Provided by: ntpsec_1.1.0+dfsg1-1ubuntu0.2_amd64 
NAME
ntp.conf - Network Time Protocol (NTP) daemon configuration file format
SYNOPSIS
/etc/ntp.conf
DESCRIPTION
The ntp.conf configuration file is read at initial startup by the ntpd(8) daemon in order
to specify the synchronization sources, modes and other related information. Usually, it
is installed in the /etc directory, but could be installed elsewhere (see the daemon’s -c
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.
Configuration files may have inclusion lines. The syntax is include followed by whitespace
followed by a file or directory name. The configuration is evaluated as though the text of
the file - or all files of the directory with the extension ".conf" - were textually
spliced in at the point of the include. Relative paths will work, even when the -c option
changes the config directory root.
The rest of this page describes the configuration and control options. The "Notes on
Configuring NTP and Setting up an NTP Subnet" page (available as part of the HTML
documentation provided under /usr/share/doc/ntp) contains an extended discussion of these
options. In addition to the discussion of general Configuration Options, there are
sections describing the following supported functionality and the options used to control
it:
• Authentication Support
• Monitoring Support
• Access Control Support
• Automatic NTP Configuration Options
• Reference Clock Support
• Miscellaneous Options
Following these is a section describing Miscellaneous Options. While there is a rich set
of options available, the only required option is one or more pool, server, peer, or
broadcast commands.
CONFIGURATION SUPPORT
Following is a description of the configuration commands in NTPv4. There are two classes
of commands, association commands that configure a persistent association with a remote
server or peer or reference clock, and auxiliary commands that specify environmental
variables that control various related operations.
Association Commands
Only those options applicable to each command are listed below. Use of options not listed
may not be caught as an error, but may result in some weird and even destructive behavior.
In contexts where a host name is expected, a -4 or --ipv4 qualifier preceding the host
name forces DNS resolution to the IPv4 namespace, while a -6 or --ipv6 qualifier forces
DNS resolution to the IPv6 namespace.
In these commands, an address can be any of (a) an IPV4 address in a.b.c.d format, (b) an
IPV6 address in [a:b:c:d:e:f:g] format, (c) a link-local IPV6 address with an interface
specified in [a:b:c:d:e:f:g]%device format, or (d) a DNS hostname.
pool address [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll
maxpoll] [preempt]
server address [key key] [burst] [iburst] [version version] [prefer] [minpoll minpoll]
[maxpoll maxpoll]
peer address [key key] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]
unpeer [address | associd | clock clocktype [ unit unitnum]]
These six 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 an IP address in dotted-quad
notation. If it is a refclock, it can be clock followed by a type-unit pair as in the
refclock directive; a missing unit clause is interpreted as unit 0.
pool
For server addresses, this command mobilizes a persistent client mode association with
a number of remote servers. In this mode the local clock can synchronized to the
remote server, but the remote server can never be synchronized to the local clock.
server
For server addresses, this command mobilizes a persistent client mode association with
the specified remote server or local radio clock. In this mode the local clock can
synchronized to the remote server, but the remote server can never be synchronized to
the local clock.
peer
NTP peer mode has been removed for security reasons. peer is now just an alias for the
server keyword. See above.
unpeer
This command removes a previously configured association. An address or association ID
can be used to identify the association. Either an IP address or DNS name can be used.
This command is most useful when supplied via ntpq runtime configuration commands
config and config-from-file.
Association Options
bias
Add the command argument, a floating-point value in seconds, to the time offset ()
computed for this server. May be useful if you are a client on a network connection
such as an ADSL line where there is a predictable asymmetry between upstream and
downstream flight times. One way you night see this is if you use a fixed set of
others and one has a stable offset that is an outlier from the others; in that case
you might want to use bias to compensate out the offset.
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 is designed to improve timekeeping quality with the
server command.
iburst
When the server is unreachable, send a burst of six packets instead of the usual one.
The packet spacing is normally 2 s; however, the spacing between the first two packets
can be changed with the calldelay command to allow additional time for a modem or ISDN
call to complete. This is designed to speed the initial synchronization acquisition
with the server command, and when ntpd(8) is started with the -q option.
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.
minpoll minpoll, maxpoll maxpoll
These options specify the minimum and maximum poll intervals for NTP messages, as a
power of 2 in seconds. 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 0 (1 s).
mode option
Pass the option to a reference clock driver. This option is valid only with refclock
addresses.
noselect
Marks the server as unused, except for display purposes. The server is discarded by
the selection algorithm.
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.
true
Mark 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 command.
version version
Specifies the version number to be used for outgoing NTP packets. Versions 1-4 are the
choices, with version 4 the default.
Association Auxiliary Commands
mdnstries number
If we are participating in mDNS, after we have synched for the first time we attempt
to register with the mDNS system. If that registration attempt fails, we try again at
one minute intervals for up to number times. After all, ntpd may be starting before
mDNS. The default value for mdnstries is 5.
AUTHENTICATION COMMANDS
controlkey key
Specifies the key identifier to use with the ntpq(1) utility, which uses the standard
protocol defined in RFC 5905. The key argument is the key identifier for a trusted
key, where the value can be in the range 1 to 65,534, inclusive.
keys keyfile
Specifies the complete path and location of the MD5/SHA1 key file containing the keys
and key identifiers used by ntpd(8), and ntpq(1) when operating with symmetric-key
cryptography. This is the same operation as the -k command line option.
trustedkey key...
Specifies the key identifiers which are trusted for the purposes of authenticating
peers with symmetric key cryptography, as well as keys used by the ntpq(1) program.
