7d73fa34d5
can only be used when ntpd is compiled with DEBUG support. PR: docs/138206 Submitted by: Oliver Pinter (oliver dot pntr at gmail dot com) MFC after: 5 days Approved by: roberto
613 lines
18 KiB
Groff
613 lines
18 KiB
Groff
.\"
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd May 18, 2010
|
|
.Dt NTPD 8
|
|
.Os
|
|
.Sh NAME
|
|
.Nm ntpd
|
|
.Nd Network Time Protocol (NTP) daemon
|
|
.Sh SYNOPSIS
|
|
.Nm
|
|
.Op Fl aAbDdgLmnPqx
|
|
.Op Fl c Ar conffile
|
|
.Op Fl f Ar driftfile
|
|
.Op Fl k Ar keyfile
|
|
.Op Fl l Ar logfile
|
|
.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
|
|
.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, 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.
|
|
.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 8
|
|
and
|
|
.Xr ntpdc 8
|
|
utility programs.
|
|
.Pp
|
|
When
|
|
.Nm
|
|
starts it looks at the value of
|
|
.Cm umask 2 ,
|
|
and if zero
|
|
.Nm
|
|
will set the
|
|
.Cm umask 2
|
|
to 022.
|
|
.Pp
|
|
The following options are available:
|
|
.Bl -tag -width indent
|
|
.It Fl a
|
|
Require cryptographic authentication for broadcast client,
|
|
multicast client and symmetric passive associations.
|
|
This is the default.
|
|
.It Fl A
|
|
Do not require cryptographic authentication for broadcast client,
|
|
multicast client and symmetric passive associations.
|
|
This is almost never a good idea.
|
|
.It Fl b
|
|
Enable the client to synchronize to broadcast servers.
|
|
.It Fl c Ar conffile
|
|
Specify the name and path of the configuration file, default
|
|
.Pa /etc/ntp.conf .
|
|
.It Fl d
|
|
Specify debugging mode.
|
|
This option may occur more than once,
|
|
with each occurrence indicating greater detail of display.
|
|
You need to compile
|
|
.Nm
|
|
with DEBUG in order to use this.
|
|
.It Fl D Ar level
|
|
Specify debugging level directly.
|
|
.It Fl f Ar driftfile
|
|
Specify the name and path of the frequency file, default
|
|
.Pa /etc/ntp.drift .
|
|
This is the same operation as the
|
|
.Ic driftfile Ar driftfile
|
|
configuration command.
|
|
.It Fl g
|
|
Normally,
|
|
.Nm
|
|
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,
|
|
.Nm
|
|
will exit with a message to the system log.
|
|
This option can be used with the
|
|
.Fl q
|
|
and
|
|
.Fl x
|
|
options.
|
|
See the
|
|
.Ic tinker
|
|
command for other options.
|
|
.It Fl k Ar keyfile
|
|
Specify the name and path of the symmetric key file, default
|
|
.Pa /etc/ntp.keys .
|
|
This is the same operation as the
|
|
.Ic keys Ar keyfile
|
|
configuration command.
|
|
.It Fl l Ar logfile
|
|
Specify the name and path of the log file.
|
|
The default is the system log file.
|
|
This is the same operation as the
|
|
.Ic logfile Ar logfile
|
|
configuration command.
|
|
.It Fl L
|
|
Do not listen to virtual IPs.
|
|
The default is to listen.
|
|
.It Fl m
|
|
Enable the client to synchronize to multicast servers at the IPv4 multicast
|
|
group address 224.0.1.1.
|
|
.It Fl n
|
|
Do not fork.
|
|
.It Fl N
|
|
To the extent permitted by the operating system, run the
|
|
.Nm
|
|
at the highest priority.
|
|
.It Fl p Ar pidfile
|
|
Specify the name and path of the file used to record the
|
|
.Nm
|
|
process ID.
|
|
This is the same operation as the
|
|
.Ic pidfile Ar pidfile
|
|
configuration command.
|
|
.It Fl P Ar priority
|
|
To the extent permitted by the operating system, run the
|
|
.Nm
|
|
at the specified priority.
|
|
.It Fl q
|
|
Exit the
|
|
.Nm
|
|
just after the first time the clock is
|
|
set.
|
|
This behavior mimics that of the
|
|
.Xr ntpdate 8
|
|
program,
|
|
which is to be retired.
|
|
The
|
|
.Fl g
|
|
and
|
|
.Fl x
|
|
options can
|
|
be used with this option.
|
|
Note: The kernel time discipline is disabled with this option.
|
|
.It Fl r Ar broadcastdelay
|
|
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 s Ar statsdir
|
|
Specify the directory path for files created by the statistics
|
|
facility.
|
|
This is the same operation as the
|
|
.Ic statsdir Ar statsdir
|
|
configuration command.
|
|
.It Fl t Ar key
|
|
Add a key number to the trusted key list.
|
|
This option can occur more than once.
|
|
.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 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
|
|
.Fl g
|
|
and
|
|
.Fl q
|
|
options.
|
|
See the
|
|
.Ic tinker
|
|
command for other options.
|
|
Note: The kernel time discipline is disabled with this option.
|
|
.El
|
|
.Ss "How NTP Operates"
|
|
The
|
|
.Nm
|
|
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 a few seconds.
|
|
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
|
|
.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 case there is no TOY chip or for some
|
|
reason its time 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.
|
|
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.
|
|
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, 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
|
|
.Fl x
|
|
option is included on the command line, the clock will
|
|
never be stepped and only slew corrections will be used.
|
|
.Pp
|
|
The issues 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
|
|
(
|
|
.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.
|
|
.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 8
|
|
program from a
|
|
.Xr cron 8
|
|
job at designated
|
|
times.
|
|
However, this program does not have the crafted signal
|
|
processing, error checking and 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 8
|
|
program may 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 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 SEE ALSO
|
|
.Xr ntp.conf 5 ,
|
|
.Xr ntpdate 8 ,
|
|
.Xr ntpdc 8 ,
|
|
.Xr ntpq 8
|
|
.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
|
|
.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.
|