Network Time Protocol (NTP)
ntpd daemon synchronizes the local
clock to one or more remote NTP servers or local timedelta sensors.
ntpd can also act as an NTP server itself,
redistributing the local time. It implements the Simple Network Time
Protocol version 4, as described in RFC 5905, and the Network Time Protocol
version 3, as described in RFC 1305. Time can also be fetched from TLS HTTPS
servers to reduce the impact of unauthenticated NTP man-in-the-middle
The options are as follows:
- Do not daemonize. If this option is specified,
ntpdwill run in the foreground and log to stderr.
- Use file as the configuration file, instead of the default /etc/ntpd.conf.
- Configtest mode. Only check the configuration file for validity.
- This option allows
ntpdto send DEBUG priority messages to syslog.
ntpd uses the
adjtime(2) system call to correct the local system time without
causing time jumps. Adjustments of 32ms and greater are logged using
The threshold value is chosen to avoid having local clock drift thrash the
log files. Should
ntpd be started with the
-v option, all calls
to adjtime(2) will be logged.
ntpd will stay for a maximum of
15 seconds in the foreground and make efforts to verify and correct the time
if constraints are configured and satisfied or if trusted servers or sensors
return results, and if the clock is not being moved backwards.
After the local clock is synchronized,
ntpd adjusts the clock frequency using the
adjfreq(2) system call to compensate for systematic drift.
ntpd is started at boot time by default
via ntpd_flags in
rc.conf(8) for more information on the boot process and enabling
ntpd starts up, it reads settings
from its configuration file, typically
ntpd.conf(5), and its initial clock drift from
/var/db/ntpd.drift. Clock drift is periodically
written to the drift file thereafter.
- Default configuration file.
- Drift file.
- Socket file for communication with ntpctl(8).
date(1), adjfreq(2), adjtime(2), ntpd.conf(5), ntpctl(8), rc(8), rc.conf(8), rdate(8)
David L. Mills, Network Time Protocol (Version 3): Specification, Implementation and Analysis, RFC 1305, March 1992.
David L. Mills, Jim Martin, Jack Burbank, and William Kasch, Network Time Protocol Version 4: Protocol and Algorithms Specification, RFC 5905, June 2010.
ntpd program first appeared in