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NETDATE(8L)							   NETDATE(8L)

NAME
       netdate - set date and time by ARPA Internet RFC 868

SYNOPSIS
       netdate [ -v ] [ -l limit ] [ protocol ] hostname...

DESCRIPTION
       Netdate	takes  a list of names of Internet hosts as arguments, selects
       the one which supplies the best time, and sets the system time  accord‐
       ingly.	The  invoker  must  be	the super-user for the time to be set.
       Protocol names (either udp or tcp) may be interspersed  with  the  host
       names,  and determine the protocol which will be used to connect to the
       hosts whose names follow, up to the next protocol name or  the  end  of
       the arguments.  The default protocol is udp.

       The  ``best''  time  is	chosen by polling the named hosts once each to
       find their times and taking their differences  from  the	 local	host's
       time.   These  differences  are used to find the largest group of hosts
       whose times agree with each other within a certain  limit.   The	 first
       host  in the largest group is picked as the best host.  (The assumption
       is that the hosts which are usually most accurate will be named first.)
       That  host  is  polled  again  and  the local host's time is set to the
       result.	The chosen host's time is  checked  on	this  second  poll  to
       insure  that  its  difference from the local host's time has not varied
       more than the limit from its difference at the first poll.

       The default limit is five seconds.  It may be set with the  -l  option.
       The  -v	option causes the groups to be shown.  The host name localhost
       is recognized as a synonym for the name of the local host,  no  network
       connection  is made for it, and its time difference is always zero.  If
       localhost is chosen as having the best time, the system time  will  not
       be  set.	 Hosts which do not respond are not counted in the groups.  If
       the limit is set to zero, the time is set to that of the first host  to
       respond	and  no	 other checking is done.  Supplying only one host name
       argument also sets the limit to zero.

       While the RFC868 protocol only returns 32 bits of data, containing  the
       time  in	 seconds,  netdate  will  accept  an extra 32 bits, containing
       microseconds (expected to be accurate to no  more  than	milliseconds).
       Delays on long haul networks may make this extra precision useless, but
       it is useful on local area networks.  The extra precision is  not  used
       on  the	first poll of a host, but it is used on the second poll of the
       chosen host, if that host supplies it.

EXAMPLE
       The most accurate hosts are named first in  each	 example.   Some  such
       call  on netdate should be put at the end of /etc/init.d/boot.local, so
       that the time will be set properly on system startup.  It is also  use‐
       ful  to	have  a	 shell script, e.g., /sbin/timehosts, which contains a
       call on netdate with arguments appropriate to the local system, so that
       it is easy to set the time manually.

netdate -l 30 udp dcn-gate tcp neighbor
       Dcn-gate	 is  a	hypothetical host which usually keeps time accurate to
       within milliseconds of Coordinated Universal Time, but may occasionally
       be  eight  hours	 off.	Neighbor is a neighbor of the local host which
       keeps time with moderate accuracy.  The time will be  set  to  that  of
       dcn-gate	 if  that and neighbor agree to within thirty seconds, else it
       will not be set at all.	This is almost good enough  for	 most  circum‐
       stances,	 but  won't do when the local host's time is known to be wrong
       (e.g., after a long downtime or a bad crash) and must be set  to	 some‐
       thing.	If  one	 of  the  hosts named is inaccurate or not responding,
       there is a problem.

netdate -l 30 udp dcn-gate tcp neighbor neighbor2
       Only two of the three hosts named must agree on	the  time.   The  time
       will  still be set (to that of the first neighbor), even if dcn-gate is
       far off as long as the two neighbors  agree.   This  is	probably  good
       enough  for  most  cases.  One can arbitrarily gerrymander the vote for
       more insurance (and less clarity), as in the following example.

netdate udp dcn-gate dcn1 tcp bbn-unix localhost neighbor
       Here dcn1 and bbn-unix are more hypothetical very accurate timekeepers,
       at  least  one  of  which  keeps	 time independently from dcn-gate, one
       hopes.  It is very likely that the time will be	set  to	 that  one  of
       those three very accurate hosts, as long as at least two of them agree,
       or at least one of them agrees with the neighbor or  the	 local	host's
       time.   If  all	the  foreign hosts disagree, the time will not be set,
       since localhost will be chosen as best.

netdate -l 3 localhost localhost udp dcn-gate dcn1 tcp bbn-unix
       This example gives localhost two votes and declares it to usually  have
       the  most  accurate  time.   All	 three foreign hosts must agree within
       three seconds and also differ from localhosts by more than  three  sec‐
       onds  for  the  time  to	 be set.  Thus the time will be set only if it
       really needs to be.

FILES
       /etc/services  for the time service port number
       /etc/protocols for the protocol numbers
       /var/log/wtmp  to record time-setting

SEE ALSO
       ARPANET Request for Comments 868, gettimeofday(2), date(1), WWV	(USA):
       2.5,5,10,15 MHz AM for Coordinated Universal Time (UCT).

DIAGNOSTICS
4th Berkeley Distribution	   85/08/21			   NETDATE(8L)
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