ntpd.8

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)nfsd.8nfs	5.2	4/5/89
.TH ntpd 8
.SH Name
ntpd \- network time protocol (NTP) daemon
.SH Syntax
.B /usr/etc/ntpd
[
.B \-a
.I threshold
][
.B \-c
.I file
][
.B \-d
][
.B \-D \fIlevel\fP
][
.B \-l
][
.B \-n
][
.B \-s
]
.SH Description
.NXR "ntpd daemon"
The University of Maryland's
.PN ntpd 
daemon synchronizes the local clock with a set of distributed
time servers.  The
.PN ntpd
daemon distributes accurate, reliable time from the best time source
available at your site to hosts on wide area networks (WAN)
and local area networks (LAN).  The three recommended time
sources in decreasing order of accuracy are:  Internet NTP service,
local radio clock, and wristwatch.  Note that the 
.PN ntpd
daemon does not require time servers to be on the same LAN as
time clients, and does not create a heavy broadcast load on the
network.
.NT
If the NTP servers are not on your Local Area Network (LAN),
you must run the 
.PN routed
daemon before running the 
.PN ntpd
daemon.  To run 
.PN routed ,
remove the number signs (#) from in front of the following lines
in your
.PN /etc/rc.local 
file:
.EX
#[ \-f /etc/routed ] && {
#       /etc/routed & echo 'routed'             >/dev/console
#}
.EE
The 
.PN routed 
daemon will be invoked when you reboot your system.  To start 
.PN routed 
without rebooting, type the following on the command line:
.EX 
\f(CB/etc/routed\f(CW
.EE
.NE
For information on setting up the network time services, see
the 
.I Guide to System and Network Setup.
.PP
The
.PN ntpd 
daemon automatically splits the nodes running the
.PN ntpd
daemon into a dynamically reconfigurable
hierarchy of nodes.  The nodes at the top level of the hierarchy 
(low stratum numbers) are
connected to the most accurate sources available.
This information is transferred to the lower-level nodes
(higher stratum numbers) which set their clocks based
on the calculated offset from a remote server, and then
distribute this time to lower levels of the hierarchy.  
.PP
The 
.PN ntpd
daemon provides a solution for distributing time to a large number of
individual workstations.  It
can also be used in conjunction with a master
.PN timed 
daemon to distribute NTP time to workstations running
.PN timed .
If 
.PN timed
is run with the 
.PN \-E
and 
.PN \-M 
options on at least one system that is also running
.PN ntpd ,
then all other systems on the network running
.PN timed
can receive time updates from a host
running
.PN ntpd .
Although
.PN timed
is easier to set up on clients, NTP is recommended
because it is more accurate and more secure.
.PP
Normally, the
.PN ntpd 
daemon is invoked at boot time from the
.PN /etc/rc.local
file.  When 
.PN ntpd
is started, it reads configuration information from 
the 
.PN /etc/ntp.conf
file, unless you have specified another configuration file with the
.PN \-c
option.  The configuration file either specifies the list of NTP
servers with which this host should synchronize, or identifies
this host as a local reference clock.  See the
.MS ntp.conf 5
reference page for more information on the
.PN /etc/ntp.conf
configuration file.
.PP
The 
.PN ntpd
daemon uses the
.MS adjtime 2
system call to gradually adjust the local clock for
small clock offsets (< 0.128 seconds).
If the local clock
lags the time on the server by more than 0.128 seconds, the
.MS settimeofday 2
system call is used to make a forward step adjustment of the local 
clock.  Clocks are never stepped
backwards; they are adjusted gradually, which 
can take a very long time.  Therefore, 
it is important to initialize the time using the
.PN ntp
command before running the
.PN ntpd
daemon. 
.SH Options
.IP "\-a \fIthreshold\fP" 5
Sets the threshold (in seconds) which limits how
far the
.PN ntpd
daemon can change the local clock.
By default, the threshold is 1000 seconds.  This 
is set to avoid propagating major mistakes throughout the
network.  If you specify the string
.PN any
instead of a number, the
.PN ntpd
daemon can change the local clock by any amount.  
.IP "\-c \fIfile\fP" 5
Specifies a configuration file for the
.PN ntpd
daemon.  By default, the configuration file is
.PN /etc/ntp.conf .
.IP \-d
Increments the debug level by one.  
The
.PN \-d
option can be specified more than once.
Higher debug levels provide more diagnostic information.
.IP "\-D \fIlevel\fP"
Sets the debug level to the specified value.
.IP \-l
Causes the 
.PN ntpd
daemon to log a message each time the local clock is adjusted.
Specify this option only if you want to gather statistical
information to analyze the local clock behavior.  If the
.PN \-l
option is set, a message may be logged every two minutes.
Messages are logged to 
.PN /usr/spool/mqueue/syslog .
.IP \-n
Inhibits the 
.PN ntpd
program from being swapped out of memory.  Using the
.PN \-n
option is recommended for both time servers and time clients.
.IP \-s 5
Prevents the
.PN ntpd 
daemon from altering the time on the local host.
The 
.PN ntpd
daemon participates as an NTP server with the
.PN \-s
flag set, but it does not change the time of the local host.
.SH Examples
Before starting the 
.PN ntpd
daemon, either manually or from the
.PN /etc/rc.local
file, you must edit the 
.PN /etc/ntp.conf
file with the appropriate information.  If your system is a client
you must specify the time servers for it to query.  If it is a
time server, you must specify the time servers with which it
peers.  See the
.MS ntp.conf 5
reference page for more information.
.PP
To start the 
.PN ntpd
daemon manually (on a time client), enter the following commands:
.EX
# \f(CB/etc/rdate \-s\fP
# \f(CB/usr/etc/ntp \-s \-f \fIserver1\fP \fIserver2\fP \fIserver3\fP
# \f(CB/usr/etc/ntpd \-n\fP
.EE
The 
.PN /etc/rdate
command initializes your time to the average network time.  The 
.PN /usr/etc/ntp
command further refines the initial time to the NTP time.
The servers specified on the command line
are the same as those specified in the
.PN /etc/ntp.conf
file.
.PP
To start the 
.PN ntpd 
daemon from the 
.PN /etc/rc.local
file (on a time client), place the following entries in the
.PN /etc/rc.local
file.  Multiple servers are included in case one of the servers crashes, or
is brought down.  The servers specified in the
.PN /etc/rc.local
file are the same as those specified in the 
.PN /etc/ntp.conf 
file.  These entries
should be placed after the
.PN syslog
entry:
.EX

