DragonFly On-Line Manual Pages
ntp.conf(5) File Formats ntp.conf(5)
NAME
ntp.conf - Network Time Protocol (NTP) daemon configuration file format
SYNOPSIS
ntp.conf [--option-name] [--option-name value]
All arguments must be options.
DESCRIPTION
The ntp.conf configuration file is read at initial startup by the
ntpd(1) 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 ntpdc, IPv6 addresses are
automatically generated. IPv6 addresses can be identified by the
presence of colons ":" in the address field. IPv6 addresses can be
used almost everywhere where IPv4 addresses can be used, with the
exception of reference clock addresses, which are always IPv4.
Note that in contexts where a host name is expected, a -4 qualifier
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]
peer address [key key | autokey] [version version] [prefer] [minpoll
minpoll] [maxpoll maxpoll]
broadcast address [key key | autokey] [version version] [prefer]
[minpoll minpoll] [ttl ttl]
manycastclient address [key key | autokey] [version version] [prefer]
[minpoll minpoll] [maxpoll maxpoll] [ttl ttl]
These 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(1) is started with the -q option.
key key
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified key
identifier with values from 1 to 65534, inclusive. The default
is to include no encryption field.
minpoll minpoll
maxpoll maxpoll
These options specify the minimum and maximum poll intervals for
NTP messages, as a power of 2 in seconds The maximum poll
interval defaults to 10 (1,024 s), but can be increased by the
maxpoll option to an upper limit of 17 (36.4 h). The minimum
poll interval defaults to 6 (64 s), but can be decreased by the
minpoll option to a lower limit of 4 (16 s).
noselect
Marks the server as unused, except for display purposes. The
server is discarded by the selection algroithm.
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.
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.
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 in another
machine. If this flag is enabled, which is the default case, new
broadcast client and symmetric passive associations and remote
configuration commands must be cryptographically authenticated using
either symmetric key or public key cryptography. If this flag is
disabled, these operations are effective even if not cryptographic
authenticated. It should be understood that operating with the auth
flag disabled invites a significant vulnerability where a rogue hacker
can masquerade as a falseticker and seriously disrupt system
timekeeping. It is important to note that this flag has no purpose
other than to allow or disallow a new association in response to new
broadcast and symmetric active messages and remote configuration
commands and, in particular, the flag has no effect on the
authentication process itself.
An attractive alternative where multicast support is available is
manycast mode, in which clients periodically troll for servers as
described in the Automatic NTP Configuration Options page. Either
symmetric key or public key cryptographic authentication can be used in
this mode. The principle advantage of manycast mode is that potential
servers need not be configured in advance, since the client finds them
during regular operation, and the configuration files for all clients
can be identical.
The security model and protocol schemes for both symmetric key and
public key cryptography are summarized below; further details are in
the briefings, papers and reports at the NTP project page linked from
http://www.ntp.org/.
Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly 65,534
keys, each distinguished by a 32-bit key identifier, to authenticate an
association. The servers and clients involved must agree on the key
and key identifier to authenticate NTP packets. Keys and related
information are specified in a key file, usually called ntp.keys, which
must be distributed and stored using secure means beyond the scope of
the NTP protocol itself. Besides the keys used for ordinary NTP
associations, additional keys can be used as passwords for the ntpq(1)
and ntpdc(1) utility programs.
When ntpd(1) 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 destinatino IP
addresses in message digest computations and so the same addresses must
be available at both the server and client. For this reason operation
with network address translation schemes is not possible. This
reflects the intended robust security model where government and
corporate NTP servers are operated outside firewall perimeters.
Operation
A specific combination of authentication scheme (none, symmetric key,
public key) and identity scheme is called a cryptotype, although not
all combinations are compatible. There may be management
configurations where the clients, servers and peers may not all support
the same cryptotypes. A secure NTPv4 subnet can be configured in many
ways while keeping in mind the principles explained above and in this
section. Note however that some cryptotype combinations may
successfully interoperate with each other, but may not represent good
security practice.
The cryptotype of an association is determined at the time of
mobilization, either at configuration time or some time later when a
message of appropriate cryptotype arrives. When mobilized by a server
or peer configuration command and no key or autokey subcommands are
present, the association is not authenticated; if the key subcommand is
present, the association is authenticated using the symmetric key ID
specified; if the autokey subcommand is present, the association is
authenticated using Autokey.
When multiple identity schemes are supported in the Autokey protocol,
the first message exchange determines which one is used. The client
request message contains bits corresponding to which schemes it has
available. The server response message contains bits corresponding to
which schemes it has available. Both server and client match the
received bits with their own and select a common scheme.
