@node ntp.conf Notes
@section Notes about ntp.conf
@pindex ntp.conf
@cindex Network Time Protocol (NTP) daemon configuration file format
@ignore
#
# EDIT THIS FILE WITH CAUTION  (invoke-ntp.conf.texi)
#
# It has been AutoGen-ed  March  3, 2020 at 05:40:57 PM by AutoGen 5.18.5
# From the definitions    ntp.conf.def
# and the template file   agtexi-file.tpl
@end ignore



The
@code{ntp.conf}
configuration file is read at initial startup by the
@code{ntpd(1ntpdmdoc)}
daemon in order to specify the synchronization sources,
modes and other related information.
Usually, it is installed in the
@file{/etc}
directory,
but could be installed elsewhere
(see the daemon's
@code{-c}
command line option).

The file format is similar to other
@sc{unix}
configuration files.
Comments begin with a
@quoteleft{}#@quoteright{}
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
@file{/usr/share/doc/ntp})
contains an extended discussion of these options.
In addition to the discussion of general
@ref{Configuration Options},
there are sections describing the following supported functionality
and the options used to control it:
@itemize @bullet
@item 
@ref{Authentication Support}
@item 
@ref{Monitoring Support}
@item 
@ref{Access Control Support}
@item 
@ref{Automatic NTP Configuration Options}
@item 
@ref{Reference Clock Support}
@item 
@ref{Miscellaneous Options}
@end itemize

Following these is a section describing
@ref{Miscellaneous Options}.
While there is a rich set of options available,
the only required option is one or more
@code{pool},
@code{server},
@code{peer},
@code{broadcast}
or
@code{manycastclient}
commands.
@node Configuration Support
@subsection 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.
@subsubsection 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
@code{reslist}
billboard generated
by
@code{ntpq(1ntpqmdoc)}
or
@code{ntpdc(1ntpdcmdoc)},
IPv6 addresses are automatically generated.
IPv6 addresses can be identified by the presence of colons
@quotedblleft{}:@quotedblright{}
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
@code{-4}
qualifier preceding
the host name forces DNS resolution to the IPv4 namespace,
while a
@code{-6}
qualifier forces DNS resolution to the IPv6 namespace.
See IPv6 references for the
equivalent classes for that address family.
@table @asis
@item @code{pool} @kbd{address} @code{[@code{burst}]} @code{[@code{iburst}]} @code{[@code{version} @kbd{version}]} @code{[@code{prefer}]} @code{[@code{minpoll} @kbd{minpoll}]} @code{[@code{maxpoll} @kbd{maxpoll}]} @code{[@code{xmtnonce}]}
@item @code{server} @kbd{address} @code{[@code{key} @kbd{key} @kbd{|} @code{autokey}]} @code{[@code{burst}]} @code{[@code{iburst}]} @code{[@code{version} @kbd{version}]} @code{[@code{prefer}]} @code{[@code{minpoll} @kbd{minpoll}]} @code{[@code{maxpoll} @kbd{maxpoll}]} @code{[@code{true}]} @code{[@code{xmtnonce}]}
@item @code{peer} @kbd{address} @code{[@code{key} @kbd{key} @kbd{|} @code{autokey}]} @code{[@code{version} @kbd{version}]} @code{[@code{prefer}]} @code{[@code{minpoll} @kbd{minpoll}]} @code{[@code{maxpoll} @kbd{maxpoll}]} @code{[@code{true}]} @code{[@code{xleave}]}
@item @code{broadcast} @kbd{address} @code{[@code{key} @kbd{key} @kbd{|} @code{autokey}]} @code{[@code{version} @kbd{version}]} @code{[@code{prefer}]} @code{[@code{minpoll} @kbd{minpoll}]} @code{[@code{ttl} @kbd{ttl}]} @code{[@code{xleave}]}
@item @code{manycastclient} @kbd{address} @code{[@code{key} @kbd{key} @kbd{|} @code{autokey}]} @code{[@code{version} @kbd{version}]} @code{[@code{prefer}]} @code{[@code{minpoll} @kbd{minpoll}]} @code{[@code{maxpoll} @kbd{maxpoll}]} @code{[@code{ttl} @kbd{ttl}]}
@end table

These five commands specify the time server name or address to
be used and the mode in which to operate.
The
@kbd{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
@file{/usr/share/doc/ntp}).
@table @asis
@item @code{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.
@item @code{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
@emph{not}
be used for type
b or m addresses.
@item @code{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.
@item @code{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
@kbd{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
@code{broadcastclient}
or
@code{multicastclient}
commands
below.
@item @code{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
@code{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
@code{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
@kbd{address}
and specifically enabled
servers respond to these messages.
The client selects the servers
providing the best time and continues as with the
@code{server}
command.
The remaining servers are discarded as if never
heard.
@end table

