ntpd manpage

Search topic Section

ntpd(8)			    System Manager's Manual		       ntpd(8)

       ntpd - Network Time Protocol (NTP) daemon

       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 ]

       The  ntpd  program  is an operating system daemon that synchronises the
       system clock with remote NTP time servers or local reference clocks. 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. The program can operate  in  any  of  several  modes,  as
       described  on  the Association Management page, and with both symmetric
       key and public key cryptography, as  described  on  the	Authentication
       Options page.

       The  ntpd program ordinarily requires a configuration file as described
       on the Configuration Commands and Options collection above.  However  a
       client  can  discover  remote servers and configure them automatically.
       This makes it possible to deploy a fleet of workstations without speci-
       fying  configuration details specific to the local environment. Further
       details are on the Automatic Server Discovery page.

       Once the NTP software distribution has been compiled and installed  and
       the  configuration file constructed, the next step is to verify correct
       operation and fix any bugs that may result. Usually, the	 command  line
       that starts the daemon is included in the system startup file, so it is
       executed only at system boot time; however, the daemon can  be  stopped
       and  restarted  from  root  at  any time. Once started, the daemon will
       begin sending and receiving messages, as specified in the configuration

       The  ntpd  program  operates  by	 exchanging  messages with one or more
       servers at designated intervals ranging from about one minute to	 about
       17  minutes. When started, the program requires several exchanges while
       the algorithms accumulate and groom the data before setting the	clock.
       The  initial delay to set the clock can be reduced using options on the
       Server Options page.

       Most computers 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.  In
       case  there is no TOY chip or the TOY time is more than 1000 s from the
       server time, ntpd assumes something must be terribly  wrong  and	 exits
       with  a	panic  message	to the system operator. With the -g option the
       clock will be initially set to the server time regardless of  the  chip
       time.  However, once the clock has been set, an error greater than 1000
       s will cause ntpd to exit anyway.

       Under ordinary conditions, ntpd slews the clock so  that	 the  time  is
       effectively continuous and never runs backwards. If due to extreme net-
       work congestion an error spike exceeds the step threshold,  by  default
       128 ms, the spike is discarded. However, if the error persists for more
       than the stepout threshold, by default  900  s,	the  system  clock  is
       stepped	to  the	 correct value. In practice the need for a step has is
       extremely rare and almost always the result of a hardware failure. With
       the  -x	option the step threshold is increased to 600 s. Other options
       are available using the tinker command  on  the	Miscellaneous  Options

       The  issues  should be carefully considered before using these options.
       The maximum slew rate possible  is  limited  to	500  parts-per-million
       (PPM)  by  the  Unix kernel. As a result, the clock can take 2000 s for
       each second the clock is outside	 the  acceptable  range.  During  this
       interval	 the 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.

       The frequency file, usually called ntp.drift, contains the latest esti-
       mate of clock frequency. If this file  does  not	 exist	when  ntpd  is
       started,	 it  enters  a special mode designed to measure the particular
       frequency directly. The measurement takes 15 minutes, after  which  the
       frequency  is  set and ntpd resumes normal mode where the time and fre-
       quency are continuously adjusted. The  frequency	 file  is  updated  at
       intervals of an hour or more depending on the measured clock stability.

       The  ntpd  program  normally  operates continuously while adjusting the
       time and frequency, but in some cases it may not be practical to run it
       continuously.  With  the	 -q option ntpd operates as in continous mode,
       but exits just after setting the clock for the first time with the con-
       figured	servers.  Most	applications will probably want to specify the
       iburst option with the server command. With this	 option	 a  volley  of
       messages	 is  exchanged to groom the data and set the clock in about 10
       s. If nothing is heard after a few minutes, the daemon  times  out  and

       NTP  uses an intricate heuristic algorithm to automatically control the
       poll interval for maximum  accuracy  consistent	with  minimum  network
       overhead.  The  algorithm  measures the incidental offset and jitter to
       determine the best poll interval. When ntpd starts, the interval is the
       default minimum 64 s. Under normal conditions when the clock discipline
       has stabilized, the interval increases in steps to the default  maximum
       1024  s.	 In  addition,	should	a server become unreachable after some
       time, the interval increases in steps to the maximum in order to reduce
       network overhead.

       The  default  poll  interval range is suitable for most conditions, but
       can be changed using options on the Server  Options  and	 Miscellaneous
       Options	pages.	However, when using maximum intervals much larger than
       the default, the residual clock frequency error must  be	 small	enough
       for  the	 discipline  loop to capture and correct. The capture range is
       500 PPM with a 64-s interval decreasing by a factor  of	two  for  each
       interval	 doubling. At a 36-hr interval, for example, the capture range
       is only 0.24 PPM.

       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.

