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ntp_acc(5)		      File Formats Manual		    ntp_acc(5)

       ntp_acc - Access Control Options

       The  ntpd daemon implements a general purpose access control list (ACL)
       containing address/match entries sorted	first  by  increasing  address
       values and then by increasing mask values. A match occurs when the bit-
       wise AND of the mask and the packet source address is equal to the bit-
       wise  AND  of the mask and address in the list. The list is searched in
       order with the last match found defining the restriction flags  associ-
       ated with the entry.

       An  example  may	 clarify how it works. Our campus has two class-B net-
       works, 128.4 for the ECE and CIS departments and 128.175 for  the  rest
       of  campus. Let's assume (not true!) that subnet 128.4.1 homes critical
       services like class rosters and spread sheets. A suitable ACL might be

       restrict default nopeer			    # deny new associations
       restrict mask	    # allow campus access
       restrict mask none	    # allow ECE and CIS access
       restrict mask notrust # require authentication on subnet 1
       restrict time.nist.gov				 # allow access

       While this facility may be useful for keeping unwanted, broken or mali-
       cious  clients  from congesting innocent servers, it should not be con-
       sidered an alternative to the  NTP  authentication  facilities.	Source
       address	based  restrictions  are  easily  circumvented by a determined

       discard [ average avg ][ minimum min ] [ monitor prob ]
	       Set the parameters of the rate control facility which  protects
	       the server from client abuse. If the limited flag is present in
	       the ACL, packets that violate these limits are discarded. If in
	       addition the kod restriction is present, a kiss-o'-death packet
	       is returned.

	       average avg
		       Specify the minimum average interpacket spacing	(mini-
		       mum average headway time) in log2 s with default 3.

	       minimum min
		       Specify the minimum interpacket spacing (guard time) in
		       log2 s with default 1.

	       monitor Specify the probability of  discard  for	 packets  that
		       overflow the rate-control window. This is a performance
		       optimization for servers	 with  aggregate  arrivals  of
		       1000 packets per second or more.

       restrict address [mask mask] [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, but it must be resolvable at
	       the time when ntpd is started and if it's resolved to  multiple
	       addresses,  only	 the  first address will be added to the list.
	       The mask argument expressed in  dotted-quad  form  defaults  to,	 meaning  that	the  address is treated as the
	       address	of  an	individual  host.  A  default  entry  (address,	 mask	 is  always included and is always the
	       first entry in the list. Note that  the	text  string  default,
	       with no mask option, may be used to indicate the default entry.
	       Some flags have the effect  to  deny  service,  some  have  the
	       effect  to  enable  service  and	 some are conditioned by other
	       flags. The flags. are not orthogonal, in that more  restrictive
	       flags  will  often  make	 less  restrictive ones redundant. The
	       flags that deny service are classed in  two  categories,	 those
	       that  restrict  time  service  and those that restrict informa-
	       tional queries and attempts to do run-time  reconfiguration  of
	       the  server.  One  or more of the following flags may be speci-

	       flake   Discard received NTP packets with probability 0.1; that
		       is,  on	average	 drop  one  packet in ten. This is for
		       testing and amusement. The name comes from Bob Braden's
		       flakeway,  which	 once  did  a  similar thing for early
		       Internet testing.

	       ignore  Deny packets of all kinds,  including  ntpq  and	 ntpdc

	       kod     Send  a	kiss-o'-death (KoD) packet if the limited flag
		       is present and a packet violates the rate limits estab-
		       lished  by  the	discard command. KoD packets are them-
		       selves rate limited for each source address separately.
		       If  this	 flag is not present, packets that violate the
		       rate limits are discarded.

	       limited Deny time service if the packet violates the rate  lim-
		       its  established	 by the discard command. This does not
		       apply to ntpq and ntpdc queries.

		       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

	       mssntp  Enable  Microsoft  Windows MS-SNTP authentication using
		       Active Directory services. Note: Potential users should
		       be  aware  that these services involve a TCP connection
		       to another process that could potentially block,	 deny-
		       ing  services  to  other	 users.	 Therefore,  this flag
		       should be used only for	a  dedicated  server  with  no
		       clients other than MS-SNTP.

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

	       noquery Deny  ntpq  and	ntpdc  queries.	 Time  service	is not

	       nopeer  Deny packets that might mobilize an association	unless
		       authenticated.	This  includes	broadcast,  symmetric-
		       active and manycast server packets  when	 a  configured
		       association  does  not  exist. Note that this flag does
		       not apply to packets that do not attempt to mobilize an

	       noserve Deny all packets except ntpq and ntpdc queries.

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

	       notrust Deny packets that are not  cryptographically  authenti-
		       cated.  Note carefully how this flag interacts with the
		       auth option of the enable and disable commands. If auth
		       is  enabled,  which  is	the default, authentication is
		       required for all packets that might mobilize an associ-
		       ation. If auth is disabled, but the notrust flag is not
		       present, an association can be mobilized whether or not
		       authenticated.  If  auth	 is  disabled, but the notrust
		       flag is present, authentication is  required  only  for
		       the specified address/mask range.


		       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

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

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


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