Options:
@table @asis
@item @code{autokey}
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey scheme
described in
@ref{Authentication Options}.
@item @code{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
@code{calldelay}
command to allow
additional time for a modem or ISDN call to complete.
This is designed to improve timekeeping quality
with the
@code{server}
command and s addresses.
@item @code{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
@code{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
@code{server}
command and s addresses and when
@code{ntpd(1ntpdmdoc)}
is started with the
@code{-q}
option.
@item @code{key} @kbd{key}
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified
@kbd{key}
identifier with values from 1 to 65535, inclusive.
The
default is to include no encryption field.
@item @code{minpoll} @kbd{minpoll}
@item @code{maxpoll} @kbd{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
@code{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
@code{minpoll}
option to a lower limit of 4 (16 s).
@item @code{noselect}
Marks the server as unused, except for display purposes.
The server is discarded by the selection algroithm.
@item @code{preempt}
Says the association can be preempted.
@item @code{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
@file{/usr/share/doc/ntp})
for further information.
@item @code{true}
Marks the server as a truechimer,
forcing the association to always survive the selection and clustering algorithms.
This option should almost certainly
@emph{only}
be used while testing an association.
@item @code{ttl} @kbd{ttl}
This option is used only with broadcast server and manycast
client modes.
It specifies the time-to-live
@kbd{ttl}
to
use on broadcast server and multicast server and the maximum
@kbd{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.
@item @code{version} @kbd{version}
Specifies the version number to be used for outgoing NTP
packets.
Versions 1-4 are the choices, with version 4 the
default.
@item @code{xleave}
Valid in
@code{peer}
and
@code{broadcast}
modes only, this flag enables interleave mode.
@item @code{xmtnonce}
Valid only for
@code{server}
and
@code{pool}
modes, this flag puts a random number in the packet's transmit timestamp.

@end table
@subsubsection Auxiliary Commands
@table @asis
@item @code{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
@ref{Authentication Options}.
@item @code{manycastserver} @kbd{address} @kbd{...}
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
@ref{Authentication Options}.
@item @code{multicastclient} @kbd{address} @kbd{...}
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
@ref{Authentication Options}.
@item @code{mdnstries} @kbd{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
@code{mdnstries}
times.
After all,
@code{ntpd}
may be starting before mDNS.
The default value for
@code{mdnstries}
is 5.
@end table
@node Authentication Support
@subsection 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
@code{key}
or
@code{autokey}
subcommand on the
@code{peer},
@code{server},
@code{broadcast}
and
@code{manycastclient}
configuration commands as described in
@ref{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
@code{auth}
flag controls whether new associations or
remote configuration commands require cryptographic authentication.
This flag can be set or reset by the
@code{enable}
and
@code{disable}
commands and also by remote
configuration commands sent by a
@code{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
@code{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
@ref{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
@code{http://www.ntp.org/}.
@subsubsection Symmetric-Key Cryptography
The original RFC-1305 specification allows any one of possibly
65,535 keys, each distinguished by a 32-bit key identifier, to
authenticate an association.
The servers and clients involved must
agree on the key and key identifier to
authenticate NTP packets.
Keys and
related information are specified in a key
file, usually called
@file{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
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
utility programs.

