NTP.CONF(5) BSD File Formats Manual NTP.CONF(5)
NAME
ntp.conf -- Network Time Protocol (NTP) daemon configuration file
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. Com-mands Commands
mands 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.
The rest of this page describes the configuration and control options. The "Notes on Configuring NTP
and Setting up a NTP Subnet" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp) contains an extended discussion of these options. In addition to the discussion of
general Configuration Support, there are sections describing the following supported functionality and
the options used to control it:
oo Authentication Support
oo Monitoring Support
oo Access Control Support
oo Reference Clock Support
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 server, peer, broadcast or manycastclient commands.
Configuration Support
Following is a description of the configuration commands in NTPv4. These commands have the same basic
functions as in NTPv3 and in some cases new functions and new arguments. There are two classes of com-mands: commands:
mands: configuration 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.
Configuration Commands
The various modes are determined by the command keyword and the type of the required IP address.
Addresses are classed by type as (s) a remote server or peer (IP class A, B and C), (b) the broadcast
address of a local interface, (m) a multicast address (IP class D), or (r) a reference clock address
(127.127.x.x). Note that 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.
server address [key key | autokey] [burst] [iburst] [version version] [prefer] [minpoll minpoll]
[maxpoll maxpoll] [noselect] [preempt]
peer address [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]
[noselect]
broadcast address [key key | autokey] [version version] [prefer] [minpoll minpoll] [ttl ttl]
manycastclient address [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll
maxpoll] [ttl ttl]
These four commands specify the time server name or address to be used and the mode in which to oper-ate. operate.
ate. The address can be either a DNS name or an IP address in dotted-quad notation. Additional infor-mation information
mation on association behavior can be found in the "Association Management" page.
server For type s and r 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 synchro-nized synchronized
nized to the remote server, but the remote server can never be synchronized to the local 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 associa-tion association
tion 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 associa-tion. association.
tion. 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
224.0.1.1 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 manycast client mode association for the
multicast address specified. In this case 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 manycastserver command specifies that the local server 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.
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 Authentication Support.
burst when the server is reachable and at each poll interval, send a burst of eight packets instead
of the usual one packet. The spacing between the first and the second packets is about 16s to
allow a modem call to complete, while the spacing between the remaining packets is about 2s.
This is designed to improve timekeeping quality with the server command and s addresses.
iburst When the server is unreachable and at each poll interval, send a burst of eight packets instead
of the usual one. As long as the server is unreachable, the spacing between packets is about
16s to allow a modem call to complete. Once the server is reachable, the spacing between pack-ets packets
ets is about 2s. This is designed to speed the initial synchronization acquisition with the
server command and s addresses 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, in seconds to
the 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).
noselect
Marks the server as unused, except for display purposes. The server is discarded by the selec-tion selection
tion 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 syn-chronization synchronization
chronization among a set of correctly operating hosts.
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) sig-nals. signals.
nals. 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. See the "Mitigation Rules and the prefer Keyword" page for further information.
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 admin-istrator. administrator.
istrator.
version version
Specifies the version number to be used for outgoing NTP packets. Versions 1-4 are the
choices, with version 4 the default.
Auxiliary Commands
broadcastclient
This command enables reception of broadcast server messages to any local interface (type b)
address. Upon receiving a message for the first time, the broadcast client measures the nomi-nal nominal
nal server propagation delay using a brief client/server exchange with the server, then enters
the broadcast client mode, in which it synchronizes to succeeding broadcast 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
Authentication Support.
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, but 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 Authentication
Support.
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
Authentication Support.
Authentication Support
Authentication support allows the NTP client to verify that the server is in fact known and trusted and
not an intruder intending accidentally or on purpose to masquerade as that server. The NTPv3 specifi-cation specification
cation RFC-1305 defines a scheme which provides cryptographic authentication of received NTP packets.
Originally, this was done using the Data Encryption Standard (DES) algorithm operating in Cipher Block
Chaining (CBC) mode, commonly called DES-CBC. Subsequently, this was augmented by the RSA Message
Digest 5 (MD5) algorithm using a private key, commonly called keyed-MD5. Either algorithm computes a
message digest, or one-way hash, which can be used to verify the server has the correct private key and
key identifier.
