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ntp.conf(5)			 File Formats			   ntp.conf(5)



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

SYNOPSIS
       ntp.conf [--option-name] [--option-name value]

       All arguments must be options.


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

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

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

       o Authentication Support

       o Monitoring Support

       o Access Control Support

       o Automatic NTP Configuration Options

       o Reference Clock Support

       o Miscellaneous Options

       Following these is a section describing Miscellaneous  Options.	 While
       there  is  a rich set of options available, the only required option is
       one or more pool, 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  com-
       mands,  configuration  commands that configure a persistent association
       with a remote server or peer or reference clock, and auxiliary commands
       that specify environmental variables that control various related oper-
       ations.

   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 ref-
       erence clock address  (127.127.x.x).   Note  that  only	those  options
       applicable to each command are listed below.  Use of options not listed
       may not be caught as an error, but may result in some  weird  and  even
       destructive behavior.

       If  the	Basic  Socket  Interface  Extensions  for  IPv6	 (RFC-2553) is
       detected, support for the IPv6 address family is generated in  addition
       to  the	default	 support  of the IPv4 address family.  In a few cases,
       including the reslist billboard generated by ntpq(8) or ntpdc(8),  IPv6
       addresses  are  automatically generated.	 IPv6 addresses can be identi-
       fied by the presence of colons : in the address field.  IPv6  addresses
       can  be	used  almost everywhere where IPv4 addresses can be used, with
       the exception of reference clock addresses, which are always IPv4.

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

       pool  address [burst] [iburst] [version version] [prefer] [minpoll min-
       poll] [maxpoll maxpoll]

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

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

       broadcast address [key key | autokey] [version version] [prefer]	 [min-
       poll minpoll] [ttl ttl] [xleave]

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

       These five commands specify the time server name or address to be  used
       and the mode in which to operate.  The address can be either a DNS name
       or an IP address in dotted-quad notation.   Additional  information  on
       association  behavior can be found in the "Association Management" page
       (available  as	part   of   the	  HTML	 documentation	 provided   in
       /usr/share/doc/ntp).

       pool   For type s addresses, this command mobilizes a persistent client
	      mode association with a number of remote servers.	 In this  mode
	      the  local  clock can synchronized to the remote server, but the
	      remote server can never be synchronized to the local clock.

       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  synchro-
	      nized  to	 the remote server, but the remote server can never be
	      synchronized to the local clock.	This  command  should  not  be
	      used for type b or m addresses.

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

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

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

       Options:

       autokey
	      All  packets sent to and received from the server or peer are to
	      include authentication fields encrypted using the autokey scheme
	      described in Authentication 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 65535, 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 max-
	      poll option to an upper limit of 17 (36.4 h).  The minimum  poll
	      interval	defaults to 6 (64 s), but can be decreased by the min-
	      poll option to a lower limit of 4 (16 s).

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

       preempt
	      Says the association can be preempted.

       true   Marks  the  server  as  a	 truechimer.  Use this option only for
	      testing.

       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  pre-
	      fer  Keyword"  page (available as part of the HTML documentation
	      provided in /usr/share/doc/ntp) for further information.

       true   Forces the association to always survive the selection and clus-
	      tering  algorithms.  This option should almost certainly only be
	      used while testing an association.

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

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

       xleave Valid in peer and broadcast modes only, this flag enables inter-
	      leave mode.

   Auxiliary Commands
       broadcastclient
	      This command enables reception of broadcast server  messages  to
	      any  local interface (type b) address.  Upon receiving a message
	      for the first time, the broadcast client	measures  the  nominal
	      server  propagation  delay  using a brief client/server exchange
	      with the server, then enters the broadcast client mode, in which
	      it synchronizes to succeeding broadcast messages.	 Note that, in
	      order to avoid accidental or malicious disruption in this	 mode,
	      both the server and client should operate using symmetric-key or
	      public-key  authentication  as   described   in	Authentication
	      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 224.0.1.1
	      assigned by the IANA should NOT be used, unless  specific	 means
	      are  taken  to  limit the span of the reply and avoid a possibly
	      massive implosion at the original sender.	 Note that,  in	 order
	      to  avoid	 accidental or malicious disruption in this mode, both
	      the server and client should operate using symmetric-key or pub-
	      lic-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
	      receiving	 a  message  for  the first time, the multicast client
	      measures the nominal server  propagation	delay  using  a	 brief
	      client/server  exchange  with the server, then enters the broad-
	      cast client mode, in which it synchronizes to succeeding	multi-
	      cast messages.  Note that, in order to avoid accidental or mali-
	      cious disruption in this mode, both the server and client should
	      operate  using  symmetric-key  or	 public-key  authentication as
	      described in Authentication Options.

       mdnstries number
	      If we are participating in mDNS, after we have synched  for  the
	      first time we attempt to register with the mDNS system.  If that
	      registration attempt fails, we try again at one minute intervals
	      for  up  to  mdnstries  times.   After all, ntpd may be starting
	      before mDNS.  The default value for mdnstries is 5.

