
, exists. This file contains the
latest estimate of clock frequency error. When the
is started and the file does not
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
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
is started and the file does
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.
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
). 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.
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
to run continuously. A common
workaround has been to run the
programs from a
job at designated times. However, these programs do not have the crafted
signal processing, error checking or mitigation algorithms of
option is intended for this purpose.
Setting this option will cause
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
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
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
useful feature is available to discipline the clock frequency. First,
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
stopped and run in one-time mode as required. At each startup, the frequency
is read from the file and initializes the kernel frequency.
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
command to a value not less than 16
s. This value is used for all configured associations, unless overridden by
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
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.
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
keyword, as described in
See OPTION PRESETS
for configuration environment variables.
- the default name of the configuration file
- the default name of the drift file
- the default name of the key file
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
firstname.lastname@example.org. Thank you.
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
David L. Mills,
Network Time Protocol (Version 1),
David L. Mills,
Network Time Protocol (Version 2),
David L. Mills,
Network Time Protocol (Version 3),
David L. Mills,
J. Martin, Ed., J. Burbank,
and W. Kasch, Network Time Protocol
Version 4: Protocol and Algorithms Specification,
David L. Mills and
B. Haberman, Ed., Network Time
Protocol Version 4: Autokey Specification,
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,
The University of Delaware and Network Time Foundation
Copyright (C) 1992-2017 The University of Delaware and Network Time Foundation
all rights reserved. This program is released under the terms of the NTP
utility has gotten rather fat. While
not huge, it has gotten larger than might be desirable for an
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, email@example.com
Portions of this document came from FreeBSD.
This manual page was AutoGen
-erated from the ntpd