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NTPD.CONF(5)                BSD File Formats Manual               NTPD.CONF(5)

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 sen-
             sor that actually exists.  Non-existent sensors are ignored.  If
             '*' is given as device name, ntpd(8) will use all timedelta sen-
             sors 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 rela-
             tive 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 exam-
             ple, will have five times more influence on time offset calcula-
             tion 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 sug-
             gests 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 syn-
             chronize 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 exam-
             ple:

                   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 informa-
     tion 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 host-
             name 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/search?q=openntpd"

FILES
     /etc/ntpd.conf     default ntpd(8) configuration file

SEE ALSO
     ntpd(8), sysctl(8)

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.

BSD                            February 16, 2015                           BSD