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NTP_CONF(5) File Formats Manual NTP_CONF(5)

NAME
ntp.conf -- Network Time Protocol (NTP) daemon configuration file format

SYNOPSIS
ntp.conf [--option-name] [--option-name value]

All arguments must be options.

DESCRIPTION
The ntp.conf configuration file is read at initial startup by the ntpd(1nt-
pdmdoc) 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 be-
gin 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 white-
space. Commands may not be continued over multiple lines. Arguments may
be host names, host addresses written in numeric, dotted-quad form, inte-
gers, 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 an 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 Options", there are sections describing the follow-
ing 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, 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 commands, configura-
tion commands that configure a persistent association with a remote server
or peer or reference clock, and auxiliary commands that specify environmen-
tal 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 (IPv4 class A, B and C), (b) the broadcast address of a local in-
terface, (m) a multicast address (IPv4 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.

If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is detected,
support for the IPv6 address family is generated in addition to the default
support of the IPv4 address family. In a few cases, including the reslist
billboard generated by ntpq(1ntpqmdoc) or ntpdc(1ntpdcmdoc), IPv6 addresses
are automatically generated. IPv6 addresses can be identified by the pres-
ence of colons ":" in the address field. IPv6 addresses can be used almost
everywhere where IPv4 addresses can be used, with the exception of refer-
ence clock addresses, which are always IPv4.

Note that in contexts where a host name is expected, a -4 qualifier preced-
ing the host name forces DNS resolution to the IPv4 namespace, while a -6
qualifier forces DNS resolution to the IPv6 namespace. See IPv6 references
for the equivalent classes for that address family.

pool address [burst] [iburst] [version version] [prefer] [minpoll minpoll]
[maxpoll maxpoll] [xmtnonce]

server address [key key | autokey] [burst] [iburst] [version version]
[prefer] [minpoll minpoll] [maxpoll maxpoll] [true] [xmtnonce]

peer address [key key | autokey] [version version] [prefer] [minpoll
minpoll] [maxpoll maxpoll] [true] [xleave]

broadcast address [key key | autokey] [version version] [prefer] [minpoll
minpoll] [ttl ttl] [xleave]

manycastclient address [key key | autokey] [version version] [prefer]
[minpoll minpoll] [maxpoll maxpoll] [ttl ttl]

These five 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. Additional information on association
behavior can be found in the "Association Management" page (available as
part of the HTML documentation provided in /usr/share/doc/ntp).

pool For type s 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 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 synchronized 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 association with the specified remote peer.
In this mode the local clock can be synchronized to the remote peer
or the remote peer can be synchronized to the local clock. This is
useful in a network of servers where, depending on various failure
scenarios, either the local or remote peer may be the better source
of time. This command should NOT be used for type b, m or r ad-
dresses.

broadcast
For type b and m addresses (only), this command mobilizes a persis-
tent broadcast mode association. Multiple commands can be used to
specify multiple local broadcast interfaces (subnets) and/or multi-
ple multicast groups. Note that local broadcast messages go only
to the interface associated with the subnet specified, but multi-
cast messages go to all interfaces. In broadcast mode the local
server sends periodic broadcast messages to a client population at
the address specified, which is usually the broadcast address on
(one of) the local network(s) or a multicast address assigned to
NTP. The IANA has assigned the multicast group address IPv4
224.0.1.1 and IPv6 ff05::101 (site local) exclusively to NTP, but
other nonconflicting addresses can be used to contain the messages
within administrative boundaries. Ordinarily, this specification
applies only to the local server operating as a sender; for opera-
tion 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 ad-
dress used on the manycastserver command for the designated many-
cast 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 caus-
ing a possibly massive implosion of replies at the sender. The
manycastserver command specifies that the local server is to oper-
ate 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 speci-
fied address and specifically enabled servers respond to these mes-
sages. The client selects the servers providing the best time and
continues as with the server command. The remaining servers are
discarded as if never heard.

Options:

autokey
All packets sent to and received from the server or peer are to in-
clude authentication fields encrypted using the autokey scheme de-
scribed in "Authentication Options".

burst when the server is reachable, send a burst of six packets instead
of the usual one. The packet spacing is 2 s. This is designed to
improve timekeeping quality with the server command and s ad-
dresses.

iburst When the server is unreachable, send a burst of eight packets in-
stead of the usual one. The packet spacing is 2 s. This is de-
signed to speed the initial synchronization acquisition with the
server command and s addresses and when ntpd(1ntpdmdoc) is started
with the -q option.

key key
All packets sent to and received from the server or peer are to in-
clude authentication fields encrypted using the specified key iden-
tifier with values from 1 to 65535, 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 op-
tion 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 selection algroithm.

preempt
Says the association can be preempted.

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 (available as part of the HTML documentation provided in
/usr/share/doc/ntp) for further information.

true Marks the server as a truechimer, forcing the association to always
survive the selection and clustering algorithms. This option
should almost certainly only be used while testing an association.

ttl ttl
This option is used only with broadcast server and manycast client
modes. It specifies the time-to-live ttl to use on broadcast
server and multicast server and the maximum ttl for the expanding
ring search with manycast client packets. Selection of the proper
value, which defaults to 127, is something of a black art and
should be coordinated with the network administrator.

version version
Specifies the version number to be used for outgoing NTP packets.
Versions 1-4 are the choices, with version 4 the default.

xleave Valid in peer and broadcast modes only, this flag enables inter-
leave mode.

xmtnonce
Valid only for server and pool modes, this flag puts a random num-
ber in the packet's transmit timestamp.

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 nominal server propa-
gation 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 acci-
dental or malicious disruption in this mode, both the server and
client should operate using symmetric-key or public-key authentica-
tion as described in "Authentication Options".

manycastserver address ...
This command enables reception of manycast client messages to the
multicast group address(es) (type m) specified. At least one ad-
dress 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 Options".

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 nomi-
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 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 Options".

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 mdnstries times. After all, ntpd may be starting before
mDNS. The default value for mdnstries is 5.

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 specification RFC-1305 de-
fines 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 replaced 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 scheme, properly described as symmetric key cryp-
tography and, in addition, provides a new Autokey scheme based on public
key cryptography. Public key cryptography is generally considered more se-
cure than symmetric key cryptography, since the security is based on a pri-
vate value which is generated by each server and never revealed. With Au-
tokey all key distribution and management functions involve only public
values, which considerably simplifies key distribution and storage. Public
key management is based on X.509 certificates, which can be provided by
commercial services or produced by utility programs in the OpenSSL software
library or the NTPv4 distribution.

While the algorithms for symmetric key cryptography are included in the
NTPv4 distribution, public key cryptography requires the OpenSSL software
library to be installed before building the NTP distribution. Directions
for doing that are on the Building and Installing the Distribution page.