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

       NTPv4 retains the NTPv3 scheme, properly	 described  as	symmetric  key
       cryptography  and,  in addition, provides a new Autokey scheme based on
       public key cryptography.	 Public key cryptography is generally  consid-
       ered 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 distribu-
       tion  and  storage.   Public  key management is based on X.509 certifi-
       cates, which can be provided by	commercial  services  or  produced  by
       utility programs in the OpenSSL software library or the NTPv4 distribu-
       tion.

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

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

       Authentication is always enabled, although ineffective if  not  config-
       ured  as	 described below.  If a NTP packet arrives including a message
       authentication code (MAC), it is accepted only if it passes all crypto-
       graphic	checks.	 The checks require correct key ID, key value and mes-
       sage 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  configura-
       tion  commands  sent  by a ntpdc(8) program running on another machine.
       If this flag is enabled, which  is  the	default	 case,	new  broadcast
       client and symmetric passive associations and remote configuration com-
       mands must be cryptographically authenticated  using  either  symmetric
       key or public key cryptography.	If this flag is disabled, these opera-
       tions are effective even if not cryptographic authenticated.  It should
       be understood that operating with the auth flag disabled invites a sig-
       nificant vulnerability where a rogue hacker can masquerade as a falset-
       icker  and  seriously  disrupt  system timekeeping.  It is important to
       note that this flag has no purpose other than to allow  or  disallow  a
       new  association in response to new broadcast and symmetric active mes-
       sages and remote configuration commands and, in	particular,  the  flag
       has no effect on the authentication process itself.

       An attractive alternative where multicast support is available is 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	 regu-
       lar operation, and the configuration files for all clients can be iden-
       tical.

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

   Symmetric-Key Cryptography
       The  original  RFC-1305 specification allows any one of possibly 65,535
       keys, each distinguished by a 32-bit key identifier, to authenticate an
       association.   The  servers  and clients involved must agree on the key
       and key identifier to  authenticate  NTP	 packets.   Keys  and  related
       information are specified in a key file, usually called ntp.keys, which
       must be distributed and stored using secure means beyond the  scope  of
       the  NTP protocol itself.  Besides the keys used for ordinary NTP asso-
       ciations, additional keys can be used as passwords for the ntpq(8)  and
       ntpdc(8) utility programs.

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

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

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

       The specific cryptographic environment  used  by	 Autokey  servers  and
       clients is determined by a set of files and soft links generated by the
       ntp-keygen(1ntpkeygenmdoc) 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	speci-
       fied  in the X.509 certificate along with the matching sign key.	 There
       are several schemes available in the  OpenSSL  software	library,  each
       identified  by  a  specific  string such as md5WithRSAEncryption, which
       stands for the MD5 message digest with RSA encryption scheme.  The cur-
       rent  NTP distribution supports all the schemes in the OpenSSL library,
       including those based on RSA and DSA digital signatures.

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

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

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

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

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

       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
       configuration  command  and  no key or autokey subcommands are present,
       the association is not authenticated; if the key subcommand is present,
       the  association is authenticated using the symmetric key ID specified;
       if the autokey subcommand is present, the association is	 authenticated
       using Autokey.

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

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

       Some  examples  may help to reduce confusion.  Client Alice has no spe-
       cific cryptotype selected.  Server Bob has both a  symmetric  key  file
       and  minimal Autokey files.  Alice's unauthenticated messages arrive at
       Bob, who replies with unauthenticated messages.	Cathy has  a  copy  of
       Bob's  symmetric key file and has selected key ID 4 in messages to Bob.
       Bob verifies the message with his key ID 4.  If it's the same  key  and
       the  message  is	 verified,  Bob sends Cathy a reply authenticated with
       that key.  If verification fails, Bob sends  Cathy  a  thing  called  a
       crypto-NAK,  which tells her something broke.  She can see the evidence
       using the ntpq(8) 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 authen-
       tication 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(1ntpkeygenmdoc) 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	gener-
       ated  by	 the  OpenSSL  utilities and certificates can be imported from
       public certificate authorities.	Note that symmetric keys are necessary
       for the ntpq(8) and ntpdc(8) utility programs.  The remaining files are
       necessary only for the Autokey protocol.