When
@code{ntpd(1ntpdmdoc)}
is first started, it reads the key file specified in the
@code{keys}
configuration command and installs the keys
in the key cache.
However,
individual keys must be activated with the
@code{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
@code{ntpdc(1ntpdcmdoc)}.
This also provides a revocation capability that can be used
if a key becomes compromised.
The
@code{requestkey}
command selects the key used as the password for the
@code{ntpdc(1ntpdcmdoc)}
utility, while the
@code{controlkey}
command selects the key used as the password for the
@code{ntpq(1ntpqmdoc)}
utility.
@subsubsection 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
@ref{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
@code{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
@code{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.
@subsubsection 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
@code{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.
@subsubsection 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
@code{server}
or
@code{peer}
configuration command and no
@code{key}
or
@code{autokey}
subcommands are present, the association is not
authenticated; if the
@code{key}
subcommand is present, the association is authenticated
using the symmetric key ID specified; if the
@code{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
@code{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.
@subsubsection Key Management
The cryptographic values used by the Autokey protocol are
incorporated as a set of files generated by the
@code{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
@code{ntpq(1ntpqmdoc)}
and
@code{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
@code{trustRoot};.
Other extension fields are ignored.
@subsubsection Authentication Commands
@table @asis
@item @code{autokey} @code{[@kbd{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.
@item @code{controlkey} @kbd{key}
Specifies the key identifier to use with the
@code{ntpq(1ntpqmdoc)}
utility, which uses the standard
protocol defined in RFC-1305.
The
@kbd{key}
argument is
the key identifier for a trusted key, where the value can be in the
range 1 to 65,535, inclusive.
@item @code{crypto} @code{[@code{cert} @kbd{file}]} @code{[@code{leap} @kbd{file}]} @code{[@code{randfile} @kbd{file}]} @code{[@code{host} @kbd{file}]} @code{[@code{sign} @kbd{file}]} @code{[@code{gq} @kbd{file}]} @code{[@code{gqpar} @kbd{file}]} @code{[@code{iffpar} @kbd{file}]} @code{[@code{mvpar} @kbd{file}]} @code{[@code{pw} @kbd{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
@code{keysdir}
command or default
@file{/usr/local/etc}.
Following are the subcommands:
@table @asis
@item @code{cert} @kbd{file}
Specifies the location of the required host public certificate file.
This overrides the link
@file{ntpkey_cert_}@kbd{hostname}
in the keys directory.
@item @code{gqpar} @kbd{file}
Specifies the location of the optional GQ parameters file.
This
overrides the link
@file{ntpkey_gq_}@kbd{hostname}
in the keys directory.
@item @code{host} @kbd{file}
Specifies the location of the required host key file.
This overrides
the link
@file{ntpkey_key_}@kbd{hostname}
in the keys directory.
@item @code{iffpar} @kbd{file}
Specifies the location of the optional IFF parameters file.
This overrides the link
@file{ntpkey_iff_}@kbd{hostname}
in the keys directory.
@item @code{leap} @kbd{file}
Specifies the location of the optional leapsecond file.
This overrides the link
@file{ntpkey_leap}
in the keys directory.
@item @code{mvpar} @kbd{file}
Specifies the location of the optional MV parameters file.
This overrides the link
@file{ntpkey_mv_}@kbd{hostname}
in the keys directory.
@item @code{pw} @kbd{password}
Specifies the password to decrypt files containing private keys and
identity parameters.
This is required only if these files have been
encrypted.
@item @code{randfile} @kbd{file}
Specifies the location of the random seed file used by the OpenSSL
library.
The defaults are described in the main text above.
@item @code{sign} @kbd{file}
Specifies the location of the optional sign key file.
This overrides
the link
@file{ntpkey_sign_}@kbd{hostname}
in the keys directory.
If this file is
not found, the host key is also the sign key.
@end table
@item @code{keys} @kbd{keyfile}
Specifies the complete path and location of the MD5 key file
containing the keys and key identifiers used by
@code{ntpd(1ntpdmdoc)},
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
when operating with symmetric key cryptography.
This is the same operation as the
@code{-k}
command line option.
@item @code{keysdir} @kbd{path}
This command specifies the default directory path for
cryptographic keys, parameters and certificates.
The default is
@file{/usr/local/etc/}.
@item @code{requestkey} @kbd{key}
Specifies the key identifier to use with the
@code{ntpdc(1ntpdcmdoc)}
utility program, which uses a
proprietary protocol specific to this implementation of
@code{ntpd(1ntpdmdoc)}.
The
@kbd{key}
argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65,535, inclusive.
@item @code{revoke} @kbd{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.
@item @code{trustedkey} @kbd{key} @kbd{...}
Specifies the key identifiers which are trusted for the
purposes of authenticating peers with symmetric key cryptography,
as well as keys used by the
@code{ntpq(1ntpqmdoc)}
and
@code{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
@kbd{key}
arguments are 32-bit unsigned
integers with values from 1 to 65,535.
@end table
@subsubsection Error Codes
The following error codes are reported via the NTP control
and monitoring protocol trap mechanism.
@table @asis
@item 101
(bad field format or length)
The packet has invalid version, length or format.
@item 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.
@item 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.
@item 104
(bad or missing public key)
The public key is missing, has incorrect format or is an unsupported type.
@item 105
(unsupported digest type)
The server requires an unsupported digest/signature scheme.
@item 106
(mismatched digest types)
Not used.
@item 107
(bad signature length)
The signature length does not match the current public key.
@item 108
(signature not verified)
The message fails the signature check.
It could be bogus or signed by a
different private key.
@item 109
(certificate not verified)
The certificate is invalid or signed with the wrong key.
@item 110
(certificate not verified)
The certificate is not yet valid or has expired or the signature could not
be verified.
@item 111
(bad or missing cookie)
The cookie is missing, corrupted or bogus.
@item 112
(bad or missing leapseconds table)
The leapseconds table is missing, corrupted or bogus.
@item 113
(bad or missing certificate)
The certificate is missing, corrupted or bogus.
@item 114
(bad or missing identity)
The identity key is missing, corrupt or bogus.
@end table
@node Monitoring Support
@subsection Monitoring Support
@code{ntpd(1ntpdmdoc)}
includes a comprehensive monitoring facility suitable
for continuous, long term recording of server and client
timekeeping performance.
See the
@code{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
@file{./scripts}
directory of the source code distribution.
Using
these facilities and
@sc{unix}
@code{cron(8)}
jobs, the data can be
automatically summarized and archived for retrospective analysis.
@subsubsection Monitoring Commands
@table @asis
@item @code{statistics} @kbd{name} @kbd{...}
Enables writing of statistics records.
Currently, eight kinds of
@kbd{name}
statistics are supported.
@table @asis
@item @code{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
@code{clockstats}:
@verbatim
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
@end verbatim

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.
@item @code{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
@code{cryptostats}:
@verbatim
49213 525.624 127.127.4.1 message
@end verbatim

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
@ref{Authentication Options}
section for further information.
@item @code{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
@code{loopstats}:
@verbatim
50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806
@end verbatim

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.
@item @code{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
@code{peerstats}:
@verbatim
48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674
@end verbatim

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.
@item @code{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
@code{rawstats}:
@verbatim
50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
@end verbatim

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.
@item @code{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
@code{sysstats}:
@verbatim
50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147
@end verbatim

