FreeBSD Man Pages

ntp.conf(5)

NAME

     ntp.conf -- Network Time Protocol (NTP) daemon configuration file

SYNOPSIS

     /etc/ntp.conf

DESCRIPTION

     The ntp.conf configuration file is read at initial startup by the ntpd(8)
     daemon in order to specify the synchronization sources, modes and other
     related information.  Usually, it is installed in the /etc directory, but
     could be installed elsewhere (see the daemon's -c command line option).

     The file format is similar to other UNIX configuration files.  Comments
     begin with a `#' character and extend to the end of the line; blank lines
     are ignored.  Configuration commands consist of an initial keyword fol-
     lowed by a list of arguments, some of which may be optional, separated by
     whitespace.  Commands may not be continued over multiple lines.  Argu-
     ments 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 a NTP Subnet" page (avail-
     able as part of the HTML documentation provided in /usr/share/doc/ntp)
     contains an extended discussion of these options.	In addition to the
     discussion of general Configuration Options, there are sections describ-
     ing the following supported functionality and the options used to control
     it:

	   o   Authentication Support

	   o   Monitoring Support

	   o   Access Control Support

	   o   Reference Clock Support

     Following these is a section describing Miscellaneous Options.  While
     there is a rich set of options available, the only required option is one
     or more server, peer, broadcast or manycastclient commands.

Configuration Support

     Following is a description of the configuration commands in NTPv4.  These
     commands have the same basic functions as in NTPv3 and in some cases new
     functions and new arguments.  There are two classes of commands, configu-
     ration commands that configure a persistent association with a remote
     server or peer or reference clock, and auxiliary commands that specify
     environmental variables that control various related operations.

   Configuration Commands
     The various modes are determined by the command keyword and the type of
     the required IP address.  Addresses are classed by type as (s) a remote
     server or peer (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
     broadcast address [key key | autokey] [version version] [prefer] [minpoll
	     minpoll] [ttl ttl]

     manycastclient address [key key | autokey] [version version] [prefer]
	     [minpoll minpoll] [maxpoll maxpoll] [ttl ttl]

     These four commands specify the time server name or address to be used
     and the mode in which to operate.	The address can be either a DNS name
     or an IP address in dotted-quad notation.	Additional information on
     association behavior can be found in the "Association Management" page.

     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 synchro-
	     nized to the local clock.	This command should not be used for
	     type b or m addresses.

     peer    For type s addresses (only), this command mobilizes a persistent
	     symmetric-active mode association with the specified remote peer.
	     In this mode the local clock can be synchronized to the remote
	     peer or the remote peer can be synchronized to the local clock.
	     This is useful in a network of servers where, depending on vari-
	     ous failure scenarios, either the local or remote peer may be the
	     better source of time.  This command should NOT be used for type
	     b, m or r addresses.

     broadcast
	     For type b and m addresses (only), this command mobilizes a per-
	     sistent broadcast mode association.  Multiple commands can be
	     used to specify multiple local broadcast interfaces (subnets)
	     and/or multiple multicast groups.	Note that local broadcast mes-
	     sages go only to the interface associated with the subnet speci-
	     fied, but multicast messages go to all interfaces.  In broadcast
	     mode the local server sends periodic broadcast messages to a
	     client population at the address specified, which is usually the
	     broadcast address on (one of) the local network(s) or a multicast
	     address assigned to NTP.  The IANA has assigned the multicast
	     group address 224.0.1.1 exclusively to NTP, but other noncon-
	     flicting 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 oper-
	     ation as a broadcast client, see the broadcastclient or
	     multicastclient commands below.

     manycastclient
	     For type m addresses (only), this command mobilizes a manycast
	     client mode association for the multicast address specified.  In
	     this case a specific address must be supplied which matches the
	     address used on the manycastserver command for the designated
	     manycast servers.	The NTP multicast address 224.0.1.1 assigned
	     by the IANA should NOT be used, unless specific means are taken
	     to avoid spraying large areas of the Internet with these messages
	     and causing a possibly massive implosion of replies at the
	     sender.  The manycastserver command specifies that the local
	     server is to operate in client mode with the remote servers that
	     are discovered as the result of broadcast/multicast messages.
	     The client broadcasts a request message to the group address
	     All packets sent to and received from the server or peer are to
	     include authentication fields encrypted using the autokey scheme
	     described in Authentication Options.

     burst   when the server is reachable 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 time-
	     keeping quality with the server command and s addresses.

