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ntpd(8)								       ntpd(8)

NAME
       ntpd - Network Time Protocol (NTP) daemon

SYNOPSIS
       ntpd  [ -46aAbdDgLmnNqx ] [ -c conffile ] [ -f driftfile ] [ -i jaildir
       ] [ -I iface ] [ -k keyfile ] [ -l logfile ] [ -p pidfile ] [ -P prior‐
       ity  ]  [  -r  broadcastdelay  ]	 [  -s	statsdir  ]  [	-t  key ] [ -u
       user[:group] ] [ -U interface_update_interval ] [ -v variable  ]	 [  -V
       variable ]

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 opera‐
       tions only when necessary to preserve the ultimate precision, about 232
       picoseconds.  While the ultimate precision is not achievable with ordi‐
       nary 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 con‐
       figured 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 mitiga‐
       tion 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 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 operat‐
       ing 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 ordinary conditions, ntpd adjusts the clock  in  small  steps  so
       that  the  timescale  is effectively continuous and without discontinu‐
       ities. 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-mil‐
       lion  (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. Dur‐
       ing this interval the local clock will not be consistent with any other
       network clock and the system cannot be used  for	 distributed  applica‐
       tions 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 fol‐
       lowing 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 con‐
       figuring 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 fre‐
       quency 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  fre‐
       quency 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 con‐
       tinuously 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 automati‐
       cally. 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  key‐
       word  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  10  s. 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 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 tai‐
       lor 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 maxi‐
       mum of 1,024 s. The default minimum can be changed with the tinker min‐
       poll  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
       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	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 direc‐
       tions 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 usu‐
       ally  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 page.

NOTES
       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.

       In contexts where a host name is expected, a -4 qualifier preceding the
       host name forces DNS resolution to the IPv4 namespace, while a -6 qual‐
       ifier forces DNS resolution to the IPv6 namespace.

       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.

       Unless  the -n, -d or -D option is used, ntpd changes the current work‐
       ing directory to the root directory, so any options or commands	speci‐
       fying  paths  need  to  use  an absolute path or a path relative to the
       root.

COMMAND LINE OPTIONS
       -4      Force DNS resolution of host names to the IPv4 namespace.

       -6      Force DNS resolution of host names to the IPv6 namespace.

       -a      Require cryptographic authentication for broadcast client, mul‐
	       ticast  client  and symmetric passive associations. This is the
	       default.

       -A      Do  not	require	 cryptographic	authentication	for  broadcast
	       client,	multicast  client  and symmetric passive associations.
	       This is almost never a good idea.

       -b      Enable the client to synchronize to broadcast servers.

       -c conffile
	       Specify the name and path of the	 configuration	file,  default
	       /etc/ntp.conf.

       -d      Specify	debugging  mode. This option may occur more than once,
	       with each occurrence indicating greater detail of display.

       -D level
	       Specify debugging level directly.

       -f driftfile
	       Specify the name and path of the frequency file.	 This  is  the
	       same  operation	as  the driftfile driftfile configuration com‐
	       mand.

       -g      Normally, ntpd 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 thresh‐
	       old is exceeded after that, ntpd will exit with	a  message  to
	       the  system  log.  This	option	can be used with the -q and -x
	       options. See the tinker command for other options.

       -i jaildir
	       Chroot the server to the directory jaildir.  This  option  also
	       implies	that  the  server  attempts to drop root privileges at
	       startup (otherwise, chroot gives very little  additional	 secu‐
	       rity),  and  it is only available if the OS supports to run the
	       server without full root privileges. You may need to also spec‐
	       ify a -u option.

       -I iface
	       Listen on interface. This option may appear an unlimited number
	       of times.

       -k keyfile
	       Specify the name and path of the symmetric key  file.  This  is
	       the same operation as the keys keyfile configuration command.

       -l logfile
	       Specify	the  name and path of the log file. The default is the
	       system log file. This is the same operation as the logfile log‐
	       file configuration command.

       -L      Do not listen to virtual IPs. The default is to listen.

       -m      Lock memory.

       -n      Don't fork.

       -N      To  the	extent permitted by the operating system, run the ntpd
	       at the highest priority.

       -p pidfile
	       Specify the name and path of the file used to record  the  ntpd
	       process	ID.  This is the same operation as the pidfile pidfile
	       configuration command.

       -P priority
	       To the extent permitted by the operating system, run  the  ntpd
	       at the specified priority.

       -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.
	       Note: The kernel time discipline is disabled with this option.

       -r broadcastdelay
	       Specify the default propagation delay from the broadcast/multi‐
	       cast server to this client. 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.  This	is the same operation as the statsdir statsdir
	       configuration command.

       -t key  Add a key number to the trusted key list. This option can occur
	       more than once.

       -u user[:group]
	       Specify	a  user,  and  optionally  a group, to switch to. This
	       option is only available if the OS supports to run  the	server
	       without	full  root  privileges. Currently, this option is sup‐
	       ported under  NetBSD  (configure	 with  --enable-clockctl)  and
	       Linux (configure with --enable-linuxcaps).

       -U interface update interval
	       Number  of seconds to wait between interface list scans to pick
	       up new and delete  network  interface.  Set  to	0  to  disable
	       dynamic interface list updating. The default is to scan every 5
	       minutes.

       -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 sets the threshold to  600  s,
	       which is well within the accuracy window to set the clock manu‐
	       ally. Note: Since the slew rate of typical Unix kernels is lim‐
	       ited to 0.5 ms/s, each second of adjustment requires an amorti‐
	       zation 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 -g and -q options. See the tinker  command  for	 other
	       options. Note: The kernel time discipline is disabled with this
	       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,  configura‐
       tion  entirely on the command line, obviating the need for a configura‐
       tion file. This may be particularly useful when the local host is to be
       configured as a broadcast/multicast client, with all peers being deter‐
       mined by listening to broadcasts at run time.

       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, inte‐
       gers, 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
       [ ... ].

EXIT CODES
       A non-zero exit code indicates an error. Any error messages are	logged
       to the system log by default.

       The  exit  code	is 0 only when ntpd is terminated by a signal, or when
       the -q option is used and ntpd successfully sets the system clock.

SEE ALSO
       ntp.conf(5), ntpq(8), ntpdc(8)

       Primary source of documentation: /usr/share/doc/ntp-*

       This file was automatically generated from HTML source.

								       ntpd(8)
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