.\" $OpenBSD: ntpd.8,v 1.49 2023/03/02 17:09:53 jmc Exp $
.\"
.\" Copyright (c) 2003, 2004, 2006 Henning Brauer <henning@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.Dd $Mdocdate: March 2 2023 $
.Dt NTPD 8
.Os
.Sh NAME
.Nm ntpd
.Nd Network Time Protocol (NTP) daemon
.Sh SYNOPSIS
.Nm ntpd
.Bk -words
.Op Fl dnv
.Op Fl f Ar file
.Ek
.Sh DESCRIPTION
The
.Nm
daemon synchronizes the local clock to one or more remote NTP servers
or local timedelta sensors.
.Nm
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 attacks.
.Pp
The options are as follows:
.Bl -tag -width "-f fileXXX"
.It Fl d
Do not daemonize.
If this option is specified,
.Nm
will run in the foreground and log to
.Em stderr .
.It Fl f Ar file
Use
.Ar file
as the configuration file,
instead of the default
.Pa /etc/ntpd.conf .
.It Fl n
Configtest mode.
Only check the configuration file for validity.
.It Fl v
This option allows
.Nm
to send DEBUG priority messages to syslog.
.El
.Pp
.Nm
uses the
.Xr adjtime 2
system call to correct the local system time without causing time jumps.
Adjustments of 32ms and greater are logged using
.Xr syslog 3 .
The threshold value is chosen to avoid having local clock drift
thrash the log files.
Should
.Nm
be started with the
.Fl d
or
.Fl v
option, all calls to
.Xr adjtime 2
will be logged.
.Pp
At boot,
.Nm
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.
.Pp
After the local clock is synchronized,
.Nm
adjusts the clock frequency using the
.Xr adjfreq 2
system call to compensate for systematic drift.
.Pp
.Nm
is started at boot time by default via
.Va ntpd_flags
in
.Pa /etc/rc.conf .
See
.Xr rc 8
and
.Xr rc.conf 8
for more information on the boot process
and enabling daemons.
.Pp
When
.Nm
starts up, it reads settings from its configuration file,
typically
.Xr ntpd.conf 5 ,
and its initial clock drift from
.Pa /var/db/ntpd.drift .
Clock drift is periodically written to the drift file thereafter.
.Sh FILES
.Bl -tag -width "/var/db/ntpd.driftXXX" -compact
.It Pa /etc/ntpd.conf
Default configuration file.
.It Pa /var/db/ntpd.drift
Drift file.
.It Pa /var/run/ntpd.sock
Socket file for communication with
.Xr ntpctl 8 .
.El
.Sh SEE ALSO
.Xr date 1 ,
.Xr adjfreq 2 ,
.Xr adjtime 2 ,
.Xr ntpd.conf 5 ,
.Xr ntpctl 8 ,
.Xr rc 8 ,
.Xr rc.conf 8 ,
.Xr rdate 8
.Sh STANDARDS
.Rs
.%A David L. Mills
.%D March 1992
.%R RFC 1305
.%T Network Time Protocol (Version 3): Specification, Implementation and Analysis
.Re
.Pp
.Rs
.%A David L. Mills
.%A Jim Martin
.%A Jack Burbank
.%A William Kasch
.%D June 2010
.%R RFC 5905
.%T Network Time Protocol Version 4: Protocol and Algorithms Specification
.Re
.Sh HISTORY
The
.Nm
program first appeared in
.Ox 3.6 .