     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 server
	     command and s addresses and when ntpd(8) is started with the -q
	     option.

     key key
	     All packets sent to and received from the server or peer are to
	     include authentication fields encrypted using the specified key
	     identifier with values from 1 to 65534, inclusive.  The default
	     is to include no encryption field.

     minpoll minpoll

     maxpoll maxpoll
	     These options specify the minimum and maximum poll intervals for
	     NTP messages, in seconds to the power of two.  The maximum poll
	     interval defaults to 10 (1,024 s), but can be increased by the
	     maxpoll option to an upper limit of 17 (36.4 h).  The minimum
	     poll interval defaults to 6 (64 s), but can be decreased by the
	     minpoll option to a lower limit of 4 (16 s).

     prefer  Marks the server as preferred.  All other things being equal,
	     this host will be chosen for synchronization among a set of cor-
	     rectly operating hosts.  See the "Mitigation Rules and the prefer
	     Keyword" page for further information.

     ttl ttl
	     This option is used only with broadcast server and manycast
	     client modes.  It specifies the time-to-live ttl to use on broad-
	     cast server and multicast server and the maximum ttl for the
	     expanding ring search with manycast client packets.  Selection of
	     the proper value, which defaults to 127, is something of a black
	     art and should be coordinated with the network administrator.

     version version
	     Specifies the version number to be used for outgoing NTP packets.
	     Versions 1-4 are the choices, with version 4 the default.

   Auxiliary Commands
     broadcastclient
	     This command enables reception of broadcast server messages to
	     any local interface (type b) address.  Upon receiving a message
	     for the first time, the broadcast client measures the nominal
	     This command enables reception of manycast client messages to the
	     multicast group address(es) (type m) specified.  At least one
	     address is required, but the NTP multicast address 224.0.1.1
	     assigned by the IANA should NOT be used, unless specific means
	     are taken to limit the span of the reply and avoid a possibly
	     massive implosion at the original sender.	Note that, in order to
	     avoid accidental or malicious disruption in this mode, both the
	     server and client should operate using symmetric-key or public-
	     key authentication as described in Authentication Options.

     multicastclient address ...
	     This command enables reception of multicast server messages to
	     the multicast group address(es) (type m) specified.  Upon receiv-
	     ing 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 sym-
	     metric-key or public-key authentication as described in
	     Authentication Options.

Authentication Support

     Authentication support allows the NTP client to verify that the server is
     in fact known and trusted and not an intruder intending accidentally or
     on purpose to masquerade as that server.  The NTPv3 specification
     RFC-1305 defines a scheme which provides cryptographic authentication of
     received NTP packets.  Originally, this was done using the Data Encryp-
     tion 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.

     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 stor-
     age.

     Authentication is configured separately for each association using the
     key or autokey subcommands on the peer, server, broadcast and
     manycastclient commands as described in Configuration Options.  The
     authentication options described below specify the suite of keys, select
     the key for each configured association and manage the configuration
     operations.

     The auth flag controls whether new associations or remote configuration
     commands require cryptographic authentication.  This flag can be set or
     reset by the enable and disable configuration commands and also by remote
     configuration commands sent by a ntpdc(8) program running in another
     machine.  If this flag is enabled, which is the default case, new broad-
     cast client and symmetric passive associations and remote configuration
     commands must be cryptographically authenticated using either symmetric-
     file contains host names, or when a server or client is configured
     remotely, host names are resolved using the DNS and a separate name reso-
     lution 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.

     An attractive alternative where multicast support is available is many-
     cast mode, in which clients periodically troll for servers.  Crypto-
     graphic 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.

     In addition to the default symmetric-key cryptographic support, support
     for public-key cryptography is available if the requisite rsaref20 soft-
     ware 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.

   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 informa-
     tion are specified in a key file, usually called 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 associa-
     tions, additional keys can be used as passwords for the ntpq(8) and
     ntpdc(8) utility programs.

