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NTP_CONF(5) File Formats Manual NTP_CONF(5)

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

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(1ntpdmdoc) daemon in order to specify the synchronization sources,
modes and other related information. Usually, it is installed in the
/etc directory, but could be installed elsewhere (see the daemon's -c
command line option).

The file format is similar to other Unix configuration files. Comments
begin with a `#' character and extend to the end of the line; blank
lines are ignored. Configuration commands consist of an initial key-
word followed by a list of arguments, some of which may be optional,
separated by whitespace. Commands may not be continued over multiple
lines. Arguments may be host names, host addresses written in numeric,
dotted-quad form, integers, floating point numbers (when specifying
times in seconds) and text strings.

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 following supported functionality and the
options used to control it:

• "Authentication Support"

• "Monitoring Support"

• "Access Control Support"

• "Automatic NTP Configuration Options"

• "Reference Clock Support"

• "Miscellaneous Options"

Following these is a section describing "Miscellaneous Options". While
there is a rich set of options available, the only required option is
one or more pool, server, peer, broadcast or manycastclient commands.

Configuration Support
Following is a description of the configuration commands in NTPv4.
These commands have the same basic functions as in NTPv3 and in some
cases new functions and new arguments. There are two classes of com-
mands, configuration commands that configure a persistent association
with a remote server or peer or reference clock, and auxiliary commands
that specify environmental variables that control various related oper-
ations.

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 interface, (m) a multicast address (IPv4 class D), or (r) a ref-
erence clock address (127.127.x.x). Note that only those options ap-
plicable 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 de-
tected, support for the IPv6 address family is generated in addition to
the default support of the IPv4 address family. In a few cases, in-
cluding the reslist billboard generated by ntpq(1ntpqmdoc) or
ntpdc(1ntpdcmdoc), IPv6 addresses are automatically generated. IPv6
addresses can be identified by the presence of colons ":" in the ad-
dress field. IPv6 addresses can be used almost everywhere where IPv4
addresses can be used, with the exception of reference clock addresses,
which are always IPv4.

Note that in contexts where a host name is expected, a -4 qualifier
preceding the host name forces DNS resolution to the IPv4 namespace,
while a -6 qualifier forces DNS resolution to the IPv6 namespace. 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 lo-
cal radio clock. In this mode the local clock can synchronized
to the remote server, but the remote server can never be syn-
chronized 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 persis-
tent symmetric-active mode association with the specified re-
mote 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, de-
pending on various failure scenarios, either the local or re-
mote peer may be the better source of time. This command
should NOT be used for type b, m or r addresses.

broadcast
For type b and m addresses (only), this command mobilizes a
persistent broadcast mode association. Multiple commands can
be used to specify multiple local broadcast interfaces (sub-
nets) and/or multiple multicast groups. Note that local broad-
cast messages go only to the interface associated with the sub-
net specified, but multicast messages go to all interfaces. In
broadcast mode the local server sends periodic broadcast mes-
sages 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 ad-
dresses can be used to contain the messages within administra-
tive boundaries. Ordinarily, this specification applies only
to the local server operating as a sender; for operation as a
broadcast client, see the broadcastclient or multicastclient
commands below.

manycastclient
For type m addresses (only), this command mobilizes a manycast
client mode association for the multicast address specified.
In this case a specific address must be supplied which matches
the address used on the manycastserver command for the desig-
nated manycast servers. The NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means
are taken to avoid spraying large areas of the Internet with
these messages and causing a possibly massive implosion of
replies at the sender. The manycastserver command specifies
that the local server is to operate in client mode with the re-
mote servers that are discovered as the result of broad-
cast/multicast messages. The client broadcasts a request mes-
sage to the group address associated with the specified address
and specifically enabled servers respond to these messages.
The client selects the servers providing the best time and con-
tinues as with the server command. The remaining servers are
discarded as if never heard.

Options:

autokey
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey
scheme described in "Authentication Options".

burst when the server is reachable, send a burst of six packets in-
stead of the usual one. The packet spacing is 2 s. This is de-
signed to improve timekeeping quality with the server command
and s addresses.

iburst When the server is unreachable, send a burst of eight packets
instead of the usual one. The packet spacing is 2 s. This is
designed 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
include authentication fields encrypted using the specified key
identifier 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 option to an upper limit of 17 (36.4 h). The minimum
poll interval defaults to 6 (64 s), but can be decreased by the
minpoll option to a lower limit of 4 (16 s).

noselect
Marks the server as unused, except for display purposes. The
server is discarded by the 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 documenta-
tion 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. Selec-
tion of the proper value, which defaults to 127, is something
of a black art and should be coordinated with the network ad-
ministrator.