       Certificates imported from OpenSSL or  public  certificate  authorities
       have  certian  limitations.  The certificate should be in ASN.1 syntax,
       X.509 Version 3 format and encoded in PEM, which	 is  the  same	format
       used  by	 OpenSSL.   The	 overall  length of the certificate encoded in
       ASN.1 must not exceed 1024 bytes.  The subject distinguished name field
       (CN)  is	 the fully qualified name of the host on which it is used; the
       remaining subject fields are ignored.  The certificate extension fields
       must  not contain either a subject key identifier or a issuer key iden-
       tifier 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 65,535, 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-
	      mands:

	      cert file
		     Specifies	the  location of the required host public cer-
		     tificate file.  This overrides the link ntpkey_cert_host-
		     name in the keys directory.

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

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

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

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

	      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
	      containing the keys and key identifiers used by ntpd(8), ntpq(8)
	      and ntpdc(8) 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
	      /usr/local/etc/.

       requestkey key
	      Specifies	 the  key  identifier to use with the ntpdc(8) utility
	      program, which uses a  proprietary  protocol  specific  to  this
	      implementation of ntpd(8).  The key argument is a key identifier
	      for the trusted key, where the value can be in the  range	 1  to
	      65,535, 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;
	      however,	updating  some values is a relatively expensive opera-
	      tion.  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  authen-
	      tication	procedures  require  that  both	 the  local and remote
	      servers share the same key and key identifier for this  purpose,
	      although different keys can be used with different servers.  The
	      key arguments are 32-bit unsigned integers with values from 1 to
	      65,535.

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

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

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

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

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

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

       106    (mismatched digest types) Not used.

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

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

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

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

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

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

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

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

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

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

	      clockstats
		     Enables recording of clock driver statistics information.
		     Each update received from a clock driver appends  a  line
		     of	 the  following	 form to the file generation set named
		     clockstats:
			 49213 525.624 127.127.4.1 93 226 00:08:29.606 D

		     The first two fields show the date (Modified Julian  Day)
		     and  time	(seconds and fraction past UTC midnight).  The
		     next field shows the clock address in  dotted-quad	 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.

	      cryptostats
		     This  option  requires the OpenSSL cryptographic software
		     library.  It enables recording  of	 cryptographic	public
		     key  protocol  information.  Each message received by the
		     protocol module appends a line of the following  form  to
		     the file generation set named cryptostats:
			 49213 525.624 127.127.4.1 message

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

	      loopstats
		     Enables recording of loop filter statistics  information.
		     Each update of the local clock outputs a line of the fol-
		     lowing form to the file generation set named loopstats:
			 50935 75440.031 0.000006019 13.778190 0.000351733 0.0133806

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

	      peerstats
		     Enables recording of peer statistics  information.	  This
		     includes  statistics records of all peers of a NTP server
		     and of special signals,  where  present  and  configured.
		     Each valid update appends a line of the following form to
		     the current element of a file generation set named	 peer-
		     stats:
			 48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674

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

	      rawstats
		     Enables recording of  raw-timestamp  statistics  informa-
		     tion.  This includes statistics records of all peers of a
		     NTP server and of special signals, where present and con-
		     figured.	Each NTP message received from a peer or clock
		     driver appends a line of the following form to  the  file
		     generation set named rawstats:
			 50928 2132.543 128.4.1.1 128.4.1.20 3102453281.584327000 3102453281.58622800031 02453332.540806000 3102453332.541458000

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

	      sysstats
		     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 previous
			    packets sent

		     Current version 9546
			    Number of packets matching the  current  NTP  ver-
			    sion.

		     Previous version 56
			    Number  of	packets matching the previous NTP ver-
			    sion.

		     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-
			    tion.

	      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 con-
			    catenated elements prefix, filename and suffix:

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

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

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

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

			    none   The file set is  actually  a	 single	 plain
				   file.

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

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

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

			    month  One	generation  file set element is gener-
				   ated per month.  The file name suffix  con-
				   sists  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 run-
				   ning at  the	 start	of  the	 corresponding
				   24-hour  period.  Information is only writ-
				   ten to  a  file  generation	by  specifying
				   enable;  output  is prevented by specifying
				   disable.

		     link | nolink
			    It is convenient to be able to access the  current
			    element  of a file generation set by a fixed name.
			    This feature is enabled  by	 specifying  link  and
			    disabled  using  nolink.   If link is specified, a
			    hard link from the current file set element	 to  a
			    file  without  suffix  is  created.	 When there is
			    already a file with this name and  the  number  of
			    links of this file is one, it is renamed appending
			    a dot, the letter C, and the pid  of  the  ntpd(8)
			    server  process.   When  the  number  of  links is
			    greater than one,  the  file  is  unlinked.	  This
			    allows  the	 current file to be accessed by a 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  informa-
       tion  and  examples  can	 be found in the "Notes on Configuring NTP and
       Setting up a NTP Subnet" page (available as part of the HTML documenta-
       tion provided in /usr/share/doc/ntp).

