freebsd-nq/usr.sbin/ntp/doc/ntp.conf.5

.\"
.\" $FreeBSD$
.\"
.Dd January 13, 2000
.Dt NTP.CONF 5
.Os
.Sh NAME
.Nm ntp.conf
.Nd Network Time Protocol (NTP) daemon configuration file
.Sh SYNOPSIS
.Nm /etc/ntp.conf
.Sh DESCRIPTION
The
.Nm
configuration file is read at initial startup by the
.Xr ntpd 8
daemon in order to specify the synchronization sources,
modes and other related information.
Usually, it is installed in the
.Pa /etc
directory,
but could be installed elsewhere
(see the daemon's
.Fl c
command line option).
.Pp
The file format is similar to other
.Ux
configuration files.
Comments begin with a
.Ql #
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.
.Pp
The rest of this page describes the configuration and control options.
The
.Qq "Notes on Configuring NTP and Setting up a NTP Subnet"
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp )
contains an extended discussion of these options.
In addition to the discussion of general
.Sx Configuration Options ,
there are sections describing the following supported functionality
and the options used to control it:
.Bl -bullet -offset indent
.It
.Sx Authentication Support
.It
.Sx Monitoring Support
.It
.Sx Access Control Support
.It
.Sx Reference Clock Support
.El
.Pp
Following these is a section describing
.Sx Miscellaneous Options .
While there is a rich set of options available,
the only required option is one or more
.Ic server ,
.Ic peer ,
.Ic broadcast
or
.Ic manycastclient
commands.
.Sh 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 auxilliary commands that specify environmental variables
that control various related operations.
.Ss 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 (IP class A, B and C), (b) the
broadcast address of a local interface, (m) a multicast address (IP
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.
.Bl -tag -width indent
.It Xo Ic server Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm burst
.Op Cm iburst
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Xc
.It Xo Ic peer Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Xc
.It Xo Ic broadcast Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm ttl Ar ttl
.Xc
.It Xo Ic manycastclient Ar address
.Op Cm key Ar key \&| Cm autokey
.Op Cm version Ar version
.Op Cm prefer
.Op Cm minpoll Ar minpoll
.Op Cm maxpoll Ar maxpoll
.Op Cm ttl Ar ttl
.Xc
.El
.Pp
These four commands specify the time server name or address to
be used and the mode in which to operate.
The
.Ar address
can be
either a DNS name or a IP address in dotted-quad notation.
Additional information on association behavior can be found in the
.Qq "Association Management"
page.
.Bl -tag -width indent
.It Ic 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
.Em not
be used for type
b or m addresses.
.It Ic 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.
.It Ic 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
.Ar 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 224.0.1.1 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
.Ic broadcastclient
or
.Ic multicastclient
commands
below.
.It Ic 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
.Ic 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
.Ic 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
.Ar address
and specifically enabled
servers respond to these messages.
The client selects the servers
providing the best time and continues as with the
.Ic server
command.
The remaining servers are discarded as if never
heard.
.El
.Pp
Options:
.Bl -tag -width indent
.It Cm autokey
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the autokey scheme
described in
.Sx Authentication Options .
.It Cm burst
when the server is reachable and at each poll interval, send a
burst of eight packets instead of the usual one packet.
The spacing
between the first and the second packets is about 16s to allow a
modem call to complete, while the spacing between the remaining
packets is about 2s.
This is designed to improve timekeeping
quality with the
.Ic server
command and s
addresses.
.It Cm iburst
When the server is unreachable and at each poll interval, send
a burst of eight packets instead of the usual one.
As long as the
server is unreachable, the spacing between packets is about 16s to
allow a modem call to complete.
Once the server is reachable, the
spacing between packets is about 2s.
This is designed to speed the
initial synchronization acquisition with the
.Ic server
command and s addresses and when
.Xr ntpd 8
is started
with the
.Fl q
option.
.It Cm key Ar key
All packets sent to and received from the server or peer are to
include authentication fields encrypted using the specified
.Ar key
identifier with values from 1 to 65534, inclusive.
The
default is to include no encryption field.
.It Cm minpoll Ar minpoll
.It Cm maxpoll Ar maxpoll
These options specify the minimum and maximum poll intervals
for NTP messages, in seconds to the power of two.
The maximum poll
interval defaults to 10 (1,024 s), but can be increased by the
.Cm 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
.Cm minpoll
option to a lower limit of 4 (16 s).
.It Cm 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
.Qq "Mitigation Rules and the prefer Keyword"
page for further
information.
.It Cm ttl Ar ttl
This option is used only with broadcast server and manycast
client modes.
It specifies the time-to-live
.Cm ttl
to
use on broadcast server and multicast server and the maximum
.Cm 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.
.It Cm version Ar version
Specifies the version number to be used for outgoing NTP
packets.
Versions 1-4 are the choices, with version 4 the
default.
.El
.Ss Auxilliary Commands
.Bl -tag -width indent
.It Ic 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
.Sx Authentication Options .
.It Ic manycastserver Ar address ...
This command enables reception of manycast client messages to
the multicast group address(es) (type m) specified.
At least one
address is required, but the NTP multicast address 224.0.1.1
assigned by the IANA should NOT be used, unless specific means are
taken to limit the span of the reply and avoid a possibly massive
implosion at the original sender.
