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NTP.CONF(5)               FreeBSD File Formats Manual              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 /etc/rc.d/ntpdate script reads this file to get a list of NTP servers
     to use if the variable ``ntpdate_hosts'' was not declared.  Refer to the
     rc.conf(5) man page for further info about this.

     The file format is similar to other UNIX configuration files.  Comments
     begin with a `#' character and extend to the end of the line; blank lines
     are ignored.  Configuration commands consist of an initial keyword
     followed by a list of arguments, some of which may be optional, separated
     by whitespace.  Commands may not be continued over multiple lines.
     Arguments may be host names, host addresses written in numeric, dotted-
     quad form, integers, floating point numbers (when specifying times in
     seconds) and text strings.

     The rest of this page describes the configuration and control options.
     The "Notes on Configuring NTP and Setting up a NTP Subnet" page
     (available as part of the HTML documentation provided in
     /usr/share/doc/ntp) contains an extended discussion of these options.  In
     addition to the discussion of general Configuration Options, there are
     sections describing the following supported functionality and the options
     used to control it:

           +o   Authentication Support

           +o   Monitoring Support

           +o   Access Control Support

           +o   Automatic NTP Configuration Options

           +o   Reference Clock Support

           +o   Miscellaneous Options

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

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

   Configuration Commands
     The various modes are determined by the command keyword and the type of
     the required IP address.  Addresses are classed by type as (s) a remote
     server or peer (IPv4 class A, B and C), (b) the broadcast address of a
     local interface, (m) a multicast address (IPv4 class D), or (r) a
     reference clock address (127.127.x.x).  Note that only those options
     applicable to each command are listed below.  Use of options not listed
     may not be caught as an error, but may result in some weird and even
     destructive behavior.

     If the Basic Socket Interface Extensions for IPv6 (RFC-2553) is detected,
     support for the IPv6 address family is generated in addition to the
     default support of the IPv4 address family.  In a few cases, including
     the reslist billboard generated by ntpdc, IPv6 addresses are
     automatically generated.  IPv6 addresses can be identified by the
     presence of colons ``:'' in the address field.  IPv6 addresses can be
     used almost everywhere where IPv4 addresses can be used, with the
     exception of reference clock addresses, which are always IPv4.

     Note that in contexts where a host name is expected, a -4 qualifier
     preceding the host name forces DNS resolution to the IPv4 namespace,
     while a -6 qualifier forces DNS resolution to the IPv6 namespace.  See
     IPv6 references for the equivalent classes for that address family.

     server address [key key | autokey] [burst] [iburst] [version version]
             [prefer] [minpoll minpoll] [maxpoll maxpoll]

     peer address [key key | autokey] [version version] [prefer] [minpoll
             minpoll] [maxpoll maxpoll]

     broadcast address [key key | autokey] [version version] [prefer] [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
     (available as part of the HTML documentation provided in
     /usr/share/doc/ntp).

     server  For type s and r addresses, this command mobilizes a persistent
             client mode association with the specified remote server or local
             radio clock.  In this mode the local clock can synchronized to
             the remote server, but the remote server can never be
             synchronized to the local clock.  This command should not be used
             for type b or m addresses.

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

     broadcast
             For type b and m addresses (only), this command mobilizes a
             persistent broadcast mode association.  Multiple commands can be
             used to specify multiple local broadcast interfaces (subnets)
             and/or multiple multicast groups.  Note that local broadcast
             messages go only to the interface associated with the subnet
             specified, but multicast messages go to all interfaces.  In
             broadcast mode the local server sends periodic broadcast messages
             to a client population at the address specified, which is usually
             the broadcast address on (one of) the local network(s) or a
             multicast address assigned to NTP.  The IANA has assigned the
             multicast group address IPv4 224.0.1.1 and IPv6 ff05::101 (site
             local) exclusively to NTP, but other nonconflicting addresses can
             be used to contain the messages within administrative boundaries.
             Ordinarily, this specification applies only to the local server
             operating as a sender; for operation as a broadcast client, see
             the broadcastclient or multicastclient commands below.

     manycastclient
             For type m addresses (only), this command mobilizes a manycast
             client mode association for the multicast address specified.  In
             this case a specific address must be supplied which matches the
             address used on the manycastserver command for the designated
             manycast servers.  The NTP multicast address 224.0.1.1 assigned
             by the IANA should NOT be used, unless specific means are taken
             to avoid spraying large areas of the Internet with these messages
             and causing a possibly massive implosion of replies at the
             sender.  The manycastserver command specifies that the local
             server is to operate in client mode with the remote servers that
             are discovered as the result of broadcast/multicast messages.
             The client broadcasts a request message to the group address
             associated with the specified address and specifically enabled
             servers respond to these messages.  The client selects the
             servers providing the best time and continues as with the server
             command.  The remaining servers are discarded as if never heard.

     Options:

     autokey
             All packets sent to and received from the server or peer are to
             include authentication fields encrypted using the autokey scheme
             described in Authentication Commands.

     burst   when the server is reachable, send a burst of eight packets
             instead of the usual one.  The packet spacing is normally 2 s;
             however, the spacing between the first and second packets can be
             changed with the calldelay command to allow additional time for a
             modem or ISDN call to complete.  This is designed to improve
             timekeeping quality with the server command and s addresses.

     iburst  When the server is unreachable, send a burst of eight packets
             instead of the usual one.  The packet spacing is normally 2 s;
             however, the spacing between the first two packets can be changed
             with the calldelay command to allow additional time for a modem
             or ISDN call to complete.  This is designed to speed the initial
             synchronization acquisition with the server command and s
             addresses and when ntpd(8) is started with the -q option.

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

     minpoll minpoll

     maxpoll maxpoll
             These options specify the minimum and maximum poll intervals for
             NTP messages, as a power of 2 in seconds.  The maximum poll
             interval defaults to 10 (1,024 s), but can be increased by the
             maxpoll option to an upper limit of 17 (36.4 h).  The minimum
             poll interval defaults to 6 (64 s), but can be decreased by the
             minpoll option to a lower limit of 4 (16 s).

     noselect
             Marks the server as unused, except for display purposes.  The
             server is discarded by the selection algorithm.

     prefer  Marks the server as preferred.  All other things being equal,
             this host will be chosen for synchronization among a set of
             correctly operating hosts.  See the "Mitigation Rules and the
             prefer Keyword" page (available as part of the HTML documentation
             provided in /usr/share/doc/ntp) for further information.

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

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

   Auxiliary Commands
     broadcastclient
             This command enables reception of broadcast server messages to
             any local interface (type b) address.  Upon receiving a message
             for the first time, the broadcast client measures the nominal
             server propagation delay using a brief client/server exchange
             with the server, then enters the broadcast client mode, in which
             it synchronizes to succeeding broadcast messages.  Note that, in
             order to avoid accidental or malicious disruption in this mode,
             both the server and client should operate using symmetric-key or
             public-key authentication as described in Authentication
             Commands.

