FreeBSD Manual Pages

NTP.CONF(5) BSD File Formats Manual NTP.CONF(5)

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

SYNOPSIS
/etc/ntp.conf

DESCRIPTION
The ntp.conf configuration file is read at initial startup by the ntpd(8)
daemon in order to specify the synchronization sources, modes and other
related information. Usually, it is installed in the /etc directory, but
could be installed elsewhere (see the daemon's -c command line option).

The /etc/rc.d/ntpdate script reads this file to get a list of NTP servers
to use if the variable "ntpdate_hosts" was not declared. Refer to the
rc.conf(5) man page for further info about this.

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

The rest of this page describes the configuration and control options.
The "Notes on Configuring NTP and Setting up a NTP Subnet" page (avail-
able as part of the HTML documentation provided in /usr/share/doc/ntp)
contains an extended discussion of these options. In addition to the
discussion of general Configuration Options, there are sections describ-
ing 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 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, configu-
ration commands that configure a persistent association with a remote
server or peer or reference clock, and auxiliary commands that specify
environmental variables that control various related operations.

Configuration Commands
The various modes are determined by the command keyword and the type of
the required IP address. Addresses are classed by type as (s) a remote
server or peer (IPv4 class A, B and C), (b) the broadcast address of a
local interface, (m) a multicast address (IPv4 class D), or (r) a refer-
ence clock address (127.127.x.x). Note that only those options applica-
ble 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 de-
fault support of the IPv4 address family. In a few cases, including the
reslist billboard generated by ntpdc, 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 pre-
ceding the host name forces DNS resolution to the IPv4 namespace, while a
-6 qualifier forces DNS resolution to the IPv6 namespace. See IPv6 ref-
erences for the equivalent classes for that address family.

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

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

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

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

These four commands specify the time server name or address to be used
and the mode in which to operate. The address can be either a DNS name
or an IP address in dotted-quad notation. Additional information on as-
sociation behavior can be found in the "Association Management" page
(available as part of the HTML documentation provided in
/usr/share/doc/ntp).

server For type s and r addresses, this command mobilizes a persistent
client mode association with the specified remote server or local
radio clock. In this mode the local clock can synchronized to
the remote server, but the remote server can never be synchro-
nized to the local clock. This command should not be used for
type b or m addresses.

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

broadcast
For type b and m addresses (only), this command mobilizes a per-
sistent broadcast mode association. Multiple commands can be
used to specify multiple local broadcast interfaces (subnets)
and/or multiple multicast groups. Note that local broadcast mes-
sages go only to the interface associated with the subnet speci-
fied, but multicast messages go to all interfaces. In broadcast
mode the local server sends periodic broadcast messages to a
client population at the address specified, which is usually the
broadcast address on (one of) the local network(s) or a multicast
address assigned to NTP. The IANA has assigned the multicast
group address IPv4 224.0.1.1 and IPv6 ff05::101 (site local) ex-
clusively to NTP, but other nonconflicting addresses can be used
to contain the messages within administrative boundaries. Ordi-
narily, this specification applies only to the local server oper-
ating as a sender; for operation as a broadcast client, see the
broadcastclient or multicastclient commands below.

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

Options:

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

burst when the server is reachable, send a burst of eight packets in-
stead of the usual one. The packet spacing is normally 2 s; how-
ever, the spacing between the first and second packets can be
changed with the calldelay command to allow additional time for a
modem or ISDN call to complete. This is designed to improve
timekeeping quality with the server command and s addresses.

iburst When the server is unreachable, send a burst of eight packets in-
stead of the usual one. The packet spacing is normally 2 s; how-
ever, the spacing between the first two packets can be changed
with the calldelay command to allow additional time for a modem
or ISDN call to complete. This is designed to speed the initial
synchronization acquisition with the server command and s ad-
dresses and when ntpd(8) is started with the -q option.

key key
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified key
identifier with values from 1 to 65534, inclusive. The default
is to include no encryption field.

minpoll minpoll

maxpoll maxpoll
These options specify the minimum and maximum poll intervals for
NTP messages, as a power of 2 in seconds The maximum poll inter-
val 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 inter-
val defaults to 6 (64 s), but can be decreased by the minpoll op-
tion 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.