Authentication is configured separately for each association using the key
or autokey subcommand on the peer, server, broadcast and manycastclient
configuration commands as described in "Configuration Options" page. The
authentication options described below specify the locations of the key
files, if other than default, which symmetric keys are trusted and the in-
terval between various operations, if other than default.

Authentication is always enabled, although ineffective if not configured as
described below. If a NTP packet arrives including a message authentica-
tion code (MAC), it is accepted only if it passes all cryptographic checks.
The checks require correct key ID, key value and message digest. If the
packet has been modified in any way or replayed by an intruder, it will
fail one or more of these checks and be discarded. Furthermore, the Au-
tokey scheme requires a preliminary protocol exchange to obtain the server
certificate, verify its credentials and initialize the protocol

The auth flag controls whether new associations or remote configuration
commands require cryptographic authentication. This flag can be set or re-
set by the enable and disable commands and also by remote configuration
commands sent by a ntpdc(1ntpdcmdoc) program running on 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
cryptography. If this flag is disabled, these operations are effective
even if not cryptographic authenticated. It should be understood that op-
erating with the auth flag disabled invites a significant vulnerability
where a rogue hacker can masquerade as a falseticker and seriously disrupt
system timekeeping. It is important to note that this flag has no purpose
other than to allow or disallow a new association in response to new broad-
cast and symmetric active messages and remote configuration commands and,
in particular, the flag has no effect on the authentication process itself.

An attractive alternative where multicast support is available is manycast
mode, in which clients periodically troll for servers as described in the
"Automatic NTP Configuration Options" page. Either symmetric key or public
key cryptographic authentication can be used in this mode. The principle
advantage of 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.

The security model and protocol schemes for both symmetric key and public
key cryptography are summarized below; further details are in the brief-
ings, papers and reports at the NTP project page linked from
http://www.ntp.org/.

Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly 65,535 keys,
each distinguished by a 32-bit key identifier, to authenticate an associa-
tion. The servers and clients involved must agree on the key and key iden-
tifier to authenticate NTP packets. Keys and related information are spec-
ified in a key file, usually called ntp.keys, which must be distributed and
stored using secure means beyond the scope of the NTP protocol itself. Be-
sides the keys used for ordinary NTP associations, additional keys can be
used as passwords for the ntpq(1ntpqmdoc) and ntpdc(1ntpdcmdoc) utility
programs.

When ntpd(1ntpdmdoc) is first started, it reads the key file specified in
the keys configuration command and installs the keys in the key cache.
However, individual 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 deactivating each batch remotely us-
ing ntpdc(1ntpdcmdoc). 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(1ntpdcmdoc) utility, while the
controlkey command selects the key used as the password for the ntpq(1nt-
pqmdoc) utility.

Public Key Cryptography
NTPv4 supports the original NTPv3 symmetric key scheme described in
RFC-1305 and in addition the Autokey protocol, which is based on public key
cryptography. The Autokey Version 2 protocol described on the Autokey Pro-
tocol page verifies packet integrity using MD5 message digests and verifies
the source with digital signatures and any of several digest/signature
schemes. Optional identity schemes described on the Identity Schemes page
and based on cryptographic challenge/response algorithms are also avail-
able. Using all of these schemes provides strong security against replay
with or without modification, spoofing, masquerade and most forms of clog-
ging attacks.

The Autokey protocol has several modes of operation corresponding to the
various NTP modes supported. Most modes use a special cookie which can be
computed independently by the client and server, but encrypted in transmis-
sion. 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,
briefing slides and reading list on the "Autonomous Authentication" page.

The specific cryptographic environment used by Autokey servers and clients
is determined by a set of files and soft links generated by the ntp-key-
gen(1ntpkeygenmdoc) program. This includes a required host key file, re-
quired certificate file and optional sign key file, leapsecond file and
identity scheme files. The digest/signature scheme is specified in the
X.509 certificate along with the matching sign key. There are several
schemes available in the OpenSSL software library, each identified by a
specific string such as md5WithRSAEncryption, which stands for the MD5 mes-
sage digest with RSA encryption scheme. The current NTP distribution sup-
ports all the schemes in the OpenSSL library, including those based on RSA
and DSA digital signatures.

NTP secure groups can be used to define cryptographic compartments and se-
curity hierarchies. It is important that every host in the group be able
to construct a certificate trail to one or more trusted hosts in the same
group. Each group host runs the Autokey protocol to obtain the certifi-
cates for all hosts along the trail to one or more trusted hosts. This re-
quires the configuration file in all hosts to be engineered so that, even
under anticipated failure conditions, the NTP subnet will form such that
every group host can find a trail to at least one trusted host.

Naming and Addressing
It is important to note that Autokey does not use DNS to resolve addresses,
since DNS can't be completely trusted until the name servers have synchro-
nized clocks. The cryptographic name used by Autokey to bind the host
identity credentials and cryptographic values must be independent of inter-
face, network and any other naming convention. The name appears in the
host certificate in either or both the subject and issuer fields, so pro-
tection against DNS compromise is essential.

By convention, the name of an Autokey host is the name returned by the Unix
gethostname(2) system call or equivalent in other systems. By the system
design model, there are no provisions to allow alternate names or aliases.
However, this is not to say that DNS aliases, different names for each in-
terface, etc., are constrained in any way.

It is also important to note that Autokey verifies authenticity using the
host name, network address and public keys, all of which are bound together
by the protocol specifically to deflect masquerade attacks. For this rea-
son Autokey includes the source and destination IP addresses in message di-
gest computations and so the same addresses must be available at both the
server and client. For this reason operation with network address transla-
tion schemes is not possible. This reflects the intended robust security
model where government and corporate NTP servers are operated outside fire-
wall perimeters.

Operation
A specific combination of authentication scheme (none, symmetric key, pub-
lic key) and identity scheme is called a cryptotype, although not all com-
binations are compatible. There may be management configurations where the
clients, servers and peers may not all support the same cryptotypes. A se-
cure NTPv4 subnet can be configured in many ways while keeping in mind the
principles explained above and in this section. Note however that some
cryptotype combinations may successfully interoperate with each other, but
may not represent good security practice.

The cryptotype of an association is determined at the time of mobilization,
either at configuration time or some time later when a message of appropri-
ate cryptotype arrives. When mobilized by a server or peer configuration
command and no key or autokey subcommands are present, the association is
not authenticated; if the key subcommand is present, the association is au-
thenticated using the symmetric key ID specified; if the autokey subcommand
is present, the association is authenticated using Autokey.

When multiple identity schemes are supported in the Autokey protocol, the
first message exchange determines which one is used. The client request
message contains bits corresponding to which schemes it has available. The
server response message contains bits corresponding to which schemes it has
available. Both server and client match the received bits with their own
and select a common scheme.