NTPv4 retains the NTPv3 schemes, properly described as symmetric-key cryptography and, in addition,
provides a new Autokey scheme based on public-key cryptography. Public-key cryptography is generally
considered more secure than symmetric-key cryptography, since the security is based on a private value
which is generated by each server and never revealed. With Autokey all key distribution and management
functions involve only public values, which considerably simplifies key distribution and storage.
Authentication is configured separately for each association using the key or autokey subcommands on
the peer, server, broadcast and manycastclient commands as described in Configuration Options. The
authentication options described below specify the suite of keys, select the key for each configured
association and manage the configuration operations.
The auth flag controls whether new associations or remote configuration commands require cryptographic
authentication. This flag can be set or reset by the enable and disable configuration commands and
also by remote configuration commands sent by a ntpdc(8) program running in another machine. If this
flag is enabled, which is the default case, new broadcast client and symmetric passive associations and
remote configuration commands must be cryptographically authenticated using either symmetric-key or
public-key schemes. If this flag is disabled, these operations are effective even if not cryptographic
authenticated. It should be understood that operating in the latter mode invites a significant vulner-ability vulnerability
ability where a rogue hacker can seriously disrupt client timekeeping.
In networks with firewalls and large numbers of broadcast clients it may be acceptable to disable
authentication, since that avoids key distribution and simplifies network maintenance. However, when
the configuration file contains host names, or when a server or client is configured remotely, host
names are resolved using the DNS and a separate name resolution process. In order to protect against
bogus name server messages, name resolution messages are authenticated using an internally generated
key which is normally invisible to the user. However, if cryptographic support is disabled, the name
resolution process will fail. This can be avoided either by specifying IP addresses instead of host
names, which is generally inadvisable, or by enabling the flag for name resolution and disabled it once
the name resolution process is complete.
An attractive alternative where multicast support is available is manycast mode, in which clients peri-odically periodically
odically troll for servers. Cryptographic authentication in this mode uses public-key schemes as
described below. The principle advantage of this manycast mode is that potential servers need not be
configured in advance, since the client finds them during regular operation, and the configuration
files for all clients can be identical.
In addition to the default symmetric-key cryptographic support, support for public-key cryptography is
available if the requisite rsaref20 software distribution has been installed before building the dis-tribution. distribution.
tribution. Public-key cryptography provides secure authentication of servers without compromising
accuracy and stability. The security model and protocol schemes for both symmetric-key and public-key
cryptography are described below.
Symmetric-Key Scheme
The original RFC-1305 specification allows any one of possibly 65,534 keys, each distinguished by a
32-bit key identifier, to authenticate an association. The servers and clients involved must agree on
the key and key identifier to authenticate their messages. Keys and related information are specified
in a key file, usually called ntp.keys, which should be exchanged and stored using secure procedures
beyond the scope of the NTP protocol itself. Besides the keys used for ordinary NTP associations,
additional keys can be used as passwords for the ntpq(8) and ntpdc(8) utility programs.
When ntpd(8) is first started, it reads the key file specified in the keys command and installs the
keys in the key cache. However, the keys must be activated with the trusted command before use. This
allows, for instance, the installation of possibly several batches of keys and then activating or deac-tivating deactivating
tivating each batch remotely using ntpdc(8). This also provides a revocation capability that can be
used if a key becomes compromised. The requestkey command selects the key used as the password for the
ntpdc(8) utility, while the controlkey command selects the key used as the password for the ntpq(8)
utility.
Public-Key Scheme
The original NTPv3 authentication scheme described in RFC-1305 continues to be supported; however, in
NTPv4 an additional authentication scheme called Autokey is available. It uses MD5 message digest, RSA
public-key signature and Diffie-Hellman key agreement algorithms available from several sources, but
not included in the NTPv4 software distribution. In order to be effective, the rsaref20 package must
be installed as described in the README.rsa file. Once installed, the configure and build process
automatically detects it and compiles the routines required.
The Autokey scheme has several modes of operation corresponding to the various NTP modes supported.