Note that, in order to avoid
accidental or malicious disruption in this mode, both the server
and client should operate using symmetric-key or public-key
authentication as described in
.Sx Authentication Options .
.It Ic multicastclient Ar address ...
This command enables reception of multicast server messages to
the multicast group address(es) (type m) specified.
Upon receiving
a message for the first time, the multicast client measures the
nominal server propagation delay using a brief client/server
exchange with the server, then enters the broadcast client mode, in
which it synchronizes to succeeding multicast messages.
Note that,
in order to avoid accidental or malicious disruption in this mode,
both the server and client should operate using symmetric-key or
public-key authentication as described in
.Sx Authentication Options .
.El
.Sh 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 an 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 augmented 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.
.Pp
NTPv4 retains the NTPv3 schemes, 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.
.Pp
Authentication is configured separately for each association
using the
.Cm key
or
.Cm autokey
subcommands on the
.Ic peer  ,
.Ic server  ,
.Ic broadcast
and
.Ic manycastclient
commands as described in
.Sx Configuration Options .
The authentication
options described below specify the suite of keys, select the key
for each configured association and manage the configuration
operations.
.Pp
The
.Cm auth
flag controls whether new associations or
remote configuration commands require cryptographic authentication.
This flag can be set or reset by the
.Ic enable
and
.Ic disable
configuration commands and also by remote
configuration commands sent by a
.Xr ntpdc 8
program running in
another machine.
If this flag is enabled, which is the default
case, new broadcast client and symmetric passive associations and
remote configuration commands must be cryptographically
authenticated using either symmetric-key or public-key schemes.
If
this flag is disabled, these operations are effective even if not
cryptographic authenticated.
It should be understood that operating
in the latter mode invites a significant vulnerability where a
rogue hacker can seriously disrupt client timekeeping.
.Pp
In networks with firewalls and large numbers of broadcast
clients it may be acceptable to disable authentication, since that
avoids key distribution and simplifies network maintenance.
However, when the configuration file contains host names, or when a
server or client is configured remotely, host names are resolved
using the DNS and a separate name resolution process.
In order to
protect against bogus name server messages, name resolution
messages are authenticated using an internally generated key which
is normally invisible to the user.
However, if cryptographic
support is disabled, the name resolution process will fail.
This
can be avoided either by specifying IP addresses instead of host
names, which is generally inadvisable, or by enabling the flag for
name resolution and disabled it once the name resolution process is
complete.
.Pp
An attractive alternative where multicast support is available
is manycast mode, in which clients periodically troll for servers.
Cryptographic authentication in this mode uses public-key schemes
as described below.
The principle advantage of this 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.
.Pp
In addition to the default symmetric-key cryptographic support,
support for public-key cryptography is available if the requisite
.Sy rsaref20
software distribution has been installed before
building the distribution.
Public-key cryptography provides secure
authentication of servers without compromising accuracy and
stability.
The security model and protocol schemes for both
symmetric-key and public-key cryptography are described below.
.Ss Symmetric-Key Scheme
The original RFC-1305 specification allows any one of possibly
65,534 keys, each distinguished by a 32-bit key identifier, to
authenticate an association.
The servers and clients involved must
agree on the key and key identifier to authenticate their messages.
Keys and related information are specified in a key file, usually
called
.Pa ntp.keys  ,
which should be exchanged and stored
using secure procedures 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
.Xr ntpq 8
and
.Xr ntpdc 8
utility programs.
.Pp
When
.Xr ntpd 8
is first started, it reads the key file
specified in the
.Ic keys
command and installs the keys in the
key cache.
However, the keys must be activated with the
.Ic 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
.Xr ntpdc 8  .
This also provides a revocation capability that can
be used if a key becomes compromised.
The
.Ic requestkey
command selects the key used as the password for the
.Xr ntpdc 8
utility, while the
.Ic controlkey
command selects the key used
as the password for the
.Xr ntpq 8
utility.
.Ss Public-Key Scheme
The original NTPv3 authentication scheme described in RFC-1305
continues to be supported; however, in NTPv4 an additional
authentication scheme called Autokey is available.
It uses MD5
message digest, RSA public-key signature and Diffie-Hellman key
agreement algorithms available from several sources, but not
included in the NTPv4 software distribution.
In order to be
effective, the
.Sy rsaref20
package must be installed as
described in the
.Pa README.rsa
file.
Once installed, the
configure and build process automatically detects it and compiles
the routines required.
The Autokey scheme has several modes of
operation corresponding to the various NTP modes supported.
RSA
signatures with timestamps are used in all modes to verify the
source of cryptographic values.
All modes use a special cookie
which can be computed independently by the client and server.
In
symmetric modes the cookie is constructed using the Diffie-Hellman
key agreement algorithm.
In other modes the cookie is constructed
from the IP addresses and a private value known only to the server.
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, in the
.Qq "Autonomous Authentication"
page.
.Pp
The cryptographic values used by the Autokey scheme are
incorporated as a set of files generated by the
.Xr ntp-genkeys 8
program, including the
symmetric private keys, public/private key pair, and the agreement
parameters.
See the
.Xr ntp.keys 5
page for a description of
the formats of these files.