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

     multicastclient address ...
             This command enables reception of multicast server messages to
             the multicast group address(es) (type m) specified.  Upon
             receiving a message for the first time, the multicast client
             measures the nominal server propagation delay using a brief
             client/server exchange with the server, then enters the broadcast
             client mode, in which it synchronizes to succeeding multicast
             messages.  Note that, in order to avoid accidental or malicious
             disruption in this mode, both the server and client should
             operate using symmetric-key or public-key authentication as
             described in Authentication Commands.

Authentication Support
     Authentication support allows the NTP client to verify that the server is
     in fact known and trusted and not an intruder intending accidentally or
     on purpose to masquerade as that server.  The NTPv3 specification
     RFC-1305 defines a scheme which provides cryptographic authentication of
     received NTP packets.  Originally, this was done using the Data
     Encryption Standard (DES) algorithm operating in Cipher Block Chaining
     (CBC) mode, commonly called DES-CBC.  Subsequently, this was replaced by
     the RSA Message Digest 5 (MD5) algorithm using a private key, commonly
     called keyed-MD5.  Either algorithm computes a message digest, or one-way
     hash, which can be used to verify the server has the correct private key
     and key identifier.

     NTPv4 retains the NTPv3 scheme, properly described as symmetric key
     cryptography and, in addition, provides a new Autokey scheme based on
     public key cryptography.  Public key cryptography is generally considered
     more secure than symmetric key cryptography, since the security is based
     on a private value which is generated by each server and never revealed.
     With Autokey all key distribution and management functions involve only
     public values, which considerably simplifies key distribution and
     storage.  Public key management is based on X.509 certificates, which can
     be provided by commercial services or produced by utility programs in the
     OpenSSL software library or the NTPv4 distribution.

     While the algorithms for symmetric key cryptography are included in the
     NTPv4 distribution, public key cryptography requires the OpenSSL software
     library to be installed before building the NTP distribution.  Directions
     for doing that are on the Building and Installing the Distribution page.

     Authentication is configured separately for each association using the
     key or autokey subcommand on the peer, server, broadcast and
     manycastclient configuration commands as described in Configuration
     Options page.  The authentication options described below specify the
     locations of the key files, if other than default, which symmetric keys
     are trusted and the interval between various operations, if other than
     default.

     Authentication is always enabled, although ineffective if not configured
     as described below.  If a NTP packet arrives including a message
     authentication code (MAC), it is accepted only if it passes all
     cryptographic checks.  The checks require correct key ID, key value and
     message digest.  If the packet has been modified in any way or replayed
     by an intruder, it will fail one or more of these checks and be
     discarded.  Furthermore, the Autokey scheme requires a preliminary
     protocol exchange to obtain the server certificate, verify its
     credentials and initialize the protocol.

     The auth flag controls whether new associations or remote configuration
     commands require cryptographic authentication.  This flag can be set or
     reset by the enable and disable commands and also by remote configuration
     commands sent by a ntpdc(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
     cryptography.  If this flag is disabled, these operations are effective
     even if not cryptographic authenticated.  It should be understood that
     operating with the auth flag disabled invites a significant vulnerability
     where a rogue hacker can masquerade as a falseticker and seriously
     disrupt system timekeeping.  It is important to note that this flag has
     no purpose other than to allow or disallow a new association in response
     to new broadcast and symmetric active messages and remote configuration
     commands and, in particular, the flag has no effect on the authentication
     process itself.

     An attractive alternative where multicast support is available is
     manycast mode, in which clients periodically troll for servers as
     described in the Automatic NTP Configuration Options page.  Either
     symmetric key or public key cryptographic authentication can be used in
     this mode.  The principle advantage of manycast mode is that potential
     servers need not be configured in advance, since the client finds them
     during regular operation, and the configuration files for all clients can
     be identical.

     The security model and protocol schemes for both symmetric key and public
     key cryptography are summarized below; further details are in the
     briefings, papers and reports at the NTP project page linked from
     http://www.ntp.org/.

   Symmetric-Key Cryptography
     The original RFC-1305 specification allows any one of possibly 65,534
     keys, each distinguished by a 32-bit key identifier, to authenticate an
     association.  The servers and clients involved must agree on the key and
     key identifier to authenticate NTP packets.  Keys and related information
     are specified in a key file, usually called ntp.keys, which must be
     distributed and stored using secure means beyond the scope of the NTP
     protocol itself.  Besides the keys used for ordinary NTP associations,
     additional keys can be used as passwords for the ntpq(8) and ntpdc(8)
     utility programs.

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

   Public Key Cryptography
     NTPv4 supports the original NTPv3 symmetric key scheme described in
     RFC-1305 and in addition the Autokey protocol, which is based on public
     key cryptography.  The Autokey Version 2 protocol described on the
     Autokey Protocol page verifies packet integrity using MD5 message digests
     and verifies the source with digital signatures and any of several
     digest/signature schemes.  Optional identity schemes described on the
     Identity Schemes page and based on cryptographic challenge/response
     algorithms are also available.  Using all of these schemes provides
     strong security against replay with or without modification, spoofing,
     masquerade and most forms of clogging attacks.

     The Autokey protocol has several modes of operation corresponding to the
     various NTP modes supported.  Most modes use a special cookie which can
     be computed independently by the client and server, but encrypted in
     transmission.  All modes use in addition a variant of the S-KEY scheme,
     in which a pseudo-random key list is generated and used in reverse order.
     These schemes are described along with an executive summary, current
     status, briefing slides and reading list on the Autonomous Authentication
     page.

     The specific cryptographic environment used by Autokey servers and
     clients is determined by a set of files and soft links generated by the
     ntp-keygen(8) program.  This includes a required host key file, required
     certificate file and optional sign key file, leapsecond file and identity
     scheme files.  The digest/signature scheme is specified in the X.509
     certificate along with the matching sign key.  There are several schemes
     available in the OpenSSL software library, each identified by a specific
     string such as md5WithRSAEncryption, which stands for the MD5 message
     digest with RSA encryption scheme.  The current NTP distribution supports
     all the schemes in the OpenSSL library, including those based on RSA and
     DSA digital signatures.

     NTP secure groups can be used to define cryptographic compartments and
     security hierarchies.  It is important that every host in the group be
     able to construct a certificate trail to one or more trusted hosts in the
     same group.  Each group host runs the Autokey protocol to obtain the
     certificates for all hosts along the trail to one or more trusted hosts.
     This requires the configuration file in all hosts to be engineered so
     that, even under anticipated failure conditions, the NTP subnet will form
     such that every group host can find a trail to at least one trusted host.

   Naming and Addressing
     It is important to note that Autokey does not use DNS to resolve
     addresses, since DNS can't be completely trusted until the name servers
     have synchronized clocks.  The cryptographic name used by Autokey to bind
     the host identity credentials and cryptographic values must be
     independent of interface, network and any other naming convention.  The
     name appears in the host certificate in either or both the subject and
     issuer fields, so protection against DNS compromise is essential.