RSA signatures with timestamps are used in all modes to verify the source of cryptographic values. All
modes use a special cookie which can be computed independently by the client and server. In symmetric
modes the cookie is constructed using the Diffie-Hellman key agreement algorithm. In other modes the
cookie is constructed from the IP addresses and a private value known only to the server. All modes
use in addition a variant of the S-KEY scheme, in which a pseudo-random key list is generated and used
in reverse order. These schemes are described along with an executive summary, current status, brief-ing briefing
ing slides and reading list, in the "Autonomous Authentication" page.
The cryptographic values used by the Autokey scheme are incorporated as a set of files generated by the
ntp-genkeys(8) program, including the symmetric private keys, public/private key pair, and the agree-ment agreement
ment parameters. See the ntp.keys(5) page for a description of the formats of these files. They con-tain contain
tain cryptographic values generated by the algorithms of the rsaref20 package and are in printable
ASCII format. All file names include the timestamp, in NTP seconds, following the default names given
below. Since the file data are derived from random values seeded by the system clock and the file name
includes the timestamp, every generation produces a different file and different file name.
The ntp.keys file contains the DES/MD5 private keys. It must be distributed by secure means to other
servers and clients sharing the same security compartment and made visible only to root. While this
file is not used with the Autokey scheme, it is needed to authenticate some remote configuration com-mands commands
mands used by the ntpdc(8), ntpq(8) utilities. The ntpkey file contains the RSA private key. It is
useful only to the machine that generated it and never shared with any other daemon or application pro-gram, program,
gram, so must be made visible only to root.
The ntp_dh file contains the agreement parameters, which are used only in symmetric (active and pas-sive) passive)
sive) modes. It is necessary that both peers beginning a symmetric-mode association share the same
parameters, but it does not matter which ntp_dh file generates them. If one of the peers contains the
parameters, the other peer obtains them using the Autokey protocol. If both peers contain the parame-ters, parameters,
ters, the most recent copy is used by both peers. If a peer does not have the parameters, they will be
requested by all associations, either configured or not; but, none of the associations can proceed
until one of them has received the parameters. Once loaded, the parameters can be provided on request
to other clients and servers. The ntp_dh file can be also be distributed using insecure means, since
the data are public values.
The ntpkey_host file contains the RSA public key, where host is the name of the host. Each host must
have its own ntpkey_host file, which is normally provided to other hosts using the Autokey protocol.
Each server or peer association requires the public key associated with the particular server or peer
to be loaded either directly from a local file or indirectly from the server using the Autokey proto-col. protocol.
col. These files can be widely distributed and stored using insecure means, since the data are public
values.
The optional ntpkey_certif_host file contains the PKI certificate for the host. This provides a bind-ing binding
ing between the host hame and RSA public key. In the current implementation the certificate is
obtained by a client, if present, but the contents are ignored.
Due to the widespread use of interface-specific naming, the host names used in configured and mobilized
associations are determined by the UNIX gethostname(3) library routine. Both the ntp-genkeys(8) pro-gram program
gram and the Autokey protocol derive the name of the public key file using the name returned by this
routine. While every server and client is required to load their own public and private keys, the pub-lic public
lic keys for each client or peer association can be obtained from the server or peer using the Autokey
protocol. Note however, that at the current stage of development the authenticity of the server or
peer and the cryptographic binding of the server name, address and public key is not yet established by
a certificate authority or web of trust.
Leapseconds Table
The NIST provides a table showing the epoch for all historic occasions of leap second insertion since
1972. The leapsecond table shows each epoch of insertion along with the offset of International Atomic
Time (TAI) with respect to Coordinated Universal Time (UTC), as disseminated by NTP. The table can be
obtained directly from NIST national time servers using FTP as the ASCII file pub/leap-seconds.
While not strictly a security function, the Autokey scheme provides means to securely retrieve the
leapsecond table from a server or peer. Servers load the leapsecond table directly from the file spec-ified specified
ified in the crypto command, while clients can load the table indirectly from the servers using the
Autokey protocol. Once loaded, the table can be provided on request to other clients and servers.