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.
@table @asis
@item Time since restart @code{36000}
Time in hours since the system was last rebooted.
@item Packets received @code{81965}
Total number of packets received.
@item Packets processed @code{0}
Number of packets received in response to previous packets sent
@item Current version @code{9546}
Number of packets matching the current NTP version.
@item Previous version @code{56}
Number of packets matching the previous NTP version.
@item Bad version @code{71793}
Number of packets matching neither NTP version.
@item Access denied @code{512}
Number of packets denied access for any reason.
@item Bad length or format @code{540}
Number of packets with invalid length, format or port number.
@item Bad authentication @code{10}
Number of packets not verified as authentic.
@item Rate exceeded @code{147}
Number of packets discarded due to rate limitation.
@end table
@item @code{statsdir} @kbd{directory_path}
Indicates the full path of a directory where statistics files
should be created (see below).
This keyword allows
the (otherwise constant)
@code{filegen}
filename prefix to be modified for file generation sets, which
is useful for handling statistics logs.
@item @code{filegen} @kbd{name} @code{[@code{file} @kbd{filename}]} @code{[@code{type} @kbd{typename}]} @code{[@code{link} | @code{nolink}]} @code{[@code{enable} | @code{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
@code{ntpdc(1ntpdcmdoc)}
program running at a remote location.
@table @asis
@item @code{name}
This is the type of the statistics records, as shown in the
@code{statistics}
command.
@item @code{file} @kbd{filename}
This is the file name for the statistics records.
Filenames of set
members are built from three concatenated elements
@code{prefix},
@code{filename}
and
@code{suffix}:
@table @asis
@item @code{prefix}
This is a constant filename path.
It is not subject to
modifications via the
@kbd{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
@kbd{loopstats}
and
@kbd{peerstats}
generation can be configured using the
@kbd{statsdir}
option explained above.
@item @code{filename}
This string is directly concatenated to the prefix mentioned
above (no intervening
@quoteleft{}/@quoteright{}).
This can be modified using
the file argument to the
@kbd{filegen}
statement.
No
@file{..}
elements are
allowed in this component to prevent filenames referring to
parts outside the filesystem hierarchy denoted by
@kbd{prefix}.
@item @code{suffix}
This part is reflects individual elements of a file set.
It is
generated according to the type of a file set.
@end table
@item @code{type} @kbd{typename}
A file generation set is characterized by its type.
The following
types are supported:
@table @asis
@item @code{none}
The file set is actually a single plain file.
@item @code{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
@code{ntpd(1ntpdmdoc)}
server incarnations.
The set member filename is built by appending a
@quoteleft{}.@quoteright{}
to concatenated
@kbd{prefix}
and
@kbd{filename}
strings, and
appending the decimal representation of the process ID of the
@code{ntpd(1ntpdmdoc)}
server process.
@item @code{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
@quoteleft{}.@quoteright{}
and a day specification in
the form
@code{YYYYMMdd}.
@code{YYYY}
is a 4-digit year number (e.g., 1992).
@code{MM}
is a two digit month number.
@code{dd}
is a two digit day number.
Thus, all information written at 10 December 1992 would end up
in a file named
@kbd{prefix}
@kbd{filename}.19921210.
@item @code{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
@code{W},
and a 2-digit week number.
For example, information from January,
10th 1992 would end up in a file with suffix
.No . Ns Ar 1992W1 .
@item @code{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.
@item @code{year}
One generation file element is generated per year.
The filename
suffix consists of a dot and a 4 digit year number.
@item @code{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
@code{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
@code{enable};
output is prevented by specifying
@code{disable}.
@end table
@item @code{link} | @code{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
@code{link}
and disabled using
@code{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
@code{C},
and the pid of the
@code{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.
@item @code{enable} @code{|} @code{disable}
Enables or disables the recording function.
@end table
@end table
@end table
@node Access Control Support
@subsection Access Control Support
The
@code{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
@file{/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
@code{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.
@subsubsection 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
@code{noserve}
or
@code{notrust}
flag of the matching restrict list entry is set,
the code is "DENY"; if the
@code{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.
@subsubsection Access Control Commands
@table @asis
@item @code{discard} @code{[@code{average} @kbd{avg}]} @code{[@code{minimum} @kbd{min}]} @code{[@code{monitor} @kbd{prob}]}
Set the parameters of the
@code{limited}
facility which protects the server from
client abuse.
The
@code{average}
subcommand specifies the minimum average packet
spacing, while the
@code{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
@code{monitor}
subcommand specifies the probability of discard
for packets that overflow the rate-control window.
@item @code{restrict} @code{address} @code{[@code{mask} @kbd{mask}]} @code{[@code{ippeerlimit} @kbd{int}]} @code{[@kbd{flag} @kbd{...}]}
The
@kbd{address}
argument expressed in
dotted-quad form is the address of a host or network.
Alternatively, the
@kbd{address}
argument can be a valid host DNS name.
The
@kbd{mask}
argument expressed in dotted-quad form defaults to
@code{255.255.255.255},
meaning that the
@kbd{address}
is treated as the address of an individual host.
A default entry (address
@code{0.0.0.0},
mask
@code{0.0.0.0})
is always included and is always the first entry in the list.
Note that text string
@code{default},
with no mask option, may
be used to indicate the default entry.
The
@code{ippeerlimit}
directive limits the number of peer requests for each IP to
@kbd{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,
@code{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:
@table @asis
@item @code{ignore}
Deny packets of all kinds, including
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
queries.
@item @code{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.
@item @code{limited}
Deny service if the packet spacing violates the lower limits specified
in the
@code{discard}
command.
A history of clients is kept using the
monitoring capability of
@code{ntpd(1ntpdmdoc)}.
Thus, monitoring is always active as
long as there is a restriction entry with the
@code{limited}
flag.
@item @code{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.
@item @code{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
@file{ntp.keys}
file.
This restriction is not enabled by default,
to maintain backward compatability.
Expect
@code{noepeer}
to become the default in ntp-4.4.
@item @code{nomodify}
Deny
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
queries which attempt to modify the state of the
server (i.e., run time reconfiguration).
Queries which return
information are permitted.
@item @code{noquery}
Deny
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
queries.
Time service is not affected.
@item @code{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
@code{pool}
associations, so if you want to use servers from a 
@code{pool}
directive and also want to use
@code{nopeer}
by default, you'll want a
@code{restrict source ...}
line as well that does
@emph{not}
include the
@code{nopeer}
directive.
@item @code{noserve}
Deny all packets except
@code{ntpq(1ntpqmdoc)}
and
@code{ntpdc(1ntpdcmdoc)}
queries.
@item @code{notrap}
Decline to provide mode 6 control message trap service to matching
hosts.
The trap service is a subsystem of the
@code{ntpq(1ntpqmdoc)}
control message
protocol which is intended for use by remote event logging programs.
@item @code{notrust}
Deny service unless the packet is cryptographically authenticated.
@item @code{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
@code{ntpport}
and
@code{non-ntpport}
may
be specified.
The
@code{ntpport}
is considered more specific and
is sorted later in the list.
@item @code{serverresponse fuzz}
When reponding to server requests,
fuzz the low order bits of the
@code{reftime}.
@item @code{version}
Deny packets that do not match the current NTP version.
@end table

