Provided by: ntp_4.2.8p10+dfsg-5ubuntu7.3_amd64 
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 command line option).
The file format is similar to other Unix configuration files. Comments begin with a ‘#’ character and
extend to the end of the line; blank lines are ignored. Configuration commands consist of an initial
keyword followed by a list of arguments, some of which may be optional, separated by whitespace.
Commands may not be continued over multiple lines. Arguments may be host names, host addresses written
in numeric, dotted-quad form, integers, floating point numbers (when specifying times in seconds) and
text strings.
The rest of this page describes the configuration and control options. The "Notes on Configuring NTP and
Setting up an NTP Subnet" page (available as part of the HTML documentation provided in
/usr/share/doc/ntp) contains an extended discussion of these options. In addition to the discussion of
general “Configuration Options”, there are sections describing the following supported functionality and
the options used to control it:
• “Authentication Support”
• “Monitoring Support”
• “Access Control Support”
• “Automatic NTP Configuration Options”
• “Reference Clock Support”
• “Miscellaneous Options”
Following these is a section describing “Miscellaneous Options”. While there is a rich set of options
available, the only required option is one or more pool, server, peer, broadcast or manycastclient
commands.
Configuration Support
Following is a description of the configuration commands in NTPv4. These commands have the same basic
functions as in NTPv3 and in some cases new functions and new arguments. There are two classes of
commands, configuration commands that configure a persistent association with a remote server or peer or
reference clock, and auxiliary commands that specify environmental variables that control various related
operations.
Configuration Commands
The various modes are determined by the command keyword and the type of the required IP address.
Addresses are classed by type as (s) a remote server or peer (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 family. 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]
server address [key key | autokey] [burst] [iburst] [version version] [prefer] [minpoll minpoll] [maxpoll
maxpoll] [true]
peer address [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll] [true]
[xleave]
broadcast address [key key | autokey] [version version] [prefer] [minpoll minpoll] [ttl ttl] [xleave]
manycastclient address [key key | autokey] [version version] [prefer] [minpoll minpoll] [maxpoll maxpoll]
[ttl ttl]
These five commands specify the time server name or address to be used and the mode in which to operate.
The address can be either a DNS name or an IP address in dotted-quad notation. Additional information on
association behavior can be found in the "Association Management" page (available as part of the HTML
documentation provided in /usr/share/doc/ntp).
pool For type s addresses, this command mobilizes a persistent client mode association with a number
of remote servers. In this mode the local clock can synchronized to the remote server, but the
remote server can never be synchronized to the local clock.
server For type s and r addresses, this command mobilizes a persistent client mode association with the
specified remote server or local radio clock. In this mode the local clock can synchronized to
the remote server, but the remote server can never be synchronized to the local clock. This
command should not be used for type b or m addresses.
peer For type s addresses (only), this command mobilizes a persistent symmetric-active mode
association with the specified remote peer. In this mode the local clock can be synchronized to
the remote peer or the remote peer can be synchronized to the local clock. This is useful in a
network of servers where, depending on various failure scenarios, either the local or remote peer
may be the better source of time. This command should NOT be used for type b, m or r 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 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) exclusively to NTP, but other nonconflicting addresses
can be used to contain the messages within administrative boundaries. Ordinarily, this
specification applies only to the local server operating as a sender; for operation as a
broadcast client, see the broadcastclient or multicastclient commands below.
manycastclient
For type m addresses (only), this command mobilizes a manycast client mode association for the
multicast address specified. In this case a specific address must be supplied which matches the
address used on the manycastserver command for the designated manycast servers. The NTP
multicast address 224.0.1.1 assigned by the IANA should NOT be used, unless specific means are
taken to avoid spraying large areas of the Internet with these messages and causing a possibly
massive implosion of replies at the sender. The manycastserver command specifies that the local
server is to operate in client mode with the remote servers that are discovered as the result of
broadcast/multicast messages. The client broadcasts a request message to the group address
associated with the specified address and specifically enabled servers respond to these messages.
The client selects the servers providing the best time and continues as with the server command.
The remaining servers are discarded as if never heard.
