ntp.conf(5) File Formats ntp.conf(5)NAMEntp.conf - Network Time Protocol (NTP) daemon configuration file format
SYNOPSISntp.conf [--option-name] [--option-name value]
All arguments must be options.
DESCRIPTION
The ntp.conf configuration file is read at initial startup by the
ntpd(8) daemon in order to specify the synchronization sources, modes
and other related information. Usually, it is installed in the /etc
directory, but could be installed elsewhere (see the daemon's -c com‐
mand 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 key‐
word followed by a list of arguments, some of which may be optional,
separated by whitespace. Commands may not be continued over multiple
lines. Arguments may be host names, host addresses written in numeric,
dotted-quad form, integers, floating point numbers (when specifying
times in seconds) and text strings.
The rest of this page describes the configuration and control options.
The "Notes on Configuring NTP and Setting up an NTP Subnet" page
(available as part of the HTML documentation provided in
/usr/share/doc/ntp) contains an extended discussion of these options.
In addition to the discussion of general Configuration Options, there
are sections describing the following supported functionality and the
options used to control it:
· Authentication Support
· Monitoring Support
· Access Control Support
· Automatic NTP Configuration Options
· Reference Clock Support
· Miscellaneous Options
Following these is a section describing Miscellaneous Options. While
there is a rich set of options available, the only required option is
one or more pool, server, peer, broadcast or manycastclient commands.
Configuration Support
Following is a description of the configuration commands in NTPv4.
These commands have the same basic functions as in NTPv3 and in some
cases new functions and new arguments. There are two classes of com‐
mands, 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 oper‐
ations.
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 ref‐
erence clock address (127.127.x.x). Note that only those options
applicable to each command are listed below. Use of options not listed
may not be caught as an error, but may result in some weird and even
destructive behavior.
If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is
detected, support for the IPv6 address family is generated in addition
to the default support of the IPv4 address family. In a few cases,
including the reslist billboard generated by ntpdc, IPv6 addresses are
automatically generated. IPv6 addresses can be identified by the pres‐
ence 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 min‐
poll] [maxpoll maxpoll]
server address [key key | autokey] [burst] [iburst] [version version]
[prefer] [minpoll minpoll] [maxpoll maxpoll]
peer address [key key | autokey] [version version] [prefer] [minpoll
minpoll] [maxpoll maxpoll]
broadcast address [key key | autokey] [version version] [prefer] [min‐
poll minpoll] [ttl ttl]
manycastclient address [key key | autokey] [version version] [prefer]
[minpoll minpoll] [maxpoll maxpoll] [ttl ttl]
These five commands specify the time server name or address to be used
and the mode in which to operate. The address can be either a DNS name
or an IP address in dotted-quad notation. Additional information on
association behavior can be found in the "Association Management" page
(available as part of the HTML documentation provided in
/usr/share/doc/ntp).
pool For type s addresses, this command mobilizes a persistent client
mode association with a number of remote servers. In this mode
the local clock can synchronized to the remote server, but the
remote server can never be synchronized to the local clock.
server For type s and r addresses, this command mobilizes a persistent
client mode association with the specified remote server or
local radio clock. In this mode the local clock can synchro‐
nized 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 per‐
sistent broadcast mode association. Multiple commands can be
used to specify multiple local broadcast interfaces (subnets)
and/or multiple multicast groups. Note that local broadcast
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 mes‐
sages 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 administra‐
tive 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 mes‐
sages and causing a possibly massive implosion of replies at the
sender. The manycastserver command specifies that the local
server is to operate in client mode with the remote servers that
are discovered as the result of broadcast/multicast messages.
The client broadcasts a request message to the group address
associated with the specified address and specifically enabled
servers respond to these messages. The client selects the
servers providing the best time and continues as with the server
command. The remaining servers are discarded as if never heard.
Options:
autokey
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey scheme
described in Authentication Options.
burst when the server is reachable, send a burst of eight packets
instead of the usual one. The packet spacing is normally 2 s;
however, the spacing between the first and second packets can be
changed with the calldelay command to allow additional time for
a modem or ISDN call to complete. This is designed to improve
timekeeping quality with the server command and s addresses.
iburst When the server is unreachable, send a burst of eight packets
instead of the usual one. The packet spacing is normally 2 s;
however, the spacing between the first two packets can be
changed with the calldelay command to allow additional time for
a modem or ISDN call to complete. This is designed to speed the
initial synchronization acquisition with the server command and
s addresses and when ntpd(8) is started with the -q option.
key key
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified key
identifier with values from 1 to 65534, inclusive. The default
is to include no encryption field.
minpoll minpoll
maxpoll maxpoll
These options specify the minimum and maximum poll intervals for
NTP messages, as a power of 2 in seconds The maximum poll inter‐
val defaults to 10 (1,024 s), but can be increased by the max‐
poll 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 min‐
poll option to a lower limit of 4 (16 s).