Key Management
All key files are installed by default in /usr/local/etc, which is normally in a shared file system in
NFS-mounted networks and avoids installing them in each machine separately. The default can be over-ridden overridden
ridden by the keysdir configuration command. However, this is not a good place to install the private
key file, since each machine needs its own file. A suitable place to install it is in /etc, which is
normally not in a shared file system.
The recommended practice is to keep the timestamp extensions when installing a file and to install a
link from the default name (without the timestamp extension) to the actual file. This allows new file
generations to be activated simply by changing the link. However, ntpd(8) parses the link name when
present to extract the extension value and sends it along with the public key and host name when
requested. This allows clients to verify that the file and generation time are always current. How-ever, However,
ever, the actual location of each file can be overridden by the crypto configuration command.
All cryptographic keys and related parameters should be regenerated on a periodic and automatic basis,
like once per month. The ntp-genkeys(8) program uses the same timestamp extension for all files gener-ated generated
ated at one time, so each generation is distinct and can be readily recognized in monitoring data.
While a public/private key pair must be generated by every server and client, the public keys and
agreement parameters do not need to be explicitly copied to all machines in the same security compart-ment, compartment,
ment, since they can be obtained automatically using the Autokey protocol. However, it is necessary
that all primary servers have the same agreement parameter file. The recommended way to do this is for
one of the primary servers to generate that file and then copy it to the other primary servers in the
same compartment using the UNIX rdist(1) command. Future versions of the Autokey protocol are to con-tain contain
tain provisions for an agreement protocol to do this automatically.
Servers and clients can make a new generation in the following way. All machines have loaded the old
generation at startup and are operating normally. At designated intervals, each machine generates a
new public/private key pair and makes links from the default file names to the new file names. The
ntpd(8) is then restarted and loads the new generation, with result clients no longer can authenticate
correctly. The Autokey protocol is designed so that after a few minutes the clients time out and
restart the protocol from the beginning, with result the new generation is loaded and operation contin-ues continues
ues as before. A similar procedure can be used for the agreement parameter file, but in this case pre-cautions precautions
cautions must be take to be sure that all machines with this file have the same copy.
Authentication Commands
autokey [logsec]
Specifies the interval between regenerations of the session key list used with the Autokey pro-tocol. protocol.
tocol. 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 regener-ated regenerated
ated for every message sent.
controlkey key
Specifies the key identifier to use with the ntpq(8) utility, which uses the standard protocol
defined in RFC-1305. The key argument is the key identifier for a trusted key, where the value
can be in the range 1 to 65534, inclusive.
crypto [flags flags] [privatekey file] [publickey file] [dhparms file] [leap file]
This command requires the NTP daemon build process be configured with the RSA library. This
command activates public-key cryptography and loads the required RSA private and public key
files and the optional Diffie-Hellman agreement parameter file, if present. If one or more
files are left unspecified, the default names are used as described below. Following are the
subcommands:
privatekey file
Specifies the location of the RSA private key file, which otherwise defaults to
/usr/local/etc/ntpkey.
publickey file
Specifies the location of the RSA public key file, which otherwise defaults to
/usr/local/etc/ntpkey_host, where host is the name of the generating machine.
dhparms file
Specifies the location of the Diffie-Hellman parameters file, which otherwise defaults
to /usr/local/etc/ntpkey_dh.
leap file
Specifies the location of the leapsecond table file, which otherwise defaults to
/usr/local/etc/ntpkey_leap.
keys keyfile
Specifies the location of the DES/MD5 private key file containing the keys and key identifiers
used by ntpd(8), ntpq(8) and ntpdc(8) when operating in symmetric-key mode.
keysdir path
This command requires the NTP daemon build process be configured with the RSA library. It
specifies the default directory path for the private key file, agreement parameters file and
one or more public key files. The default when this command does not appear in the configura-tion configuration
tion file is /usr/local/etc.
requestkey key
Specifies the key identifier to use with the ntpdc(8) utility program, which uses a proprietary
protocol specific to this implementation of ntpd(8). The key argument is a key identifier for
the trusted key, where the value can be in the range 1 to 65534, inclusive.
revoke logsec
Specifies the interval between re-randomization of certain cryptographic values used by the
Autokey scheme, as a power of 2 in seconds. These values need to be updated frequently in
order to deflect brute-force attacks on the algorithms of the scheme; however, updating some
values is a relatively expensive operation. The default interval is 16 (65,536 s or about 18
hours). For poll intervals above the specified interval, the values will be updated for every
message sent.