     When ntpd(8) is first started, it reads the key file specified in the
     keys command and installs the keys in the key cache.  However, the keys
     must be activated with the trusted command before use.  This allows, for
     instance, the installation of possibly several batches of keys and then
     activating or deactivating each batch remotely using ntpdc(8).  This also
     provides a revocation capability that can be used if a key becomes com-
     promised.	The requestkey command selects the key used as the password
     for the ntpdc(8) utility, while the controlkey command selects the key
     used as the password for the ntpq(8) utility.

   Public-Key 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 sev-
     eral sources, but not included in the NTPv4 software distribution.  In
     order to be effective, the rsaref20 package must be installed as
     described in the 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 correspond-
     ing to the various NTP modes supported.  RSA signatures with timestamps
     tus, briefing slides and reading list, in the "Autonomous Authentication"
     page.

     The cryptographic values used by the Autokey scheme are incorporated as a
     set of files generated by the ntp-genkeys(8) program, including the sym-
     metric private keys, public/private key pair, and the agreement parame-
     ters.  See the ntp.keys(5) page for a description of the formats of these
     files.  They contain cryptographic values generated by the algorithms of
     the 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.

     The ntp.keys file contains the DES/MD5 private keys.  It must be distrib-
     uted by secure means to other servers and clients sharing the same secu-
     rity 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 ntpdc(8), ntpq(8) utilities.  The
     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.

     The 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 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 ntp_dh file can be also be distributed using insecure
     means, since the data are public values.

     The ntpkey_host file contains the RSA public key, where host is the name
     of the host.  Each host must have its own ntpkey_host file, which is nor-
     mally provided to other hosts using the Autokey protocol.	Each server or
     peer association requires the public key associated with the particular
     server or peer to be loaded either directly from a local file or indi-
     rectly from the server using the Autokey protocol.  These files can be
     widely distributed and stored using insecure means, since the data are
     public values.

     The optional ntpkey_certif_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.

     Due to the widespread use of interface-specific naming, the host names
     used in configured and mobilized associations are determined by the UNIX
     gethostname(3) library routine.  Both the 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
     with respect to Coordinated Universal Time (UTC), as disseminated by NTP.
     The table can be obtained directly from NIST national time servers using
     FTP as the ASCII file pub/leap-seconds.

     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 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.

   Key Management
     All key files are installed by default in /usr/local/etc, which is nor-
     mally in a shared file system in NFS-mounted networks and avoids
     installing them in each machine separately.  The default can be overrid-
     den by the 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 /etc, which is normally not
     in a shared file system.

     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 gener-
     ations to be activated simply by changing the link.  However, 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 crypto configuration command.

     All cryptographic keys and related parameters should be regenerated on a
     periodic and automatic basis, like once per month.  The 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 com-
     partment, since they can be obtained automatically using the Autokey pro-
     tocol.  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 UNIX rdist(1) command.
     Future versions of the Autokey protocol are to contain provisions for an
     agreement protocol to do this automatically.

     Servers and clients can make a new generation in the following way.  All
     machines have loaded the old generation at startup and are operating nor-
     mally.  At designated intervals, each machine generates a new public/pri-
     vate key pair and makes links from the default file names to the new file
     names.  The ntpd(8) is then restarted and loads the new generation, with
     result clients no longer can authenticate correctly.  The Autokey proto-
     col 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.

   Authentication Commands

     controlkey key
	     Specifies the key identifier to use with the ntpq(8) utility,
	     which uses the standard protocol defined in RFC-1305.  The key
	     argument is the key identifier for a trusted key, where the value
	     can be in the range 1 to 65534, inclusive.

     crypto [flags flags] [privatekey file] [publickey file] [dhparms file]
	     [leap file]
	     This command requires the NTP daemon build process be configured
	     with the RSA library.  This command activates public-key cryptog-
	     raphy 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:

	     privatekey file
		     Specifies the location of the RSA private key file, which
		     otherwise defaults to /usr/local/etc/ntpkey.

	     publickey file
		     Specifies the location of the RSA public key file, which
		     otherwise defaults to /usr/local/etc/ntpkey_host, where
		     host is the name of the generating machine.

	     dhparms file
		     Specifies the location of the Diffie-Hellman parameters
		     file, which otherwise defaults to
		     /usr/local/etc/ntpkey_dh.

	     leap file
		     Specifies the location of the leapsecond table file,
		     which otherwise defaults to /usr/local/etc/ntpkey_leap.

