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16 <h3>Access Control Commands and Options</h3>
17 <img src="pic/pogo6.gif" alt="gif"
18 align="left"><a href="http://www.eecis.udel.edu/~mills/pictures.html">from <i>Pogo</i>,
20 <p>The skunk watches for intruders and sprays.</p>
21 <p>Last update: <!-- #BeginDate format:En2m -->7-Jan-2018 23:56<!-- #EndDate
24 <h4>Related Links</h4>
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30 <h4>Commands and Options</h4>
31 <p>Unless noted otherwise, further information about these ccommands is on
32 the <a href="accopt.html">Access Control Support</a> page.</p>
34 <dt id="discard"><tt>discard [ average <i>avg</i> ][ minimum <i>min</i> ]
35 [ monitor <i>prob</i> ]</tt></dt>
36 <dd>Set the parameters of the rate control facility which protects the
37 server from client abuse. If the <tt>limited</tt> flag is present in the
38 ACL, packets that violate these limits are discarded. If, in addition,
39 the <tt>kod</tt> flag is present, a kiss-o'-death packet is
40 returned. See the <a href="rate.html">Rate Management</a> page for
41 further information. The options are:
43 <dt><tt>average <i>avg</i></tt></dt>
44 <dd>Specify the minimum average interpacket spacing (minimum average
45 headway time) in log<sub>2</sub> s with default 3.</dd>
46 <dt><tt>minimum <i>min</i></tt></dt>
47 <dd>Specify the minimum interpacket spacing (guard time) in seconds
49 <dt><tt>monitor</tt></dt>
50 <dd>Specify the probability of being recorded for packets that
51 overflow the MRU list size limit set by <tt>mru maxmem</tt>
52 or <tt>mru maxdepth</tt>. This is a performance optimization for
53 servers with aggregate arrivals of 1000 packets per second or
57 <dt id="restrict"><tt>restrict [-4 | -6] default [ippeerlimit <i>num</i>]
58 [<i>flag</i>][...]<br> restrict source [ippeerlimit <i>num</i>]
59 [<i>flag</i>][...]<br> restrict <i>address</i> [mask <i>mask</i>]
60 [ippeerlimit <i>num</i>] [<i>flag</i>][...]</tt></dt>
61 <dd>The <tt><i>address</i></tt> argument expressed in IPv4 or IPv6 numeric
62 address form is the address of a host or network. Alternatively,
63 the <tt><i>address</i></tt> argument can be a valid host DNS
64 name. The <tt><i>mask</i></tt> argument expressed in IPv4 or IPv6
65 numeric address form defaults to all mask bits on, meaning that
66 the <tt><i>address</i></tt> is treated as the address of an individual
67 host. A default entry (address 0.0.0.0, mask 0.0.0.0 for IPv4 and
68 address :: mask :: for IPv6) is always the first entry in the
69 list. <tt>restrict default</tt>, with no mask option, modifies both IPv4
70 and IPv6 default entries. <tt>restrict source</tt> configures a template
71 restriction automatically added at runtime for each association, whether
72 configured, ephemeral, or preemptible, and removed when the association
74 <dd>The optional <tt>ippeerlimit</tt> takes a numeric argument that
75 indicates how many incoming (at present) peer requests will be permitted
76 for each IP, regardless of whether or not the request comes from an
77 authenticated source. A value of -1 means "unlimited", which is the
78 current default. A value of 0 means "none". Ordinarily one would
79 expect at most 1 of these sessions to exist per IP, however if the
80 remote side is operating thru a proxy there would be one association for
81 each remote peer at that IP.</dd>
82 <dd>Some flags have the effect to deny service, some have the effect to
83 enable service and some are conditioned by other flags. The flags are
84 not orthogonal, in that more restrictive flags will often make less
85 restrictive ones redundant. The flags that deny service are classed in
86 two categories, those that restrict time service and those that restrict
87 informational queries and attempts to do run-time reconfiguration of the
88 server. One or more of the following flags may be specified:</dd>
91 <dt><tt>flake</tt></dt>
92 <dd>Discard received NTP packets with probability 0.1; that is, on
93 average drop one packet in ten. This is for testing and
94 amusement. The name comes from Bob Braden's <i>flakeway</i>, which
95 once did a similar thing for early Internet testing.</dd>
96 <dt><tt>ignore</tt></dt>
97 <dd>Deny packets of all kinds, including <tt>ntpq</tt>
98 and <tt>ntpdc</tt> queries.</dd>
100 <dd>Send a kiss-o'-death (KoD) packet if the <tt>limited</tt> flag is
101 present and a packet violates the rate limits established by
102 the <tt>discard</tt> command. KoD packets are themselves rate