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

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

xmtnonce
Valid only for server and pool modes, this flag puts a random
number 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 propagation delay using a brief client/server exchange
with the server, then enters the broadcast client mode, in
which it synchronizes to succeeding broadcast messages. Note
that, in order to avoid accidental or malicious disruption in
this mode, both the server and client should operate using sym-
metric-key or public-key authentication 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 address is required, but the NTP multicast address
224.0.1.1 assigned by the IANA should NOT be used, unless spe-
cific 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 sym-
metric-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 re-
ceiving a message for the first time, the multicast client mea-
sures the nominal server propagation delay using a brief
client/server exchange with the server, then enters the broad-
cast client mode, in which it synchronizes to succeeding multi-
cast messages. Note that, in order to avoid accidental or ma-
licious 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 in-
tervals 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 defines a scheme which provides cryptographic authentication
of received NTP packets. Originally, this was done using the Data En-
cryption 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, com-
monly 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
cryptography and, in addition, provides a new Autokey scheme based on
public key cryptography. Public key cryptography is generally consid-
ered more secure than symmetric key cryptography, since the security is
based on a private value which is generated by each server and never
revealed. With Autokey all key distribution and management functions
involve only public values, which considerably simplifies key distribu-
tion and storage. Public key management is based on X.509 certifi-
cates, which can be provided by commercial services or produced by
utility programs in the OpenSSL software library or the NTPv4 distribu-
tion.

While the algorithms for symmetric key cryptography are included in the
NTPv4 distribution, public key cryptography requires the OpenSSL soft-
ware library to be installed before building the NTP distribution. Di-
rections for doing that are on the Building and Installing the Distrib-
ution 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 interval between various operations, if other than
default.

Authentication is always enabled, although ineffective if not config-
ured as described below. If a NTP packet arrives including a message
authentication code (MAC), it is accepted only if it passes all crypto-
graphic checks. The checks require correct key ID, key value and mes-
sage 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 Autokey scheme requires a preliminary protocol ex-
change 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
reset by the enable and disable commands and also by remote configura-
tion 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 configu-
ration 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 operating with the auth flag disabled in-
vites a significant vulnerability where a rogue hacker can masquerade
as a falseticker and seriously disrupt system timekeeping. It is im-
portant to note that this flag has no purpose other than to allow or
disallow a new association in response to new broadcast 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 many-
cast 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 pub-
lic key cryptography are summarized below; further details are in the
briefings, 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
association. The servers and clients involved must agree on the key
and key identifier to authenticate NTP packets. Keys and related in-
formation are specified 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. Besides the keys used for ordinary NTP asso-
ciations, 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 using ntpdc(1ntpdcmdoc). This also provides a re-
vocation 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(1ntpqmdoc) 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 Au-
tokey Protocol page verifies packet integrity using MD5 message digests
and verifies the source with digital signatures and any of several di-
gest/signature schemes. Optional identity schemes described on the
Identity Schemes page and based on cryptographic challenge/response al-
gorithms are also available. Using all of these schemes provides
strong security against replay with or without modification, spoofing,
masquerade and most forms of clogging 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 transmission. All modes use in addition a variant of the S-KEY
scheme, in which a pseudo-random key list is generated and used in re-
verse order. These schemes are described along with an executive sum-
mary, 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-keygen(1ntpkeygenmdoc) program. This includes a required host key
file, required certificate file and optional sign key file, leapsecond
file and identity scheme files. The digest/signature scheme is speci-
fied 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 message digest with RSA encryption scheme. The cur-
rent NTP distribution supports 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
security 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 certificates for all hosts along the trail to one or more trusted
hosts. This requires the configuration file in all hosts to be engi-
neered so that, even under anticipated failure conditions, the NTP sub-
net 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 ad-
dresses, since DNS can't be completely trusted until the name servers
have synchronized clocks. The cryptographic name used by Autokey to
bind the host identity credentials and cryptographic values must be in-
dependent of interface, network and any other naming convention. The
name appears in the host certificate in either or both the subject and
issuer fields, so protection 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 interface, 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 reason Autokey includes the source and destination IP ad-
dresses in message digest computations and so the same addresses must
be available at both the server and client. For this reason operation
with network address translation schemes is not possible. This re-
flects the intended robust security model where government and corpo-
rate NTP servers are operated outside firewall perimeters.

