NAME¶
ntp.conf - NTP server configuration file
SYNOPSIS¶
ntp.conf
DESCRIPTION¶
Ordinarily,
ntpd reads the
ntp.conf configuration file at startup
time in order to determine the synchronization sources and operating modes. It
is also possible to specify a working, although limited, configuration
entirely on the command line, obviating the need for a configuration file.
This may be particularly useful when the local host is to be configured as a
broadcast/multicast client, with all peers being determined by listening to
broadcasts at run time.
Usually, the configuration file is installed in the
/etc directory, but
could be installed elsewhere (see the -c
conffile command line option).
The file format is similar to other Unix configuration files - comments begin
with a # character and extend to the end of the line; blank lines are ignored.
Configuration commands consist of an initial keyword followed by a list of
arguments, some of which may be optional, separated by whitespace. Commands
may not be continued over multiple lines. Arguments may be host names, host
addresses written in numeric, dotted-quad form, integers, floating point
numbers (when specifying times in seconds) and text strings. Optional
arguments are delimited by [ ] in the following descriptions, while
alternatives are separated by |. The notation [ ... ] means an optional,
indefinite repetition of the last item before the [ ... ].
Following is a description of the configuration commands in NTPv4. There are two
classes of commands, configuration commands that configure an association with
a remote server, peer or reference clock, and auxiliary 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 reference 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 qualifier preceding the host name forces DNS
resolution to the IPv4 namespace, 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 associations 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 normally 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 mobilizes a persistent
broadcast mode association. Multiple commands can be used to specify
multiple local broadcast 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 boundaries. Ordinarily, this specification applies only to
the local server operating as a sender; for operation as a broadcast
client, see the broadcastclient or multicastclient 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 specific address must be supplied which matches the address used on
the manycastserver command for the designated 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 messages. 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 minimum 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.
- mode option
- Pass the option to a reference clock driver, where option is
an integer in the range from 0 to 255, inclusive. This option is valid
only with type r addresses.
- 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 precision 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. Selection 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 packets. Versions
1-4 are the choices, with version 4 the default. This option is valid only
with the server, peer and broadcast commands.
- xleave
- Operate in interleaved mode (symmetric and broadcast modes only). (see NTP
Interleaved Modes)
Auxiliary Commands¶
- broadcastclient [novolley]
- This command enables reception of broadcast server messages to any local
interface (type b) address. Ordinarily, upon receiving 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
continues in listen-only mode. If the novolley keyword is present,
the exchange is not used and the value specified in the
broadcastdelay 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 public 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 broadcast client mode, in which it synchronizes to
succeeding multicast messages. 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.
Authentication Commands¶
- autokey [logsec]
- Specifies the interval between regenerations of the session key list used
with the autokey feature. Note that the size of the key list for each
association depends on this interval and the current poll interval. The
default value is 12 (4096 s or about 1.1 hours). For poll intervals above
the specified interval, a session key list with a single entry will be
regenerated for every message sent.
- revoke [logsec]
- Specifies the interval between recomputations of the private value used
with the autokey feature, which ordinarily requires an expensive public-
key computation. The default value is 12 (65,536 s or about 18 hours). For
poll intervals above the specified interval, a new private value will be
recomputed for every message sent.
Miscellaneous Options¶
- driftfile driftfile
- This command specifies the name of the file use to record the frequency
offset of the local clock oscillator. If the file exists, it is read at
startup in order to set the initial frequency offset and then updated once
per hour with the current frequency offset computed by the daemon. If the
file does not exist or this command is not given, the initial frequency
offset is assumed to be zero. In this case, it may take some hours for the
frequency to stabilize and the residual timing errors to subside.
The file format consists of a single line containing a single floating point
number, which records the frequency offset measured in parts-per-million
(PPM). The file is updated by first writing the current drift value into a
temporary file and then renaming this file to replace the old version.
This implies that ntpd must have write permission for the directory the
drift file is located in, and that file system links, symbolic or
otherwise, should be avoided.
- enable [auth | bclient | calibrate | kernel | monitor | ntp | pps |
stats]
- disable [auth | bclient | calibrate | kernel | monitor | ntp | pps |
stats]
- Provides a way to enable or disable various server options. Flags not
mentioned are unaffected. Note that all of these flags can be controlled
remotely using the ntpdc utility program.
- auth
- Enables the server to synchronize with unconfigured peers only if the peer
has been correctly authenticated using either public key or private key
cryptography. The default for this flag is enable.
- bclient
- Enables the server to listen for a message from a broadcast or multicast
server, as in the multicastclient command with default address. The
default for this flag is disable.
- calibrate
- Enables the calibrate feature for reference clocks. The default for this
flag is disable.
- kernel
- Enables the kernel time discipline, if available. The default for this
flag is enable if support is available, otherwise disable.
- monitor
- Enables the monitoring facility. See the ntpdc program and the
monlist command or further information. The default for this flag
is enable.
- ntp
- Enables time and frequency discipline. In effect, this switch opens and
closes the feedback loop, which is useful for testing. The default for
this flag is enable.
- pps
- Enables the pulse-per-second (PPS) signal when frequency and time is
disciplined by the precision time kernel modifications. See the A Kernel
Model for Precision Timekeeping page for further information. The default
for this flag is disable.
- stats
- Enables the statistics facility. See the Monitoring Options page for
further information. The default for this flag is disable.
- includefile includefile
- This command allows additional configuration commands to be included from
a separate file. Include files may be nested to a depth of five; upon
reaching the end of any include file, command processing resumes in the
previous configuration file. This option is useful for sites that run
ntpd on multiple hosts, with (mostly) common options (e.g., a
restriction list).
- interface [listen | ignore | drop] [all |
ipv4 | ipv6 | wildcard | name |
address[/prefixlen]]
- This command controls which network addresses ntpd opens, and
whether input is dropped without processing. The first parameter
determines the action for addresses which match the second parameter. That
parameter specifies a class of addresses, or a specific interface name, or
an address. In the address case, prefixlen determines how many bits
must match for this rule to apply. ignore prevents opening matching
addresses, drop causes ntpd to open the address and drop all
received packets without examination. Multiple interface commands
can be used. The last rule which matches a particular address determines
the action for it. interface commands are disabled if any
-I, --interface, -L, or --novirtualips
command-line options are used. If none of those options are used and no
interface actions are specified in the configuration file, all
available network addresses are opened. The nic command is an alias
for interface.
FILES¶
/etc/ntp.conf
NOTES¶
Note that this manual page shows only the most important configuration commands.
The full documentation (see below) contains more details.
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)
The complete documentation can be found at
/usr/share/doc/ntp-doc/html/ntpd.html#cfg in the package ntp-doc.