ntp.conf(5) - phpMan

NTP_CONF(5) BSD File Formats Manual NTP_CONF(5)

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

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

All arguments must be options.

DESCRIPTION
The ntp.conf configuration file is read at initial startup by the ntpd(8) daemon in order to
specify the synchronization sources, modes and other related information. Usually, it is
installed in the /etc directory, but could be installed elsewhere (see the daemon's -c com-
mand 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 com-
mands consist of an initial keyword followed by a list of arguments, some of which may be
optional, separated by whitespace. Commands may not be continued over multiple lines. Ar-
guments 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 documenta-
tion 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:

o Authentication Support

o Monitoring Support

o Access Control Support

o Automatic NTP Configuration Options

o Reference Clock Support

o Miscellaneous Options

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

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

Configuration Commands
The various modes are determined by the command keyword and the type of the required IP ad-
dress. Addresses are classed by type as (s) a remote server or peer (IPv4 class A, B and
C), (b) the broadcast address of a local interface, (m) a multicast address (IPv4 class D),
or (r) a reference clock address (127.127.x.x). Note that only those options applicable to
each command are listed below. Use of options not listed may not be caught as an error, but
may result in some weird and even destructive behavior.

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

Note that in contexts where a host name is expected, a -4 qualifier preceding the host name
forces DNS resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to
the IPv6 namespace. 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 associa-
tion with the specified remote server or local radio clock. In this mode the local
clock can synchronized to the remote server, but the remote server can never be syn-
chronized to the local clock. This command should not be used for type b or m ad-
dresses.

peer For type s addresses (only), this command mobilizes a persistent symmetric-active
mode association with the specified remote peer. In this mode the local clock can
be synchronized to the remote peer or the remote peer can be synchronized to the lo-
cal clock. This is useful in a network of servers where, depending on various fail-
ure scenarios, either the local or remote peer may be the better source of time.
This command should NOT be used for type b, m or r addresses.

broadcast
For type b and m addresses (only), this command mobilizes a persistent broadcast
mode association. Multiple commands can be used to specify multiple local broadcast
interfaces (subnets) and/or multiple multicast groups. Note that local broadcast
messages go only to the interface associated with the subnet specified, but multi-
cast messages go to all interfaces. In broadcast mode the local server sends peri-
odic broadcast messages to a client population at the address specified, which is
usually the broadcast address on (one of) the local network(s) or a multicast ad-
dress 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 nonconflict-
ing addresses can be used to contain the messages within administrative boundaries.
Ordinarily, this specification applies only to the local server operating as a
sender; for operation as a broadcast client, see the broadcastclient or
multicastclient commands below.

manycastclient
For type m addresses (only), this command mobilizes a manycast client mode associa-
tion 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 op-
erate in client mode with the remote servers that are discovered as the result of
broadcast/multicast messages. The client broadcasts a request message to the group
address associated with the specified address and specifically enabled servers re-
spond to these messages. The client selects the servers providing the best time and
continues as with the server command. The remaining servers are discarded as if
never heard.

Options:

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

burst when the server is reachable, send a burst of eight packets instead of the usual
one. The packet spacing is normally 2 s; however, the spacing between the first and
second packets can be changed with the calldelay command to allow additional time
for a modem or ISDN call to complete. This is designed to improve timekeeping qual-
ity 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 normally 2 s; however, the spacing between the first two
packets can be changed with the calldelay command to allow additional time for a mo-
dem or ISDN call to complete. This is designed to speed the initial synchronization
acquisition with the server command and s addresses and when ntpd(8) is started with
the -q option.

key key
All packets sent to and received from the server or peer are to include authentica-
tion 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 cho-
sen for synchronization among a set of correctly operating hosts. See the
"Mitigation Rules and the prefer Keyword" page (available as part of the HTML docu-
mentation provided in /usr/share/doc/ntp) for further information.

true Marks the server as a truechimer, forcing the association to always survive the se-
lection 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 speci-
fies the time-to-live ttl to use on broadcast server and multicast server and the
maximum ttl for the expanding ring search with manycast client packets. Selection
of the proper value, which defaults to 127, is something of a black art and should
be coordinated with the network administrator.

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

xleave Valid in peer and broadcast modes only, this flag enables interleave 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 symmet-
ric-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 mul-
ticast address 224.0.1.1 assigned by the IANA should NOT be used, unless specific
means are taken to limit the span of the reply and avoid a possibly massive implo-
sion 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 symmet-
ric-key or public-key authentication as described in Authentication Options.

multicastclient address ...
This command enables reception of multicast server messages to the multicast group
address(es) (type m) specified. Upon receiving a message for the first time, the
multicast client measures the nominal server propagation delay using a brief
client/server exchange with the server, then enters the broadcast client mode, in
which it synchronizes to succeeding multicast messages. Note that, in order to
avoid accidental or malicious disruption in this mode, both the server and client
should operate using symmetric-key or public-key authentication as described in
Authentication Options.

mdnstries number
If we are participating in mDNS, after we have synched for the first time we attempt
to register with the mDNS system. If that registration attempt fails, we try again
at one minute intervals for up to mdnstries times. After all, ntpd may be starting
before mDNS. The default value for mdnstries is 5.