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

ttl ttl
This option is used only with broadcast server and manycast
client modes. It specifies the time-to-live ttl to use on broad-
cast server and multicast server and the maximum ttl for the ex-
panding 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.

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 symmetric-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 ad-
dress is required, but the NTP multicast address 224.0.1.1 as-
signed by the IANA should NOT be used, unless specific means are
taken to limit the span of the reply and avoid a possibly massive
implosion at the original sender. Note that, in order to avoid
accidental or malicious disruption in this mode, both the server
and client should operate using symmetric-key or public-key au-
thentication 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 receiv-
ing 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 sym-
metric-key or public-key authentication as described in
Authentication Options.

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

NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryp-
tography and, in addition, provides a new Autokey scheme based on public
key cryptography. Public key cryptography is generally considered more
secure than symmetric key cryptography, since the security is based on a
private value which is generated by each server and never revealed. With
Autokey all key distribution and management functions involve only public
values, which considerably simplifies key distribution and storage. Pub-
lic key management is based on X.509 certificates, which can be provided
by commercial services or produced by utility programs in the OpenSSL
software library or the NTPv4 distribution.

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

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

Authentication is always enabled, although ineffective if not configured
as described below. If a NTP packet arrives including a message authen-
tication 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 requires a preliminary protocol exchange to obtain the
server certificate, verify its credentials and initialize the protocol

The auth flag controls whether new associations or remote configuration
commands require cryptographic authentication. This flag can be set or
reset by the enable and disable commands and also by remote configuration
commands sent by a ntpdc(8) program running in another machine. If this
flag is enabled, which is the default case, new broadcast client and sym-
metric passive associations and remote configuration commands must be
cryptographically authenticated using either symmetric key or public key
cryptography. If this flag is disabled, these operations are effective
even if not cryptographic authenticated. It should be understood that
operating with the auth flag disabled invites a significant vulnerability
where a rogue hacker can masquerade as a falseticker and seriously dis-
rupt system timekeeping. It is important to note that this flag has no
purpose other than to allow or disallow a new association in response to
new broadcast and symmetric active messages and remote configuration com-
mands and, in particular, the flag has no effect on the authentication
process itself.

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

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

Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly 65,534
keys, each distinguished by a 32-bit key identifier, to authenticate an
association. The servers and clients involved must agree on the key and
key identifier to authenticate NTP packets. Keys and related 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 proto-
col itself. Besides the keys used for ordinary NTP associations, addi-
tional keys can be used as passwords for the ntpq(8) and ntpdc(8) utility
programs.

When ntpd(8) is first started, it reads the key file specified in the
keys configuration command and installs the keys in the key cache. How-
ever, individual keys must be activated with the trusted command before
use. This allows, for instance, the installation of possibly several
batches of keys and then activating or deactivating each batch remotely
using ntpdc(8). This also provides a revocation capability that can be
used if a key becomes compromised. The requestkey command selects the
key used as the password for the ntpdc(8) utility, while the controlkey
command selects the key used as the password for the ntpq(8) utility.