Operation
A specific combination of authentication scheme (none, symmetric key,
public key) and identity scheme is called a cryptotype, although not
all combinations are compatible. There may be management configura-
tions where the clients, servers and peers may not all support the same
cryptotypes. A secure NTPv4 subnet can be configured in many ways
while keeping in mind the principles explained above and in this sec-
tion. Note however that some cryptotype combinations may successfully
interoperate with each other, but may not represent good security prac-
tice.

The cryptotype of an association is determined at the time of mobiliza-
tion, either at configuration time or some time later when a message of
appropriate 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 authenticated 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 re-
ceived 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 unau-
thenticated packet, while the same server receiving a packet of a cryp-
totype it supports will respond with packets of that cryptotype. How-
ever, unconfigured broadcast or manycast client associations or symmet-
ric passive associations will not be mobilized unless the server sup-
ports a cryptotype compatible with the first packet received. By de-
fault, unauthenticated associations will not be mobilized unless over-
ridden in a decidedly dangerous way.

Some examples may help to reduce confusion. Client Alice has no spe-
cific 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 message 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 something 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
credentials. Now, Bob can act just like the girls in his own choice of
servers; he can run multiple configured associations with multiple dif-
ferent servers (or the same server, although that might not be useful).
But, wise security policy might preclude some cryptotype combinations;
for instance, running an identity scheme with one server and no authen-
tication 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 gener-
ated by the OpenSSL utilities and certificates can be imported from
public certificate authorities. Note that symmetric keys are necessary
for the ntpq(1ntpqmdoc) and ntpdc(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 Version 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 ignored. The certificate extension fields
must not contain either a subject key identifier or a issuer key iden-
tifier 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 in-
terval, a session key list with a single entry will be regener-
ated for every message sent.

controlkey key
Specifies the key identifier to use with the ntpq(1ntpqmdoc)
utility, which uses the standard protocol defined in RFC-1305.
The key argument is the key identifier for a trusted key, where
the value can be in the range 1 to 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 en-
cryption scheme and loads the required private and public val-
ues described above. If one or more files are left unspeci-
fied, the default names are used as described above. Unless
the complete path and name of the file are specified, the loca-
tion 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 cer-
tificate 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 directory.

host file
Specifies the location of the required host key file.
This overrides the link ntpkey_key_hostname in the keys
directory.

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

leap file
Specifies the location of the optional leapsecond file.
This overrides the link ntpkey_leap in the keys direc-
tory.

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

pw password
Specifies the password to decrypt files containing pri-
vate 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
containing the keys and key identifiers used by
ntpd(1ntpdmdoc), ntpq(1ntpqmdoc) and ntpdc(1ntpdcmdoc) when op-
erating with symmetric key cryptography. This is the same op-
eration as the -k command line option.

keysdir path
This command specifies the default directory path for crypto-
graphic 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 identifier 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
cryptographic values used by the Autokey scheme, as a power of
2 in seconds. These values need to be updated frequently in
order to deflect brute-force attacks on the algorithms of the
scheme; however, updating some values is a relatively expensive
operation. The default interval is 16 (65,536 s or about 18
hours). For poll intervals above the specified interval, the
values will be updated for every message sent.

trustedkey key ...
Specifies the key identifiers which are trusted for the pur-
poses of authenticating peers with symmetric key cryptography,
as well as keys used by the ntpq(1ntpqmdoc) and
ntpdc(1ntpdcmdoc) programs. The authentication procedures re-
quire that both the local and remote servers share the same key
and key identifier for this purpose, although different keys
can be used with different servers. The key arguments are
32-bit unsigned integers with values from 1 to 65,535.

Error Codes
The following error codes are reported via the NTP control and monitor-
ing 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 in-
correct format or is an unsupported type.

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

106 (mismatched digest types) Not used.

107 (bad signature length) The signature length does not match the
current 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 bo-
gus.

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

113 (bad or missing certificate) The certificate is missing, cor-
rupted 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
performance. See the statistics command below for a listing and exam-
ple of each type of statistics currently supported. Statistic files
are managed using file generation sets and scripts in the ./scripts di-
rectory 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 informa-
tion. 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 re-
ceived from the clock in decoded ASCII format, where
meaningful. In some clock drivers a good deal of addi-
tional information can be gathered and displayed as
well. See information specific to each clock for fur-
ther details.

cryptostats
This option requires the OpenSSL cryptographic software
library. It enables recording of cryptographic public
key protocol information. Each message received by the
protocol 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 certain ancillary information. See the
"Authentication Options" section for further informa-
tion.

