Provided by: ntp_4.2.8p12+dfsg-3ubuntu4.20.04.1_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 65535, inclusive. The default is to include no encryption field.
minpoll minpoll
maxpoll maxpoll
These options specify the minimum and maximum poll intervals for NTP messages, as a
power of 2 in seconds The maximum poll interval defaults to 10 (1,024 s), but can be
increased by the maxpoll option to an upper limit of 17 (36.4 h). The minimum poll
interval defaults to 6 (64 s), but can be decreased by the minpoll option to a lower
limit of 4 (16 s).
noselect
Marks the server as unused, except for display purposes. The server is discarded by
the selection algroithm.
preempt
Says the association can be preempted.
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 65,535 keys, each
distinguished by a 32-bit key identifier, to authenticate an association. The servers and
clients involved must agree on the key and key identifier to authenticate NTP packets. Keys
and related 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 65,535, inclusive.
crypto [cert file] [leap file] [randfile file] [host file] [sign file] [gq file] [gqpar
file] [iffpar file] [mvpar file] [pw password]
This command requires the OpenSSL library. It activates public key cryptography,
selects the message digest and signature encryption scheme and loads the required
private and public values described above. If one or more files are left
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
65,535, inclusive.
revoke logsec
Specifies the interval between re-randomization of certain cryptographic values used
by the Autokey scheme, as a power of 2 in seconds. These values need to be updated
frequently in order to deflect brute-force attacks on the algorithms of the scheme;
however, updating some values is a relatively expensive operation. The default
interval is 16 (65,536 s or about 18 hours). For poll intervals above the specified
interval, the values will be updated for every message sent.
trustedkey key ...
Specifies the key identifiers which are trusted for the 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 65,535.
Error Codes
The following error codes are reported via the NTP control and monitoring protocol trap
mechanism.
101 (bad field format or length) The packet has invalid version, length or format.
102 (bad timestamp) The packet timestamp is the same or older than the most recent
received. This could be due to a replay or a server clock time step.
103 (bad filestamp) The packet filestamp is the same or older than the most recent
received. This could be due to a replay or a key file generation error.
104 (bad or missing public key) The public key is missing, has 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] [ippeerlimit int] [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. The ippeerlimit directive limits the number of peer
requests for each IP to int, where a value of -1 means "unlimited", the current
default. A value of 0 means "none". There would usually be at most 1 peering
request per IP, but if the remote peering requests are behind a proxy there could
well be more than 1 per IP. In the current implementation, flag always restricts
access, i.e., an entry with no flags indicates that free access to the server is to
be given. The flags are not orthogonal, in that more restrictive flags will often
make less restrictive ones 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.
noepeer
Deny ephemeral peer requests, even if they come from an authenticated
source. Note that the ability to use a symmetric key for authentication may
be restricted to one or more IPs or subnets via the third field of the
ntp.keys file. This restriction is not enabled by default, to maintain
backward compatability. Expect noepeer to become the default in ntp-4.4.
nomodify
Deny ntpq(1) and ntpdc(1) queries which attempt to modify the state of the
server (i.e., run time reconfiguration). Queries which return information
are permitted.
noquery
Deny ntpq(1) and ntpdc(1) queries. Time service is not affected.
nopeer Deny unauthenticated packets which would result in mobilizing a new
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).
interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard name | address [/
prefixlen]]
The interface directive controls which network addresses ntpd(8) opens, and whether
input is dropped without processing. The first parameter determines the action for
addresses which match the second parameter. The second parameter specifies a class
of addresses, or a specific interface name, or an address. In the address case,
prefixlen determines how many bits must match for this rule to apply. ignore
prevents opening matching addresses, drop causes ntpd(8) to open the address and
drop all received packets without examination. Multiple interface directives can be
used. The last rule which matches a particular address determines the action for
it. interface directives are disabled if any -I, --interface, -L, or --novirtualips
command-line options are specified in the configuration file, all available network
addresses are opened. The nic directive is an alias for interface.
leapfile leapfile
This command loads the IERS leapseconds file and initializes the leapsecond values
for the next leapsecond event, leapfile expiration time, and TAI offset. The file
can be obtained directly from the IERS at
https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list or
ftp://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list. The leapfile is scanned
when ntpd(8) processes the leapfile directive or when ntpd detects that the leapfile
has changed. ntpd checks once a day to see if the leapfile has changed. The
update-leap(1update_leapmdoc) script can be run to see if the leapfile should be
updated.
