NAME

ntpdate − set the date and time via NTP

SYNOPSIS

/usr/sbin/ntpdate [ −bdosu ] [ −a key# ] [ −e authdelay ] [ −k keyfile ] [ −m ]

[ −o version ] [ −p samples ] [ −t timeout ] [ −w ] server...

DESCRIPTION

The ntpdate utility sets the local date and time. To determine the correct time, it polls the Network Time Protocol servers on the hosts given as arguments.  This utility must be run as root on the local host.  It obtains a number of samples from each of the servers and applies the standard NTP clock filter and selection algorithms to select the best of these.  Typically, ntpdate can be inserted in the /etc/rc.local startup up script to set the time of day at boot time. It can also be run from time−to−time via cron(1M). 

The reliability and precision of ntpdate improve dramatically with a greater number of servers.  While a single server may be used, better performance and greater resistance to inaccuracy on the part of any one server can be obtained by providing at least three or four servers, if not more. 

The ntpdate utility makes time adjustments in one of two ways.  If it determines that your clock is off by more than 0.5 seconds it simply steps the time by calling gettimeofday(3C).  If the error is less than 0.5 seconds,  by default, it slews the clock’s time with the offset, via a call to adjtime(2).  The latter technique is less disruptive and more accurate when the offset is small; it works quite well when ntpdate is run by cron every hour or two.  The adjustment made in the latter case is actually 50% larger than the measured offset. This adjustment tends to keep a badly drifting clock more accurate, at some expense to stability. This tradeoff is usually advantageous.  At boot time, however, it is usually better to step the time.  This can be forced in all cases by specifying the −b switch on the command line. 

The ntpdate utility will decline to set the date if an NTP server daemon (for example, xntpd(1M)) is running on the same host.  It can be run on a regular basis from cron(1M) 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

The following options are supported:

−s Log actions via the syslog(3) facility rather than to the standard output — a useful option when running the program from cron(1M). 

−d Display what will be done without actually doing it.  Information useful for general debugging is also printed. 

−a key# Authenticate transactions, using the key number, key#.

−e authdelay Specify an authentication processing delay, authdelay in seconds (see xntpd(1M) for details).  This number is usually small enough to be negligible for ntpdate’s purposes. However, specifying a value may improve timekeeping on very slow CPU’s. 

−k keyfile Read keys from the file keyfile instead of the default file, /etc/ntp.keys.  keyfile should be in the format described in xntpd(1M). 

−m Join multicast group specified in server and synchronize to multicast NTP packets. The standard NTP group is 224.0.1.1. 

−o Force the program to poll as a version 1 or version 2 implementation.  By default ntpdate claims to be an NTP version 3 implementation in its outgoing packets. However, some older software declines to respond to version 3 queries. This option can be used in these cases. 

−p samples Set the number of samples ntpdate acquires from each server.  samples can be between 1 and 8 inclusive.  The default is 4. 

−t timeout Set the time ntpdate spends, waiting for a response.  timeout is rounded to a multiple of 0.2 seconds.  The default is 1 second, a value suitable for polling across a LAN. 

−u Use an unprivileged port to send the packets from. This option is useful when you are behind a firewall that blocks incoming traffic to privileged ports, and you want to synchronize with hosts beyond the firewall. Note that the −d option always uses unprivileged ports. 

−w When used together with -m, wait until able to join group and synchronize. 

FILES

/etc/ntp.keys contains the encryption keys used by ntpdate. 

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

SEE ALSO

cron(1M), xntpd(1M), adjtime(2), gettimeofday(3C), syslog(3), attributes(5)

BUGS

The technique used for improving accuracy by compensating for clock oscillator errors is inadequate. However, doing better would require the program to save state from previous runs. 

SunOS 5.6  —  Last change: 20 May 1997