Authentication Support
Authentication support allows the NTP client to verify that the server is in fact known and
trusted and not an intruder intending accidentally or on purpose to masquerade as that
server. The NTPv3 specification RFC-1305 defines a scheme which provides cryptographic au-
thentication of received NTP packets. Originally, this was done using the Data Encryption
Standard (DES) algorithm operating in Cipher Block Chaining (CBC) mode, commonly called
DES-CBC. Subsequently, this was replaced by the RSA Message Digest 5 (MD5) algorithm using
a private key, commonly called keyed-MD5. Either algorithm computes a message digest, or
one-way hash, which can be used to verify the server has the correct private key and key
identifier.

NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryptography and, in ad-
dition, provides a new Autokey scheme based on public key cryptography. Public key cryptog-
raphy is generally considered more secure than symmetric key cryptography, since the secu-
rity 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 con-
siderably simplifies key distribution and storage. Public key management is based on X.509
certificates, which can be provided by commercial services or produced by utility programs
in the OpenSSL software library or the NTPv4 distribution.

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

Authentication is configured separately for each association using the key or autokey sub-
command on the peer, server, broadcast and manycastclient configuration commands as de-
scribed 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 configured as described below.
If a NTP packet arrives including a message authentication code (MAC), it is accepted only
if it passes all cryptographic checks. The checks require correct key ID, key value and
message digest. If the packet has been modified in any way or replayed by an intruder, it
will fail one or more of these checks and be discarded. Furthermore, the Autokey scheme re-
quires a preliminary protocol exchange to obtain the server certificate, verify its creden-
tials 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 com-
mands and also by remote configuration commands sent by a ntpdc(1) program running on an-
other machine. If this flag is enabled, which is the default case, new broadcast client and
symmetric passive associations and remote configuration commands must be cryptographically
authenticated using either symmetric key or public key cryptography. If this flag is dis-
abled, these operations are effective even if not cryptographic authenticated. It should be
understood that operating with the auth flag disabled invites a significant vulnerability
where a rogue hacker can masquerade as a falseticker and seriously disrupt system timekeep-
ing. It is important 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 con-
figuration commands and, in particular, the flag has no effect on the authentication process
itself.

An attractive alternative where multicast support is available is manycast mode, in which
clients periodically troll for servers as described in the Automatic NTP Configuration
Options page. Either symmetric key or public key cryptographic authentication can be used
in this mode. The principle advantage of manycast mode is that potential servers need not
be configured in advance, since the client finds them during regular operation, and the con-
figuration files for all clients can be identical.

The security model and protocol schemes for both symmetric key and public key cryptography
are summarized below; further details are in the 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 distin-
guished 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 re-
lated information are specified in a key file, usually called ntp.keys, which must be dis-
tributed and stored using secure means beyond the scope of the NTP protocol itself. Besides
the keys used for ordinary NTP associations, additional keys can be used as passwords for
the ntpq(1) and ntpdc(1) utility programs.

When ntpd(8) 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 possi-
bly several batches of keys and then activating or deactivating each batch remotely using
ntpdc(1). This also provides a revocation capability that can be used if a key becomes com-
promised. The requestkey command selects the key used as the password for the ntpdc(1)
utility, while the controlkey command selects the key used as the password for the ntpq(1)
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 pro-
tocol described on the Autokey Protocol page verifies packet integrity using MD5 message di-
gests and verifies the source with digital signatures and any of several digest/signature
schemes. Optional identity schemes described on the Identity Schemes page and based on
cryptographic challenge/response algorithms are also 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 reverse order.
These schemes are described along with an executive summary, current status, briefing slides
and reading list on the Autonomous Authentication page.

The specific cryptographic environment used by Autokey servers and clients is determined by
a set of files and soft links generated by the ntp-keygen(1ntpkeygenmdoc) program. This in-
cludes a required host key file, required certificate file and optional sign key file,
leapsecond file and identity scheme files. The digest/signature scheme is specified in the
X.509 certificate along with the matching sign key. There are several schemes available in
the OpenSSL software library, each identified by a specific string such as
md5WithRSAEncryption, which stands for the MD5 message digest with RSA encryption scheme.
The current 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 ob-
tain the certificates for all hosts along the trail to one or more trusted hosts. This re-
quires the configuration file in all hosts to be engineered so that, even under anticipated
failure conditions, the NTP subnet will form such that every group host can find a trail to
at least one trusted host.

Naming and Addressing
It is important to note that Autokey does not use DNS to resolve addresses, since DNS can't
be completely trusted until the name servers have synchronized clocks. The cryptographic
name used by Autokey to bind the host identity credentials and cryptographic values must be
independent 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 provi-
sions 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 de-
flect masquerade attacks. For this reason Autokey includes the source and destination IP
addresses 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 reflects the intended robust security model where government and cor-
porate NTP servers are operated outside firewall perimeters.

Operation
A specific combination of authentication scheme (none, symmetric key, public key) and iden-
tity scheme is called a cryptotype, although not all combinations are compatible. There may
be management configurations 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 section. Note however that some cryptotype
combinations may successfully interoperate with each other, but may not represent good secu-
rity practice.

The cryptotype of an association is determined at the time of mobilization, either at con-
figuration 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 associ-
ation 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 ex-
change determines which one is used. The client request message contains bits corresponding
to which schemes it has available. The server response message contains bits corresponding
to which schemes it has available. Both server and client match the received bits with
their own and select a common scheme.

