openntpd-openbsd/src/usr.sbin/ntpd/ntpd.conf.5

.\" $OpenBSD: ntpd.conf.5,v 1.44 2020/02/10 13:18:21 schwarze Exp $
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.\" Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
.\"
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.Dd $Mdocdate: February 10 2020 $
.Dt NTPD.CONF 5
.Os
.Sh NAME
.Nm ntpd.conf
.Nd Network Time Protocol daemon configuration file
.Sh DESCRIPTION
This manual page describes the format of the
.Xr ntpd 8
configuration file.
.Pp
.Nm
has the following format:
.Pp
Empty lines and lines beginning with the
.Sq #
character are ignored.
.Pp
Keywords may be specified multiple times within the configuration file.
The basic configuration options are as follows:
.Bl -tag -width Ds
.It Xo Ic listen on Ar address
.Op Ic rtable Ar table-id
.Xc
Specify a local IP address or a hostname the
.Xr ntpd 8
daemon should listen on.
If it appears multiple times,
.Xr ntpd 8
will listen on each given address.
If
.Sq *
is given as an address,
.Xr ntpd 8
will listen on all local addresses using the specified routing table.
.Xr ntpd 8
does not listen on any address by default.
The optional
.Ic rtable
keyword will specify which routing table to listen on.
By default
.Xr ntpd 8
will listen using the current routing table.
For example:
.Bd -literal -offset indent
listen on *
.Ed
.Pp
or
.Bd -literal -offset indent
listen on 127.0.0.1
listen on ::1
listen on 127.0.0.1 rtable 4
.Ed
.It Ic query from Ar address
Specify a local IP address the
.Xr ntpd 8
daemon should use for outgoing queries to subsequently specified servers.
For example:
.Bd -literal -offset indent
query from 192.0.2.1
query from 2001:db8::1
.Ed
.It Xo Ic sensor Ar device
.Op Ic correction Ar microseconds
.Op Ic refid Ar ID-string
.Op Ic stratum Ar stratum-value
.Op Ic trusted
.Op Ic weight Ar weight-value
.Xc
Specify a timedelta sensor device
.Xr ntpd 8
should use.
The sensor can be specified multiple times:
.Xr ntpd 8
will use each given sensor that actually exists.
Non-existent sensors are ignored.
If
.Sq *
is given as device name,
.Xr ntpd 8
will use all timedelta sensors it finds.
.Xr ntpd 8
does not use any timedelta sensor by default.
For example:
.Bd -literal -offset indent
sensor *
sensor nmea0
.Ed
.Pp
A
.Ic correction
in microseconds can be given to compensate
for the sensor's offset.
The maximum correction is 127 seconds.
For example, if a DCF77 receiver is lagging 70ms behind
actual time:
.Bd -literal -offset indent
sensor udcf0 correction 70000
.Ed
.Pp
A
.Ic refid
.Ar ID-string
of up up to 4 ASCII characters can be
given to publish the sensor type to clients.
RFC 2030 suggests some common reference identifiers, but new identifiers
"can be contrived as appropriate."
If an
.Ar ID-string
is not given,
.Xr ntpd 8
will use a generic reference ID.
For example:
.Bd -literal -offset indent
sensor nmea0 refid GPS
.Ed
.Pp
The
.Ic stratum
keyword can be used to change the stratum value from the default of 1.
.Pp
The
.Ic trusted
keyword indicates the time learned is secure, trustworthy,
and not vulnerable to man-in-the-middle attacks, so
.Ic constraints
validation is skipped.
This is useful for boot-time correction in environments where
.Ic constraints
cannot be used.
.Pp
The
.Ic weight
keyword permits finer control over the relative importance
of time sources (servers or sensor devices).
Weights are specified in the range 1 to 10;
if no weight is given,
the default is 1.
A server with a weight of 5, for example,
will have five times more influence on time offset calculation
than a server with a weight of 1.
.It Xo Ic server Ar address
.Op Ic trusted
.Op Ic weight Ar weight-value
.Xc
Specify the IP address or the hostname of an NTP
server to synchronize to.
If it appears multiple times,
.Xr ntpd 8
will try to synchronize to all of the servers specified.
If a hostname resolves to multiple IPv4 and/or IPv6 addresses,
.Xr ntpd 8
uses the first address.
If it does not get a reply,
.Xr ntpd 8
retries with the next address and continues to do so until a working address
is found.
For example:
.Bd -literal -offset indent
server 10.0.0.2 weight 5
server ntp.example.org weight 1
.Ed
.Pp
To provide redundancy, it is good practice to configure multiple servers.
In general, best accuracy is obtained by using servers that have a low
network latency.
.It Xo Ic servers Ar address
.Op Ic trusted
.Op Ic weight Ar weight-value
.Xc
As with
.Cm server ,
specify the IP address or hostname of an NTP server to synchronize to.
If it appears multiple times,
.Xr ntpd 8
will try to synchronize to all of the servers specified.
Should the hostname resolve to multiple IP addresses,
.Xr ntpd 8
will try to synchronize to all of them.
For example:
.Bd -literal -offset indent
servers pool.ntp.org
servers pool.ntp.org weight 5
.Ed
.El
.Sh CONSTRAINTS
.Xr ntpd 8
can be configured to query the
.Sq Date
from trusted HTTPS servers via TLS.
This time information is not used for precision but acts as an
authenticated constraint,
thereby reducing the impact of unauthenticated NTP
man-in-the-middle attacks.
Received NTP packets with time information falling outside of a range
near the constraint will be discarded and such NTP servers
will be marked as invalid.
.Bl -tag -width Ds
.It Ic constraint from Ar url [ip...]
Specify the URL, IP address or the hostname of an HTTPS server to
provide a constraint.
If the url is followed by one or more addresses the url and addresses will be
tried until a working one is found.
The url path and expected certificate name is always taken from the
url specified.
If
.Ic constraint from
is used more than once,
.Xr ntpd 8
will calculate a median constraint from all the servers specified.
.Bd -literal -offset indent
server ntp.example.org
constraint from www.example.com
constraint from "https://9.9.9.9" "2620:fe::9"
.Ed
.It Ic constraints from Ar url
As with
.Ic constraint from ,
specify the URL, IP address or the hostname of an HTTPS server to
provide a constraint.
Should the hostname resolve to multiple IP addresses,
.Xr ntpd 8
will calculate a median constraint from all of them.
For example:
.Bd -literal -offset indent
servers pool.ntp.org
constraints from "https://www.google.com/"
.Ed
.El
.Sh FILES
.Bl -tag -width /etc/examples/ntpd.conf -compact
.It Pa /etc/ntpd.conf
default
.Xr ntpd 8
configuration file
.It Pa /etc/examples/ntpd.conf
example configuration file
.El
.Sh SEE ALSO
.Xr ntpctl 8 ,
.Xr ntpd 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
file format first appeared in
.Ox 3.6 .