noselect
Marks the server as unused, except for display purposes. The
server is discarded by the selection algroithm.
prefer Marks the server as preferred. All other things being equal,
this host will be chosen for synchronization among a set of cor‐
rectly operating hosts. See the "Mitigation Rules and the pre‐
fer Keyword" page (available as part of the HTML documentation
provided in /usr/share/doc/ntp) for further information.
ttl ttl
This option is used only with broadcast server and manycast
client modes. It specifies the time-to-live ttl to use on
broadcast server and multicast server and the maximum ttl for
the expanding ring search with manycast client packets. Selec‐
tion of the proper value, which defaults to 127, is something of
a black art and should be coordinated with the network adminis‐
trator.
version version
Specifies the version number to be used for outgoing NTP pack‐
ets. Versions 1-4 are the choices, with version 4 the default.
Auxiliary Commands
broadcastclient
This command enables reception of broadcast server messages to
any local interface (type b) address. Upon receiving a message
for the first time, the broadcast client measures the nominal
server propagation delay using a brief client/server exchange
with the server, then enters the broadcast client mode, in which
it synchronizes to succeeding broadcast messages. Note that, in
order to avoid accidental or malicious disruption in this mode,
both the server and client should operate using symmetric-key or
public-key authentication as described in Authentication
Options.
manycastserver address ...
This command enables reception of manycast client messages to
the multicast group address(es) (type m) specified. At least
one address is required, but the NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means
are taken to limit the span of the reply and avoid a possibly
massive implosion at the original sender. Note that, in order
to avoid accidental or malicious disruption in this mode, both
the server and client should operate using symmetric-key or pub‐
lic-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 broad‐
cast client mode, in which it synchronizes to succeeding multi‐
cast messages. Note that, in order to avoid accidental or mali‐
cious disruption in this mode, both the server and client should
operate using symmetric-key or public-key authentication as
described in Authentication Options.
Authentication Support
Authentication support allows the NTP client to verify that the server
is in fact known and trusted and not an intruder intending accidentally
or on purpose to masquerade as that server. The NTPv3 specification
RFC-1305 defines a scheme which provides cryptographic authentication
of received NTP packets. Originally, this was done using the Data
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, com‐
monly 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 consid‐
ered 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 distribu‐
tion and storage. Public key management is based on X.509 certifi‐
cates, which can be provided by commercial services or produced by
utility programs in the OpenSSL software library or the NTPv4 distribu‐
tion.
While the algorithms for symmetric key cryptography are included in the
NTPv4 distribution, public key cryptography requires the OpenSSL soft‐
ware library to be installed before building the NTP distribution.
Directions for doing that are on the Building and Installing the Dis‐
tribution page.
Authentication is configured separately for each association using the
key or autokey subcommand on the peer, server, broadcast and manycast‐
client 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 config‐
ured as described below. If a NTP packet arrives including a message
authentication code (MAC), it is accepted only if it passes all crypto‐
graphic checks. The checks require correct key ID, key value and mes‐
sage 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 configura‐
tion commands sent by a ntpdc(8) program running in another machine.
If this flag is enabled, which is the default case, new broadcast
client and symmetric passive associations and remote configuration com‐
mands must be cryptographically authenticated using either symmetric
key or public key cryptography. If this flag is disabled, these opera‐
tions are effective even if not cryptographic authenticated. It should
be understood that operating with the auth flag disabled invites a sig‐
nificant vulnerability where a rogue hacker can masquerade as a falset‐
icker 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 mes‐
sages 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 many‐
cast mode, in which clients periodically troll for servers as described
in the Automatic NTP Configuration Options page. Either symmetric key
or public key cryptographic authentication can be used in this mode.
The principle advantage of manycast mode is that potential servers need
not be configured in advance, since the client finds them during regu‐
lar operation, and the configuration files for all clients can be iden‐
tical.
The security model and protocol schemes for both symmetric key and pub‐
lic 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 asso‐
ciations, additional keys can be used as passwords for the ntpq(8) and
ntpdc(8) utility programs.
When ntpd(8) is first started, it reads the key file specified in the
keys configuration command and installs the keys in the key cache.
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(8). This also provides a revocation capability
that can be used if a key becomes compromised. The requestkey command
selects the key used as the password for the ntpdc(8) utility, while
the controlkey command selects the key used as the password for the
ntpq(8) utility.
Public Key Cryptography
NTPv4 supports the original NTPv3 symmetric key scheme described in
RFC-1305 and in addition the Autokey protocol, which is based on public
key cryptography. The Autokey Version 2 protocol described on the
Autokey Protocol page verifies packet integrity using MD5 message
digests and verifies the source with digital signatures and any of sev‐
eral 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 Auton‐
omous 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 speci‐
fied 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 cur‐
rent 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 engi‐
neered so that, even under anticipated failure conditions, the NTP sub‐
net will form such that every group host can find a trail to at least
one trusted host.
