ntpdate


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

DESCRIPTION
       ntpdate sets the local date and time by polling the Network Time Proto-
       col (NTP) server(s) given as the server arguments to determine the cor-
       rect 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  sophisti-
       cated  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 limited.

       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  ntp-
       date 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.

OPTIONS
       -a key Enable  the  authentication function and specify the key identi-
              fier 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.

              for details). This number is usually small enough to be negligi-
              ble  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 ver-
              sion, 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 conve-
              nience 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 identifica-
              tion string to be logged.

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

FILES
       /etc/ntp.keys
              - encryption keys used by ntpdate.

BUGS
       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.
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