Multiple keys on the same line should be separated by spaces. Key ranges can be
specified as (first ... last). The spaces around the ... are necessary. Multiple
trustedkey lines are supported and trusted keys can also be specified on the command
line.
The authentication procedures require that both the local and remote servers share the
same key and key identifier for this purpose, although different keys can be used with
different servers. The key arguments are 32-bit unsigned integers with values from 1 to
65,534.
MONITORING SUPPORT
ntpd(8) includes a comprehensive monitoring facility suitable for continuous, long term
recording of server and client timekeeping performance. See the statistics command below
for a listing and example of each type of statistics currently supported. Statistic files
are managed using file generation sets and scripts in the ./scripts directory of this
distribution. Using these facilities and UNIX cron(8) jobs, the data can be automatically
summarized and archived for retrospective analysis.
Monitoring Commands
statistics name...
Enables writing of statistics records. Currently, eight kinds of name statistics are
supported.
clockstats
Enables recording of clock driver statistics information. Each update received
from a clock driver appends a line of the following form to the file generation
set named clockstats:
49213 525.624 SPECTRACOM(1) 93 226 00:08:29.606
┌────────────────────┬───────┬─────────────────────────┐
│Item │ Units │ Description │
├────────────────────┼───────┼─────────────────────────┤
│49213 │ MJD │ modified Julian day │
│ │ │ number │
├────────────────────┼───────┼─────────────────────────┤
│525.624 │ s │ time of day (s) past │
│ │ │ midnight UTC │
├────────────────────┼───────┼─────────────────────────┤
│SPECTRACOM(1) │ │ receiver identifier │
│ │ │ (Spectracom unit 1) │
├────────────────────┼───────┼─────────────────────────┤
│93 226 00:08:29.606 │ │ timecode (format varies │
│ │ │ by refclock) │
└────────────────────┴───────┴─────────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and fraction
past UTC midnight). The next normally shows clock type and unit (but if you are running in
strict Classic compatibility mode it will show the magic clock address in dotted-quad
notation). The final field last timecode received from the clock in decoded ASCII format,
where meaningful. In some clock drivers a good deal of additional information can be
gathered and displayed as well. See information specific to each clock for further
details.
loopstats
Enables recording of loop filter statistics information. Each update of the local
clock outputs a line of the following form to the file generation set named loopstats:
50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806
┌────────────┬────────┬──────────────────────────┐
│Item │ Units │ Description │
├────────────┼────────┼──────────────────────────┤
│50935 │ MJD │ date │
├────────────┼────────┼──────────────────────────┤
│75440.031 │ s │ time past midnight │
├────────────┼────────┼──────────────────────────┤
│0.000006019 │ s │ clock offset │
├────────────┼────────┼──────────────────────────┤
│13.778 │ PPM │ drift (frequency offset) │
├────────────┼────────┼──────────────────────────┤
│0.000351733 │ s │ RMS jitter │
├────────────┼────────┼──────────────────────────┤
│0.013380 │ PPM │ RMS frequency jitter │
│ │ │ (aka wander) │
├────────────┼────────┼──────────────────────────┤
│6 │ log2 s │ clock discipline loop │
│ │ │ time constant │
└────────────┴────────┴──────────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and
fraction past UTC midnight). The next five fields show time offset (seconds),
frequency offset (parts per million - PPM), RMS jitter (seconds), Allan deviation
(PPM) and clock discipline time constant.
protostats
Record significant peer and system events. Each significant event appends one line to
the protostats file set:
49213 525.624 128.4.1.1 963a 8a message
┌──────────┬───────┬─────────────────────────┐
│Item │ Units │ Description │
├──────────┼───────┼─────────────────────────┤
│49213 │ MJD │ date │
├──────────┼───────┼─────────────────────────┤
│525.624 │ s │ time past midnight │
├──────────┼───────┼─────────────────────────┤
│128.4.1.1 │ IP │ source address (0.0.0.0 │
│ │ │ for system) │
├──────────┼───────┼─────────────────────────┤
│963a │ code │ status word │
├──────────┼───────┼─────────────────────────┤
│8a │ code │ event message code │
├──────────┼───────┼─────────────────────────┤
│message │ text │ event message │
└──────────┴───────┴─────────────────────────┘
The event message code and message field are described on the "Event Messages and
Status Words" page.
peerstats
Enables recording of peer statistics information. This includes statistics records of
all peers of a NTP server and of special signals, where present and configured. Each
valid update appends a line of the following form to the current element of a file
generation set named peerstats:
48773 10847.650 SPECTRACOM(4) 9714 -0.001605376 0.000000000
0.001424877 0.000958674
┌──────────────┬───────┬──────────────────────┐
│Item │ Units │ Description │
├──────────────┼───────┼──────────────────────┤
│48773 │ MJD │ date │
├──────────────┼───────┼──────────────────────┤
│10847.650 │ s │ time past midnight │
├──────────────┼───────┼──────────────────────┤
│SPECTRACOM(4) │ │ clock name (unit) or │
│ │ │ source address │
├──────────────┼───────┼──────────────────────┤
│9714 │ hex │ status word │
├──────────────┼───────┼──────────────────────┤
│-0.001605376 │ s │ clock offset │
├──────────────┼───────┼──────────────────────┤
│0.000000000 │ s │ roundtrip delay │
├──────────────┼───────┼──────────────────────┤
│0.001424877 │ s │ dispersion │
├──────────────┼───────┼──────────────────────┤
│0.000958674 │ s │ RMS jitter │
└──────────────┴───────┴──────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and
fraction past UTC midnight). The third field shows the reference clock type and unit
number (but if you are running in the peer address in dotted-quad notation instead)
The fourth field is a status word, encoded in hex in the format described in Appendix
A of the NTP specification RFC 1305. The final four fields show the offset, delay,
dispersion and RMS jitter, all in seconds.