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

50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

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), fre-
quency offset (parts per million - PPM), RMS jitter
(seconds), Allan deviation (PPM) and clock discipline
time constant.

peerstats
Enables recording of peer statistics information. This
includes statistics records of all peers of a NTP
server and of special signals, where present and con-
figured. Each valid update appends a line of the fol-
lowing 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 dot-
ted-quad notation and status, respectively. The status
field is encoded in hex in the format described in Ap-
pendix 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 informa-
tion. This includes statistics records of all peers of
a NTP server and of special signals, where present and
configured. Each NTP message received from a peer or
clock driver appends a line of the following form to
the file generation set named rawstats:

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 ad-
dress followed by the local address in dotted-quad no-
tation. The final four fields show the originate, re-
ceive, transmit and final NTP timestamps in order. The
timestamp values are as received and before processing
by the various data smoothing and mitigation algo-
rithms.

sysstats
Enables recording of ntpd statistics counters on a pe-
riodic 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 remaining ten fields show the statistics counter
values accumulated since the last generated line.

Time since restart 36000
Time in hours since the system was last re-
booted.

Packets received 81965
Total number of packets received.

Packets processed 0
Number of packets received in response to pre-
vious packets sent

Current version 9546
Number of packets matching the current NTP ver-
sion.

Previous version 56
Number of packets matching the previous NTP
version.

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 limita-
tion.

statsdir directory_path
Indicates the full path of a directory where statistics
files should be created (see below). This keyword al-
lows the (otherwise constant) filegen filename prefix
to be modified for file generation sets, which is use-
ful for handling statistics logs.

filegen name [file filename] [type typename] [link | nolink]
[enable | disable]
Configures setting of generation file set name. Gener-
ation file sets provide a means for handling files that
are continuously growing during the lifetime of a
server. Server statistics are a typical example for
such files. Generation file sets provide access to a
set of files used to store the actual data. At any
time at most one element of the set is being written
to. The type given specifies when and how data will be
directed to a new element of the set. This way, infor-
mation stored in elements of a file set that are cur-
rently unused are available for administrational opera-
tions 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(1ntpdcmdoc) program running at a remote location.

name This is the type of the statistics records, as
shown in the statistics command.

file filename
This is the file name for the statistics
records. Filenames of set members are built
from three concatenated elements 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 com-
pile-time constant. It may, however,
be configurable for individual file
generation sets via other commands.
For example, the prefix used with
loopstats and peerstats generation can
be configured using the statsdir option
explained above.

filename
This string is directly concatenated to
the prefix mentioned above (no inter-
vening `/'). This can be modified us-
ing the file argument to the filegen
statement. No .. elements are allowed
in this component to prevent filenames
referring to parts outside the filesys-
tem hierarchy denoted by prefix.

suffix This part is reflects individual ele-
ments 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 in-
carnation of a ntpd server. This type
does not perform any changes to file
set members during runtime, however it
provides an easy way of separating
files belonging to different
ntpd(1ntpdmdoc) server incarnations.
The set member filename is built by ap-
pending 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 cre-
ated 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 re-
lated to a certain week of a year. The
term week is defined by computing
day-of-year modulo 7. Elements of such
a file generation set are distinguished
by appending the following suffix to
the file set filename base: A dot, a
4-digit year number, the letter W, and
a 2-digit week number. For example,
information from January, 10th 1992
would end up in a file with suffix
.1992W1.

month One generation file set element is gen-
erated per month. The file name suffix
consists of a dot, a 4-digit year num-
ber, and a 2-digit month.

year One generation file element is gener-
ated per year. The filename suffix
consists of a dot and a 4 digit year
number.

age This type of file generation sets
changes to a new element of the file
set every 24 hours of server operation.
The filename suffix consists of a dot,
the letter a, and an 8-digit number.
This number is taken to be the number
of seconds the server is running at the
start of the corresponding 24-hour pe-
riod. Information is only written to a
file generation by specifying enable;
output is prevented by specifying
disable.

link | nolink
It is convenient to be able to access the cur-
rent element of a file generation set by a
fixed name. This feature is enabled by speci-
fying link and disabled using nolink. If link
is specified, a hard link from the current file
set element to a file without suffix is cre-
ated. 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 val-
ues. 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 docu-
mentation 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 fa-
cility was expanded to deflect cryptographic and clogging attacks.
While this facility may be useful for keeping unwanted or broken or ma-
licious clients from congesting innocent servers, it should not be con-
sidered an alternative to the NTP authentication facilities. Source
address based restrictions are easily circumvented by a determined
cracker.

Clients can be denied service because they are explicitly included in
the restrict list created by the restrict command or implicitly as the
result of cryptographic or rate limit violations. Cryptographic viola-
tions include certificate or identity verification failure; rate limit
violations generally result from defective NTP implementations that
send packets at abusive rates. Some violations cause denied service
only for the offending packet, others cause denied service for a timed
period and others cause the denied service for an indefinite period.
When a client or network is denied access for an indefinite period, the
only way at present to remove the restrictions is by restarting the
server.

