ntpdate - set the date and time via NTP


   ntpdate  [-bBdoqsuv]  [-a key] [-e authdelay] [-k keyfile] [-o version]
   [-p samples] [-t timeout] server [...]


   ntpdate sets the local date  and  time  by  polling  the  Network  Time
   Protocol (NTP) server(s) given as the server arguments to determine the
   correct time. It must be run as root on  the  local  host  (unless  the
   option  -q  is used). A number of samples are obtained from each of the
   servers specified and a subset of the NTP clock  filter  and  selection
   algorithms  are  applied  to  select  the  best of these. Note that the
   accuracy and reliability of ntpdate depends on the number  of  servers,
   the number of polls each time it is run and the interval between runs.

   ntpdate  can  be run manually as necessary to set the host clock, or it
   can be run from the host startup script to set the clock at boot  time.
   This is useful in some cases to set the clock initially before starting
   the NTP daemon ntpd. It is also possible to run  ntpdate  from  a  cron
   script.  However,  it  is important to note that ntpdate with contrived
   cron  scripts  is  no  substitute  for  the  NTP  daemon,  which   uses
   sophisticated  algorithms  to  maximize  accuracy and reliability while
   minimizing resource use. Finally, since ntpdate does not discipline the
   host  clock  frequency  as  does  ntpd,  the  accuracy using ntpdate is

   Time adjustments are made by ntpdate in one of  two  ways.  If  ntpdate
   determines  the  clock  is in error more than 0.5 second it will simply
   step the time by calling the  system  settimeofday()  routine.  If  the
   error  is  less  than 0.5 seconds, it will slew the time by calling the
   system adjtime() routine. The latter technique is less  disruptive  and
   more  accurate  when  the  error  is  small,  and works quite well when
   ntpdate is run by cron every hour or two.

   ntpdate will decline to set the date if an  NTP  server  daemon  (e.g.,
   ntpd)  is  running  on the same host. When running ntpdate on a regular
   basis from cron as an alternative to running a daemon,  doing  so  once
   every  hour  or  two will result in precise enough timekeeping to avoid
   stepping the clock.


   -a key Enable  the  authentication  function  and   specify   the   key
          identifier  to  be  used  for  authentication  as  the  argument
          keyntpdate. The keys and key identifiers must match in both  the
          client  and  server  key  files.  The  default is to disable the
          authentication function.

   -B     Force the time to always be slewed using  the  adjtime()  system
          call,  even if the measured offset is greater than +-128 ms. The
          default is to step the time using settimeofday() if  the  offset
          is  greater  than  +-128  ms.  Note  that, if the offset is much
          greater than +-128 ms in this case, that it can take a long time
          (hours)  to  slew  the  clock  to the correct value. During this
          time, the host should not be used to synchronize clients.

   -b     Force the time to be stepped  using  the  settimeofday()  system
          call,  rather  than  slewed (default) using the adjtime() system
          call. This option should be used when called from a startup file
          at boot time.

   -d     Enable  the debugging mode, in which ntpdate will go through all
          the steps, but not adjust the local  clock.  Information  useful
          for general debugging will also be printed.

   -e authdelay
          Specify  the  processing  delay  to  perform  an  authentication
          function as the value authdelay, in seconds  and  fraction  (see
          ntpd  for  details).  This  number is usually small enough to be
          negligible for most purposes,  though  specifying  a  value  may
          improve timekeeping on very slow CPU's.

   -k keyfile
          Specify  the  path for the authentication key file as the string
          keyfile. The default is /etc/ntp.keys. This file  should  be  in
          the format described in ntpd.

   -o version
          Specify  the  NTP  version  for  outgoing packets as the integer
          version, which can be 1 or 2. The  default  is  3.  This  allows
          ntpdate to be used with older NTP versions.

   -p samples
          Specify the number of samples to be acquired from each server as
          the integer samples, with values from  1  to  8  inclusive.  The
          default is 4.

   -q     Query only - don't set the clock.

   -s     Divert  logging output from the standard output (default) to the
          system  syslog  facility.  This  is   designed   primarily   for
          convenience of cron scripts.

   -t timeout
          Specify  the  maximum  time waiting for a server response as the
          value timeout, in seconds and fraction. The value is is  rounded
          to  a  multiple of 0.2 seconds. The default is 1 second, a value
          suitable for polling across a LAN.

   -u     Direct ntpdate to use an unprivileged port for outgoing packets.
          This  is most useful when behind a firewall that blocks incoming
          traffic to privileged ports, and you want  to  synchronise  with
          hosts  beyond  the firewall. Note that the -d option always uses
          unprivileged ports.

   -v     Be  verbose.  This   option   will   cause   ntpdate's   version
          identification string to be logged.


   ntpdate's exit status is zero if it found a server and could update the
   clock, and nonzero otherwise.


          - encryption keys used by ntpdate.


   The slew adjustment is actually 50% larger than  the  measured  offset,
   since this (it is argued) will tend to keep a badly drifting clock more
   accurate. This is probably not a good idea and may  cause  a  troubling
   hunt for some values of the kernel variables tick and tickadj.


   David L. Mills (mills@udel.edu)
   This   manpage   converted  from  html  to  roff  by  Fabrizio  Polacco





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