Public Key Cryptography
NTPv4 supports the original NTPv3 symmetric key scheme described in
RFC-1305 and in addition the Autokey protocol, which is based on public
key cryptography. The Autokey Version 2 protocol described on the Au-
tokey Protocol page verifies packet integrity using MD5 message digests
and verifies the source with digital signatures and any of several di-
gest/signature schemes. Optional identity schemes described on the Iden-
tity Schemes page and based on cryptographic challenge/response algo-
rithms are also available. Using all of these schemes provides strong
security against replay with or without modification, spoofing, masquer-
ade 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 sta-
tus, 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(8) program. This includes a required host key file, required
certificate file and optional sign key file, leapsecond file and identity
scheme files. The digest/signature scheme is specified in the X.509 cer-
tificate 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 di-
gest 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 obtain the cer-
tificates for all hosts along the trail to one or more trusted hosts.
This requires 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 ad-
dresses, since DNS can't be completely trusted until the name servers
have synchronized clocks. The cryptographic name used by Autokey to bind
the host identity credentials and cryptographic values must be indepen-
dent of interface, network and any other naming convention. The name ap-
pears in the host certificate in either or both the subject and issuer
fields, so protection against DNS compromise is essential.

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

It is also important to note that Autokey verifies authenticity using the
host name, network address and public keys, all of which are bound to-
gether by the protocol specifically to deflect masquerade attacks. For
this reason Autokey includes the source and destinatino 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 corporate NTP servers are op-
erated outside firewall perimeters.

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

The cryptotype of an association is determined at the time of mobiliza-
tion, either at configuration time or some time later when a message of
appropriate cryptotype arrives. When mobilized by a server or peer con-
figuration command and no key or autokey subcommands are present, the as-
sociation is not authenticated; if the key subcommand is present, the as-
sociation is authenticated using the symmetric key ID specified; if the
autokey subcommand is present, the association is authenticated using Au-
tokey.

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

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

Some examples may help to reduce confusion. Client Alice has no specific
cryptotype selected. Server Bob has both a symmetric key file and mini-
mal Autokey files. Alice's unauthenticated messages arrive at Bob, who
replies with unauthenticated messages. Cathy has a copy of Bob's symmet-
ric 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 veri-
fication fails, Bob sends Cathy a thing called a crypto-NAK, which tells
her something broke. She can see the evidence using the ntpq program.

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

It should be clear from the above that Bob can support all the girls at
the same time, as long as he has compatible authentication and identity
credentials. Now, Bob can act just like the girls in his own choice of
servers; he can run multiple configured associations with multiple dif-
ferent servers (or the same server, although that might not be useful).
But, wise security policy might preclude some cryptotype combinations;
for instance, running an identity scheme with one server and no authenti-
cation 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(8) 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(8) and ntpdc(8) util-
ity programs. The remaining files are necessary only for the Autokey
protocol.

Certificates imported from OpenSSL or public certificate authorities have
certian limitations. The certificate should be in ASN.1 syntax, X.509
Version 3 format and encoded in PEM, which is the same format used by
OpenSSL. The overall length of the certificate encoded in ASN.1 must not
exceed 1024 bytes. The subject distinguished name field (CN) is the
fully qualified name of the host on which it is used; the remaining sub-
ject fields are ignored. The certificate extension fields must not con-
tain 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 ev-
ery message sent.

controlkey key
Specifies the key identifier to use with the ntpq(8) utility,
which uses the standard protocol defined in RFC-1305. The key
argument is the key identifier for a trusted key, where the value
can be in the range 1 to 65,534, 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 en-
cryption scheme and loads the required private and public values
described above. If one or more files are left unspecified, the
default names are used as described above. Unless the complete
path and name of the file are specified, the location of a file
is relative to the keys directory specified in the keysdir com-
mand or default /usr/local/etc. Following are the subcommands:

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

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

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

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

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

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

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

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

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 con-
taining the keys and key identifiers used by ntpd(8), ntpq(8) and
ntpdc 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 crypto-
graphic keys, parameters and certificates. The default is
/usr/local/etc/.

requestkey key
Specifies the key identifier to use with the ntpdc(8) utility
program, which uses a proprietary protocol specific to this im-
plementation of ntpd(8). The key argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,534, inclusive.

revoke logsec
Specifies the interval between re-randomization of certain cryp-
tographic 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; how-
ever, updating some values is a relatively expensive operation.
The default interval is 16 (65,536 s or about 18 hours). For
poll intervals above the specified interval, the values will be
updated for every message sent.

trustedkey key ...
Specifies the key identifiers which are trusted for the purposes
of authenticating peers with symmetric key cryptography, as well
as keys used by the ntpq(8) and ntpdc(8) programs. The authenti-
cation procedures require that both the local and remote servers
share the same key and key identifier for this purpose, although
different keys can be used with different servers. The key argu-
ments are 32-bit unsigned integers with values from 1 to 65,534.

