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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

	   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, 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 de-
     fault 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 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 as-
     sociation 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) ex-
	     clusively to NTP, but other nonconflicting	addresses can be used
	     to	contain	the messages within administrative boundaries.	Ordi-
	     narily, this specification	applies	only to	the local server oper-
	     ating 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 as-
	     sociated 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 in-
	     stead of the usual	one.  The packet spacing is normally 2 s; how-
	     ever, 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 in-
	     stead of the usual	one.  The packet spacing is normally 2 s; how-
	     ever, 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 ad-
	     dresses 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 op-
	     tion 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 ex-
	     panding 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 ad-
	     dress is required,	but the	NTP multicast address	as-
	     signed 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 au-
	     thentication 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 lo-
     cations of	the key	files, if other	than default, which symmetric keys are
     trusted and the interval between various operations, if other than	de-

     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 op-
     eration, 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	Au-
     tokey Protocol page verifies packet integrity using MD5 message digests
     and verifies the source with digital signatures and any of	several	di-
     gest/signature schemes.  Optional identity	schemes	described on the Iden-
     tity 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 di-
     gest 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 ad-
     dresses, 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 ap-
     pears 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	to-
     gether 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 op-
     erated 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 as-
     sociation is not authenticated; if	the key	subcommand is present, the as-
     sociation is authenticated	using the symmetric key	ID specified; if the
     autokey subcommand	is present, the	association is authenticated using Au-

     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, un-
     configured	broadcast or manycast client associations or symmetric passive
     associations will not be mobilized	unless the server supports a crypto-
     type compatible with the first packet received.  By default, unauthenti-
     cated 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 ev-
	     ery 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	en-
	     cryption 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	com-
	     mand or default /usr/local/etc.  Following	are the	subcommands:

	     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 im-
	     plementation 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 di-
	     gest/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 bo-

     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 no-
		     tation and	status,	respectively.  The status field	is en-
		     coded 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 sec-

		     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 el-
		     ement 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 in-
				     carnation 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 be-
				     longing to	different ntpd(8) server in-
				     carnations.  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 De-
				     cember 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 al-
			     lows 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	re-
     striction 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 ad-
     dress 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 re-
     striction 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 in-
     clude 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 offend-
     ing packet, others	cause denied service for a timed period	and others
     cause the denied service for an indefinate	period.	 When a	client or net-
     work 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 ac-
     tion 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 unsynchro-
     nized 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 re-
     strict 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	cryp-
     tographic violation occurs, the code is "CRYP".

     A client receiving	a KoD performs a set of	sanity checks to minimize se-
     curity 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 en-
	     abled.  The default minimum average and minimum are 5 and 2, re-
	     spectively.  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 re-
	     strictive 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 informa-
	     tional 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 re-
		     questors 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 au-

		     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 be-
	     sides 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 in-
     tended result is that each	manycast client	mobilizes client associations
     with some number of the "best" of the nearby manycast servers, yet	auto-
     matically 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 de-
     fined 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  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 im-
     mediately set to eight times maxpoll.  If less than minclock servers are
     found when	the TTL	has reached the	maximum	hops, the manycast poll	inter-
     val 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 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 as-
     sociation is mobilized, subsequent	manycast server	replies	are discarded,
     since that	would result in	a duplicate association.  If during a poll in-
     terval the	number of client associations falls below minclock, all	many-
     cast 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	de-
		     faults to 1, but can be changed to	any number from	1 to

	     minclock minclock
		     The clustering algorithm repeatedly casts out outlyer as-
		     sociations	until no more than minclock associations re-
		     main.  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 de-
		     tect 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, sat-
     ellite 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 de-
     fault.  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 ad-
     dresses 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 op-
     tion 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 or-
     dinarily displays an external stratum of one.  In order to	provide	engi-
     neered 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 in-
		     terval 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 er-
		     ror or bias due to	serial port or operating system	laten-
		     cies, different cable lengths or receiver internal	delay.
		     The specified offset is in	addition to the	propagation
		     delay provided by other means, such as internal DIP-
		     switches.	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 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	de-
		     scriptions	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 de-
		     fault 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 or-
		     dinarily 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 ac-
	     cess 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 de-
		     fault 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 in-
	     cluded 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 op-
	     tion 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 ac-
	     cess 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	addi-
	     tional 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 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 in-
		     crease 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 re-
		     cent interval the algorithm will search for a minimum de-
		     lay.  The lower limit is 900 s (15	m), but	a more reason-
		     able 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 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 in-
	     terface 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 in-
	     terface used may vary from	time to	time with routing changes.

	     The trap receiver will generally log event	messages and other in-
	     formation from the	server in a log	file.  While such monitor pro-
	     grams 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  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

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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