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15 <p class="apache">Apache HTTP Server Version 2.0</p>
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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 2.0</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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29 <p>This page could be summarized with the statement: don't
30 configure Apache in such a way that it relies on DNS resolution
31 for parsing of the configuration files. If Apache requires DNS
32 resolution to parse the configuration files then your server
33 may be subject to reliability problems (ie. it might not boot),
34 or denial and theft of service attacks (including users able
35 to steal hits from other users).</p>
37 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
39 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
40 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
41 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
43 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
45 <h2><a name="example" id="example">A Simple Example</a></h2>
48 <div class="example"><p><code>
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 10.0.0.1.
68 Then consider this configuration snippet:</p>
70 <div class="example"><p><code>
71 <VirtualHost 10.0.0.1> <br />
72 ServerAdmin webgirl@abc.dom <br />
73 DocumentRoot /www/abc <br />
77 <p>This time Apache needs to use reverse DNS to find the
78 <code>ServerName</code> for this virtualhost. If that reverse
79 lookup fails then it will partially disable the virtualhost
80 (prior to Apache version 1.2 the server would not even boot).
81 If the virtual host is name-based then it will effectively be
82 totally disabled, but if it is IP-based then it will mostly
83 work. However, if Apache should ever have to generate a full
84 URL for the server which includes the server name, then it will
85 fail to generate a valid URL.</p>
87 <p>Here is a snippet that avoids both of these problems:</p>
89 <div class="example"><p><code>
90 <VirtualHost 10.0.0.1> <br />
91 ServerName www.abc.dom <br />
92 ServerAdmin webgirl@abc.dom <br />
93 DocumentRoot /www/abc <br />
96 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
98 <h2><a name="denial" id="denial">Denial of Service</a></h2>
101 <p>There are (at least) two forms that denial of service
102 can come in. If you are running a version of Apache prior to
103 version 1.2 then your server will not even boot if one of the
104 two DNS lookups mentioned above fails for any of your virtual
105 hosts. In some cases this DNS lookup may not even be under your
106 control; for example, if <code>abc.dom</code> is one of your
107 customers and they control their own DNS, they can force your
108 (pre-1.2) server to fail while booting simply by deleting the
109 <code>www.abc.dom</code> record.</p>
111 <p>Another form is far more insidious. Consider this
112 configuration snippet:</p>
114 <div class="example"><p><code>
115 <VirtualHost www.abc.dom> <br />
116 ServerAdmin webgirl@abc.dom <br />
117 DocumentRoot /www/abc <br />
118 </VirtualHost> <br />
120 <VirtualHost www.def.dom> <br />
121 ServerAdmin webguy@def.dom <br />
122 DocumentRoot /www/def <br />
126 <p>Suppose that you've assigned 10.0.0.1 to
127 <code>www.abc.dom</code> and 10.0.0.2 to
128 <code>www.def.dom</code>. Furthermore, suppose that
129 <code>def.dom</code> has control of their own DNS. With this
130 config you have put <code>def.dom</code> into a position where
131 they can steal all traffic destined to <code>abc.dom</code>. To
132 do so, all they have to do is set <code>www.def.dom</code> to
133 10.0.0.1. Since they control their own DNS you can't stop them
134 from pointing the <code>www.def.dom</code> record wherever they
137 <p>Requests coming in to 10.0.0.1 (including all those where
138 users typed in URLs of the form
139 <code>http://www.abc.dom/whatever</code>) will all be served by
140 the <code>def.dom</code> virtual host. To better understand why
141 this happens requires a more in-depth discussion of how Apache
142 matches up incoming requests with the virtual host that will
143 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
144 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
145 <div class="section">
146 <h2><a name="main" id="main">The "main server" Address</a></h2>
149 <p>The addition of <a href="vhosts/name-based.html">name-based
150 virtual host support</a> in Apache 1.1 requires Apache to know
151 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code> is running
152 on. To get this address it uses either the global
153 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
154 (if present) or calls the C function <code>gethostname</code>
155 (which should return the same as typing "hostname" at the
156 command prompt). Then it performs a DNS lookup on this address.
157 At present there is no way to avoid this lookup.</p>
159 <p>If you fear that this lookup might fail because your DNS
160 server is down then you can insert the hostname in
161 <code>/etc/hosts</code> (where you probably already have it so
162 that the machine can boot properly). Then ensure that your
163 machine is configured to use <code>/etc/hosts</code> in the
164 event that DNS fails. Depending on what OS you are using this
165 might be accomplished by editing <code>/etc/resolv.conf</code>,
166 or maybe <code>/etc/nsswitch.conf</code>.</p>
168 <p>If your server doesn't have to perform DNS for any other
169 reason then you might be able to get away with running Apache
170 with the <code>HOSTRESORDER</code> environment variable set to
171 "local". This all depends on what OS and resolver libraries you
172 are using. It also affects CGIs unless you use
173 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
174 to consult the man pages or FAQs for your OS.</p>
175 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
176 <div class="section">
177 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
183 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
188 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
192 ensure all virtual hosts have an explicit
193 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
196 <li>create a <code><VirtualHost _default_:*></code>
197 server that has no pages to serve</li>
199 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
200 <div class="section">
201 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
204 <p>The situation regarding DNS is highly undesirable. For
205 Apache 1.2 we've attempted to make the server at least continue
206 booting in the event of failed DNS, but it might not be the
207 best we can do. In any event, requiring the use of explicit IP
208 addresses in configuration files is highly undesirable in
209 today's Internet where renumbering is a necessity.</p>
211 <p>A possible work around to the theft of service attack
212 described above would be to perform a reverse DNS lookup on the
213 IP address returned by the forward lookup and compare the two
214 names -- in the event of a mismatch, the virtualhost would be
215 disabled. This would require reverse DNS to be configured
216 properly (which is something that most admins are familiar with
217 because of the common use of "double-reverse" DNS lookups by
218 FTP servers and TCP wrappers).</p>
220 <p>In any event, it doesn't seem possible to reliably boot a
221 virtual-hosted web server when DNS has failed unless IP
222 addresses are used. Partial solutions such as disabling
223 portions of the configuration might be worse than not booting
224 at all depending on what the webserver is supposed to
227 <p>As HTTP/1.1 is deployed and browsers and proxies start
228 issuing the <code>Host</code> header it will become possible to
229 avoid the use of IP-based virtual hosts entirely. In this case,
230 a webserver has no requirement to do DNS lookups during
231 configuration. But as of March 1997 these features have not
232 been deployed widely enough to be put into use on critical
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243 <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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