     By convention, the name of an Autokey host is the name returned by the
     Unix gethostname(2) system call or equivalent in other systems.  By the
     system design model, there are no provisions to allow alternate names or
     aliases.  However, this is not to say that DNS aliases, different names
     for each interface, etc., are constrained in any way.

     It is also important to note that Autokey verifies authenticity using the
     host name, network address and public keys, all of which are bound
     together by the protocol specifically to deflect masquerade attacks.  For
     this reason Autokey includes the source and destination IP addresses in
     message digest computations and so the same addresses must be available
     at both the server and client.  For this reason operation with network
     address translation schemes is not possible.  This reflects the intended
     robust security model where government and corporate NTP servers are
     operated outside firewall perimeters.

   Operation
     A specific combination of authentication scheme (none, symmetric key,
     public key) and identity scheme is called a cryptotype, although not all
     combinations are compatible.  There may be management configurations
     where the clients, servers and peers may not all support the same
     cryptotypes.  A secure NTPv4 subnet can be configured in many ways while
     keeping in mind the principles explained above and in this section.  Note
     however that some cryptotype combinations may successfully interoperate
     with each other, but may not represent good security practice.

     The cryptotype of an association is determined at the time of
     mobilization, either at configuration time or some time later when a
     message of appropriate cryptotype arrives.  When mobilized by a server or
     peer configuration command and no key or autokey subcommands are present,
     the association is not authenticated; if the key subcommand is present,
     the association is authenticated using the symmetric key ID specified; if
     the autokey subcommand is present, the association is authenticated using
     Autokey.

     When multiple identity schemes are supported in the Autokey protocol, the
     first message exchange determines which one is used.  The client request
     message contains bits corresponding to which schemes it has available.
     The server response message contains bits corresponding to which schemes
     it has available.  Both server and client match the received bits with
     their own and select a common scheme.

     Following the principle that time is a public value, a server responds to
     any client packet that matches its cryptotype capabilities.  Thus, a
     server receiving an unauthenticated packet will respond with an
     unauthenticated packet, while the same server receiving a packet of a
     cryptotype it supports will respond with packets of that cryptotype.
     However, unconfigured broadcast or manycast client associations or
     symmetric passive associations will not be mobilized unless the server
     supports a cryptotype compatible with the first packet received.  By
     default, unauthenticated associations will not be mobilized unless
     overridden in a decidedly dangerous way.

     Some examples may help to reduce confusion.  Client Alice has no specific
     cryptotype selected.  Server Bob has both a symmetric key file and
     minimal Autokey files.  Alice's unauthenticated messages arrive at Bob,
     who replies with unauthenticated messages.  Cathy has a copy of Bob's
     symmetric key file and has selected key ID 4 in messages to Bob.  Bob
     verifies the message with his key ID 4.  If it's the same key and the
     message is verified, Bob sends Cathy a reply authenticated with that key.
     If verification fails, Bob sends Cathy a thing called a crypto-NAK, which
     tells her something broke.  She can see the evidence using the ntpq
     program.

     Denise has rolled her own host key and certificate.  She also uses one of
     the identity schemes as Bob.  She sends the first Autokey message to Bob
     and they both dance the protocol authentication and identity steps.  If
     all comes out okay, Denise and Bob continue as described above.

     It should be clear from the above that Bob can support all the girls at
     the same time, as long as he has compatible authentication and identity
     credentials.  Now, Bob can act just like the girls in his own choice of
     servers; he can run multiple configured associations with multiple
     different servers (or the same server, although that might not be
     useful).  But, wise security policy might preclude some cryptotype
     combinations; for instance, running an identity scheme with one server
     and no authentication with another might not be wise.

   Key Management
     The cryptographic values used by the Autokey protocol are incorporated as
     a set of files generated by the ntp-keygen(8) utility program, including
     symmetric key, host key and public certificate files, as well as sign
     key, identity parameters and leapseconds files.  Alternatively, host and
     sign keys and certificate files can be generated by the OpenSSL utilities
     and certificates can be imported from public certificate authorities.
     Note that symmetric keys are necessary for the ntpq(8) and ntpdc(8)
     utility programs.  The remaining files are necessary only for the Autokey
     protocol.

     Certificates imported from OpenSSL or public certificate authorities have
     certian limitations.  The certificate should be in ASN.1 syntax, X.509
     Version 3 format and encoded in PEM, which is the same format used by
     OpenSSL.  The overall length of the certificate encoded in ASN.1 must not
     exceed 1024 bytes.  The subject distinguished name field (CN) is the
     fully qualified name of the host on which it is used; the remaining
     subject fields are ignored.  The certificate extension fields must not
     contain either a subject key identifier or a issuer key identifier field;
     however, an extended key usage field for a trusted host must contain the
     value trustRoot.  Other extension fields are ignored.

   Authentication Commands
     autokey [logsec]
             Specifies the interval between regenerations of the session key
             list used with the Autokey protocol.  Note that the size of the
             key list for each association depends on this interval and the
             current poll interval.  The default value is 12 (4096 s or about
             1.1 hours).  For poll intervals above the specified interval, a
             session key list with a single entry will be regenerated for
             every message sent.

     controlkey key
             Specifies the key identifier to use with the ntpq(8) utility,
             which uses the standard protocol defined in RFC-1305.  The key
             argument is the key identifier for a trusted key, where the value
             can be in the range 1 to 65,534, inclusive.

     crypto [cert file] [leap file] [randfile file] [host file] [sign file]
             [gq file] [gqpar file] [iffpar file] [mvpar file] [pw password]
             This command requires the OpenSSL library.  It activates public
             key cryptography, selects the message digest and signature
             encryption scheme and loads the required private and public
             values described above.  If one or more files are left
             unspecified, the default names are used as described above.
             Unless the complete path and name of the file are specified, the
             location of a file is relative to the keys directory specified in
             the keysdir command or default /usr/local/etc.  Following are the
             subcommands:

             cert file
                     Specifies the location of the required host public
                     certificate file.  This overrides the link
                     ntpkey_cert_hostname in the keys directory.

             gqpar file
                     Specifies the location of the optional GQ parameters
                     file.  This overrides the link ntpkey_gq_hostname in the
                     keys directory.

             host file
                     Specifies the location of the required host key file.
                     This overrides the link ntpkey_key_hostname in the keys
                     directory.

             iffpar file
                     Specifies the location of the optional IFF parameters
                     file.This overrides the link ntpkey_iff_hostname in the
                     keys directory.

             leap file
                     Specifies the location of the optional leapsecond file.
                     This overrides the link ntpkey_leap in the keys
                     directory.

             mvpar file
                     Specifies the location of the optional MV parameters
                     file.  This overrides the link ntpkey_mv_hostname in the
                     keys directory.

             pw password
                     Specifies the password to decrypt files containing
                     private keys and identity parameters.  This is required
                     only if these files have been encrypted.

             randfile file
                     Specifies the location of the random seed file used by
                     the OpenSSL library.  The defaults are described in the
                     main text above.

             sign file
                     Specifies the location of the optional sign key file.
                     This overrides the link ntpkey_sign_hostname in the keys
                     directory.  If this file is not found, the host key is
                     also the sign key.