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

Some examples may help to reduce confusion. Client Alice has no specific cryptotype se-
lected. Server Bob has both a symmetric key file and minimal Autokey files. Alice's unau-
thenticated 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(1) 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 de-
scribed 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 different 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 authentication with another might not be wise.

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

Certificates imported from OpenSSL or public certificate authorities have certian limita-
tions. 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 en-
coded in ASN.1 must not exceed 1024 bytes. The subject distinguished name field (CN) is the
fully qualified name of the host on which it is used; the remaining subject fields are ig-
nored. The certificate extension fields must not contain either a subject key identifier or
a issuer key identifier field; however, an extended key usage field for a trusted host must
contain the value trustRoot;. Other extension fields are ignored.

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

controlkey key
Specifies the key identifier to use with the ntpq(1) utility, which uses the stan-
dard 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] [sign file] [gq file] [gqpar
file] [iffpar file] [mvpar file] [pw password]
This command requires the OpenSSL library. It activates public key cryptography,
selects the message digest and signature encryption scheme and loads the required
private and public values described above. If one or more files are left unspeci-
fied, the default names are used as described above. Unless the complete path and
name of the file are specified, the location of a file is relative to the keys di-
rectory specified in the keysdir command or default /usr/local/etc. Following are
the subcommands:

cert file
Specifies the location of the required host public certificate 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 directory.

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 private keys and identity
parameters. This is required only if these files have been encrypted.

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

sign file
Specifies the location of the optional sign key file. This overrides the
link ntpkey_sign_hostname in the keys directory. If this file is not found,
the host key is also the sign key.

keys keyfile
Specifies the complete path and location of the MD5 key file containing the keys and
key identifiers used by ntpd(8), ntpq(1) and ntpdc(1) when operating with symmetric
key cryptography. This is the same operation as the -k command line option.

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

requestkey key
Specifies the key identifier to use with the ntpdc(1) utility program, which uses a
proprietary protocol specific to this implementation of ntpd(8). The key argument
is a key identifier for the trusted key, where the value can be in the range 1 to
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 in-
terval is 16 (65,536 s or about 18 hours). For poll intervals above the specified
interval, the values will be updated for every message sent.

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

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

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

102 (bad timestamp) The packet timestamp is the same or older than the most recent re-
ceived. 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 re-
ceived. 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 incorrect 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 bogus.

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

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

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

Monitoring Support
ntpd(8) includes a comprehensive monitoring facility suitable for continuous, long term
recording of server and client timekeeping performance. See the statistics command below
for a listing and example of each type of statistics currently supported. Statistic files
are managed using file generation sets and scripts in the ./scripts directory of the source
code distribution. Using these facilities and UNIX cron(8) jobs, the data can be automati-
cally summarized and archived for retrospective analysis.

Monitoring Commands
statistics name ...
Enables writing of statistics records. Currently, eight kinds of name statistics
are supported.

clockstats
Enables recording of clock driver statistics information. Each update re-
ceived from a clock driver appends a line of the following form to the file
generation set named clockstats:

49213 525.624 127.127.4.1 93 226 00:08:29.606 D

The first two fields show the date (Modified Julian Day) and time (seconds
and fraction past UTC midnight). The next field shows the clock address in
dotted-quad notation. The final field shows the last timecode received from
the clock in decoded ASCII format, where meaningful. In some clock drivers
a good deal of additional information can be gathered and displayed as well.
See information specific to each clock for further details.

cryptostats
This option requires the OpenSSL cryptographic software 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 information.

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

50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

The first two fields show the date (Modified Julian Day) and time (seconds
and fraction past UTC midnight). The next five fields show time offset
(seconds), frequency offset (parts per million - PPM), RMS jitter (seconds),
Allan deviation (PPM) and clock discipline time constant.

peerstats
Enables recording of peer statistics information. This includes statistics
records of all peers of a NTP server and of special signals, where present
and configured. Each valid update appends a line of the following form to
the current element of a file generation set named peerstats:

48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674

The first two fields show the date (Modified Julian Day) and time (seconds
and fraction past UTC midnight). The next two fields show the peer address
in dotted-quad notation and status, respectively. The status field is en-
coded in hex in the format described in Appendix A of the NTP specification
RFC 1305. The final four fields show the offset, delay, dispersion and RMS
jitter, all in seconds.

rawstats
Enables recording of raw-timestamp statistics information. This includes
statistics records of all peers of a NTP server and of special signals,
where present and configured. Each NTP message received from a peer or
clock driver appends a line of the following form to the file generation set
named rawstats:

50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000

The first two fields show the date (Modified Julian Day) and time (seconds
and fraction past UTC midnight). The next two fields show the remote peer
or clock address followed by the local address in dotted-quad notation. The
final four fields show the originate, receive, transmit and final NTP time-
stamps in order. The timestamp values are as received and before processing
by the various data smoothing and mitigation algorithms.

sysstats
Enables recording of ntpd statistics counters on a periodic basis. Each
hour a line of the following form is appended to the file generation set
named sysstats:

50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147

The first two fields show the date (Modified Julian Day) and time (seconds
and fraction past UTC midnight). The remaining ten fields show the statis-
tics counter values accumulated since the last generated line.

