Network Time Protocol. Version 4 of the NTP server.
Now lets see the manual of ntpd service.Manual ntpd:NAME
ntpd - Network Time Protocol (NTP) daemonSYNOPSIS
ntpd [ -aAbdgLmNPqx ] [ -c conffile ] [ -f driftfile ] [ -g ] [ -k keyfile ] [ -l logfile ] [ -N high ] [ -p
pidfile ] [ -r broadcastdelay ] [ -s statsdir ] [ -t key ] [ -v variable ] [ -V variable ] [ -i chroot_dir ]
[ -u server_user ] [ -x ]DESCRIPTION
The ntpd program 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. ntpd 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.HOW NTP OPERATES
The ntpd program 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 exahanges 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 on the Configuration Options page.
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 ordinariy 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 configuring 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 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
ntpd can operate in any of several modes, including symmetric active/passive, client/server broadcast/multicast
and manycast, as described in the Association Management page. 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.
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
program from a cron 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 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 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 onetime
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 overhead by increasing it. However, the
user is advised to carefully consider the consequenses 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 overriden 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
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 minumum 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 the Miscellaneous Options
If NetInfo support is built into ntpd , then ntpd will attempt to read its configuration from the NetInfo if the
default ntp.conf 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 and ntpdc utility programs.
When ntpd starts it looks at the value of umask , and if zero ntpd will set the umask to 022 .COMMAND LINE OPTIONS
-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
-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 126.96.36.199 (requires multicast
-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 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.
-i chroot_dir Chroot the ntpd server process into chroot_dir. To use this option you have to copy all the
files that ntpd process needs into the chroot directory. This option adds security only if the server also
drops root privileges (see -u option).
-u server_user Ntpd process drops root privileges and changes user ID to server_user and group ID to the primary
group of server_user.
-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.THE CONFIGURATION FILE
Ordinarily, ntpd reads the ntp.conf 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
Usually, the configuration file is installed in the /etc directory, but could be installed elsewhere (see the -c
conffile command line option). The file format is similar to other Unix configuration files - comments begin with
a # character and extend to the end of the line; blank lines are ignored.
Configuration commands consist of an initial keyword followed by a list of arguments, some of which may be
optional, separated by whitespace. Commands may not be continued over multiple lines. Arguments may be host names,
host addresses written in numeric, dotted-quad form, integers, floating point numbers (when specifying times in
seconds) and text strings. Optional arguments are delimited by [ ] in the following descriptions, while alternatives
are separated by | . The notation [ ... ] means an optional, indefinite repetition of the last item before
the [ ... ] .FILES
/etc/ntp/ntp.conf - the default name of the configuration file
/var/lib/ntp/drift - the default name of the drift file
/etc/ntp/keys - the default name of the key fileBUGS
ntpd 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.SEE ALSO
Primary source of documentation: /usr/share/doc/ntp-*/ntpd.htmlAUTHOR
David L. Mills <email@example.com>