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

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

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

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

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

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

106 (mismatched digest types) Not used.

107 (bad signature length) The signature length does not match the
current public key.

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

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

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

111 (bad or missing cookie) The cookie is missing, corrupted or bo-
gus.

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

113 (bad or missing certificate) The certificate is missing, cor-
rupted or bogus.

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

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

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

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

49213 525.624 127.127.4.1 93 226 00:08:29.606 D

The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next field shows the clock address in dotted-quad nota-
tion. 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 informa-
tion can be gathered and displayed as well. See informa-
tion 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 nota-
tion, 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 fol-
lowing form to the file generation set named loopstats:

50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next five fields show time offset (seconds), frequency
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 no-
tation 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 sec-
onds.

rawstats
Enables recording of raw-timestamp statistics informa-
tion. This includes statistics records of all peers of a
NTP server and of special signals, where present and con-
figured. 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, trans-
mit and final NTP timestamps in order. The timestamp
values are as received and before processing by the vari-
ous data smoothing and mitigation algorithms.

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

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

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

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

Packets received 81965
Total number of packets received.

Packets processed 0
Number of packets received in response to previ-
ous packets sent

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

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

Bad version 71793
Number of packets matching neither NTP version.

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

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

Bad authentication 10
Number of packets not verified as authentic.

Rate exceeded 147
Number of packets discarded due to rate limita-
tion.

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

filegen name [file filename] [type typename] [link | nolink]
[enable | disable]
Configures setting of generation file set name. Genera-
tion file sets provide a means for handling files that
are continuously growing during the lifetime of a server.
Server statistics are a typical example for such files.
Generation file sets provide access to a set of files
used to store the actual data. At any time at most one
element of the set is being written to. The type given
specifies when and how data will be directed to a new el-
ement of the set. This way, information stored in ele-
ments of a file set that are currently unused are avail-
able 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(8) pro-
gram running at a remote location.

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

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

prefix This is a constant filename path. It is
not subject to modifications via the
filegen option. It is defined by the
server, usually specified as a compile-
time constant. It may, however, be con-
figurable 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 interven-
ing `/'). This can be modified using the
file argument to the filegen statement.
No .. elements are allowed in this compo-
nent 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 in-
carnation of a ntpd server. This type
does not perform any changes to file set
members during runtime, however it pro-
vides an easy way of separating files be-
longing 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 cre-
ated per day. A day is defined as the
period between 00:00 and 24:00 UTC. The
file set member suffix consists of a `.'
and a day specification in the form
YYYYMMdd. YYYY is a 4-digit year number
(e.g., 1992). MM is a two digit month
number. dd is a two digit day number.
Thus, all information written at 10 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 gener-
ation set are distinguished by appending
the following suffix to the file set
filename base: A dot, a 4-digit year num-
ber, the letter W, and a 2-digit week
number. For example, information from
January, 10th 1992 would end up in a file
with suffix .1992W1.

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

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

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

link | nolink
It is convenient to be able to access the current
element of a file generation set by a fixed name.
This feature is enabled by specifying link and
disabled using nolink. If link is specified, a
hard link from the current file set element to a
file without 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 append-
ing a dot, the letter C, and the pid of the ntpd
server process. When the number of links is
greater than one, the file is unlinked. This al-
lows the current file to be accessed by a con-
stant name.

enable | disable
Enables or disables the recording function.