Following the principle that time is a public value, a server responds to
any client packet that matches its cryptotype capabilities. Thus, a server
receiving an unauthenticated packet will respond with an unauthenticated
packet, while the same server receiving a packet of a cryptotype it sup-
ports will respond with packets of that cryptotype. However, unconfigured
broadcast or manycast client associations or symmetric passive associations
will not be mobilized unless the server supports a cryptotype compatible
with the first packet received. By default, unauthenticated associations
will not be mobilized unless overridden in a decidedly dangerous way.

Some examples may help to reduce confusion. Client Alice has no specific
cryptotype selected. Server Bob has both a symmetric key file and minimal
Autokey files. Alice's unauthenticated messages arrive at Bob, who replies
with unauthenticated messages. Cathy has a copy of Bob's symmetric key
file and has selected key ID 4 in messages to Bob. Bob verifies the mes-
sage with his key ID 4. If it's the same key and the message is verified,
Bob sends Cathy a reply authenticated with that key. If verification
fails, Bob sends Cathy a thing called a crypto-NAK, which tells her some-
thing broke. She can see the evidence using the ntpq(1ntpqmdoc) program.

Denise has rolled her own host key and certificate. She also uses one of
the identity schemes as Bob. She sends the first Autokey message to Bob
and they both dance the protocol authentication and identity steps. If all
comes out okay, Denise and Bob continue as described above.

It should be clear from the above that Bob can support all the girls at the
same time, as long as he has compatible authentication and identity creden-
tials. Now, Bob can act just like the girls in his own choice of servers;
he can run multiple configured associations with multiple different servers
(or the same server, although that might not be useful). But, wise secu-
rity policy might preclude some cryptotype combinations; for instance, run-
ning an identity scheme with one server and no authentication with another
might not be wise.

Key Management
The cryptographic values used by the Autokey protocol are incorporated as a
set of files generated by the ntp-keygen(1ntpkeygenmdoc) utility program,
including symmetric key, host key and public certificate files, as well as
sign key, identity parameters and leapseconds files. Alternatively, host
and sign keys and certificate files can be generated by the OpenSSL utili-
ties and certificates can be imported from public certificate authorities.
Note that symmetric keys are necessary for the ntpq(1ntpqmdoc) and nt-
pdc(1ntpdcmdoc) utility programs. The remaining files are necessary only
for the Autokey protocol.

Certificates imported from OpenSSL or public certificate authorities have
certian limitations. The certificate should be in ASN.1 syntax, X.509 Ver-
sion 3 format and encoded in PEM, which is the same format used by OpenSSL.
The overall length of the certificate encoded in ASN.1 must not exceed 1024
bytes. The subject distinguished name field (CN) is the fully qualified
name of the host on which it is used; the remaining subject fields are ig-
nored. The certificate extension fields must not contain either a subject
key identifier or a issuer key identifier field; however, an extended key
usage field for a trusted host must contain the value trustRoot;. Other
extension fields are ignored.

Authentication Commands
autokey [logsec]
Specifies the interval between regenerations of the session key
list used with the Autokey protocol. Note that the size of the key
list for each association depends on this interval and the current
poll interval. The default value is 12 (4096 s or about 1.1
hours). For poll intervals above the specified interval, a session
key list with a single entry will be regenerated for every message
sent.

controlkey key
Specifies the key identifier to use with the ntpq(1ntpqmdoc) util-
ity, 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 65,535, inclusive.

crypto [cert file] [leap file] [randfile file] [host file] [gq file] [gqpar
file] [iffpar file] [mvpar file] [pw password]
This command requires the OpenSSL library. It activates public key
cryptography, selects the message digest and signature encryption
scheme and loads the required private and public values described
above. If one or more files are left unspecified, the default
names are used as described above. Unless the complete path and
name of the file are specified, the location of a file is relative
to the keys directory specified in the keysdir command or default
/usr/local/etc. Following are the subcommands:

cert file
Specifies the location of the required host public certifi-
cate file. This overrides the link ntpkey_cert_hostname in
the keys directory.

gqpar file
Specifies the location of the optional GQ parameters file.
This overrides the link ntpkey_gq_hostname in the keys di-
rectory.

host file
Specifies the location of the required host key file. This
overrides the link ntpkey_key_hostname in the keys direc-
tory.

iffpar file
Specifies the location of the optional IFF parameters file.
This overrides the link ntpkey_iff_hostname in the keys di-
rectory.

leap file
Specifies the location of the optional leapsecond file.
This overrides the link ntpkey_leap in the keys directory.

mvpar file
Specifies the location of the optional MV parameters file.
This overrides the link ntpkey_mv_hostname in the keys di-
rectory.

pw password
Specifies the password to decrypt files containing private
keys and identity parameters. This is required only if
these files have been encrypted.

randfile file
Specifies the location of the random seed file used by the
OpenSSL library. The defaults are described in the main
text above.

keys keyfile
Specifies the complete path and location of the MD5 key file con-
taining the keys and key identifiers used by ntpd(1ntpdmdoc),
ntpq(1ntpqmdoc) and ntpdc(1ntpdcmdoc) when operating with symmetric
key cryptography. This is the same operation as the -k command
line option.

keysdir path
This command specifies the default directory path for cryptographic
keys, parameters and certificates. The default is /usr/local/etc/.

requestkey key
Specifies the key identifier to use with the ntpdc(1ntpdcmdoc)
utility program, which uses a proprietary protocol specific to this
implementation of ntpd(1ntpdmdoc). The key argument is a key iden-
tifier for the trusted key, where the value can be in the range 1
to 65,535, inclusive.

revoke logsec
Specifies the interval between re-randomization of certain crypto-
graphic values used by the Autokey scheme, as a power of 2 in sec-
onds. These values need to be updated frequently in order to de-
flect brute-force attacks on the algorithms of the scheme; however,
updating some values is a relatively expensive operation. The de-
fault interval is 16 (65,536 s or about 18 hours). For poll inter-
vals 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(1ntpqmdoc) and ntpdc(1ntpdcmdoc) programs.
The authentication procedures require that both the local and re-
mote servers share the same key and key identifier for this pur-
pose, although different keys can be used with different servers.
The key arguments are 32-bit unsigned integers with values from 1
to 65,535.

Error Codes
The following error codes are reported via the NTP control and monitoring
protocol trap mechanism.

101 (bad field format or length) The packet has invalid version, length
or format.

102 (bad timestamp) The packet timestamp is the same or older than the
most recent received. This could be due to a replay or a server
clock time step.

103 (bad filestamp) The packet filestamp is the same or older than the
most recent received. This could be due to a replay or a key file
generation error.