rawstats
Enables recording of raw-timestamp statistics information. This includes statistics
records of all peers of a NTP server and of special signals, where present and
configured. Each NTP message received from a peer or clock driver appends a line of
the following form to the file generation set named rawstats:
56285 54575.160 128.4.1.1 192.168.1.5 3565350574.400229473
3565350574.442385200 3565350574.442436000
3565350575.154505763 0 4 4 1 8 -21 0.000000 0.000320
PPS 0
┌─────────────────────┬──────────────────────────┬──────────────────────────┐
│Item │ Units │ Description │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│56285 │ MJD │ date │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│54575.160 │ s │ time past midnight │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│128.4.1.1 │ IP │ source address │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│192.168.1.5 │ IP │ destination address │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│3565350574.400229473 │ NTP s │ origin timestamp │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│3565350574.442385200 │ NTP s │ receive timestamp │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│3565350574.442436000 │ NTP s │ transmit timestamp │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│3565350575.154505763 │ NTP s │ destination timestamp │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│0 │ 0: OK, 1: insert │ leap warning indicator │
│ │ pending, 2: delete │ │
│ │ pending, 3: not synced │ │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│4 │ 4 was current in 2012 │ NTP version │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│4 │ 3: client, 4: server, 6: │ mode │
│ │ ntpq │ │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│1 │ 1-15, 16: not synced │ stratum │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│8 │ log2 seconds │ poll │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│-21 │ log2 seconds │ precision │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│0.000000 │ seconds │ total roundtrip delay │
│ │ │ from the remote server │
│ │ │ to the primary reference │
│ │ │ clock │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│0.000320 │ seconds │ total dispersion from │
│ │ │ the remote server to the │
│ │ │ primary reference clock │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│.PPS. │ IP or text │ refid, association ID │
├─────────────────────┼──────────────────────────┼──────────────────────────┤
│0 │ integer │ lost packets since last │
│ │ │ response │
└─────────────────────┴──────────────────────────┴──────────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and
fraction past UTC midnight). The next two fields show the remote peer or clock
identification followed by the local address in dotted-quad notation. The final four
fields show the originate, receive, transmit and final NTP timestamps in order. The
timestamp values are as received and before processing by the various data smoothing
and mitigation algorithms.
sysstats
Enables recording of ntpd statistics counters on a periodic basis. Each hour a line of
the following form is appended to the file generation set named sysstats:
50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147 1
┌─────────┬───────┬───────────────────────┐
│Item │ Units │ Description │
├─────────┼───────┼───────────────────────┤
│50928 │ MJD │ date │
├─────────┼───────┼───────────────────────┤
│2132.543 │ s │ time past midnight │
├─────────┼───────┼───────────────────────┤
│3600 │ s │ time since reset │
├─────────┼───────┼───────────────────────┤
│81965 │ # │ packets received │
├─────────┼───────┼───────────────────────┤
│0 │ # │ packets for this host │
├─────────┼───────┼───────────────────────┤
│9546 │ # │ current versions │
├─────────┼───────┼───────────────────────┤
│56 │ # │ old version │
├─────────┼───────┼───────────────────────┤
│512 │ # │ access denied │
├─────────┼───────┼───────────────────────┤
│540 │ # │ bad length or format │
├─────────┼───────┼───────────────────────┤
│10 │ # │ bad authentication │
├─────────┼───────┼───────────────────────┤
│4 │ # │ declined │
├─────────┼───────┼───────────────────────┤
│147 │ # │ rate exceeded │
├─────────┼───────┼───────────────────────┤
│1 │ # │ kiss-o'-death packets │
│ │ │ sent │
└─────────┴───────┴───────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and
fraction past UTC midnight). The remaining ten fields show the statistics counter
values accumulated since the last generated line.
usestats
Enables recording of ntpd resource usage statistics. Each hour a line of the following
form is appended to the file generation set named usestats:
57570 83399.541 3600 0.902 1.451 164 0 0 0 2328 64226 1 0 4308
┌──────────┬───────┬──────────────────────────┐
│Item │ Units │ Description │
├──────────┼───────┼──────────────────────────┤
│57570 │ MJD │ date │
├──────────┼───────┼──────────────────────────┤
│83399.541 │ s │ time past midnight │
├──────────┼───────┼──────────────────────────┤
│3600 │ s │ time since reset │
├──────────┼───────┼──────────────────────────┤
│0.902 │ s │ ru_utime: CPU seconds - │
│ │ │ user mode │
├──────────┼───────┼──────────────────────────┤
│1.451 │ s │ ru_stime: CPU seconds - │
│ │ │ system │
├──────────┼───────┼──────────────────────────┤
│164 │ # │ ru_minflt: page faults - │
│ │ │ reclaim/soft (no I/O) │
├──────────┼───────┼──────────────────────────┤
│0 │ # │ ru_majflt: page faults - │
│ │ │ I/O │
├──────────┼───────┼──────────────────────────┤
│0 │ # │ ru_nswap: process │
│ │ │ swapped out │
├──────────┼───────┼──────────────────────────┤
│0 │ # │ ru_inblock: file blocks │
│ │ │ in │
├──────────┼───────┼──────────────────────────┤
│2328 │ # │ ru_oublock: file blocks │
│ │ │ out │
├──────────┼───────┼──────────────────────────┤
│64226 │ # │ ru_nvcsw: context │
│ │ │ switches, wait │
├──────────┼───────┼──────────────────────────┤
│1 │ # │ ru_nivcsw: context │
│ │ │ switches, preempts │
├──────────┼───────┼──────────────────────────┤
│0 │ # │ ru_nsignals: signals │
├──────────┼───────┼──────────────────────────┤
│4308 │ # │ ru_maxrss: resident set │
│ │ │ size, kilobytes │
└──────────┴───────┴──────────────────────────┘
The first two fields show the date (Modified Julian Day) and time (seconds and
fraction past UTC midnight). The ru_ tags are the names from the rusage struct. See
man getrusage for details. (The NetBSD and FreeBSD man pages have more details.) The
maxrss column is the high water mark since the process was started. The remaining
fields show the values used since the last report.