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).
@end table
@node Automatic NTP Configuration Options
@subsection Automatic NTP Configuration Options
@subsubsection 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
@code{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
@code{manycastclient}
command, which is similar to the
@code{server}
command but with a multicast (IPv4 class
@code{D}
or IPv6 prefix
@code{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
@code{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
@code{manycastclient},
@code{tos}
and
@code{ttl}
configuration commands.
The manycast poll interval is
normally eight times the system poll interval,
which starts out at the
@code{minpoll}
value specified in the
@code{manycastclient},
command and, under normal circumstances, increments to the
@code{maxpolll}
value specified in this command.
Initially, the TTL is
set at the minimum hops specified by the
@code{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
@code{minclock}
and
@code{minsane}
values specified in the
@code{tos}
configuration command.
At least
@code{minsane}
candidate servers must be available and the mitigation
algorithms produce at least
@code{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,
@code{minsane}
defaults to 1 and
@code{minclock}
defaults to 3.
For manycast service
@code{minsane}
should be explicitly set to 4, assuming at least that
number of servers are available.

If at least
@code{minclock}
servers are found, the manycast poll interval is immediately
set to eight times
@code{maxpoll}.
If less than
@code{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
@code{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
@code{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
@code{tos}
command can be used to modify this behavior.
Servers with stratum below
@code{floor}
or above
@code{ceiling}
specified in the
@code{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
@code{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
@code{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
@code{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
@code{multicastclient}
and
@code{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
@code{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
@code{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
@code{ntpd(1ntpdmdoc)}
continuously and run either
@code{sntp(1sntpmdoc)}
or
@code{ntpd(1ntpdmdoc)}
@code{-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
@code{ntpd(1ntpdmdoc)}
@code{-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.
@subsubsection 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.
@subsubsection Broadcast Options
@table @asis
@item @code{tos} @code{[@code{bcpollbstep} @kbd{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.
@end table
@subsubsection Manycast Options
@table @asis
@item @code{tos} @code{[@code{ceiling} @kbd{ceiling} | @code{cohort} @code{@{} @code{0} | @code{1} @code{@}} | @code{floor} @kbd{floor} | @code{minclock} @kbd{minclock} | @code{minsane} @kbd{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:
@table @asis
@item @code{ceiling} @kbd{ceiling}
Peers with strata above
@code{ceiling}
will be discarded if there are at least
@code{minclock}
peers remaining.
This value defaults to 15, but can be changed
to any number from 1 to 15.
@item @code{cohort} @code{@{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.
@item @code{floor} @kbd{floor}
Peers with strata below
@code{floor}
will be discarded if there are at least
@code{minclock}
peers remaining.
This value defaults to 1, but can be changed
to any number from 1 to 15.
@item @code{minclock} @kbd{minclock}
The clustering algorithm repeatedly casts out outlier
associations until no more than
@code{minclock}
associations remain.
This value defaults to 3,
but can be changed to any number from 1 to the number of
configured sources.
@item @code{minsane} @kbd{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,
@code{minsane}
should be at least 4 in order to detect and discard
a single falseticker.
@end table
@item @code{ttl} @kbd{hop} @kbd{...}
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.
@end table
@node Reference Clock Support
@subsection 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
@file{/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
@file{/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
@file{/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
@file{/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,
@code{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
@code{127.127.}@kbd{t}.@kbd{u},
where
@kbd{t}
is an integer
denoting the clock type and
@kbd{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
@code{server}
command is used to configure a reference
clock, where the
@kbd{address}
argument in that command
is the clock address.
The
@code{key},
@code{version}
and
@code{ttl}
options are not used for reference clock support.
The
@code{mode}
option is added for reference clock support, as
described below.
The
@code{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
@file{/usr/share/doc/ntp})
page.
The
@code{minpoll}
and
@code{maxpoll}
options have
meaning only for selected clock drivers.
See the individual clock
driver document pages for additional information.

The
@code{fudge}
command is used to provide additional
information for individual clock drivers and normally follows
immediately after the
@code{server}
command.
The
@kbd{address}
argument specifies the clock address.
The
@code{refid}
and
@code{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
@code{fudge}
command as well.