104 (bad or missing public key) The public key is missing, has incor-
rect format or is an unsupported type.

105 (unsupported digest type) The server requires an unsupported di-
gest/signature scheme.

106 (mismatched digest types) Not used.

107 (bad signature length) The signature length does not match the cur-
rent public key.

108 (signature not verified) The message fails the signature check. It
could be bogus or signed by a different private key.

109 (certificate not verified) The certificate is invalid or signed
with the wrong key.

110 (certificate not verified) The certificate is not yet valid or has
expired or the signature could not be verified.

111 (bad or missing cookie) The cookie is missing, corrupted or bogus.

112 (bad or missing leapseconds table) The leapseconds table is miss-
ing, corrupted or bogus.

113 (bad or missing certificate) The certificate is missing, corrupted
or bogus.

114 (bad or missing identity) The identity key is missing, corrupt or
bogus.

Monitoring Support
ntpd(1ntpdmdoc) includes a comprehensive monitoring facility suitable for
continuous, long term recording of server and client timekeeping perfor-
mance. 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 the source
code 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 525.624 127.127.4.1 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 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.

cryptostats
This option requires the OpenSSL cryptographic software li-
brary. It enables recording of cryptographic public key
protocol information. Each message received by the proto-
col module appends a line of the following form to the file
generation set named cryptostats:

49213 525.624 127.127.4.1 message

The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next field shows the peer address in dotted-quad notation,
The final message field includes the message type and cer-
tain ancillary information. See the "Authentication
Options" section for further information.

loopstats
Enables recording of loop filter statistics information.
Each update of the local clock outputs a line of the fol-
lowing form to the file generation set named loopstats:

50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

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 off-
set (parts per million - PPM), RMS jitter (seconds), Allan
deviation (PPM) and clock discipline time constant.

peerstats
Enables recording of peer statistics information. This in-
cludes 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 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674

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 nota-
tion 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 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 config-
ured. Each NTP message received from a peer or clock dri-
ver appends a line of the following form to the file gener-
ation 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 fol-
lowed 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 smooth-
ing 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

The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The re-
maining ten fields show the statistics counter values accu-
mulated since the last generated line.

Time since restart 36000
Time in hours since the system was last rebooted.

Packets received 81965
Total number of packets received.

Packets processed 0
Number of packets received in response to previous
packets sent

Current version 9546
Number of packets matching the current NTP version.

Previous version 56
Number of packets matching the previous NTP ver-
sion.

Bad version 71793
Number of packets matching neither NTP version.

Access denied 512
Number of packets denied access for any reason.

Bad length or format 540
Number of packets with invalid length, format or
port number.

Bad authentication 10
Number of packets not verified as authentic.

Rate exceeded 147
Number of packets discarded due to rate limitation.

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 mod-
ified for file generation sets, which is useful for han-
dling 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 con-
tinuously growing during the lifetime of a server. Server
statistics are a typical example for such files. Genera-
tion 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 op-
erations 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 ntpdc(1ntpdcm-
doc) 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 con-
catenated elements file ... prefix, file ...
filename and file ... 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 genera-
tion 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 out-
side 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 incar-
nation 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 dif-
ferent ntpd(1ntpdmdoc) server incarnations.
The set member filename is built by append-
ing a `.' to concatenated prefix and
filename strings, and appending the decimal
representation of the process ID of the
ntpd(1ntpdmdoc) 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 follow-
ing 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 gener-
ated per month. The file name suffix con-
sists 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 run-
ning at the start of the corresponding
24-hour period. Information is only writ-
ten 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 dis-
abled 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(1ntpdmdoc) 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
The ntpd(1ntpdmdoc) daemon implements a general purpose address/mask based
restriction list. The list contains address/match entries sorted first by
increasing address values and 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 provided in
/usr/share/doc/ntp).

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 fa-
cility may be useful for keeping unwanted or broken or malicious clients
from congesting innocent servers, it should not be considered an alterna-
tive to the NTP authentication facilities. Source address based restric-
tions 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 in-
clude 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 de-
nied 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 ac-
tion 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 spe-
cial packet format has been created for this purpose called the
"kiss-of-death" (KoD) packet. KoD packets have the leap bits set unsyn-
chronized 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 re-
strict 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 crypto-
graphic violation occurs, the code is "CRYP".

A client receiving a KoD performs a set of sanity checks to minimize secu-
rity exposure, then updates the stratum and reference identifier peer vari-
ables, sets the access denied (TEST4) bit in the peer flash variable and
sends a message to the log. As long as the TEST4 bit is set, the client
will send no further packets to the server. The only way at present to re-
cover 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 coop-
erates.

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 in log2 seconds, defaulting to 3
(8s), while the minimum subcommand specifies the minimum packet
spacing in seconds, defaulting to 2. Packets that violate these
minima are discarded and a kiss-o'-death packet returned if en-
abled. The monitor subcommand indirectly specifies the probability
of replacing the oldest entry from the monitor (MRU) list of recent
requests used to enforce rate controls, when that list is at its
maximum size. The probability of replacing the oldest entry is the
age of that entry in seconds divided by the monitor value, default
3000. For example, if the oldest entry in the MRU list represents a
request 300 seconds ago, by default the probability of replacing it
with an entry representing the client request being processed now
is 10%. Conversely, if the oldest entry is more than 3000 seconds
old, the probability is 100%.

restrict address [mask mask] [ippeerlimit int] [flag ...]
The address argument expressed in numeric form is the address of a
host or network. Alternatively, the address argument can be a
valid hostname. When a hostname is provided, a restriction entry
is created for each address the hostname resolves to, and any pro-
vided mask is ignored and an individual host mask is used for each
entry. The mask argument expressed in numeric form defaults to all
bits lit, meaning that the address is treated as the address of an
individual host. A default entry with address and mask all zeroes
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 indi-
cate the default entry. The ippeerlimit directive limits the num-
ber of peer requests for each IP to int, where a value of -1 means
"unlimited", the current default. A value of 0 means "none".
There would usually be at most 1 peering request per IP, but if the
remote peering requests are behind a proxy there could well be more
than 1 per IP. In the current implementation, flag always re-
stricts 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 cate-
gories, 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 speci-
fied:

ignore Deny packets of all kinds, including ntpq(1ntpqmdoc) and
ntpdc(1ntpdcmdoc) queries.

kod If this flag is set when a rate violation occurs, a
kiss-o'-death (KoD) packet is sometimes sent. KoD packets
are rate limited to no more than one per minimum average
interpacket spacing, set by discard average defaulting to
8s. Otherwise, no response is sent.

limited
Deny service if the packet spacing violates the lower lim-
its specified in the discard command. A history of clients
is kept using the monitoring capability of ntpd(1ntpdmdoc).
Thus, monitoring is always active as long as there is a re-
striction entry with the limited flag.

