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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> |
22 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
23 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
24 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
27 <p>This page could be summarized with the statement: don't
28 configure Apache in such a way that it relies on DNS resolution
29 for parsing of the configuration files. If Apache requires DNS
30 resolution to parse the configuration files then your server
31 may be subject to reliability problems (ie. it might not boot),
32 or denial and theft of service attacks (including users able
33 to steal hits from other users).</p>
35 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
36 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
37 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
39 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
41 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
43 <h2><a name="example" id="example">A Simple Example</a></h2>
46 <div class="example"><p><code>
47 <VirtualHost www.abc.dom> <br />
48 ServerAdmin webgirl@abc.dom <br />
49 DocumentRoot /www/abc <br />
53 <p>In order for Apache to function properly, it absolutely needs
54 to have two pieces of information about each virtual host: the
55 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
56 IP address that the server will bind and respond to. The above
57 example does not include the IP address, so Apache must use DNS
58 to find the address of <code>www.abc.dom</code>. If for some
59 reason DNS is not available at the time your server is parsing
60 its config file, then this virtual host <strong>will not be
61 configured</strong>. It won't be able to respond to any hits
62 to this virtual host (prior to Apache version 1.2 the server
63 would not even boot).</p>
65 <p>Suppose that <code>www.abc.dom</code> has address 192.0.2.1.
66 Then consider this configuration snippet:</p>
68 <div class="example"><p><code>
69 <VirtualHost 192.0.2.1> <br />
70 ServerAdmin webgirl@abc.dom <br />
71 DocumentRoot /www/abc <br />
75 <p>This time Apache needs to use reverse DNS to find the
76 <code>ServerName</code> for this virtualhost. If that reverse
77 lookup fails then it will partially disable the virtualhost
78 (prior to Apache version 1.2 the server would not even boot).
79 If the virtual host is name-based then it will effectively be
80 totally disabled, but if it is IP-based then it will mostly
81 work. However, if Apache should ever have to generate a full
82 URL for the server which includes the server name, then it will
83 fail to generate a valid URL.</p>
85 <p>Here is a snippet that avoids both of these problems:</p>
87 <div class="example"><p><code>
88 <VirtualHost 192.0.2.1> <br />
89 ServerName www.abc.dom <br />
90 ServerAdmin webgirl@abc.dom <br />
91 DocumentRoot /www/abc <br />
94 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
96 <h2><a name="denial" id="denial">Denial of Service</a></h2>
99 <p>There are (at least) two forms that denial of service
100 can come in. If you are running a version of Apache prior to
101 version 1.2 then your server will not even boot if one of the
102 two DNS lookups mentioned above fails for any of your virtual
103 hosts. In some cases this DNS lookup may not even be under your
104 control; for example, if <code>abc.dom</code> is one of your
105 customers and they control their own DNS, they can force your
106 (pre-1.2) server to fail while booting simply by deleting the
107 <code>www.abc.dom</code> record.</p>
109 <p>Another form is far more insidious. Consider this
110 configuration snippet:</p>
112 <div class="example"><p><code>
113 <VirtualHost www.abc.dom><br />
114 <span class="indent">
115 ServerAdmin webgirl@abc.dom<br />
116 DocumentRoot /www/abc<br />
118 </VirtualHost><br />
120 <VirtualHost www.def.dom><br />
121 <span class="indent">
122 ServerAdmin webguy@def.dom<br />
123 DocumentRoot /www/def<br />
128 <p>Suppose that you've assigned 192.0.2.1 to
129 <code>www.abc.dom</code> and 192.0.2.2 to
130 <code>www.def.dom</code>. Furthermore, suppose that
131 <code>def.dom</code> has control of their own DNS. With this
132 config you have put <code>def.dom</code> into a position where
133 they can steal all traffic destined to <code>abc.dom</code>. To
134 do so, all they have to do is set <code>www.def.dom</code> to
135 192.0.2.1. Since they control their own DNS you can't stop them
136 from pointing the <code>www.def.dom</code> record wherever they
139 <p>Requests coming in to 192.0.2.1 (including all those where
140 users typed in URLs of the form
141 <code>http://www.abc.dom/whatever</code>) will all be served by
142 the <code>def.dom</code> virtual host. To better understand why
143 this happens requires a more in-depth discussion of how Apache
144 matches up incoming requests with the virtual host that will
145 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
146 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
147 <div class="section">
148 <h2><a name="main" id="main">The "main server" Address</a></h2>
151 <p>The addition of <a href="vhosts/name-based.html">name-based
152 virtual host support</a> in Apache 1.1 requires Apache to know
153 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
154 is running on. To get this address it uses either the global
155 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
156 (if present) or calls the C function <code>gethostname</code>
157 (which should return the same as typing "hostname" at the
158 command prompt). Then it performs a DNS lookup on this address.
159 At present there is no way to avoid this lookup.</p>
161 <p>If you fear that this lookup might fail because your DNS
162 server is down then you can insert the hostname in
163 <code>/etc/hosts</code> (where you probably already have it so
164 that the machine can boot properly). Then ensure that your
165 machine is configured to use <code>/etc/hosts</code> in the
166 event that DNS fails. Depending on what OS you are using this
167 might be accomplished by editing <code>/etc/resolv.conf</code>,
168 or maybe <code>/etc/nsswitch.conf</code>.</p>
170 <p>If your server doesn't have to perform DNS for any other
171 reason then you might be able to get away with running Apache
172 with the <code>HOSTRESORDER</code> environment variable set to
173 "local". This all depends on what OS and resolver libraries you
174 are using. It also affects CGIs unless you use
175 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
176 to consult the man pages or FAQs for your OS.</p>
177 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
178 <div class="section">
179 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
185 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
190 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
194 ensure all virtual hosts have an explicit
195 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
198 <li>create a <code><VirtualHost _default_:*></code>
199 server that has no pages to serve</li>
201 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
202 <div class="section">
203 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
206 <p>The situation regarding DNS is highly undesirable. For
207 Apache 1.2 we've attempted to make the server at least continue
208 booting in the event of failed DNS, but it might not be the
209 best we can do. In any event, requiring the use of explicit IP
210 addresses in configuration files is highly undesirable in
211 today's Internet where renumbering is a necessity.</p>
213 <p>A possible work around to the theft of service attack
214 described above would be to perform a reverse DNS lookup on the
215 IP address returned by the forward lookup and compare the two
216 names -- in the event of a mismatch, the virtualhost would be
217 disabled. This would require reverse DNS to be configured
218 properly (which is something that most admins are familiar with
219 because of the common use of "double-reverse" DNS lookups by
220 FTP servers and TCP wrappers).</p>
222 <p>In any event, it doesn't seem possible to reliably boot a
223 virtual-hosted web server when DNS has failed unless IP
224 addresses are used. Partial solutions such as disabling
225 portions of the configuration might be worse than not booting
226 at all depending on what the webserver is supposed to
229 <p>As HTTP/1.1 is deployed and browsers and proxies start
230 issuing the <code>Host</code> header it will become possible to
231 avoid the use of IP-based virtual hosts entirely. In this case,
232 a webserver has no requirement to do DNS lookups during
233 configuration. But as of March 1997 these features have not
234 been deployed widely enough to be put into use on critical
237 <div class="bottomlang">
238 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
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240 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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242 </div><div id="footer">
243 <p class="apache">Copyright 2008 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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