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NTP.CONF(5)		    BSD File Formats Manual		   NTP.CONF(5)

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


     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 fol‐
     lowed by a list of arguments, some of which may be optional, separated by
     whitespace.  Commands may not be continued over multiple lines.  Argu‐
     ments may be host names, host addresses written in numeric, dotted-quad
     form, integers, floating point numbers (when specifying times in seconds)
     and text strings.

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

	   ·   Authentication Support

	   ·   Monitoring Support

	   ·   Access Control Support

	   ·   Automatic NTP Configuration Options

	   ·   Reference Clock Support

	   ·   Miscellaneous Options

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

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

   Configuration Commands
     The various modes are determined by the command keyword and the type of
     the required IP address.  Addresses are classed by type as (s) a remote
     server or peer (IPv4 class A, B and C), (b) the broadcast address of a
     local interface, (m) a multicast address (IPv4 class D), or (r) a refer‐
     ence clock address (127.127.x.x).	Note that only those options applica‐
     ble 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

     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 automati‐
     cally 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 refer‐
     ence clock addresses, which are always IPv4.

     Note that in contexts where a host name is expected, a -4 qualifier pre‐
     ceding the host name forces DNS resolution to the IPv4 namespace, while a
     -6 qualifier forces DNS resolution to the IPv6 namespace.	See IPv6 ref‐
     erences 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

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

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

	     For type b and m addresses (only), this command mobilizes a per‐
	     sistent broadcast mode association.  Multiple commands can be
	     used to specify multiple local broadcast interfaces (subnets)
	     and/or multiple multicast groups.	Note that local broadcast mes‐
	     sages go only to the interface associated with the subnet speci‐
	     fied, but multicast messages go to all interfaces.	 In broadcast
	     mode the local server sends periodic broadcast messages to a
	     client population at the address specified, which is usually the
	     broadcast address on (one of) the local network(s) or a multicast
	     address assigned to NTP.  The IANA has assigned the multicast
	     group address IPv4 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.

	     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 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.


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

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

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

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

     minpoll minpoll

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

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

     prefer  Marks the server as preferred.  All other things being equal,
	     this host will be chosen for synchronization among a set of cor‐
	     rectly operating hosts.  See the "Mitigation Rules and the prefer
	     Keyword" page (available as part of the HTML documentation pro‐
	     vided 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 broad‐
	     cast server and multicast server and the maximum ttl for the
	     expanding ring search with manycast client packets.  Selection of
	     the proper value, which defaults to 127, is something of a black
	     art and should be coordinated with the network administrator.

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

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

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

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

Authentication Support
     Authentication support allows the NTP client to verify that the server is
     in fact known and trusted and not an intruder intending accidentally or
     on purpose to masquerade as that server.  The NTPv3 specification
     RFC-1305 defines a scheme which provides cryptographic authentication of
     received NTP packets.  Originally, this was done using the Data Encryp‐
     tion Standard (DES) algorithm operating in Cipher Block Chaining (CBC)
     mode, commonly called DES-CBC.  Subsequently, this was 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 cryp‐
     tography 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.  Pub‐
     lic 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

     Authentication is always enabled, although ineffective if not configured
     as described below.  If a NTP packet arrives including a message authen‐
     tication 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 sym‐
     metric 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 dis‐
     rupt 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 com‐
     mands and, in particular, the flag has no effect on the authentication
     process itself.

     An attractive alternative where multicast support is available is many‐
     cast mode, in which clients periodically troll for servers as described
     in the Automatic NTP Configuration Options page.  Either symmetric key or
     public key cryptographic authentication can be used in this mode.	The
     principle advantage of manycast mode is that potential servers need not
     be configured in advance, since the client finds them during 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 brief‐
     ings, papers and reports at the NTP project page linked from

   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 dis‐
     tributed and stored using secure means beyond the scope of the NTP proto‐
     col itself.  Besides the keys used for ordinary NTP associations, addi‐
     tional keys can be used as passwords for the ntpq(8) and ntpdc(8) utility

     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.	 How‐
     ever, 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 algo‐
     rithms are also available.	 Using all of these schemes provides strong
     security against replay with or without modification, spoofing, masquer‐
     ade 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 sta‐
     tus, briefing slides and reading list on the Autonomous Authentication

     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 cer‐
     tificate 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 cer‐
     tificates 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 indepen‐
     dent of interface, network and any other naming convention.  The name
     appears in the host certificate in either or both the subject and issuer
     fields, so protection against DNS compromise is essential.