Following the principle that time is a public value, a server responds
to any client packet that matches its cryptotype capabilities. Thus, a
server receiving an unauthenticated packet will respond with an
unauthenticated packet, while the same server receiving a packet of a
cryptotype it supports will respond with packets of that cryptotype.
However, unconfigured broadcast or manycast client associations or
symmetric passive associations will not be mobilized unless the server
supports a cryptotype compatible with the first packet received. By
default, unauthenticated associations will not be mobilized unless
overridden in a decidedly dangerous way.
Some examples may help to reduce confusion. Client Alice has no
specific cryptotype selected. Server Bob has both a symmetric key file
and minimal Autokey files. Alice's unauthenticated messages arrive at
Bob, who replies with unauthenticated messages. Cathy has a copy of
Bob's symmetric key file and has selected key ID 4 in messages to Bob.
Bob verifies the message with his key ID 4. If it's the same key and
the message is verified, Bob sends Cathy a reply authenticated with
that key. If verification fails, Bob sends Cathy a thing called a
crypto-NAK, which tells her something broke. She can see the evidence
using the ntpq(1) program.
Denise has rolled her own host key and certificate. She also uses one
of the identity schemes as Bob. She sends the first Autokey message to
Bob and they both dance the protocol authentication and identity steps.
If all comes out okay, Denise and Bob continue as described above.
It should be clear from the above that Bob can support all the girls at
the same time, as long as he has compatible authentication and identity
credentials. Now, Bob can act just like the girls in his own choice of
servers; he can run multiple configured associations with multiple
different servers (or the same server, although that might not be
useful). But, wise security policy might preclude some cryptotype
combinations; for instance, running an identity scheme with one server
and no authentication with another might not be wise.
Key Management
The cryptographic values used by the Autokey protocol are incorporated
as a set of files generated by the ntp-keygen(1ntpkeygenmdoc) utility
program, including symmetric key, host key and public certificate
files, as well as sign key, identity parameters and leapseconds files.
Alternatively, host and sign keys and certificate files can be
generated by the OpenSSL utilities and certificates can be imported
from public certificate authorities. Note that symmetric keys are
necessary for the ntpq(1) and ntpdc(1) utility programs. The remaining
files are necessary only for the Autokey protocol.
Certificates imported from OpenSSL or public certificate authorities
have certian limitations. The certificate should be in ASN.1 syntax,
X.509 Version 3 format and encoded in PEM, which is the same format
used by OpenSSL. The overall length of the certificate encoded in
ASN.1 must not exceed 1024 bytes. The subject distinguished name field
(CN) is the fully qualified name of the host on which it is used; the
remaining subject fields are ignored. The certificate extension fields
must not contain either a subject key identifier or a issuer key
identifier field; however, an extended key usage field for a trusted
host must contain the value trustRoot;. Other extension fields are
ignored.
Authentication Commands
autokey [logsec]
Specifies the interval between regenerations of the session key
list used with the Autokey protocol. Note that the size of the
key list for each association depends on this interval and the
current poll interval. The default value is 12 (4096 s or about
1.1 hours). For poll intervals above the specified interval, a
session key list with a single entry will be regenerated for
every message sent.
controlkey key
Specifies the key identifier to use with the ntpq(1) utility,
which uses the standard protocol defined in RFC-1305. The key
argument is the key identifier for a trusted key, where the
value can be in the range 1 to 65,534, inclusive.
crypto [cert file] [leap file] [randfile file] [host file] [sign file]
[gq file] [gqpar file] [iffpar file] [mvpar file] [pw password]
This command requires the OpenSSL library. It activates public
key cryptography, selects the message digest and signature
encryption scheme and loads the required private and public
values described above. If one or more files are left
unspecified, the default names are used as described above.
Unless the complete path and name of the file are specified, the
location of a file is relative to the keys directory specified
in the keysdir command or default /usr/local/etc. Following are
the subcommands:
cert file
Specifies the location of the required host public
certificate file. This overrides the link
ntpkey_cert_hostname in the keys directory.
gqpar file
Specifies the location of the optional GQ parameters
file. This overrides the link ntpkey_gq_hostname in the
keys directory.
host file
Specifies the location of the required host key file.
This overrides the link ntpkey_key_hostname in the keys
directory.
iffpar file
Specifies the location of the optional IFF parameters
file.This overrides the link ntpkey_iff_hostname in the
keys directory.
leap file
Specifies the location of the optional leapsecond file.
