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(1ntpdmdoc) 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:
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(1ntpqmdoc) or
ntpdc(1ntpdcmdoc), 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(1ntpdmdoc) is
started with the -q option.
-
-
- key
key
- All packets sent to and received from the server or peer
are to include authentication fields encrypted using the specified
key identifier with values from 1 to 65534,
inclusive. The default is to include no encryption field.
-
-
- minpoll
minpoll
-
- maxpoll
maxpoll
- These options specify the minimum and maximum poll
intervals for NTP messages, as a power of 2 in seconds The maximum poll
interval defaults to 10 (1,024 s), but can be increased by the
maxpoll option to an upper limit of 17 (36.4 h). The
minimum poll interval defaults to 6 (64 s), but can be decreased by the
minpoll option to a lower limit of 4 (16 s).
-
-
- noselect
- Marks the server as unused, except for display purposes.
The server is discarded by the selection algroithm.
-
-
- preempt
- Says the association can be preempted.
-
-
- true
- Marks the server as a truechimer. Use this option only for
testing.
-
-
- prefer
- Marks the server as preferred. All other things being
equal, this host will be chosen for synchronization among a set of
correctly operating hosts. See the “Mitigation Rules and the prefer
Keyword” page (available as part of the HTML documentation provided
in /usr/share/doc/ntp) for further information.
-
-
- true
- Forces the association to always survive the selection and
clustering algorithms. This option should almost certainly
only be used while testing an association.
-
-
- ttl
ttl
- This option is used only with broadcast server and manycast
client modes. It specifies the time-to-live ttl to
use on broadcast server and multicast server and the maximum
ttl for the expanding ring search with manycast
client packets. Selection of the proper value, which defaults to 127, is
something of a black art and should be coordinated with the network
administrator.
-
-
- version
version
- Specifies the version number to be used for outgoing NTP
packets. Versions 1-4 are the choices, with version 4 the default.
-
-
- xleave
- Valid in peer and
broadcast modes only, this flag enables interleave
mode.
Auxiliary Commands
-
-
- broadcastclient
- This command enables reception of broadcast server messages
to any local interface (type b) address. Upon receiving a message for the
first time, the broadcast client measures the nominal server propagation
delay using a brief client/server exchange with the server, then enters
the broadcast client mode, in which it synchronizes to succeeding
broadcast messages. Note that, in order to avoid accidental or malicious
disruption in this mode, both the server and client should operate using
symmetric-key or public-key authentication as described in
Authentication
Options.
-
-
- manycastserver
address ...
- This command enables reception of manycast client messages
to the multicast group address(es) (type m) specified. At least one
address is required, but the NTP multicast address 224.0.1.1 assigned by
the IANA should NOT be used, unless specific means are taken to limit the
span of the reply and avoid a possibly massive implosion at the original
sender. Note that, in order to avoid accidental or malicious disruption in
this mode, both the server and client should operate using symmetric-key
or public-key authentication as described in
Authentication
Options.
-
-
- multicastclient
address ...
- This command enables reception of multicast server messages
to the multicast group address(es) (type m) specified. Upon receiving a
message for the first time, the multicast client measures the nominal
server propagation delay using a brief client/server exchange with the
server, then enters the broadcast client mode, in which it synchronizes to
succeeding multicast messages. Note that, in order to avoid accidental or
malicious disruption in this mode, both the server and client should
operate using symmetric-key or public-key authentication as described in
Authentication
Options.
-
-
- mdnstries
number
- If we are participating in mDNS, after we have synched for
the first time we attempt to register with the mDNS system. If that
registration attempt fails, we try again at one minute intervals for up to
mdnstries times. After all, ntpd may
be starting before mDNS. The default value for mdnstries
is 5.
Authentication Support
Authentication support allows the NTP client to verify that the server is in
fact known and trusted and not an intruder intending accidentally or on
purpose to masquerade as that server. The NTPv3 specification RFC-1305 defines
a scheme which provides cryptographic authentication of received NTP packets.
Originally, this was done using the Data Encryption Standard (DES) algorithm
operating in Cipher Block Chaining (CBC) mode, commonly called DES-CBC.
Subsequently, this was replaced by the RSA Message Digest 5 (MD5) algorithm
using a private key, commonly called keyed-MD5. Either algorithm computes a
message digest, or one-way hash, which can be used to verify the server has
the correct private key and key identifier.
NTPv4 retains the NTPv3 scheme, properly described as symmetric key cryptography
and, in addition, provides a new Autokey scheme based on public key
cryptography. Public key cryptography is generally considered more secure than
symmetric key cryptography, since the security is based on a private value
which is generated by each server and never revealed. With Autokey all key
distribution and management functions involve only public values, which
considerably simplifies key distribution and storage. Public key management is
based on X.509 certificates, which can be provided by commercial services or
produced by utility programs in the OpenSSL software library or the NTPv4
distribution.
While the algorithms for symmetric key cryptography are included in the NTPv4
distribution, public key cryptography requires the OpenSSL software library to
be installed before building the NTP distribution. Directions for doing that
are on the Building and Installing the Distribution page.
