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<h3>Access Control Commands and Options</h3>
<img src="pic/pogo6.gif" alt="gif"
align="left"><a href="http://www.eecis.udel.edu/~mills/pictures.html">from <i>Pogo</i>,
Walt Kelly</a>
<p>The skunk watches for intruders and sprays.</p>
<p>Last update: <!-- #BeginDate format:En2m -->7-Jan-2018 23:56<!-- #EndDate
  --> UTC</p>
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<h4>Related Links</h4>
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<h4>Commands and Options</h4>
<p>Unless noted otherwise, further information about these ccommands is on
the <a href="accopt.html">Access Control Support</a> page.</p>
<dl>
  <dt id="discard"><tt>discard [ average <i>avg</i> ][ minimum <i>min</i> ]
      [ monitor <i>prob</i> ]</tt></dt>
  <dd>Set the parameters of the rate control facility which protects the
    server from client abuse. If the <tt>limited</tt> flag is present in the
    ACL, packets that violate these limits are discarded. If, in addition,
    the <tt>kod</tt> flag is present, a kiss-o'-death packet is
    returned. See the <a href="rate.html">Rate Management</a> page for
    further information. The options are:
    <dl>
      <dt><tt>average <i>avg</i></tt></dt>
      <dd>Specify the minimum average interpacket spacing (minimum average
        headway time) in log<sub>2</sub> s with default 3.</dd>
      <dt><tt>minimum <i>min</i></tt></dt>
      <dd>Specify the minimum interpacket spacing (guard time) in seconds
	with default 2.</dd>
      <dt><tt>monitor</tt></dt>
      <dd>Specify the probability of being recorded for packets that
	overflow the MRU list size limit set by <tt>mru maxmem</tt>
	or <tt>mru maxdepth</tt>. This is a performance optimization for
	servers with aggregate arrivals of 1000 packets per second or
	more.</dd>
    </dl>
  </dd>
  <dt id="restrict"><tt>restrict [-4 | -6] default [ippeerlimit <i>num</i>]
      [<i>flag</i>][...]</tt></dt>
  <dt><tt>restrict source [ippeerlimit <i>num</i>]
      [<i>flag</i>][...]</tt></dt>
  <dt><tt>restrict <i>address</i> [mask <i>mask</i>]
      [ippeerlimit <i>num</i>] [<i>flag</i>][...]</tt></dt>
  <dd>The <tt><i>address</i></tt> argument expressed in IPv4 or IPv6 numeric
    address form is the address of a host or network. Alternatively,
    the <tt><i>address</i></tt> argument can be a valid host DNS
    name. The <tt><i>mask</i></tt> argument expressed in IPv4 or IPv6
    numeric address form defaults to all mask bits on, meaning that
    the <tt><i>address</i></tt> is treated as the address of an individual
    host. A default entry (address 0.0.0.0, mask 0.0.0.0 for IPv4 and
    address :: mask :: for IPv6) is always the first entry in the
    list. <tt>restrict default</tt>, with no mask option, modifies both IPv4
    and IPv6 default entries. <tt>restrict source</tt> configures a template
    restriction automatically added at runtime for each association, whether
    configured, ephemeral, or preemptible, and removed when the association
    is demobilized.</dd>
  <dd>The optional <tt>ippeerlimit</tt> takes a numeric argument that
    indicates how many incoming (at present) peer requests will be permitted
    for each IP, regardless of whether or not the request comes from an
    authenticated source.  A value of -1 means "unlimited", which is the
    current default.  A value of 0 means "none".  Ordinarily one would
    expect at most 1 of these sessions to exist per IP, however if the
    remote side is operating thru a proxy there would be one association for
    each remote peer at that IP.</dd>
  <dd>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
    informational queries and attempts to do run-time reconfiguration of the
    server. One or more of the following flags may be specified:</dd>
  <dd>
    <dl>
      <dt><tt>flake</tt></dt>
      <dd>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 <i>flakeway</i>, which
	once did a similar thing for early Internet testing.</dd>
      <dt><tt>ignore</tt></dt>
      <dd>Deny packets of all kinds, including <tt>ntpq</tt>
	and <tt>ntpdc</tt> queries.</dd>
      <dt><tt>kod</tt></dt>