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

Packets received 81965
Total number of packets received.

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

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

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

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

filegen name [file filename] [type typename] [link | nolink] [enable | disable]
Configures setting of generation file set name. Generation file sets pro-
vide 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 writ-
ten to. The type given specifies when and how data will be directed to a
new element of the set. This way, information stored in elements of a file
set that are currently unused are available for administrational operations
without the risk of disturbing the operation of ntpd. (Most important: they
can be removed to free space for new data produced.)

Note that this command can be sent from the ntpdc(1) program running at a
remote location.

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

file filename
This is the file name for the statistics records. Filenames of set
members are built from three concatenated elements file ... prefix,
file ... filename and file ... suffix:

prefix This is a constant filename path. It is not subject to mod-
ifications via the filegen option. It is defined by the
server, usually specified as a compile-time constant. It
may, however, be configurable for individual file generation
sets via other commands. For example, the prefix used with
loopstats and peerstats generation can be configured using
the statsdir option explained above.

filename
This string is directly concatenated to the prefix mentioned
above (no intervening '/'). This can be modified using the
file argument to the filegen statement. No .. elements are
allowed in this component to prevent filenames referring to
parts outside the filesystem hierarchy denoted by prefix.

suffix This part is reflects individual elements of a file set. It
is generated according to the type of a file set.

type typename
A file generation set is characterized by its type. The following
types are supported:

none The file set is actually a single plain file.

pid One element of file set is used per incarnation of a ntpd
server. This type does not perform any changes to file set
members during runtime, however it provides an easy way of
separating files belonging to different ntpd(8) server in-
carnations. The set member filename is built by appending a
'.' to concatenated prefix and filename strings, and append-
ing the decimal representation of the process ID of the
ntpd(8) server process.

day One file generation set element is created per day. A day
is defined as the period between 00:00 and 24:00 UTC. The
file set member suffix consists of a '.' and a day specifi-
cation 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 De-
cember 1992 would end up in a file named prefix
filename.19921210.

week Any file set member contains data related to a certain week
of a year. The term week is defined by computing
day-of-year modulo 7. Elements of such a file generation
set are distinguished by appending the following suffix to
the file set filename base: A dot, a 4-digit year number,
the letter W, and a 2-digit week number. For example, in-
formation from January, 10th 1992 would end up in a file
with suffix .1992W1.

month One generation file set element is generated per month. The
file name suffix consists of a dot, a 4-digit year number,
and a 2-digit month.

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

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

link | nolink
It is convenient to be able to access the current element of a file
generation set by a fixed name. This feature is enabled by specify-
ing link and disabled using nolink. If link is specified, a hard
link from the current file set element to a file without suffix is
created. When there is already a file with this name and the number
of links of this file is one, it is renamed appending a dot, the
letter C, and the pid of the ntpd(8) server process. When the num-
ber 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(8) daemon implements a general purpose address/mask based restriction list. The
list contains address/match entries sorted first by increasing address values and and then
by increasing mask values. A match occurs when the bitwise AND of the mask and the packet
source address is equal to the bitwise AND of the mask and address in the list. The list is
searched in order with the last match found defining the restriction flags associated with
the entry. Additional information and examples can be found in the "Notes on Configuring
NTP and Setting up a NTP Subnet" page (available as part of the HTML documentation provided
in /usr/share/doc/ntp).

The restriction facility was implemented in conformance with the access policies for the
original NSFnet backbone time servers. Later the facility was expanded to deflect crypto-
graphic and clogging attacks. While this facility may be useful for keeping unwanted or
broken or malicious clients from congesting innocent servers, it should not be considered an
alternative to the NTP authentication facilities. Source address based restrictions are
easily circumvented by a determined cracker.

Clients can be denied service because they are explicitly included in the restrict list cre-
ated by the restrict command or implicitly as the result of cryptographic or rate limit vio-
lations. Cryptographic violations include certificate or identity verification failure;
rate limit violations generally result from defective NTP implementations that send packets
at abusive rates. Some violations cause denied service only for the offending packet, oth-
ers cause denied service for a timed period and others cause the denied service for an in-
definite 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 incre-
menting statistics counters. Sometimes a more proactive response is needed, such as a
server message that explicitly requests the client to stop sending and leave a message for
the system operator. A special packet format has been created for this purpose called the
"kiss-of-death" (KoD) packet. KoD packets have the leap bits set unsynchronized and stratum
set to zero and the reference identifier field set to a four-byte ASCII code. If the
noserve or notrust flag of the matching restrict list entry is set, the code is "DENY"; if
the limited flag is set and the rate limit is exceeded, the code is "RATE". Finally, if a
cryptographic violation occurs, the code is "CRYP".

A client receiving a KoD performs a set of sanity checks to minimize security exposure, then
updates the stratum and reference identifier peer variables, sets the access denied (TEST4)
bit in the peer flash variable and sends a message to the log. As long as the TEST4 bit is
set, the client will send no further packets to the server. The only way at present to re-
cover from this condition is to restart the protocol at both the client and server. This
happens automatically at the client when the association times out. It will happen at the
server only if the server operator cooperates.