Access Control Support
The ntpd(8) daemon implements a general purpose address/mask based re-
striction 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 ad-
dress 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 re-
striction 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 facil-
ity was expanded to deflect cryptographic and clogging attacks. While
this facility may be useful for keeping unwanted or broken or malicious
clients from congesting innocent servers, it should not be considered an
alternative to the NTP authentication facilities. Source address based
restrictions are easily circumvented by a determined cracker.

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

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

A client receiving a KoD performs a set of sanity checks to minimize se-
curity exposure, then updates the stratum and reference identifier peer
variables, sets the access denied (TEST4) bit in the peer flash variable
and sends a message to the log. As long as the TEST4 bit is set, the
client will send no further packets to the server. The only way at
present to recover from this condition is to restart the 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 en-
abled. The default minimum average and minimum are 5 and 2, re-
spectively. The monitor subcommand specifies the probability of
discard for packets that overflow the rate-control window.

restrict address [mask mask] [flag ...]
The address argument expressed in dotted-quad form is the address
of a host or network. Alternatively, the address argument can be
a valid host DNS name. The mask argument expressed in 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 default entry. In
the current implementation, flag always restricts access, i.e.,
an entry with no flags indicates that free access to the server
is to be given. The flags are not orthogonal, in that more re-
strictive flags will often make less restrictive ones redundant.
The flags can generally be classed into two categories, those
which restrict time service and those which restrict informa-
tional 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(8) and ntpdc(8)
queries.

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

limited
Deny service if the packet spacing violates the lower
limits specified in the discard command. A history of
clients is kept using the monitoring capability of
ntpd(8). Thus, monitoring is always active as long as
there is a restriction entry with the limited flag.

lowpriotrap
Declare traps set by matching hosts to be low priority.
The number of traps a server can maintain is limited (the
current limit is 3). Traps are usually assigned on a
first come, first served basis, with later trap 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.

nomodify
Deny ntpq(8) and ntpdc(8) queries which attempt to modify
the state of the server (i.e., run time reconfiguration).
Queries which return information are permitted.

noquery
Deny ntpq(8) and ntpdc(8) queries. Time service is not
affected.

nopeer Deny packets which would result in mobilizing a new asso-
ciation. This includes broadcast and symmetric active
packets when a configured association does not exist.

noserve
Deny all packets except ntpq(8) and ntpdc(8) queries.

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

notrust
Deny service unless the packet is cryptographically au-
thenticated.

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

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

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

Automatic NTP Configuration Options
Manycasting
Manycasting is a automatic discovery and configuration paradigm new to
NTPv4. It is intended as a means for a multicast client to troll the
nearby network neighborhood to find cooperating manycast servers, vali-
date them using cryptographic means and evaluate their time values with
respect to other servers that might be lurking in the vicinity. The in-
tended result is that each manycast client mobilizes client associations
with some number of the "best" of the nearby manycast servers, yet auto-
matically 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 para-
digm is designed to find a plurality of redundant servers satisfying de-
fined optimality criteria.

Manycasting can be used with either symmetric key or public key cryptog-
raphy. The public key infrastructure (PKI) offers the best protection
against compromised keys and is generally considered stronger, at least
with relatively large key sizes. It is implemented using the Autokey
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 many-
castclient 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 feasi-
ble time-to-live (TTL) hops, depending on how many servers have already
been found. There can be as many manycast client associations as differ-
ent 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 distinc-
tion 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 many-
cast 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 tem-
plate, but only if cryptographically authenticated and the server stratum
is less than or equal to the client stratum. Authentication is explic-
itly required and either symmetric key or public key (Autokey) can be
used. Then, the client polls the server at its unicast address in burst
mode in order to reliably set the host clock and validate the source.
This normally results in a volley of eight client/server at 2-s intervals
during which both the synchronization and cryptographic protocols run
concurrently. Following the volley, the client runs the NTP intersection
and clustering algorithms, which act to discard all but the "best" asso-
ciations according to stratum and synchronization distance. The surviv-
ing 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 implo-
sion 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 retransmission
the TTL is increased until reaching the maximum hops specified by this
command or a sufficient number client associations have been found. Fur-
ther retransmissions use the same TTL.