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 algo-
rithm by allowing low priority traps to be overridden by
later requests for normal priority traps.

noepeer
Deny ephemeral peer requests, even if they come from an au-
thenticated source. Note that the ability to use a symmet-
ric key for authentication may be restricted to one or more
IPs or subnets via the third field of the ntp.keys file.
This restriction is not enabled by default, to maintain
backward compatability. Expect noepeer to become the de-
fault in ntp-4.4.

nomodify
Deny ntpq(1ntpqmdoc) and ntpdc(1ntpdcmdoc) queries which
attempt to modify the state of the server (i.e., run time
reconfiguration). Queries which return information are
permitted.

noquery
Deny ntpq(1ntpqmdoc) and ntpdc(1ntpdcmdoc) queries. Time
service is not affected.

nopeer Deny unauthenticated packets which would result in mobiliz-
ing a new association. This includes broadcast and symmet-
ric active packets when a configured association does not
exist. It also includes pool associations, so if you want
to use servers from a pool directive and also want to use
nopeer by default, you'll want a restrict source ... line
as well that does not include the nopeer directive.

noserve
Deny all packets except ntpq(1ntpqmdoc) and ntpdc(1ntpdcm-
doc) queries.

notrap Decline to provide mode 6 control message trap service to
matching hosts. The trap service is a subsystem of the
ntpq(1ntpqmdoc) control message protocol which is intended
for use by remote event logging programs.

notrust
Deny service unless the packet is cryptographically authen-
ticated.

ntpport
This is actually a match algorithm modifier, rather than a
restriction flag. Its presence causes the restriction en-
try to be matched only if the source port in the packet is
the standard NTP UDP port (123). There can be two restric-
tion entries with the same IP address if one specifies
ntpport and the other does not. The ntpport entry is con-
sidered more specific and is sorted later in the list.

serverresponse fuzz
When reponding to server requests, fuzz the low order bits
of the reftime.

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 in-
serted into the table at startup to prevent ntpd from attempting to
synchronize to itself, such as with manycastclient when manycast is
also specified with the same multicast address. A default entry is
also always present, though if it is otherwise unconfigured; no
flags are associated with the default entry (i.e., everything be-
sides your own NTP server is unrestricted).

delrestrict [source] address
Remove a previously-set restriction. This is useful for runtime
configuration via ntpq(1ntpqmdoc) infinity.Cm source is specified,
a dynamic restriction created from the restrict source template at
the time an association was added is removed. Without source a
static restriction is removed.

Automatic NTP Configuration Options
Manycasting
Manycasting is a automatic discovery and configuration paradigm new to
NTPv4. It is intended as a means for a multicast client to troll the
nearby network neighborhood to find cooperating manycast servers, validate
them using cryptographic means and evaluate their time values with respect
to other servers that might be lurking in the vicinity. The intended re-
sult is that each manycast client mobilizes client associations with some
number of the "best" of the nearby manycast servers, yet automatically re-
configures to sustain this number of servers should one or another fail.

Note that the manycasting paradigm does not coincide with the anycast para-
digm described in RFC-1546, which is designed to find a single server from
a clique of servers providing the same service. The manycast paradigm is
designed to find a plurality of redundant servers satisfying defined opti-
mality criteria.

Manycasting can be used with either symmetric key or public key cryptogra-
phy. The public key infrastructure (PKI) offers the best protection
against compromised keys and is generally considered stronger, at least
with relatively large key sizes. It is implemented using the Autokey pro-
tocol and the OpenSSL cryptographic library available from
http://www.openssl.org/. The library can also be used with other NTPv4
modes as well and is highly recommended, especially for broadcast modes.

A persistent manycast client association is configured using the
manycastclient command, which is similar to the server command but with a
multicast (IPv4 class D or IPv6 prefix FF) group address. The IANA has
designated IPv4 address 224.1.1.1 and IPv6 address FF05::101 (site local)
for NTP. When more servers are needed, it broadcasts manycast client mes-
sages to this address at the minimum feasible rate and minimum feasible
time-to-live (TTL) hops, depending on how many servers have already been
found. There can be as many manycast client associations as different
group address, each one serving as a template for a future ephemeral uni-
cast client/server association.

Manycast servers configured with the manycastserver command listen on the
specified group address for manycast client messages. Note the distinction
between manycast client, which actively broadcasts messages, and manycast
server, which passively responds to them. If a manycast server is in scope
of the current TTL and is itself synchronized to a valid source and operat-
ing at a stratum level equal to or lower than the manycast client, it
replies to the manycast client message with an ordinary unicast server mes-
sage.

The manycast client receiving this message mobilizes an ephemeral
client/server association according to the matching manycast client tem-
plate, but only if cryptographically authenticated and the server stratum
is less than or equal to the client stratum. Authentication is explicitly
required and either symmetric key or public key (Autokey) can be used.
Then, the client polls the server at its unicast address in burst mode in
order to reliably set the host clock and validate the source. This nor-
mally results in a volley of eight client/server at 2-s intervals during
which both the synchronization and cryptographic protocols run concur-
rently. Following the volley, the client runs the NTP intersection and
clustering algorithms, which act to discard all but the "best" associations
according to stratum and synchronization distance. The surviving associa-
tions then continue in ordinary client/server mode.

The manycast client polling strategy is designed to reduce as much as pos-
sible the volume of manycast client messages and the effects of implosion
due to near-simultaneous arrival of manycast server messages. The strategy
is determined by the manycastclient, tos and ttl configuration commands.
The manycast poll interval is normally eight times the system poll inter-
val, which starts out at the minpoll value specified in the manycastclient,
command and, under normal circumstances, increments to the maxpolll value
specified in this command. Initially, the TTL is set at the minimum hops
specified by the ttl command. At each retransmission the TTL is increased
until reaching the maximum hops specified by this command or a sufficient
number client associations have been found. Further retransmissions use
the same TTL.

The quality and reliability of the suite of associations discovered by the
manycast client is determined by the NTP mitigation algorithms and the
minclock and minsane values specified in the tos configuration command. At
least minsane candidate servers must be available and the mitigation algo-
rithms produce at least minclock survivors in order to synchronize the
clock. Byzantine agreement principles require at least four candidates in
order to correctly discard a single falseticker. For legacy purposes,
minsane defaults to 1 and minclock defaults to 3. For manycast service
minsane should be explicitly set to 4, assuming at least that number of
servers are available.

If at least minclock servers are found, the manycast poll interval is imme-
diately set to eight times maxpoll. If less than minclock servers are
found when the TTL has reached the maximum hops, the manycast poll interval
is doubled. For each transmission after that, the poll interval is doubled
again until reaching the maximum of eight times maxpoll. Further transmis-
sions use the same poll interval and TTL values. Note that while all this
is going on, each client/server association found is operating normally it
the system poll interval.

