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 unltimate precision, about 232 picoseconds.
While the ultimate precision, is not achievable with ordinary workstations
and networks of today, it may be required with future nanosecond CPU clocks
and gigabit LANs.
The daemon can operate in any of several modes, including symmetric
active/passive, client/server broadcast/multicast and manycast. A broadcast/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 environment.
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
Various internal ntpd variables can be displayed and configuration
options altered while the daemon is running using the ntpq
and ntpdc 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
Enable authentication mode (default).
Disable authentication mode.
Synchronize using NTP broadcast messages.
Specify the name and path of the configuration file.
Specify debugging mode. This flag may occur multiple times, with each occurrence
indicating greater detail of display.
Specify the name and path of the drift file.
Specify the name and path of the file containing the NTP authentication
Specify the name and path of the log file. The default is the system log
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.
Specify the default propagation delay from the broadcast/multicast 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 facility.
Add a key number to the trusted key list.
Add a system variable.
Add a system variable listed by default.
The Configuration File
The ntpd configuration file is read at initial startup in order
to specify 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 # character and extend to the end of the line; blank lines
are ignored. Configuration commands consist of an initial keyword followed
by a list of arguments, some of which may be optional, separated by whitespace.
Commands 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 contains an extended
discussion of these options.
/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
ntpd has gotten rather fat. While not huge, it has gotten larger
than might be desireable for an elevated-priority daemon running on a workstation,
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,
David L. Mills (email@example.com)