The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further
action except incrementing statistics counters. Sometimes a more
proactive response is needed, such as a server message that explicitly
requests the client to stop sending and leave a message for the system
operator. A special packet format has been created for this purpose
called the "kiss-of-death" (KoD) packet. KoD packets have the leap
bits set unsynchronized and stratum set to zero and the reference iden-
tifier field set to a four-byte ASCII code. If the noserve or notrust
flag of the matching restrict list entry is set, the code is "DENY"; if
the limited flag is set and the rate limit is exceeded, the code is
"RATE". Finally, if a cryptographic violation occurs, the code is
"CRYP".

A client receiving a KoD performs a set of sanity checks to minimize
security exposure, then updates the stratum and reference identifier
peer variables, sets the access denied (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 recover from this condition is to restart the proto-
col at both the client and server. This happens automatically at the
client when the association times out. It will happen at the server
only if the server operator cooperates.

Access Control Commands
discard [average avg] [minimum min] [monitor prob]
Set the parameters of the limited facility which protects the
server from client abuse. The average subcommand specifies the
minimum average packet spacing 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 enabled. 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 re-
placing 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 restric-
tion entry is created for each address the hostname resolves
to, and any provided 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 en-
try 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 indicate the de-
fault entry. The ippeerlimit directive limits the number 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 al-
ways restricts access, i.e., an entry with no flags indicates
that free access to the server is to be given. The flags are
not orthogonal, in that more restrictive flags will often make
less restrictive ones redundant. The flags can generally be
classed into two categories, those which restrict time service
and those which restrict informational queries and attempts to
do run-time reconfiguration of the server. One or more of the
following flags may be specified:

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 mini-
mum 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
limits 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 restriction 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 algorithm 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
authenticated source. Note that the ability to use a
symmetric 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 de-
fault, to maintain backward compatability. Expect
noepeer to become the default 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 infor-
mation are permitted.

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

nopeer Deny unauthenticated packets which would result in mo-
bilizing a new association. This includes broadcast
and symmetric active packets when a configured associa-
tion does not exist. It also includes pool associa-
tions, so if you want to use servers from a pool direc-
tive 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(1ntpdcmdoc) 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 au-
thenticated.

ntpport
This is actually a match algorithm modifier, rather
than a restriction flag. Its presence causes the re-
striction entry to be matched only if the source port
in the packet is the standard NTP UDP port (123).
There can be two restriction entries with the same IP
address if one specifies ntpport and the other does
not. The ntpport entry is considered 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, inter-
face, ntpport, for each of the local host's interface addresses
are inserted into the table at startup to prevent ntpd from at-
tempting to synchronize to itself, such as with manycastclient
when manycast is also specified with the same multicast ad-
dress. A default entry is also always present, though if it is
otherwise unconfigured; no flags are associated with the de-
fault entry (i.e., everything besides your own NTP server is
unrestricted).

delrestrict [source] address
Remove a previously-set restriction. This is useful for run-
time 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 re-
moved. 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, vali-
date them using cryptographic means and evaluate their time values with
respect to other servers that might be lurking in the vicinity. The
intended result is that each manycast client mobilizes client associa-
tions with some number of the "best" of the nearby manycast servers,
yet automatically reconfigures to sustain this number of servers should
one or another fail.

Note that the manycasting paradigm does not coincide with the anycast
paradigm described in RFC-1546, which is designed to find a single
server from a clique of servers providing the same service. The many-
cast paradigm is designed to find a plurality of redundant servers sat-
isfying defined optimality criteria.

Manycasting can be used with either symmetric key or public key cryp-
tography. The public key infrastructure (PKI) offers the best protec-
tion against compromised keys and is generally considered stronger, at
least with relatively large key sizes. It is implemented using the Au-
tokey protocol 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 messages to this address at the minimum feasible rate and mini-
mum feasible time-to-live (TTL) hops, depending on how many servers
have already been found. There can be as many manycast client associa-
tions as different group address, each one serving as a template for a
future ephemeral unicast 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 mes-
sages, and manycast server, which passively responds to them. If a
manycast server is in scope of the current TTL and is itself synchro-
nized to a valid source and operating at a stratum level equal to or
lower than the manycast client, it replies to the manycast client mes-
sage with an ordinary unicast server message.