This overrides the link ntpkey_leap in the keys
directory.
mvpar file
Specifies the location of the optional MV parameters
file. This overrides the link ntpkey_mv_hostname in the
keys directory.
pw password
Specifies the password to decrypt files containing
private keys and identity parameters. This is required
only if these files have been encrypted.
randfile file
Specifies the location of the random seed file used by
the OpenSSL library. The defaults are described in the
main text above.
sign file
Specifies the location of the optional sign key file.
This overrides the link ntpkey_sign_hostname in the keys
directory. If this file is not found, the host key is
also the sign key.
keys keyfile
Specifies the complete path and location of the MD5 key file
containing the keys and key identifiers used by ntpd(1), 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(1). The key argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,534, inclusive.
revoke logsec
Specifies the interval between re-randomization of certain
cryptographic values used by the Autokey scheme, as a power of 2
in seconds. These values need to be updated frequently in order
to deflect brute-force attacks on the algorithms of the scheme;
however, updating some values is a relatively expensive
operation. The default interval is 16 (65,536 s or about 18
hours). For poll intervals above the specified interval, the
values will be updated for every message sent.
trustedkey key ...
Specifies the key identifiers which are trusted for the purposes
of authenticating peers with symmetric key cryptography, as well
as keys used by the ntpq(1) and ntpdc(1) programs. The
authentication procedures require that both the local and remote
servers share the same key and key identifier for this purpose,
although different keys can be used with different servers. The
key arguments are 32-bit unsigned integers with values from 1 to
65,534.
Error Codes
The following error codes are reported via the NTP control and
monitoring protocol trap mechanism.
101
(bad field format or length) The packet has invalid version,
length or format.
102
(bad timestamp) The packet timestamp is the same or older than
the most recent received. This could be due to a replay or a
server clock time step.
103
(bad filestamp) The packet filestamp is the same or older than
the most recent received. This could be due to a replay or a
key file generation error.
104
(bad or missing public key) The public key is missing, has
incorrect format or is an unsupported type.
105
(unsupported digest type) The server requires an unsupported
digest/signature scheme.
106
(mismatched digest types) Not used.
107
(bad signature length) The signature length does not match the
current public key.
108
(signature not verified) The message fails the signature check.
It could be bogus or signed by a different private key.
109
(certificate not verified) The certificate is invalid or signed
with the wrong key.
110
(certificate not verified) The certificate is not yet valid or
has expired or the signature could not be verified.
111
(bad or missing cookie) The cookie is missing, corrupted or
bogus.
112
(bad or missing leapseconds table) The leapseconds table is
missing, corrupted or bogus.
113
(bad or missing certificate) The certificate is missing,
corrupted or bogus.
114
(bad or missing identity) The identity key is missing, corrupt
or bogus.
Monitoring Support
ntpd(1) 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 this 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 prefix, filename and 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(1) 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(1) 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
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
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(1) 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 indefinate
period. When a client or network is denied access for an indefinate
period, the only way at present to remove the restrictions is by
restarting the server.
The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further
action except incrementing statistics counters. Sometimes a more
proactive response is needed, such as a server message that explicitly
requests the client to stop sending and leave a message for the system
operator. A special packet format has been created for this purpose
called the "kiss-of-death" (KoD) packet. KoD packets have the leap
bits set unsynchronized and stratum set to zero and the reference
identifier field set to a four-byte ASCII code. If the noserve or
notrust flag of the matching restrict list entry is set, the code is
"DENY"; if the limited flag is set and the rate limit is exceeded, the
code is "RATE". Finally, if a cryptographic violation occurs, the code
is "CRYP".
A client receiving a KoD performs a set of sanity checks to minimize
security exposure, then updates the stratum and reference identifier
peer variables, sets the access denied (TEST4) bit in the peer flash
variable and sends a message to the log. As long as the TEST4 bit is
set, the client will send no further packets to the server. The only
way at present to recover from this condition is to restart the
protocol at both the client and server. This happens automatically at
the client when the association times out. It will happen at the
server only if the server operator cooperates.
Access Control Commands
discard [average avg] [minimum min] [monitor prob]
Set the parameters of the limited facility which protects the
server from client abuse. The average subcommand specifies the
minimum average packet spacing, while the minimum subcommand
specifies the minimum packet spacing. Packets that violate
these minima are discarded and a kiss-o'-death packet returned
if enabled. The default minimum average and minimum are 5 and
2, respectively. The monitor subcommand specifies the
probability of discard for packets that overflow the rate-
control window.
restrict address [mask mask] [flag ...]
