NAME
ntpd.conf
—
Network Time Protocol daemon
configuration file
DESCRIPTION
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]- Specify a local IP address or a hostname the
ntpd(8) daemon should listen on. If it appears multiple times,
ntpd(8) will listen on each given address. If ‘*’ is
given as an address,
ntpd(8) will listen on all local addresses using the specified
routing table. ntpd(8) does not listen on any address by default. The
optional
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
sensor
device [correction
microseconds] [weight
weight-value] [refid
string] [stratum
stratum-value]- Specify a timedelta sensor device
ntpd(8) should use. The sensor can be specified multiple times:
ntpd(8) will use each given sensor that actually exists.
Non-existent sensors are ignored. If ‘*’ is given as device
name, ntpd(8) will use all timedelta sensors it finds.
ntpd(8) does not use any timedelta sensor by default. For example:
sensor * sensor nmea0
An optional 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
The optional
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.An optional reference ID string - 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
A stratum value other than the default of 1 can be assigned using the stratum keyword.
server
address [weight
weight-value] [rtable
table-id]- Specify the IP address or the 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.
The
rtable
option specifies which routing table should be used for connection attempts. Hostname resolution will still happen using the default routing table. If a hostname resolves to multiple IPv4 and/or IPv6 addresses, ntpd(8) uses the first address. If it does not get a reply, ntpd(8) retries with the next address and continues to do so until a working address is found. For example:server 10.0.0.2 weight 5 server ntp.example.org weight 1 rtable 4
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 [weight
weight-value] [rtable
table-id]- As with
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 rtable 5
CONSTRAINTS
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- Specify the URL, IP address or the hostname of an HTTPS server to provide
a constraint. If
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
constraints from
url- As with
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, ntpd(8) will calculate a median constraint from all of them. For example:servers pool.ntp.org constraints from "https://www.google.com/"
FILES
- /etc/ntpd.conf
- default ntpd(8) configuration file
SEE ALSO
HISTORY
The ntpd.conf
file format first appeared
in OpenBSD 3.6.
CAVEATS
When using different rtable
options,
ntpd(8) must be started in rtable 0.