     keys keyfile
	     Specifies the location of the DES/MD5 private key file containing
	     the keys and key identifiers used by ntpd(8), ntpq(8) and
	     ntpdc(8) when operating in symmetric-key mode.

     keysdir 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 /usr/local/etc.

     requestkey key
	     Specifies the key identifier to use with the ntpdc(8) utility
	     program, which uses a proprietary protocol specific to this
	     implementation of ntpd(8).  The key argument is a key identifier
	     for the trusted key, where the value can be in the range 1 to
	     65534, inclusive.

     revoke logsec
	     Specifies the interval between re-randomization of certain cryp-
	     tographic values used by the Autokey scheme, as a power of 2 in
	     seconds.  These values need to be updated frequently in order to
	     deflect brute-force attacks on the algorithms of the scheme; how-
	     as keys used by the ntpq(8) and ntpdc(8) programs.  The authenti-
	     cation procedures require that both the local and remote servers
	     share the same key and key identifier for this purpose, although
	     different keys can be used with different servers.  The key argu-
	     ments are 32-bit unsigned integers with values from 1 to 65,534.

Monitoring Support

     ntpd(8) includes a comprehensive monitoring facility suitable for contin-
     uous, long term recording of server and client timekeeping performance.
     See the statistics command below for a listing and example of each type
     of statistics currently supported.  Statistic files are managed using
     file generation sets and scripts in the ./scripts directory of this dis-
     tribution.  Using these facilities and UNIX cron(8) jobs, the data can be
     automatically summarized and archived for retrospective analysis.

   Monitoring Commands
     statistics name ...
	     Enables writing of statistics records.  Currently, four kinds of
	     name statistics are supported.

	     loopstats
		     Enables recording of loop filter statistics information.
		     Each update of the local clock outputs a line of the fol-
		     lowing form to the file generation set named loopstats:

		     50935 75440.031 0.000006019 13.778190 0.000351733 0.013380 6

		     The first two fields show the date (Modified Julian Day)
		     and time (seconds and fraction past UTC midnight).  The
		     next five fields show time offset (seconds), frequency
		     offset (parts per million - PPM), RMS jitter (seconds),
		     Allan deviation (PPM) and clock discipline time constant.

	     peerstats
		     Enables recording of peer statistics information.	This
		     includes statistics records of all peers of a NTP server
		     and of special signals, where present and configured.
		     Each valid update appends a line of the following form to
		     the current element of a file generation set named peer-
		     stats:

		     48773 10847.650 127.127.4.1 9714 -0.001605 0.00000 0.00142

		     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.

	     clockstats
		     Enables recording of clock driver statistics information.
		     Each update received from a clock driver appends a line
		     of the following form to the file generation set named
		     clockstats:

		     49213 525.624 127.127.4.1 93 226 00:08:29.606 D
		     tion specific to each clock for further details.

	     rawstats
		     Enables recording of raw-timestamp statistics informa-
		     tion.  This includes statistics records of all peers of a
		     NTP server and of special signals, where present and con-
		     figured.  Each NTP message received from a peer or clock
		     driver appends a line of the following form to the file
		     generation set named rawstats:

		     50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000
		     The first two fields show the date (Modified Julian Day)
		     and time (seconds and fraction past UTC midnight).  The
		     next two fields show the remote peer or clock address
		     followed by the local address in dotted-quad notation.
		     The final four fields show the originate, receive, trans-
		     mit and final NTP timestamps in order.  The timestamp
		     values are as received and before processing by the vari-
		     ous data smoothing and mitigation algorithms.

     statsdir directory_path
	     Indicates the full path of a directory where statistics files
	     should be created (see below).  This keyword allows the (other-
	     wise constant) filegen filename prefix to be modified for file
	     generation sets, which is useful for handling statistics logs.

     filegen name [file filename] [type typename] [link | nolink] [enable |
	     disable]
	     Configures setting of generation file set name.  Generation file
	     sets provide a means for handling files that are continuously
	     growing during the lifetime of a server.  Server statistics are a
	     typical example for such files.  Generation file sets provide
	     access to a set of files used to store the actual data.  At any
	     time at most one element of the set is being written to.  The
	     type given specifies when and how data will be directed to a new
	     element of the set.  This way, information stored in elements of
	     a file set that are currently unused are available for adminis-
	     trational operations without the risk of disturbing the operation
	     of ntpd(8).  (Most important: they can be removed to free space
	     for new data produced.)  Note that this command can be sent from
	     the ntpdc(8) program running at a remote location.