     keys keyfile
             Specifies the complete path and location of the MD5 key file
             containing the keys and key identifiers used by ntpd(8), ntpq(8)
             and ntpdc when operating with symmetric key cryptography.  This
             is the same operation as the -k command line option.

     keysdir path
             This command specifies the default directory path for
             cryptographic keys, parameters and certificates.  The default is
             /usr/local/etc/.

     requestkey key
             Specifies the key identifier to use with the ntpdc(8) utility
             program, which uses a proprietary protocol specific to this
             implementation of ntpd(8).  The key argument is a key identifier
             for the trusted key, where the value can be in the range 1 to
             65,534, inclusive.

     revoke logsec
             Specifies the interval between re-randomization of certain
             cryptographic values used by the Autokey scheme, as a power of 2
             in seconds.  These values need to be updated frequently in order
             to deflect brute-force attacks on the algorithms of the scheme;
             however, updating some values is a relatively expensive
             operation.  The default interval is 16 (65,536 s or about 18
             hours).  For poll intervals above the specified interval, the
             values will be updated for every message sent.

     trustedkey key ...
             Specifies the key identifiers which are trusted for the purposes
             of authenticating peers with symmetric key cryptography, as well
             as keys used by the ntpq(8) and 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
             key arguments are 32-bit unsigned integers with values from 1 to
             65,534.

   Error Codes
     The following error codes are reported via the NTP control and monitoring
     protocol trap mechanism.

     101     (bad field format or length) The packet has invalid version,
             length or format.

     102     (bad timestamp) The packet timestamp is the same or older than
             the most recent received.  This could be due to a replay or a
             server clock time step.

     103     (bad filestamp) The packet filestamp is the same or older than
             the most recent received.  This could be due to a replay or a key
             file generation error.

     104     (bad or missing public key) The public key is missing, has
             incorrect format or is an unsupported type.

     105     (unsupported digest type) The server requires an unsupported
             digest/signature scheme.

     106     (mismatched digest types) Not used.

     107     (bad signature length) The signature length does not match the
             current public key.

     108     (signature not verified) The message fails the signature check.
             It could be bogus or signed by a different private key.

     109     (certificate not verified) The certificate is invalid or signed
             with the wrong key.

     110     (certificate not verified) The certificate is not yet valid or
             has expired or the signature could not be verified.

     111     (bad or missing cookie) The cookie is missing, corrupted or
             bogus.

     112     (bad or missing leapseconds table) The leapseconds table is
             missing, corrupted or bogus.

     113     (bad or missing certificate) The certificate is missing,
             corrupted or bogus.

     114     (bad or missing identity) The identity key is missing, corrupt or
             bogus.

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

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

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

                     49213 525.624 127.127.4.1 93 226 00:08:29.606 D

                     The first two fields show the date (Modified Julian Day)
                     and time (seconds and fraction past UTC midnight).  The
                     next field shows the clock address in dotted-quad
                     notation.  The final field shows the last timecode
                     received from the clock in decoded ASCII format, where
                     meaningful.  In some clock drivers a good deal of
                     additional information can be gathered and displayed as
                     well.  See information specific to each clock for further
                     details.

             cryptostats
                     This option requires the OpenSSL cryptographic software
                     library.  It enables recording of cryptographic public
                     key protocol information.  Each message received by the
                     protocol module appends a line of the following form to
                     the file generation set named cryptostats:

                     49213 525.624 127.127.4.1 message

                     The first two fields show the date (Modified Julian Day)
                     and time (seconds and fraction past UTC midnight).  The
                     next field shows the peer address in dotted-quad
                     notation.  The final message field includes the message
                     type and certain ancillary information.  See the
                     Authentication Commands section for further information.

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

                     50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

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

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

                     48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674

                     The first two fields show the date (Modified Julian Day)
                     and time (seconds and fraction past UTC midnight).  The
                     next two fields show the peer address in dotted-quad
                     notation and status, respectively.  The status field is
                     encoded in hex in the format described in Appendix A of
                     the NTP specification RFC 1305.  The final four fields
                     show the offset, delay, dispersion and RMS jitter, all in
                     seconds.

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

                     50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000

                     The first two fields show the date (Modified Julian Day)
                     and time (seconds and fraction past UTC midnight).  The
                     next two fields show the remote peer or clock address
                     followed by the local address in dotted-quad notation.
                     The final four fields show the originate, receive,
                     transmit and final NTP timestamps in order.  The
                     timestamp values are as received and before processing by
                     the various data smoothing and mitigation algorithms.

             sysstats
                     Enables recording of ntpd statistics counters on a
                     periodic basis.  Each hour a line of the following form
                     is appended to the file generation set named sysstats:

                     50928 2132.543 36000 81965 0 9546 56 71793 512 540 10 147

                     The first two fields show the date (Modified Julian Day)
                     and time (seconds and fraction past UTC midnight).  The
                     remaining ten fields show the statistics counter values
                     accumulated since the last generated line.

                     Time since restart 36000
                             Time in hours since the system was last rebooted.

                     Packets received 81965
                             Total number of packets received.

                     Packets processed 0
                             Number of packets received in response to
                             previous packets sent.

                     Current version 9546
                             Number of packets matching the current NTP
                             version.

                     Previous version 56
                             Number of packets matching the previous NTP
                             version.

                     Bad version 71793
                             Number of packets matching neither NTP version.

                     Access denied 512
                             Number of packets denied access for any reason.

                     Bad length or format 540
                             Number of packets with invalid length, format or
                             port number.

                     Bad authentication 10
                             Number of packets not verified as authentic.

                     Rate exceeded 147
                             Number of packets discarded due to rate
                             limitation.

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

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

                     Note that this command can be sent from the ntpdc(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.
                             Filenames of set members are built from three
                             concatenated elements file ... prefix, file ...
                             filename and file ... suffix:

                             prefix  This is a constant filename path.  It is
                                     not subject to modifications via the
                                     filegen option.  It is defined by the
                                     server, usually specified as a compile-
                                     time constant.  It may, however, be
                                     configurable for individual file
                                     generation sets via other commands.  For
                                     example, the prefix used with loopstats
                                     and peerstats generation can be
                                     configured using the statsdir option
                                     explained above.

                             filename
                                     This string is directly concatenated to
                                     the prefix mentioned above (no
                                     intervening `/').  This can be modified
                                     using the file argument to the filegen
                                     statement.  No .. elements are allowed in
                                     this component to prevent filenames
                                     referring to parts outside the filesystem
                                     hierarchy denoted by prefix.

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

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

                             none    The file set is actually a single plain
                                     file.

                             pid     One element of file set is used per
                                     incarnation of a ntpd server.  This type
                                     does not perform any changes to file set
                                     members during runtime, however it
                                     provides an easy way of separating files
                                     belonging to different ntpd(8) server
                                     incarnations.  The set member filename is
                                     built by appending a `.' to concatenated
                                     prefix and filename strings, and
                                     appending the decimal representation of
                                     the process ID of the ntpd(8) server
                                     process.