The stratum number of a reference clock is by default zero.
Since the
@code{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
@code{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
@code{refid}
option is used for this purpose.
Except where noted,
these options apply to all clock drivers.
@subsubsection Reference Clock Commands
@table @asis
@item @code{server} @code{127.127.}@kbd{t}.@kbd{u} @code{[@code{prefer}]} @code{[@code{mode} @kbd{int}]} @code{[@code{minpoll} @kbd{int}]} @code{[@code{maxpoll} @kbd{int}]}
This command can be used to configure reference clocks in
special ways.
The options are interpreted as follows:
@table @asis
@item @code{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
@file{/usr/share/doc/ntp})
for further information.
@item @code{mode} @kbd{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.
@item @code{minpoll} @kbd{int}
@item @code{maxpoll} @kbd{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
@code{minpoll}
and
@code{maxpoll}
default to 6 (64 s).
For modem reference clocks,
@code{minpoll}
defaults to 10 (17.1 m) and
@code{maxpoll}
defaults to 14 (4.5 h).
The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.
@end table
@item @code{fudge} @code{127.127.}@kbd{t}.@kbd{u} @code{[@code{time1} @kbd{sec}]} @code{[@code{time2} @kbd{sec}]} @code{[@code{stratum} @kbd{int}]} @code{[@code{refid} @kbd{string}]} @code{[@code{mode} @kbd{int}]} @code{[@code{flag1} @code{0} @code{|} @code{1}]} @code{[@code{flag2} @code{0} @code{|} @code{1}]} @code{[@code{flag3} @code{0} @code{|} @code{1}]} @code{[@code{flag4} @code{0} @code{|} @code{1}]}
This command can be used to configure reference clocks in
special ways.
It must immediately follow the
@code{server}
command which configures the driver.
Note that the same capability
is possible at run time using the
@code{ntpdc(1ntpdcmdoc)}
program.
The options are interpreted as
follows:
@table @asis
@item @code{time1} @kbd{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
@code{enable}
command described in
@ref{Miscellaneous Options}
page and operates as described in the
"Reference Clock Drivers"
page
(available as part of the HTML documentation
provided in
@file{/usr/share/doc/ntp}).
@item @code{time2} @kbd{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
@file{/usr/share/doc/ntp} @file{).}
@item @code{stratum} @kbd{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.
@item @code{refid} @kbd{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.
@item @code{mode} @kbd{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.
@item @code{flag1} @code{0} @code{|} @code{1}
@item @code{flag2} @code{0} @code{|} @code{1}
@item @code{flag3} @code{0} @code{|} @code{1}
@item @code{flag4} @code{0} @code{|} @code{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
@code{flag4}
is used to enable recording monitoring
data to the
@code{clockstats}
file configured with the
@code{filegen}
command.
Further information on the
@code{filegen}
command can be found in
@ref{Monitoring Options}.
@end table
@end table
@node Miscellaneous Options
@subsection Miscellaneous Options
@table @asis
@item @code{broadcastdelay} @kbd{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.
@item @code{calldelay} @kbd{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.
@item @code{driftfile} @kbd{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
@code{-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
@code{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.
@item @code{dscp} @kbd{value}
This option specifies the Differentiated Services Control Point (DSCP) value,
a 6-bit code.
The default value is 46, signifying Expedited Forwarding.
@item @code{enable} @code{[@code{auth} | @code{bclient} | @code{calibrate} | @code{kernel} | @code{mode7} | @code{monitor} | @code{ntp} | @code{stats} | @code{peer_clear_digest_early} | @code{unpeer_crypto_early} | @code{unpeer_crypto_nak_early} | @code{unpeer_digest_early}]}
@item @code{disable} @code{[@code{auth} | @code{bclient} | @code{calibrate} | @code{kernel} | @code{mode7} | @code{monitor} | @code{ntp} | @code{stats} | @code{peer_clear_digest_early} | @code{unpeer_crypto_early} | @code{unpeer_crypto_nak_early} | @code{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
@code{ntpdc(1ntpdcmdoc)}
utility program.
@table @asis
@item @code{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
@code{enable}.
@item @code{bclient}
Enables the server to listen for a message from a broadcast or
multicast server, as in the
@code{multicastclient}
command with default
address.
The default for this flag is
@code{disable}.
@item @code{calibrate}
Enables the calibrate feature for reference clocks.
The default for
this flag is
@code{disable}.
@item @code{kernel}
Enables the kernel time discipline, if available.
The default for this
flag is
@code{enable}
if support is available, otherwise
@code{disable}.
@item @code{mode7}
Enables processing of NTP mode 7 implementation-specific requests
which are used by the deprecated
@code{ntpdc(1ntpdcmdoc)}
program.
The default for this flag is disable.
This flag is excluded from runtime configuration using
@code{ntpq(1ntpqmdoc)}.
The
@code{ntpq(1ntpqmdoc)}
program provides the same capabilities as
@code{ntpdc(1ntpdcmdoc)}
using standard mode 6 requests.
@item @code{monitor}
Enables the monitoring facility.
See the
@code{ntpdc(1ntpdcmdoc)}
program
and the
@code{monlist}
command or further information.
The
default for this flag is
@code{enable}.
@item @code{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
@code{enable}.
@item @code{peer_clear_digest_early}
By default, if
@code{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
@code{peerstats}
file for evidence of any of these attacks.
The
default for this flag is
@code{enable}.
@item @code{stats}
Enables the statistics facility.
See the
@ref{Monitoring Options}
section for further information.
The default for this flag is
@code{disable}.
@item @code{unpeer_crypto_early}
By default, if
@code{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
.B still
using autokey
.B and
you are seeing this sort of DoS attack
disabling this flag will delay
tearing down the association until the reachability counter
becomes zero.
You can check your
@code{peerstats}
file for evidence of any of these attacks.
The
default for this flag is
@code{enable}.
@item @code{unpeer_crypto_nak_early}
By default, if
@code{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
@code{peerstats}
file for evidence of any of these attacks.
The
default for this flag is
@code{enable}.
@item @code{unpeer_digest_early}
By default, if
@code{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
@code{peerstats}
file for evidence of any of these attacks.
The
default for this flag is
@code{enable}.
@end table
@item @code{includefile} @kbd{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
@code{ntpd(1ntpdmdoc)}
on multiple hosts, with (mostly) common options (e.g., a
restriction list).
@item @code{interface} @code{[@code{listen} | @code{ignore} | @code{drop}]} @code{[@code{all} | @code{ipv4} | @code{ipv6} | @code{wildcard} @kbd{name} | @kbd{address} @code{[@code{/} @kbd{prefixlen}]}]}
The
@code{interface}
directive controls which network addresses
@code{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,
@kbd{prefixlen}
determines how many bits must match for this rule to apply.
@code{ignore}
prevents opening matching addresses,
@code{drop}
causes
@code{ntpd(1ntpdmdoc)}
to open the address and drop all received packets without examination.
Multiple
@code{interface}
directives can be used.
The last rule which matches a particular address determines the action for it.
@code{interface}
directives are disabled if any
@code{-I},
@code{--interface},
@code{-L},
or
@code{--novirtualips}
command-line options are specified in the configuration file,
all available network addresses are opened.
The
@code{nic}
directive is an alias for
@code{interface}.
@item @code{leapfile} @kbd{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
@code{https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list}
or
@code{ftp://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list}.
The
@code{leapfile}
is scanned when
@code{ntpd(1ntpdmdoc)}
processes the
@code{leapfile} @code{directive} @code{or} @code{when}
@code{ntpd} @code{detects} @code{that} @code{the}
@kbd{leapfile}
has changed.
@code{ntpd}
checks once a day to see if the
@kbd{leapfile}
has changed.
The
@code{update-leap(1update_leapmdoc)}
script can be run to see if the
@kbd{leapfile}
should be updated.
@item @code{leapsmearinterval} @kbd{seconds}
This EXPERIMENTAL option is only available if
@code{ntpd(1ntpdmdoc)}
was built with the
@code{--enable-leap-smear}
option to the
@code{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).
.Sy DO NOT USE THIS OPTION ON PUBLIC-ACCESS SERVERS!
See http://bugs.ntp.org/2855 for more information.
@item @code{logconfig} @kbd{configkeyword}
This command controls the amount and type of output written to
the system
@code{syslog(3)}
facility or the alternate
@code{logfile}
log file.
By default, all output is turned on.
All
@kbd{configkeyword}
keywords can be prefixed with
@quoteleft{}=@quoteright{},
@quoteleft{}+@quoteright{}
and
@quoteleft{}-@quoteright{},
where
@quoteleft{}=@quoteright{}
sets the
@code{syslog(3)}
priority mask,
@quoteleft{}+@quoteright{}
adds and
@quoteleft{}-@quoteright{}
removes
messages.
@code{syslog(3)}
messages can be controlled in four
classes
(@code{clock}, @code{peer}, @code{sys} and @code{sync}).
Within these classes four types of messages can be
controlled: informational messages
(@code{info}),
event messages
(@code{events}),
statistics messages
(@code{statistics})
and
status messages
(@code{status}).

