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<h3>Access Control 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 -->30-Sep-2009  17:16<!-- #EndDate -->
		UTC</p>
<br clear="left">

<h4>Related Links</h4>

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<h4>Table of Contents</h4>

<ul>
<li class="inline"><a href="#acx">Access Control Support</a></li>
<li class="inline"><a href="#cmd">Access Control Commands</a></li>
</ul>

<hr>

<h4 id="acx">Access Control Support</h4>

<p>The <tt>ntpd</tt> 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 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.</p>

<p>An example may clarify how it works. Our campus has two class-B networks,
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</p>
<pre>
restrict default nopeer					# deny new associations
restrict 128.175.0.0 mask 255.255.0.0 		# allow campus access
restrict 128.4.0.0 mask 255.255.0.0 none	# allow ECE and CIS access
restrict 128.4.1.0 mask 255.255.255.0 notrust # require authentication on subnet 1
restrict time.nist.gov						# allow access
</pre>

<p>While this facility may be useful for keeping unwanted, 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.</p>

<h4 id="cmd">Access Control Commands</h4>

<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> restriction
	is present, a kiss-o'-death packet is returned.</dd>

<dd><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 log<sub>2</sub> s
	with default 1.</dd>

<dt><tt>monitor</tt></dt>
<dd>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.</dd>

</dl></dd>

<dt id="restrict"><tt>restrict <i>address</i> [mask <i>mask</i>] [<i>flag</i>][...]</tt></dt>
<dd>The <tt><i>address</i></tt> argument expressed in dotted-quad 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
	dotted-quad form defaults to 255.255.255.255, 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) is always included and is always the first entry in the list.
	Note that the text string <tt>default</tt>, with no mask option, may be used
	to indicate the default entry.</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 this flag is not present, packets that violate the rate limits are discarded.</dd>

<dt><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">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.</span></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. 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>
			<dt><tt>non-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). Both <tt>ntpport</tt> and <tt>non-ntpport</tt> may
				be specified. The <tt>ntpport</tt> is considered more specific and is sorted
				later in the list.</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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