NTPD.CONF(5) | File Formats Manual | NTPD.CONF(5) |
ntpd.conf
—
Network Time Protocol daemon configuration file
This manual page describes the format of the ntpd(8) configuration file.
ntpd.conf
has the following format:
Empty lines and lines beginning with the ‘#’ character are ignored.
Keywords may be specified multiple times within the configuration file. The basic configuration options are as follows:
listen
on
address [rtable
table-id]rtable
keyword will specify
which routing table to listen on. By default
ntpd(8) will listen using the current
routing table. For example:
listen on *
or
listen on 127.0.0.1 listen on ::1 listen on 127.0.0.1 rtable 4
query
from
sourceaddrquery from 192.0.2.1 query from 2001:db8::1
sensor
device [correction
microseconds] [refid
ID-string] [stratum
stratum-value] [trusted
]
[weight
weight-value]sensor * sensor nmea0
A 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:
sensor udcf0 correction 70000
A refid
ID-string of 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 ID-string is not given,
ntpd(8) will use a generic reference
ID. For example:
sensor nmea0 refid GPS
The stratum
keyword can be used to
change the stratum value from the default of 1.
The trusted
keyword indicates the time
learned is secure, trustworthy, and not vulnerable to man-in-the-middle
attacks, so constraints
validation is skipped.
This is useful for boot-time correction in environments where
constraints
cannot be used.
The 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.
server
address [trusted
]
[weight
weight-value]server 10.0.0.2 weight 5 server ntp.example.org weight 1
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.
servers
address [trusted
]
[weight
weight-value]server
, specify the IP address or hostname
of an NTP server to synchronize to. If it appears multiple times,
ntpd(8) will try to synchronize to all of
the servers specified. Should the hostname resolve to multiple IP
addresses, ntpd(8) will try to
synchronize to all of them. For example:
servers pool.ntp.org servers pool.ntp.org weight 5
ntpd(8) can be configured to query the ‘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.
constraint
from
url [ip...]constraint from
is used more than once,
ntpd(8) will calculate a median
constraint from all the servers specified.
server ntp.example.org constraint from www.example.com constraint from "https://9.9.9.9" "2620:fe::9"
constraints
from
urlconstraint 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,
ntpd(8) will calculate a median
constraint from all of them. For example:
servers pool.ntp.org constraints from "https://www.google.com/"
The ntpd.conf
file format first appeared
in OpenBSD 3.6.
January 6, 2021 | OpenBSD-current |