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(8) and ntpdc(8) programs. 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, four kinds of name statistics are supported.
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 0.013380 6
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 disci-pline discipline
pline time constant.
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 gen-eration generation
eration set named peerstats:
48773 10847.650 127.127.4.1 9714 -0.001605 0.00000 0.00142
The first two fields show the date (Modified Julian Day) and time (seconds and fraction
past UTC midnight). The next two fields show the peer address in dotted-quad notation
and status, respectively. The status field is encoded in hex in the format described
in Appendix A of the NTP specification RFC 1305. The final three fields show the off-set, offset,
set, delay and RMS jitter, all in seconds.
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 525.624 127.127.4.1 93 226 00:08:29.606 D
The first two fields show the date (Modified Julian Day) and time (seconds and fraction
past UTC midnight). The next field shows the clock address in dotted-quad notation.
The final field shows the last timecode received from the clock in decoded ASCII for-mat, format,
mat, 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.
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 config-ured. configured.
ured. Each NTP message received from a peer or clock driver appends a line of the fol-lowing following
lowing form to the file generation set named rawstats:
50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
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 address followed
by the local address in dotted-quad notation. The final four fields show the origi-nate, originate,
nate, 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.
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 han-dling handling
dling 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
administrational operations without the risk of disturbing the operation of ntpd(8). (Most
important: they can be removed to free space for new data produced.) Note that this command
can be sent from the ntpdc(8) 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 compiletime
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 inter-vening intervening
vening `/' (slash)). 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 file system hierarchy denoted by pre-fix. prefix.
fix.
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(8) 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 `.' (dot) to
concatenated prefix and filename strings, and appending the decimal representa-tion representation
tion 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
`.' (dot) 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 Ql 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 con-sists consists
sists 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.
Information is only written to a file generation by specifying enable; output
is prevented by specifying disable.
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 suf-fix suffix
fix 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(8) 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.
Access Control Support
ntpd(8) implements a general purpose address-and-mask based restriction list. The list is sorted by
address and by mask, and the list is searched in this order for matches, with the last match found
defining the restriction flags associated with the incoming packets. The source address of incoming
packets is used for the match, with the 32- bit address being and'ed with the mask associated with the
restriction entry and then compared with the entry's address (which has also been and'ed with the mask)
to look for a match. Additional information and examples can be found in the "Notes on Configuring NTP
and Setting up a NTP Subnet" page.
The restriction facility was implemented in conformance with the access policies for the original
NSFnet backbone time servers. While this facility may be otherwise useful for keeping unwanted or bro-ken broken
ken remote time servers from affecting your own, it should not be considered an alternative to the
standard NTP authentication facility. Source address based restrictions are easily circumvented by a
determined cracker.
The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further action except incrementing sta-tistics statistics
tistics counters. Sometimes a more proactive response is needed, such as a server message that explic-itly explicitly
itly 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 packet. If the kod flag is set and
either service is denied or the client limit is exceeded, the server returns the packet and sets the
leap bits unsynchronized, stratum zero and the ASCII string "DENY" in the reference source identifier
field. If the kod flag is not set, the server simply drops the packet.
A client or peer receiving a kiss-of-death packet performs a set of sanity checks to minimize security
exposure. If this is the first packet received from the server, the client assumes an access denied
condition at the server. It updates the stratum and reference identifier peer variables and sets the
access denied (test 4) bit in the peer flash variable. If this bit is set, the client sends no packets
to the server. If this is not the first packet, the client assumes a client limit condition at the
server, but does not update the peer variables. In either case, a message is sent to the system log.
Access Control Commands
restrict numeric_address [mask numeric_mask] [flag ...]