Administratively scoped multicast boundaries are normally specified by the
network router configuration and, in the case of IPv6, the link/site scope
prefix. By default, the increment for TTL hops is 32 starting from 31;
however, the ttl configuration command can be used to modify the values to
match the scope rules.

It is often useful to narrow the range of acceptable servers which can be
found by manycast client associations. Because manycast servers respond
only when the client stratum is equal to or greater than the server stra-
tum, primary (stratum 1) servers fill find only primary servers in TTL
range, which is probably the most common objective. However, unless con-
figured otherwise, all manycast clients in TTL range will eventually find
all primary servers in TTL range, which is probably not the most common ob-
jective in large networks. The tos command can be used to modify this be-
havior. Servers with stratum below floor or above ceiling specified in the
tos command are strongly discouraged during the selection process; however,
these servers may be temporally accepted if the number of servers within
TTL range is less than minclock.

The above actions occur for each manycast client message, which repeats at
the designated poll interval. However, once the ephemeral client associa-
tion is mobilized, subsequent manycast server replies are discarded, since
that would result in a duplicate association. If during a poll interval
the number of client associations falls below minclock, all manycast client
prototype associations are reset to the initial poll interval and TTL hops
and operation resumes from the beginning. It is important to avoid fre-
quent manycast client messages, since each one requires all manycast
servers in TTL range to respond. The result could well be an implosion,
either minor or major, depending on the number of servers in range. The
recommended value for maxpoll is 12 (4,096 s).

It is possible and frequently useful to configure a host as both manycast
client and manycast server. A number of hosts configured this way and
sharing a common group address will automatically organize themselves in an
optimum configuration based on stratum and synchronization distance. For
example, consider an NTP subnet of two primary servers and a hundred or
more dependent clients. With two exceptions, all servers and clients have
identical configuration files including both multicastclient and
multicastserver commands using, for instance, multicast group address
239.1.1.1. The only exception is that each primary server configuration
file must include commands for the primary reference source such as a GPS
receiver.

The remaining configuration files for all secondary servers and clients
have the same contents, except for the tos command, which is specific for
each stratum level. For stratum 1 and stratum 2 servers, that command is
not necessary. For stratum 3 and above servers the floor value is set to
the intended stratum number. Thus, all stratum 3 configuration files are
identical, all stratum 4 files are identical and so forth.

Once operations have stabilized in this scenario, the primary servers will
find the primary reference source and each other, since they both operate
at the same stratum (1), but not with any secondary server or client, since
these operate at a higher stratum. The secondary servers will find the
servers at the same stratum level. If one of the primary servers loses its
GPS receiver, it will continue to operate as a client and other clients
will time out the corresponding association and re-associate accordingly.

Some administrators prefer to avoid running ntpd(1ntpdmdoc) continuously
and run either sntp(1sntpmdoc) or ntpd(1ntpdmdoc) -q as a cron job. In ei-
ther case the servers must be configured in advance and the program fails
if none are available when the cron job runs. A really slick application
of manycast is with ntpd(1ntpdmdoc) -q. The program wakes up, scans the
local landscape looking for the usual suspects, selects the best from among
the rascals, sets the clock and then departs. Servers do not have to be
configured in advance and all clients throughout the network can have the
same configuration file.

Manycast Interactions with Autokey
Each time a manycast client sends a client mode packet to a multicast group
address, all manycast servers in scope generate a reply including the host
name and status word. The manycast clients then run the Autokey protocol,
which collects and verifies all certificates involved. Following the burst
interval all but three survivors are cast off, but the certificates remain
in the local cache. It often happens that several complete signing trails
from the client to the primary servers are collected in this way.

About once an hour or less often if the poll interval exceeds this, the
client regenerates the Autokey key list. This is in general transparent in
client/server mode. However, about once per day the server private value
used to generate cookies is refreshed along with all manycast client asso-
ciations. In this case all cryptographic values including certificates is
refreshed. If a new certificate has been generated since the last refresh
epoch, it will automatically revoke all prior certificates that happen to
be in the certificate cache. At the same time, the manycast scheme starts
all over from the beginning and the expanding ring shrinks to the minimum
and increments from there while collecting all servers in scope.

Broadcast Options
tos [bcpollbstep gate]
This command provides a way to delay, by the specified number of
broadcast poll intervals, believing backward time steps from a
broadcast server. Broadcast time networks are expected to be
trusted. In the event a broadcast server's time is stepped back-
wards, there is clear benefit to having the clients notice this
change as soon as possible. Attacks such as replay attacks can
happen, however, and even though there are a number of protections
built in to broadcast mode, attempts to perform a replay attack are
possible. This value defaults to 0, but can be changed to any num-
ber of poll intervals between 0 and 4.

Manycast Options
tos [ceiling ceiling | cohort { 0 | 1 } | 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.

cohort {0 | 1}
This is a binary flag which enables (0) or disables (1)
manycast server replies to manycast clients with the same
stratum level. This is useful to reduce implosions where
large numbers of clients with the same stratum level are
present. The default is to enable these replies.

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 asso-
ciations 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.

ttl hop ...
This command specifies a list of TTL values in increasing order, up
to 8 values can be specified. In manycast mode these values are
used in turn in an expanding-ring search. The default is eight
multiples of 32 starting at 31.

Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio, satel-
lite and modem reference clocks plus a special pseudo-clock used for backup
or when no other clock source is available. Detailed descriptions of indi-
vidual 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 (available as part of the
HTML documentation provided in /usr/share/doc/ntp). In addition, support
for a PPS signal is available as described in the "Pulse-per-second (PPS)
Signal Interfacing" page (available as part of the HTML documentation pro-
vided in /usr/share/doc/ntp). Many drivers support special line disci-
pline/streams modules which can significantly improve the accuracy using
the driver. These are described in the "Line Disciplines and Streams
Drivers" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp).