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

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

   The Kiss-of-Death Packet
       Ordinarily, packets denied service are simply dropped with  no  further
       action  except  incrementing  statistics	 counters.   Sometimes	a more
       proactive response is needed, such as a server message that  explicitly
       requests	 the client to stop sending and leave a message for the system
       operator.  A special packet format has been created  for	 this  purpose
       called  the  "kiss-of-death"  (KoD)  packet.  KoD packets have the leap
       bits set unsynchronized and stratum set to zero and the reference iden-
       tifier  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	proto-
       col  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 probabil-
	      ity of discard for packets that overflow the  rate-control  win-
	      dow.

       restrict address [mask mask] [ippeerlimit int] [flag ...]
	      The  address  argument  expressed	 in  dotted-quad  form	is the
	      address of a host or network.  Alternatively, the address	 argu-
	      ment  can be a valid host DNS name.  The mask argument expressed
	      in dotted-quad form defaults to  255.255.255.255,	 meaning  that
	      the  address is treated as the address of an individual host.  A
	      default entry (address 0.0.0.0, mask 0.0.0.0) is always included
	      and  is  always  the  first  entry  in the list.	Note that text
	      string default, with no mask option, may be used to indicate the
	      default  entry.	The ippeerlimit directive limits the number of
	      peer requests for each IP to int, where  a  value	 of  -1	 means
	      "unlimited",  the	 current  default.  A value of 0 means "none".
	      There would usually be at most 1 peering request per IP, but  if
	      the  remote peering requests are behind a proxy there could well
	      be more than 1 per IP.   In  the	current	 implementation,  flag
	      always  restricts access, i.e., an entry with no flags indicates
	      that free access to the server is to be given.   The  flags  are
	      not  orthogonal,	in that more restrictive flags will often make
	      less restrictive ones redundant.	The  flags  can	 generally  be
	      classed  into  two categories, those which restrict time service
	      and those which restrict informational queries and  attempts  to
	      do  run-time  reconfiguration of the server.  One or more of the
	      following flags may be specified:

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

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

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

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

	      noepeer
		     Deny  ephemeral  peer requests, even if they come from an
		     authenticated source.  Note that the  ability  to	use  a
		     symmetric key for authentication may be restricted to one
		     or more IPs  or  subnets  via  the	 third	field  of  the
		     ntp.keys  file.   This  restriction  is  not  enabled  by
		     default,  to  maintain  backward  compatability.	Expect
		     noepeer to become the default in ntp-4.4.

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

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

	      nopeer Deny unauthenticated packets which would result in	 mobi-
		     lizing  a	new  association.  This includes broadcast and
		     symmetric active packets when  a  configured  association
		     does  not	exist.	It also includes pool associations, so
		     if you want to use servers from a pool directive and also
		     want  to  use  nopeer  by default, you'll want a restrict
		     source ...	 line as well that does not include the nopeer
		     directive.

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

	      notrap Decline to provide mode 6 control message trap service to
		     matching hosts.  The trap service is a subsystem  of  the
		     ntpq(8)  control  message	protocol which is intended for
		     use by remote event logging programs.

	      notrust
		     Deny  service  unless  the	 packet	 is  cryptographically
		     authenticated.

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

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

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

Automatic NTP Configuration Options
   Manycasting
       Manycasting is a automatic discovery and configuration paradigm new  to
       NTPv4.	It  is intended as a means for a multicast client to troll the
       nearby network neighborhood to find cooperating manycast servers, 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	 many-
       cast paradigm is designed to find a plurality of redundant servers sat-
       isfying defined optimality criteria.

       Manycasting can be used with either symmetric key or public  key	 cryp-
       tography.   The public key infrastructure (PKI) offers the best protec-
       tion 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 224.1.1.1  and  IPv6  address  FF05::101	 (site
       local)  for  NTP.  When more servers are needed, it broadcasts manycast
       client messages to this address at the minimum feasible rate and	 mini-
       mum  feasible  time-to-live  (TTL)  hops, depending on how many servers
       have already been found.	 There can be as many manycast client associa-
       tions  as different group address, each one serving as a template for a
       future ephemeral unicast client/server association.

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

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

       The manycast client polling strategy is designed to reduce as  much  as
       possible	 the  volume  of  manycast  client messages and the effects of
       implosion due to near-simultaneous arrival of manycast server messages.
       The  strategy is determined by the manycastclient, tos and ttl configu-
       ration commands.	 The manycast poll interval is	normally  eight	 times
       the  system poll interval, which starts out at the minpoll value speci-
       fied 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  associa-
       tions have been found.  Further retransmissions use the same TTL.