Naming and Addressing
It is important to note that Autokey does not use DNS to resolve
addresses, since DNS can't be completely trusted until the name servers
have synchronized clocks. The cryptographic name used by Autokey to
bind the host identity credentials and cryptographic values must be
independent of interface, network and any other naming convention. The
name appears in the host certificate in either or both the subject and
issuer fields, so protection against DNS compromise is essential.
By convention, the name of an Autokey host is the name returned by the
Unix gethostname(2) system call or equivalent in other systems. By the
system design model, there are no provisions to allow alternate names
or aliases. However, this is not to say that DNS aliases, different
names for each interface, etc., are constrained in any way.
It is also important to note that Autokey verifies authenticity using
the host name, network address and public keys, all of which are bound
together by the protocol specifically to deflect masquerade attacks.
For this reason Autokey includes the source and destinatino IP
addresses in message digest computations and so the same addresses must
be available at both the server and client. For this reason operation
with network address translation schemes is not possible. This
reflects the intended robust security model where government and corpo‐
rate 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 configura‐
tions 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 sec‐
tion. Note however that some cryptotype combinations may successfully
interoperate with each other, but may not represent good security prac‐
tice.
The cryptotype of an association is determined at the time of mobiliza‐
tion, either at configuration time or some time later when a message of
appropriate cryptotype arrives. When mobilized by a server or peer
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 unau‐
thenticated packet, while the same server receiving a packet of a cryp‐
totype it supports will respond with packets of that cryptotype. How‐
ever, unconfigured broadcast or manycast client associations or symmet‐
ric passive associations will not be mobilized unless the server sup‐
ports 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 spe‐
cific 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(8) program.
Denise has rolled her own host key and certificate. She also uses one
of the identity schemes as Bob. She sends the first Autokey message to
Bob and they both dance the protocol authentication and identity steps.
If all comes out okay, Denise and Bob continue as described above.
It should be clear from the above that Bob can support all the girls at
the same time, as long as he has compatible authentication and identity
credentials. Now, Bob can act just like the girls in his own choice of
servers; he can run multiple configured associations with multiple dif‐
ferent servers (or the same server, although that might not be useful).
But, wise security policy might preclude some cryptotype combinations;
for instance, running an identity scheme with one server and no authen‐
tication 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 gener‐
ated by the OpenSSL utilities and certificates can be imported from
public certificate authorities. Note that symmetric keys are necessary
for the ntpq(8) and ntpdc(8) 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 iden‐
tifier 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(8) utility,
which uses the standard protocol defined in RFC-1305. The key
argument is the key identifier for a trusted key, where the
value can be in the range 1 to 65,534, inclusive.
crypto [cert file] [leap file] [randfile file] [host file] [sign file]
[gq file] [gqpar file] [iffpar file] [mvpar file] [pw password]
This command requires the OpenSSL library. It activates public
key cryptography, selects the message digest and signature
encryption scheme and loads the required private and public val‐
ues described above. If one or more files are left unspecified,
the default names are used as described above. Unless the com‐
plete 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 subcom‐
mands:
cert file
Specifies the location of the required host public cer‐
tificate file. This overrides the link ntpkey_cert_host‐
name in the keys directory.
gqpar file
Specifies the location of the optional GQ parameters
file. This overrides the link ntpkey_gq_hostname in the
keys directory.
host file
Specifies the location of the required host key file.
This overrides the link ntpkey_key_hostname in the keys
directory.
iffpar file
Specifies the location of the optional IFF parameters
file.This overrides the link ntpkey_iff_hostname in the
keys directory.
leap file
Specifies the location of the optional leapsecond file.
This overrides the link ntpkey_leap in the keys direc‐
tory.
mvpar file
Specifies the location of the optional MV parameters
file. This overrides the link ntpkey_mv_hostname in the
keys directory.
pw password
Specifies the password to decrypt files containing pri‐
vate keys and identity parameters. This is required only
if these files have been encrypted.
randfile file
Specifies the location of the random seed file used by
the OpenSSL library. The defaults are described in the
main text above.
sign file
Specifies the location of the optional sign key file.
This overrides the link ntpkey_sign_hostname in the keys
directory. If this file is not found, the host key is
also the sign key.
keys keyfile
Specifies the complete path and location of the MD5 key file
containing the keys and key identifiers used by ntpd(8), ntpq(8)
and ntpdc(8) when operating with symmetric key cryptography.
This is the same operation as the -k command line option.
keysdir path
This command specifies the default directory path for crypto‐
graphic keys, parameters and certificates. The default is
/usr/local/etc/.
requestkey key
Specifies the key identifier to use with the ntpdc(8) utility
program, which uses a proprietary protocol specific to this
implementation of ntpd(8). The key argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,534, inclusive.
revoke logsec
Specifies the interval between re-randomization of certain cryp‐
tographic values used by the Autokey scheme, as a power of 2 in
seconds. These values need to be updated frequently in order to
deflect brute-force attacks on the algorithms of the scheme;
however, updating some values is a relatively expensive opera‐
tion. The default interval is 16 (65,536 s or about 18 hours).