They contain cryptographic values
generated by the algorithms of the
.Sy rsaref20
package and
are in printable ASCII format.
All file names include the
timestamp, in NTP seconds, following the default names given below.
Since the file data are derived from random values seeded by the
system clock and the file name includes the timestamp, every
generation produces a different file and different file name.
.Pp
The
.Pa ntp.keys
file contains the DES/MD5 private keys.
It
must be distributed by secure means to other servers and clients
sharing the same security compartment and made visible only to
root.
While this file is not used with the Autokey scheme, it is
needed to authenticate some remote configuration commands used by
the
.Xr ntpdc 8 ,
.Xr ntpq 8
utilities.
The
.Pa ntpkey
file
contains the RSA private key.
It is useful only to the machine that
generated it and never shared with any other daemon or application
program, so must be made visible only to root.
.Pp
The
.Pa ntp_dh
file contains the agreement parameters,
which are used only in symmetric (active and passive) modes.
It is
necessary that both peers beginning a symmetric-mode association
share the same parameters, but it does not matter which
.Pa ntp_dh
file generates them.
If one of the peers contains
the parameters, the other peer obtains them using the Autokey
protocol.
If both peers contain the parameters, the most recent
copy is used by both peers.
If a peer does not have the parameters,
they will be requested by all associations, either configured or
not; but, none of the associations can proceed until one of them
has received the parameters.
Once loaded, the parameters can be
provided on request to other clients and servers.
The
.Pa ntp_dh
file can be also be distributed using insecure
means, since the data are public values.
.Pp
The
.Pa ntpkey_ Ns Ar host
file contains the RSA public
key, where
.Ar host
is the name of the host.
Each host
must have its own
.Pa ntpkey_ Ns Ar host
file, which is
normally provided to other hosts using the Autokey protocol.
Each
.Ic server
or
.Ic peer
association requires the public
key associated with the particular server or peer to be loaded
either directly from a local file or indirectly from the server
using the Autokey protocol.
These files can be widely distributed
and stored using insecure means, since the data are public
values.
.Pp
The optional
.Pa ntpkey_certif_ Ns Ar host
file contains
the PKI certificate for the host.
This provides a binding between
the host hame and RSA public key.
In the current implementation the
certificate is obtained by a client, if present, but the contents
are ignored.
.Pp
Due to the widespread use of interface-specific naming, the host
names used in configured and mobilized associations are determined
by the
.Ux
.Xr gethostname 3
library routine.
Both the
.Xr ntp-genkeys 8
program and the Autokey protocol derive the
name of the public key file using the name returned by this
routine.
While every server and client is required to load their
own public and private keys, the public keys for each client or
peer association can be obtained from the server or peer using the
Autokey protocol.
Note however, that at the current stage of
development the authenticity of the server or peer and the
cryptographic binding of the server name, address and public key is
not yet established by a certificate authority or web of trust.
.Ss Leapseconds Table
The NIST provides a table showing the epoch for all historic
occasions of leap second insertion since 1972.
The leapsecond table
shows each epoch of insertion along with the offset of
International Atomic Time (TAI) with respect to Coordinated
Universtal Time (UTC), as disseminated by NTP.
The table can be
obtained directly from NIST national time servers using
FTP as the ASCII file
.Pa pub/leap-seconds .
.Pp
While not strictly a security function, the Autokey scheme
provides means to securely retrieve the leapsecond table from a
server or peer.
Servers load the leapsecond table directly from the
file specified in the
.Ic crypto
command, while clients can
load the table indirectly from the servers using the Autokey
protocol.
Once loaded, the table can be provided on request to
other clients and servers.
.Ss Key Management
All key files are installed by default in
.Pa /usr/local/etc ,
which is normally in a shared filesystem
in NFS-mounted networks and avoids installing them in each machine
separately.
The default can be overridden by the
.Ic keysdir   
configuration command.
However, this is not a good place to install
the private key file, since each machine needs its own file.
A
suitable place to install it is in
.Pa /etc ,
which is normally
not in a shared filesystem.
.Pp
The recommended practice is to keep the timestamp extensions
when installing a file and to install a link from the default name
(without the timestamp extension) to the actual file.
This allows
new file generations to be activated simply by changing the link.
However,
.Xr ntpd 8
parses the link name when present to extract
the extension value and sends it along with the public key and host
name when requested.
This allows clients to verify that the file
and generation time are always current.
However, the actual
location of each file can be overridden by the
.Ic crypto
configuration command.
.Pp
All cryptographic keys and related parameters should be
regenerated on a periodic and automatic basis, like once per month.
The
.Xr ntp-genkeys 8
program uses the same timestamp extension
for all files generated at one time, so each generation is distinct
and can be readily recognized in monitoring data.
While a
public/private key pair must be generated by every server and
client, the public keys and agreement parameters do not need to be
explicitly copied to all machines in the same security compartment,
since they can be obtained automatically using the Autokey
protocol.
However, it is necessary that all primary servers have
the same agreement parameter file.
The recommended way to do this
is for one of the primary servers to generate that file and then
copy it to the other primary servers in the same compartment using
the
.Ux
.Xr rdist 1
command.
Future versions of the Autokey
protocol are to contain provisions for an agreement protocol to do
this automatically.
.Pp
Servers and clients can make a new generation in the following
way.