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

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

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

       It  is often useful to narrow the range of acceptable servers which can
       be found by manycast client  associations.   Because  manycast  servers
       respond	only  when  the client stratum is equal to or greater than the
       server stratum, primary (stratum 1)  servers  fill  find	 only  primary
       servers	in  TTL	 range,	 which	is probably the most common objective.
       However, unless configured otherwise, all manycast clients in TTL range
       will  eventually find all primary servers in TTL range, which is proba-
       bly 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  min-
       clock.

       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	 many-
       cast client and manycast server.	 A number of hosts configured this way
       and sharing a common group address will	automatically  organize	 them-
       selves 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, mul-
       ticast group address 239.1.1.1.	The only exception is that  each  pri-
       mary  server  configuration  file must include commands for the primary
       reference source such as a GPS receiver.

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

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

       Some  administrators  prefer  to avoid running ntpd(8) continuously and
       run either sntp(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	 many-
       cast  is	 with ntpd(8) -q.  The program wakes up, scans the local land-
       scape looking for the usual suspects, selects the best from  among  the
       rascals,	 sets  the  clock and then departs.  Servers do not have to be
       configured in advance and all clients throughout the network  can  have
       the same configuration file.

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

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

   Broadcast Options
       tos [bcpollbstep gate]
	      This command provides a way to delay, by the specified number of
	      broadcast	 poll  intervals, believing backward time steps from a
	      broadcast server.	 Broadcast time networks are  expected	to  be
	      trusted.	 In  the  event	 a  broadcast server's time is stepped
	      backwards, there is clear benefit to having the  clients	notice
	      this change as soon as possible.	Attacks such as replay attacks
	      can happen, however, and even though there are a number of  pro-
	      tections	built  in  to  broadcast  mode,	 attempts to perform a
	      replay attack are possible.  This value defaults to 0,  but  can
	      be changed to any number of poll intervals between 0 and 4.

   Manycast Options
       tos  [ceiling  ceiling | cohort { 0 | 1 } | floor floor | minclock min-
       clock | 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
		     15.

	      minclock minclock
		     The  clustering  algorithm	 repeatedly  casts out outlier
		     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 docu-
       mentation  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 avail-
       able 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	disci-
       pline/streams  modules  which  can  significantly  improve the accuracy
       using the driver.  These are described in  the  "Line  Disciplines  and
       Streams Drivers" page (available as part of the HTML documentation pro-
       vided in /usr/share/doc/ntp).

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

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

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

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

       The  stratum number of a reference clock is by default zero.  Since the
       ntpd(8) daemon adds one to the stratum of each peer, a  primary	server
       ordinarily  displays  an	 external stratum of one.  In order to provide
       engineered backups, it is often useful to specify the  reference	 clock
       stratum as greater than zero.  The stratum option is used for this 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  spe-
	      cial 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 "Miti-
		     gation Rules and the prefer Keyword" page	(available  as
		     part    of	   the	  HTML	 documentation	 provided   in
		     /usr/share/doc/ntp) for further information.

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

	      minpoll int

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

       fudge  127.127.t.u [time1 sec] [time2 sec] [stratum int] [refid string]
       [mode int] [flag1 0 | 1] [flag2 0 | 1] [flag3 0 | 1] [flag4 0 | 1]
	      This command can be used to configure reference clocks  in  spe-
	      cial  ways.  It must immediately follow the server command which
	      configures the driver.  Note that the same capability is	possi-
	      ble  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 Driv-
		     ers" page (available as part of  the  HTML	 documentation
		     provided in /usr/share/doc/ntp).

	      time2 secs
		     Specifies	a fixed-point decimal number in seconds, which
		     is	 interpreted  in  a  driver-dependent  way.   See  the
		     descriptions  of specific drivers in the "Reference Clock
		     Drivers" page (available as part of the  HTML  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
		     Options.

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

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

       driftfile driftfile
	      This command specifies the complete path and name	 of  the  file
	      used  to	record	the  frequency	of the local clock oscillator.
	      This is the same operation as the -f command  line  option.   If
	      the  file exists, it is read at startup in order to set the ini-
	      tial frequency and then updated once per hour with  the  current
	      frequency	 computed  by  the daemon.  If the file name is speci-
	      fied, 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 otherwise, should be avoided.

       dscp value
	      This option specifies the Differentiated Services Control	 Point
	      (DSCP) value, a 6-bit code.  The default value is 46, signifying
	      Expedited Forwarding.

       enable [auth | bclient | calibrate | kernel | mode7 | monitor |	ntp  |
       stats	 |    peer_clear_digest_early	 |    unpeer_crypto_early    |
       unpeer_crypto_nak_early | unpeer_digest_early]

       disable [auth | bclient | calibrate | kernel | mode7 | monitor | ntp  |
       stats	 |    peer_clear_digest_early	 |    unpeer_crypto_early    |
       unpeer_crypto_nak_early | unpeer_digest_early]
	      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 pro-
	      gram.