Options:
autokey
All packets sent to and received from the server or peer are to include authentication fields
encrypted using the autokey scheme described in “Authentication 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 quality with the server command and s addresses.
iburst When the server is unreachable, send a burst of eight packets instead of the usual one. The
packet spacing is normally 2 s; however, 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
addresses and when ntpd(8) is started with the -q option.
key key
All packets sent to and received from the server or peer are to include authentication fields
encrypted using the specified key identifier with values from 1 to 65534, inclusive. The default
is to include no encryption field.
minpoll minpoll
maxpoll maxpoll
These options specify the minimum and maximum poll intervals for NTP messages, 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.
true Marks the server as a truechimer. Use this option only for testing.
prefer Marks the server as preferred. All other things being equal, this host will be chosen for
synchronization among a set of correctly operating hosts. See the "Mitigation Rules and the
prefer Keyword" page (available as part of the HTML documentation provided in /usr/share/doc/ntp)
for further information.
true Forces the association to always survive the selection and clustering algorithms. This option
should almost certainly only be used while testing an association.
ttl ttl
This option is used only with broadcast server and manycast client modes. It specifies the
time-to-live ttl to use on broadcast server and multicast server and the maximum ttl for the
expanding ring search with manycast client packets. Selection of the proper value, which
defaults to 127, is something of a black art and should be coordinated with the network
administrator.
version version
Specifies the version number to be used for outgoing NTP packets. Versions 1-4 are the choices,
with version 4 the default.
xleave Valid in peer and broadcast modes only, this flag enables interleave mode.
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 address is required, but the NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means are taken to limit the span of the
reply and avoid a possibly massive implosion at the original sender. Note that, in order to
avoid accidental or malicious disruption in this mode, both the server and client should operate
using symmetric-key or public-key authentication as described in “Authentication 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 authentication of received NTP
packets. Originally, this was done using the Data Encryption Standard (DES) algorithm operating in
Cipher Block Chaining (CBC) mode, commonly called DES-CBC. Subsequently, this was replaced by the RSA
Message Digest 5 (MD5) algorithm using a private key, commonly called keyed-MD5. Either algorithm
computes a message digest, or one-way hash, which can be used to verify the server has the correct
private key and key identifier.
NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryptography and, in addition,
provides a new Autokey scheme based on public key cryptography. Public key cryptography is generally
considered more secure than symmetric key cryptography, since the security is based on a private value
which is generated by each server and never revealed. With Autokey all key distribution and management
functions involve only public values, which considerably simplifies key distribution and storage. Public
key management is based on X.509 certificates, which can be provided by commercial services or produced
by utility programs in the OpenSSL software library or the NTPv4 distribution.
While the algorithms for symmetric key cryptography are included in the NTPv4 distribution, public key
cryptography requires the OpenSSL software library to be installed before building the NTP distribution.
Directions for doing that are on the Building and Installing the Distribution page.
Authentication is configured separately for each association using the key or autokey subcommand on the
peer, server, broadcast and manycastclient configuration commands as described in “Configuration Options”
page. The authentication options described below specify the locations of the key files, if other than
default, which symmetric keys are trusted and the 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 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(1) program running on another machine. If this flag is enabled,
which is the default case, new broadcast client and symmetric passive associations and remote
configuration commands must be cryptographically authenticated using either symmetric key or public key
cryptography. If this flag is disabled, these operations are effective even if not cryptographic
authenticated. It should be understood that operating with the auth flag disabled invites a significant
vulnerability where a rogue hacker can masquerade as a falseticker and seriously disrupt system
timekeeping. It is important to note that this flag has no purpose other than to allow or disallow a new
association in response to new broadcast and symmetric active messages and remote configuration commands
and, in particular, the flag has no effect on the authentication process itself.
An attractive alternative where multicast support is available is manycast mode, in which clients
periodically troll for servers as described in the “Automatic NTP Configuration Options” page. Either
symmetric key or public key cryptographic authentication can be used in this mode. The principle
advantage of manycast mode is that potential servers need not be configured in advance, since the client
finds them during regular operation, and the configuration files for all clients can be identical.