The manycast client receiving this message mobilizes an ephemeral
client/server association according to the matching manycast client
template, 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 normally results in a volley of eight client/server at
2-s intervals during which both the synchronization and cryptographic
protocols run concurrently. 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 associations then continue in ordinary
client/server mode.

The manycast client polling strategy is designed to reduce as much as
possible the volume of manycast client messages and the effects of im-
plosion due to near-simultaneous arrival of manycast server messages.
The strategy is determined by the manycastclient, tos and ttl configu-
ration commands. The manycast poll interval is normally eight times
the system poll interval, which starts out at the minpoll value speci-
fied 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 associa-
tions 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 com-
mand. At least minsane candidate servers must be available and the
mitigation algorithms 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
immediately 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 inter-
val is doubled again until reaching the maximum of eight times maxpoll.
Further transmissions 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 re-
spond only when the client stratum is equal to or greater than the
server stratum, primary (stratum 1) servers fill find only primary
servers in TTL range, which is probably the most common objective.
However, unless configured otherwise, all manycast clients in TTL range
will eventually find all primary servers in TTL range, which is proba-
bly not the most common objective in large networks. The tos command
can be used to modify this behavior. 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
association is mobilized, subsequent manycast server replies are dis-
carded, 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 frequent 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 many-
cast client and manycast server. A number of hosts configured this way
and sharing a common group address will automatically organize them-
selves 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, mul-
ticast group address 239.1.1.1. The only exception is that each pri-
mary 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 com-
mand is not necessary. For stratum 3 and above servers the floor value
is set to the intended stratum number. Thus, all stratum 3 configura-
tion 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) continu-
ously and run either sntp(1sntpmdoc) or ntpd(1ntpdmdoc) -q as a cron
job. In either 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, se-
lects 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 Au-
tokey 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 sev-
eral 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 transpar-
ent in client/server mode. However, about once per day the server pri-
vate value used to generate cookies is refreshed along with all many-
cast client associations. In this case all cryptographic values in-
cluding certificates is refreshed. If a new certificate has been gen-
erated 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
backwards, there is clear benefit to having the clients notice
this change as soon as possible. Attacks such as replay at-
tacks 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 number 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 algo-
rithms. 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 implo-
sions 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
associations until no more than minclock associations
remain. This value defaults to 3, but can be changed
to any number from 1 to the number of configured
sources.

minsane minsane
This is the minimum number of candidates available to
the clock selection algorithm in order to produce one
or more truechimers for the clustering algorithm. If
fewer than this number are available, the clock is
undisciplined and allowed to run free. The default is
1 for legacy purposes. However, according to princi-
ples 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 or-
der, up to 8 values can be specified. In manycast mode these
values are used in turn in an expanding-ring search. The de-
fault is eight multiples of 32 starting at 31.

Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio,
satellite and modem reference clocks plus a special pseudo-clock used
for backup or when no other clock source is available. Detailed de-
scriptions of individual device drivers and options can be found in the
"Reference Clock Drivers" page (available as part of the HTML documen-
tation 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 avail-
able as described in the "Pulse-per-second (PPS) Signal Interfacing"
page (available as part of the HTML documentation provided in
/usr/share/doc/ntp). Many drivers support special line disci-
pline/streams modules which can significantly improve the accuracy us-
ing the driver. These are described in the "Line Disciplines and
Streams Drivers" page (available as part of the HTML documentation pro-
vided in /usr/share/doc/ntp).

A reference clock will generally (though not always) be a radio time-
code receiver which is synchronized to a source of standard time such
as the services offered by the NRC in Canada and NIST and USNO in the
US. The interface between the computer and the timecode receiver is
device dependent, but is usually a serial port. A device driver spe-
cific to each reference clock must be selected and compiled in the dis-
tribution; however, most common radio, satellite and modem clocks are
included by default. Note that an attempt to configure a reference
clock when the driver has not been compiled or the hardware port has
not been appropriately configured results in a scalding remark to the
system log file, but is otherwise non hazardous.

For the purposes of configuration, ntpd(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. Refer-
ence clock addresses are of the form 127.127.t.u, where t is an integer
denoting the clock type and u indicates the unit number in the range
0-3. While it may seem overkill, it is in fact sometimes useful to
configure multiple reference clocks of the same type, in which case the
unit numbers must be unique.

The server command is used to configure a reference clock, where the
address argument in that command is the clock address. The key,
version and ttl options are not used for reference clock support. The
mode option is added for reference clock support, as described below.
The prefer option can be useful to persuade the server to cherish a
reference clock with somewhat more enthusiasm than other reference
clocks or peers. Further information on this option can be found in
the "Mitigation Rules and the prefer Keyword" (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 indi-
vidual 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 de-
vice. 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 stratum as greater than zero. The stratum option is used for
this purpose. Also, in cases involving both a reference clock and a
pulse-per-second (PPS) discipline signal, it is useful to specify the
reference clock identifier as other than the default, depending on the
driver. The refid option is used for this purpose. Except where
noted, these options apply to all clock drivers.