	     name    This is the type of the statistics records, as shown in
		     the statistics command.

	     file filename
		     This is the file name for the statistics records.	File-
		     names of set members are built from three concatenated
		     elements prefix, filename and suffix:

		     prefix  This is a constant filename path.	It is not sub-
			     ject to modifications via the filegen option.  It
			     is defined by the server, usually specified as a
			     compile-time constant.  It may, however, be con-
			     figurable for individual file generation sets via
			     other commands.  For example, the prefix used
			     with loopstats and peerstats generation can be
			     configured using the statsdir option explained
			     referring to parts outside the file system hier-
			     archy denoted by prefix.

		     suffix  This part is reflects individual elements of a
			     file set.	It is generated according to the type
			     of a file set.

	     type typename
		     A file generation set is characterized by its type.  The
		     following types are supported:

		     none    The file set is actually a single plain file.

		     pid     One element of file set is used per incarnation
			     of a ntpd(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 ntpd(8) server
			     incarnations.  The set member filename is built
			     by appending a `.' (dot) to concatenated prefix
			     and filename strings, and appending the decimal
			     representation of the process ID of the ntpd(8)
			     server process.

		     day     One file generation set element is created per
			     day.  A day is defined as the period between
			     00:00 and 24:00 UTC.  The file set member suffix
			     consists of a `.' (dot) and a day specification
			     in the form YYYYMMdd.  YYYY is a 4-digit year
			     number (e.g., 1992).  MM is a two digit month
			     number.  dd is a two digit day number.  Thus, all
			     information written at 10 December 1992 would end
			     up in a file named ~prefix/filename/19921210.

		     week    Any file set member contains data related to a
			     certain week of a year.  The term week is defined
			     by computing day-of-year modulo 7.  Elements of
			     such a file generation set are distinguished by
			     appending the following suffix to the file set
			     filename base: A dot, a 4-digit year number, the
			     letter Ql W , and a 2-digit week number.  For
			     example, information from January, 10th 1992
			     would end up in a file with suffix .1992W1.

		     month   One generation file set element is generated per
			     month.  The file name suffix consists of a dot, a
			     4-digit year number, and a 2-digit month.

		     year    One generation file element is generated per
			     year.  The filename suffix consists of a dot and
			     a 4 digit year number.

		     age     This type of file generation sets changes to a
			     new element of the file set every 24 hours of
			     server operation.	The filename suffix consists
			     of a dot, the letter `a', and an 8-digit number.
			     This number is taken to be the number of seconds
			     the server is running at the start of the corre-

		     is enabled by specifying link and disabled using nolink.
		     If link is specified, a hard link from the current file
		     set element to a file without suffix is created.  When
		     there is already a file with this name and the number of
		     links of this file is one, it is renamed appending a dot,
		     the letter `C', and the pid of the ntpd(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.

	     enable | disable
		     Enables or disables the recording function.

Access Control Support

     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 "Notes on Configuring NTP and Setting up a NTP
     Subnet" page.

     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.

   The Kiss-of-Death Packet
     Ordinarily, packets denied service are simply dropped with no further
     action except incrementing statistics counters.  Sometimes a more proac-
     tive 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 kod flag is set and either ser-
     vice 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 kod flag
     is not set, the server simply drops the packet.

     A client or peer receiving a kiss-of-death packet performs a set of san-
     ity 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 vari-
     able.  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.

   Access Control Commands
     restrict numeric_address [mask numeric_mask] [flag ...]
	     The numeric_address argument, expressed in dotted-quad form, is
	     the address of a host or network.	The mask, also expressed in
	     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:

	     kod     If access is denied, send a kiss-of-death packet.

	     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.

	     noquery
		     Ignore all NTP mode 6 and 7 packets (i.e., information
		     queries and configuration requests) from the source.
		     Time service is not affected.

	     nomodify
		     Ignore all NTP mode 6 and 7 packets which attempt to mod-
		     ify the state of the server (i.e., run time reconfigura-
		     tion).  Queries which return information are permitted.

	     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.

	     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.

	     noserve
		     Ignore NTP packets whose mode is other than 6 or 7.  In
		     effect, time service is denied, though queries may still
		     be permitted.