The security model and protocol schemes for both symmetric key and public key cryptography are summarized
below; further details are in the 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 65,534 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 distributed 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 possibly 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 compromised. 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 protocol described on the
Autokey Protocol page verifies packet integrity using MD5 message digests and verifies the source with
digital signatures and any of several digest/signature schemes. Optional identity schemes described on
the Identity Schemes page and based on cryptographic challenge/response algorithms are also 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 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 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 obtain the certificates for all
hosts along the trail to one or more trusted hosts. This requires the configuration file in all hosts to
be 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 provisions to allow alternate
names or aliases. However, this is not to say that DNS aliases, different names for each interface,
etc., are constrained in any way.
It is also important to note that Autokey verifies authenticity using the host name, network address and
public keys, all of which are bound together by the protocol specifically to deflect masquerade attacks.
For this reason Autokey includes the source and destination IP 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 operated outside firewall perimeters.
Operation
A specific combination of authentication scheme (none, symmetric key, public key) and identity scheme is
called a cryptotype, although not all combinations are compatible. There may be management
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 security practice.
The cryptotype of an association is determined at the time of mobilization, either at configuration time
or some time later when a message of appropriate cryptotype arrives. When mobilized by a server or peer
configuration command and no key or autokey subcommands are present, the association is not
authenticated; if the key subcommand is present, the association is authenticated using the symmetric key
ID specified; if the autokey subcommand is present, the association is authenticated using Autokey.
When multiple identity schemes are supported in the Autokey protocol, the first message exchange
determines which one is used. The client request message contains bits corresponding to which schemes it
has available. The server response message contains bits corresponding to which schemes it has
available. Both server and client match the 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, unconfigured broadcast or manycast client associations or
symmetric passive associations will not be mobilized unless the server supports a cryptotype compatible
with the first packet received. By default, unauthenticated associations will not be mobilized unless
overridden in a decidedly dangerous way.
Some examples may help to reduce confusion. Client Alice has no specific cryptotype selected. Server
Bob has both a symmetric key file and minimal Autokey files. Alice's unauthenticated messages arrive at
Bob, who replies with unauthenticated messages. Cathy has a copy of Bob's symmetric key file and has
selected key ID 4 in messages to Bob. Bob verifies the 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 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 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 limitations. The
certificate should be in ASN.1 syntax, X.509 Version 3 format and encoded in PEM, which is the same
format used by OpenSSL. The overall length of the certificate encoded in ASN.1 must not exceed 1024
bytes. The subject distinguished name field (CN) is the fully qualified name of the host on which it is
used; the remaining subject fields are ignored. The certificate extension fields must not contain either
a subject key identifier or a issuer key 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 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 encryption scheme and loads the required private and public values
described above. If one or more files are left unspecified, the default names are used as
described above. Unless the complete path and name of the file are specified, the location of a
file is relative to the keys directory specified in the keysdir command or default
/usr/local/etc. Following are the subcommands:
cert file
Specifies the location of the required host public 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 65,534, inclusive.
revoke logsec
Specifies the interval between re-randomization of certain cryptographic values used by the
Autokey scheme, as a power of 2 in seconds. These values need to be updated frequently in order
to deflect brute-force attacks on the algorithms of the scheme; however, updating some values is
a relatively expensive operation. The default interval is 16 (65,536 s or about 18 hours). For
poll intervals above the specified interval, the values will be updated for every message sent.
trustedkey key ...
Specifies the key identifiers which are trusted for the purposes of authenticating peers with
symmetric key cryptography, as well as keys used by the ntpq(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 different keys can be used with different servers. The
key arguments 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 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 automatically summarized and archived for retrospective analysis.
Monitoring Commands
statistics name ...