[\-f /etc/syslog] && {
     /etc/syslog   & echo \-n ' syslog'           >/dev/console
}
[\-f /etc/rdate] && {
     /etc/rdate \-s   & echo \-n ' rdate'          >/dev/console
}
[\-f /usr/etc/ntp] && {
     /usr/etc/ntp \-s \-f \fIserver1\fP \fIserver2\fP \fIserver3\fP \\
                      & echo \-n ' ntp'           >/dev/console
}
[\-f /usr/etc/ntpd] && {
     /usr/etc/ntpd \-n & echo \-n ' ntpd'          >/dev/console
     
}
.EE
.SH Diagnostics
The 
.PN ntpd 
daemon logs errors, major state changes, and statistics
reports using the 
.PN syslog 
daemon; the log entries appear in the file
.PN /usr/spool/mqueue/syslog 
with the word 
.PN ntpd:
on each	relevant line.  Normal log entries show when 
.PN ntpd 
gains or loses synchronization with a lower-stratum host.  Also,
once an hour 
.PN ntpd 
issues a 
.PN "ntpd: stats:" 
entry that gives information about its state.
.PP
Once an hour, if 
.PN ntpd 
is synchronized, it updates the
.PN /etc/ntp.drift 
file.  This file shows the estimated
clock drift for each of the past 5 hours, with the most recent
hour listed first.  (The 6th number in this file is the
number of hours 
.PN ntpd 
has been running).  To convert the
drift values to parts per million (ppm), divide them
by 4096 and multiply by 1000000.  For example, +0.0107001161
means that 
.PN ntpd 
estimates that the clock is drifting
by about 2.61 ppm, or is losing about 0.226 seconds per day.
[(2.61/1000000) * 24 * 60 * 60 = 0.226].  Negative drift
values mean that the clock is gaining time.  If 
.PN ntpd
is working, your computer's clock should be
accurate to within a few seconds per day.
.PP
Another diagnostic tool is the 
.PN ntpdc 
command.  You can use this to look at any host running 
.PN ntpd .
The following command line returns the state of the remote host's 
.PN ntpd 
server:
.EX 
% \f(CB/usr/etc/ntpdc \fIhostname\fP
.EE
.PP
The value that is returned for the 
.PN offset 
should contain values not greater than
100 milliseconds.  See the 
.MS ntpdc 8
reference page for more information.
.PP
If the
.PN ntpd
daemon sets the time as frequently as every 10 minutes (indicated
by messages in the
.PN /usr/spool/mqueue/syslog
file), then you should kill the
.PN ntpd 
daemon, remove the 
.PN /etc/ntp.drift 
file, run the
.PN ntp
command to initialize the time, and restart the 
.PN ntpd
daemon.
.PP
If your system clock is ahead of the server time by more than 1
second, you should kill the
.PN ntpd
daemon, remove the
.PN /etc/ntp.drift
file, run the
.PN ntp
command to initialize the time, and restart the
.PN ntpd
daemon.
.PP
If your clock is more than 1000 seconds off from the server
time, and you did not specify the 
.PN "\-a any"
option, 
.PN ntpd
will not change your system time.  Rather, 
.PN ntpd
will repeatedly log messages to the
.PN /usr/spool/mqueue/syslog
file, indicating that the time is too far off to reset.
.SH See Also
ntp(1), adjtime(2), settimeofday(2), ntp.conf(5), ntpdc(8), timed(8)
.br
\fIRFC 1129\(emInternet Time Synchronization:  the Network Time Protocol\fP
.br
.I "Guide to System and Network Setup"
.br
.I "Introduction to Networking and Distributed System Services"