	     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 syn-
		     chronization partners.

	     notrust
		     Treat these hosts normally in other respects, but never
		     use them as synchronization sources.

	     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 client_limit hosts that have shown up at
		     the server and that have been active during the last

	     ntpport
		     This is actually a match algorithm modifier, rather than
		     a restriction flag.  Its presence causes the restriction
		     entry to be matched only if the source port in the packet
		     is the standard NTP UDP port (123).  Both ntpport and
		     non-ntpport may be specified.  The ntpport is considered
		     more specific and is sorted later in the list.

	     version
		     Ignore these hosts if not the current NTP version.

	     Default restriction list entries, with the flags ignore,
	     interface, ntpport, for each of the local host's interface
	     addresses are inserted into the table at startup to prevent the
	     server from attempting to synchronize to its own time.  A default
	     entry is also always present, though if it is otherwise unconfig-
	     ured; no flags are associated with the default entry (i.e.,
	     everything besides your own NTP server is unrestricted).

     clientlimit limit
	     Set the client_limit variable, which limits the number of simul-
	     taneous access-controlled clients.  The default value for this
	     variable is 3.

     clientperiod period
	     Set the 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.

Reference Clock Support

     The NTP Version 4 daemon supports some three dozen different radio,
     satellite and modem reference clocks plus a special pseudo-clock used for
     backup or when no other clock source is available.  Detailed descriptions
     of individual device drivers and options can be found in the "Reference
     Clock Drivers" page (available as part of the HTML documentation provided
     in /usr/share/doc/ntp).  Additional information can be found in the pages
     linked there, including the "Debugging Hints for Reference Clock Drivers"
     and "How To Write a Reference Clock Driver" pages.  In addition, support
     for a PPS signal is available as described in the "Pulse-per-second (PPS)
     Signal Interfacing" page.	Many drivers support special line disci-
     pline/streams modules which can significantly improve the accuracy using
     the driver.  These are described in the "Line Disciplines and Streams
     Drivers" page.

     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 depen-
     dent, but is usually a serial port.  A device driver specific to each
     reference clock must be selected and compiled in the distribution; how-
     ever, 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 appropri-
     ately configured results in a scalding remark to the system log file, but
     is otherwise non hazardous.

     unique.

     The server command is used to configure a reference clock, where the
     address argument in that command is the clock address.  The key, version
     and ttl options are not used for reference clock support.	The mode
     option is added for reference clock support, as described below.  The
     prefer option can be useful to persuade the server to cherish a reference
     clock with somewhat more enthusiasm than other reference clocks or peers.
     Further information on this option can be found in the "Mitigation Rules
     and the prefer Keyword" page.  The minpoll and maxpoll options have mean-
     ing only for selected clock drivers.  See the individual clock driver
     document pages for additional information.

     The fudge command is used to provide additional information for individ-
     ual clock drivers and normally follows immediately after the server com-
     mand.  The address argument specifies the clock address.  The refid and
     stratum options can be used to override the defaults for the device.
     There are two optional device-dependent time offsets and four flags that
     can be included in the fudge command as well.

     The stratum number of a reference clock is by default zero.  Since the
     ntpd(8) daemon adds one to the stratum of each peer, a primary server
     ordinarily displays an external stratum of one.  In order to provide
     engineered backups, it is often useful to specify the reference clock
     stratum as greater than zero.  The stratum option is used for this pur-
     pose.  Also, in cases involving both a reference clock and a pulse-per-
     second (PPS) discipline signal, it is useful to specify the reference
     clock identifier as other than the default, depending on the driver.  The
     refid option is used for this purpose.  Except where noted, these options
     apply to all clock drivers.