Enables writing of statistics records. Currently, eight kinds of name statistics are supported.
clockstats
Enables recording of clock driver statistics information. Each update received from a
clock driver appends a line of the following form to the file generation set named
clockstats:
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
The first two fields show the date (Modified Julian Day) and time (seconds and fraction
past UTC midnight). The next field shows the clock address in dotted-quad notation. The
final field shows the last timecode received from the clock in decoded ASCII format,
where meaningful. In some clock drivers a good deal of additional information can be
gathered and displayed as well. See information specific to each clock for further
details.
cryptostats
This option requires the OpenSSL cryptographic software 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 encoded in hex in the format described in
Appendix A of the NTP specification RFC 1305. The final four fields show the offset,
delay, dispersion and RMS jitter, all in seconds.
rawstats
Enables recording of raw-timestamp statistics information. This includes statistics
records of all peers of a NTP server and of special signals, where present and
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 timestamps 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 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 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 created (see
below). This keyword allows the (otherwise constant) filegen filename prefix to be
modified for file generation sets, which is useful for handling statistics logs.
filegen name [file filename] [type typename] [link | nolink] [enable | disable]
Configures setting of generation file set name. Generation file sets provide a means for
handling files that are continuously growing during the lifetime of a server. Server
statistics are a typical example for such files. Generation file sets provide access to
a set of files used to store the actual data. At any time at most one element of the set
is being written to. The type given specifies when and how data will be directed to a
new element of the set. This way, 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 modifications via
the filegen option. It is defined by the server, usually specified as a
compile-time constant. It may, however, be configurable for individual
file generation sets via other commands. For example, the prefix used
with loopstats and peerstats 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 incarnations. The set member filename is built
by appending a ‘.’ to concatenated prefix and filename strings, and
appending 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 specification in the form YYYYMMdd. YYYY is
a 4-digit year number (e.g., 1992). MM is a two digit month number. dd
is a two digit day number. Thus, all information written at 10 December
1992 would end up in a file named prefix filename.19921210.
week Any file set member contains data related to a certain week of a year.
The term week is defined by computing day-of-year modulo 7. Elements of
such a file generation set are distinguished by appending the following
suffix to the file set filename base: A dot, a 4-digit year number, the
letter W, and a 2-digit week number. For example, information from
January, 10th 1992 would end up in a file with suffix .1992W1.
month One generation file set element is generated per month. The file name
suffix consists of a dot, a 4-digit year number, and a 2-digit month.
year One generation file element is generated per year. The filename suffix
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. Information is only written to a file
generation by specifying enable; output is prevented by specifying
disable.
link | nolink
It is convenient to be able to access the current element of a file generation
set by a fixed name. This feature is enabled by specifying link and disabled
using nolink. If link is specified, a hard link from the current file set
element to a file without 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 number of
links is greater than one, the file is unlinked. This allows the current file to
be accessed by a constant name.
enable | disable
Enables or disables the recording function.
Access Control Support
The ntpd(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 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 include certificate or identity verification failure; rate limit violations generally result
from defective NTP implementations that send packets at abusive rates. Some violations cause denied
service only for the offending packet, others cause denied service for a timed period and others cause
the denied service for an indefinite period. When a client or network is denied access for an indefinite
period, the only way at present to remove the restrictions is by restarting the server.
The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further action except incrementing
statistics counters. Sometimes a more proactive response is needed, such as a server message that
explicitly requests the client to stop sending and leave a message for the system operator. A special
packet format has been created for this purpose called the "kiss-of-death" (KoD) packet. KoD packets
have the leap bits set unsynchronized and stratum set to zero and the reference 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 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 enabled. The default minimum average and minimum are 5 and 2,
respectively. 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 restrictive flags will often make less restrictive ones
redundant. The flags can generally be classed into two categories, those which restrict time
service and those which restrict informational queries and attempts to do run-time
reconfiguration of the server. One or more of the following flags may be specified:
ignore Deny packets of all kinds, including ntpq(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 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 requestors being denied service. This flag
modifies the assignment algorithm by allowing low priority traps to be overridden by
later requests for normal priority traps.
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 packets which would result in mobilizing a new association. This includes broadcast
and symmetric active packets when a configured association does not exist. It also
includes pool associations, so if you want to use servers from a pool directive and also
want to use nopeer by default, you'll want a restrict source ... line as well that does
not include the nopeer directive.
noserve
Deny all packets except ntpq(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.