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

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

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

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

                             age     This type of file generation sets changes
                                     to a new element of the file set every 24
                                     hours of server operation.  The filename
                                     suffix consists of a dot, the letter a,
                                     and an 8-digit number.  This number is
                                     taken to be the number of seconds the
                                     server is running at the start of the
                                     corresponding 24-hour period.
                                     Information is only written to a file
                                     generation by specifying enable; output
                                     is prevented by specifying disable.

                     link | nolink
                             It is convenient to be able to access the current
                             element of a file generation set by a fixed name.
                             This feature is enabled by specifying link and
                             disabled using nolink.  If link is specified, a
                             hard link from the current file set element to a
                             file without suffix is created.  When there is
                             already a file with this name and the number of
                             links of this file is one, it is renamed
                             appending a dot, the letter C, and the pid of the
                             ntpd server process.  When the number of links is
                             greater than one, the file is unlinked.  This
                             allows the current file to be accessed by a
                             constant name.

                     enable | disable
                             Enables or disables the recording function.

Access Control Support
     The ntpd(8) daemon implements a general purpose address/mask based
     restriction list.  The list contains address/match entries sorted first
     by increasing address values and then by increasing mask values.  A match
     occurs when the bitwise AND of the mask and the packet source address is
     equal to the bitwise AND of the mask and address in the list.  The list
     is searched in order with the last match found defining the restriction
     flags associated with the entry.  Additional information and examples can
     be found in the "Notes on Configuring NTP and Setting up a NTP Subnet"
     page (available as part of the HTML documentation provided in
     /usr/share/doc/ntp).

     The restriction facility was implemented in conformance with the access
     policies for the original NSFnet backbone time servers.  Later the
     facility was expanded to deflect cryptographic and clogging attacks.
     While this facility may be useful for keeping unwanted or broken or
     malicious clients from congesting innocent servers, it should not be
     considered an alternative to the NTP authentication facilities.  Source
     address based restrictions are easily circumvented by a determined
     cracker.

     Clients can be denied service because they are explicitly included in the
     restrict list created by the restrict command or implicitly as the result
     of cryptographic or rate limit violations.  Cryptographic violations
     include certificate or identity verification failure; rate limit
     violations generally result from defective NTP implementations that send
     packets at abusive rates.  Some violations cause denied service only for
     the offending packet, others cause denied service for a timed period and
     others cause the denied service for an indefinite period.  When a client
     or network is denied access for an indefinite period, the only way at
     present to remove the restrictions is by restarting the server.

   The Kiss-of-Death Packet
     Ordinarily, packets denied service are simply dropped with no further
     action except incrementing statistics counters.  Sometimes a more
     proactive response is needed, such as a server message that explicitly
     requests the client to stop sending and leave a message for the system
     operator.  A special packet format has been created for this purpose
     called the "kiss-of-death" (KoD) packet.  KoD packets have the leap bits
     set unsynchronized and stratum set to zero and the reference identifier
     field set to a four-byte ASCII code.  If the noserve or notrust flag of
     the matching restrict list entry is set, the code is "DENY"; if the
     limited flag is set and the rate limit is exceeded, the code is "RATE".
     Finally, if a cryptographic violation occurs, the code is "CRYP".

     A client receiving a KoD performs a set of sanity checks to minimize
     security exposure, then updates the stratum and reference identifier peer
     variables, sets the access denied (TEST4) bit in the peer flash variable
     and sends a message to the log.  As long as the TEST4 bit is set, the
     client will send no further packets to the server.  The only way at
     present to recover from this condition is to restart the protocol at both
     the client and server.  This happens automatically at the client when the
     association times out.  It will happen at the server only if the server
     operator cooperates.

   Access Control Commands
     discard [average avg] [minimum min] [monitor prob]
             Set the parameters of the limited facility which protects the
             server from client abuse.  The average subcommand specifies the
             minimum average packet spacing, while the minimum subcommand
             specifies the minimum packet spacing.  Packets that violate these
             minimum are discarded and a kiss-o'-death packet returned if
             enabled.  The default minimum average and minimum are 5 and 2,
             respectively.  The monitor subcommand specifies the probability
             of discard for packets that overflow the rate-control window.

     restrict address [mask mask] [flag ...]
             The address argument expressed in dotted-quad form is the address
             of a host or network.  Alternatively, the address argument can be
             a valid host DNS name.  The mask argument expressed in dotted-
             quad form defaults to 255.255.255.255, meaning that the address
             is treated as the address of an individual host.  A default entry
             (address 0.0.0.0, mask 0.0.0.0) is always included and is always
             the first entry in the list.  Note that text string default, with
             no mask option, may be used to indicate the default entry.  In
             the current implementation, flag always restricts access, i.e.,
             an entry with no flags indicates that free access to the server
             is to be given.  The flags are not orthogonal, in that more
             restrictive flags will often make less restrictive ones
             redundant.  The flags can generally be classed into two
             categories, those which restrict time service and those which
             restrict informational queries and attempts to do run-time
             reconfiguration of the server.  One or more of the following
             flags may be specified:

             ignore  Deny packets of all kinds, including ntpq(8) and ntpdc(8)
                     queries.

             kod     If this flag is set when an access violation occurs, a
                     kiss-o'-death (KoD) packet is sent.  KoD packets are rate
                     limited to no more than one per second.  If another KoD
                     packet occurs within one second after the last one, the
                     packet is dropped.

             limited
                     Deny service if the packet spacing violates the lower
                     limits specified in the discard command.  A history of
                     clients is kept using the monitoring capability of
                     ntpd(8).  Thus, monitoring is always active as long as
                     there is a restriction entry with the limited flag.

             lowpriotrap
                     Declare traps set by matching hosts to be low priority.
                     The number of traps a server can maintain is limited (the
                     current limit is 3).  Traps are usually assigned on a
                     first come, first served basis, with later trap
                     requestors being denied service.  This flag modifies the
                     assignment algorithm by allowing low priority traps to be
                     overridden by later requests for normal priority traps.

             nomodify
                     Deny ntpq(8) and ntpdc(8) queries which attempt to modify
                     the state of the server (i.e., run time reconfiguration).
                     Queries which return information are permitted.

             noquery
                     Deny ntpq(8) and ntpdc(8) queries.  Time service is not
                     affected.

             nopeer  Deny packets which would result in mobilizing a new
                     association.  This includes broadcast and symmetric
                     active packets when a configured association does not
                     exist.

             noserve
                     Deny all packets except ntpq(8) and ntpdc(8) queries.

             notrap  Decline to provide mode 6 control message trap service to
                     matching hosts.  The trap service is a subsystem of the
                     ntpdq control message protocol which is intended for use
                     by remote event logging programs.

             notrust
                     Deny service unless the packet is cryptographically
                     authenticated.

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

             version
                     Deny packets that do not match the current NTP version.