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

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

     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 crypto‐
     types.  A secure NTPv4 subnet can be configured in many ways while keep‐
     ing 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 mobiliza‐
     tion, either at configuration time or some time later when a message of
     appropriate cryptotype arrives.  When mobilized by a server or peer con‐
     figuration 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

     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 unauthen‐
     ticated 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 pas‐
     sive associations will not be mobilized unless the server supports a
     cryptotype compatible with the first packet received.  By default, unau‐
     thenticated 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 mini‐
     mal Autokey files.	 Alice's unauthenticated messages arrive at Bob, who
     replies with unauthenticated messages.  Cathy has a copy of Bob's symmet‐
     ric 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 veri‐
     fication 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 dif‐
     ferent servers (or the same server, although that might not be useful).
     But, wise security policy might preclude some cryptotype combinations;
     for instance, running an identity scheme with one server and no authenti‐
     cation 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) util‐
     ity programs.  The remaining files are necessary only for the Autokey

     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 sub‐
     ject fields are ignored.  The certificate extension fields must not con‐
     tain 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 val‐
	     ues described above.  If one or more files are left unspecified,
	     the default names are used as described above.  Unless the com‐
	     plete path and name of the file are specified, the location of a
	     file is relative to the keys directory specified in the keysdir
	     command or default /usr/local/etc.	 Following are the subcom‐

	     cert file
		     Specifies the location of the required host public cer‐
		     tificate 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

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

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

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

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

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

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

     keys keyfile
	     Specifies the complete path and location of the MD5 key file con‐
	     taining 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 crypto‐
	     graphic keys, parameters and certificates.	 The default is

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

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

   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 incor‐
	     rect 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

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

     113     (bad or missing certificate) The certificate is missing, cor‐
	     rupted or bogus.

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

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

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

		     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

		     49213 525.624 93 226 00:08:29.606 D

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

		     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 message

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

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

		     50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

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

		     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

		     48773 10847.650 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

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

		     50928 2132.543 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000

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

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

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

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

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

		     Packets received 81965
			     Total number of packets received.

		     Packets processed 0
			     Number of packets received in response to previ‐
			     ous packets sent

		     Current version 9546
			     Number of packets matching the current NTP ver‐

		     Previous version 56
			     Number of packets matching the previous NTP ver‐

		     Bad version 71793
			     Number of packets matching neither NTP version.

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

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

		     Bad authentication 10
			     Number of packets not verified as authentic.

		     Rate exceeded 147
			     Number of packets discarded due to rate limita‐

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

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

		     Note that this command can be sent from the ntpdc(8) pro‐
		     gram running at a remote location.

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

		     file filename
			     This is the file name for the statistics records.
			     Filenames of set members are built from three
			     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 con‐
				     figurable for individual file generation
				     sets via other commands.  For example,
				     the prefix used with loopstats and
				     peerstats generation can be configured
				     using the statsdir option explained

				     This string is directly concatenated to
				     the prefix mentioned above (no interven‐
				     ing ‘/’).	This can be modified using the
				     file argument to the filegen statement.
				     No .. elements are allowed in this compo‐
				     nent 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

			     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 pro‐
				     vides 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 append‐
				     ing the decimal representation of the
				     process ID of the ntpd(8) server process.

			     day     One file generation set element is cre‐
				     ated 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 gener‐
				     ation set are distinguished by appending
				     the following suffix to the file set
				     filename base: A dot, a 4-digit year num‐
				     ber, 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 gener‐
				     ated 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.  Informa‐
				     tion 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 append‐
			     ing a dot, the letter C, and the pid of the ntpd
			     server process.  When the number of links is
			     greater than one, the file is unlinked.  This
			     allows the current file to be accessed by a con‐
			     stant name.

		     enable | disable
			     Enables or disables the recording function.