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 associated with the default entry (i.e., everything
besides 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,
validate them using cryptographic means and evaluate their time values with respect to other servers that
might be lurking in the vicinity. The intended result is that each manycast client mobilizes client
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 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 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 authenticated and the server stratum
is less than or equal to the client stratum. Authentication is explicitly required and either symmetric
key or public key (Autokey) can be used. Then, the client polls the server at its unicast address in
burst mode in order to reliably set the host clock and validate the source. This normally results in a
volley of eight client/server at 2-s intervals during which both the synchronization and cryptographic
protocols run concurrently. Following the volley, the client runs the NTP intersection and clustering
algorithms, which act to discard all but the "best" associations according to stratum and synchronization
distance. The surviving associations then continue in ordinary client/server mode.
The manycast client polling strategy is designed to reduce as much as possible the volume of manycast
client messages and the effects of 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 retransmission the TTL is increased until reaching the maximum hops specified by
this command or a sufficient number client associations have been found. Further retransmissions use the
same TTL.
The quality and reliability of the suite of associations discovered by the manycast client is determined
by the NTP mitigation algorithms and the minclock and minsane values specified in the tos configuration
command. At least minsane candidate servers must be available and the mitigation algorithms produce at
least minclock survivors in order to synchronize the clock. Byzantine agreement principles require at
least four candidates in order to correctly discard a single falseticker. For legacy purposes, minsane
defaults to 1 and minclock defaults to 3. For manycast service minsane should be explicitly set to 4,
assuming at least that number of servers are available.
If at least minclock servers are found, the manycast poll interval is immediately set to eight times
maxpoll. If less than minclock servers are found when the TTL has reached the maximum hops, the manycast
poll interval is doubled. For each transmission after that, the poll 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 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 configured 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 manycast client prototype associations are reset to the
initial poll interval and TTL hops and operation resumes from the beginning. It is important to avoid
frequent manycast client messages, since each one requires all manycast servers in TTL range to respond.
The result could well be an implosion, either minor or major, depending on the number of servers in
range. The recommended value for maxpoll is 12 (4,096 s).
It is possible and frequently useful to configure a host as both manycast client and manycast server. A
number of hosts configured this way and sharing a common group address will automatically organize
themselves in an optimum configuration based on stratum and synchronization distance. For example,
consider an NTP subnet of two primary servers and a hundred or more dependent clients. With two
exceptions, all servers and clients have identical configuration files including both multicastclient and
multicastserver commands using, for instance, multicast group address 239.1.1.1. The only exception is
that each primary server configuration file must include commands for the primary reference source such
as a GPS receiver.
The remaining configuration files for all secondary servers and clients have the same contents, except
for the tos command, which is specific for each stratum level. For stratum 1 and stratum 2 servers, that
command is not necessary. For stratum 3 and above servers the floor value is set to the intended stratum
number. Thus, all stratum 3 configuration files are identical, all stratum 4 files are identical and so
forth.
Once operations have stabilized in this scenario, the primary servers will find the primary reference
source and each other, since they both operate at the same stratum (1), but not with any secondary server
or client, since these operate at a higher stratum. The secondary servers will find the servers at the
same stratum level. If one of the primary servers loses its GPS receiver, it will continue to operate as
a client and other clients will time out the corresponding association and re-associate accordingly.
Some administrators prefer to avoid running ntpd(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 departs. Servers do not have to be configured in advance and all
clients throughout the network can have the same configuration file.
Manycast Interactions with Autokey
Each time a manycast client sends a client mode packet to a multicast group address, all manycast servers
in scope generate a reply including the host name and status word. The manycast clients then run the
Autokey protocol, which collects and verifies all certificates involved. Following the burst interval
all but three survivors are cast off, but the certificates remain in the local cache. It often happens
that several complete signing trails from the client to the primary servers are collected in this way.
About once an hour or less often if the poll interval exceeds this, the client regenerates the Autokey
key list. This is in general transparent in client/server mode. However, about once per day the server
private value used to generate cookies is refreshed along with all manycast client 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 intervals,
believing backward time steps from a broadcast server. Broadcast time networks are expected to
be trusted. In the event a broadcast server's time is stepped backwards, there is clear benefit
to having the clients notice this change as soon as possible. Attacks such as replay 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
algorithm. If fewer than this number are available, the clock is undisciplined
and allowed to run free. The default is 1 for legacy purposes. However,
according to principles of Byzantine agreement, minsane should be at least 4 in
order to detect and discard a single falseticker.
ttl hop ...