The address argument expressed in dotted-quad form is the
address of a host or network. Alternatively, the address
argument can be a valid host DNS name. The mask argument
expressed in dotted-quad form defaults to 255.255.255.255,
meaning that the address is treated as the address of an
individual host. A default entry (address 0.0.0.0, mask
0.0.0.0) is always included and is always the first entry in the
list. Note that text string default, with no mask option, may
be used to indicate the default entry. In the current
implementation, flag always restricts access, i.e., an entry
with no flags indicates that free access to the server is to be
given. The flags are not orthogonal, in that more restrictive
flags will often make less restrictive ones redundant. The
flags can generally be classed into two categories, those which
restrict time service and those which restrict informational
queries and attempts to do run-time reconfiguration of the
server. One or more of the following flags may be specified:
ignore
Deny packets of all kinds, including ntpq(1) and ntpdc(1)
queries.
kod
If this flag is set when an access violation occurs, a
kiss-o'-death (KoD) packet is sent. KoD packets are rate
limited to no more than one per second. If another KoD
packet occurs within one second after the last one, the
packet is dropped.
limited
Deny service if the packet spacing violates the lower
limits specified in the discard command. A history of
clients is kept using the monitoring capability of
ntpd(1). Thus, monitoring is always active as long as
there is a restriction entry with the limited flag.
lowpriotrap
Declare traps set by matching hosts to be low priority.
The number of traps a server can maintain is limited (the
current limit is 3). Traps are usually assigned on a
first come, first served basis, with later trap
requestors being denied service. This flag modifies the
assignment algorithm by allowing low priority traps to be
overridden by later requests for normal priority traps.
nomodify
Deny ntpq(1) and ntpdc(1) queries which attempt to modify
the state of the server (i.e., run time reconfiguration).
Queries which return information are permitted.
noquery
Deny ntpq(1) and ntpdc(1) queries. Time service is not
affected.
nopeer
Deny packets which would result in mobilizing a new
association. This includes broadcast and symmetric
active packets when a configured association does not
exist. It also includes pool associations, so if you
want to use servers from a pool directive and also want
to use nopeer by default, you'll want a restrict source
... line as well that does
not
include the nopeer directive.
noserve
Deny all packets except ntpq(1) and ntpdc(1) queries.
notrap
Decline to provide mode 6 control message trap service to
matching hosts. The trap service is a subsystem of the
ntpdq 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(1) continuously and
run either sntp(1) or ntpd(1) -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(1) -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.
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(1) 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(1) 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(1) 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 | unpeer_crypto_early | unpeer_crypto_nak_early |
unpeer_digest_early]
disable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp |
stats | 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.
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(1) 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 still using autokey and 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(1) 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(1) 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(1) on multiple
hosts, with (mostly) common options (e.g., a restriction list).
leapsmearinterval seconds
This EXPERIMENTAL option is only available if ntpd(1) 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). 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(1) and
the major system events. For a simple reference server, the
following minimum message configuration could be useful:
logconfig =syncall +clockall
This configuration will list all clock information and
synchronization information. All other events and messages
about peers, system events and so on is suppressed.
logfile logfile
This command specifies the location of an alternate log file to
be used instead of the default system syslog(3) facility. This
is the same operation as the -l command line option.
setvar variable [default]
This command adds an additional system variable. These
variables can be used to distribute additional information such
as the access policy. If the variable of the form name=value is
followed by the default keyword, the variable will be listed as
part of the default system variables (ntpq(1) rv command)).
These additional variables serve informational purposes only.
They are not related to the protocol other that they can be
listed. The known protocol variables will always override any
variables defined via the setvar mechanism. There are three
special variables that contain the names of all variable of the
same group. The sys_var_list holds the names of all system
variables. The peer_var_list holds the names of all peer
variables and the clock_var_list holds the names of the
reference clock variables.
tinker [allan allan | dispersion dispersion | freq freq | huffpuff
huffpuff | panic panic | step step | stepback stepback | stepfwd
stepfwd | stepout stepout]
This command can be used to alter several system variables in
very exceptional circumstances. It should occur in the
configuration file before any other configuration options. The
default values of these variables have been carefully optimized
for a wide range of network speeds and reliability expectations.
In general, they interact in intricate ways that are hard to
predict and some combinations can result in some very nasty
behavior. Very rarely is it necessary to change the default
values; but, some folks cannot resist twisting the knobs anyway
and this command is for them. Emphasis added: twisters are on
their own and can expect no help from the support group.