Reference Clock Commands
server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
This command can be used to configure reference clocks in spe-
cial 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 syn-
chronization 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 di-
aling protocol in the ACTS driver and a device subtype
in the parse drivers.

minpoll int

maxpoll int
These options specify the minimum and maximum polling
interval for reference clock 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 reference clocks, minpoll defaults to 10
(17.1 m) and maxpoll defaults to 14 (4.5 h). The al-
lowable 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 spe-
cial ways. It must immediately follow the server command which
configures the driver. Note that the same capability is possi-
ble at run time using the ntpdc(1ntpdcmdoc) program. The op-
tions are interpreted as follows:

time1 sec
Specifies a constant to be added to the time offset
produced by the driver, a fixed-point decimal number in
seconds. This is used as a calibration constant to ad-
just 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 sys-
tematic 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 indi-
vidual system and driver is available, an approximate
correction is noted in the driver documentation pages.
Note: in order to facilitate calibration when more than
one radio clock or PPS signal is supported, a special
calibration feature is available. It takes the form of
an argument to the enable command described in
"Miscellaneous Options" page and operates as described
in the "Reference Clock Drivers" page (available as
part of the HTML documentation 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 descriptions of specific drivers in the "Reference
Clock Drivers" page (available as part of the HTML doc-
umentation provided in /usr/share/doc/ntp ).

stratum int
Specifies the stratum number assigned to the driver, an
integer between 0 and 15. This number overrides the
default stratum number ordinarily assigned by the dri-
ver itself, usually zero.

refid string
Specifies an ASCII string of from one to four charac-
ters which defines the reference identifier used by the
driver. This string overrides the default identifier
ordinarily assigned by the driver itself.

mode int
Specifies a mode number which is interpreted in a de-
vice-specific fashion. For instance, it selects a di-
aling 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 par-
ticular clock driver. However, by convention flag4 is
used to enable recording monitoring data to the
clockstats file configured with the filegen command.
Further information on the filegen command can be found
in "Monitoring Options".

Miscellaneous Options
broadcastdelay seconds
The broadcast and multicast modes require a special calibration
to determine the network delay between the local and remote
servers. Ordinarily, this is done automatically by the initial
protocol exchanges between the client and server. In some
cases, the calibration procedure may fail due to network or
server access controls, for example. This command specifies
the default delay to be used under these circumstances. Typi-
cally (for Ethernet), a number between 0.003 and 0.007 seconds
is appropriate. The default when this command is not used is
0.004 seconds.

driftfile driftfile
This command specifies the 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 ini-
tial frequency and then updated once per hour with the current
frequency computed by the daemon. If the file name is speci-
fied, 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 command is not given, the daemon
will always start with an initial frequency of zero.

The file format consists of a single line containing a single
floating point number, which records the frequency offset mea-
sured 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(1ntpdmdoc) must have write permission for the direc-
tory 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, signify-
ing Expedited Forwarding.

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

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

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

kernel Enables the kernel time discipline, if available. The
default for this flag is enable if support is avail-
able, otherwise disable.

mode7 Enables processing of NTP mode 7 implementation-spe-
cific requests which are used by the deprecated
ntpdc(1ntpdcmdoc) program. The default for this flag
is disable. This flag is excluded from runtime config-
uration using ntpq(1ntpqmdoc). The ntpq(1ntpqmdoc)
program provides the same capabilities as
ntpdc(1ntpdcmdoc) using standard mode 6 requests.

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
receives a crypto-NAK packet that passes the duplicate
packet and origin timestamp checks the peer variables
are 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 deliv-
ered 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 op-
tion. 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 associa-
tion is immediately cleared. This is almost certainly
a feature, but if, in spite of the current recommenda-
tion of not using autokey, 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 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.

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 immediately cleared. While this is generally a fea-
ture as it allows for quick recovery, if this type of
packet is carefully forged and sent during an appropri-
ate window it can be used for 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.

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, com-
mand 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 restric-
tion list).

interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard name |
address [/ prefixlen]]
The interface directive controls which network addresses
ntpd(1ntpdmdoc) opens, and whether input is dropped without
processing. The first parameter determines the action for ad-
dresses which match the second parameter. The second parameter
specifies a class of addresses, or a specific interface name,
or an address. In the address case, prefixlen determines how
many bits must match for this rule to apply. ignore prevents
opening matching addresses, drop causes ntpd(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 specified 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
expiration time, and TAI offset. The file can be obtained di-
rectly 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
update-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 information.