This command specifies a list of TTL values in increasing order, up to 8 values can be
specified. In manycast mode these values are used in turn in an expanding-ring search.
The default is eight multiples of 32 starting at 31.
Reference Clock Support
The NTP Version 4 daemon supports some three dozen different radio, satellite and modem reference clocks
plus a special pseudo-clock used for backup or when no other clock source is available. Detailed
descriptions of individual device drivers and options can be found in the "Reference Clock Drivers" page
(available as part of the HTML documentation provided in /usr/share/doc/ntp). Additional information can
be found in the pages linked there, including the "Debugging Hints for Reference Clock Drivers" and "How
To Write a Reference Clock Driver" pages (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 reference clock must be selected and compiled in the
distribution; however, most common radio, satellite and modem clocks are included by default. Note that
an attempt to configure a reference clock when the driver has not been compiled or the hardware port has
not been appropriately configured results in a scalding remark to the system log file, but is otherwise
non hazardous.
For the purposes of configuration, ntpd(8) treats reference clocks in a manner analogous to normal NTP
peers as much as possible. Reference clocks are identified by a syntactically correct but invalid IP
address, in order to distinguish them from normal NTP peers. Reference clock addresses are of the form
127.127.t.u, where t is an integer denoting the clock type and u indicates the unit number in the range
0-3. While it may seem overkill, it is in fact sometimes useful to configure multiple reference clocks
of the same type, in which case the unit numbers must be unique.
The server command is used to configure a reference clock, where the address argument in that command is
the clock address. The key, version and ttl options are not used for reference clock support. The mode
option is added for reference clock support, as described below. The prefer option can be useful to
persuade the server to cherish a reference clock with somewhat more enthusiasm than other reference
clocks or peers. Further information on this option can be found in the "Mitigation Rules and the prefer
Keyword" (available as part of the HTML documentation provided in /usr/share/doc/ntp) page. The minpoll
and maxpoll options have meaning only for selected clock drivers. See the individual clock driver
document pages for additional information.
The fudge command is used to provide additional information for individual clock drivers and normally
follows immediately after the server command. The address argument specifies the clock address. The
refid and stratum options can be used to override the defaults for the device. There are two optional
device-dependent time offsets and four flags that can be included in the fudge command as well.
The stratum number of a reference clock is by default zero. Since the ntpd(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 reference clocks, both
minpoll and maxpoll default to 6 (64 s). For modem reference clocks, minpoll defaults to
10 (17.1 m) and maxpoll defaults to 14 (4.5 h). The allowable range is 4 (16 s) to 17
(36.4 h) inclusive.
fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string] [mode int] [flag1 0 | 1] [flag2 0
| 1] [flag3 0 | 1] [flag4 0 | 1]
This command can be used to configure reference clocks in special ways. It must immediately
follow the server command which configures the driver. Note that the same capability is possible
at run time using the ntpdc(1) program. The options are interpreted as follows:
time1 sec
Specifies a constant to be added to the time offset produced by the driver, a fixed-point
decimal number in seconds. This is used as a calibration constant to adjust the nominal
time offset of a particular clock to agree with an external standard, such as a precision
PPS signal. It also provides a way to correct a systematic error or bias due to serial
port or operating system latencies, different cable lengths or receiver internal delay.
The specified offset is in addition to the propagation delay provided by other means,
such as internal DIPswitches. Where a calibration for an individual system and driver is
available, an approximate correction is noted in the driver documentation pages. Note:
in order to facilitate calibration when more than one radio clock or PPS signal is
supported, a special calibration feature is available. It takes the form of an argument
to the enable command described in “Miscellaneous Options” page and operates as described
in the "Reference Clock Drivers" page (available as part of the HTML documentation
provided in /usr/share/doc/ntp).
time2 secs
Specifies a fixed-point decimal number in seconds, which is interpreted in a
driver-dependent way. See the descriptions of specific drivers in the "Reference Clock
Drivers" page (available as part of the HTML 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 assigned by the driver itself,
usually zero.
refid string
Specifies an ASCII string of from one to four characters which defines the reference
identifier used by the driver. This string overrides the default identifier ordinarily
assigned by the driver itself.
mode int
Specifies a mode number which is interpreted in a device-specific fashion. For instance,
it selects a dialing protocol in the ACTS driver and a device subtype in the parse
drivers.
flag1 0 | 1
flag2 0 | 1
flag3 0 | 1
flag4 0 | 1
These four flags are used for customizing the clock driver. The interpretation of these
values, and whether they are used at all, is a function of the particular clock driver.