   Reference Clock Commands
     server 127.127.t.u [prefer] [mode int] [minpoll int] [maxpoll int]
	     This command can be used to configure reference clocks in special
	     ways.  The options are interpreted as follows:

	     prefer  Marks the reference clock as preferred.  All other things
		     being equal, this host will be chosen for synchronization
		     among a set of correctly operating hosts.	See the
		     "Mitigation Rules and the prefer Keyword" page for fur-
		     ther information.

	     mode int
		     Specifies a mode number which is interpreted in a device-
		     specific fashion.	For instance, it selects a dialing
		     protocol in the ACTS driver and a device subtype in the
		     parse drivers.

	     minpoll int

	     maxpoll int
		     These options specify the minimum and maximum polling
		     interval for reference clock messages, in seconds to the
		     power of two.  For most directly connected reference
		     clocks, both minpoll and maxpoll default to 6 (64 s).
		     For modem reference clocks, minpoll defaults to 10 (17.1
		     m) and maxpoll defaults to 14 (4.5 h).  The allowable
		     range is 4 (16 s) to 17 (36.4 h) inclusive.

	     as follows:

	     time1 sec
		     Specifies a constant to be added to the time offset pro-
		     duced by the driver, a fixed-point decimal number in sec-
		     onds.  This is used as a calibration constant to adjust
		     the nominal time offset of a particular clock to agree
		     with an external standard, such as a precision PPS sig-
		     nal.  It also provides a way to correct a systematic
		     error or bias due to serial port or operating system
		     latencies, different cable lengths or receiver internal
		     delay.  The specified offset is in addition to the propa-
		     gation delay provided by other means, such as internal
		     DIPswitches.  Where a calibration for an individual sys-
		     tem and driver is available, an approximate correction is
		     noted in the driver documentation pages.  Note: in order
		     to facilitate calibration when more than one radio clock
		     or PPS signal is supported, a special calibration feature
		     is available.  It takes the form of an argument to the
		     enable command described in Miscellaneous Options page
		     and operates as described in the "Reference Clock
		     Drivers" page.

	     time2 secs
		     Specifies a fixed-point decimal number in seconds, which
		     is interpreted in a driver-dependent way.	See the
		     descriptions of specific drivers in the "reference clock
		     drivers" page.

	     stratum int
		     Specifies the stratum number assigned to the driver, an
		     integer between 0 and 15.	This number overrides the
		     default stratum number ordinarily assigned by the driver
		     itself, usually zero.

	     refid string
		     Specifies an ASCII string of from one to four characters
		     which defines the reference identifier used by the
		     driver.  This string overrides the default identifier
		     ordinarily assigned by the driver itself.

	     mode int
		     Specifies a mode number which is interpreted in a device-
		     specific fashion.	For instance, it selects a dialing
		     protocol in the ACTS driver and a device subtype in the
		     parse drivers.

	     flag1 0 | 1

	     flag2 0 | 1

	     flag3 0 | 1

	     flag4 0 | 1
		     These four flags are used for customizing the clock
		     driver.  The interpretation of these values, and whether
		     they are used at all, is a function of the particular
		     clock driver.  However, by convention flag4 is used to

	     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 Eth-
	     ernet), a number between 0.003 and 0.007 seconds is appropriate.
	     The default when this command is not used is 0.004 seconds.

     driftfile 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 fre-
	     quency 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.

	     The file format consists of a single line containing a single
	     floating point number, which records the frequency offset mea-
	     sured in parts-per-million (PPM).	The file is updated by first
	     writing the current drift value into a temporary file and then
	     renaming this file to replace the old version.  This implies that
	     ntpd(8) must have write permission for the directory the drift
	     file is located in, and that file system links, symbolic or oth-
	     erwise, should be avoided.

     enable [auth | bclient | calibrate | kernel | monitor | ntp | stats]

     disable [auth | bclient | calibrate | kernel | monitor | ntp | stats]
	     Provides a way to enable or disable various server options.
	     Flags not mentioned are unaffected.  Note that all of these flags
	     can be controlled remotely using the ntpdc(8) utility program.

	     bclient
		     When enabled, this is identical to the broadcastclient
		     command.  The default for this flag is disable.

	     calibrate
		     Enables the calibration facility, which automatically
		     adjusts the time1 values for each clock driver to display
		     the same offset as the currently selected source or ker-
		     nel discipline signal.  See the "Reference Clock Drivers"
		     page for further information.  The default for this flag
		     is disable.

	     kernel  Enables the precision-time kernel support for the
		     adjtime(2) system call, if implemented.  Ordinarily, sup-
		     port 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 devel-
		     opment.  The default for this flag is enable.

	     monitor
		     Enables the monitoring facility.  See the ntpdc(8) pro-
		     gram and the monlist command or further information.  The
		     clock driver can be used to provide this function and
		     also certain time variables for error estimates and leap-
		     indicators.  See the "Reference Clock Drivers" page for
		     further information.  The default for this flag is
		     enable.