The numeric_address argument, expressed in dotted-quad form, is the address of a host or net-work. network.
work. The mask, also expressed in dotted-quad form, defaults to 255.255.255.255, meaning that
the numeric_address is treated as the address of an individual host. A default entry (address
0.0.0.0, mask 0.0.0.0) is always included and, given the sort algorithm, is always the first
entry in the list. Note that, while numeric_address is normally given in dotted-quad format,
the 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 indi-cates indicates
cates 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 gener-ally generally
ally 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:
kod If access is denied, send a kiss-of-death packet.
ignore Ignore all packets from hosts which match this entry. If this flag is specified nei-ther neither
ther queries nor time server polls will be responded to.
noquery
Ignore all NTP mode 6 and 7 packets (i.e., information queries and configuration
requests) from the source. Time service is not affected.
nomodify
Ignore all NTP mode 6 and 7 packets which attempt to modify the state of the server
(i.e., run time reconfiguration). Queries which return information are permitted.
notrap Decline to provide mode 6 control message trap service to matching hosts. The trap
service is a subsystem of the mode 6 control message protocol which is intended for use
by remote event logging programs.
lowpriotrap
Declare traps set by matching hosts to be low priority. The number of traps a server
can maintain is limited (the current limit is 3). Traps are usually assigned on a
first come, first served basis, with later trap requestors being denied service. This
flag modifies the assignment algorithm by allowing low priority traps to be overridden
by later requests for normal priority traps.
noserve
Ignore NTP packets whose mode is other than 6 or 7. In effect, time service is denied,
though queries may still be permitted.
nopeer Provide stateless time service to polling hosts, but do not allocate peer memory
resources to these hosts even if they otherwise might be considered useful as future
synchronization partners.
notrust
Treat these hosts normally in other respects, but never use them as synchronization
sources.
limited
These hosts are subject to limitation of number of clients from the same net. Net in
this context refers to the IP notion of net (class A, class B, class C, etc.). Only
the first client_limit hosts that have shown up at the server and that have been active
during the last client_limit_period seconds are accepted. Requests from other clients
from the same net are rejected. Only time request packets are taken into account.
Query packets sent by the ntpq(8) and ntpdc(8) programs are not subject to these lim-its. limits.
its. 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.
ntpport
This is actually a match algorithm modifier, rather than a restriction flag. Its pres-ence presence
ence 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.
version
Ignore these hosts if not 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., every-thing everything
thing besides your own NTP server is unrestricted).
clientlimit limit
Set the client_limit variable, which limits the number of simultaneous access-controlled
clients. The default value for this variable is 3.
clientperiod period
Set the client_limit_period variable, which specifies the number of seconds after which a
client is considered inactive and thus no longer is counted for client limit restriction. The
default value for this variable is 3600 seconds.
Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio, satellite and modem reference
clocks plus a special pseudo-clock used for backup or when no other clock source is available.
Detailed descriptions of individual device drivers and options can be found in the "Reference Clock
Drivers" page (available as part of the HTML documentation provided in /usr/share/doc/ntp). Additional
information can be found in the pages linked there, including the "Debugging Hints for Reference Clock
Drivers" and "How To Write a Reference Clock Driver" pages. In addition, support for a PPS signal is
available as described in the "Pulse-per-second (PPS) Signal Interfacing" page. Many drivers support
special line discipline/streams modules which can significantly improve the accuracy using the driver.
These are described in the "Line Disciplines and Streams Drivers" page.
A reference clock will generally (though not always) be a radio timecode receiver which is synchronized
to a source of standard time such as the services offered by the NRC in Canada and NIST and USNO in the
US. The interface between the computer and the timecode receiver is device dependent, but is usually a
serial port. A device driver specific to each reference clock must be selected and compiled in the
distribution; however, most common radio, satellite and modem clocks are included by default. Note
that an attempt to configure a reference clock when the driver has not been compiled or the hardware
port has not been appropriately configured results in a scalding remark to the system log file, but is
otherwise non hazardous.
For the purposes of configuration, ntpd(8) treats reference clocks in a manner analogous to normal NTP
peers as much as possible. Reference clocks are identified by a syntactically correct but invalid IP
address, in order to distinguish them from normal NTP peers. Reference clock addresses are of the form
127.127.t.u, where t is an integer denoting the clock type and u indicates the unit number in the range
0-3. While it may seem overkill, it is in fact sometimes useful to configure multiple reference clocks
of the same type, in which case the unit numbers must be unique.