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) messages can be controlled in four classes (clock,
peer, sys and sync). Within these classes four types of mes-
sages can be controlled: informational messages (info), event
messages (events), statistics messages (statistics) and status
messages (status).

Configuration keywords are formed by concatenating the message
class with the event class. The all prefix can be used instead
of a message class. A message class may also be followed by
the all keyword to enable/disable all messages of the respec-
tive 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 ref-
erence server, the following minimum message configuration
could be useful:

logconfig =syncall +clockall

This configuration will list all clock information and synchro-
nization information. All other events and messages about
peers, system events and so on is suppressed.

logfile logfile
This command specifies the location of an alternate log file to
be used instead of the default system syslog(3) facility. This
is the same operation as the -l command line option.

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 en-
tries or kilobytes, if both maxdepth and maxmem are
used, the last one used controls. The default is 1024
kilobytes.

mindepth count
Lower limit on the MRU list size. When the MRU list
has fewer than mindepth entries, existing entries are
never removed to make room for newer ones, regardless
of their age. The default is 600 entries.

maxage seconds
Once the MRU list has mindepth entries and an addi-
tional 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 monitoringfa-
cility is first enabled, in terms of the number of en-
tries 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 de-
fault 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 writ-
ten and the threshold becomes the new threshold value. If the
threshold is not exceeeded, it is reduced by half. This is in-
tended 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 service. 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
number between 3 and 17 inclusive, identifying a specific poll
interval. 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 numbers must be between 0 and one-half of the poll
interval, inclusive. Ar early specifies how early the poll may
start, while Ar late specifies how late the poll may be de-
layed. With no arguments, 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 ex-
posed 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 option). 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 what-
ever memory the process wants into memory".

stacksize N4kPages
Specifies the maximum size of the process stack on sys-
tems 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
requested 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 modifi-
cations given with :config or config-from-file to the ntpd
host's filename in the saveconfigdir. This command will be re-
jected unless the saveconfigdir directive appears in ntpd 's
configuration file. filename can use strftime(3) format direc-
tives 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 re-
quired.

setvar variable [default]
This command adds an additional system variable. These vari-
ables 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 pur-
poses 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 vari-
able 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 config-
uration file before any other configuration options. The de-
fault values of these variables have been carefully optimized
for a wide range of network speeds and reliability expecta-
tions. In general, they interact in intricate ways that are
hard to predict and some combinations can result in some very
nasty behavior. Very rarely is it necessary to change the de-
fault values; but, some folks cannot resist twisting the knobs
anyway and this command is for them. Emphasis added: twisters
are on their own and can expect no help from the support group.

The variables operate as follows:

allan allan
The argument becomes the new value for the minimum Al-
lan intercept, which is a parameter of the PLL/FLL
clock discipline 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
increase rate, normally .000015 s/s.

freq freq
The argument becomes the initial value of the frequency
offset in parts-per-million. This overrides the value
in the frequency file, if present, and avoids the ini-
tial 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 re-
cent 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 de-
fault, since the filter is not enabled unless this com-
mand 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 sec-
onds. If set to zero, step adjustments will never oc-
cur. Note: The kernel time discipline is disabled if
the step threshold is set to zero or greater than the
default.

stepback stepback
The argument is the step threshold for the backward di-
rection, which by default is 0.128 s. It can be set to
any positive number in seconds. If both the forward
and backward step thresholds are set to zero, step ad-
justments will never occur. Note: The kernel time dis-
cipline 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 sec-
onds. 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 ad-
dress and port number for sending messages with the specified
local interface address. If the port number is unspecified, a
value of 18447 is used. If the interface address is not speci-
fied, the message is sent with a source address of the local
interface the message is sent through. Note that on a multi-
homed 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 or-
der. Up to 8 values can be specified. In manycast mode these
values are used in-turn in an expanding-ring search. The de-
fault is eight multiples of 32 starting at 31.

The trap receiver will generally log event messages and other
information from the server in a log file. While such monitor
programs may also request their own trap dynamically, configur-
ing 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 or-
der, up to 8 values can be specified. In manycast mode these
values are used in turn in an expanding-ring search. The de-
fault 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 load-
ing 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
autogen-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
obtained via secure directory services when they become universally
available.

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 defini-
tions.

FreeBSD 15.0 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&sektion=5&manpath=FreeBSD+15.0-RELEASE+and+Ports>

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