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

Automatic NTP Configuration Options
   Manycasting
     Manycasting is a automatic discovery and configuration paradigm new to
     NTPv4.  It is intended as a means for a multicast client to troll the
     nearby network neighborhood to find cooperating manycast servers,
     validate them using cryptographic means and evaluate their time values
     with respect to other servers that might be lurking in the vicinity.  The
     intended result is that each manycast client mobilizes client
     associations with some number of the "best" of the nearby manycast
     servers, yet automatically reconfigures to sustain this number of servers
     should one or another fail.

     Note that the manycasting paradigm does not coincide with the anycast
     paradigm described in RFC-1546, which is designed to find a single server
     from a clique of servers providing the same service.  The manycast
     paradigm is designed to find a plurality of redundant servers satisfying
     defined optimality criteria.

     Manycasting can be used with either symmetric key or public key
     cryptography.  The public key infrastructure (PKI) offers the best
     protection against compromised keys and is generally considered stronger,
     at least with relatively large key sizes.  It is implemented using the
     Autokey protocol and the OpenSSL cryptographic library available from
     http://www.openssl.org/.  The library can also be used with other NTPv4
     modes as well and is highly recommended, especially for broadcast modes.

     A persistent manycast client association is configured using the
     manycastclient command, which is similar to the server command but with a
     multicast (IPv4 class D or IPv6 prefix FF) group address.  The IANA has
     designated IPv4 address 224.1.1.1 and IPv6 address FF05::101 (site local)
     for NTP.  When more servers are needed, it broadcasts manycast client
     messages to this address at the minimum feasible rate and minimum
     feasible time-to-live (TTL) hops, depending on how many servers have
     already been found.  There can be as many manycast client associations as
     different group address, each one serving as a template for a future
     ephemeral unicast client/server association.

     Manycast servers configured with the manycastserver command listen on the
     specified group address for manycast client messages.  Note the
     distinction between manycast client, which actively broadcasts messages,
     and manycast server, which passively responds to them.  If a manycast
     server is in scope of the current TTL and is itself synchronized to a
     valid source and operating at a stratum level equal to or lower than the
     manycast client, it replies to the manycast client message with an
     ordinary unicast server message.

     The manycast client receiving this message mobilizes an ephemeral
     client/server association according to the matching manycast client
     template, but only if cryptographically authenticated and the server
     stratum is less than or equal to the client stratum.  Authentication is
     explicitly required and either symmetric key or public key (Autokey) can
     be used.  Then, the client polls the server at its unicast address in
     burst mode in order to reliably set the host clock and validate the
     source.  This normally results in a volley of eight client/server at 2-s
     intervals during which both the synchronization and cryptographic
     protocols run concurrently.  Following the volley, the client runs the
     NTP intersection and clustering algorithms, which act to discard all but
     the "best" associations according to stratum and synchronization
     distance.  The surviving associations then continue in ordinary
     client/server mode.

     The manycast client polling strategy is designed to reduce as much as
     possible the volume of manycast client messages and the effects of
     implosion due to near-simultaneous arrival of manycast server messages.
     The strategy is determined by the manycastclient, tos and ttl
     configuration commands.  The manycast poll interval is normally eight
     times the system poll interval, which starts out at the minpoll value
     specified in the manycastclient, command and, under normal circumstances,
     increments to the maxpolll value specified in this command.  Initially,
     the TTL is set at the minimum hops specified by the ttl command.  At each
     retransmission the TTL is increased until reaching the maximum hops
     specified by this command or a sufficient number client associations have
     been found.  Further retransmissions use the same TTL.

     The quality and reliability of the suite of associations discovered by
     the manycast client is determined by the NTP mitigation algorithms and
     the minclock and minsane values specified in the tos configuration
     command.  At least minsane candidate servers must be available and the
     mitigation algorithms produce at least minclock survivors in order to
     synchronize the clock.  Byzantine agreement principles require at least
     four candidates in order to correctly discard a single falseticker.  For
     legacy purposes, minsane defaults to 1 and minclock defaults to 3.  For
     manycast service minsane should be explicitly set to 4, assuming at least
     that number of servers are available.

     If at least minclock servers are found, the manycast poll interval is
     immediately set to eight times maxpoll.  If less than minclock servers
     are found when the TTL has reached the maximum hops, the manycast poll
     interval is doubled.  For each transmission after that, the poll interval
     is doubled again until reaching the maximum of eight times maxpoll.
     Further transmissions use the same poll interval and TTL values.  Note
     that while all this is going on, each client/server association found is
     operating normally it the system poll interval.

     Administratively scoped multicast boundaries are normally specified by
     the network router configuration and, in the case of IPv6, the link/site
     scope prefix.  By default, the increment for TTL hops is 32 starting from
     31; however, the ttl configuration command can be used to modify the
     values to match the scope rules.

     It is often useful to narrow the range of acceptable servers which can be
     found by manycast client associations.  Because manycast servers respond
     only when the client stratum is equal to or greater than the server
     stratum, primary (stratum 1) servers will find only primary servers in
     TTL range, which is probably the most common objective.  However, unless
     configured otherwise, all manycast clients in TTL range will eventually
     find all primary servers in TTL range, which is probably not the most
     common objective in large networks.  The tos command can be used to
     modify this behavior.  Servers with stratum below floor or above ceiling
     specified in the tos command are strongly discouraged during the
     selection process; however, these servers may be temporally accepted if
     the number of servers within TTL range is less than minclock.

     The above actions occur for each manycast client message, which repeats
     at the designated poll interval.  However, once the ephemeral client
     association is mobilized, subsequent manycast server replies are
     discarded, since that would result in a duplicate association.  If during
     a poll interval the number of client associations falls below minclock,
     all manycast client prototype associations are reset to the initial poll
     interval and TTL hops and operation resumes from the beginning.  It is
     important to avoid frequent manycast client messages, since each one
     requires all manycast servers in TTL range to respond.  The result could
     well be an implosion, either minor or major, depending on the number of
     servers in range.  The recommended value for maxpoll is 12 (4,096 s).

     It is possible and frequently useful to configure a host as both manycast
     client and manycast server.  A number of hosts configured this way and
     sharing a common group address will automatically organize themselves in
     an optimum configuration based on stratum and synchronization distance.
     For example, consider an NTP subnet of two primary servers and a hundred
     or more dependent clients.  With two exceptions, all servers and clients
     have identical configuration files including both multicastclient and
     multicastserver commands using, for instance, multicast group address
     239.1.1.1.  The only exception is that each primary server configuration
     file must include commands for the primary reference source such as a GPS
     receiver.

     The remaining configuration files for all secondary servers and clients
     have the same contents, except for the tos command, which is specific for
     each stratum level.  For stratum 1 and stratum 2 servers, that command is
     not necessary.  For stratum 3 and above servers the floor value is set to
     the intended stratum number.  Thus, all stratum 3 configuration files are
     identical, all stratum 4 files are identical and so forth.

     Once operations have stabilized in this scenario, the primary servers
     will find the primary reference source and each other, since they both
     operate at the same stratum (1), but not with any secondary server or
     client, since these operate at a higher stratum.  The secondary servers
     will find the servers at the same stratum level.  If one of the primary
     servers loses its GPS receiver, it will continue to operate as a client
     and other clients will time out the corresponding association and re-
     associate accordingly.

