ntpd(8)ntpd(8)Namentpd - network time protocol (NTP) daemon
Syntax
/usr/etc/ntpd [ -a threshold ][ -c file ][ -d ][ -D level ][ -l ][ -n
][ -s ]
Description
The University of Maryland's daemon synchronizes the local clock with a
set of distributed time servers. The 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 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. If the NTP
servers are not on your Local Area Network (LAN), you must run the dae‐
mon before running the daemon. To run remove the number signs (#) from
in front of the following lines in your file:
#[ -f /etc/routed ] && {
# /etc/routed & echo 'routed' >/dev/console
#}
The daemon will be invoked when you reboot your system. To start with‐
out rebooting, type the following on the command line:
/etc/routed
For information on setting up the network time services, see the Guide
to System and Network Setup.
The daemon automatically splits the nodes running the 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.
The daemon provides a solution for distributing time to a large number
of individual workstations. It can also be used in conjunction with a
master daemon to distribute NTP time to workstations running If is run
with the and options on at least one system that is also running then
all other systems on the network running can receive time updates from
a host running Although is easier to set up on clients, NTP is recom‐
mended because it is more accurate and more secure.
Normally, the daemon is invoked at boot time from the file. When is
started, it reads configuration information from the file, unless you
have specified another configuration file with the option. The config‐
uration 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 reference page for more information on the configura‐
tion file.
The daemon uses the 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 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
command before running the daemon.
Options-a threshold
Sets the threshold (in seconds) which limits how far the daemon
can change the local clock. By default, the threshold is 1000
seconds. This is set to avoid propagating major mistakes through‐
out the network. If you specify the string instead of a number,
the daemon can change the local clock by any amount.
-c file
Specifies a configuration file for the daemon. By default, the
configuration file is
-d Increments the debug level by one. The option can be specified
more than once. Higher debug levels provide more diagnostic
information.
-D level
Sets the debug level to the specified value.
-l Causes the daemon to log a message each time the local clock is
adjusted. Specify this option only if you want to gather statis‐
tical information to analyze the local clock behavior. If the
option is set, a message may be logged every two minutes. Mes‐
sages are logged to
-n Inhibits the program from being swapped out of memory. Using the
option is recommended for both time servers and time clients.
-s Prevents the daemon from altering the time on the local host. The
daemon participates as an NTP server with the flag set, but it
does not change the time of the local host.
Examples
Before starting the daemon, either manually or from the file, you must
edit the 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 reference page for more information.
To start the daemon manually (on a time client), enter the following
commands:
# /etc/rdate -s
# /usr/etc/ntp -s -f server1 server2 server3
# /usr/etc/ntpd -n
The command initializes your time to the average network time. The
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
file.
To start the daemon from the file (on a time client), place the follow‐
ing entries in the file. Multiple servers are included in case one of
the servers crashes, or is brought down. The servers specified in the
file are the same as those specified in the file. These entries should
be placed after the entry:
[-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 server1 server2 server3 \
& echo -n ' ntp' >/dev/console
}
[-f /usr/etc/ntpd] && {
/usr/etc/ntpd -n & echo -n ' ntpd' >/dev/console
}
Diagnostics
The daemon logs errors, major state changes, and statistics reports
using the daemon; the log entries appear in the file with the word on
each relevant line. Normal log entries show when gains or loses syn‐
chronization with a lower-stratum host. Also, once an hour issues a
entry that gives information about its state.
Once an hour, if is synchronized, it updates the 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 has been running). To convert the drift values to parts per
million (ppm), divide them by 4096 and multiply by 1000000. For exam‐
ple, +0.0107001161 means that 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 is working, your computer's clock
should be accurate to within a few seconds per day.
Another diagnostic tool is the command. You can use this to look at
any host running The following command line returns the state of the
remote host's server:
% /usr/etc/ntpdc hostname
The value that is returned for the should contain values not greater
than 100 milliseconds. See the reference page for more information.
If the daemon sets the time as frequently as every 10 minutes (indi‐
cated by messages in the file), then you should kill the daemon, remove
the file, run the command to initialize the time, and restart the dae‐
mon.
If your system clock is ahead of the server time by more than 1 second,
you should kill the daemon, remove the file, run the command to ini‐
tialize the time, and restart the daemon.
If your clock is more than 1000 seconds off from the server time, and
you did not specify the option, will not change your system time.
Rather, will repeatedly log messages to the file, indicating that the
time is too far off to reset.
See Alsontp(1), adjtime(2), settimeofday(2), ntp.conf(5), ntpdc(8), timed(8)
RFC 1129—Internet Time Synchronization: the Network Time Protocol
Guide to System and Network Setup
Introduction to Networking and Distributed System Services
ntpd(8)
