ntpd - Network Time Protocol (NTP) daemon
ntpd [ -46aAbgLmnNqx ] [ -c conffile ] [ -f driftfile ] [ -i jaildir ]
[ -k keyfile ] [ -l logfile ] [ -p pidfile ] [ -P priority ] [ -r
broadcastdelay ] [ -s statsdir ] [ -t key ] [ -u user[:group] ] [ -U
interface_update_interval ] [ -v variable ] [ -V variable ]
The ntpd program is an operating system daemon which sets and maintains
the system time of day in synchronism with Internet standard time
servers. It 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 ultimate precision is not achievable
with ordinary workstations and networks of today, it may be required
with future gigahertz 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 time.
Various internal ntpd variables can be displayed and configuration
options altered while the daemon is running using the ntpq and ntpd
When ntpd starts it looks at the value of umask, and if it is zero ntpd
will set the umask to 0222.
-a Require cryptographic authentication for broadcast client,
multicast client and symmetric passive associations. This is
-A Do not require cryptographic authentication for broadcast
client, multicast client and symmetric passive associations.
This is almost never a good idea.
-b Enable the client to synchronize to broadcast servers.
Specify the name and path of the configuration file, default
Specify the name and path of the frequency file, default
/etc/ntp.drift. This is the same operation as the driftfile
driftfile configuration command.
-g Normally, ntpd exits with a message to the system log if the
offset exceeds the panic threshold, which is 1000 s by default.
This option allows the time to be set to any value without
restriction; however, this can happen only once. If the
threshold is exceeded after that, ntpd will exit with a message
to the system log. This option can be used with the -q and -x
Chroot the server to the directory jaildir. This option also
implies that the server attempts to drop root privileges at
startup (otherwise, chroot gives very little additional
security). You may need to also specify a -u option.
Specify the name and path of the symmetric key file, default
/etc/ntp.keys. This is the same operation as the keys keyfile
Specify the name and path of the log file. The default is the
system log file. This is the same operation as the logfile
logfile configuration command.
-L Do not listen to virtual IPs. The default is to listen.
-n Don't fork.
-N To the extent permitted by the operating system, run the ntpd at
the highest priority.
Specify the name and path of the file used to record the ntpd
process ID. This is the same operation as the pidfile pidfile
To the extent permitted by the operating system, run the ntpd at
the specified priority.
-q Exit the ntpd just after the first time the clock is set. This
behavior mimics that of the ntpdate program, which is to be
retired. The -g and -x options can be used with this option.
Note: The kernel time discipline is disabled with this option.
Specify the default propagation delay from the
broadcast/multicast server to this client. This is necessary
only if the delay cannot be computed automatically by the
Specify the directory path for files created by the statistics
facility. This is the same operation as the statsdir statsdir
-t key Add a key number to the trusted key list. This option can occur
more than once.
Specify a user, and optionally a group, to switch to.
Number of seconds to wait between interface list scans to pick
up new and delete network interface. Set to 0 to disable
dynamic interface list updating. The default is to scan every 5
-v variable, -V variable
Add a system variable listed by default.
-x Normally, the time is slewed if the offset is less than the step
threshold, which is 128 ms by default, and stepped if above the
threshold. This option sets the threshold to 600 s, which is
well within the accuracy window to set the clock manually.
Note: Since the slew rate of typical Unix kernels is limited to
0.5 ms/s, each second of adjustment requires an amortization
interval of 2000 s. Thus, an adjustment as much as 600 s will
take almost 14 days to complete. This option can be used with
the -g and -q options. Note: The kernel time discipline is
disabled with this option.
default name of the configuration file
default name of the drift file
The complete documentation can be found at
/usr/share/doc/ntp-doc/html/ntpd.html in the package ntp-doc.