     Some administrators prefer to avoid running ntpd(8) continuously and run
     either ntpdate(8) or ntpd(8) -q as a cron job.  In either case the
     servers must be configured in advance and the program fails if none are
     available when the cron job runs.  A really slick application of manycast
     is with ntpd(8) -q.  The program wakes up, scans the local landscape
     looking for the usual suspects, selects the best from among the rascals,
     sets the clock and then departs.  Servers do not have to be configured in
     advance and all clients throughout the network can have the same
     configuration file.

   Manycast Interactions with Autokey
     Each time a manycast client sends a client mode packet to a multicast
     group address, all manycast servers in scope generate a reply including
     the host name and status word.  The manycast clients then run the Autokey
     protocol, which collects and verifies all certificates involved.
     Following the burst interval all but three survivors are cast off, but
     the certificates remain in the local cache.  It often happens that
     several complete signing trails from the client to the primary servers
     are collected in this way.

     About once an hour or less often if the poll interval exceeds this, the
     client regenerates the Autokey key list.  This is in general transparent
     in client/server mode.  However, about once per day the server private
     value used to generate cookies is refreshed along with all manycast
     client associations.  In this case all cryptographic values including
     certificates is refreshed.  If a new certificate has been generated since
     the last refresh epoch, it will automatically revoke all prior
     certificates that happen to be in the certificate cache.  At the same
     time, the manycast scheme starts all over from the beginning and the
     expanding ring shrinks to the minimum and increments from there while
     collecting all servers in scope.

   Manycast Options
     tos [ceiling ceiling | cohort { 0 | 1 } | floor floor | minclock minclock
             | minsane minsane]
             This command affects the clock selection and clustering
             algorithms.  It can be used to select the quality and quantity of
             peers used to synchronize the system clock and is most useful in
             manycast mode.  The variables operate as follows:

             ceiling ceiling
                     Peers with strata above ceiling will be discarded if
                     there are at least minclock peers remaining.  This value
                     defaults to 15, but can be changed to any number from 1
                     to 15.

             cohort {0 | 1}
                     This is a binary flag which enables (0) or disables (1)
                     manycast server replies to manycast clients with the same
                     stratum level.  This is useful to reduce implosions where
                     large numbers of clients with the same stratum level are
                     present.  The default is to enable these replies.

             floor floor
                     Peers with strata below floor will be discarded if there
                     are at least minclock peers remaining.  This value
                     defaults to 1, but can be changed to any number from 1 to
                     15.

             minclock minclock
                     The clustering algorithm repeatedly casts out outerlayer
                     associations until no more than minclock associations
                     remain.  This value defaults to 3, but can be changed to
                     any number from 1 to the number of configured sources.

             minsane minsane
                     This is the minimum number of candidates available to the
                     clock selection algorithm in order to produce one or more
                     true chimers for the clustering algorithm.  If fewer than
                     this number are available, the clock is undisciplined and
                     allowed to run free.  The default is 1 for legacy
                     purposes.  However, according to principles of Byzantine
                     agreement, minsane should be at least 4 in order to
                     detect and discard a single falseticker.

     ttl hop ...
             This command specifies a list of TTL values in increasing order,
             up to 8 values can be specified.  In manycast mode these values
             are used in turn in an expanding-ring search.  The default is
             eight multiples of 32 starting at 31.

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

     A reference clock will generally (though not always) be a radio timecode
     receiver which is synchronized to a source of standard time such as the
     services offered by the NRC in Canada and NIST and USNO in the US.  The
     interface between the computer and the timecode receiver is device
     dependent, but is usually a serial port.  A device driver specific to
     each reference clock must be selected and compiled in the distribution;
     however, most common radio, satellite and modem clocks are included by
     default.  Note that an attempt to configure a reference clock when the
     driver has not been compiled or the hardware port has not been
     appropriately configured results in a scalding remark to the system log
     file, but is otherwise non hazardous.

     For the purposes of configuration, ntpd(8) treats reference clocks in a
     manner analogous to normal NTP peers as much as possible.  Reference
     clocks are identified by a syntactically correct but invalid IP address,
     in order to distinguish them from normal NTP peers.  Reference clock
     addresses are of the form 127.127.t.u, where t is an integer denoting the
     clock type and u indicates the unit number in the range 0-3.  While it
     may seem overkill, it is in fact sometimes useful to configure multiple
     reference clocks of the same type, in which case the unit numbers must be
     unique.

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

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

     The stratum number of a reference clock is by default zero.  Since the
     ntpd(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
     purpose.  Also, in cases involving both a reference clock and a pulse-
     per-second (PPS) discipline signal, it is useful to specify the reference
     clock identifier as other than the default, depending on the driver.  The
     refid option is used for this purpose.  Except where noted, these options
     apply to all clock drivers.

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

             prefer  Marks the reference clock as preferred.  All other things
                     being equal, this host will be chosen for synchronization
                     among a set of correctly operating hosts.  See the
                     "Mitigation Rules and the prefer Keyword" page (available
                     as part of the HTML documentation provided in
                     /usr/share/doc/ntp) for further information.

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

             minpoll int

             maxpoll int
                     These options specify the minimum and maximum polling
                     interval for reference clock messages, as a power of 2 in
                     seconds For most directly connected reference clocks,
                     both minpoll and maxpoll default to 6 (64 s).  For modem
                     reference clocks, minpoll defaults to 10 (17.1 m) and
                     maxpoll defaults to 14 (4.5 h).  The allowable range is 4
                     (16 s) to 17 (36.4 h) inclusive.

     fudge 127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string]
             [mode int] [flag1 0 | 1] [flag2 0 | 1] [flag3 0 | 1] [flag4 0 |
             1]
             This command can be used to configure reference clocks in special
             ways.  It must immediately follow the server command which
             configures the driver.  Note that the same capability is possible
             at run time using the ntpdc(8) program.  The options are
             interpreted as follows:

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

             time2 secs
                     Specifies a fixed-point decimal number in seconds, which
                     is interpreted in a driver-dependent way.  See the
                     descriptions of specific drivers in the "Reference Clock
                     Drivers" page (available as part of the HTML
                     documentation provided in /usr/share/doc/ntp).

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

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

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

             flag1 0 | 1

             flag2 0 | 1

             flag3 0 | 1

             flag4 0 | 1
                     These four flags are used for customizing the clock
                     driver.  The interpretation of these values, and whether
                     they are used at all, is a function of the particular
                     clock driver.  However, by convention flag4 is used to
                     enable recording monitoring data to the clockstats file
                     configured with the filegen command.  Further information
                     on the filegen command can be found in Monitoring
                     Options.