For poll intervals above the specified interval, the values will
be updated for every message sent.
trustedkey key ...
Specifies the key identifiers which are trusted for the purposes
of authenticating peers with symmetric key cryptography, as well
as keys used by the ntpq(8) and ntpdc(8) programs. The authen‐
tication 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 monitor‐
ing 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, cor‐
rupted or bogus.
114 (bad or missing identity) The identity key is missing, corrupt
or bogus.
Monitoring Supportntpd(8) includes a comprehensive monitoring facility suitable for con‐
tinuous, long term recording of server and client timekeeping perfor‐
mance. See the statistics command below for a listing and example of
each type of statistics currently supported. Statistic files are man‐
aged using file generation sets and scripts in the ./scripts directory
of this distribution. Using these facilities and UNIX cron(8) jobs,
the data can be automatically summarized and archived for retrospective
analysis.
Monitoring Commands
statistics name ...
Enables writing of statistics records. Currently, four kinds of
name statistics are supported.
clockstats
Enables recording of clock driver statistics information.
Each update received from a clock driver appends a line
of the following form to the file generation set named
clockstats:
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next field shows the clock address in dotted-quad nota‐
tion. The final field shows the last timecode received
from the clock in decoded ASCII format, where meaningful.
In some clock drivers a good deal of additional informa‐
tion can be gathered and displayed as well. See informa‐
tion specific to each clock for further details.
cryptostats
This option requires the OpenSSL cryptographic software
library. It enables recording of cryptographic public
key protocol information. Each message received by the
protocol module appends a line of the following form to
the file generation set named cryptostats:
49213 525.624 127.127.4.1 message
The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next field shows the peer address in dotted-quad nota‐
tion, The final message field includes the message type
and certain ancillary information. See the Authentica‐
tion Options section for further information.
loopstats
Enables recording of loop filter statistics information.
Each update of the local clock outputs a line of the fol‐
lowing form to the file generation set named loopstats:
50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806
The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next five fields show time offset (seconds), frequency
offset (parts per million - PPM), RMS jitter (seconds),
Allan deviation (PPM) and clock discipline time constant.
peerstats
Enables recording of peer statistics information. This
includes statistics records of all peers of a NTP server
and of special signals, where present and configured.
Each valid update appends a line of the following form to
the current element of a file generation set named peer‐
stats:
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 informa‐
tion. This includes statistics records of all peers of a
NTP server and of special signals, where present and con‐
figured. Each NTP message received from a peer or clock
driver appends a line of the following form to the file
generation set named rawstats:
50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
next two fields show the remote peer or clock address
followed by the local address in dotted-quad notation.
The final four fields show the originate, receive, trans‐
mit and final NTP timestamps in order. The timestamp
values are as received and before processing by the vari‐
ous data smoothing and mitigation algorithms.
sysstats
Enables recording of ntpd statistics counters on a peri‐
odic basis. Each hour a line of the following form is
appended to the file generation set named sysstats:
50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147
The first two fields show the date (Modified Julian Day)
and time (seconds and fraction past UTC midnight). The
remaining ten fields show the statistics counter values
accumulated since the last generated line.
Time since restart 36000
Time in hours since the system was last rebooted.
Packets received 81965
Total number of packets received.
Packets processed 0
Number of packets received in response to previous
packets sent
Current version 9546
Number of packets matching the current NTP ver‐
sion.
Previous version 56
Number of packets matching the previous NTP ver‐
sion.
Bad version 71793
Number of packets matching neither NTP version.
Access denied 512
Number of packets denied access for any reason.
Bad length or format 540
Number of packets with invalid length, format or
port number.
Bad authentication 10
Number of packets not verified as authentic.
Rate exceeded 147
Number of packets discarded due to rate limita‐
tion.
statsdir directory_path
Indicates the full path of a directory where statistics
files should be created (see below). This keyword allows
the (otherwise constant) filegen filename prefix to be
modified for file generation sets, which is useful for
handling statistics logs.
filegen name [file filename] [type typename] [link | nolink]
[enable | disable]
Configures setting of generation file set name. Genera‐
tion file sets provide a means for handling files that
are continuously growing during the lifetime of a server.
Server statistics are a typical example for such files.
Generation file sets provide access to a set of files
used to store the actual data. At any time at most one
element of the set is being written to. The type given
specifies when and how data will be directed to a new
element of the set. This way, information stored in ele‐
ments of a file set that are currently unused are avail‐
able for administrational operations without the risk of
disturbing the operation of ntpd. (Most important: they
can be removed to free space for new data produced.)
Note that this command can be sent from the ntpdc(8) pro‐
gram running at a remote location.
name This is the type of the statistics records, as
shown in the statistics command.
file filename
This is the file name for the statistics records.