Authentication is configured separately for each association using the
key or
autokey subcommand on the
peer,
server,
broadcast
and
manycastclient configuration commands as described in
Configuration Options page.
The authentication options described below specify the locations of the key
files, if other than default, which symmetric keys are trusted and the
interval between various operations, if other than default.
Authentication is always enabled, although ineffective if not configured as
described below. If a NTP packet arrives including a message authentication
code (MAC), it is accepted only if it passes all cryptographic checks. The
checks require correct key ID, key value and message digest. If the packet has
been modified in any way or replayed by an intruder, it will fail one or more
of these checks and be discarded. Furthermore, the Autokey scheme requires a
preliminary protocol exchange to obtain the server certificate, verify its
credentials and initialize the protocol
The
auth flag controls whether new associations or remote
configuration commands require cryptographic authentication. This flag can be
set or reset by the
enable and
disable
commands and also by remote configuration commands sent by a
ntpdc(1ntpdcmdoc)
program running on another machine. If this flag is enabled, which is the
default case, new broadcast client and symmetric passive associations and
remote configuration commands must be cryptographically authenticated using
either symmetric key or public key cryptography. If this flag is disabled,
these operations are effective even if not cryptographic authenticated. It
should be understood that operating with the
auth flag
disabled invites a significant vulnerability where a rogue hacker can
masquerade as a falseticker and seriously disrupt system timekeeping. It is
important to note that this flag has no purpose other than to allow or
disallow a new association in response to new broadcast and symmetric active
messages and remote configuration commands and, in particular, the flag has no
effect on the authentication process itself.
An attractive alternative where multicast support is available is manycast mode,
in which clients periodically troll for servers as described in the
Automatic NTP
Configuration Options page. Either symmetric key or public key
cryptographic authentication can be used in this mode. The principle advantage
of manycast mode is that potential servers need not be configured in advance,
since the client finds them during regular operation, and the configuration
files for all clients can be identical.
The security model and protocol schemes for both symmetric key and public key
cryptography are summarized below; further details are in the briefings,
papers and reports at the NTP project page linked from
http://www.ntp.org/
.
Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly 65,534 keys, each
distinguished by a 32-bit key identifier, to authenticate an association. The
servers and clients involved must agree on the key and key identifier to
authenticate NTP packets. Keys and related information are specified in a key
file, usually called
ntp.keys, which must be distributed and
stored using secure means beyond the scope of the NTP protocol itself. Besides
the keys used for ordinary NTP associations, additional keys can be used as
passwords for the
ntpq(1ntpqmdoc) and
ntpdc(1ntpdcmdoc)
utility programs.
When
ntpd(1ntpdmdoc) 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(1ntpdcmdoc). 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(1ntpdcmdoc)
utility, while the
controlkey command selects the key used
as the password for the
ntpq(1ntpqmdoc) 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(1ntpqmdoc) 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(1ntpqmdoc) and
ntpdc(1ntpdcmdoc)
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(1ntpqmdoc)
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(1ntpdmdoc),
ntpq(1ntpqmdoc) and
ntpdc(1ntpdcmdoc)
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(1ntpdcmdoc)
utility program, which uses a proprietary protocol specific to this
implementation of
ntpd(1ntpdmdoc). 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(1ntpqmdoc) and
ntpdc(1ntpdcmdoc)
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(1ntpdmdoc) 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(1ntpdcmdoc)
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(1ntpdmdoc)
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(1ntpdmdoc)
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(1ntpdmdoc)
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(1ntpdmdoc) 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(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
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(1ntpdmdoc).
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(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
queries which attempt to modify the state of the server (i.e., run
time reconfiguration). Queries which return information are
permitted.
-
-
- noquery
- Deny
ntpq(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
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(1ntpqmdoc)
and
ntpdc(1ntpdcmdoc)
queries.
-
-
- notrap
- Decline to provide mode 6 control message trap service
to matching hosts. The trap service is a subsystem of the
ntpq(1ntpqmdoc)
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(1ntpdmdoc)
continuously and run either
sntp(1sntpmdoc) or
ntpd(1ntpdmdoc)
-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(1ntpdmdoc)
-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(1ntpdmdoc) 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(1ntpdmdoc) 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(1ntpdcmdoc)
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(1ntpdmdoc) 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(1ntpdcmdoc)
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(1ntpdcmdoc)
program. The default for this flag is disable. This flag is excluded
from runtime configuration using
ntpq(1ntpqmdoc).
The
ntpq(1ntpqmdoc)
program provides the same capabilities as
ntpdc(1ntpdcmdoc)
using standard mode 6 requests.
-
-
- monitor
- Enables the monitoring facility. See the
ntpdc(1ntpdcmdoc)
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(1ntpdmdoc) 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(1ntpdmdoc)
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(1ntpdmdoc)
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(1ntpdmdoc)
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(1ntpdmdoc) 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(1ntpdmdoc) 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(1ntpdmdoc) 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(1ntpdmdoc)
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(1ntpdmdoc) 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(1ntpdmdoc) 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(1ntpqmdoc)
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(1ntpdmdoc),
ntpdc(1ntpdcmdoc),
ntpq(1ntpqmdoc)
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.