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

If at least minclock servers are found, the manycast poll interval is im-
mediately set to eight times maxpoll. If less than minclock servers are
found when the TTL has reached the maximum hops, the manycast poll inter-
val 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 increment for TTL hops is 32 starting from
31; however, the ttl configuration command can be used to modify the val-
ues to match the scope rules.

It is often useful to narrow the range of acceptable servers which can be
found by manycast client associations. Because manycast servers respond
only when the client stratum is equal to or greater than the server stra-
tum, primary (stratum 1) servers fill find only primary servers in TTL
range, which is probably the most common objective. However, unless con-
figured otherwise, all manycast clients in TTL range will eventually find
all primary servers in TTL range, which is probably not the most common
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 as-
sociation is mobilized, subsequent manycast server replies are discarded,
since that would result in a duplicate association. If during a poll in-
terval 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 manycast server. A number of hosts configured this way and
sharing a common group address will automatically organize themselves in
an optimum configuration based on stratum and synchronization distance.
For example, consider an NTP subnet of two primary servers and a hundred
or more dependent clients. With two exceptions, all servers and clients
have identical configuration files including both multicastclient and
multicastserver commands using, for instance, multicast group address
239.1.1.1. The only exception is that each primary server configuration
file must include commands for the primary reference source such as a GPS
receiver.

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

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

Some administrators prefer to avoid running ntpd(8) continuously and run
either ntpdate(8) 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 departs. Servers do not have to be configured in
advance and all clients throughout the network can have the same configu-
ration 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. Follow-
ing the burst interval all but three survivors are cast off, but the cer-
tificates remain in the local cache. It often happens that several com-
plete 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 certifi-
cates 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.

Manycast Options
tos [ceiling ceiling | cohort { 0 | 1 } | floor floor | minclock minclock
| minsane minsane]
This command affects the clock selection and clustering algo-
rithms. It can be used to select the quality and quantity of
peers used to synchronize the system clock and is most useful in
manycast mode. The variables operate as follows:

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

cohort {0 | 1}
This is a binary flag which enables (0) or disables (1)
manycast server replies to manycast clients with the same
stratum level. This is useful to reduce 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 de-
faults to 1, but can be changed to any number from 1 to
15.

minclock minclock
The clustering algorithm repeatedly casts out outlyer as-
sociations until no more than minclock associations re-
main. This value defaults to 3, but can be changed to
any number from 1 to the number of configured sources.

minsane minsane
This is the minimum number of candidates available to the
clock selection algorithm in order to produce one or more
truechimers for the clustering algorithm. If fewer than
this number are available, the clock is undisciplined and
allowed to run free. The default is 1 for legacy pur-
poses. However, according to principles of Byzantine
agreement, minsane should be at least 4 in order to de-
tect 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, sat-
ellite and modem reference clocks plus a special pseudo-clock used for
backup or when no other clock source is available. Detailed descriptions
of individual device drivers and options can be found in the "Reference
Clock Drivers" page (available as part of the HTML documentation provided
in /usr/share/doc/ntp). Additional information can be found in the pages
linked there, including the "Debugging Hints for Reference Clock Drivers"
and "How To Write a Reference Clock Driver" pages (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 doc-
umentation 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 depen-
dent, but is usually a serial port. A device driver specific to each
reference clock must be selected and compiled in the distribution; how-
ever, most common radio, satellite and modem clocks are included by de-
fault. Note that an attempt to configure a reference clock when the
driver has not been compiled or the hardware port has not been appropri-
ately configured results in a scalding remark to the system log file, but
is otherwise non hazardous.

