Radix cross Linux

     5         kx .TH NETDATE 8L 85/08/21
     5         kx .UC 4
     5         kx .SH NAME
     5         kx netdate \- set date and time by ARPA Internet RFC 868
     5         kx .SH SYNOPSIS
     5         kx .B
     5         kx netdate
     5         kx [ \-v ] [ \-l limit ] [ protocol ] hostname...
     5         kx .SH DESCRIPTION
     5         kx \fINetdate\fP takes a list of names of Internet hosts as arguments,
     5         kx selects the one which supplies the best time, and sets
     5         kx the system time accordingly.
     5         kx The invoker must be the super-user for the time to be set.
     5         kx Protocol names (either \fBudp\fP or \fBtcp\fP) may be
     5         kx interspersed with the host names, and determine the
     5         kx protocol which will be used to connect to the hosts
     5         kx whose names follow, up to the next protocol name or
     5         kx the end of the arguments.
     5         kx The default protocol is \fBudp\fP.
     5         kx .PP
     5         kx The ``best'' time is chosen by polling the named hosts
     5         kx once each to find their times and taking their differences
     5         kx from the local host's time.
     5         kx These differences are used to find the largest group of hosts
     5         kx whose times agree with each other within a certain limit.
     5         kx The first host in the largest group is picked as the best host.
     5         kx (The assumption is that the hosts which are usually most accurate
     5         kx will be named first.)
     5         kx That host is polled again and the local host's time is set to the result.
     5         kx The chosen host's time is checked on this second poll to insure that
     5         kx its difference from the local host's time has not varied more than the limit
     5         kx from its difference at the first poll.
     5         kx .PP
     5         kx The default limit is five seconds.  It may be set with the \fB\-l\fP option.
     5         kx The \fB\-v\fP option causes the groups to be shown.
     5         kx The host name \fBlocalhost\fP is recognized as a synonym for the name
     5         kx of the local host, no network connection is made for it, and its
     5         kx time difference is always zero.
     5         kx If \fBlocalhost\fP is chosen as having the best time,
     5         kx the system time will not be set.
     5         kx Hosts which do not respond are not counted in the groups.
     5         kx If the limit is set to zero, the time is set to that of the
     5         kx first host to respond and no other checking is done.
     5         kx Supplying only one host name argument also sets the limit to zero.
     5         kx .PP
     5         kx While the RFC868 protocol only returns 32\ bits of data, containing
     5         kx the time in seconds, \fInetdate\fP will accept an extra 32\ bits,
     5         kx containing microseconds (expected to be accurate to no more than milliseconds).
     5         kx Delays on long haul networks may make this extra precision useless,
     5         kx but it is useful on local area networks.
     5         kx The extra precision is not used on the first poll of a host,
     5         kx but it is used on the second poll of the chosen host,
     5         kx if that host supplies it.
     5         kx .SH EXAMPLE
     5         kx The most accurate hosts are named first in each example.
     5         kx Some such call on \fInetdate\fP should be put at the end of
     5         kx \fB/etc/rc.d/rc.local\fP,
     5         kx so that the time will be set properly on system startup.
     5         kx It is also useful to have a shell script, e.g., \fB/sbin/timehosts\fP,
     5         kx which contains a call on \fInetdate\fP with arguments appropriate
     5         kx to the local system, so that it is easy to set the time manually.
     5         kx .SH "netdate -l 30 udp dcn\-gate tcp neighbor"
     5         kx \fIDcn\-gate\fP is a hypothetical host which usually keeps time
     5         kx accurate to within milliseconds of Coordinated Universal Time,
     5         kx but may occasionally be eight hours off.
     5         kx \fINeighbor\fP is a neighbor of the local host which keeps time
     5         kx with moderate accuracy.
     5         kx The time will be set to that of \fIdcn\-gate\fP if that and \fIneighbor\fP
     5         kx agree to within thirty seconds, else it will not be set at all.
     5         kx This is almost good enough for most circumstances, but won't do
     5         kx when the local host's time is known to be wrong (e.g., after
     5         kx a long downtime or a bad crash) and must be set to something.
     5         kx If one of the hosts named is inaccurate or not responding, there is a problem.
     5         kx .SH "netdate -l 30 udp dcn\-gate tcp neighbor neighbor2"
     5         kx Only two of the three hosts named must agree on the time.
     5         kx The time will still be set (to that of the first neighbor),
     5         kx even if \fIdcn\-gate\fP is far off as long as the two neighbors agree.
     5         kx This is probably good enough for most cases.
     5         kx One can arbitrarily gerrymander the vote for more insurance
     5         kx (and less clarity), as in the following example.
     5         kx .SH "netdate\ udp\ dcn\-gate\ dcn1\ tcp\ bbn\-unix\ localhost\ neighbor"
     5         kx Here \fIdcn1\fP and \fIbbn\-unix\fP are more hypothetical very accurate
     5         kx timekeepers, at least one of which keeps time independently from
     5         kx \fIdcn\-gate\fP, one hopes.
     5         kx It is very likely that the time will be set to that one of those three
     5         kx very accurate hosts, as long as at least two of them agree, or at least
     5         kx one of them agrees with the neighbor or the local host's time.
     5         kx If all the foreign hosts disagree, the time will not be set,
     5         kx since \fBlocalhost\fP will be chosen as best.
     5         kx .SH "netdate\ \-l\ 3\ localhost\ localhost\ udp\ dcn\-gate\ dcn1\ tcp\ bbn\-unix"
     5         kx This example gives \fBlocalhost\fP two votes and declares it to usually
     5         kx have the most accurate time.
     5         kx All three foreign hosts must agree within three seconds
     5         kx and also differ from \fBlocalhosts\fP by more than three seconds
     5         kx for the time to be set.
     5         kx Thus the time will be set only if it really needs to be.
     5         kx .SH FILES
     5         kx .nf
     5         kx /etc/services	for the time service port number
     5         kx /etc/protocols	for the protocol numbers
     5         kx /var/log/wtmp	to record time-setting
     5         kx .SH SEE ALSO
     5         kx ARPANET Request for Comments 868, gettimeofday(2), date(1),
     5         kx WWV (USA):
     5         kx 2.5,5,10,15 MHz AM for Coordinated Universal Time (UCT).
     5         kx .SH DIAGNOSTICS