timingstats
(Only available when the daemon is compiled with process time debugging support
(--enable-debug-timing - costs performance). Record processing time statistics for
various selected code paths.
53876 36.920 10.0.3.5 1 0.000014592 input processing delay
┌────────────┬───────┬───────────────────────┐
│Item │ Units │ Description │
├────────────┼───────┼───────────────────────┤
│53876 │ MJD │ date │
├────────────┼───────┼───────────────────────┤
│36.920 │ s │ time past midnight │
├────────────┼───────┼───────────────────────┤
│10.0.3.5 │ IP │ server address │
├────────────┼───────┼───────────────────────┤
│1 │ # │ event count │
├────────────┼───────┼───────────────────────┤
│0.000014592 │ s │ total time │
├────────────┼───────┼───────────────────────┤
│message │ text │ code path description │
│ │ │ (see source) │
└────────────┴───────┴───────────────────────┘
statsdir directory_path
Indicates the full path of a directory where statistics files should be created (see
below). This keyword allows the (otherwise constant) filegen filename prefix to be
modified for file generation sets, which is useful for handling statistics logs.
filegen name [file filename] [type typename] [link | nolink] [enable | disable]
Configures setting of generation file set name. Generation file sets provide a means
for handling files that are continuously growing during the lifetime of a server.
Server statistics are a typical example for such files. Generation file sets provide
access to a set of files used to store the actual data. At any time at most one
element of the set is being written to. The type given specifies when and how data
will be directed to a new element of the set. This way, information stored in elements
of a file set that are currently unused are available for administrative operations
without the risk of disturbing the operation of ntpd. (Most important: they can be
removed to free space for new data produced.)
Note that this command can be sent from the ntpq(1) program running at a remote
location.
name
This is the type of the statistics records, as shown in the statistics command.
file filename
This is the file name for the statistics records. Filenames of set members are
built from three concatenated elements prefix, filename and suffix:
prefix
This is a constant filename path. It is not subject to modifications via the filegen
option. It is defined by the server, usually specified as a compile-time constant. It
may, however, be configurable for individual file generation sets via other commands.
For example, the prefix used with loopstats and peerstats generation can be configured
using the statsdir option explained above.
filename
This string is directly concatenated to the prefix mentioned above (no intervening
‘/’). This can be modified using the file argument to the filegen statement. No ..
elements are allowed in this component to prevent filenames referring to parts outside
the filesystem hierarchy denoted by prefix.
suffix
This part is reflects individual elements of a file set. It is generated according to
the type of a file set.
type typename
A file generation set is characterized by its type. The following types are
supported:
none
The file set is actually a single plain file.
pid
One element of file set is used per incarnation of a ntpd server. This type does not
perform any changes to file set members during runtime, however it provides an easy
way of separating files belonging to different ntpd(8) server incarnations. The set
member filename is built by appending a ‘.’ to concatenated prefix and filename
strings, and appending the decimal representation of the process ID of the ntpd(8)
server process.
day
One file generation set element is created per day. A day is defined as the period
between 00:00 and 24:00 UTC. The file set member suffix consists of a ‘.’ and a day
specification in the form YYYYMMdd. YYYY is a 4-digit year number (e.g., 1992). MM
is a two digit month number. dd is a two digit day number. Thus, all information
written at 10 December 1992 would end up in a file named prefix filename.19921210.
week
Any file set member contains data related to a certain week of a year. The term week
is defined by computing day-of-year modulo 7. Elements of such a file generation set
are distinguished by appending the following suffix to the file set filename base: A
dot, a 4-digit year number, the letter W, and a 2-digit week number. For example,
information from January, 10th 1992 would end up in a file with suffix 1992W1.
month
One generation file set element is generated per month. The file name suffix consists
of a dot, a 4-digit year number, and a 2-digit month.
year
One generation file element is generated per year. The filename suffix consists of a
dot and a 4 digit year number.
age
This type of file generation sets changes to a new element of the file set every 24
hours of server operation. The filename suffix consists of a dot, the letter a, and an
8-digit number. This number is taken to be the number of seconds the server is running
at the start of the corresponding 24-hour period.
link | nolink
It is convenient to be able to access the current element of a file generation set
by a fixed name. This feature is enabled by specifying link and disabled using
nolink. If link is specified, a hard link from the current file set element to a
file without suffix is created. When there is already a file with this name and
the number of links of this file is one, it is renamed appending a dot, the letter
C, and the pid of the ntpd server process. When the number of links is greater
than one, the file is unlinked. This allows the current file to be accessed by a
constant name.
enable | disable
Enables or disables the recording function. Information is only written to a file
generation by specifying enable; output is prevented by specifying disable.
