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       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 (
       This  manpage  converted  from  html  to  roff  by   Fabrizio   Polacco