ntp/doc/ntpd.8

.\"
.\" $FreeBSD$
.\"
.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.