       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, such as during other than work hours. The
       filter remembers the minimum delay over the most recent	interval  mea-
       sured  usually  in  hours. Under conditions of severe delay, the filter
       corrects the apparent offset using the sign of the offset and the  dif-
       ference	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 huffpuff command, as  described  in  the	Miscellaneous  Options

       As  provided  by	 international agreement, an extra second is sometimes
       inserted in Coordinated Universal Time (UTC) at the end of  a  selected
       month,  usually	June or December. The National Institutes of Standards
       and  Technology	(NIST)	provides  an  historic	leapseconds  file   at
       time.nist.gov  for  retrieval  via  FTP. When this file, usually called
       ntp-leapseconds.list, is copied and installed in a directory. The leap-
       file configuration command specifies the path to this file. At startup,
       ntpd reads it and initializes three leapsecond values: the NTP  seconds
       at  the	next  leap  event, the offset of UTC relative to International
       Atomic Time (TAI) after the leap and the NTP seconds when the  leapsec-
       onds file expires and should be retrieved again.

       If  a  host  does  not have the leapsecond values, they can be obtained
       over the net using  the	Autokey	 security  protocol.  Ordinarily,  the
       leapseconds  file  is  installed	 on the primary servers and the values
       flow from them via secondary servers  to	 the  clients.	When  multiple
       servers	are  involved,	the values with the latest expiration time are

       If the latest leap is in the past, nothing further is done  other  than
       to  install  the	 TAI offset. If the leap is in the future less than 28
       days, the leap warning bits are set. If in  the	future	less  than  23
       hours,  the kernel is armed to insert one second at the end of the cur-
       rent day. If the kernel is enabled, the leap is done  automatically  at
       that  time;  otherwise, the clock is effectively stopped for one second
       at the leap. Additional details are in the The NTP Timescale  and  Leap
       Seconds white paper

       If  none	 of the above provisions are available, dsependent servers and
       clients tally the leap warning bits of surviving servers and  reference
       clocks.	When  a majority of the survivors show warning, a leap is pro-
       grammed at the end of the current month. During the month  and  day  of
       insertion, they operate as above. In this way the leap is propagated at
       all dependent servers and clients.

       A new experimental feature called interleaved modes can be used in  NTP
       symmetric  or  broadcast	 modes.	 It is designed to improve accuracy by
       avoiding kernel latency and queueing delay, as  described  on  the  NTP
       Interleaved  Modes  page. It is activated by the xleave option with the
       peer or broadcast configuration commands. The  NTP  protocol  automati-
       cally  reconfigures in normal or interleaved mode as required. Ordinary
       broadcast clients can use the same servers as  interleaved  clients  at
       the  same  time. Further details are in the white paper NTP Interleaved
       On-Wire Protocol and the briefing Interleaved Synchronization Protocols
       for LANs and Space Data Links.

       If  ntpd,  is  configured with NetInfo support, it will attempt to read
       its configuration from the NetInfo service 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

       -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
	       same operation as the enable auth command and is the default.

       -A      Do  not	require	 cryptographic	authentication	for  broadcast
	       client, multicast client and  symmetric	passive	 associations.
	       This  is	 the  same  operation  as the disable auth command and
	       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

       -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 command.

       -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 [address | interface name]
	       Open the network address given, or all the addresses associated
	       with  the given interface name. This option may appear multiple
	       times. This option also implies not  opening  other  addresses,
	       except  wildcard	 and  localhost.  This	option	is deprecated.
	       Please consider using the configuration file interface command,
	       which is more versatile.

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

       -L      Do not listen to virtual	 interfaces,  defined  as  those  with
	       names  containing  a  colon.  This option is deprecated. Please
	       consider using the configuration file interface command,	 which
	       is more versatile.

       -M      Raise scheduler precision to its maximum (1 msec) using timeBe-
	       ginPeriod. (Windows only)

       -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

       -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

       -t key  Add a key number to the trusted key list. This option can occur
	       more than once. This is the same operation  as  the  trustedkey
	       key command.

       -u user[:group]
	       Specify	a  user,  and  optionally  a group, to switch to. This
	       option is only available if the OS supports running 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

       -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 and the step threshold is applied also  to  leap	second

       Ordinarily,  ntpd  reads	 the ntp.conf configuration file at startup 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 con-
       figured as a broadcast client, with servers determined by listening  to
       broadcasts at run time.

       Usually, the configuration file is installed as/etc/ntp.conf, 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 command keyword followed
       by a list of option keywords separated by whitespace. Commands may  not
       be  continued  over  multiple  lines.  Options  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	options	 pages,	 while
       alternatives  are  separated  by	 |.  The  notation  [  ...  ] means an
       optional, indefinite repetition of the last item before the [ ... ].

       |File		     | Default		    |  Option  |  Command     |
       |configuration file   | /etc/ntp.conf	    |  -c      |  none	      |
       |frequency file	     | none		    |  -f      |  driftfile   |
       |leapseconds file     | none		    |	       |  leapfile    |
       |process ID file	     | none		    |  -p      |  pidfile     |
       |log file	     | system log	    |  -l      |  logfile     |
       |include file	     | none		    |  none    |  includefile |
       |statistics path	     | /var/log/ntpstats/   |  -s      |  statsdir    |
       |keys path	     | /etc/ntp/crypto	    |  none    |  keysdir     |

       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.

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

       The official HTML documentation.

       This file was automatically generated from HTML source.