ACCESS CONTROL SUPPORT
The ntpd(8) daemon implements a general purpose address/mask based restriction list. The
list contains address/match entries sorted first by increasing address values and then by
increasing mask values. A match occurs when the bitwise AND of the mask and the packet
source address is equal to the bitwise AND of the mask and address in the list. The list
is searched in order with the last match found defining the restriction flags associated
with the entry. Additional information and examples can be found in the "Notes on
Configuring NTP and Setting up a NTP Subnet" page (available as part of the HTML
documentation).
The restriction facility was implemented in conformance with the access policies for the
original NSFnet backbone time servers. Later the facility was expanded to deflect
cryptographic and clogging attacks. While this facility may be useful for keeping unwanted
or broken or malicious clients from congesting innocent servers, it should not be
considered an alternative to the NTP authentication facilities. Source address based
restrictions are easily circumvented by a determined cracker.
Clients can be denied service because they are explicitly included in the restrict list
created by the restrict command or implicitly as the result of cryptographic or rate limit
violations. Cryptographic violations include certificate or identity verification failure;
rate limit violations generally result from defective NTP implementations that send
packets at abusive rates. Some violations cause denied service only for the offending
packet, others cause denied service for a timed period and others cause the denied service
for an indefinite period. When a client or network is denied access for an indefinite
period, the only way at present to remove the restrictions is by restarting the server.
The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further action except
incrementing statistics counters. Sometimes a more proactive response is needed, such as a
server message that explicitly requests the client to stop sending and leave a message for
the system operator. A special packet format has been created for this purpose called the
"kiss-of-death" (KoD) packet. KoD packets have the leap bits set unsynchronized and
stratum set to zero and the reference identifier field set to a four-byte ASCII code. If
the noserve or notrust flag of the matching restrict list entry is set, the code is
"DENY"; if the limited flag is set and the rate limit is exceeded, the code is "RATE".
Finally, if a cryptographic violation occurs, the code is "CRYP".
A client receiving a KoD performs a set of sanity checks to minimize security exposure,
then updates the stratum and reference identifier peer variables, sets the access denied
(BOGON4) bit in the peer flash variable and sends a message to the log. As long as the
BOGON4 bit is set, the client will send no further packets to the server. The only way at
present to recover from this condition is to restart the protocol at both the client and
server. This happens automatically at the client when the association times out. It will
happen at the server only if the server operator cooperates.
ACCESS CONTROL COMMANDS
discard [average avg] [minimum min] [monitor prob]
Set the parameters of the limited facility which protects the server from client
abuse. The average subcommand specifies the minimum average packet spacing, while the
minimum subcommand specifies the minimum packet spacing. Packets that violate these
minima are discarded and a kiss-o'-death packet returned if enabled. The default
minimum average and minimum are 5 and 2, respectively. The monitor subcommand
specifies the probability of discard for packets that overflow the rate-control
window. The options are:
average avg
Specify the minimum average interpacket spacing (minimum average headway time) in
log2 s with default 3.
minimum min
Specify the minimum interpacket spacing (guard time) in seconds with default 2.
monitor
Specify the probability of being recorded for packets that overflow the MRU list
size limit set by mru maxmem or mru maxdepth. This is a performance optimization
for servers with aggregate arrivals of 1000 packets per second or more.
restrict address[/cidr] [mask mask] [flag ...]
The address argument expressed in dotted-quad (for IPv4) or :-delimited (for IPv6)
form is the address of a host or network. Alternatively, the address argument can be a
valid host DNS name. The mask argument expressed in IPv4 or IPv6 numeric address form
defaults to all mask bits on, meaning that the address is treated as the address of an
individual host. Instead of an explicit mask the address/cidr may be specified in CIDR
notation. A default entry (address 0.0.0.0, mask 0.0.0.0) is always included and is
always the first entry in the list. Note that text string default, with no mask
option, may be used to indicate the default entry. In the current implementation, flag
always restricts access, i.e., an entry with no flags indicates that free access to
the server is to be given. The flags are not orthogonal, in that more restrictive
flags will often make less restrictive ones redundant. The flags can generally be
classed into two categories, those which restrict time service and those which
restrict informational queries and attempts to do run-time reconfiguration of the
server. One or more of the following flags may be specified:
flake
Discard received NTP packets with probability 0.1; that is, on average drop one
packet in ten. This is for testing and amusement. The name comes from Bob Braden’s
flakeway, which once did a similar thing for early Internet testing.
ignore
Deny packets of all kinds, including ntpq(1) queries.
kod
If this flag is set when an access violation occurs, a kiss-o'-death (KoD) packet
is sent. KoD packets are rate limited to no more than one per second. If another
KoD packet occurs within one second after the last one, the packet is dropped.
limited
Deny service if the packet spacing violates the lower limits specified in the
discard command. A history of clients is kept using the monitoring capability of
ntpd(8). Thus, monitoring is always active as long as there is a restriction entry
with the limited flag.
mssntp
Enable Microsoft Windows MS-SNTP authentication using Active Directory services.
Note: Potential users should be aware that these services involve a TCP connection
to another process that could potentially block, denying services to other users.