Filenames of set members are built from three con‐
catenated elements prefix, filename and suffix:
prefix This is a constant filename path. It is
not subject to modifications via the file‐
gen option. It is defined by the server,
usually specified as a compile-time con‐
stant. It may, however, be configurable
for individual file generation sets via
other commands. For example, the prefix
used with loopstats and peerstats genera‐
tion 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 out‐
side 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 incar‐
nation 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 dif‐
ferent ntpd(8) server incarnations. The
set member filename is built by appending a
‘.’ to concatenated prefix and filename
strings, and appending the decimal repre‐
sentation of the process ID of the ntpd(8)
server process.
day One file generation set element is created
per day. A day is defined as the period
between 00:00 and 24:00 UTC. The file set
member suffix consists of a ‘.’ and a day
specification in the form YYYYMMdd. YYYY
is a 4-digit year number (e.g., 1992). MM
is a two digit month number. dd is a two
digit day number. Thus, all information
written at 10 December 1992 would end up in
a file named prefix filename.19921210.
week Any file set member contains data related
to a certain week of a year. The term week
is defined by computing day-of-year modulo
7. Elements of such a file generation set
are distinguished by appending the follow‐
ing suffix to the file set filename base: A
dot, a 4-digit year number, the letter W,
and a 2-digit week number. For example,
information from January, 10th 1992 would
end up in a file with suffix
month One generation file set element is gener‐
ated per month. The file name suffix con‐
sists 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 run‐
ning at the start of the corresponding
24-hour period. Information is only writ‐
ten to a file generation by specifying
enable; output is prevented by specifying
disable.
link | nolink
It is convenient to be able to access the current
element of a file generation set by a fixed name.
This feature is enabled by specifying link and
disabled using nolink. If link is specified, a
hard link from the current file set element to a
file without suffix is created. When there is
already a file with this name and the number of
links of this file is one, it is renamed appending
a dot, the letter C, and the pid of the ntpd
server process. When the number of links is
greater than one, the file is unlinked. This
allows the current file to be accessed by a con‐
stant name.
enable | disable
Enables or disables the recording function.
Access Control Support
The ntpd(8) daemon implements a general purpose address/mask based
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 informa‐
tion and examples can be found in the "Notes on Configuring NTP and
Setting up a NTP Subnet" page (available as part of the HTML documenta‐
tion 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 viola‐
tions include certificate or identity verification failure; rate limit
violations generally result from defective NTP implementations that
send packets at abusive rates. Some violations cause denied service
only for the offending packet, others cause denied service for a timed
period and others cause the denied service for an indefinate period.
When a client or network is denied access for an indefinate period, the
only way at present to remove the restrictions is by restarting the
server.
The Kiss-of-Death Packet
Ordinarily, packets denied service are simply dropped with no further
action except incrementing statistics counters. Sometimes a more
proactive response is needed, such as a server message that explicitly
requests the client to stop sending and leave a message for the system
operator. A special packet format has been created for this purpose
called the "kiss-of-death" (KoD) packet. KoD packets have the leap
bits set unsynchronized and stratum set to zero and the reference iden‐
tifier 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 proto‐
col 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 probabil‐
ity of discard for packets that overflow the rate-control win‐
dow.
restrict address [mask mask] [flag ...]
The address argument expressed in dotted-quad form is the
address of a host or network. Alternatively, the address argu‐
ment can be a valid host DNS name. The mask argument expressed
in dotted-quad form defaults to 255.255.255.255, meaning that
the address is treated as the address of an individual host. A
default entry (address 0.0.0.0, mask 0.0.0.0) is always included
and is always the first entry in the list. Note that text
string default, with no mask option, may be used to indicate the
default entry. In the current implementation, flag always
restricts access, i.e., an entry with no flags indicates that
free access to the server is to be given. The flags are not
orthogonal, in that more restrictive flags will often make less
restrictive ones redundant. The flags can generally be classed
into two categories, those which restrict time service and those
which restrict informational queries and attempts to do run-time
reconfiguration of the server. One or more of the following
flags may be specified:
ignore Deny packets of all kinds, including ntpq(8) and ntpdc(8)
queries.
kod If this flag is set when an access violation occurs, a
kiss-o'-death (KoD) packet is sent. KoD packets are rate
limited to no more than one per second. If another KoD
packet occurs within one second after the last one, the
packet is dropped.
limited
Deny service if the packet spacing violates the lower
limits specified in the discard command. A history of
clients is kept using the monitoring capability of
ntpd(8). Thus, monitoring is always active as long as
there is a restriction entry with the limited flag.
lowpriotrap
Declare traps set by matching hosts to be low priority.
The number of traps a server can maintain is limited (the
current limit is 3). Traps are usually assigned on a
first come, first served basis, with later trap
requestors being denied service. This flag modifies the
assignment algorithm by allowing low priority traps to be
overridden by later requests for normal priority traps.
nomodify
Deny ntpq(8) and ntpdc(8) queries which attempt to modify
the state of the server (i.e., run time reconfiguration).
