ntp.conf(5) ntp.conf(5)
NAME
ntp.conf - Server Options
Following is a description of the configuration commands in NTPv4.
There are two classes of commands, configuration commands that config-
ure an association with a remote server, peer or reference clock, and
auxilliary commands that specify environmental variables that control
various related operations.
CONFIGURATION COMMANDS
The various modes are determined by the command keyword and the
required IP address. Addresses are classed by type as (s) a remote
server or peer (IPv4 class A, B and C), (b) the broadcast address of a
local interface, (m) a multicast address (IPv4 class D), or (r) a ref-
erence clock address (127.127.x.x). The options that can be used with
these commands are listed below.
If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is
detected, support for the IPv6 address family is generated in addition
to the default support of the IPv4 address family. IPv6 addresses can
be identified by the presence of colons ":" in the address field. IPv6
addresses can be used almost everywhere where IPv4 addresses can be
used, with the exception of reference clock addresses, which are always
IPv4. Note that in contexts where a host name is expected, a -4 quali-
fier preceding the host name forces DNS resolution to the IPv4 names-
pace, while a -6 qualifier forces DNS resolution to the IPv6 namespace.
There are three types of associations: persistent, preemptable and
ephemeral. Persistent associations are mobilized by a configuration
command and never demobilized. Preemptable associations, which are new
to NTPv4, are mobilized by a configuration command which includes the
prempt flag and are demobilized by timeout or error. Ephemeral associa-
tions are mobilized upon arrival of designated messages and demobilized
by timeout or error.
server address [options ...]
peer address [options ...]
broadcast address [options ...]
manycastclient address [options ...]
These four commands specify the time server name or address to
be used and the mode in which to operate. The address can be
either a DNS name or a IP address in dotted-quad notation.
Additional information on association behavior can be found in
the Association Management page.
server For type s and r addresses (only), this command nor-
mally mobilizes a persistent client mode association
with the specified remote server or local reference
clock. If the preempt flag is specified, a preemptable
association is mobilized instead. In client mode the
client clock can synchronize to the remote server or
local reference clock, but the remote server can never
be synchronized to the client clock. This command
should NOT be used for type b or m addresses.
peer For type s addresses (only), this command mobilizes a
persistent symmetric-active mode association with the
specified remote peer. In this mode the local clock can
be synchronized to the remote peer or the remote peer
can be synchronized to the local clock. This is useful
in a network of servers where, depending on various
failure scenarios, either the local or remote peer may
be the better source of time. This command should NOT
be used for type b, m or r addresses.
broadcast
For type b and m addresses (only), this command mobi-
lizes a persistent broadcast mode association. Multiple
commands can be used to specify multiple local broad-
cast interfaces (subnets) and/or multiple multicast
groups. Note that local broadcast messages go only to
the interface associated with the subnet specified, but
multicast messages go to all interfaces. In broadcast
mode the local server sends periodic broadcast messages
to a client population at the address specified, which
is usually the broadcast address on (one of) the local
network(s) or a multicast address assigned to NTP. The
IANA has assigned the multicast group address IPv4
224.0.1.1 and IPv6 ff05::101 (site local) exclusively
to NTP, but other nonconflicting addresses can be used
to contain the messages within administrative bound-
aries. Ordinarily, this specification applies only to
the local server operating as a sender; for operation
as a broadcast client, see the broadcastclient or mul-
ticastclient commands below.
manycastclient
For type m addresses (only), this command mobilizes a
preemptable manycast client mode association for the
multicast group address specified. In this mode a spe-
cific address must be supplied which matches the
address used on the manycastserver command for the des-
ignated manycast servers. The NTP multicast address
224.0.1.1 assigned by the IANA should NOT be used,
unless specific means are taken to avoid spraying large
areas of the Internet with these messages and causing a
possibly massive implosion of replies at the sender.
The manycastclient command specifies that the host is
to operate in client mode with the remote servers that
are discovered as the result of broadcast/multicast
messages. The client broadcasts a request message to
the group address associated with the specified address
and specifically enabled servers respond to these mes-
sages. The client selects the servers providing the
best time and continues as with the server command. The
remaining servers are discarded as if never heard.