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.
mru [maxdepth count | maxmem kilobytes | mindepth count | maxage seconds | initialloc count
| initmem kilobytes | incalloc count | incmem kilobytes]
Controls size limite of the monitoring facility's Most Recently Used (MRU) list of
client addresses, which is also used by the rate control facility.
maxdepth count
maxmem kilobytes
Equivalent upper limits on the size of the MRU list, in terms of entries or
kilobytes. The acutal limit will be up to incalloc entries or incmem
kilobytes larger. As with all of the mru options offered in units of
entries or kilobytes, if both maxdepth and maxmem are used, the last one
used controls. The default is 1024 kilobytes.
mindepth count
Lower limit on the MRU list size. When the MRU list has fewer than mindepth
entries, existing entries are never removed to make room for newer ones,
regardless of their age. The default is 600 entries.
maxage seconds
Once the MRU list has mindepth entries and an additional client is to ba
added to the list, if the oldest entry was updated more than maxage seconds
ago, that entry is removed and its storage is reused. If the oldest entry
was updated more recently the MRU list is grown, subject to maxdepth /
moxmem. The default is 64 seconds.
initalloc count
initmem kilobytes
Initial memory allocation at the time the monitoringfacility is first
enabled, in terms of the number of entries or kilobytes. The default is 4
kilobytes.
incalloc count
incmem kilobytes
Size of additional memory allocations when growing the MRU list, in entries
or kilobytes. The default is 4 kilobytes.
nonvolatile threshold
Specify the threshold delta in seconds before an hourly change to the driftfile
(frequency file) will be written, with a default value of 1e-7 (0.1 PPM). The
frequency file is inspected each hour. If the difference between the current
frequency and the last value written exceeds the threshold, the file is written and
the threshold becomes the new threshold value. If the threshold is not exceeeded,
it is reduced by half. This is intended to reduce the number of file writes for
embedded systems with nonvolatile memory.
phone dial ...
This command is used in conjunction with the ACTS modem driver (type 18) or the JJY
driver (type 40, mode 100 - 180). For the ACTS modem driver (type 18), the
arguments consist of a maximum of 10 telephone numbers used to dial USNO, NIST, or
European time service. For the JJY driver (type 40 mode 100 - 180), the argument is
one telephone number used to dial the telephone JJY service. The Hayes command ATDT
is normally prepended to the number. The number can contain other modem control
codes as well.
reset [allpeers] [auth] [ctl] [io] [mem] [sys] [timer]
Reset one or more groups of counters maintained by ntpd and exposed by ntpq and
ntpdc.
rlimit [memlock Nmegabytes | stacksize N4kPages filenum Nfiledescriptors]
memlock Nmegabytes
Specify the number of megabytes of memory that should be allocated and
locked. Probably only available under Linux, this option may be useful when
dropping root (the -i option). The default is 32 megabytes on non-Linux
machines, and -1 under Linux. -1 means "do not lock the process into
memory". 0 means "lock whatever memory the process wants into memory".
stacksize N4kPages
Specifies the maximum size of the process stack on systems with the
mlockall() function. Defaults to 50 4k pages (200 4k pages in OpenBSD).
filenum Nfiledescriptors
Specifies the maximum number of file descriptors ntpd may have open at once.
Defaults to the system default.
saveconfigdir directory_path
Specify the directory in which to write configuration snapshots requested with ntpq
's saveconfig command. If saveconfigdir does not appear in the configuration file,
saveconfig requests are rejected by ntpd.
saveconfig filename
Write the current configuration, including any runtime modifications given with
:config or config-from-file to the ntpd host's filename in the saveconfigdir. This
command will be rejected unless the saveconfigdir directive appears in ntpd 's
configuration file. filename can use strftime(3) format directives to substitute
the current date and time, for example, saveconfig ntp-%Y%m%d-%H%M%S.conf. The
filename used is stored in the system variable savedconfig. Authentication is
required.
setvar variable [default]
This command adds an additional system variable. These variables can be used to
distribute additional information such as the access policy. If the variable of the
form name=value is followed by the default keyword, the variable will be listed as
part of the default system variables (ntpq(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.
sysinfo
Display operational summary.
sysstats
Show statistics counters maintained in the protocol module.
tinker [allan allan | dispersion dispersion | freq freq | huffpuff huffpuff | panic panic |
step step | stepback stepback | stepfwd stepfwd | stepout stepout]
This command can be used to alter several system variables in very exceptional
circumstances. It should occur in the configuration file before any other
configuration options. The default values of these variables have been carefully
optimized for a wide range of network speeds and reliability expectations. In
general, they 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.
writevar assocID name = value [,...]
Write (create or update) the specified variables. If the assocID is zero, the
variablea re from the system variables name space, otherwise they are from the peer
variables name space. The assocID is required, as the same name can occur in both
name spaces.
trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host address and port number
for sending messages with the specified local 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.
ttl hop ...
This command specifies a list of TTL values in increasing order. Up to 8 values can
be specified. In manycast mode these values are used in-turn in an expanding-ring
search. The default is eight multiples of 32 starting at 31.
The trap receiver will generally log event messages and other information from the
server in a log file. While such monitor programs may also request their own trap
dynamically, configuring a trap receiver will ensure that no messages are lost when
the server is started.
hop ...
This command specifies a list of TTL values in increasing order, up to 8 values can
be specified. In manycast mode these values are used in turn in an expanding-ring
search. The default is eight multiples of 32 starting at 31.
OPTIONS
--help Display usage information and exit.
--more-help
Pass the extended usage information through a pager.
--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a simple version. The
`c' mode will print copyright information and `n' will print the full copyright
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.