Therefore, this flag should be used only for a dedicated server with no clients
other than MS-SNTP.
nomodify
Deny ntpq(1) queries which attempt to modify the state of the server (i.e., run
time reconfiguration). Queries which return information are permitted.
noquery
Deny ntpq(1) queries. Time service is not affected.
nopeer
Deny packets which would result in mobilizing a new association. This includes
symmetric active packets when a configured association does not exist. That used
to happen when the remote client used the peer command in its config file. We
don’t support that mode. It used to include pool servers but they now poke a hole
in any restrictions.
noserve
Deny all packets except ntpq(1) and queries.
notrust
Deny service unless the packet is cryptographically authenticated.
ntpport
This is actually a match algorithm modifier, rather than a restriction flag. Its
presence causes the restriction entry to be matched only if the source port in the
packet is the standard NTP UDP port (123). Both ntpport and non-ntpport may be
specified. The ntpport is considered more specific and is sorted later in the
list.
nomrulist
Do not accept MRU-list requests. These can be expensive to service, and may
generate a high volume of response traffic.
version
Deny packets that do not match the current NTP version.
Default restriction list entries with the flags ignore, interface, ntpport, for each
of the local host’s interface addresses are inserted into the table at startup to
prevent the server from attempting to synchronize to its own time. A default entry is
also always present, though if it is otherwise unconfigured; no flags are associated
with the default entry (i.e., everything besides your own NTP server is unrestricted).
unrestrict address[/cidr] [mask mask] [flag ...]
Like a restrict command, but turns off the specified flags rather than turning them on
(expected to be useful mainly with ntpq :config). An unrestrict with no flags
specified removes any rule with matching address and mask. Use only on an address/mask
or CIDR-format address mentioned in a previous restrict statement.
AUTOMATIC NTP CONFIGURATION OPTIONS
Manycasting
For a detailed description of manycast operation, see the "Server Discovery" page
(available as part of the HTML documentation).
Manycast Options
tos [ceiling ceiling | floor floor | minclock minclock | minsane minsane]
This command affects the clock selection and clustering algorithms. It can be used to
select the quality and quantity of peers used to synchronize the system clock and is
most useful in manycast mode. The variables operate as follows:
ceiling ceiling
Peers with strata above ceiling will be discarded if there are at least minclock
peers remaining. This value defaults to 15, but can be changed to any number from
1 to 15.
floor floor
Peers with strata below floor will be discarded if there are at least minclock
peers remaining. This value defaults to 1, but can be changed to any number from 1
to 15.
minclock minclock
The clustering algorithm repeatedly casts out outlier associations until no more
than minclock associations remain. This value defaults to 3, but can be changed to
any number from 1 to the number of configured sources.
minsane minsane
This is the minimum number of candidates available to the clock selection
algorithm in order to produce one or more truechimers for the clustering
algorithm. If fewer than this number are available, the clock is undisciplined and
allowed to run free. The default is 1 for legacy purposes. However, according to
principles of Byzantine agreement, minsane should be at least 4 in order to detect
and discard a single falseticker.
REFERENCE CLOCK SUPPORT
For a detailed description of reference-clock configuration, see the "Reference Clock
Drivers" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp).
REFERENCE CLOCK COMMANDS
refclock drivername [unit u] [prefer] [subtype int] [mode int] [minpoll int] [maxpoll int]
[time1 sec] [time2 sec] [stratum int] [refid string] [path filename] [ppspath filename]
[baud number] [flag1 {0 | 1}] [flag2 {0 | 1}] [flag3 {0 | 1}] [flag4 {0 | 1}]
This command is used to configure reference clocks. The required drivername argument
is the shortname of a driver type (e.g. shm, nmea, generic; see the
link:refclock.html [Reference Clock Drivers] page for a full list. The options are
interpreted as follows:
unit
The 0-origin unit number of the device. Modifies the devicename. If not specified,
defaults to zero.
prefer
Marks the reference clock as preferred. All other things being equal, this host
will be chosen for synchronization among a set of correctly operating hosts and
clocks. See the "Mitigation Rules and the prefer Keyword" page (available as part
of the HTML documentation provided in /usr/share/doc/ntp) for further information.
subtype int
Some drivers (notably the generic and jjy drivers) support multiple device types.
This option selects among them in a driver-dependent way.
mode int
Specifies a mode number which is interpreted in a device-specific fashion. For
instance, it selects a dialing protocol in the ACTS driver and a sentence mix in
the nmea driver.
minpoll int; maxpoll int
These options specify the minimum and maximum polling interval for reference clock
messages, as a power of 2 in seconds. For most directly connected reference
clocks, both minpoll and maxpoll default to 6 (64 sec). For modem reference
clocks, minpoll defaults to 10 (17.1 min) and maxpoll defaults to 14 (4.5 hours).
The allowable range is 0 (1 sec) to 17 (36.4 hours) inclusive.
time1 sec
Specifies a constant to be added to the time offset produced by the driver, a
fixed-point decimal number in seconds. This is used as a calibration constant to
adjust the nominal time offset of a particular clock to agree with an external
standard, such as a precision PPS signal. It also provides a way to correct a
systematic error or bias due to serial port or operating system latencies,
different cable lengths or receiver internal delay. The specified offset is in
addition to the propagation delay provided by other means, such as internal DIP
switches. Where a calibration for an individual system and driver is available, an
approximate correction is noted in the driver documentation pages. Note: in order
to facilitate calibration when more than one radio clock or PPS signal is
supported, a special calibration feature is available. It takes the form of an
argument to the enable command described in "Miscellaneous Options" page and
operates as described in the "Reference Clock Drivers" page.
time2 secs
Specifies a fixed-point decimal number in seconds, which is interpreted in a
driver-dependent way. See the descriptions of specific drivers in the "Reference
Clock Drivers" page.
stratum int
Specifies the stratum number assigned to the driver, an integer between 0 and 15.