Access Control Commands
discard [average avg] [minimum min] [monitor prob]
Set the parameters of the limited facility which protects the server from client
abuse. The average subcommand specifies the minimum average packet spacing, while
the minimum subcommand specifies the minimum packet spacing. Packets that violate
these minima are discarded and a kiss-o'-death packet returned if enabled. The de-
fault minimum average and minimum are 5 and 2, respectively. The monitor subcommand
specifies the probability of discard for packets that overflow the rate-control win-
dow.

restrict address [mask mask] [ippeerlimit int] [flag ...]
The address argument expressed in dotted-quad form is the address of a host or net-
work. Alternatively, the address argument can be a valid host DNS name. The mask
argument expressed in dotted-quad form defaults to 255.255.255.255, meaning that the
address is treated as the address of an individual host. A default entry (address
0.0.0.0, mask 0.0.0.0) is always included and 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 always restricts access, i.e., an entry with no
flags indicates that free access to the server is to be given. The flags are not
orthogonal, in that more restrictive flags will often make less restrictive ones re-
dundant. The flags can generally be classed into two categories, those which re-
strict 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(1) and ntpdc(1) queries.

kod If this flag is set when an access violation occurs, a kiss-o'-death (KoD)
packet is sent. KoD packets are rate limited to no more than one per sec-
ond. If another KoD packet occurs within one second after the last one, the
packet is dropped.

limited
Deny service if the packet spacing violates the lower limits specified in
the discard command. A history of clients is kept using the monitoring ca-
pability of ntpd(8). 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 usu-
ally assigned on a first come, first served basis, with later trap re-
questors 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 default, to maintain
backward compatability. Expect noepeer to become the default in ntp-4.4.

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

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

nopeer Deny unauthenticated packets which would result in mobilizing a new associa-
tion. This includes broadcast and symmetric active packets when a config-
ured association does not exist. It also includes pool associations, so if
you want to use servers from a pool directive and also want to use nopeer by
default, you'll want a restrict source ... line as well that does not in-
clude the nopeer directive.

noserve
Deny all packets except ntpq(1) and ntpdc(1) queries.

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

notrust
Deny service unless the packet is cryptographically authenticated.

ntpport
This is actually a match algorithm modifier, rather than a restriction flag.
Its presence causes the restriction entry to be matched only if the source
port in the packet is the standard NTP UDP port (123). Both ntpport and
non-ntpport may be specified. The ntpport is considered more specific and
is sorted later in the list.

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

version
Deny packets that do not match the current NTP version.

Default restriction list entries with the flags ignore, interface, ntpport, for each
of the local host's interface addresses are inserted into the table at startup to
prevent the server from attempting to synchronize to its own time. A default entry
is also always present, though if it is otherwise unconfigured; no flags are associ-
ated with the default entry (i.e., everything besides your own NTP server is unre-
stricted).

Automatic NTP Configuration Options
Manycasting
Manycasting is a automatic discovery and configuration paradigm new to NTPv4. It is in-
tended as a means for a multicast client to troll the nearby network neighborhood to find
cooperating manycast servers, validate them using cryptographic means and evaluate their
time values with respect to other servers that might be lurking in the vicinity. The in-
tended result is that each manycast client mobilizes client associations 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 manycast paradigm is designed to find a plurality of redundant servers
satisfying defined optimality criteria.

Manycasting can be used with either symmetric key or public key cryptography. The public
key infrastructure (PKI) offers the best protection against compromised keys and is gener-
ally considered stronger, at least with relatively large key sizes. It is implemented using
the Autokey 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 minimum feasible time-to-live (TTL) hops,
depending on how many servers have already been found. There can be as many manycast client
associations as different group address, each one serving as a template for a future
ephemeral 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 messages, and manycast server, which passively responds to them. If a
manycast server is in scope of the current TTL and is itself synchronized to a valid source
and operating at a stratum level equal to or lower than the manycast client, it replies to
the manycast client message 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 authenti-
cated 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 pro-
tocols 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 implosion due to near-simultaneous arrival of
manycast server messages. The strategy is determined by the manycastclient, tos and ttl
configuration commands. The manycast poll interval is normally eight times the system poll
interval, which starts out at the minpoll value specified in the manycastclient, command
and, under normal circumstances, increments to the maxpolll value specified in this command.
Initially, the TTL is set at the minimum hops specified by the ttl command. At each re-
transmission the TTL is increased until reaching the maximum hops specified by this command
or a sufficient number client associations have been found. Further retransmissions use the
same TTL.

The quality and reliability of the suite of associations discovered by the manycast client
is determined by the NTP mitigation algorithms and the minclock and minsane values specified
in the tos configuration command. At least minsane candidate servers must be available and
the mitigation algorithms produce at least minclock survivors in order to synchronize the
clock. Byzantine agreement principles require at least four candidates in order to cor-
rectly 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, assum-
ing 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 interval 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 incre-
ment for TTL hops is 32 starting from 31; however, the ttl configuration command can be used
to modify the values to match the scope rules.

