NTPD(8) BSD System Manager's Manual NTPD(8)
ntpd -- Network Time Protocol (NTP) daemon
ntpd [-aAbdm] [-c conffile] [-f driftfile] [-g] [-i chrootdir]
[-k keyfile] [-l logfile] [-p pidfile] [-r broadcastdelay]
[-s statsdir] [-t key] [-u user[:group]] [-v variable] [-V variable]
ntpd is an operating system daemon which sets and maintains the system
time-of-day in synchronism with Internet standard time servers. ntpd is
a complete implementation of the Network Time Protocol (NTP) version 4,
but also retains compatibility with version 3, as defined by RFC-1305,
and version 1 and 2, as defined by RFC-1059 and RFC-1119, respectively.
ntpd does most computations in 64-bit floating point arithmetic and does
relatively clumsy 64-bit fixed point operations only when necessary to
preserve the ultimate precision, about 232 picoseconds. While the ulti-
mate precision is not achievable with ordinary workstations and networks
of today, it may be required with future nanosecond CPU clocks and giga-
The daemon can operate in any of several modes, including symmetric
active/passive, client/server broadcast/multicast and manycast. A broad-
cast/multicast or manycast client can discover remote servers, compute
server-client propagation delay correction factors and configure itself
automatically. This makes it possible to deploy a fleet of workstations
without specifying configuration details specific to the local environ-
Ordinarily, ntpd reads the ntp.conf configuration file at startup time in
order to determine the synchronization sources and operating modes. It is
also possible to specify a working, although limited, configuration
entirely on the command line, obviating the need for a configuration
file. This may be particularly appropriate when the local host is to be
configured as a broadcast/multicast client or manycast client, with all
peers being determined by listening to broadcasts at run time.
If NetInfo support is built into ntpd, then ntpd will attempt to read its
configuration from the NetInfo if the default ntp.conf file cannot be
read and no file is specified by the -c option.
Various internal ntpd variables can be displayed and configuration
options altered while the daemon is running using the ntpq(8) and
ntpdc(8) utility programs.
When ntpd starts it looks at the value of umask , and if it's zero ntpd
will set the umask to 022
COMMAND LINE OPTIONS
-a Enable authentication mode (default).
-A Disable authentication mode.
-b Synchronize using NTP broadcast messages.
Specify the name and path of the configuration file.
-d Specify debugging mode. This flag may occur multiple times, with
each occurrence indicating greater detail of display.
Specify debugging level directly.
Specify the name and path of the drift file.
-g Normally, the daemon exits if the offset exceeds a 1000-s sanity
limit. This option overrides this limit and allows the time to be
set to any value without restriction; however, this can happen
only once. After that, the daemon will exit if the limit is
Specify the path to a directory in which ntpd will run chrooted.
Specify the name and path of the file containing the NTP authen-
Specify the name and path of the log file. The default is the
system log facility.
-m Synchronize using NTP multicast messages on the IP multicast
group address 126.96.36.199 (requires multicast kernel).
Specify the name and path to record the daemon's process ID.
-P Override the priority limit set by the operating system. Not rec-
ommended for sissies.
Specify the default propagation delay from the broadcast/multi-
cast server and this computer. This is necessary only if the
delay cannot be computed automatically by the protocol.
Specify the directory path for files created by the statistics
-t key Add a key number to the trusted key list.
Specify user (and optionally group) as which ntpd will run.
Add a system variable listed by default.
-x Ordinarily, if the time is to be adjusted more than 128 ms, it is
stepped, not gradually slewed. This option forces the time to be
slewed in all cases. Note: Since the slew rate is limited to 0.5
ms/s, each second of adjustment requires an amortization interval
of 2000 s. Thus, an adjustment of many seconds can take hours or
days to amortize.
THE CONFIGURATION FILE
The ntpd configuration file is read at initial startup in order to spec-
ify the synchronization sources, modes and other related information.
Usually, it is installed in the /etc directory, but could be installed
elsewhere (see the -c conffile command line option). The file format is
similar to other Unix configuration files - comments begin with a # char-
acter and extend to the end of the line; blank lines are ignored. Config-
uration commands consist of an initial keyword followed by a list of
arguments, some of which may be optional, separated by whitespace. Com-
mands may not be continued over multiple lines. Arguments may be host
names, host addresses written in numeric, dotted-quad form, integers,
floating point numbers (when specifying times in seconds) and text
strings. Optional arguments are delimited by [ ] in the following
descriptions, while alternatives are separated by |. The notation [...]
means an optional, indefinite repetition of the last item before the
See the following pages for configuration and control options. While
there is a rich set of options available, the only required option is one
or more server, peer, broadcast or manycastclient commands described in
the Configuration Options page. The Notes on Configuring NTP and Setting
up a NTP Subnet page in /usr/share/doc/html/ntp/notes.htm contains an
extended discussion of these options. For Configuration Options, refer
For Authentication Options, refer to /usr/share/doc/html/ntp/authopt.htm.
For Monitoring Options, refer to /usr/share/doc/html/ntp/monopt.htm.
For Access Control Options, refer to /usr/share/doc/html/ntp/accopt.htm.
For Reference Clock Options, refer to
For Miscellaneous Options, refer to /usr/share/doc/html/ntp/miscopt.htm.
/etc/ntp.conf the default name of the configuration file
/etc/ntp.drift the default name of the drift file
/etc/ntp.keys the default name of the key file
David L. Mills <millsATudel.edu>
ntpd has gotten rather fat. While not huge, it has gotten larger than
might be desirable for an elevated-priority daemon running on a worksta-
tion, particularly since many of the fancy features which consume the
space were designed more with a busy primary server, rather than a high
stratum workstation, in mind.
BSD February 21, 2002 BSD