ntpd.8   [plain text]

.\" $FreeBSD: src/usr.sbin/ntp/doc/ntpd.8,v 1.9 2002/07/14 14:44:40 charnier Exp $
.Dd August  2, 2001
.Dt NTPD 8
.Nm ntpd
.Nd Network Time Protocol (NTP) daemon
.Op Fl aAbdgLmnPqx
.Op Fl c Ar conffile
.Op Fl D Ar level
.Op Fl f Ar driftfile
.Op Fl k Ar keyfile
.Op Fl l Ar logfile
.Op Fl N Cm high
.Op Fl p Ar pidfile
.Op Fl r Ar broadcastdelay
.Op Fl s Ar statsdir
.Op Fl t Ar key
.Op Fl v Ar variable
.Op Fl V Ar variable
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 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
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.
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.
Various internal
variables can be displayed and
configuration options altered while the
is running
using the
.Xr ntpq 8
.Xr ntpdc 8
utility programs.
starts it looks at the value of
.Xr umask 2 ,
and if zero
will set the
.Xr umask 2
to 022.
The following options are available:
.Bl -tag -width indent
.It Fl a
Enable authentication mode (default).
.It Fl A
Disable authentication mode.
.It Fl b
Synchronize using NTP broadcast messages.
.It Fl c Ar conffile
Specify the name and path of the configuration file.
.It Fl d
Specify debugging mode.
This flag may occur multiple times,
with each occurrence indicating greater detail of display.
.It Fl D Ar level
Specify debugging level directly.
.It Fl f Ar driftfile
Specify the name and path of the drift file.
.It Fl g
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,
will exit if the limit is exceeded.
option can be used with the
.Fl q
.It Fl k Ar keyfile
Specify the name and path of the file containing the NTP
authentication keys.
.It Fl l Ar logfile
Specify the name and path of the log file.
The default is the
system log facility.
.It Fl L
Listen to virtual IPs.
.It Fl m
Synchronize using NTP multicast messages on the IP multicast
group address (requires multicast kernel).
.It Fl n
Don't fork.
.It Fl N Ar priority
To the extent permitted by the operating system, run the
at a high priority.
.It Fl p Ar pidfile
Specify the name and path to record the
.Nm Ns 's
.It Fl P
Override the priority limit set by the operating system.
recommended for sissies.
.It Fl q
Exit the
just after the first time the clock is
This behavior mimics that of the
.Xr ntpdate 8
which is to be retired.
.Fl g
.Fl x
options can
be used with this option.
.It Fl r Ar broadcastdelay
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
.It Fl s Ar statsdir
Specify the directory path for files created by the statistics
.It Fl t Ar key
Add a key number to the trusted key list.
.It Fl v Ar variable
.It Fl V Ar variable
Add a system variable listed by default.
.It Fl 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
If the step threshold is set to zero, all offsets are
stepped, regardless of value and regardless of the
.Fl x
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
Thus, an adjustment of many seconds can take hours or days to
This option can be used with the
.Fl q
.Ss "How NTP Operates"
utility operates by exchanging messages with
one or more configured servers at designated poll intervals.
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
The initial delay to set the clock can be reduced using the
.Cm iburst
keyword with the
.Ic server
command, as described in
.Xr ntp.conf 5 .
Most operating systems and hardware of today incorporate a
time-of-year (TOY) chip to maintain the time during periods when
the power is off.
When the machine is booted, the chip is used to
initialize the operating system time.
After the machine has
synchronized to a NTP server, the operating system corrects the
chip from time to time.
In case there is no TOY chip or for some
reason its time 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.
This causes
to exit with a panic message to
the system log.
.Fl 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
to exit anyway.
Under ordinary conditions,
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.
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, the error might exceed 128 ms.
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
.Fl 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
.Fl x
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
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,
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
(time.ien.it comes to mind), there may be occasional
step/slew corrections and subsequent frequency corrections.
helps in these cases to use the
.Cm burst
keyword when
configuring the server.
.Ss "Frequency Discipline"
behavior at startup depends on whether the
frequency file, usually
.Pa ntp.drift ,
This file
contains the latest estimate of clock frequency error.
When the
is started and the file does not exist, the
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
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
is started and the file does exist, the
frequency is initialized from the file and enters normal mode
After that the current frequency offset is written to
the file at hourly intervals.
.Ss "Operating Modes"
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
(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.
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,
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
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
A common workaround has been to run the
.Xr ntpdate 8
program from a
.Xr cron 8
job at designated
However, this program does not have the crafted signal
processing, error checking and mitigation algorithms of
.Nm .
.Fl q
option is intended for this purpose.
Setting this option will cause
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 a minute.
If nothing is heard after a
couple of minutes, the daemon times out and exits.
After a suitable
period of mourning, the
.Xr ntpdate 8
program may be
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
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
.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.
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.
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
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
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. If the system is conserving energy by spinning
down the disk or sleeping when idle then the update is deferred until 
ntpd terminates.
.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.
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
The filter is activated by the
.Ic tinker command and
.Cm huffpuff
keyword, as described in
.Xr ntp.conf 5 .
.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
.Xr ntp.conf 5 ,
.Xr ntpdate 8 ,
.Xr ntpdc 8 ,
.Xr ntpq 8
In addition to the manual pages provided,
comprehensive documentation is available on the world wide web
.Li http://www.ntp.org/ .
A snapshot of this documentation is available in HTML format in
.Pa /usr/share/doc/ntp .
.%A David L. Mills
.%T Network Time Protocol (Version 1)
.%O RFC1059
.%A David L. Mills
.%T Network Time Protocol (Version 2)
.%O RFC1119
.%A David L. Mills
.%T Network Time Protocol (Version 3)
.%O RFC1305
utility has gotten rather fat.
While not huge, it has gotten
larger than might be desirable for an elevated-priority
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