All machines have loaded the old generation at startup and are
operating normally.
At designated intervals, each machine generates
a new public/private key pair and makes links from the default file
names to the new file names.
The
.Xr ntpd 8
is then restarted
and loads the new generation, with result clients no longer can
authenticate correctly.
The Autokey protocol is designed so that
after a few minutes the clients time out and restart the protocol
from the beginning, with result the new generation is loaded and
operation continues as before.
A similar procedure can be used for
the agreement parameter file, but in this case precautions must be
take to be sure that all machines with this file have the same
copy.
.Ss Authentication Commands
.Bl -tag -width indent
.It Ic autokey Op Ar 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.
.It Ic controlkey Ar key
Specifies the key identifier to use with the
.Xr ntpq 8
utility, which uses the standard
protocol defined in RFC-1305.
The
.Ar key
argument is
the key identifier for a trusted key, where the value can be in the
range 1 to 65534, inclusive.
.It Xo Ic crypto
.Op Cm flags Ar flags
.Op Cm privatekey Ar file
.Op Cm publickey Ar file
.Op Cm dhparms Ar file
.Op Cm leap Ar file
.Xc
This command requires the NTP daemon build process be
configured with the RSA library.
This command activates public-key
cryptography and loads the required RSA private and public key
files and the optional Diffie-Hellman agreement parameter file, if
present.
If one or more files are left unspecified, the default
names are used as described below.
Following are the
subcommands:
.Bl -tag -width indent
.It Cm privatekey Ar file
Specifies the location of the RSA private key file, which
otherwise defaults to
.Pa /usr/local/etc/ntpkey .
.It Cm publickey Ar file
Specifies the location of the RSA public key file, which
otherwise defaults to
.Pa /usr/local/etc/ntpkey_ Ns Ar host ,
where
.Ar host
is the name of the generating machine.
.It Cm dhparms Ar file
Specifies the location of the Diffie-Hellman parameters file,
which otherwise defaults to
.Pa /usr/local/etc/ntpkey_dh .
.It Cm leap Ar file
Specifies the location of the leapsecond table file, which
otherwise defaults to
.Pa /usr/local/etc/ntpkey_leap .
.El
.It Ic keys Ar keyfile
Specifies the location of the DES/MD5 private key file
containing the keys and key identifiers used by
.Xr ntpd 8 ,
.Xr ntpq 8
and
.Xr ntpdc 8
when operating in symmetric-key
mode.
.It Ic keysdir Ar path
This command requires the NTP daemon build process be
configured with the RSA library.
It specifies the default directory
path for the private key file, agreement parameters file and one or
more public key files.
The default when this command does not
appear in the configuration file is
.Pa /usr/local/etc .
.It Ic requestkey Ar key
Specifies the key identifier to use with the
.Xr ntpdc 8
utility program, which uses a
proprietary protocol specific to this implementation of
.Xr ntpd 8  .
The
.Ar key
argument is a key identifier
for the trusted key, where the value can be in the range 1 to
65534, inclusive.
.It Ic revoke Ar 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.
.It Ic trustedkey Ar key ...
Specifies the key identifiers which are trusted for the
purposes of authenticating peers with symmetric-key cryptography,
as well as keys used by the
.Xr ntpq 8
and
.Xr ntpdc 8
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
.Ar key
arguments are 32-bit unsigned
integers with values from 1 to 65,534.
.El
.Sh Monitoring Support
.Xr ntpd 8
includes a comprehensive monitoring facility suitable
for continuous, long term recording of server and client
timekeeping performance.
See the
.Ic 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
.Pa ./scripts
directory of this distribution.
Using
these facilities and
.Ux
.Xr cron 8   
jobs, the data can be
automatically summarized and archived for retrospective analysis.
.Ss Monitoring Commands
.Bl -tag -width indent
.It Ic statistics Ar name ...
Enables writing of statistics records.
Currently, four kinds of
.Ar name
statistics are supported.
.Bl -tag -width indent
.It Cm loopstats   
Enables recording of loop filter statistics information.
Each
update of the local clock outputs a line of the following form to
the file generation set named loopstats:
.Bd -literal
50935 75440.031 0.000006019 13.778190 0.000351733 0.013380 6
.Ed
.Pp
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.
.It Cm peerstats   
Enables recording of peer statistics information.
This includes
statistics records of all peers of a NTP server and of special
signals, where present and configured.
Each valid update appends a
line of the following form to the current element of a file
generation set named peerstats:
.Bd -literal
48773 10847.650 127.127.4.1 9714 -0.001605 0.00000 0.00142
.Ed
.Pp
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 three fields show the offset, delay and RMS jitter, all in
seconds.
.It Cm clockstats
Enables recording of clock driver statistics information.
Each
update received from a clock driver appends a line of the following
form to the file generation set named clockstats:
.Bd -literal
49213 525.624 127.127.4.1 93 226 00:08:29.606 D
.Ed
.Pp
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.
.It Cm rawstats
Enables recording of raw-timestamp statistics information.
This
includes statistics records of all peers of a NTP server and of
special signals, where present and configured.
Each NTP message
received from a peer or clock driver appends a line of the
following form to the file generation set named rawstats:
.Bd -literal
50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
.Ed
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.