It is often useful to narrow the range of acceptable servers which can be found by manycast
client associations. Because manycast servers respond only when the client stratum is equal
to or greater than the server stratum, primary (stratum 1) servers fill find only primary
servers in TTL range, which is probably the most common objective. However, unless config-
ured otherwise, all manycast clients in TTL range will eventually find all primary servers
in TTL range, which is probably 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 discarded, since that would result in a duplicate association.
If during a poll interval the number of client associations falls below minclock, all many-
cast 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 manycast client and many-
cast server. A number of hosts configured this way and sharing a common group address will
automatically organize themselves in an optimum configuration based on stratum and synchro-
nization 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 con-
figuration files including both multicastclient and multicastserver commands using, for in-
stance, multicast group address 239.1.1.1. The only exception is that each primary server
configuration file must include commands for the primary reference source such as a GPS re-
ceiver.

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

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

Some administrators prefer to avoid running ntpd(8) continuously and run either sntp(1) or
ntpd(8) -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(8) -q. The program wakes up, scans the local landscape looking for
the usual suspects, selects the best from among the rascals, sets the clock and then de-
parts. Servers do not have to be configured in advance and all clients throughout the net-
work can have the same configuration file.

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

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

Broadcast Options
tos [bcpollbstep gate]
This command provides a way to delay, by the specified number of broadcast poll in-
tervals, believing backward time steps from a broadcast server. Broadcast time net-
works 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 attacks can happen, however, and even though
there are a number of protections built in to broadcast mode, attempts to perform a
replay attack are possible. This value defaults to 0, but can be changed to any
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 algorithms. It can be used
to select the quality and quantity of peers used to synchronize the system clock and
is most useful in manycast mode. The variables operate as follows:

ceiling ceiling
Peers with strata above ceiling will be discarded if there are at least
minclock peers remaining. This value defaults to 15, but can be changed to
any number from 1 to 15.

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

floor floor
Peers with strata below floor will be discarded if there are at least
minclock peers remaining. This value defaults to 1, but can be changed to
any number from 1 to 15.

minclock minclock
The clustering algorithm repeatedly casts out outlier 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 al-
gorithm. If fewer than this number are available, the clock is undisci-
plined and allowed to run free. The default is 1 for legacy purposes. How-
ever, according to principles of Byzantine agreement, minsane should be at
least 4 in order to detect and discard a single falseticker.

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

Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio, satellite and modem ref-
erence clocks plus a special pseudo-clock used for backup or when no other clock source is
available. Detailed descriptions of individual device drivers and options can be found in
the "Reference Clock Drivers" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp). Additional information can be found in the pages linked there, includ-
ing the "Debugging Hints for Reference Clock Drivers" and "How To Write a Reference Clock
Driver" pages (available as part of the HTML documentation provided in /usr/share/doc/ntp).
In addition, support for a PPS signal is available as described in the "Pulse-per-second
(PPS) Signal Interfacing" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp). Many drivers support special line discipline/streams modules which can
significantly improve the accuracy using the driver. These are described in the "Line
Disciplines and Streams Drivers" page (available as part of the HTML documentation provided
in /usr/share/doc/ntp).

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

For the purposes of configuration, ntpd(8) treats reference clocks in a manner analogous to
normal NTP peers as much as possible. Reference clocks are identified by a syntactically
correct but invalid IP address, in order to distinguish them from normal NTP peers. 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 refer-
ence clock support. The mode option is added for reference clock support, as described be-
low. 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 op-
tions have meaning only for selected clock drivers. See the individual clock driver docu-
ment pages for additional information.

The fudge command is used to provide additional information for individual clock drivers and
normally follows immediately after the server command. The address argument specifies the
clock address. The refid and stratum options can be used to override the defaults for the
device. There are two optional device-dependent time offsets and four flags that can be in-
cluded in the fudge command as well.

The stratum number of a reference clock is by default zero. Since the ntpd(8) daemon adds
one to the stratum of each peer, a primary server ordinarily displays an external stratum of
one. In order to provide engineered backups, it is often useful to specify the reference
clock stratum as greater than zero. The stratum option is used for this purpose. Also, in
cases involving both a reference clock and a pulse-per-second (PPS) discipline signal, it is
useful to specify the reference clock identifier as other than the default, depending on the
driver. The refid option is used for this purpose. Except where noted, these options apply
to all clock drivers.

Reference Clock Commands
server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
This command can be used to configure reference clocks in special ways. The options
are interpreted as follows:

prefer Marks the reference clock as preferred. All other things being equal, this
host will be chosen for synchronization among a set of correctly operating
hosts. See the "Mitigation Rules and the prefer Keyword" page (available as
part of the HTML documentation provided in /usr/share/doc/ntp) for further
information.

mode int
Specifies a mode number which is interpreted in a device-specific fashion.
For instance, it selects a dialing protocol in the ACTS driver and a device
subtype in the parse drivers.

minpoll int

maxpoll int
These options specify the minimum and maximum polling interval for reference
clock messages, as a power of 2 in seconds For most directly connected ref-
erence clocks, both minpoll and maxpoll default to 6 (64 s). For modem ref-
erence clocks, minpoll defaults to 10 (17.1 m) and maxpoll defaults to 14
(4.5 h). The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.

fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [mode int] [flag1 0 |
1] [flag2 0 | 1] [flag3 0 | 1] [flag4 0 | 1]
This command can be used to configure reference clocks in special ways. It must im-
mediately follow the server command which configures the driver. Note that the same
capability is possible at run time using the ntpdc(1) program. The options are in-
terpreted 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 con-
stant to adjust the nominal time offset of a particular clock to agree with
an external standard, such as a precision PPS signal. It also provides a
way to correct a systematic error or bias due to serial port or operating
system latencies, different cable lengths or receiver internal delay. The
specified offset is in addition to the propagation delay provided by other
means, such as internal DIPswitches. Where a calibration for an individual
system and driver is available, an approximate correction is noted in the
driver 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 com-
mand described in Miscellaneous Options page and operates as described in
the "Reference Clock Drivers" page (available as part of the HTML documenta-
tion 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 documentation
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 as-
signed by the driver itself, usually zero.