Miscellaneous Options
     broadcastdelay seconds
             The broadcast and multicast modes require a special calibration
             to determine the network delay between the local and remote
             servers.  Ordinarily, this is done automatically by the initial
             protocol exchanges between the client and server.  In some cases,
             the calibration procedure may fail due to network or server
             access controls, for example.  This command specifies the default
             delay to be used under these circumstances.  Typically (for
             Ethernet), a number between 0.003 and 0.007 seconds is
             appropriate.  The default when this command is not used is 0.004
             seconds.

     calldelay delay
             This option controls the delay in seconds between the first and
             second packets sent in burst or iburst mode to allow additional
             time for a modem or ISDN call to complete.

     driftfile driftfile
             This command specifies the complete path and name of the file
             used to record the frequency of the local clock oscillator.  This
             is the same operation as the -f command line option.  If the file
             exists, it is read at startup in order to set the initial
             frequency and then updated once per hour with the current
             frequency computed by the daemon.  If the file name is specified,
             but the file itself does not exist, the starts with an initial
             frequency of zero and creates the file when writing it for the
             first time.  If this command is not given, the daemon will always
             start with an initial frequency of zero.

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

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

     disable [auth | bclient | calibrate | kernel | monitor | ntp | pps |
             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.

             auth    Enables the server to synchronize with unconfigured peers
                     only if the peer has been correctly authenticated using
                     either public key or private key cryptography.  The
                     default for this flag is enable.

             bclient
                     Enables the server to listen for a message from a
                     broadcast or multicast server, as in the multicastclient
                     command with default address.  The default for this flag
                     is disable.

             calibrate
                     Enables the calibrate feature for reference clocks.  The
                     default for this flag is disable.

             kernel  Enables the kernel time discipline, if available.  The
                     default for this flag is enable if support is available,
                     otherwise disable.

             monitor
                     Enables the monitoring facility.  See the ntpdc(8)
                     program and the monlist command or further information.
                     The default for this flag is enable.

             ntp     Enables time and frequency discipline.  In effect, this
                     switch opens and closes the feedback loop, which is
                     useful for testing.  The default for this flag is enable.

             pps     Enables the pulse-per-second (PPS) signal when frequency
                     and time is disciplined by the precision time kernel
                     modifications.  See the "A Kernel Model for Precision
                     Timekeeping" (available as part of the HTML documentation
                     provided in /usr/share/doc/ntp) page for further
                     information.  The default for this flag is disable.

             stats   Enables the statistics facility.  See the Monitoring
                     Options section for further information.  The default for
                     this flag is disable.

     includefile includefile
             This command allows additional configuration commands to be
             included from a separate file.  Include files may be nested to a
             depth of five; upon reaching the end of any include file, command
             processing resumes in the previous configuration file.  This
             option is useful for sites that run ntpd(8) on multiple hosts,
             with (mostly) common options (e.g., a restriction list).

     logconfig configkeyword
             This command controls the amount and type of output written to
             the system syslog(3) facility or the alternate logfile log file.
             By default, all output is turned on.  All configkeyword keywords
             can be prefixed with `=', `+' and `-', where `=' sets the
             syslog(3) priority mask, `+' adds and `-' removes messages.
             syslog(3) messages can be controlled in four classes (clock,
             peer, sys and sync).  Within these classes four types of messages
             can be controlled: informational messages (info), event messages
             (events), statistics messages (statistics) and status messages
             (status).

             Configuration keywords are formed by concatenating the message
             class with the event class.  The all prefix can be used instead
             of a message class.  A message class may also be followed by the
             all keyword to enable/disable all messages of the respective
             message class.Thus, a minimal log configuration could look like
             this:

             logconfig =syncstatus +sysevents

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

             logconfig =syncall +clockall

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

     logfile logfile
             This command specifies the location of an alternate log file to
             be used instead of the default system syslog(3) facility.  This
             is the same operation as the -l command line option.

     setvar variable [default]
             This command adds an additional system variable.  These variables
             can be used to distribute additional information such as the
             access policy.  If the variable of the form name=value is
             followed by the default keyword, the variable will be listed as
             part of the default system variables (ntpq(8) rv command)).
             These additional variables serve informational purposes only.
             They are not related to the protocol other that they can be
             listed.  The known protocol variables will always override any
             variables defined via the setvar mechanism.  There are three
             special variables that contain the names of all variable of the
             same group.  The sys_var_list holds the names of all system
             variables.  The peer_var_list holds the names of all peer
             variables and the clock_var_list holds the names of the reference
             clock variables.

     tinker [allan allan | dispersion dispersion | freq freq | huffpuff
             huffpuff | panic panic | step srep | stepout stepout]
             This command can be used to alter several system variables in
             very exceptional circumstances.  It should occur in the
             configuration file before any other configuration options.  The
             default values of these variables have been carefully optimized
             for a wide range of network speeds and reliability expectations.
             In general, they interact in intricate ways that are hard to
             predict and some combinations can result in some very nasty
             behavior.  Very rarely is it necessary to change the default
             values; but, some folks cannot resist twisting the knobs anyway
             and this command is for them.  Emphasis added: twisters are on
             their own and can expect no help from the support group.

             The variables operate as follows:

             allan allan
                     The argument becomes the new value for the minimum Allan
                     intercept, which is a parameter of the PLL/FLL clock
                     discipline algorithm.  The value in log2 seconds defaults
                     to 7 (1024 s), which is also the lower limit.

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

             freq freq
                     The argument becomes the initial value of the frequency
                     offset in parts-per-million.  This overrides the value in
                     the frequency file, if present, and avoids the initial
                     training state if it is not.

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

             panic panic
                     The argument is the panic threshold, normally 1000 s.  If
                     set to zero, the panic sanity check is disabled and a
                     clock offset of any value will be accepted.

             step step
                     The argument is the step threshold, which by default is
                     0.128 s.  It can be set to any positive number in
                     seconds.  If set to zero, step adjustments will never
                     occur.  Note: The kernel time discipline is disabled if
                     the step threshold is set to zero or greater than the
                     default.

             stepout stepout
                     The argument is the stepout timeout, which by default is
                     900 s.  It can be set to any positive number in seconds.
                     If set to zero, the stepout pulses will not be
                     suppressed.

     trap host_address [port port_number] [interface interface_address]
             This command configures a trap receiver at the given host address
             and port number for sending messages with the specified local
             interface address.  If the port number is unspecified, a value of
             18447 is used.  If the interface address is not specified, the
             message is sent with a source address of the local interface the
             message is sent through.  Note that on a multihomed host the
             interface used may vary from time to time with routing changes.

             The trap receiver will generally log event messages and other
             information from the server in a log file.  While such monitor
             programs may also request their own trap dynamically, configuring
             a trap receiver will ensure that no messages are lost when the
             server is started.

     hop ...
             This command specifies a list of TTL values in increasing order,
             up to 8 values can be specified.  In manycast mode these values
             are used in turn in an expanding-ring search.  The default is
             eight multiples of 32 starting at 31.

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
     rc.conf(5), 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 11.0-PRERELEASE        December 21, 2006       FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | Configuration Support | Authentication Support | Monitoring Support | Access Control Support | Automatic NTP Configuration Options | Reference Clock Support | Miscellaneous Options | FILES | SEE ALSO | BUGS

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