However, by convention flag4 is used to enable recording monitoring data to the
clockstats file configured with the filegen command. Further information on the filegen
command can be found in “Monitoring Options”.
Miscellaneous Options
broadcastdelay seconds
The broadcast and multicast modes require a special calibration to determine the network delay
between the local and remote servers. Ordinarily, this is done automatically by the initial
protocol exchanges between the client and server. In some cases, the calibration procedure may
fail due to network or server access controls, for example. This command specifies the default
delay to be used under these circumstances. Typically (for Ethernet), a number between 0.003 and
0.007 seconds is appropriate. The default when this command is not used is 0.004 seconds.
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 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 renaming this file to replace the
old version. This implies that ntpd(8) must have write permission for the directory the drift
file is located in, and that file system links, symbolic or otherwise, should be avoided.
dscp value
This option specifies the Differentiated Services Control Point (DSCP) value, a 6-bit code. The
default value is 46, signifying Expedited Forwarding.
enable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp | stats | peer_clear_digest_early |
unpeer_crypto_early | unpeer_crypto_nak_early | unpeer_digest_early]
disable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp | stats | peer_clear_digest_early |
unpeer_crypto_early | unpeer_crypto_nak_early | unpeer_digest_early]
Provides a way to enable or disable various server options. Flags not mentioned are unaffected.
Note that all of these flags can be controlled remotely using the ntpdc(1) utility program.
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 disable. 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 variables are immediately
cleared. While this is generally a feature as it allows for quick recovery if a server
key has changed, a properly forged and appropriately 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 autokey, you are using autokey you are seeing
this sort of DoS attack disabling this flag will delay tearing down the association until
the reachability counter becomes zero. You can check your peerstats file for evidence of
any of these attacks. The default for this flag is enable.
unpeer_crypto_nak_early
By default, if ntpd(8) receives a crypto-NAK packet that passes the duplicate packet and
origin timestamp checks the association is immediately cleared. While this is generally
a feature as it allows for quick recovery if a server key has changed, a properly forged
and appropriately delivered crypto-NAK packet can be used in a DoS attack. If you have
active noticable problems with this type of DoS attack then you should consider disabling
this option. You can check your peerstats file for evidence of any of these attacks.
The default for this flag is enable.
unpeer_digest_early
By default, if ntpd(8) receives what should be an authenticated packet that passes other
packet sanity checks but contains an invalid digest the association is immediately
cleared. While this is generally a feature as it allows 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 consider disabling this option. You can check your peerstats file for evidence of
any of these attacks. The default for this flag is enable.
includefile includefile
This command allows additional configuration commands to be 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).
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 messages (events), statistics messages (statistics) and status messages (status).
Configuration keywords are formed by concatenating the message class with the event class. The
all prefix can be used instead of a message class. A message class may also be followed by the
all keyword to enable/disable all messages of the respective message class. Thus, a minimal log
configuration could look like this:
logconfig =syncstatus +sysevents
This would just list the synchronizations state of ntpd(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.
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 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 step |
stepback stepback | stepfwd stepfwd | stepout stepout]
This command can be used to alter several system variables in very exceptional circumstances. It
should occur in the configuration file before any other configuration options. The default
values of these variables have been carefully optimized for a wide range of network speeds and
reliability expectations. In general, they 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
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, normally .000015
s/s.
freq freq
The argument becomes the initial value of the frequency offset in parts-per-million.