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

mode int
Specifies a mode number which is interpreted in a device-specific fashion.
For instance, it selects a dialing protocol in the ACTS driver and a device
subtype in the parse drivers.

flag1 0 | 1

flag2 0 | 1

flag3 0 | 1

flag4 0 | 1
These four flags are used for customizing the clock driver. The interpreta-
tion of these values, and whether they are used at all, is a function of the
particular clock driver. However, by convention flag4 is used to enable
recording monitoring data to the clockstats file configured with the filegen
command. Further information on the filegen command can be found in
Monitoring Options.

Miscellaneous Options
broadcastdelay seconds
The broadcast and multicast modes require a special calibration to determine the
network delay between the local and remote servers. Ordinarily, this is done auto-
matically 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 cir-
cumstances. Typically (for Ethernet), a number between 0.003 and 0.007 seconds is
appropriate. The default when this command is not used is 0.004 seconds.

calldelay delay
This option controls the delay in seconds between the first and second packets sent
in burst or iburst mode to allow additional time for a modem or ISDN call to com-
plete.

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 com-
mand line option. If the file exists, it is read at startup in order to set the
initial frequency and then updated once per hour with the current frequency computed
by the daemon. If the file name is specified, but the file itself does not exist,
the starts with an initial frequency of zero and creates the file when writing it
for the first time. If this command is not given, the daemon will always start with
an initial frequency of zero.

The file format consists of a single line containing a single floating point number,
which records the frequency offset measured in parts-per-million (PPM). The file is
updated by first writing the current drift value into a temporary file and then re-
naming this file to replace the old version. This implies that ntpd(8) must have
write permission for the directory the drift file is located in, and that file sys-
tem links, symbolic or otherwise, should be avoided.

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

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

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

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

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

mode7 Enables processing of NTP mode 7 implementation-specific requests which are
used by the deprecated ntpdc(1) program. The default for this flag is dis-
able. This flag is excluded from runtime configuration using ntpq(1). The
ntpq(1) program provides the same capabilities as ntpdc(1) using standard
mode 6 requests.

monitor
Enables the monitoring facility. See the ntpdc(1) 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(8) is using autokey and it receives a crypto-NAK packet
that passes the duplicate packet and origin timestamp checks the peer vari-
ables are 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.

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(8) receives an autokey packet that fails TEST9, a crypto
failure, the association is immediately cleared. This is almost certainly a
feature, but if, in spite of the current recommendation of not using au-
tokey, you are using autokey you are seeing this sort of DoS attack dis-
abling this flag will delay tearing down the association until the reacha-
bility 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(8) receives a crypto-NAK packet that passes the dupli-
cate packet and origin timestamp checks the association is immediately
cleared. While this is generally a feature as it allows for quick recovery
if a server key has changed, a properly forged and appropriately delivered
crypto-NAK packet can be used in a DoS attack. If you have active noticable
problems with this type of DoS attack then you should consider disabling
this option. You can check your peerstats file for evidence of any of these
attacks. The default for this flag is enable.

unpeer_digest_early
By default, if ntpd(8) receives what should be an authenticated packet that
passes other packet sanity checks but contains an invalid digest the associ-
ation is immediately cleared. While this is generally a feature as it al-
lows for quick recovery, if this type of packet is carefully forged and sent
during an appropriate window it can be used for a DoS attack. If you have
active noticable problems with this type of DoS attack then you should con-
sider disabling this option. You can check your peerstats file for evidence
of any of these attacks. The default for this flag is enable.

includefile includefile
This command allows additional configuration commands to be included from a separate
file. Include files may be nested to a depth of five; upon reaching the end of any
include file, command processing resumes in the previous configuration file. This
option is useful for sites that run ntpd(8) on multiple hosts, with (mostly) common
options (e.g., a restriction list).

interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard name | address [/
prefixlen]]
The interface directive controls which network addresses ntpd(8) opens, and whether
input is dropped without processing. The first parameter determines the action for
addresses which match the second parameter. The second parameter specifies a class
of addresses, or a specific interface name, or an address. In the address case,
prefixlen determines how many bits must match for this rule to apply. ignore pre-
vents opening matching addresses, drop causes ntpd(8) 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 directly from the IERS at
https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list or
ftp://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list. The leapfile is scanned
when ntpd(8) 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 up-
dated.

leapsmearinterval seconds
This EXPERIMENTAL option is only available if ntpd(8) 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 messages can be controlled: informational messages (info), event mes-
sages (events), statistics messages (statistics) and status messages (status).

