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8 <title>Issues Regarding DNS and Apache - Apache HTTP Server</title>
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15 <p class="apache">Apache HTTP Server Version 2.3</p>
16 <img alt="" src="./images/feather.gif" /></div>
17 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
19 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.3</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1>
21 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
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28 <p>This page could be summarized with the statement: don't
29 configure Apache in such a way that it relies on DNS resolution
30 for parsing of the configuration files. If Apache requires DNS
31 resolution to parse the configuration files then your server
32 may be subject to reliability problems (ie. it might not boot),
33 or denial and theft of service attacks (including users able
34 to steal hits from other users).</p>
36 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
37 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
39 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
40 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
42 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
44 <h2><a name="example" id="example">A Simple Example</a></h2>
47 <div class="example"><p><code>
48 # This is a misconfiguration example, do not use on your server <br />
49 <VirtualHost www.abc.dom> <br />
50 ServerAdmin webgirl@abc.dom <br />
51 DocumentRoot /www/abc <br />
55 <p>In order for Apache to function properly, it absolutely needs
56 to have two pieces of information about each virtual host: the
57 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
58 IP address that the server will bind and respond to. The above
59 example does not include the IP address, so Apache must use DNS
60 to find the address of <code>www.abc.dom</code>. If for some
61 reason DNS is not available at the time your server is parsing
62 its config file, then this virtual host <strong>will not be
63 configured</strong>. It won't be able to respond to any hits
64 to this virtual host (prior to Apache version 1.2 the server
65 would not even boot).</p>
67 <p>Suppose that <code>www.abc.dom</code> has address 192.0.2.1.
68 Then consider this configuration snippet:</p>
70 <div class="example"><p><code>
71 # This is a misconfiguration example, do not use on your server <br />
72 <VirtualHost 192.0.2.1> <br />
73 ServerAdmin webgirl@abc.dom <br />
74 DocumentRoot /www/abc <br />
78 <p>This time Apache needs to use reverse DNS to find the
79 <code>ServerName</code> for this virtualhost. If that reverse
80 lookup fails then it will partially disable the virtualhost
81 (prior to Apache version 1.2 the server would not even boot).
82 If the virtual host is name-based then it will effectively be
83 totally disabled, but if it is IP-based then it will mostly
84 work. However, if Apache should ever have to generate a full
85 URL for the server which includes the server name, then it will
86 fail to generate a valid URL.</p>
88 <p>Here is a snippet that avoids both of these problems:</p>
90 <div class="example"><p><code>
91 <VirtualHost 192.0.2.1> <br />
92 ServerName www.abc.dom <br />
93 ServerAdmin webgirl@abc.dom <br />
94 DocumentRoot /www/abc <br />
97 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
99 <h2><a name="denial" id="denial">Denial of Service</a></h2>
102 <p>There are (at least) two forms that denial of service
103 can come in. If you are running a version of Apache prior to
104 version 1.2 then your server will not even boot if one of the
105 two DNS lookups mentioned above fails for any of your virtual
106 hosts. In some cases this DNS lookup may not even be under your
107 control; for example, if <code>abc.dom</code> is one of your
108 customers and they control their own DNS, they can force your
109 (pre-1.2) server to fail while booting simply by deleting the
110 <code>www.abc.dom</code> record.</p>
112 <p>Another form is far more insidious. Consider this
113 configuration snippet:</p>
115 <div class="example"><p><code>
116 <VirtualHost www.abc.dom><br />
117 <span class="indent">
118 ServerAdmin webgirl@abc.dom<br />
119 DocumentRoot /www/abc<br />
121 </VirtualHost><br />
123 <VirtualHost www.def.dom><br />
124 <span class="indent">
125 ServerAdmin webguy@def.dom<br />
126 DocumentRoot /www/def<br />
131 <p>Suppose that you've assigned 192.0.2.1 to
132 <code>www.abc.dom</code> and 192.0.2.2 to
133 <code>www.def.dom</code>. Furthermore, suppose that
134 <code>def.dom</code> has control of their own DNS. With this
135 config you have put <code>def.dom</code> into a position where
136 they can steal all traffic destined to <code>abc.dom</code>. To
137 do so, all they have to do is set <code>www.def.dom</code> to
138 192.0.2.1. Since they control their own DNS you can't stop them
139 from pointing the <code>www.def.dom</code> record wherever they
142 <p>Requests coming in to 192.0.2.1 (including all those where
143 users typed in URLs of the form
144 <code>http://www.abc.dom/whatever</code>) will all be served by
145 the <code>def.dom</code> virtual host. To better understand why
146 this happens requires a more in-depth discussion of how Apache
147 matches up incoming requests with the virtual host that will
148 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
149 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
150 <div class="section">
151 <h2><a name="main" id="main">The "main server" Address</a></h2>
154 <p>The addition of <a href="vhosts/name-based.html">name-based
155 virtual host support</a> in Apache 1.1 requires Apache to know
156 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
157 is running on. To get this address it uses either the global
158 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
159 (if present) or calls the C function <code>gethostname</code>
160 (which should return the same as typing "hostname" at the
161 command prompt). Then it performs a DNS lookup on this address.
162 At present there is no way to avoid this lookup.</p>
164 <p>If you fear that this lookup might fail because your DNS
165 server is down then you can insert the hostname in
166 <code>/etc/hosts</code> (where you probably already have it so
167 that the machine can boot properly). Then ensure that your
168 machine is configured to use <code>/etc/hosts</code> in the
169 event that DNS fails. Depending on what OS you are using this
170 might be accomplished by editing <code>/etc/resolv.conf</code>,
171 or maybe <code>/etc/nsswitch.conf</code>.</p>
173 <p>If your server doesn't have to perform DNS for any other
174 reason then you might be able to get away with running Apache
175 with the <code>HOSTRESORDER</code> environment variable set to
176 "local". This all depends on what OS and resolver libraries you
177 are using. It also affects CGIs unless you use
178 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
179 to consult the man pages or FAQs for your OS.</p>
180 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
181 <div class="section">
182 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
188 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
193 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
197 ensure all virtual hosts have an explicit
198 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
201 <li>create a <code><VirtualHost _default_:*></code>
202 server that has no pages to serve</li>
204 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
205 <div class="section">
206 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
209 <p>The situation regarding DNS is highly undesirable. For
210 Apache 1.2 we've attempted to make the server at least continue
211 booting in the event of failed DNS, but it might not be the
212 best we can do. In any event, requiring the use of explicit IP
213 addresses in configuration files is highly undesirable in
214 today's Internet where renumbering is a necessity.</p>
216 <p>A possible work around to the theft of service attack
217 described above would be to perform a reverse DNS lookup on the
218 IP address returned by the forward lookup and compare the two
219 names -- in the event of a mismatch, the virtualhost would be
220 disabled. This would require reverse DNS to be configured
221 properly (which is something that most admins are familiar with
222 because of the common use of "double-reverse" DNS lookups by
223 FTP servers and TCP wrappers).</p>
225 <p>In any event, it doesn't seem possible to reliably boot a
226 virtual-hosted web server when DNS has failed unless IP
227 addresses are used. Partial solutions such as disabling
228 portions of the configuration might be worse than not booting
229 at all depending on what the webserver is supposed to
232 <p>As HTTP/1.1 is deployed and browsers and proxies start
233 issuing the <code>Host</code> header it will become possible to
234 avoid the use of IP-based virtual hosts entirely. In this case,
235 a webserver has no requirement to do DNS lookups during
236 configuration. But as of March 1997 these features have not
237 been deployed widely enough to be put into use on critical
240 <div class="bottomlang">
241 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
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247 <p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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