The server command is used to configure a reference clock, where the address argument in that command
is the clock address. The key, version and ttl options are not used for reference clock support. The
mode option is added for reference clock support, as described below. The prefer option can be useful
to persuade the server to cherish a reference clock with somewhat more enthusiasm than other reference
clocks or peers. Further information on this option can be found in the "Mitigation Rules and the
prefer Keyword" page. The minpoll and maxpoll options have meaning only for selected clock drivers.
See the individual clock driver document pages for additional information.
The fudge command is used to provide additional information for individual clock drivers and normally
follows immediately after the server command. The address argument specifies the clock address. The
refid and stratum options can be used to override the defaults for the device. There are two optional
device-dependent time offsets and four flags that can be included in the fudge command as well.
The stratum number of a reference clock is by default zero. Since the ntpd(8) daemon adds one to the
stratum of each peer, a primary server ordinarily displays an external stratum of one. In order to
provide engineered backups, it is often useful to specify the reference clock stratum as greater than
zero. The stratum option is used for this purpose. Also, in cases involving both a reference clock
and a pulse-per-second (PPS) discipline signal, it is useful to specify the reference clock identifier
as other than the default, depending on the driver. The refid option is used for this purpose. Except
where noted, these options apply to all clock drivers.
Reference Clock Commands
server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
This command can be used to configure reference clocks in special ways. The options are inter-preted interpreted
preted as follows:
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. See the
"Mitigation Rules and the prefer Keyword" page for further information.
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 device subtype in the
parse drivers.
minpoll int
maxpoll int
These options specify the minimum and maximum polling interval for reference clock mes-sages, messages,
sages, in seconds to the power of two. For most directly connected reference clocks,
both minpoll and maxpoll default to 6 (64 s). For modem reference clocks, minpoll
defaults to 10 (17.1 m) and maxpoll defaults to 14 (4.5 h). The allowable range is 4
(16 s) to 17 (36.4 h) inclusive.
fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [mode int] [flag1 0 | 1] [flag2
0 | 1] [flag3 0 | 1] [flag4 0 | 1]
This command can be used to configure reference clocks in special ways. It must immediately
follow the server command which configures the driver. Note that the same capability is possi-ble possible
ble at run time using the ntpdc(8) program. The options are interpreted as follows:
time1 sec
Specifies a constant to be added to the time offset produced by the driver, a fixed-point fixedpoint
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 DIPswitches. Where a calibration for an individual
system and driver is available, an approximate correction is noted in the driver docu-mentation documentation
mentation 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 driverdependent
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.
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 device subtype in the
parse drivers.
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
broadcastdelay seconds
The broadcast and multicast modes require a special calibration to determine the network delay
between the local and remote servers. Ordinarily, this is done automatically by the initial
protocol exchanges between the client and server. In some cases, the calibration procedure may
fail due to network or server access controls, for example. This command specifies the default
delay to be used under these circumstances. Typically (for Ethernet), a number between 0.003
and 0.007 seconds is appropriate. The default when this command is not used is 0.004 seconds.
driftfile driftfile
This command specifies the name of the file used 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 fre-quency frequency
quency 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 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(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 | bclient | calibrate | kernel | monitor | ntp | stats]
disable [auth | bclient | calibrate | kernel | monitor | ntp | stats]
Provides a way to enable or disable various server options. Flags not mentioned are unaf-fected. unaffected.
fected. Note that all of these flags can be controlled remotely using the ntpdc(8) utility
program.
bclient
When enabled, this is identical to the broadcastclient command. The default for this
flag is disable.
calibrate
Enables the calibration facility, which automatically adjusts the time1 values for each
clock driver to display the same offset as the currently selected source or kernel dis-cipline discipline
cipline signal. See the "Reference Clock Drivers" page for further information. The
default for this flag is disable.
kernel Enables the precision-time kernel support for the adjtime(2) system call, if imple-mented. implemented.
mented. Ordinarily, support for this routine is detected automatically when the NTP
daemon is compiled, so it is not necessary for the user to worry about this flag. It
is provided primarily so that this support can be disabled during kernel development.
