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8 <title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server</title>
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14 <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
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 HTTP Server</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 HTTP Server in such a way that it relies on DNS resolution
30 for parsing of the configuration files. If HTTPD 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 HTTPD 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 HTTPD 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 HTTPD 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 HTTPD 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 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 HTTPD 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 192.0.2.1> <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 HTTPD 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 <span class="indent">
117 ServerAdmin webgirl@abc.dom<br />
118 DocumentRoot /www/abc<br />
120 </VirtualHost><br />
122 <VirtualHost www.def.dom><br />
123 <span class="indent">
124 ServerAdmin webguy@def.dom<br />
125 DocumentRoot /www/def<br />
130 <p>Suppose that you've assigned 192.0.2.1 to
131 <code>www.abc.dom</code> and 192.0.2.2 to
132 <code>www.def.dom</code>. Furthermore, suppose that
133 <code>def.dom</code> has control of their own DNS. With this
134 config you have put <code>def.dom</code> into a position where
135 they can steal all traffic destined to <code>abc.dom</code>. To
136 do so, all they have to do is set <code>www.def.dom</code> to
137 192.0.2.1. Since they control their own DNS you can't stop them
138 from pointing the <code>www.def.dom</code> record wherever they
141 <p>Requests coming in to 192.0.2.1 (including all those where
142 users typed in URLs of the form
143 <code>http://www.abc.dom/whatever</code>) will all be served by
144 the <code>def.dom</code> virtual host. To better understand why
145 this happens requires a more in-depth discussion of how HTTPD
146 matches up incoming requests with the virtual host that will
147 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
148 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
149 <div class="section">
150 <h2><a name="main" id="main">The "main server" Address</a></h2>
153 <p><a href="vhosts/name-based.html">Name-based
154 virtual host support</a> requires HTTPD to know
155 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
156 is running on. To get this address it uses either the global
157 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
158 (if present) or calls the C function <code>gethostname</code>
159 (which should return the same as typing "hostname" at the
160 command prompt). Then it performs a DNS lookup on this address.
161 At present there is no way to avoid this lookup.</p>
163 <p>If you fear that this lookup might fail because your DNS
164 server is down then you can insert the hostname in
165 <code>/etc/hosts</code> (where you probably already have it so
166 that the machine can boot properly). Then ensure that your
167 machine is configured to use <code>/etc/hosts</code> in the
168 event that DNS fails. Depending on what OS you are using this
169 might be accomplished by editing <code>/etc/resolv.conf</code>,
170 or maybe <code>/etc/nsswitch.conf</code>.</p>
172 <p>If your server doesn't have to perform DNS for any other
173 reason then you might be able to get away with running HTTPD
174 with the <code>HOSTRESORDER</code> environment variable set to
175 "local". This all depends on what OS and resolver libraries you
176 are using. It also affects CGIs unless you use
177 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
178 to consult the man pages or FAQs for your OS.</p>
179 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
180 <div class="section">
181 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
187 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
192 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
196 ensure all virtual hosts have an explicit
197 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
200 <li>create a <code><VirtualHost _default_:*></code>
201 server that has no pages to serve</li>
203 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
204 <div class="section">
205 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
208 <p>The situation regarding DNS is highly undesirable. Although
209 we've attempted to make the server at least continue
210 booting in the event of failed DNS, it might not be the
211 best we can do. In any event, requiring the use of explicit IP
212 addresses in configuration files is highly undesirable in
213 today's Internet where renumbering is a necessity.</p>
215 <p>A possible work around to the theft of service attack
216 described above would be to perform a reverse DNS lookup on the
217 IP address returned by the forward lookup and compare the two
218 names -- in the event of a mismatch, the virtualhost would be
219 disabled. This would require reverse DNS to be configured
220 properly (which is something that most admins are familiar with
221 because of the common use of "double-reverse" DNS lookups by
222 FTP servers and TCP wrappers).</p>
224 <p>In any event, it doesn't seem possible to reliably boot a
225 virtual-hosted web server when DNS has failed unless IP
226 addresses are used. Partial solutions such as disabling
227 portions of the configuration might be worse than not booting
228 at all depending on what the webserver is supposed to
231 <p>As HTTP/1.1 is deployed and browsers and proxies start
232 issuing the <code>Host</code> header it will become possible to
233 avoid the use of IP-based virtual hosts entirely. In this case,
234 a webserver has no requirement to do DNS lookups during
235 configuration. But as of March 1997 these features have not
236 been deployed widely enough to be put into use on critical
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246 <p class="apache">Copyright 2010 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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