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 in-
terface 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 com-
mon radio, satellite and modem clocks are included by default. Note that
an attempt to configure a reference clock when the driver has not been com-
piled 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(1ntpdmdoc) 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 op-
tion can be useful to persuade the server to cherish a reference clock with
somewhat more enthusiasm than other reference clocks or peers. Further in-
formation on this option can be found in the "Mitigation Rules and the
prefer Keyword" (available as part of the HTML documentation provided in
/usr/share/doc/ntp) 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(1ntpdmdoc) 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 stra-
tum 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 iden-
tifier as other than the default, depending on the driver. The refid op-
tion 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 interpreted 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 (available
as part of the HTML documentation provided in
/usr/share/doc/ntp) for further information.

mode int
Specifies a mode number which is interpreted in a de-
vice-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 in-
terval 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 s). For modem refer-
ence 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 config-
ures the driver. Note that the same capability is possible at run
time using the ntpdc(1ntpdcmdoc) program. The options are inter-
preted as follows:

time1 sec
Specifies a constant to be added to the time offset pro-
duced by the driver, a fixed-point decimal number in sec-
onds. 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 ap-
proximate 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 (available as part of the HTML documen-
tation provided in /usr/share/doc/ntp).

time2 secs
Specifies a fixed-point decimal number in seconds, which is
interpreted in a driver-dependent way. See the descrip-
tions of specific drivers in the "Reference Clock Drivers"
page (available as part of the HTML documentation provided
in /usr/share/doc/ntp ).

stratum int
Specifies the stratum number assigned to the driver, an in-
teger 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 as-
signed by the driver itself.

mode int
Specifies a mode number which is interpreted in a de-
vice-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 com-
mand 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 ex-
changes between the client and server. In some cases, the calibra-
tion 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 be-
tween 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 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, the starts with an initial frequency of zero and
creates the file when writing it for the first time. If this com-
mand 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(1nt-
pdmdoc) must have write permission for the directory the drift file
is located in, and that file system links, symbolic or otherwise,
should be avoided.

dscp value
This option specifies the Differentiated Services Control Point
(DSCP) value, a 6-bit code. The default value is 46, signifying
Expedited Forwarding.

auth Enables the server to synchronize with unconfigured peers
only if the peer has been correctly authenticated using ei-
ther public key or private key cryptography. The default
for this flag is enable.

bclient
Enables the server to listen for a message from a broadcast
or multicast server, as in the multicastclient command with
default address. The default for this flag is disable.

calibrate
Enables the calibrate feature for reference clocks. The
default for this flag is disable.

kernel Enables the kernel time discipline, if available. The de-
fault for this flag is enable if support is available, oth-
erwise disable.

mode7 Enables processing of NTP mode 7 implementation-specific
requests which are used by the deprecated ntpdc(1ntpdcmdoc)
program. The default for this flag is disable. This flag
is excluded from runtime configuration using ntpq(1ntpqm-
doc). The ntpq(1ntpqmdoc) program provides the same capa-
bilities as ntpdc(1ntpdcmdoc) using standard mode 6 re-
quests.

monitor
Enables the monitoring facility. See the ntpdc(1ntpdcmdoc)
program and the monlist command or further information.
The default for this flag is enable.

ntp Enables time and frequency discipline. In effect, this
switch opens and closes the feedback loop, which is useful
for testing. The default for this flag is enable.

peer_clear_digest_early
By default, if ntpd(1ntpdmdoc) is using autokey and it re-
ceives a crypto-NAK packet that passes the duplicate packet
and origin timestamp checks the peer variables are immedi-
ately cleared. While this is generally a feature as it al-
lows for quick recovery if a server key has changed, a
properly forged and appropriately delivered crypto-NAK
packet can be used in a DoS attack. If you have active
noticable problems with this type of DoS attack then you
should consider disabling this option. You can check your
peerstats file for evidence of any of these attacks. 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.

unpeer_crypto_early
By default, if ntpd(1ntpdmdoc) receives an autokey packet
that fails TEST9, a crypto failure, the association is im-
mediately cleared. This is almost certainly a feature, but
if, in spite of the current recommendation of not using au-
tokey, you are using autokey you are seeing this sort of
DoS attack disabling this flag will delay tearing down the
association until the reachability counter becomes zero.
You can check your peerstats file for evidence of any of
these attacks. The default for this flag is enable.

unpeer_crypto_nak_early
By default, if ntpd(1ntpdmdoc) receives a crypto-NAK packet
that passes the duplicate packet and origin timestamp
checks the association is immediately cleared. While this
is generally a feature as it allows for quick recovery if a
server key has changed, a properly forged and appropriately
delivered crypto-NAK packet can be used in a DoS attack.
If you have active noticable problems with this type of DoS
attack then you should consider disabling this option. You
can check your peerstats file for evidence of any of these
attacks. The default for this flag is enable.

unpeer_digest_early
By default, if ntpd(1ntpdmdoc) receives what should be an
authenticated packet that passes other packet sanity checks
but contains an invalid digest the association is immedi-
ately cleared. While this is generally a feature as it al-
lows for quick recovery, if this type of packet is care-
fully forged and sent during an appropriate window it can
be used for a DoS attack. If you have active noticable
problems with this type of DoS attack then you should con-
sider disabling this option. You can check your peerstats
file for evidence of any of these attacks. The default for
this flag is enable.

includefile includefile
This command allows additional configuration commands to be in-
cluded from a separate file. Include files may be nested to a
depth of five; upon reaching the end of any include file, command
processing resumes in the previous configuration file. This option
is useful for sites that run ntpd(1ntpdmdoc) on multiple hosts,
with (mostly) common options (e.g., a restriction list).

interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard name |
address [/ prefixlen]]
The interface directive controls which network addresses ntpd(1nt-
pdmdoc) opens, and whether input is dropped without processing.
The first parameter determines the action for addresses which match
the second parameter. The second parameter specifies a class of
addresses, or a specific interface name, or an address. In the ad-
dress case, prefixlen determines how many bits must match for this
rule to apply. ignore prevents opening matching addresses, drop
causes ntpd(1ntpdmdoc) to open the address and drop all received
packets without examination. Multiple interface directives can be
used. The last rule which matches a particular address determines
the action for it. interface directives are disabled if any -I,
--interface, -L, or --novirtualips command-line options are speci-
fied in the configuration file, all available network addresses are
opened. The nic directive is an alias for interface.

leapfile leapfile
This command loads the IERS leapseconds file and initializes the
leapsecond values for the next leapsecond event, leapfile expira-
tion time, and TAI offset. The file can be obtained directly from
the IERS at
https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list or
ftp://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list. The
leapfile is scanned when ntpd(1ntpdmdoc) processes the leapfile
directive or when ntpd detects that the leapfile has changed. ntpd
checks once a day to see if the leapfile has changed. The up-
date-leap(1update_leapmdoc) script can be run to see if the
leapfile should be updated.

leapsmearinterval seconds
This EXPERIMENTAL option is only available if ntpd(1ntpdmdoc) was
built with the --enable-leap-smear option to the configure script.
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 infor-
mation.