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

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

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

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

	      mode7  Enables processing of NTP mode 7  implementation-specific
		     requests  which  are used by the deprecated ntpdc(8) pro-
		     gram.  The default for this flag is disable.   This  flag
		     is	 excluded  from	 runtime  configuration using ntpq(8).
		     The ntpq(8) program provides  the	same  capabilities  as
		     ntpdc(8) using standard mode 6 requests.

	      monitor
		     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.

	      peer_clear_digest_early
		     By default, if ntpd(8) is using autokey and it receives a
		     crypto-NAK packet that passes the	duplicate  packet  and
		     origin  timestamp	checks	the peer variables are immedi-
		     ately cleared.  While this is generally a feature	as  it
		     allows  for quick recovery if a server key has changed, a
		     properly forged and  appropriately	 delivered  crypto-NAK
		     packet  can  be used in a DoS attack.  If you have active
		     noticable problems with this type of DoS attack then  you
		     should  consider  disabling  this	option.	 You can check
		     your peerstats file for evidence of any of these attacks.
		     The default for this flag is enable.

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

	      unpeer_crypto_early
		     By	 default,  if  ntpd(8) receives an autokey packet that
		     fails TEST9, a crypto failure, the association is immedi-
		     ately  cleared.   This is almost certainly a feature, but
		     if, in spite of the current recommendation of  not	 using
		     autokey,  you  are still using autokey and you are seeing
		     this sort of DoS attack disabling this  flag  will	 delay
		     tearing  down  the	 association  until  the  reachability
		     counter becomes zero.  You can check your peerstats  file
		     for  evidence  of	any of these attacks.  The default for
		     this flag is enable.

	      unpeer_crypto_nak_early
		     By default, if ntpd(8) receives a crypto-NAK packet  that
		     passes  the  duplicate packet and origin timestamp checks
		     the association is immediately cleared.   While  this  is
		     generally	a feature as it allows for quick recovery if a
		     server key has changed, a properly forged	and  appropri-
		     ately  delivered  crypto-NAK  packet can be used in a DoS
		     attack.  If you have active noticable problems with  this
		     type  of  DoS  attack  then you should consider disabling
		     this option.  You can check your peerstats file for  evi-
		     dence of any of these attacks.  The default for this flag
		     is enable.

	      unpeer_digest_early
		     By default, if ntpd(8) receives what should be an authen-
		     ticated packet that passes other packet sanity checks but
		     contains an invalid digest the association is immediately
		     cleared.	While this is generally a feature as it allows
		     for quick recovery, if this type of packet	 is  carefully
		     forged  and  sent	during an appropriate window it can be
		     used for a DoS attack.   If  you  have  active  noticable
		     problems  with  this  type	 of DoS attack then you should
		     consider disabling this option.  You can check your peer-
		     stats  file  for  evidence	 of any of these attacks.  The
		     default for this flag is enable.

       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,  com-
	      mand  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).

       interface [listen | ignore | drop] [all | ipv4 | ipv6 | wildcard name |
       address [/ prefixlen]]
	      The interface directive controls which network addresses ntpd(8)
	      opens,  and  whether  input  is dropped without processing.  The
	      first parameter determines the action for addresses which	 match
	      the second parameter.  The second parameter specifies a class of
	      addresses, or a specific interface name, or an address.  In  the
	      address  case, prefixlen determines how many bits must match for
	      this rule to apply.  ignore prevents opening matching addresses,
	      drop  causes  ntpd(8)  to open the address and drop all received
	      packets without examination.  Multiple interface directives  can
	      be  used.	  The  last  rule  which  matches a particular address
	      determines the action for it.  interface directives are disabled
	      if  any  -I,  --interface,  -L,  or  --novirtualips command-line
	      options are specified in the configuration file,	all  available
	      network addresses are opened.  The nic directive is an alias for
	      interface.

       leapfile leapfile
	      This command loads the IERS leapseconds file and initializes the
	      leapsecond  values for the next leapsecond event, leapfile expi-
	      ration time, and TAI offset.  The file can be obtained  directly
	      from the IERS at https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-
	      seconds.list  or	 ftp://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-
	      seconds.list.   The  leapfile  is scanned when ntpd(8) processes
	      the leapfile directive or when ntpd detects  that	 the  leapfile
	      has  changed.  ntpd checks once a day to see if the leapfile has
	      changed.	The update-leap(1update_leapmdoc) script can be run to
	      see if the leapfile should be updated.

       leapsmearinterval seconds
	      This  EXPERIMENTAL option is only available if ntpd(8) was built
	      with the --enable-leap-smear option to the configure script.  It
	      specifies	 the interval over which a leap second correction will
	      be applied.  Recommended values for this option are between 7200
	      (2  hours)  and  86400 (24 hours).  See http://bugs.ntp.org/2855
	      for more information.