Access Control Support
     The ntpd(8) daemon implements a general purpose address/mask based
     restriction list.	The list contains address/match entries sorted first
     by increasing address values and and then by increasing mask values.  A
     match occurs when the bitwise AND of the mask and the packet source
     address is equal to the bitwise AND of the mask and address in the list.
     The list is searched in order with the last match found defining the
     restriction flags associated with the entry.  Additional 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

     The restriction facility was implemented in conformance with the access
     policies for the original NSFnet backbone time servers.  Later the facil‐
     ity 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 viola‐
     tions generally result from defective NTP implementations that send pack‐
     ets at abusive rates.  Some violations cause denied service only for the
     offending packet, others cause denied service for a timed period and oth‐
     ers cause the denied service for an indefinate period.  When a client or
     network is denied access for an indefinate period, the only way at
     present to remove the restrictions is by restarting the server.

   The Kiss-of-Death Packet
     Ordinarily, packets denied service are simply dropped with no further
     action except incrementing statistics counters.  Sometimes a more proac‐
     tive response is needed, such as a server message that explicitly
     requests the client to stop sending and leave a message for the system
     operator.	A special packet format has been created for this purpose
     called the "kiss-of-death" (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
	     minima are discarded and a kiss-o'-death packet returned if
	     enabled.  The default minimum average and minimum are 5 and 2,
	     respectively.  The monitor subcommand specifies the 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, meaning that the address
	     is treated as the address of an individual host.  A default entry
	     (address, mask 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 redun‐
	     dant.  The flags can generally be classed into two categories,
	     those which restrict time service and those which restrict infor‐
	     mational 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)

	     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.

		     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.

		     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.

		     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.

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

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

		     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.

		     Deny service unless the packet is cryptographically

		     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.

		     Deny packets that do not match the current NTP version.

	     Default restriction list entries with the flags ignore, inter‐
	     face, 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 is a automatic discovery and configuration paradigm new to
     NTPv4.  It is intended as a means for a multicast client to troll the
     nearby network neighborhood to find cooperating manycast servers, vali‐
     date them using cryptographic means and evaluate their time values with
     respect to other servers that might be lurking in the vicinity.  The
     intended result is that each manycast client mobilizes client associa‐
     tions with some number of the "best" of the nearby manycast servers, yet
     automatically reconfigures to sustain this number of servers should one
     or another fail.

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

     Manycasting can be used with either symmetric key or public key cryptog‐
     raphy.  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 many‐
     castclient command, which is similar to the server command but with a
     multicast (IPv4 class D or IPv6 prefix FF) group address.	The IANA has
     designated IPv4 address 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 feasi‐
     ble time-to-live (TTL) hops, depending on how many servers have already
     been found.  There can be as many manycast client associations as differ‐
     ent 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 distinc‐
     tion 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 many‐
     cast 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 tem‐
     plate, but only if cryptographically authenticated and the server stratum
     is less than or equal to the client stratum.  Authentication is explic‐
     itly 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" asso‐
     ciations according to stratum and synchronization distance.  The surviv‐
     ing 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 implo‐
     sion 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.  Fur‐
     ther retransmissions use the same TTL.

     The quality and reliability of the suite of associations discovered by
     the manycast client is determined by the NTP mitigation algorithms and
     the minclock and minsane values specified in the tos configuration com‐
     mand.  At least minsane candidate servers must be available and the miti‐
     gation algorithms produce at least minclock survivors in order to syn‐
     chronize 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.  Fur‐
     ther transmissions use the same poll interval and TTL values.  Note that
     while all this is going on, each client/server association found is oper‐
     ating 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 val‐
     ues 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 stra‐
     tum, primary (stratum 1) servers fill find only primary servers in TTL
     range, which is probably the most common objective.  However, unless con‐
     figured 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 dis‐
     carded, since that would result in a duplicate association.  If during a
     poll interval the number of client associations falls below minclock, all
     manycast client prototype associations are reset to the initial poll
     interval and TTL hops and operation resumes from the beginning.  It is
     important to avoid frequent manycast client messages, since each one
     requires all manycast servers in TTL range to respond.  The result could
     well be an implosion, either minor or major, depending on the number of
     servers in range.	The recommended value for maxpoll is 12 (4,096 s).