.El
.It Ic statsdir Ar directory_path
Indicates the full path of a directory where statistics files
should be created (see below).
This keyword allows the (otherwise
constant)
.Ic filegen   
filename prefix to be modified for file
generation sets, which is useful for handling statistics logs.
.It Xo Ic filegen Ar name
.Op Cm file Ar filename
.Op Cm type Ar typename
.Op Cm link \&| Cm nolink
.Op Cm enable \&| Cm disable
.Xc
Configures setting of generation file set
.Ar 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
.Xr ntpd 8 . 
(Most important: they can be removed to
free space for new data produced.)
Note that this command can be sent from the
.Xr ntpdc 8   
program running at a remote location.
.Bl -tag -width indent
.It Ar name
This is the type of the statistics records, as shown in the
.Ic statistics   
command.
.It Cm file Ar filename
This is the file name for the statistics records.
Filenames of
set members are built from three concatenated elements
prefix, filename and
suffix:
.Bl -tag -width indent
.It prefix
This is a constant filename path.
It is not subject to
modifications via the
.Ic 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
.Cm loopstats   
and
.Cm peerstats   
generation can be
configured using the
.Ic statsdir   
option explained above.
.It filename
This string is directly concatenated to the prefix mentioned
above (no intervening
.Ql /
(slash)).
This can be modified
using the
.Ar file
argument to the
.Ic filegen   
statement.
No
.Ql \&..
elements are allowed in this component to prevent
filenames referring to parts outside the filesystem hierarchy
denoted by prefix.
.It suffix
This part is reflects individual elements of a file set.
It is
generated according to the type of a file set.
.El
.It Cm type Ar typename
A file generation set is characterized by its type.
The
following types are supported:
.Bl -tag -width indent
.It none
The file set is actually a single plain file.
.It pid
One element of file set is used per incarnation of a
.Xr ntpd 8   
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
.Xr ntpd 8   
server
incarnations.
The set member filename is built by appending a
.Ql \&.
(dot) to concatenated prefix and filename
strings, and appending the decimal representation of the process ID
of the
.Xr ntpd 8   
server process.
.It 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
.Ql \&.
(dot) and a day
specification in the form
.Ar YYYYMMdd .
.Ar YYYY
is a 4-digit year
number (e.g., 1992).
.Ar MM
is a two digit month number.
.Ar dd
is a two digit day number.
Thus, all information
written at 10 December 1992 would end up in a file named
.Sm off
.Pa Ar prefix / Ar filename / 19921210 .
.Sm on
.It 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
Ql W ,
and a 2-digit
week number.
For example, information from January, 10th 1992 would
end up in a file with suffix
.Pa .1992W1 .
.It 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.
.It year
One generation file element is generated per year.
The filename
suffix consists of a dot and a 4 digit year number.
.It 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
.Ql 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
.Ic enable  ; 
output is prevented by specifying
.Ic disable  . 
.El
.It Cm link \&| Cm 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
.Cm link
and disabled using
.Cm nolink .
If
.Cm 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
.Ql C ,
and the pid
of the
.Xr ntpd 8   
server process.
When the number of links is
greater than one, the file is unlinked.
This allows the current
file to be accessed by a constant name.
.It Cm enable \&| Cm disable
Enables or disables the recording function.
.El
.El
.Sh Access Control Support
.Xr ntpd 8   
implements a general purpose address-and-mask based
restriction list.
The list is sorted by address and by mask, and
the list is searched in this order for matches, with the last match
found defining the restriction flags associated with the incoming
packets.
The source address of incoming packets is used for the
match, with the 32- bit address being and'ed with the mask
associated with the restriction entry and then compared with the
entry's address (which has also been and'ed with the mask) to look
for a match.
Additional information and examples can be found in the
.Qq "Notes on Configuring NTP and Setting up a NTP Subnet"
page.
.Pp
The restriction facility was implemented in conformance with the
access policies for the original NSFnet backbone time servers.
While this facility may be otherwise useful for keeping unwanted or
broken remote time servers from affecting your own, it should not
be considered an alternative to the standard NTP authentication
facility.
Source address based restrictions are easily circumvented
by a determined cracker.
.Ss 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 packet.
If the
.Cm kod
flag is set and either service is denied or the client
limit is exceeded, the server returns the packet and sets the
leap bits unsynchronized, stratum zero and the ASCII string "DENY"
in the reference source identifier field.
If the
.Cm kod
flag
is not set, the server simply drops the packet.
.Pp
A client or peer receiving a kiss-of-death packet performs a set
of sanity checks to minimize security exposure.
If this is the
first packet received from the server, the client assumes an access
denied condition at the server.
It updates the stratum and
reference identifier peer variables and sets the access denied
(test 4) bit in the peer flash variable.
If this bit is set, the
client sends no packets to the server.
If this is not the first
packet, the client assumes a client limit condition at the server,
but does not update the peer variables.
In either case, a message
is sent to the system log.
.Ss Access Control Commands
.Bl -tag -width indent
.It Xo Ic restrict numeric_address
.Op Cm mask Ar numeric_mask
.Op Ar flag ...
.Xc
The
.Ar numeric_address
argument, expressed in
dotted- quad form, is the address of an host or network.
The
.Cm mask ,
also expressed in dotted-quad form,
defaults to 255.255.255.255, meaning that the
.Ar numeric_address
is treated as the address of an
individual host.