103 limited for each source address separately. If the <tt>kod</tt> flag
104 is used in a restriction which does not have the <tt>limited</tt>
105 flag, no KoD responses will result.</dd>
106 <dt id="limited"><tt>limited</tt></dt>
107 <dd>Deny time service if the packet violates the rate limits
108 established by the <tt>discard</tt> command. This does not apply
109 to <tt>ntpq</tt> and <tt>ntpdc</tt> queries.</dd>
110 <dt><tt>lowpriotrap</tt></dt>
111 <dd>Declare traps set by matching hosts to be low priority. The number
112 of traps a server can maintain is limited (the current limit is
113 3). Traps are usually assigned on a first come, first served basis,
114 with later trap requestors being denied service. This flag modifies
115 the assignment algorithm by allowing low priority traps to be
116 overridden by later requests for normal priority traps.</dd>
117 <dt><tt>mssntp</tt></dt>
118 <dd>Enable Microsoft Windows MS-SNTP authentication using Active
119 Directory services. <span class="style1"><b>Note: Potential users
120 should be aware that these services involve a TCP connection to
121 another process that could potentially block, denying services to
122 other users. Therefore, this flag should be used only for a
123 dedicated server with no clients other than MS-SNTP.</b></span></dd>
124 <dt><tt>noepeer</tt></dt>
125 <dd>Deny packets that would mobilize an ephemeral peering association,
126 even if authenticated.</dd>
127 <dt><tt>nomodify</tt></dt>
128 <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries which attempt to
129 modify the state of the server (i.e., run time
130 reconfiguration). Queries which return information are
132 <dt><tt>noquery</tt></dt>
133 <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries. Time service is not
135 <dt><tt>nopeer</tt></dt>
136 <dd>Deny packets that might mobilize an association unless
137 authenticated. This includes broadcast, symmetric-active and
138 manycast server packets when a configured association does not
139 exist. It also includes <tt>pool</tt> associations, so if you want
140 to use servers from a <tt>pool</tt> directive and also want to
141 use <tt>nopeer</tt> by default, you'll want a <tt>"restrict source
142 ..."</tt> line as well that does <i>not</i> include
143 the <tt>nopeer</tt> directive. Note that this flag does not apply
144 to packets that do not attempt to mobilize an association. </dd>
145 <dt><tt>noserve</tt></dt>
146 <dd>Deny all packets except <tt>ntpq</tt> and <tt>ntpdc</tt>
148 <dt><tt>notrap</tt></dt>
149 <dd>Decline to provide mode 6 control message trap service to matching
150 hosts. The trap service is a subsystem of the <tt>ntpdc</tt> control
151 message protocol which is intended for use by remote event logging
153 <dt><tt>notrust</tt></dt>
154 <dd>Deny packets that are not cryptographically authenticated. Note
155 carefully how this flag interacts with the <tt>auth</tt> option of
156 the <tt>enable</tt> and <tt>disable</tt> commands. If <tt>auth</tt>
157 is enabled, which is the default, authentication is required for all
158 packets that might mobilize an association. If <tt>auth</tt> is
159 disabled, but the <tt>notrust</tt> flag is not present, an
160 association can be mobilized whether or not
161 authenticated. If <tt>auth</tt> is disabled, but
162 the <tt>notrust</tt> flag is present, authentication is required
163 only for the specified address/mask range. </dd>
164 <dt><tt>ntpport</tt></dt>
165 <dd>This is actually a match algorithm modifier, rather than a
166 restriction flag. Its presence causes the restriction entry to be
167 matched only if the source port in the packet is the standard NTP
168 UDP port (123). A restrict line containing <tt>ntpport</tt> is
169 considered more specific than one with the same address and mask,
170 but lacking <tt>ntpport</tt>.</dd>
171 <dt><tt>version</tt></dt>
172 <dd>Deny packets that do not match the current NTP version.</dd>
175 <dd>Default restriction list entries with the flags <tt>ignore,
176 ntpport</tt>, for each of the local host's interface addresses are
177 inserted into the table at startup to prevent the server from
178 attempting to synchronize to its own time. A default entry is also
179 always present, though if it is otherwise unconfigured; no flags are
180 associated with the default entry (i.e., everything besides your own
181 NTP server is unrestricted).</dd>
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