     It is possible and frequently useful to configure a host as both 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	 The only exception is that each primary server configuration
     file must include commands for the primary reference source such as a GPS

     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-as‐
     sociate 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 configu‐
     ration 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.  Follow‐
     ing the burst interval all but three survivors are cast off, but the cer‐
     tificates remain in the local cache.  It often happens that several com‐
     plete 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 certifi‐
     cates 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 algo‐
	     rithms.  It can be used to select the quality and quantity of
	     peers used to synchronize the system clock and is most useful in
	     manycast mode.  The variables operate as follows:

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

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

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

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

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

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

Reference Clock Support
     The NTP Version 4 daemon supports some three dozen different radio,
     satellite and modem reference clocks plus a special pseudo-clock used for
     backup or when no other clock source is available.	 Detailed descriptions
     of individual device drivers and options can be found in the "Reference
     Clock Drivers" page (available as part of the HTML 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 doc‐
     umentation 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 depen‐
     dent, but is usually a serial port.  A device driver specific to each
     reference clock must be selected and compiled in the distribution; how‐
     ever, most common radio, satellite and modem clocks are included by
     default.  Note that an attempt to configure a reference clock when the
     driver has not been compiled or the hardware port has not been appropri‐
     ately configured results in a scalding remark to the system log file, but
     is otherwise non hazardous.

     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

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

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

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

	     prefer  Marks the reference clock as preferred.  All other things
		     being equal, this host will be chosen for synchronization
		     among a set of correctly operating hosts.	See the
		     "Mitigation Rules and the prefer Keyword" page (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 |
	     This command can be used to configure reference clocks in special
	     ways.  It must immediately follow the server command which con‐
	     figures 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 pro‐
		     duced by the driver, a fixed-point decimal number in sec‐
		     onds.  This is used as a calibration constant to adjust
		     the nominal time offset of a particular clock to agree
		     with an external standard, such as a precision PPS sig‐
		     nal.  It also provides a way to correct a systematic
		     error or bias due to serial port or operating system
		     latencies, different cable lengths or receiver internal
		     delay.  The specified offset is in addition to the propa‐
		     gation delay provided by other means, such as internal
		     DIPswitches.  Where a calibration for an individual sys‐
		     tem and driver is available, an approximate correction is
		     noted in the driver documentation pages.  Note: in order
		     to facilitate calibration when more than one radio clock
		     or PPS signal is supported, a special calibration feature
		     is available.  It takes the form of an argument to the
		     enable command described in Miscellaneous Options page
		     and operates as described in the "Reference Clock
		     Drivers" page (available as part of the HTML documenta‐
		     tion provided in /usr/share/doc/ntp).

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

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

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

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

	     flag1 0 | 1

	     flag2 0 | 1

	     flag3 0 | 1

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

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

     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 fre‐
	     quency 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 mea‐
	     sured in parts-per-million (PPM).	The file is updated by first
	     writing the current drift value into a temporary file and then
	     renaming this file to replace the old version.  This implies that
	     ntpd(8) must have write permission for the directory the drift
	     file is located in, and that file system links, symbolic or oth‐
	     erwise, should be avoided.

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

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

		     Enables the server to listen for a message from a broad‐
		     cast or multicast server, as in the multicastclient com‐
		     mand with default address.	 The default for this flag is

		     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.

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

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

	     pps     Enables the pulse-per-second (PPS) signal when frequency
		     and time is disciplined by the precision time kernel mod‐
		     ifications.  See the "A Kernel Model for Precision
		     Timekeeping" (available as part of the HTML documentation
		     provided in /usr/share/doc/ntp) page for further informa‐
		     tion.  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

	     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 mes‐
	     sage class.Thus, a minimal log configuration could look like

	     logconfig =syncstatus +sysevents

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

	     logconfig =syncall +clockall

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

     logfile logfile
	     This command specifies the location of an alternate log file to
	     be used instead of the default system syslog(3) facility.	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 fol‐
	     lowed by the default keyword, the variable will be listed as part
	     of the default system variables (ntpq(8) rv command)).  These
	     additional variables serve informational purposes only.  They are
	     not related to the protocol other that they can be listed.	 The
	     known protocol variables will always override any variables
	     defined via the setvar mechanism.	There are three special vari‐
	     ables that contain the names of all variable of the same group.
	     The sys_var_list holds the names of all system variables.	The
	     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 configu‐
	     ration file before any other configuration options.  The default
	     values of these variables have been carefully optimized for a
	     wide range of network speeds and reliability expectations.	 In
	     general, they interact in intricate ways that are hard to 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 com‐
	     mand is for them.	Emphasis added: twisters are on their own and
	     can expect no help from the support group.

	     The variables operate as follows:

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

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

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

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

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

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

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

     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.

     /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

     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.

     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

BSD			       December 21, 2006			   BSD

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