       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 sys-
	      log(3) priority mask, `+' adds and `-' removes  messages.	  sys-
	      log(3)  messages can be controlled in four classes (clock, peer,
	      sys and sync).  Within these classes four types of messages  can
	      be  controlled:  informational  messages	(info), event messages
	      (events), statistics messages (statistics) and  status  messages
	      (status).

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

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

	      This configuration will list all clock information and  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.

       mru [maxdepth count | maxmem kilobytes | mindepth count |  maxage  sec-
       onds  |	initialloc count | initmem kilobytes | incalloc count | incmem
       kilobytes]
	      Controls size limite of the monitoring facility's Most  Recently
	      Used  (MRU)  list of client addresses, which is also used by the
	      rate control facility.

	      maxdepth count

	      maxmem kilobytes
		     Equivalent upper limits on the size of the MRU  list,  in
		     terms  of entries or kilobytes.  The acutal limit will be
		     up to incalloc entries or incmem  kilobytes  larger.   As
		     with  all	of the mru options offered in units of entries
		     or kilobytes, if both maxdepth and maxmem are  used,  the
		     last one used controls.  The default is 1024 kilobytes.

	      mindepth count
		     Lower  limit on the MRU list size.	 When the MRU list has
		     fewer than mindepth entries, existing entries  are	 never
		     removed  to make room for newer ones, regardless of their
		     age.  The default is 600 entries.

	      maxage seconds
		     Once the MRU list has mindepth entries and an  additional
		     client  is	 to  ba added to the list, if the oldest entry
		     was updated more than maxage seconds ago, that  entry  is
		     removed  and  its storage is reused.  If the oldest entry
		     was updated more recently the MRU list is grown,  subject
		     to maxdepth / moxmem.  The default is 64 seconds.

	      initalloc count

	      initmem kilobytes
		     Initial  memory  allocation at the time the monitoringfa-
		     cility is first  enabled,	in  terms  of  the  number  of
		     entries or kilobytes.  The default is 4 kilobytes.

	      incalloc count

	      incmem kilobytes
		     Size  of  additional  memory allocations when growing the
		     MRU list, in entries or  kilobytes.   The	default	 is  4
		     kilobytes.

       nonvolatile threshold
	      Specify  the  threshold delta in seconds before an hourly change
	      to the driftfile	(frequency  file)  will	 be  written,  with  a
	      default  value  of  1e-7	(0.1  PPM).   The  frequency  file  is
	      inspected each hour.  If the difference between the current fre-
	      quency  and  the	last  value written exceeds the threshold, the
	      file is written and the  threshold  becomes  the	new  threshold
	      value.   If  the	threshold  is  not exceeeded, it is reduced by
	      half.  This is intended to reduce the number of file writes  for
	      embedded systems with nonvolatile memory.

       phone dial ...
	      This  command  is used in conjunction with the ACTS modem driver
	      (type 18) or the JJY driver (type 40, mode 100 - 180).  For  the
	      ACTS  modem driver (type 18), the arguments consist of a maximum
	      of 10 telephone numbers used to dial  USNO,  NIST,  or  European
	      time  service.  For the JJY driver (type 40 mode 100 - 180), the
	      argument is one telephone number used to dial the telephone  JJY
	      service.	 The  Hayes  command ATDT is normally prepended to the
	      number.  The number can contain other  modem  control  codes  as
	      well.

       reset [allpeers] [auth] [ctl] [io] [mem] [sys] [timer]
	      Reset  one  or  more  groups  of counters maintained by ntpd and
	      exposed by ntpq and ntpdc.

       rlimit [memlock Nmegabytes | stacksize N4kPages	filenum	 Nfiledescrip-
       tors]

	      memlock Nmegabytes
		     Specify  the number of megabytes of memory that should be
		     allocated and  locked.   Probably	only  available	 under
		     Linux,  this option may be useful when dropping root (the
		     -i option).  The default is  32  megabytes	 on  non-Linux
		     machines,	and -1 under Linux.  -1 means "do not lock the
		     process into memory".  0 means "lock whatever memory  the
		     process wants into memory".

	      stacksize N4kPages
		     Specifies	the  maximum size of the process stack on sys-
		     tems with the mlockall() function.	  Defaults  to	50  4k
		     pages (200 4k pages in OpenBSD).