Queries which return information are permitted.
noquery
Deny ntpq(8) and ntpdc(8) queries. Time service is not
affected.
nopeer Deny packets which would result in mobilizing a new asso‐
ciation. This includes broadcast and symmetric active
packets when a configured association does not exist.
noserve
Deny all packets except ntpq(8) and ntpdc(8) queries.
notrap Decline to provide mode 6 control message trap service to
matching hosts. The trap service is a subsystem of the
ntpdq control message protocol which is intended for use
by remote event logging programs.
notrust
Deny service unless the packet is cryptographically
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, ntp‐
port, for each of the local host's interface addresses are inserted
into the table at startup to prevent the server from attempting to syn‐
chronize 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, vali‐
date 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 associa‐
tions 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 many‐
cast paradigm is designed to find a plurality of redundant servers sat‐
isfying defined optimality criteria.
Manycasting can be used with either symmetric key or public key cryp‐
tography. The public key infrastructure (PKI) offers the best protec‐
tion against compromised keys and is generally considered stronger, at
least with relatively large key sizes. It is implemented using the
Autokey protocol and the OpenSSL cryptographic library available from
http://www.openssl.org/. The library can also be used with other NTPv4
modes as well and is highly recommended, especially for broadcast
modes.
A persistent manycast client association is configured using the many‐
castclient command, which is similar to the server command but with a
multicast (IPv4 class D or IPv6 prefix FF) group address. The IANA has
designated IPv4 address 224.1.1.1 and IPv6 address FF05::101 (site
local) for NTP. When more servers are needed, it broadcasts manycast
client messages to this address at the minimum feasible rate and mini‐
mum feasible time-to-live (TTL) hops, depending on how many servers
have already been found. There can be as many manycast client associa‐
tions 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 mes‐
sages, and manycast server, which passively responds to them. If a
manycast server is in scope of the current TTL and is itself synchro‐
nized to a valid source and operating at a stratum level equal to or
lower than the manycast client, it replies to the manycast client mes‐
sage 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 configu‐
ration commands. The manycast poll interval is normally eight times
the system poll interval, which starts out at the minpoll value speci‐
fied 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 associa‐
tions 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 com‐
mand. 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 inter‐
val 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 proba‐
bly 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 min‐
clock.
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 dis‐
carded, 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 many‐
cast client and manycast server. A number of hosts configured this way
and sharing a common group address will automatically organize them‐
selves 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, mul‐
ticast group address 239.1.1.1. The only exception is that each pri‐
mary 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 com‐
mand is not necessary. For stratum 3 and above servers the floor value
is set to the intended stratum number. Thus, all stratum 3 configura‐
tion files are identical, all stratum 4 files are identical and so
forth.
Once operations have stabilized in this scenario, the primary servers
will find the primary reference source and each other, since they both
operate at the same stratum (1), but not with any secondary server or
client, since these operate at a higher stratum. The secondary servers
will find the servers at the same stratum level. If one of the primary
servers loses its GPS receiver, it will continue to operate as a client
and other clients will time out the corresponding association and re-
associate accordingly.
Some administrators prefer to avoid running ntpd(8) continuously and
run either ntpdate(8) or ntpd(8)-q as a cron job. In either case the
servers must be configured in advance and the program fails if none are
available when the cron job runs. A really slick application of many‐
cast is with ntpd(8)-q. The program wakes up, scans the local land‐
scape 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 transpar‐
ent in client/server mode. However, about once per day the server pri‐
vate value used to generate cookies is refreshed along with all many‐
cast client associations. In this case all cryptographic values
including certificates is refreshed. If a new certificate has been
generated since the last refresh epoch, it will automatically revoke
all prior certificates that happen to be in the certificate cache. At
the same time, the manycast scheme starts all over from the beginning
and the expanding ring shrinks to the minimum and increments from there
while collecting all servers in scope.
Manycast Options
tos [ceiling ceiling | cohort { 0 | 1 } | floor floor | minclock min‐
clock | minsane minsane]
This command affects the clock selection and clustering algo‐
rithms. It can be used to select the quality and quantity of
peers used to synchronize the system clock and is most useful in
manycast mode. The variables operate as follows:
ceiling ceiling
Peers with strata above ceiling will be discarded if
there are at least minclock peers remaining. This value
defaults to 15, but can be changed to any number from 1
to 15.
cohort {0 | 1 }
This is a binary flag which enables (0) or disables (1)
manycast server replies to manycast clients with the same
stratum level. This is useful to reduce implosions where
large numbers of clients with the same stratum level are
present. The default is to enable these replies.
floor floor
Peers with strata below floor will be discarded if there
are at least minclock peers remaining. This value
defaults to 1, but can be changed to any number from 1 to
15.
minclock minclock
The clustering algorithm repeatedly casts out outlyer
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 pur‐
poses. 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 docu‐
mentation 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 avail‐
able 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 disci‐
pline/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 pro‐
vided in /usr/share/doc/ntp).