The default for this flag is enable.
monitor
Enables the monitoring facility. See the ntpdc(8) program and the monlist command or
further information. The default for this flag is enable.
ntp Enables the server to adjust its local clock by means of NTP. If disabled, the local
clock free-runs at its intrinsic time and frequency offset. This flag is useful in
case the local clock is controlled by some other device or protocol and NTP is used
only to provide synchronization to other clients. In this case, the local clock driver
can be used to provide this function and also certain time variables for error esti-mates estimates
mates and leap-indicators. See the "Reference Clock Drivers" page for further informa-tion. information.
tion. The default for this flag is enable.
stats Enables the statistics facility. See the "Monitoring Options" page for further infor-mation. information.
mation. The default for this flag is enable.
logconfig configkeyword
This command controls the amount and type of output written to the system syslog(3) facility or
the alternate logfile log file. By default, all output is turned on. All configkeyword key-words keywords
words 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. Informa-tional Informational
tional messages (info) control configuration information. Event messages (events) control log-ging logging
ging of events (reachability, synchronization, alarm conditions). Statistical output is con-trolled controlled
trolled with the statistics keyword. The final message group is the status messages. This
describes mainly the synchronizations status. Configuration keywords are formed by concatenat-ing concatenating
ing 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.
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(8) 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 vari-ables variables
ables 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 vari-ables variables
ables and the clock_var_list holds the names of the reference clock variables.
tinker [step step | panic panic | dispersion dispersion | stepout stepout | minpoll minpoll | allan
allan | huffpuff huffpuff]
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 can't resist twisting the knobs anyway and this com-mand command
mand is for them. Emphasis added: twisters are on their own and can expect no help from the
support group.
All arguments are in floating point seconds or seconds per second. The minpoll argument is an
integer in seconds to the power of two. The variables operate as follows:
step step
The argument becomes the new value for the step threshold, normally 0.128 s. If set to
zero, step adjustments will never occur. In general, if the intent is only to avoid
step adjustments, the step threshold should be left alone and the -x command line
option be used instead.
panic panic
The argument becomes the new value for 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.
dispersion dispersion
The argument becomes the new value for the dispersion increase rate, normally .000015.
stepout stepout
The argument becomes the new value for the watchdog timeout, normally 900 s.
minpoll minpoll
The argument becomes the new value for the minimum poll interval used when configuring
multicast client, manycast client and , symmetric passive mode association. The value
defaults to 6 (64 s) and has a lower limit of 4 (16 s).
allan allan
The argument becomes the new value for the minimum Allan intercept, which is a parame-ter parameter
ter of the PLL/FLL clock discipline algorithm. The value defaults to 1024 s, which is
also the lower limit.
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.
trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host address and port number for sending
messages with the specified local interface address. If the port number is unspecified, a
value of 18447 is used. If the interface address is not specified, the message is sent with a
source address of the local interface the message is sent through. Note that on a multihomed
host the interface used may vary from time to time with routing changes.
The trap receiver will generally log event messages and other information from the server in a
log file. While such monitor programs may also request their own trap dynamically, configuring
a trap receiver will ensure that no messages are lost when the server is started.
FILES
/etc/ntp.conf the default name of the configuration file
ntp.keys private MD5 keys
ntpkey RSA private key
ntpkey_host RSA public key
ntp_dh Diffie-Hellman agreement parameters
SEE ALSO
ntpd(8), ntpdc(8), ntpq(8)
In addition to the manual pages provided, comprehensive documentation is available on the world wide
web at http://www.ntp.org/ A snapshot of this documentation is available in HTML format in
/usr/share/doc/ntp.
David L. Mills, Network Time Protocol (Version 3), RFC1305.
BUGS
The syntax checking is not picky; some combinations of ridiculous and even hilarious options and modes
may not be detected.
The ntpkey_host files are really digital certificates. These should be obtained via secure directory
services when they become universally available.
BSD June 8, 2006 BSD
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