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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 2.1</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-project/">Documentation</a> > <a href="./">Version 2.1</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></p>
26 <p>This page could be summarized with the statement: don't
27 configure Apache in such a way that it relies on DNS resolution
28 for parsing of the configuration files. If Apache requires DNS
29 resolution to parse the configuration files then your server
30 may be subject to reliability problems (ie. it might not boot),
31 or denial and theft of service attacks (including users able
32 to steal hits from other users).</p>
34 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
35 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
36 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
37 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
38 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
40 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
42 <h2><a name="example" id="example">A Simple Example</a></h2>
45 <div class="example"><p><code>
46 <VirtualHost www.abc.dom> <br />
47 ServerAdmin webgirl@abc.dom <br />
48 DocumentRoot /www/abc <br />
52 <p>In order for Apache to function properly, it absolutely needs
53 to have two pieces of information about each virtual host: the
54 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
55 IP address that the server will bind and respond to. The above
56 example does not include the IP address, so Apache must use DNS
57 to find the address of <code>www.abc.dom</code>. If for some
58 reason DNS is not available at the time your server is parsing
59 its config file, then this virtual host <strong>will not be
60 configured</strong>. It won't be able to respond to any hits
61 to this virtual host (prior to Apache version 1.2 the server
62 would not even boot).</p>
64 <p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
65 Then consider this configuration snippet:</p>
67 <div class="example"><p><code>
68 <VirtualHost 10.0.0.1> <br />
69 ServerAdmin webgirl@abc.dom <br />
70 DocumentRoot /www/abc <br />
74 <p>This time Apache needs to use reverse DNS to find the
75 <code>ServerName</code> for this virtualhost. If that reverse
76 lookup fails then it will partially disable the virtualhost
77 (prior to Apache version 1.2 the server would not even boot).
78 If the virtual host is name-based then it will effectively be
79 totally disabled, but if it is IP-based then it will mostly
80 work. However, if Apache should ever have to generate a full
81 URL for the server which includes the server name, then it will
82 fail to generate a valid URL.</p>
84 <p>Here is a snippet that avoids both of these problems:</p>
86 <div class="example"><p><code>
87 <VirtualHost 10.0.0.1> <br />
88 ServerName www.abc.dom <br />
89 ServerAdmin webgirl@abc.dom <br />
90 DocumentRoot /www/abc <br />
93 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
95 <h2><a name="denial" id="denial">Denial of Service</a></h2>
98 <p>There are (at least) two forms that denial of service
99 can come in. If you are running a version of Apache prior to
100 version 1.2 then your server will not even boot if one of the
101 two DNS lookups mentioned above fails for any of your virtual
102 hosts. In some cases this DNS lookup may not even be under your
103 control; for example, if <code>abc.dom</code> is one of your
104 customers and they control their own DNS, they can force your
105 (pre-1.2) server to fail while booting simply by deleting the
106 <code>www.abc.dom</code> record.</p>
108 <p>Another form is far more insidious. Consider this
109 configuration snippet:</p>
111 <div class="example"><p><code>
112 <VirtualHost www.abc.dom> <br />
113 ServerAdmin webgirl@abc.dom <br />
114 DocumentRoot /www/abc <br />
115 </VirtualHost> <br />
117 <VirtualHost www.def.dom> <br />
118 ServerAdmin webguy@def.dom <br />
119 DocumentRoot /www/def <br />
123 <p>Suppose that you've assigned 10.0.0.1 to
124 <code>www.abc.dom</code> and 10.0.0.2 to
125 <code>www.def.dom</code>. Furthermore, suppose that
126 <code>def.dom</code> has control of their own DNS. With this
127 config you have put <code>def.dom</code> into a position where
128 they can steal all traffic destined to <code>abc.dom</code>. To
129 do so, all they have to do is set <code>www.def.dom</code> to
130 10.0.0.1. Since they control their own DNS you can't stop them
131 from pointing the <code>www.def.dom</code> record wherever they
134 <p>Requests coming in to 10.0.0.1 (including all those where
135 users typed in URLs of the form
136 <code>http://www.abc.dom/whatever</code>) will all be served by
137 the <code>def.dom</code> virtual host. To better understand why
138 this happens requires a more in-depth discussion of how Apache
139 matches up incoming requests with the virtual host that will
140 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
141 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
142 <div class="section">
143 <h2><a name="main" id="main">The "main server" Address</a></h2>
146 <p>The addition of <a href="vhosts/name-based.html">name-based
147 virtual host support</a> in Apache 1.1 requires Apache to know
148 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
149 is running on. To get this address it uses either the global
150 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
151 (if present) or calls the C function <code>gethostname</code>
152 (which should return the same as typing "hostname" at the
153 command prompt). Then it performs a DNS lookup on this address.
154 At present there is no way to avoid this lookup.</p>
156 <p>If you fear that this lookup might fail because your DNS
157 server is down then you can insert the hostname in
158 <code>/etc/hosts</code> (where you probably already have it so
159 that the machine can boot properly). Then ensure that your
160 machine is configured to use <code>/etc/hosts</code> in the
161 event that DNS fails. Depending on what OS you are using this
162 might be accomplished by editing <code>/etc/resolv.conf</code>,
163 or maybe <code>/etc/nsswitch.conf</code>.</p>
165 <p>If your server doesn't have to perform DNS for any other
166 reason then you might be able to get away with running Apache
167 with the <code>HOSTRESORDER</code> environment variable set to
168 "local". This all depends on what OS and resolver libraries you
169 are using. It also affects CGIs unless you use
170 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
171 to consult the man pages or FAQs for your OS.</p>
172 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
173 <div class="section">
174 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
180 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
185 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
189 ensure all virtual hosts have an explicit
190 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
193 <li>create a <code><VirtualHost _default_:*></code>
194 server that has no pages to serve</li>
196 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
197 <div class="section">
198 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
201 <p>The situation regarding DNS is highly undesirable. For
202 Apache 1.2 we've attempted to make the server at least continue
203 booting in the event of failed DNS, but it might not be the
204 best we can do. In any event, requiring the use of explicit IP
205 addresses in configuration files is highly undesirable in
206 today's Internet where renumbering is a necessity.</p>
208 <p>A possible work around to the theft of service attack
209 described above would be to perform a reverse DNS lookup on the
210 IP address returned by the forward lookup and compare the two
211 names -- in the event of a mismatch, the virtualhost would be
212 disabled. This would require reverse DNS to be configured
213 properly (which is something that most admins are familiar with
214 because of the common use of "double-reverse" DNS lookups by
215 FTP servers and TCP wrappers).</p>
217 <p>In any event, it doesn't seem possible to reliably boot a
218 virtual-hosted web server when DNS has failed unless IP
219 addresses are used. Partial solutions such as disabling
220 portions of the configuration might be worse than not booting
221 at all depending on what the webserver is supposed to
224 <p>As HTTP/1.1 is deployed and browsers and proxies start
225 issuing the <code>Host</code> header it will become possible to
226 avoid the use of IP-based virtual hosts entirely. In this case,
227 a webserver has no requirement to do DNS lookups during
228 configuration. But as of March 1997 these features have not
229 been deployed widely enough to be put into use on critical
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233 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
234 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
235 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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237 <p class="apache">Copyright 1995-2005 The Apache Software Foundation or its licensors, as applicable.<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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