COMMAND OPTIONS
autokey All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey
scheme described in the Authentication Options page. This
option is valid with all commands.
burst When the server is reachable, send a burst of eight packets
instead of the usual one. The packet spacing is normally 2 s;
however, the spacing between the first and second packets can
be changed with the calldelay command to allow additional time
for a modem or ISDN call to complete. This option is valid with
only the server command and is a recommended option with this
command when the maxpoll option is 11 or greater.
iburst When the server is unreachable, send a burst of eight packets
instead of the usual one. The packet spacing is normally 2 s;
however, the spacing between the first and second packets can
be changed with the calldelay command to allow additional time
for a modem or ISDN call to complete. This option is valid with
only the server command and is a recommended option with this
command.
key key All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified key
identifier with values from 1 to 65534, inclusive. The default
is to include no encryption field. This option is valid with
all commands.
minpoll minpoll
maxpoll maxpoll
These options specify the minimum and maximum poll intervals
for NTP messages, in seconds as a power of two. The maximum
poll interval defaults to 10 (1,024 s), but can be increased by
the maxpoll option to an upper limit of 17 (36.4 h). The mini-
mum poll interval defaults to 6 (64 s), but can be decreased by
the minpoll option to a lower limit of 4 (16 s). These option
are valid only with the server and peer commands.
noselect
Marks the server as unused, except for display purposes. The
server is discarded by the selection algorithm. This option is
valid only with the server and peer commands.
preempt Specifies the association as preemptable rather than the
default persistent. This option is valied only with the server
command.
prefer Marks the server as preferred. All other things being equal,
this host will be chosen for synchronization among a set of
correctly operating hosts. See the Mitigation Rules and the
prefer Keyword page for further information. This option is
valid only with the server and peer commands.
true Force the association to assume truechimer status; that is,
always survive the selection and clustering algorithms. This
option can be used with any association, but is most useful for
reference clocks with large jitter on the serial port and pre-
cision pulse-per-second (PPS) signals. Caution: this option
defeats the algorithms designed to cast out falsetickers and
can allow these sources to set the system clock. This option is
valid only with the server and peer commands.
ttl ttl This option is used only with broadcast server and manycast
client modes. It specifies the time-to-live ttl to use on
broadcast server and multicast server and the maximum ttl for
the expanding ring search with manycast client packets. Selec-
tion of the proper value, which defaults to 127, is something
of a black art and should be coordinated with the network
administrator.
version version
Specifies the version number to be used for outgoing NTP pack-
ets. Versions 1-4 are the choices, with version 4 the default.
This option is valid only with the server, peer and broadcast
commands.
AUXILLIARY COMMANDS
broadcastclient [novolley]
This command enables reception of broadcast server messages to
any local interface (type b) address. Ordinarily, upon receiv-
ing a message for the first time, the broadcast client measures
the nominal server propagation delay using a brief
client/server exchange with the server, after which it contin-
ues in listen-only mode. If the novolley keyword is present,
the exchange is not used and the value specified in the broad-
castdelay command is used or, if the broadcastdelay command is
not used, the default 4.0 ms. Note that, in order to avoid
accidental or malicious disruption in this mode, both the
server and client should operate using symmetric key or public
key authentication as described in the Authentication Options
page. Note that the novolley keyword is incompatible with pub-
lic key authentication.
manycastserver address [...]
This command enables reception of manycast client messages to
the multicast group address(es) (type m) specified. At least
one address is required. The NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means
are taken to limit the span of the reply and avoid a possibly
massive implosion at the original sender. Note that, in order
to avoid accidental or malicious disruption in this mode, both
the server and client should operate using symmetric key or
public key authentication as described in the Authentication
Options page.
multicastclient address [...]
This command enables reception of multicast server messages to
the multicast group address(es) (type m) specified. Upon
receiving a message for the first time, the multicast client
measures the nominal server propagation delay using a brief
client/server exchange with the server, then enters the broad-
cast client mode, in which it synchronizes to succeeding multi-
cast messages. Note that, in order to avoid accidental or mali-
cious disruption in this mode, both the server and client
should operate using symmetric key or public key authentication
as described in the Authentication Options page.
BUGS
The syntax checking is not picky; some combinations of ridiculous and
even hilarious options and modes may not be detected.
SEE ALSO
ntpd(8), ntp_auth(5), ntp_mon(5), ntp_acc(5), ntp_clock(5), ntp_misc(5)
Primary source of documentation: /usr/share/doc/ntp-*
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ntp.conf(5)
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