A default entry (address 0.0.0.0, mask
0.0.0.0) is always included and, given the sort algorithm,
is always the first entry in the list.
Note that, while
.Ar numeric_address
is normally given in dotted-quad
format, the text string
.Ql default ,
with no mask option, may
be used to indicate the default entry.
In the current implementation,
.Cm 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
catagories, 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:
.Bl -tag -width indent
.It Cm kod
If access is denied, send a kiss-of-death packet.
.It Cm ignore
Ignore all packets from hosts which match this entry.
If this
flag is specified neither queries nor time server polls will be
responded to.
.It Cm noquery
Ignore all NTP mode 6 and 7 packets (i.e. information queries
and configuration requests) from the source.
Time service is not
affected.
.It Cm nomodify
Ignore all NTP mode 6 and 7 packets which attempt to modify the
state of the server (i.e. run time reconfiguration).
Queries which
return information are permitted.
.It Cm notrap
Decline to provide mode 6 control message trap service to
matching hosts.
The trap service is a subsystem of the mode 6
control message protocol which is intended for use by remote event
logging programs.
.It Cm 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.
.It Cm noserve
Ignore NTP packets whose mode is other than 6 or 7.
In effect,
time service is denied, though queries may still be permitted.
.It Cm nopeer
Provide stateless time service to polling hosts, but do not
allocate peer memory resources to these hosts even if they
otherwise might be considered useful as future synchronization
partners.
.It Cm notrust
Treat these hosts normally in other respects, but never use
them as synchronization sources.
.It Cm limited
These hosts are subject to limitation of number of clients from
the same net.
Net in this context refers to the IP notion of net
(class A, class B, class C, etc.).
Only the first
.Va client_limit
hosts that have shown up at the server and
that have been active during the last
.Va client_limit_period
seconds are accepted.
Requests from other clients from the same net
are rejected.
Only time request packets are taken into account.
Query packets sent by the
.Xr ntpq 8   
and
.Xr ntpdc 8   
programs
are not subject to these limits.
A history of clients is kept using
the monitoring capability of
.Xr ntpd 8  . 
Thus, monitoring is
always active as long as there is a restriction entry with the
.Cm limited
flag.
.It Cm 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
.Cm ntpport
and
.Cm non-ntpport
may
be specified.
The
.Cm ntpport
is considered more specific and
is sorted later in the list.
.It Cm version   
Ignore these hosts if not the current NTP version.
.El
.Pp
Default restriction list entries, with the flags
.Cm ignore ,
.Cm interface ,
.Cm 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).
.It Ic clientlimit Ar limit
Set the
.Va client_limit
variable, which limits the number
of simultaneous access-controlled clients.
The default value for
this variable is 3.
.It Ic clientperiod Ar period
Set the
.Va client_limit_period
variable, which specifies
the number of seconds after which a client is considered inactive
and thus no longer is counted for client limit restriction.
The
default value for this variable is 3600 seconds.
.El
.Sh 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
.Qq "Reference Clock Drivers"
page
(available as part of the HTML documentation
provided in
.Pa /usr/share/doc/ntp ) .
Additional information can be found in the pages linked
there, including the
.Qq "Debugging Hints for Reference Clock Drivers"
and
.Qq "How To Write a Reference Clock Driver"
pages.
In addition, support for a PPS
signal is available as described in the
.Qq "Pulse-per-second (PPS) Signal Interfacing"
page.
Many
drivers support special line discipline/streams modules which can
significantly improve the accuracy using the driver.
These are
described in the
.Qq "Line Disciplines and Streams Drivers"
page.
.Pp
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.
.Pp
For the purposes of configuration,
.Xr ntpd 8
treats
reference clocks in a manner analogous to normal NTP peers as much
as possible.
Reference clocks are identified by a syntactically
correct but invalid IP address, in order to distinguish them from
normal NTP peers.
Reference clock addresses are of the form
.Sm off
.Li 127.127. Ar t . Ar u ,
.Sm on
where
.Ar t
is an integer
denoting the clock type and
.Ar 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.
.Pp
The
.Ic server
command is used to configure a reference
clock, where the
.Ar address
argument in that command
is the clock address.
The
.Cm key  ,
.Cm version
and
.Cm ttl
options are not used for reference clock support.
The
.Cm mode
option is added for reference clock support, as
described below.
The
.Cm 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
.Qq "Mitigation Rules and the prefer Keyword"
page.
The
.Cm minpoll
and
.Cm maxpoll
options have
meaning only for selected clock drivers.
See the individual clock
driver document pages for additional information.
.Pp
The
.Ic fudge
command is used to provide additional
information for individual clock drivers and normally follows
immediately after the
.Ic server
command.
The
.Ar address
argument specifies the clock address.
The
.Cm refid
and
.Cm 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
.Ic fudge   
command as well.
.Pp
The stratum number of a reference clock is by default zero.
Since the
.Xr ntpd 8
daemon adds one to the stratum of each
peer, a primary server ordinarily displays an external stratum of
one.
In order to provide engineered backups, it is often useful to
specify the reference clock stratum as greater than zero.
The
.Cm 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
.Cm refid
option is used for this purpose.
Except where noted,
these options apply to all clock drivers.