Configuration keywords are formed by concatenating the message class with
the event class.
The
@code{all}
prefix can be used instead of a message class.
A
message class may also be followed by the
@code{all}
keyword to enable/disable all
messages of the respective message class.
Thus, a minimal log configuration
could look like this:
@verbatim
logconfig =syncstatus +sysevents
@end verbatim

This would just list the synchronizations state of
@code{ntpd(1ntpdmdoc)}
and the major system events.
For a simple reference server, the
following minimum message configuration could be useful:
@verbatim
logconfig =syncall +clockall
@end verbatim

This configuration will list all clock information and
synchronization information.
All other events and messages about
peers, system events and so on is suppressed.
@item @code{logfile} @kbd{logfile}
This command specifies the location of an alternate log file to
be used instead of the default system
@code{syslog(3)}
facility.
This is the same operation as the
@code{-l}
command line option.
@item @code{mru} @code{[@code{maxdepth} @kbd{count} | @code{maxmem} @kbd{kilobytes} | @code{mindepth} @kbd{count} | @code{maxage} @kbd{seconds} | @code{initialloc} @kbd{count} | @code{initmem} @kbd{kilobytes} | @code{incalloc} @kbd{count} | @code{incmem} @kbd{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.
@table @asis
@item @code{maxdepth} @kbd{count}
@item @code{maxmem} @kbd{kilobytes}
Equivalent upper limits on the size of the MRU list, in terms of entries or kilobytes.
The acutal limit will be up to
@code{incalloc}
entries or
@code{incmem}
kilobytes larger.
As with all of the
@code{mru}
options offered in units of entries or kilobytes, if both
@code{maxdepth}
and
@code{maxmem} @code{are} @code{used,} @code{the} @code{last} @code{one} @code{used} @code{controls.}
The default is 1024 kilobytes.
@item @code{mindepth} @kbd{count}
Lower limit on the MRU list size.
When the MRU list has fewer than
@code{mindepth}
entries, existing entries are never removed to make room for newer ones,
regardless of their age.
The default is 600 entries.
@item @code{maxage} @kbd{seconds}
Once the MRU list has
@code{mindepth}
entries and an additional client is to ba added to the list,
if the oldest entry was updated more than
@code{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 
@code{maxdepth} @code{/} @code{moxmem}.
The default is 64 seconds.
@item @code{initalloc} @kbd{count}
@item @code{initmem} @kbd{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.
@item @code{incalloc} @kbd{count}
@item @code{incmem} @kbd{kilobytes}
Size of additional memory allocations when growing the MRU list, in entries or kilobytes.
The default is 4 kilobytes.
@end table
@item @code{nonvolatile} @kbd{threshold}
Specify the
@kbd{threshold}
delta in seconds before an hourly change to the
@code{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
@code{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.
@item @code{phone} @kbd{dial} @kbd{...}
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.
@item @code{pollskewlist} @code{[@kbd{poll} @kbd{value} | @kbd{value}]} @kbd{...} @code{[@code{default} @kbd{value} | @kbd{value}]}
Enable skewing of our poll requests to our servers.
@kbd{poll}
is a number between 3 and 17 inclusive, identifying a specific poll interval.
A poll interval is 2^n seconds in duration,
so a poll value of 3 corresponds to 8 seconds
and
a poll interval of 17 corresponds to
131,072 seconds, or about a day and a half.
The next two numbers must be between 0 and one-half of the poll interval,
inclusive.
The first number specifies how early the poll may start,
while
the second number specifies how late the poll may be delayed.
With no arguments, internally specified default values are chosen.
@item @code{reset} @code{[@code{allpeers}]} @code{[@code{auth}]} @code{[@code{ctl}]} @code{[@code{io}]} @code{[@code{mem}]} @code{[@code{sys}]} @code{[@code{timer}]}
Reset one or more groups of counters maintained by
@code{ntpd}
and exposed by
@code{ntpq}
and
@code{ntpdc}.
@item @code{rlimit} @code{[@code{memlock} @kbd{Nmegabytes} | @code{stacksize} @kbd{N4kPages} @code{filenum} @kbd{Nfiledescriptors}]}
@table @asis
@item @code{memlock} @kbd{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
@code{-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".
@item @code{stacksize} @kbd{N4kPages}
Specifies the maximum size of the process stack on systems with the
@code{mlockall()}
function.
Defaults to 50 4k pages (200 4k pages in OpenBSD).
@item @code{filenum} @kbd{Nfiledescriptors}
Specifies the maximum number of file descriptors ntpd may have open at once.
Defaults to the system default.
@end table
@item @code{saveconfigdir} @kbd{directory_path}
Specify the directory in which to write configuration snapshots
requested with
.Cm ntpq 's
@code{saveconfig}
command.
If
@code{saveconfigdir}
does not appear in the configuration file,