A reference clock will generally (though not always) be a radio time‐
code 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 spe‐
cific to each reference clock must be selected and compiled in the dis‐
tribution; however, most common radio, satellite and modem clocks are
included by default. Note that an attempt to configure a reference
clock when the driver has not been compiled or the hardware port has
not been appropriately configured results in a scalding remark to the
system log file, but is otherwise non hazardous.
For the purposes of configuration, ntpd(8) treats reference clocks in a
manner analogous to normal NTP peers as much as possible. Reference
clocks are identified by a syntactically correct but invalid IP
address, in order to distinguish them from normal NTP peers. Reference
clock addresses are of the form 127.127.t.u, where t is an integer
denoting the clock type and u indicates the unit number in the range
0-3. While it may seem overkill, it is in fact sometimes useful to
configure multiple reference clocks of the same type, in which case the
unit numbers must be unique.
The server command is used to configure a reference clock, where the
address argument in that command is the clock address. The key, ver‐
sion 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 indi‐
vidual clock drivers and normally follows immediately after the server
command. The address argument specifies the clock address. The refid
and stratum options can be used to override the defaults for the
device. There are two optional device-dependent time offsets and four
flags that can be included in the fudge command as well.
The stratum number of a reference clock is by default zero. Since the
ntpd(8) daemon adds one to the stratum of each peer, a primary server
ordinarily displays an external stratum of one. In order to provide
engineered backups, it is often useful to specify the reference clock
stratum as greater than zero. The stratum option is used for this pur‐
pose. 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 spe‐
cial 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 "Miti‐
gation 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 spe‐
cial ways. It must immediately follow the server command which
configures the driver. Note that the same capability is possi‐
ble at run time using the ntpdc(8) program. The options are
interpreted as follows:
time1 sec
Specifies a constant to be added to the time offset pro‐
duced by the driver, a fixed-point decimal number in sec‐
onds. This is used as a calibration constant to adjust
the nominal time offset of a particular clock to agree
with an external standard, such as a precision PPS sig‐
nal. It also provides a way to correct a systematic
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 propa‐
gation delay provided by other means, such as internal
DIPswitches. Where a calibration for an individual sys‐
tem 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 Driv‐
ers" 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 documenta‐
tion provided in /usr/share/doc/ntp).
stratum int
Specifies the stratum number assigned to the driver, an
integer between 0 and 15. This number overrides the
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 ini‐
tial frequency and then updated once per hour with the current
frequency computed by the daemon. If the file name is speci‐
fied, but the file itself does not exist, the starts with an
initial frequency of zero and creates the file when writing it
for the first time. If this command is not given, the daemon
will always start with an initial frequency of zero.
The file format consists of a single line containing a single
floating point number, which records the frequency offset mea‐
sured in parts-per-million (PPM). The file is updated by first
writing the current drift value into a temporary file and then
renaming this file to replace the old version. This implies
that ntpd(8) must have write permission for the directory the
drift file is located in, and that file system links, symbolic
or otherwise, should be avoided.
enable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp |
stats]
disable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp |
stats]
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(8) utility pro‐
gram.
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 broad‐
cast or multicast server, as in the multicastclient com‐
mand with default address. The default for this flag is
disable.
calibrate
Enables the calibrate feature for reference clocks. The
default for this flag is disable.
kernel Enables the kernel time discipline, if available. The
default for this flag is enable if support is available,
otherwise disable.
mode7 Enables processing of NTP mode 7 implementation-specific
requests which are used by the deprecated ntpdc(8) pro‐
gram. The default for this flag is disable. This flag
is excluded from runtime configuration using ntpq(8).
The ntpq(8) program provides the same capabilities as
ntpdc(8) using standard mode 6 requests.
monitor
Enables the monitoring facility. See the ntpdc(8) pro‐
gram and the monlist command or further information. The
default for this flag is enable.
ntp Enables time and frequency discipline. In effect, this
switch opens and closes the feedback loop, which is use‐
ful for testing. The default for this flag is enable.
stats Enables the statistics facility. See the Monitoring
Options section for further information. The default for
this flag is disable.
includefile includefile
This command allows additional configuration commands to be
included from a separate file. Include files may be nested to a
depth of five; upon reaching the end of any include file, com‐
mand processing resumes in the previous configuration file.
This option is useful for sites that run ntpd(8) on multiple
hosts, with (mostly) common options (e.g., a restriction list).
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 sys‐
log(3) priority mask, ‘+’ adds and ‘-’ removes messages. sys‐
log(3) messages can be controlled in four classes (clock, peer,
sys and sync). Within these classes four types of messages can
be controlled: informational messages (info), event messages
(events), statistics messages (statistics) and status messages
(status).