.Ss Reference Clock Commands
.Bl -tag -width indent
.It Xo Ic server
.Sm off
.Li 127.127. Ar t . Ar u
.Sm on
.Op Cm prefer
.Op Cm mode Ar int
.Op Cm minpoll Ar int
.Op Cm maxpoll Ar int
.Xc
This command can be used to configure reference clocks in
special ways.
The options are interpreted as follows:
.Bl -tag -width indent
.It Cm 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
.Qq "Mitigation Rules and the prefer Keyword"
page for further
information.
.It Cm mode Ar 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.
.It Cm minpoll Ar int
.It Cm maxpoll Ar int
These options specify the minimum and maximum polling interval
for reference clock messages, in seconds to the power of two.
For
most directly connected reference clocks, both
.Cm minpoll
and
.Cm maxpoll
default to 6 (64 s).
For modem reference clocks,
.Cm minpoll
defaults to 10 (17.1 m) and
.Cm maxpoll
defaults to 14 (4.5 h).
The allowable range is 4 (16 s) to 17 (36.4 h) inclusive.
.El
.It Xo Ic fudge
.Sm off
.Li 127.127. Ar t . Ar u
.Sm on
.Op Cm time1 Ar sec
.Op Cm time2 Ar sec
.Op Cm stratum Ar int
.Op Cm refid Ar string
.Op Cm mode Ar int
.Op Cm flag1 Cm 0 \&| Cm 1
.Op Cm flag2 Cm 0 \&| Cm 1
.Op Cm flag3 Cm 0 \&| Cm 1
.Op Cm flag4 Cm 0 \&| Cm 1
.Xc
This command can be used to configure reference clocks in
special ways.
It must immediately follow the
.Ic server
command which configures the driver.
Note that the same capability
is possible at run time using the
.Xr ntpdc 8
program.
The options are interpreted as
follows:
.Bl -tag -width indent
.It Cm time1 Ar 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
.Ic enable
command described in
.Sx Miscellaneous Options
page and operates as described in the
.Qq "Reference Clock Drivers"
page.
.It Cm time2 Ar 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
.Qq "reference clock drivers"
page.
.It Cm stratum Ar 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.
.It Cm refid Ar 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.
.It Cm mode Ar 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.
.It Cm flag1 Cm 0 \&| Cm 1
.It Cm flag2 Cm 0 \&| Cm 1
.It Cm flag3 Cm 0 \&| Cm 1
.It Cm flag4 Cm 0 \&| Cm 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
.Cm flag4
is used to enable recording monitoring
data to the
.Cm clockstats
file configured with the
.Ic filegen
command.
Further information on the
.Ic filegen
command can be found in
.Sx Monitoring Options .
.El
.El
.Sh Miscellaneous Options
.Bl -tag -width indent
.It Ic broadcastdelay Ar 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.
.It Ic driftfile Ar driftfile
This command specifies the name of the file used to record the
frequency offset of the local clock oscillator.
If the file exists,
it is read at startup in order to set the initial frequency offset
and then updated once per hour with the current frequency offset
computed by the daemon.
If the file does not exist or this command
is not given, the initial frequency offset is assumed zero.
In this
case, it may take some hours for the frequency to stabilize and the
residual timing errors to subside.
.Pp
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
.Xr ntpd 8
must have write permission for the directory the
drift file is located in, and that filesystem links, symbolic or
otherwise, should be avoided.
.It Xo Ic enable
.Oo
.Cm auth | Cm bclient |
.Cm calibrate | Cm kernel |
.Cm monitor | Cm ntp |
.Cm stats
.Oc
.Xc
.It Xo Ic disable
.Oo
.Cm auth | Cm bclient |
.Cm calibrate | Cm kernel |
.Cm monitor | Cm ntp |
.Cm stats
.Oc
.Xc
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
.Xr ntpdc 8
utility program.
.Bl -tag -width indent
.It Cm bclient
When enabled, this is identical to the
.Ic broadcastclient
command.
The default for this flag is
.Ic disable  .
.It Cm calibrate
Enables the calibration facility, which automatically adjusts
the
.Ic time1
values for each clock driver to display the same
offset as the currently selected source or kernel discipline
signal.
See the
.Qq "Reference Clock Drivers"
page
for further information.
The default for this flag is
.Ic disable  .
.It Cm kernel
Enables the precision-time kernel support for the
.Xr adjtime 2
system call, if implemented.
Ordinarily,
support for this routine is detected automatically when the NTP
daemon is compiled, so it is not necessary for the user to worry
about this flag.
It is provided primarily so that this support
can be disabled during kernel development.
The default for this
flag is
.Ic enable  .
.It Cm monitor
Enables the monitoring facility.
See the
.Xr ntpdc 8
program
and the
.Ic monlist
command or further information.
The
default for this flag is
.Ic enable  .
.It Cm ntp
Enables the server to adjust its local clock by means of NTP.
If disabled, the local clock free-runs at its intrinsic time and
frequency offset.
This flag is useful in case the local clock is
controlled by some other device or protocol and NTP is used only to
provide synchronization to other clients.
In this case, the local
clock driver can be used to provide this function and also certain
time variables for error estimates and leap-indicators.
See the
.Qq "Reference Clock Drivers"
page for further
information.