Configuration keywords are formed by concatenating the message class with the event
class. The all prefix can be used instead of a message class. A message class may
also be followed by the all keyword to enable/disable all messages of the respective
message class. Thus, a minimal log configuration could look like this:

logconfig =syncstatus +sysevents

This would just list the synchronizations state of ntpd(8) and the major system
events. For a simple reference server, the following minimum message configuration
could be useful:

logconfig =syncall +clockall

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

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

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 kilo-
bytes larger. As with all of the mru options offered in units of entries or
kilobytes, if both maxdepth and maxmem are used, the last one used controls.
The default is 1024 kilobytes.

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

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

initalloc count

initmem kilobytes
Initial memory allocation at the time the monitoringfacility is first en-
abled, in terms of the number of entries or kilobytes. The default is 4
kilobytes.

incalloc count

incmem kilobytes
Size of additional memory allocations when growing the MRU list, in entries
or kilobytes. The default is 4 kilobytes.

nonvolatile threshold
Specify the threshold delta in seconds before an hourly change to the driftfile
(frequency file) will be written, with a default value of 1e-7 (0.1 PPM). The fre-
quency file is inspected each hour. If the difference between the current frequency
and the last value written exceeds the threshold, the file is written and the
threshold becomes the new threshold value. If the threshold is not exceeeded, it is
reduced by half. This is intended to reduce the number of file writes for embedded
systems with nonvolatile memory.

phone dial ...
This command is used in conjunction with the ACTS modem driver (type 18) or the JJY
driver (type 40, mode 100 - 180). For the ACTS modem driver (type 18), the argu-
ments consist of a maximum of 10 telephone numbers used to dial USNO, NIST, or Euro-
pean 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 value | value] ... [default value | value]
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. The first number
specifies how early the poll may start, while the second number specifies how late
the poll may be delayed. 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 exposed by ntpq and
ntpdc.

rlimit [memlock Nmegabytes | stacksize N4kPages filenum Nfiledescriptors]

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

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

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

saveconfigdir directory_path
Specify the directory in which to write configuration snapshots 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 modifications given with
:config or config-from-file to the ntpd host's filename in the saveconfigdir. This
command will be rejected unless the saveconfigdir directive appears in ntpd 's con-
figuration file. filename can use strftime(3) format directives to substitute the
current date and time, for example, saveconfig ntp-%Y%m%d-%H%M%S.conf. The filename
used is stored in the system variable savedconfig. Authentication is required.

setvar variable [default]
This command adds an additional system variable. These variables can be used to
distribute additional information such as the access policy. If the variable of the
form name=value is followed by the default keyword, the variable will be listed as
part of the default system variables (ntpq(1) rv command)). These additional vari-
ables serve informational purposes only. They are not related to the protocol other
that they can be listed. The known protocol variables will always override any
variables defined via the setvar mechanism. There are three special variables that
contain the names of all variable of the same group. The sys_var_list holds the
names of all system variables. The peer_var_list holds the names of all peer vari-
ables 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 cir-
cumstances. It should occur in the configuration file before any other configura-
tion options. The default values of these variables have been carefully optimized
for a wide range of network speeds and reliability expectations. In general, they
interact in intricate ways that are hard to predict and some combinations can result
in some very nasty behavior. Very rarely is it necessary to change the default val-
ues; but, some folks cannot resist twisting the knobs anyway and this command is for
them. Emphasis added: twisters are on their own and can expect no help from the
support group.

The variables operate as follows:

allan allan
The argument becomes the new value for the minimum Allan intercept, which is
a parameter of the PLL/FLL clock discipline algorithm. The value in log2
seconds defaults to 7 (1024 s), which is also the lower limit.

dispersion dispersion
The argument becomes the new value for the dispersion increase rate, nor-
mally .000015 s/s.

freq freq
The argument becomes the initial value of the frequency offset in
parts-per-million. This overrides the value in the frequency file, if
present, and avoids the initial training state if it is not.

huffpuff huffpuff
The argument becomes the new value for the experimental huff-n'-puff filter
span, which determines the most recent interval the algorithm will search
for a minimum delay. The lower limit is 900 s (15 m), but a more reasonable
value is 7200 (2 hours). There is no default, since the filter is not en-
abled unless this command is given.

panic panic
The argument is the panic threshold, normally 1000 s. If set to zero, the
panic sanity check is disabled and a clock offset of any value will be ac-
cepted.

step step
The argument is the step threshold, which by default is 0.128 s. It can be
set to any positive number in seconds. If set to zero, step adjustments
will never occur. Note: The kernel time discipline is disabled if the step
threshold is set to zero or greater than the default.

stepback stepback
The argument is the step threshold for the backward direction, which by de-
fault 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 adjustments
will never occur. Note: The kernel time discipline is disabled if each di-
rection of step threshold are either set to zero or greater than .5 second.

stepfwd stepfwd
As for stepback, but for the forward direction.

stepout stepout
The argument is the stepout timeout, which by default is 900 s. It can be
set to any positive number in seconds. If set to zero, the stepout pulses
will not be suppressed.

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

trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host address and port number
for sending messages with the specified local 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 multihomed host the interface used may vary from
time to time with routing changes.

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

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

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

OPTIONS
--help Display usage information and exit.

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

--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a simple version. The
`c' mode will print copyright information and `n' will print the full copyright no-
tice.

OPTION PRESETS
Any option that is not marked as not presettable may be preset by loading values from envi-
ronment 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 auto-
gen-users AT lists.net. Thank you.