	      filenum Nfiledescriptors
		     Specifies the maximum number of file descriptors ntpd may
		     have open at once.	 Defaults to the system default.

       saveconfigdir directory_path
	      Specify the directory in which to write configuration  snapshots
	      requested	 with  saveconfig  command.  If saveconfigdir does not
	      appear  in  the  configuration  file,  saveconfig	 requests  are
	      rejected by ntpd.

       saveconfig filename
	      Write the current configuration, including any runtime modifica-
	      tions given with :config or config-from-file to the ntpd	host's
	      filename	in  the	 saveconfigdir.	 This command will be rejected
	      unless the  saveconfigdir	 directive  appears  in	 configuration
	      file.  filename can use strftime(3) format directives to substi-
	      tute  the	 current  date	and  time,   for   example,   savecon-
	      fig ntp-%Y%m%d-%H%M%S.conf.   The filename used is stored in the
	      system variable savedconfig.  Authentication is required.

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

       sysinfo
	      Display operational summary.

       sysstats
	      Show statistics counters maintained in the protocol module.

       tinker  [allan  allan  |	 dispersion  dispersion | freq freq | huffpuff
       huffpuff | panic panic |	 step  step  |	stepback  stepback  |  stepfwd
       stepfwd | 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  pre-
	      dict  and some combinations can result in some very nasty behav-
	      ior.  Very rarely is it necessary to change the default  values;
	      but, some folks cannot resist twisting the knobs anyway and this
	      command is for them.  Emphasis added: twisters are on their  own
	      and can expect no help from the support group.

	      The variables operate as follows:

	      allan allan
		     The  argument becomes the new value for the minimum Allan
		     intercept, which is a parameter of the PLL/FLL clock 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
		     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.

	      stepback stepback
		     The argument is  the  step	 threshold  for	 the  backward
		     direction, which by default is 0.128 s.  It can be set to
		     any positive number in seconds.  If both the forward  and
		     backward  step  thresholds	 are set to zero, step adjust-
		     ments will never occur.  Note: The kernel time discipline
		     is	 disabled  if  each  direction	of  step threshold are
		     either set to zero or greater than .5 second.

	      stepfwd stepfwd
		     As for stepback, but for the forward direction.

	      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-
		     pressed.

       writevar assocID name = value [,...]
	      Write  (create or update) the specified variables.  If the asso-
	      cID is zero, the variablea re from  the  system  variables  name
	      space,  otherwise	 they  are from the peer variables name space.
	      The assocID is required, as the same name can occur in both name
	      spaces.

       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	speci-
	      fied,  the  message  is  sent with a source address of the local
	      interface the message is sent through.  Note that	 on  a	multi-
	      homed  host  the	interface used may vary from time to time with
	      routing changes.

       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.

	      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, configur-
	      ing 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.

OPTIONS
       --help Display usage information and exit.

       --more-help
	      Pass the extended usage information through a pager.

       --version [{v|c|n}]
	      Output version of program and exit.  The default mode is `v',  a
	      simple  version.	 The `c' mode will print copyright information
	      and `n' will print the full copyright notice.

OPTION PRESETS
       Any option that is not marked as not presettable may be preset by load-
       ing values from environment variables named:
	 NTP_CONF_<option-name> or NTP_CONF

ENVIRONMENT
       See OPTION PRESETS for configuration environment variables.

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

EXIT STATUS
       One of the following exit values will be returned:

       0  (EXIT_SUCCESS)
	      Successful program execution.

       1  (EXIT_FAILURE)
	      The operation failed or the command syntax was not valid.

       70  (EX_SOFTWARE)
	      libopts  had an internal operational error.  Please report it to
	      autogen-users@lists.sourceforge.net.  Thank you.

SEE ALSO
       ntpd(8), ntpdc(8), ntpq(8)

       In addition to the manual pages provided,  comprehensive	 documentation
       is  available on the world wide web at http://www.ntp.org/.  A snapshot
       of   this   documentation   is	available   in	  HTML	  format    in
       /usr/share/doc/ntp.  David L. Mills, Network Time Protocol (Version 4),
       RFC5905

AUTHORS
       The University of Delaware and Network Time Foundation

COPYRIGHT
       Copyright (C) 1992-2017 The University of  Delaware  and	 Network  Time
       Foundation  all	rights	reserved.   This program is released under the
       terms of the NTP license, <http://ntp.org/license>.

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

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

       Please send bug reports to: http://bugs.ntp.org, bugs@ntp.org

NOTES
       This document was derived from FreeBSD.

       This  manual  page  was AutoGen-erated from the ntp.conf option defini-
       tions.



4.2.8p12			  14 Aug 2018			   ntp.conf(5)