For the purposes of configuration, ntpd(8) treats reference clocks in a
manner analogous to normal NTP peers as much as possible. Reference
clocks are identified by a syntactically correct but invalid IP address,
in order to distinguish them from normal NTP peers. Reference clock ad-
dresses are of the form 127.127.t.u, where t is an integer denoting the
clock type and u indicates the unit number in the range 0-3. While it
may seem overkill, it is in fact sometimes useful to configure multiple
reference clocks of the same type, in which case the unit numbers must be
unique.

The server command is used to configure a reference clock, where the
address argument in that command is the clock address. The key, version
and ttl options are not used for reference clock support. The mode op-
tion is added for reference clock support, as described below. The
prefer option can be useful to persuade the server to cherish a reference
clock with somewhat more enthusiasm than other reference clocks or peers.
Further information on this option can be found in the "Mitigation Rules
and the prefer Keyword" (available as part of the HTML documentation pro-
vided in /usr/share/doc/ntp) page. The minpoll and maxpoll options have
meaning only for selected clock drivers. See the individual clock driver
document pages for additional information.

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

The stratum number of a reference clock is by default zero. Since the
ntpd(8) daemon adds one to the stratum of each peer, a primary server or-
dinarily displays an external stratum of one. In order to provide engi-
neered 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 in-
terval for reference clock messages, as a power of 2 in
seconds For most directly connected reference clocks,
both minpoll and maxpoll default to 6 (64 s). For modem
reference clocks, minpoll defaults to 10 (17.1 m) and
maxpoll defaults to 14 (4.5 h). The allowable range is 4
(16 s) to 17 (36.4 h) inclusive.

fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string]
[mode int] [flag1 0 | 1] [flag2 0 | 1] [flag3 0 | 1] [flag4 0 |
1]
This command can be used to configure reference clocks in special
ways. It must immediately follow the server command which con-
figures the driver. Note that the same capability is possible at
run time using the ntpdc(8) program. The options are interpreted
as follows:

time1 sec
Specifies a constant to be added to the time offset pro-
duced by the driver, a fixed-point decimal number in sec-
onds. This is used as a calibration constant to adjust
the nominal time offset of a particular clock to agree
with an external standard, such as a precision PPS sig-
nal. It also provides a way to correct a systematic er-
ror or bias due to serial port or operating system laten-
cies, different cable lengths or receiver internal delay.
The specified offset is in addition to the propagation
delay provided by other means, such as internal DIP-
switches. Where a calibration for an individual system
and driver is available, an approximate correction is
noted in the driver documentation pages. Note: in order
to facilitate calibration when more than one radio clock
or PPS signal is supported, a special calibration feature
is available. It takes the form of an argument to the
enable command described in Miscellaneous Options page
and operates as described in the "Reference Clock
Drivers" page (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 de-
scriptions of specific drivers in the "Reference Clock
Drivers" page (available as part of the HTML documenta-
tion 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 de-
fault stratum number ordinarily assigned by the driver
itself, usually zero.

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

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

flag1 0 | 1

flag2 0 | 1

flag3 0 | 1

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

Miscellaneous Options
broadcastdelay seconds
The broadcast and multicast modes require a special calibration
to determine the network delay between the local and remote
servers. Ordinarily, this is done automatically by the initial
protocol exchanges between the client and server. In some cases,
the calibration procedure may fail due to network or server ac-
cess controls, for example. This command specifies the default
delay to be used under these circumstances. Typically (for Eth-
ernet), 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 complete.

driftfile driftfile
This command specifies the complete path and name of the file
used to record the frequency of the local clock oscillator. This
is the same operation as the -f command line option. If the file
exists, it is read at startup in order to set the initial fre-
quency 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 mea-
sured in parts-per-million (PPM). The file is updated by first
writing the current drift value into a temporary file and then
renaming this file to replace the old version. This implies that
ntpd(8) must have write permission for the directory the drift
file is located in, and that file system links, symbolic or oth-
erwise, should be avoided.