	     stats   Enables the statistics facility.  See the "Monitoring
		     Options" page for further information.  The default for
		     this flag is enable.

     logconfig configkeyword
	     This command controls the amount and type of output written to
	     the system syslog(3) facility or the alternate logfile log file.
	     By default, all output is turned on.  All configkeyword keywords
	     can be prefixed with `=', `+' and `-', where `=' sets the
	     syslog(3) priority mask, `+' adds and `-' removes messages.
	     syslog(3) messages can be controlled in four classes (clock,
	     peer, sys and sync).  Within these classes four types of messages
	     can be controlled.  Informational messages (info) control config-
	     uration information.  Event messages (events) control logging of
	     events (reachability, synchronization, alarm conditions).	Sta-
	     tistical output is controlled with the 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 all prefix can be used instead of a message class.  A message
	     class may also be followed by the all keyword to enable/disable
	     all messages of the respective message class.  Thus, a minimal
	     log configuration could look like this:

	     logconfig =syncstatus +sysevents

	     This would just list the synchronizations state of ntpd(8) and
	     the major system events.  For a simple reference server, the fol-
	     lowing minimum message configuration could be useful:

	     logconfig =syncall +clockall

	     This configuration will list all clock information and synchro-
	     nization information.  All other events and messages about peers,
	     system events and so on is suppressed.

     logfile logfile
	     This command specifies the location of an alternate log file to
	     be used instead of the default system syslog(3) facility.

     setvar variable [default]
	     This command adds an additional system variable.  These variables
	     can be used to distribute additional information such as the
	     access policy.  If the variable of the form name=value is fol-
	     lowed by the default keyword, the variable will be listed as part
	     of the default system variables (ntpq(8) rv command)).  These
	     additional variables serve informational purposes only.  They are
	     not related to the protocol other that they can be listed.  The
	     known protocol variables will always override any variables
	     defined via the setvar mechanism.	There are three special vari-
	     ables that contain the names of all variable of the same group.
	     The sys_var_list holds the names of all system variables.	The
	     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 com-
	     mand is for them.	Emphasis added: twisters are on their own and
	     can expect no help from the support group.

	     All arguments are in floating point seconds or seconds per sec-
	     ond.  The minpoll argument is an integer in seconds to the power
	     of two.  The variables operate as follows:

	     step step
		     The argument becomes the new value for the step thresh-
		     old, 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 -x command line option be used instead.

	     panic panic
		     The argument becomes the new value for the panic thresh-
		     old, normally 1000 s.  If set to zero, the panic sanity
		     check is disabled and a clock offset of any value will be
		     accepted.

	     dispersion dispersion
		     The argument becomes the new value for the dispersion
		     increase rate, normally .000015.

	     stepout stepout
		     The argument becomes the new value for the watchdog time-
		     out, normally 900 s.

	     minpoll 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).

	     allan allan
		     The argument becomes the new value for the minimum Allan
		     intercept, which is a parameter of the PLL/FLL clock dis-
		     cipline algorithm.  The value defaults to 1024 s, which
		     is also the lower limit.

	     huffpuff huffpuff
		     The argument becomes the new value for the experimental
		     huff-n'-puff filter span, which determines the most
		     recent interval the algorithm will search for a minimum
		     delay.  The lower limit is 900 s (15 m), but a more rea-
		     sonable value is 7200 (2 hours).  There is no default,
		     since the filter is not enabled unless this command is
		     given.

     trap host_address [port port_number] [interface interface_address]
	     This command configures a trap receiver at the given host address
	     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.

FILES

     /etc/ntp.conf   the default name of the configuration file
     ntp.keys	     private MD5 keys
     ntpkey	     RSA private key
     ntpkey_host     RSA public key
     ntp_dh	     Diffie-Hellman agreement parameters

SEE ALSO

     ntpd(8), ntpdc(8), ntpq(8)

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

     David L. Mills, Network Time Protocol (Version 3), RFC1305.

BUGS

     The syntax checking is not picky; some combinations of ridiculous and
     even hilarious options and modes may not be detected.

     The ntpkey_host files are really digital certificates.  These should be
     obtained via secure directory services when they become universally
     available.

FreeBSD 5.4		       January 13, 2000 		   FreeBSD 5.4

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