This number overrides the default stratum number ordinarily assigned by the driver
itself, usually zero.
refid string
Specifies an ASCII string of from one to four characters which defines the
reference identifier used by the driver. This string overrides the default
identifier ordinarily assigned by the driver itself.
path filepath
Overrides the default device location for this refclock.
ppspath filepath
Overrides the default PPS device location (if any) for this driver.
baud number
Overrides the defaults baud rate for this driver.
flag1 {0 | 1}; flag2 {0 | 1}; flag3 {0 | 1}; flag4 {0 | 1}
These four flags are used for customizing the clock driver. The interpretation of
these values, and whether they are used at all, is a function of the particular
clock driver. However, by convention flag4 is used to enable recording monitoring
data to the clockstats file configured with the filegen command. Further
information on the filegen command can be found in "Monitoring Options".
MISCELLANEOUS OPTIONS
calldelay delay
This option controls the delay in seconds between the first and second packets sent in
burst or iburst mode to allow additional time for a modem or ISDN call to complete.
driftfile driftfile
This command specifies the complete path and name of the file used to record the
frequency of the local clock oscillator. This is the same operation as the -f command
line option. If the file exists, it is read at startup in order to set the initial
frequency and then updated once per hour with the current frequency computed by the
daemon. If the file name is specified, but the file itself does not exist, ntpd starts
with an initial frequency of zero and creates the file when writing it for the first
time. If this command is not given, the daemon will always start with an initial
frequency of zero.
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(8) 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 | calibrate | kernel | monitor | ntp | stats]; disable [auth | calibrate |
kernel | monitor | ntp | 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 ntpq(1)
utility program.
auth
Enables the server to synchronize with unconfigured peers only if the peer has
been correctly authenticated. The default for this flag is enable.
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 ntpq(1) program and the monlist command
for 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.
stats
Enables the statistics facility. See the "Monitoring Options" section 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. Relative
pathnames are evaluated not with respect to the current working directory but with
respect to the directory name of the last pushed file in the stack. This option is
useful for sites that run ntpd(8) 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 of the -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.
leapfile leapfile
This command loads the NIST leapseconds file and initializes the leapsecond values for
the next leapsecond time, expiration time and TAI offset. The file can be obtained
directly from NIST national time servers using ftp as the ASCII file
pub/leap-seconds.list.
The leapfile is scanned when ntpd processes the leapfile directive or when ntpd
detects that leapfile has changed. ntpd checks once a day to see if the leapfile has
changed.
leapsmearinterval interval
This experimental option is only available if ntpd was built with the
--enable-leap-smear option, It specifies the interval over which a leap second
correction will be applied. Recommended values for this option are between 7200 (2
hours) and 86400 (24 hours). DO NOT USE THIS OPTION ON PUBLIC-ACCESS SERVERS! See
http://bugs.ntp.org/2855 for more information.
logconfig configkeyword
This command controls the amount and type of output written to the system syslog(3)
facility or the alternate log file. By default, all output is turned on. All
configkeyword keywords can be prefixed with ‘=’, ‘+’ and ‘-’, where ‘=’ sets the
syslog(3) priority mask, ‘+’ adds and ‘-’ removes messages. syslog(3) messages can be
controlled in four classes (clock,peer,sys and sync). Within these classes four types
of messages can be controlled: informational messages (info), event messages (events),
statistics messages (statistics) and status messages (status).
Configuration keywords are formed by concatenating the message class with the event
class. The all prefix can be used instead of a message class. A message class may also
be followed by the all keyword to enable/disable all messages of the respective
message class. Thus, a minimal log configuration could look like this:
logconfig =syncstatus +sysevents
This would just list the synchronizations state of ntpd(8) and the major system
events. For a simple reference server, the following minimum message configuration
could be useful:
logconfig =syncall +clockall
This configuration will list all clock information and synchronization information.
All other events and messages about peers, system events and so on is suppressed.
logfile logfile
This command specifies the location of an alternate log file to be used instead of the
default system syslog(3) facility. This is the same operation as the -l command line
option.
mru [maxdepth count | maxmem kilobytes | mindepth count | maxage seconds | minage seconds
| initalloc count | initmem kilobytes | incalloc count | incmem kilobytes]
Controls size limits of the monitoring facility Most Recently Used (MRU) list of
client addresses, which is also used by the rate control facility.
maxdepth count, maxmem kilobytes
Equivalent upper limits on the size of the MRU list, in terms of entries or
kilobytes. The actual limit will be up to incalloc entries or incmem kilobytes
larger. As with all of the mru options offered in units of entries or kilobytes,
if both maxdepth and maxmem are used, the last one used controls. The default is
1024 kilobytes.
mindepth count
Lower limit on the MRU list size. When the MRU list has fewer than mindepth
entries, existing entries are never removed to make room for newer ones,
regardless of their age. The default is 600 entries.
maxage seconds, minage seconds
If an address is not in the list, there are several possible ways to find a slot
for it.
1. If the list has fewer than mindepth entries, a slot is allocated from the free
list. This is the normal case for a server without a lot of clients. If
clients come and go, for example, laptops going between home and work, the
default setup shows only the long term average.
2. If the age of the oldest slot is greater than maxage, the oldest slot is
recycled (default 3600 seconds).
