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21: <h3>Access Control Options</h3>
22:
23: <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>
24:
25: <p>The skunk watches for intruders and sprays.</p>
26: <p>Last update:
27: <!-- #BeginDate format:En2m -->30-Sep-2009 17:16<!-- #EndDate -->
28: UTC</p>
29: <br clear="left">
30:
31: <h4>Related Links</h4>
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35:
36: <h4>Table of Contents</h4>
37:
38: <ul>
39: <li class="inline"><a href="#acx">Access Control Support</a></li>
40: <li class="inline"><a href="#cmd">Access Control Commands</a></li>
41: </ul>
42:
43: <hr>
44:
45: <h4 id="acx">Access Control Support</h4>
46:
47: <p>The <tt>ntpd</tt> daemon implements a general purpose access control list
48: (ACL) containing address/match entries sorted first by increasing address
49: values and then by increasing mask values. A match occurs when the bitwise
50: AND of the mask and the packet source address is equal to the bitwise AND of
51: the mask and address in the list. The list is searched in order with the last
52: match found defining the restriction flags associated with the entry.</p>
53:
54: <p>An example may clarify how it works. Our campus has two class-B networks,
55: 128.4 for the ECE and CIS departments and 128.175 for the rest of campus.
56: Let's assume (not true!) that subnet 128.4.1 homes critical services like class
57: rosters and spread sheets. A suitable ACL might be</p>
58: <pre>
59: restrict default nopeer # deny new associations
60: restrict 128.175.0.0 mask 255.255.0.0 # allow campus access
61: restrict 128.4.0.0 mask 255.255.0.0 none # allow ECE and CIS access
62: restrict 128.4.1.0 mask 255.255.255.0 notrust # require authentication on subnet 1
63: restrict time.nist.gov # allow access
64: </pre>
65:
66: <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>
67:
68: <h4 id="cmd">Access Control Commands</h4>
69:
70: <dl>
71:
72: <dt id="discard"><tt>discard [ average <i>avg</i> ][ minimum <i>min</i> ] [ monitor <i>prob</i> ]</tt></dt>
73: <dd>Set the parameters of the rate control facility which protects the server
74: from client abuse. If the <tt>limited</tt> flag is present in the ACL, packets
75: that violate these limits are discarded. If in addition the <tt>kod</tt> restriction
76: is present, a kiss-o'-death packet is returned.</dd>
77:
78: <dd><dl>
79:
80: <dt><tt>average <i>avg</i></tt></dt>
81: <dd>Specify the minimum average interpacket spacing (minimum average headway
82: time) in log<sub>2</sub> s with default 3.</dd>
83:
84: <dt><tt>minimum <i>min</i></tt></dt>
85: <dd>Specify the minimum interpacket spacing (guard time) in log<sub>2</sub> s
86: with default 1.</dd>
87:
88: <dt><tt>monitor</tt></dt>
89: <dd>Specify the probability of discard for packets that overflow the rate-control
90: window. This is a performance optimization for servers with aggregate arrivals
91: of 1000 packets per second or more.</dd>
92:
93: </dl></dd>
94:
95: <dt id="restrict"><tt>restrict <i>address</i> [mask <i>mask</i>] [<i>flag</i>][...]</tt></dt>
96: <dd>The <tt><i>address</i></tt> argument expressed in dotted-quad form is the
97: address of a host or network. Alternatively, the <tt><i>address</i></tt> argument
98: can be a valid host DNS name. The <tt><i>mask</i></tt> argument expressed in
99: dotted-quad form defaults to 255.255.255.255, meaning that the <tt><i>address</i></tt> is
100: treated as the address of an individual host. A default entry (address 0.0.0.0,
101: mask 0.0.0.0) is always included and is always the first entry in the list.
102: Note that the text string <tt>default</tt>, with no mask option, may be used
103: to indicate the default entry.</dd>
104:
105: <dd>Some flags have the effect to deny service, some have the effect to
106: enable service and some are conditioned by other flags. The flags. are
107: not orthogonal, in that more restrictive flags will often make less restrictive
108: ones redundant. The flags that deny service are classed in two categories,
109: those that restrict time service and those that restrict informational queries
110: and attempts to do run-time reconfiguration of the server. One or more of the
111: following flags may be specified:</dd>
112: <dd><dl>
113:
114: <dt><tt>flake</tt></dt>
115: <dd>Discard received NTP packets with probability 0.1; that is, on average drop
116: one packet in ten. This is for testing and amusement. The name comes from Bob
117: Braden's <i>flakeway</i>, which once did a similar thing for early Internet
118: testing.</dd>
119:
120: <dt><tt>ignore</tt></dt>
121: <dd>Deny packets of all kinds, including <tt>ntpq</tt> and <tt>ntpdc</tt> queries.</dd>
122:
123: <dt><tt>kod</tt></dt>
124: <dd>Send a kiss-o'-death (KoD) packet if the <tt>limited</tt> flag is present
125: and a packet violates the rate limits established by the <tt>discard</tt> command.