      <dd>Send a kiss-o'-death (KoD) packet if the <tt>limited</tt> flag is
	present and a packet violates the rate limits established by
	the <tt>discard</tt> command. KoD packets are themselves rate
	limited for each source address separately. If the <tt>kod</tt> flag
	is used in a restriction which does not have the <tt>limited</tt>
	flag, no KoD responses will result.</dd>
      <dt id="limited"><tt>limited</tt></dt>
      <dd>Deny time service if the packet violates the rate limits
	established by the <tt>discard</tt> command. This does not apply
	to <tt>ntpq</tt> and <tt>ntpdc</tt> queries.</dd>
      <dt><tt>lowpriotrap</tt></dt>
      <dd>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.</dd>
      <dt><tt>mssntp</tt></dt>
      <dd>Enable Microsoft Windows MS-SNTP authentication using Active
	Directory services. <span class="style1"><b>Note: Potential users
	should be aware that these services involve a TCP connection to
	another process that could potentially block, denying services to
	other users. Therefore, this flag should be used only for a
	dedicated server with no clients other than MS-SNTP.</b></span></dd>
      <dt><tt>noepeer</tt></dt>
      <dd>Deny packets that would mobilize an ephemeral peering association,
	even if authenticated.</dd>
      <dt><tt>nomodify</tt></dt>
      <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries which attempt to
	modify the state of the server (i.e., run time
	reconfiguration). Queries which return information are
	permitted.</dd>
      <dt><tt>noquery</tt></dt>
      <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries. Time service is not
	affected.</dd>
      <dt><tt>nopeer</tt></dt>
      <dd>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. It also includes <tt>pool</tt> associations, so if you want
	to use servers from a <tt>pool</tt> directive and also want to
	use <tt>nopeer</tt> by default, you'll want a <tt>"restrict source
	..."</tt> line as well that does <i>not</i> include
	the <tt>nopeer</tt> directive.  Note that this flag does not apply
	to packets that do not attempt to mobilize an association. </dd>
      <dt><tt>noserve</tt></dt>
      <dd>Deny all packets except <tt>ntpq</tt> and <tt>ntpdc</tt>
	queries.</dd>
      <dt><tt>notrap</tt></dt>
      <dd>Decline to provide mode 6 control message trap service to matching
	hosts. The trap service is a subsystem of the <tt>ntpdc</tt> control
	message protocol which is intended for use by remote event logging
	programs.</dd>
      <dt><tt>notrust</tt></dt>
      <dd>Deny packets that are not cryptographically authenticated. Note
	carefully how this flag interacts with the <tt>auth</tt> option of
	the <tt>enable</tt> and <tt>disable</tt> commands. If <tt>auth</tt>
	is enabled, which is the default, authentication is required for all
	packets that might mobilize an association. If <tt>auth</tt> is
	disabled, but the <tt>notrust</tt> flag is not present, an
	association can be mobilized whether or not
	authenticated. If <tt>auth</tt> is disabled, but
	the <tt>notrust</tt> flag is present, authentication is required
	only for the specified address/mask range. </dd>
      <dt><tt>ntpport</tt></dt>
      <dd>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). A restrict line containing <tt>ntpport</tt> is
        considered more specific than one with the same address and mask,
        but lacking <tt>ntpport</tt>.</dd>
      <dt><tt>serverresponse fuzz</tt></dt>
      <dd>When reponding to server requests, fuzz the low order bits of
	the <tt>reftime</tt>.</dd>
      <dt><tt>version</tt></dt>
      <dd>Deny packets that do not match the current NTP version.</dd>
    </dl>
  </dd>
  <dd>Default restriction list entries with the flags <tt>ignore,
      ntpport</tt>, for each of the local host's interface addresses are
      inserted into the table at startup to prevent the server from
      attempting to synchronize to its own time. A default entry is also
      always present, though if it is otherwise unconfigured; no flags are
      associated with the default entry (i.e., everything besides your own
      NTP server is unrestricted).</dd>
</dl>
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