NTPD(8) BSD System Manager's Manual NTPD(8)
NAME
ntpd -- Network Time Protocol (NTP) daemon
SYNOPSIS
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]
DESCRIPTION
The ntpd utility is an operating system daemon which sets and maintains the system time of day in syn-chronism synchronism
chronism with Internet standard time servers. It is a complete implementation of the Network Time Pro-tocol Protocol
tocol (NTP) version 4, but also retains compatibility with version 3, as defined by RFC-1305, and ver-sion version
sion 1 and 2, as defined by RFC-1059 and RFC-1119, respectively.
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 lim-ited, limited,
ited, 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 Net-Info NetInfo
Info 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) utility programs.
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.
-c conffile
Specify the name and path of the configuration file. (Disable netinfo?)
-d Specify debugging mode. This flag may occur multiple times, with each occurrence indicating
greater detail of display.
-D level
Specify debugging level directly.
-f driftfile
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.
-k keyfile
Specify the name and path of the file containing the NTP authentication keys.
-l logfile
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 224.0.1.1 (requires
multicast kernel).
-n Don't fork.
-N priority
To the extent permitted by the operating system, run the ntpd at a high priority.
-p pidfile
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.
-r 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 protocol.
-s statsdir
Specify the directory path for files created by the statistics facility.
-t key Add a key number to the trusted key list.
-v variable
-V variable
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
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 ini-tial initial
tial poll interval for each server is delayed an interval randomized over 0-16s. At the default ini-tial initial
tial 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 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 cor-rects corrects
rects 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
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 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 configur-ing configuring
ing the server.
Frequency Discipline
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 cur-rent current
rent frequency offset written to it. When the ntpd is started and the file does exist, the ntpd fre-quency frequency
quency is initialized from the file and enters normal mode immediately. After that the current fre-quency frequency
quency offset is written to the file at hourly intervals.
Operating Modes
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 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 auto-matically. automatically.
matically. 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 intro-duced introduced
duced 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 com-mand. command.
mand. 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 be retired.
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 intrin-sic intrinsic
sic 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
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 over-head overhead
head 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 configu-ration configuration
ration 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 spe-cially specially
cially 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.
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 trans-fer transfer
fer 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).
FILES
/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
SEE ALSO
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.
BUGS
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 work-
station in mind.
BSD August 2, 2001 BSD
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