This overrides the value in the frequency file, if present, and avoids the initial
training state if it is not.
huffpuff huffpuff
The argument becomes the new value for the experimental huff-n'-puff filter span, which
determines the most recent interval the algorithm will search for a minimum delay. The
lower limit is 900 s (15 m), but a more reasonable value is 7200 (2 hours). There is no
default, since the filter is not enabled unless this command is given.
panic panic
The argument is the panic threshold, normally 1000 s. If set to zero, the panic sanity
check is disabled and a clock offset of any value will be accepted.
step step
The argument is the step threshold, which by default is 0.128 s. It can be set to any
positive number in seconds. If set to zero, step adjustments will never occur. Note:
The kernel time discipline is disabled if the step threshold is set to zero or greater
than the default.
stepback stepback
The argument is the step threshold for the backward direction, which by default is 0.128
s. It can be set to any positive number in seconds. If both the forward and backward
step thresholds are set to zero, step adjustments will never occur. Note: The kernel
time discipline is disabled if each direction of step threshold are either set to zero or
greater than .5 second.
stepfwd stepfwd
As for stepback, but for the forward direction.
stepout stepout
The argument is the stepout timeout, which by default is 900 s. It can be set to any
positive number in seconds. If set to zero, the stepout pulses will not be suppressed.
rlimit [memlock Nmegabytes | stacksize N4kPages filenum Nfiledescriptors]
memlock Nmegabytes
Specify the number of megabytes of memory that should be allocated and locked. Probably
only available under Linux, this option may be useful when dropping root (the -i option).
The default is 32 megabytes on non-Linux machines, and -1 under Linux. -1 means "do not
lock the process into memory". 0 means "lock 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.
trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host address and port number for sending
messages with the specified local interface address. If the port number is unspecified, a value
of 18447 is used. If the interface address is not specified, the message is sent with a source
address of the local interface the message is sent through. Note that on a multihomed host the
interface used may vary from time to time with routing changes.
The trap receiver will generally log event messages and other information from the server in a
log file. While such monitor programs may also request their own trap dynamically, configuring a
trap receiver will ensure that no messages are lost when the server is started.
hop ...
This command specifies a list of TTL values in increasing order, up to 8 values can be specified.
In manycast mode these values are used in turn in an expanding-ring search. The default is eight
multiples of 32 starting at 31.
OPTIONS
--help Display usage information and exit.
--more-help
Pass the extended usage information through a pager.
--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a simple version. The `c' mode
will print copyright information and `n' will print the full copyright notice.
OPTION PRESETS
Any option that is not marked as not presettable may be preset by loading values from environment
variables named:
NTP_CONF_<option-name> or NTP_CONF
ENVIRONMENT
See OPTION PRESETS for configuration environment variables.
FILES
/etc/ntp.conf the default name of the configuration file
ntp.keys private MD5 keys
ntpkey RSA private key
ntpkey_host RSA public key
ntp_dh Diffie-Hellman agreement parameters
EXIT STATUS
One of the following exit values will be returned:
0 (EXIT_SUCCESS)
Successful program execution.
1 (EXIT_FAILURE)
The operation failed or the command syntax was not valid.
70 (EX_SOFTWARE)
libopts had an internal operational error. Please report it to
autogen-users@lists.sourceforge.net. Thank you.
SEE ALSO
ntpd(8), ntpdc(1), ntpq(1)
In addition to the manual pages provided, comprehensive documentation is available on the world wide web
at http://www.ntp.org/. A snapshot of this documentation is available in HTML format in
/usr/share/doc/ntp.
David L. Mills, Network Time Protocol (Version 4), RFC5905.
AUTHORS
The University of Delaware and Network Time Foundation
COPYRIGHT
Copyright (C) 1992-2017 The University of Delaware and Network Time Foundation all rights reserved. This
program is released under the terms of the NTP license, <http://ntp.org/license>.
BUGS
The syntax checking is not picky; some combinations of ridiculous and even hilarious options and modes
may not be detected.
The ntpkey_host files are really digital certificates. These should be obtained via secure directory
services when they become universally available.
Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org
NOTES
This document was derived from FreeBSD.
This manual page was AutoGen-erated from the ntp.conf option definitions.
Debian March 21 2017 NTP_CONF(5)