126: KoD packets are themselves rate limited for each source address separately.
127: If this flag is not present, packets that violate the rate limits are discarded.</dd>
128:
129: <dt><tt>limited</tt></dt>
130: <dd>Deny time service if the packet violates the rate limits established by the <tt>discard</tt> command.
131: This does not apply to <tt>ntpq</tt> and <tt>ntpdc</tt> queries.</dd>
132:
133: <dt><tt>lowpriotrap</tt></dt>
134: <dd>Declare traps set by matching hosts to be low priority. The number of traps
135: a server can maintain is limited (the current limit is 3). Traps are usually
136: assigned on a first come, first served basis, with later trap requestors being
137: denied service. This flag modifies the assignment algorithm by allowing low
138: priority traps to be overridden by later requests for normal priority traps.</dd>
139: <dt><tt>mssntp</tt></dt>
140: <dd>Enable Microsoft Windows MS-SNTP authentication using Active Directory services.
141: <span class="style1">Note: Potential users should be aware that these services
142: involve a TCP connection to another process that could potentially block,
143: denying services to other users. Therefore, this flag should be used only
144: for a dedicated server with no clients other than MS-SNTP.</span></dd>
145: <dt><tt>nomodify</tt></dt>
146: <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries which attempt to modify the
147: state of the server (i.e., run time reconfiguration). Queries which return information
148: are permitted.</dd>
149:
150: <dt><tt>noquery</tt></dt>
151: <dd>Deny <tt>ntpq</tt> and <tt>ntpdc</tt> queries. Time service is not affected.</dd>
152:
153: <dt><tt>nopeer</tt></dt>
154: <dd>Deny packets that might mobilize an association unless authenticated. This
155: includes broadcast, symmetric-active and manycast server packets when a configured
156: association does not exist. Note that this flag does not apply to packets
157: that do not attempt to mobilize an association. </dd>
158:
159: <dt><tt>noserve</tt></dt>
160: <dd>Deny all packets except <tt>ntpq</tt> and <tt>ntpdc</tt> queries.</dd>
161:
162: <dt><tt>notrap</tt></dt>
163: <dd>Decline to provide mode 6 control message trap service to matching hosts.
164: The trap service is a subsystem of the <tt>ntpdc</tt> control message protocol
165: which is intended for use by remote event logging programs.</dd>
166:
167: <dt><tt>notrust</tt></dt>
168: <dd>Deny packets that are not cryptographically authenticated. Note carefully
169: how this flag interacts with the <tt>auth</tt> option of the <tt>enable</tt> and <tt>disable</tt> commands.
170: If <tt>auth</tt> is enabled, which is the default, authentication is required
171: for all packets that might mobilize an association.
172: If <tt>auth</tt> is
173: disabled, but the <tt>notrust</tt> flag is not present, an association can be
174: mobilized whether or not authenticated. If <tt>auth</tt> is disabled, but the <tt>notrust</tt> flag
175: is present, authentication is required only for the specified address/mask
176: range. </dd>
177:
178: <dt><tt>ntpport</tt></dt>
179: <dt><tt>non-ntpport</tt></dt>
180: <dd>This is actually a match algorithm modifier, rather than a restriction
181: flag. Its presence causes the restriction entry to be matched only if the
182: source port in the packet is the standard NTP UDP port (123). Both <tt>ntpport</tt> and <tt>non-ntpport</tt> may
183: be specified. The <tt>ntpport</tt> is considered more specific and is sorted
184: later in the list.</dd>
185: <dt><tt>version</tt></dt>
186: <dd>Deny packets that do not match the current NTP version.</dd>
187: </dl>
188: </dd>
189: <dd>Default restriction list entries with the flags <tt>ignore, ntpport</tt>,
190: for each of the local host's interface addresses are inserted into the table
191: at startup to prevent the server from attempting to synchronize to its own time.
192: A default entry is also always present, though if it is otherwise unconfigured;
193: no flags are associated with the default entry (i.e., everything besides your
194: own NTP server is unrestricted).</dd>
195: </dl>
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