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 de-
fault for this flag is enable.

bclient
Enables the server to listen for a message from a broad-
cast or multicast server, as in the multicastclient com-
mand 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.

monitor
Enables the monitoring facility. See the ntpdc(8) pro-
gram 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 use-
ful for testing. The default for this flag is enable.

pps Enables the pulse-per-second (PPS) signal when frequency
and time is disciplined by the precision time kernel mod-
ifications. See the "A Kernel Model for Precision
Timekeeping" (available as part of the HTML documentation
provided in /usr/share/doc/ntp) page for further informa-
tion. The default for this flag is disable.

stats Enables the statistics facility. See the Monitoring
Options section for further information. The default for
this flag is disable.

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

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 messages
(events), statistics messages (statistics) and status messages
(status).

Configuration keywords are formed by concatenating the message
class with the event class. The all prefix can be used instead
of a message class. A message class may also be followed by the
all keyword to enable/disable all messages of the respective mes-
sage 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 fol-
lowing minimum message configuration could be useful:

logconfig =syncall +clockall

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

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

setvar variable [default]
This command adds an additional system variable. These variables
can be used to distribute additional information such as the ac-
cess policy. If the variable of the form name=value is followed
by the default keyword, the variable will be listed as part of
the default system variables (ntpq(8) rv command)). These addi-
tional variables serve informational purposes only. They are not
related to the protocol other that they can be listed. The known
protocol variables will always override any variables defined via
the setvar mechanism. There are three special variables that
contain the names of all variable of the same group. The
sys_var_list holds the names of all system variables. The
peer_var_list holds the names of all peer variables and the
clock_var_list holds the names of the reference clock variables.

tinker [allan allan | dispersion dispersion | freq freq | huffpuff
huffpuff | panic panic | step srep | stepout stepout]
This command can be used to alter several system variables in
very exceptional circumstances. It should occur in the configu-
ration file before any other configuration options. The default
values of these variables have been carefully optimized for a
wide range of network speeds and reliability expectations. In
general, they interact in intricate ways that are hard to predict
and some combinations can result in some very nasty behavior.
Very rarely is it necessary to change the default values; but,
some folks cannot resist twisting the knobs anyway and this com-
mand 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 dis-
cipline algorithm. The value in log2 seconds defaults to
7 (1024 s), which is also the lower limit.

dispersion dispersion
The argument becomes the new value for the dispersion in-
crease rate, normally .000015 s/s.

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

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

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

step step
The argument is the step threshold, which by default is
0.128 s. It can be set to any positive number in sec-
onds. 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.

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 sup-
pressed.

trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host address
and port number for sending messages with the specified local in-
terface address. If the port number is unspecified, a value of
18447 is used. If the interface address is not specified, the
message is sent with a source address of the local interface the
message is sent through. Note that on a multihomed host the in-
terface used may vary from time to time with routing changes.

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

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

FILES
/etc/ntp.conf the default name of the configuration file
ntp.keys private MD5 keys
ntpkey RSA private key
ntpkey_host RSA public key
ntp_dh Diffie-Hellman agreement parameters

SEE ALSO
rc.conf(5), ntpd(8), ntpdc(8), ntpq(8)

In addition to the manual pages provided, comprehensive documentation is
available on the world wide web at http://www.ntp.org/. A snapshot of
this documentation is available in HTML format in /usr/share/doc/ntp.

David L. Mills, Network Time Protocol (Version 3), RFC1305.

BUGS
The syntax checking is not picky; some combinations of ridiculous and
even hilarious options and modes may not be detected.

The ntpkey_host files are really digital certificates. These should be
obtained via secure directory services when they become universally
available.

BSD December 21, 2006 BSD

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=ntp.conf&sektion=5&manpath=FreeBSD+8.2-RELEASE+and+Ports>

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