ntpd -- Network Time Protocol (NTP) daemon
ntpd [-aAbdgLmnPqx] [-c conffile] [-D level] [-f driftfile] [-k keyfile]
[-l logfile] [-N high] [-p pidfile] [-r broadcastdelay]
[-s statsdir] [-t key] [-v variable] [-V variable]
The ntpd utility is an operating system daemon which sets and maintains
the system time of day in synchronism with Internet standard time
servers. It is a complete implementation of the Network Time Protocol
(NTP) version 4, but also retains compatibility with version 3, as
defined by RFC-1305, and version 1 and 2, as defined by RFC-1059 and
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(8) and ntpdc(8)
When ntpd starts it looks at the value of umask(2), and if zero ntpd will
set the umask(2) to 022.
The following options are available:
-a Enable authentication mode (default).
-A Disable authentication mode.
-b Synchronize using NTP broadcast messages.
Specify the name and path of the configuration file. (Disable
-d Specify debugging mode. This flag may occur multiple times, with
each occurrence indicating greater detail of display.
Specify debugging level directly.
Specify the name and path of the drift file.
-g Normally, ntpd exits if the offset exceeds the sanity limit,
which is 1000 s by default. If the sanity limit is set to zero,
no sanity checking is performed and any offset is acceptable.
This option overrides the limit and allows the time to be set to
any value without restriction; however, this can happen only
once. After that, ntpd will exit if the limit is exceeded. This
option can be used with the -q option.
Specify the name and path of the file containing the NTP authentication
Specify the name and path of the log file. The default is the
system log facility.
-L Listen to virtual IPs.
-m Synchronize using NTP multicast messages on the IP multicast
group address 18.104.22.168 (requires multicast kernel).
-n Don't fork.
To the extent permitted by the operating system, run the ntpd at
a high priority.
Specify the name and path to record the ntpd's process ID.
-P Override the priority limit set by the operating system. Not
recommended for sissies.
-q Exit the ntpd just after the first time the clock is set. This
behavior mimics that of the ntpdate(8) program, which is to be
retired. The -g and -x options can be used with this option.
Specify the default propagation delay from the broadcast/multicast
server and this computer. This is necessary only if the
delay cannot be computed automatically by the protocol.
Specify the directory path for files created by the statistics
-t key Add a key number to the trusted key list.
Add a system variable listed by default.
-x Normally, the time is slewed if the offset is less than the step
threshold, which is 128 ms by default, and stepped if above the
threshold. This option forces the time to be slewed in all
cases. If the step threshold is set to zero, all offsets are
stepped, regardless of value and regardless of the -x option. In
general, this is not a good idea, as it bypasses the clock state
machine which is designed to cope with large time and frequency
errors Note: Since the slew rate is limited to 0.5 ms/s, each
second of adjustment requires an amortization interval of 2000 s.
Thus, an adjustment of many seconds can take hours or days to
amortize. This option can be used with the -q option.
How NTP Operates [Toc] [Back]
The ntpd utility operates by exchanging messages with one or more configured
servers at designated poll intervals. When started, whether for the
first or subsequent times, the program requires several exchanges from
the majority of these servers so the signal processing and mitigation
algorithms can accumulate and groom the data and set the clock. In order
to protect the network from bursts, the initial poll interval for each
server is delayed an interval randomized over 0-16s. At the default initial
poll interval of 64s, several minutes can elapse before the clock is
set. The initial delay to set the clock can be reduced using the iburst
keyword with the server configuration command, as described in
Most operating systems and hardware of today incorporate a time-of-year
(TOY) chip to maintain the time during periods when the power is off.
When the machine is booted, the chip is used to initialize the operating
system time. After the machine has synchronized to a NTP server, the
operating system corrects the chip from time to time. In case there is
no TOY chip or for some reason its time is more than 1000s from the
server time, ntpd assumes something must be terribly wrong and the only
reliable action is for the operator to intervene and set the clock by
hand. This causes ntpd to exit with a panic message to the system log.
The -g option overrides this check and the clock will be set to the
server time regardless of the chip time. However, and to protect against
broken hardware, such as when the CMOS battery fails or the clock counter
becomes defective, once the clock has been set, an error greater than
1000s will cause ntpd to exit anyway.
Under ordinary conditions, ntpd adjusts the clock in small steps so that
the timescale is effectively continuous and without discontinuities.
Under conditions of extreme network congestion, the roundtrip delay 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
As the result of this behavior, once the clock has been set, it very
rarely strays more than 128 ms, even under extreme cases of network path
congestion and jitter. Sometimes, in particular when ntpd is first
started, the error might exceed 128 ms. This may on occasion cause the
clock to be set backwards if the local clock time is more than 128 s in
the future relative to the server. In some applications, this behavior
may be unacceptable. If the -x option is included on the command line,
the clock will never be stepped and only slew corrections will be used.
The issues should be carefully explored before deciding to use the -x
option. The maximum slew rate possible is limited to 500 parts-per-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 (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
Frequency Discipline [Toc] [Back]
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 [Toc] [Back]
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 onetime
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(8) program from a cron(8)
job at designated times. However, this program does not have the crafted
signal processing, error checking and mitigation algorithms of ntpd. The
-q option is intended for this purpose. Setting this option will cause
ntpd to exit just after setting the clock for the first time. The 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 a minute.
If nothing is heard after a couple of minutes, the daemon times out and
exits. After a suitable period of mourning, the ntpdate(8) program may
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 [Toc] [Back]
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).
/etc/ntp.conf the default name of the configuration file
/etc/ntp.drift the default name of the drift file
/etc/ntp.keys the default name of the key file
ntp.conf(5), ntpdate(8), ntpdc(8), ntpq(8)
In addition to the manual pages provided, comprehensive documentation is
available on the world wide web at 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.
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.
FreeBSD 5.2.1 August 2, 2001 FreeBSD 5.2.1 [ Back ]