Configuration keywords are formed by concatenating the message
class with the event class. The all prefix can be used instead
of a message class. A message class may also be followed by the
all keyword to enable/disable all messages of the respective
message class.Thus, a minimal log configuration could look like
this:
logconfig =syncstatus +sysevents
This would just list the synchronizations state of ntpd(8) and
the major system events. For a simple reference server, the
following minimum message configuration could be useful:
logconfig =syncall +clockall
This configuration will list all clock information and synchro‐
nization information. All other events and messages about
peers, system events and so on is suppressed.
logfile logfile
This command specifies the location of an alternate log file to
be used instead of the default system syslog(3) facility. This
is the same operation as the -l command line option.
setvar variable [default]
This command adds an additional system variable. These vari‐
ables 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(8) 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 vari‐
ables and the clock_var_list holds the names of the reference
clock variables.
tinker [allan allan | dispersion dispersion | freq freq | huffpuff
huffpuff | panic panic | step srep | stepout stepout]
This command can be used to alter several system variables in
very exceptional circumstances. It should occur in the configu‐
ration file before any other configuration options. The default
values of these variables have been carefully optimized for a
wide range of network speeds and reliability expectations. In
general, they interact in intricate ways that are hard to pre‐
dict and some combinations can result in some very nasty behav‐
ior. 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 dis‐
cipline algorithm. The value in log2 seconds defaults to
7 (1024 s), which is also the lower limit.
dispersion dispersion
The argument becomes the new value for the dispersion
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 rea‐
sonable value is 7200 (2 hours). There is no default,
since the filter is not enabled unless this command is
given.
panic panic
The argument is the panic threshold, normally 1000 s. If
set to zero, the panic sanity check is disabled and a
clock offset of any value will be accepted.
step step
The argument is the step threshold, which by default is
0.128 s. It can be set to any positive number in sec‐
onds. If set to zero, step adjustments will never occur.
Note: The kernel time discipline is disabled if the step
threshold is set to zero or greater than the default.
stepout stepout
The argument is the stepout timeout, which by default is
900 s. It can be set to any positive number in seconds.
If set to zero, the stepout pulses will not be sup‐
pressed.
rlimit [memlock Nmegabytes | stacksize N4kPages filenum Nfiledescrip‐
tors]
memlock Nmegabytes
Specify the number of megabytes of memory that can be
allocated. Probably only available under Linux, this
option is useful when dropping root (the -i option). The
default is 32 megabytes. Setting this to zero will pre‐
vent any attemp to lock memory.
stacksize N4kPages
Specifies the maximum size of the process stack on sys‐
tems with the
filenum Nfiledescriptors
Specifies the maximum number of file descriptors ntpd may
have open at once. Defaults to the system default.
mlockall() function. Defaults to 50 4k pages (200 4k
pages in OpenBSD).
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 speci‐
fied, the message is sent with a source address of the local
interface the message is sent through. Note that on a multi‐
homed host the interface used may vary from time to time with
routing changes.
The trap receiver will generally log event messages and other
information from the server in a log file. While such monitor
programs may also request their own trap dynamically, configur‐
ing 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 load‐
ing values from environment variables named:
NTP_CONF_<option-name> or NTP_CONF
ntp.conf(5) File Formats ntp.conf(5)NAMEntp.conf ntp.conf - Network Time Protocol (NTP) daemon configuration
file format cvt_prog='/usr/local/gnu/share/autogen/texi2man'
cvt_prog=`cd `dirname "$cvt_prog"` >/dev/null && pwd
`/`basename "$cvt_prog"` cd $tmp_dir test -x "$cvt_prog" ||
die "'$cvt_prog' is not executable" {
list='synopsis description options option-presets'
for f in $list ; do cat $f ; echo ; done
rm -f $list name
list='implementation-notes environment files examples exit-status
errors
compatibility see-also conforming-to history authors copyright
bugs
notes'
for f in $list ; do cat $f ; echo ; done > .end-doc
rm -f $list
list=`ls -1 *`' .end-doc'
for f in $list ; do cat $f ; echo ; done
rm -f $list } 1>.doc 2>/dev/null sed -f .cmds .doc |
/opt/csw/bin/ggrep -E -v '^[ ]*$' | $cvt_prog
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 ALSOntpd(8), ntpdc(8), ntpq(8)
In addition to the manual pages provided, comprehensive documentation
is available on the world wide web at http://www.ntp.org/. A snapshot
of this documentation is available in HTML format in
/usr/share/doc/ntp. David L. Mills, Network Time Protocol (Version 4),
RFC5905
AUTHORS
The University of Delaware
COPYRIGHT
Copyright (C) 1970-2013 The University of Delaware 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 corresponds to version 4.2.7p410 of NTP. This document
was derived from FreeBSD.
This manual page was AutoGen-erated from the ntp.conf option defini‐
tions.
4.2.7p410 08 Jan 2014 ntp.conf(5)