3. If the freelist is not empty, a slot is allocated from the free list.
4. If the freelist is empty but not full (see maxmem), more memory is allocated
(see incmem) and a new slot is used.
5. If the age of the oldest slot is more than minage, the oldest slot is recycled
(default 64 seconds).
6. Otherwise, no slot is available.
initalloc count, initmem kilobytes
Initial memory allocation at the time the monitoring facility is first enabled, in
terms of entries or kilobytes. The default is 4 kilobytes.
incalloc count, incmem kilobytes
Size of additional memory allocations when growing the MRU list, in entries or
kilobytes. The default is 4 kilobytes.
nonvolatile threshold
Specify the threshold in seconds to write the frequency file, with default of 1e-7
(0.1 PPM). The frequency file is inspected each hour. If the difference between the
current frequency and the last value written exceeds the threshold, the file is
written and the threshold becomes the new threshold value. If the threshold is not
exceeded, it is reduced by half. This is intended to reduce the frequency of
unnecessary file writes for embedded systems with nonvolatile memory.
phone dial ...
This command is used in conjunction with the ACTS modem driver (type modem) or the JJY
driver (type jjy). For ACTS, the arguments consist of a maximum of 10 telephone
numbers used to dial USNO, NIST or European time services. For the jjy driver in modes
100-180, the argument is one telephone number used to dial the telephone JJY service.
The Hayes command ATDT is normally prepended to the number, which can contain other
modem control codes as well.
reset [allpeers] [auth] [ctl] [io] [mem] [sys] [timer]
Reset one or more groups of counters maintained by ntpd and exposed by ntpq.
setvar variable [default]
This command adds an additional system variable. These variables can be used to
distribute additional information such as the access policy. If the variable of the
form name=value is followed by the default keyword, the variable will be listed as
part of the default system variables (ntpq(1) rv command). These additional variables
serve informational purposes only. They are not related to the protocol other that
they can be listed. The known protocol variables will always override any variables
defined via the setvar mechanism. There are three special variables that contain the
names of all variable of the same group. The sys_var_list holds the names of all
system variables. The peer_var_list holds the names of all peer variables and the
clock_var_list holds the names of the reference clock variables.
tinker [allan allan | dispersion dispersion | freq freq | huffpuff huffpuff | panic panic
| step step | stepback stepback | stepfwd stepfwd | stepout stepout]
This command can be used to alter several system variables in very exceptional
circumstances. It should occur in the configuration file before any other
configuration options. The default values of these variables have been carefully
optimized for a wide range of network speeds and reliability expectations. In general,
they interact in intricate ways that are hard to predict and some combinations can
result in some very nasty behavior. Very rarely is it necessary to change the default
values; but, some folks cannot resist twisting the knobs anyway and this command is
for them. Emphasis added: twisters are on their own and can expect no help from the
support group.
The variables operate as follows:
allan allan
The argument becomes the new value for the minimum Allan intercept, which is a
parameter of the PLL/FLL clock discipline algorithm. The value in log2 seconds
defaults to 11 (2048 s), which is also the lower limit.
dispersion dispersion
The argument becomes the new value for the dispersion increase rate, normally
.000015 s/s.
freq freq
The argument becomes the initial value of the frequency offset in
parts-per-million. This overrides the value in the frequency file, if present, and
avoids the initial training state if it is not.
huffpuff huffpuff
The argument becomes the new value for the experimental huff-n'-puff filter span,
which determines the most recent interval the algorithm will search for a minimum
delay. The lower limit is 900 s (15 m), but a more reasonable value is 7200 (2
hours). There is no default, since the filter is not enabled unless this command
is given.
panic panic
The argument is the panic threshold, normally 1000 s. If set to zero, the panic
sanity check is disabled and a clock offset of any value will be accepted.
step step
The argument is the step threshold, which by default is 0.128 sec. It can be set
to any positive number in seconds. If set to zero, step adjustments will never
occur. Note: The kernel time discipline is disabled if the step threshold is set
to zero or greater than the default.
stepback stepback
The argument is the step threshold for the backward direction, which by default is
0.128 sec. It can be set to any positive number in seconds. If both the forward
and backward step thresholds are set to zero, step adjustments will never occur.
Note: The kernel time discipline is disabled if each direction of step threshold
are either set to zero or greater than .5 second.
stepfwd stepfwd
As for stepback, but for the forward direction.
stepout stepout
The argument is the stepout timeout, which by default is 900 s. It can be set to
any positive number in seconds. If set to zero, the stepout pulses will not be
suppressed.
rlimit [memlock megabytes | stacksize 4kPages | filenum filedescriptors]
memlock megabytes
Ignored for backward compatibility.
stacksize 4kPages
Specifies the maximum size of the process stack on systems with the mlockall()
function. Defaults to 50 4k pages.
filenum filedescriptors
Specifies the maximum number of file descriptors ntpd may have open at once.
Defaults to the system default.
FILES
/etc/ntp.conf
the default name of the configuration file
ntp.keys
private keys
One of the following exit values will be returned:
0 (EXIT_SUCCESS)
Successful program execution.
1 (EXIT_FAILURE)
The operation failed or the command syntax was not valid.
SEE ALSO
ntpd(8), ntpq(1).
In addition to the manual pages provided, comprehensive documentation is available on the
world wide web at https://www.ntpsec.org. A snapshot of this documentation is available in
HTML format in /usr/share/doc/ntp.
David L. Mills, Network Time Protocol (Version 4), RFC 5905
BUGS
The syntax checking is not picky; some combinations of ridiculous and even hilarious
options and modes may not be detected.