@code{saveconfig}
requests are rejected by
@code{ntpd}.
@item @code{saveconfig} @kbd{filename}
Write the current configuration, including any runtime
modifications given with
@code{:config}
or
@code{config-from-file}
to the
@code{ntpd}
host's
@kbd{filename}
in the
@code{saveconfigdir}.
This command will be rejected unless the
@code{saveconfigdir}
directive appears in
.Cm ntpd 's
configuration file.
@kbd{filename}
can use
@code{strftime(3)}
format directives to substitute the current date and time,
for example,
@code{saveconfig\ ntp-%Y%m%d-%H%M%S.conf}.
The filename used is stored in the system variable
@code{savedconfig}.
Authentication is required.
@item @code{setvar} @kbd{variable} @code{[@code{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
@code{name}@code{=}@kbd{value}
is followed by the
@code{default}
keyword, the
variable will be listed as part of the default system variables
(@code{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
@code{setvar}
mechanism.
There are three special variables that contain the names
of all variable of the same group.
The
@code{sys_var_list}
holds
the names of all system variables.
The
@code{peer_var_list}
holds
the names of all peer variables and the
@code{clock_var_list}
holds the names of the reference clock variables.
@item @code{sysinfo}
Display operational summary.
@item @code{sysstats}
Show statistics counters maintained in the protocol module.
@item @code{tinker} @code{[@code{allan} @kbd{allan} | @code{dispersion} @kbd{dispersion} | @code{freq} @kbd{freq} | @code{huffpuff} @kbd{huffpuff} | @code{panic} @kbd{panic} | @code{step} @kbd{step} | @code{stepback} @kbd{stepback} | @code{stepfwd} @kbd{stepfwd} | @code{stepout} @kbd{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:
@table @asis
@item @code{allan} @kbd{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.
@item @code{dispersion} @kbd{dispersion}
The argument becomes the new value for the dispersion increase rate,
normally .000015 s/s.
@item @code{freq} @kbd{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.
@item @code{huffpuff} @kbd{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.
@item @code{panic} @kbd{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.
@item @code{step} @kbd{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.
@item @code{stepback} @kbd{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.
@item @code{stepfwd} @kbd{stepfwd}
As for stepback, but for the forward direction.
@item @code{stepout} @kbd{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.
@end table
@item @code{writevar} @kbd{assocID\ name} @kbd{=} @kbd{value} @kbd{[,...]}
Write (create or update) the specified variables.
If the
@code{assocID}
is zero, the variablea re from the
system variables
name space, otherwise they are from the
peer variables
name space.
The
@code{assocID}
is required, as the same name can occur in both name spaces.
@item @code{trap} @kbd{host_address} @code{[@code{port} @kbd{port_number}]} @code{[@code{interface} @kbd{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.
@item @code{ttl} @kbd{hop} @kbd{...}
This command specifies a list of TTL values in increasing order.
Up to 8 values can be specified.
In
@code{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.
@item @code{hop} @kbd{...}
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.
@end table

This section was generated by @strong{AutoGen},
using the @code{agtexi-cmd} template and the option descriptions for the @code{ntp.conf} program.
This software is released under the NTP license, <http://ntp.org/license>.

@menu
* ntp.conf Files::                  Files
* ntp.conf See Also::               See Also
* ntp.conf Bugs::                   Bugs
* ntp.conf Notes::                  Notes
@end menu

@node ntp.conf Files
@subsection ntp.conf Files
@table @asis
@item @file{/etc/ntp.conf}
the default name of the configuration file
@item @file{ntp.keys}
private MD5 keys
@item @file{ntpkey}
RSA private key
@item @file{ntpkey_}@kbd{host}
RSA public key
@item @file{ntp_dh}
Diffie-Hellman agreement parameters
@end table
@node ntp.conf See Also
@subsection ntp.conf See Also
@code{ntpd(1ntpdmdoc)},
@code{ntpdc(1ntpdcmdoc)},
@code{ntpq(1ntpqmdoc)}

In addition to the manual pages provided,
comprehensive documentation is available on the world wide web
at
@code{http://www.ntp.org/}.
A snapshot of this documentation is available in HTML format in
@file{/usr/share/doc/ntp}.
@*

@*
David L. Mills, @emph{Network Time Protocol (Version 4)}, RFC5905
@node ntp.conf Bugs
@subsection ntp.conf Bugs
The syntax checking is not picky; some combinations of
ridiculous and even hilarious options and modes may not be
detected.

The
@file{ntpkey_}@kbd{host}
files are really digital
certificates.
These should be obtained via secure directory
services when they become universally available.
@node ntp.conf Notes
@subsection ntp.conf Notes
This document was derived from FreeBSD.