The default for this flag is
.Ic enable  .
.It Cm stats
Enables the statistics facility.
See the
.Qq "Monitoring Options"
page for further information.
The default for this flag is
.Ic enable  .
.El
.It Ic logconfig Ar configkeyword
This command controls the amount and type of output written to
the system
.Xr syslog 3
facility or the alternate
.Ic logfile
log file.
By default, all output is turned on.
All
.Ar configkeyword
keywords can be prefixed with
.Ql = ,
.Ql +
and
.Ql - ,
where
.Ql =
sets the
.Xr syslog 3
priority mask,
.Ql +
adds and
.Ql -
removes
messages.
.Xr syslog 3
messages can be controlled in four
classes
.Po
.Cm clock ,
.Cm peer ,
.Cm sys
and
.Cm sync
.Pc .
Within these classes four types of messages can be
controlled.
Informational messages
.Pq Cm info
control configuration
information.
Event messages
.Pq Cm events
control logging of
events (reachability, synchronization, alarm conditions).
Statistical output is controlled with the
.Cm statistics
keyword.
The final message group is the status messages.
This
describes mainly the synchronizations status.
Configuration
keywords are formed by concatenating the message class with the
event class.
The
.Cm all
prefix can be used instead of a
message class.
A message class may also be followed by the
.Cm all
keyword to enable/disable all messages of the
respective message class.
Thus, a minimal log configuration could look like this:
.Bd -literal
logconfig=syncstatus +sysevents
.Ed
.Pp
This would just list the synchronizations state of
.Xr ntpd 8
and the major system events.
For a simple reference server, the
following minimum message configuration could be useful:
.Bd -literal
logconfig=syncall +clockall
.Ed
.Pp
This configuration will list all clock information and
synchronization information.
All other events and messages about
peers, system events and so on is suppressed.
.It Ic logfile Ar logfile
This command specifies the location of an alternate log file to
be used instead of the default system
.Xr syslog 3   
facility.
.It Ic setvar Ar variable Op Cm 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
.Sm off
.Va name = Ar value
.Sm on
is followed by the
.Cm default
keyword, the
variable will be listed as part of the default system variables
.Po
.Xr ntpq 8
.Ic rv
command
.Pc ) .
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
.Ic setvar
mechanism.
There are three special variables that contain the names
of all variable of the same group.
The
.Va sys_var_list
holds
the names of all system variables.
The
.Va peer_var_list
holds
the names of all peer variables and the
.Va clock_var_list
holds the names of the reference clock variables.
.It Xo Ic tinker
.Oo
.Cm step Ar step |
.Cm panic Ar panic |
.Cm dispersion Ar dispersion |
.Cm stepout Ar stepout |
.Cm minpoll Ar minpoll |
.Cm allan Ar allan |
.Cm huffpuff Ar huffpuff
.Oc
.Xc
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 can't 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.
.Pp
All arguments are in floating point seconds or seconds per
second.
The
.Ar minpoll
argument is an integer in seconds to
the power of two.
The variables operate as follows:
.Bl -tag -width indent
.It Cm step Ar step
The argument becomes the new value for the step threshold,
normally 0.128 s.
If set to zero, step adjustments will never
occur.
In general, if the intent is only to avoid step adjustments,
the step threshold should be left alone and the
.Fl x
command
line option be used instead.
.It Cm panic Ar panic
The argument becomes the new value for 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.
.It Cm dispersion Ar dispersion
The argument becomes the new value for the dispersion increase
rate, normally .000015.
.It Cm stepout Ar stepout
The argument becomes the new value for the watchdog timeout,
normally 900 s.
.It Cm minpoll Ar minpoll
The argument becomes the new value for the minimum poll
interval used when configuring multicast client, manycast client
and , symmetric passive mode association.
The value defaults to 6
(64 s) and has a lower limit of 4 (16 s).
.It Cm allan Ar 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 defaults to 1024 s, which is also the lower
limit.
.It Cm huffpuff Ar 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.
.El
.It Xo Ic trap Ar host_address
.Op Cm port Ar port_number
.Op Cm interface Ar interface_address
.Xc
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.
.Pp
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.
.El
.Sh FILES
.Bl -tag -width /etc/ntp.drift -compact
.It Pa /etc/ntp.conf
the default name of the configuration file
.It Pa ntp.keys
private MD5 keys
.It Pa ntpkey
RSA private key
.It Pa ntpkey_ Ns Ar host
RSA public key
.It Pa ntp_dh
Diffie-Hellman agreement parameters
.El
.Sh SEE ALSO
.Xr ntpd 8 ,
.Xr ntpdc 8 ,
.Xr ntpq 8
.Pp
In addition to the manual pages provided,
comprehensive documentation is available on the world wide web
at
.Li http://www.ntp.org/ .
A snapshot of this documentation is available in HTML format in
.Pa /usr/share/doc/ntp .
.Rs
.%A David L. Mills
.%T Network Time Protocol (Version 3)
.%O RFC1305
.Re
.Sh BUGS
The syntax checking is not picky; some combinations of
ridiculous and even hilarious options and modes may not be
detected.
.Pp
The
.Pa ntpkey_ Ns Ar host
files are really digital
certificates.
These should be obtained via secure directory
services when they become universally available.