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 keywords can
be prefixed with `=', `+' and `-', where `=' sets the syslog(3)
priority mask, `+' adds and `-' removes messages. syslog(3) mes-
sages can be controlled in four classes (clock, peer, sys and
sync). Within these classes four types of messages can be con-
trolled: 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(1ntpdmdoc)
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 synchroniza-
tion information. All other events and messages about peers, sys-
tem 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.

maxdepth count

maxmem kilobytes
Equivalent upper limits on the size of the MRU list, in
terms of entries or kilobytes. The acutal limit will be up
to incalloc entries or incmem kilobytes larger. As with
all of the mru options offered in units of entries or kilo-
bytes, 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 re-
moved to make room for newer ones, regardless of their age.
The default is 600 entries.

maxage seconds
Once the MRU list has mindepth entries and an additional
client is to ba added to the list, if the oldest entry was
updated more than maxage seconds ago, that entry is removed
and its storage is reused. If the oldest entry was updated
more recently the MRU list is grown, subject to maxdepth /
moxmem. The default is 64 seconds.

initalloc count

initmem kilobytes
Initial memory allocation at the time the monitoringfacil-
ity is first enabled, in terms of the number 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 delta in seconds before an hourly change to
the driftfile (frequency file) will be written, with a default
value 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
exceeeded, it is reduced by half. This is intended to reduce the
number of file writes for embedded systems with nonvolatile memory.

phone dial ...
This command is used in conjunction with the ACTS modem driver
(type 18) or the JJY driver (type 40, mode 100 - 180). For the
ACTS modem driver (type 18), the arguments consist of a maximum of
10 telephone numbers used to dial USNO, NIST, or European time ser-
vice. For the JJY driver (type 40 mode 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. The number
can contain other modem control codes as well.

pollskewlist [poll early late] ... [default early late]
Enable skewing of our poll requests to our servers. poll is a num-
ber between 3 and 17 inclusive, identifying a specific poll inter-
val. A poll interval is 2^n seconds in duration, so a poll value
of 3 corresponds to 8 seconds and a poll interval of 17 corresponds
to 131,072 seconds, or about a day and a half. The next two num-
bers must be between 0 and one-half of the poll interval, inclu-
sive. Ar early specifies how early the poll may start, while Ar
late specifies how late the poll may be delayed. With no argu-
ments, internally specified default values are chosen.

reset [allpeers] [auth] [ctl] [io] [mem] [sys] [timer]
Reset one or more groups of counters maintained by ntpd and exposed
by ntpq and ntpdc.

rlimit [memlock Nmegabytes | stacksize N4kPages filenum Nfiledescriptors]

memlock Nmegabytes
Specify the number of megabytes of memory that should be
allocated and locked. Probably only available under Linux,
this option may be useful when dropping root (the -i op-
tion). The default is 32 megabytes on non-Linux machines,
and -1 under Linux. -1 means "do not lock the process into
memory". 0 means "lock whatever memory the process wants
into memory".

stacksize N4kPages
Specifies the maximum size of the process stack on systems
with the mlockall() function. Defaults to 50 4k pages (200
4k pages in OpenBSD).

filenum Nfiledescriptors
Specifies the maximum number of file descriptors ntpd may
have open at once. Defaults to the system default.

saveconfigdir directory_path
Specify the directory in which to write configuration snapshots re-
quested with ntpq 's saveconfig command. If saveconfigdir does not
appear in the configuration file, saveconfig requests are rejected
by ntpd.

saveconfig filename
Write the current configuration, including any runtime modifica-
tions given with :config or config-from-file to the ntpd host's
filename in the saveconfigdir. This command will be rejected un-
less the saveconfigdir directive appears in ntpd 's configuration
file. filename can use strftime(3) format directives to substitute
the current date and time, for example,
saveconfig ntp-%Y%m%d-%H%M%S.conf. The filename used is stored in
the system variable savedconfig. Authentication is required.

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(1ntpqmdoc) 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.

sysinfo
Display operational summary.

sysstats
Show statistics counters maintained in the protocol module.

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 in-
teract in intricate ways that are hard to predict and some combina-
tions can result in some very nasty behavior. Very rarely is it
necessary to change the default values; but, some folks cannot re-
sist twisting the knobs anyway and this command is for them. Em-
phasis 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 disci-
pline algorithm. The value in log2 seconds defaults to 7
(1024 s), which is also the lower limit.

dispersion dispersion
The argument becomes the new value for the dispersion in-
crease 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 s. 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 thresh-
old is set to zero or greater than the default.

stepback stepback
The argument is the step threshold for the backward direc-
tion, which by default is 0.128 s. It can be set to any
positive number in seconds. If both the forward and back-
ward 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.

writevar assocID name = value [,...]
Write (create or update) the specified variables. If the assocID
is zero, the variablea re from the system variables name space,
otherwise they are from the peer variables name space. The assocID
is required, as the same name can occur in both name spaces.

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 in-
terface address. If the port number is unspecified, a value of
18447 is used. If the interface address is not specified, the mes-
sage is sent with a source address of the local interface the mes-
sage is sent through. Note that on a multihomed host the interface
used may vary from time to time with routing changes.

ttl hop ...
This command specifies a list of TTL values in increasing order.
Up to 8 values can be specified. In manycast mode these values are
used in-turn in an expanding-ring search. The default is eight
multiples of 32 starting at 31.

The trap receiver will generally log event messages and other in-
formation from the server in a log file. While such monitor pro-
grams may also request their own trap dynamically, configuring a
trap receiver will ensure that no messages are lost when the server
is started.

hop ...
This command specifies a list of TTL values in increasing order, up
to 8 values can be specified. In manycast mode these values are
used in turn in an expanding-ring search. The default is eight
multiples of 32 starting at 31.

OPTIONS
--help Display usage information and exit.

--more-help
Pass the extended usage information through a pager.

--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a
simple version. The `c' mode will print copyright information and
`n' will print the full copyright notice.

OPTION PRESETS
Any option that is not marked as not presettable may be preset by loading
values from environment variables named:
NTP_CONF_<option-name> or NTP_CONF

ENVIRONMENT
See OPTION PRESETS for configuration environment variables.

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

EXIT STATUS
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.

70 (EX_SOFTWARE)
libopts had an internal operational error. Please report it to au-
togen-users@lists.sourceforge.net. Thank you.

SEE ALSO
ntpd(1ntpdmdoc), ntpdc(1ntpdcmdoc), ntpq(1ntpqmdoc)

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 4), RFC5905.

AUTHORS
The University of Delaware and Network Time Foundation

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 ob-
tained via secure directory services when they become universally avail-
able.

Please send bug reports to: https://bugs.ntp.org, bugs@ntp.org

NOTES
This document was derived from FreeBSD.

This manual page was AutoGen-erated from the ntp.conf option definitions.

FreeBSD ports 15.quarterly May 25 2024 NTP_CONF(5)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=ntp.conf&manpath=FreeBSD+15.1-RELEASE+and+Ports.quarterly>

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