The variables operate as follows:
allan allan
The argument becomes the new value for the minimum Allan
intercept, which is a parameter of the PLL/FLL clock
discipline algorithm. The value in log2 seconds defaults
to 7 (1024 s), which is also the lower limit.
dispersion dispersion
The argument becomes the new value for the dispersion
increase rate, normally .000015 s/s.
freq freq
The argument becomes the initial value of the frequency
offset in parts-per-million. This overrides the value in
the frequency file, if present, and avoids the initial
training state if it is not.
huffpuff huffpuff
The argument becomes the new value for the experimental
huff-n'-puff filter span, which determines the most
recent interval the algorithm will search for a minimum
delay. The lower limit is 900 s (15 m), but a more
reasonable value is 7200 (2 hours). There is no default,
since the filter is not enabled unless this command is
given.
panic panic
The argument is the panic threshold, normally 1000 s. If
set to zero, the panic sanity check is disabled and a
clock offset of any value will be accepted.
step step
The argument is the step threshold, which by default is
0.128 s. It can be set to any positive number in
seconds. If set to zero, step adjustments will never
occur. Note: The kernel time discipline is disabled if
the step threshold is set to zero or greater than the
default.
stepback stepback
The argument is the step threshold for the backward
direction, which by default is 0.128 s. It can be set to
any positive number in seconds. If both the forward and
backward step thresholds are set to zero, step
adjustments will never occur. Note: The kernel time
discipline is disabled if each direction of step
threshold are either set to zero or greater than .5
second.
stepfwd stepfwd
As for stepback, but for the forward direction.
stepout stepout
The argument is the stepout timeout, which by default is
900 s. It can be set to any positive number in seconds.
If set to zero, the stepout pulses will not be
suppressed.
rlimit [memlock Nmegabytes | stacksize N4kPages filenum
Nfiledescriptors]
memlock Nmegabytes
Specify the number of megabytes of memory that should be
allocated and locked. Probably only available under
Linux, this option may be useful when dropping root (the
-i option). The default is 32 megabytes on non-Linux
machines, and -1 under Linux. -1 means "do not lock the
process into memory". 0 means "lock whatever memory the
process wants into memory".
stacksize N4kPages
Specifies the maximum size of the process stack on
systems with the mlockall() function. Defaults to 50 4k
pages (200 4k pages in OpenBSD).
filenum Nfiledescriptors
Specifies the maximum number of file descriptors ntpd may
have open at once. Defaults to the system default.
trap host_address [port port_number] [interface interface_address]
This command configures a trap receiver at the given host
address and port number for sending messages with the specified
local interface address. If the port number is unspecified, a
value of 18447 is used. If the interface address is not
specified, the message is sent with a source address of the
local interface the message is sent through. Note that on a
multihomed host the interface used may vary from time to time
with routing changes.
The trap receiver will generally log event messages and other
information from the server in a log file. While such monitor
programs may also request their own trap dynamically,
configuring a trap receiver will ensure that no messages are
lost when the server is started.
hop ...
This command specifies a list of TTL values in increasing order,
up to 8 values can be specified. In manycast mode these values
are used in turn in an expanding-ring search. The default is
eight multiples of 32 starting at 31.
OPTIONS
--help
Display usage information and exit.
--more-help
Pass the extended usage information through a pager.
--version [{v|c|n}]
Output version of program and exit. The default mode is `v', a
simple version. The `c' mode will print copyright information
and `n' will print the full copyright notice.
OPTION PRESETS
Any option that is not marked as not presettable may be preset by
loading values from environment variables named:
NTP_CONF_<option-name> or NTP_CONF
ENVIRONMENT
See OPTION PRESETS for configuration environment variables.
FILES
/etc/ntp.conf
the default name of the configuration file
ntp.keys
private MD5 keys
ntpkey
RSA private key
ntpkey_host
RSA public key
ntp_dh
Diffie-Hellman agreement parameters
EXIT STATUS
One of the following exit values will be returned:
0 (EXIT_SUCCESS)
Successful program execution.
1 (EXIT_FAILURE)
The operation failed or the command syntax was not valid.
70 (EX_SOFTWARE)
libopts had an internal operational error. Please report it to
autogen-users@lists.sourceforge.net. Thank you.
SEE ALSO
ntpd(1), 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) 1992-2016 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.
4.2.8p6 20 Jan 2016 ntp.conf(5)