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15 <p class="apache">Apache HTTP Server Version 2.3</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 2.3</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> |
22 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
23 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </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 start up),
33 or denial and theft of service attacks (including virtual hosts able
34 to steal hits from other virtual hosts).</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>
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 # This is a misconfiguration example, do not use on your server <br />
48 <VirtualHost www.example.dom> <br />
49 ServerAdmin webgirl@example.dom <br />
50 DocumentRoot /www/example <br />
54 <p>In order for the server to function properly, it absolutely needs
55 to have two pieces of information about each virtual host: the
56 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
57 IP address that the server will bind and respond to. The above
58 example does not include the IP address, so httpd must use DNS
59 to find the address of <code>www.example.dom</code>. If for some
60 reason DNS is not available at the time your server is parsing
61 its config file, then this virtual host <strong>will not be
62 configured</strong>. It won't be able to respond to any hits
63 to this virtual host.</p>
65 <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
66 Then consider this configuration snippet:</p>
68 <div class="example"><p><code>
69 # This is a misconfiguration example, do not use on your server <br />
70 <VirtualHost 192.0.2.1> <br />
71 ServerAdmin webgirl@example.dom <br />
72 DocumentRoot /www/example <br />
76 <p>This time httpd needs to use reverse DNS to find the
77 <code>ServerName</code> for this virtualhost. If that reverse
78 lookup fails then it will partially disable the virtualhost.
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 httpd should ever have to generate a full
82 URL for the server which includes the server name (such as when a
83 Redirect is issued), then it will 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.example.dom <br />
90 ServerAdmin webgirl@example.dom <br />
91 DocumentRoot /www/example <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>Consider this configuration snippet:</p>
101 <div class="example"><p><code>
102 <VirtualHost www.example1.dom><br />
103 <span class="indent">
104 ServerAdmin webgirl@example1.dom<br />
105 DocumentRoot /www/example1<br />
107 </VirtualHost><br />
109 <VirtualHost www.example2.dom><br />
110 <span class="indent">
111 ServerAdmin webguy@example2.dom<br />
112 DocumentRoot /www/example2<br />
117 <p>Suppose that you've assigned 192.0.2.1 to
118 <code>www.example1.dom</code> and 192.0.2.2 to
119 <code>www.example2.dom</code>. Furthermore, suppose that
120 <code>example2.dom</code> has control of their own DNS. With this
121 config you have put <code>example2.dom</code> into a position where
122 they can steal all traffic destined to <code>example1.dom</code>. To
123 do so, all they have to do is set <code>www.example2.dom</code> to
124 192.0.2.1. Since they control their own DNS you can't stop them
125 from pointing the <code>www.example2.dom</code> record wherever they
128 <p>Requests coming in to 192.0.2.1 (including all those where
129 users typed in URLs of the form
130 <code>http://www.example1.dom/whatever</code>) will all be served by
131 the <code>example2.dom</code> virtual host. To better understand why
132 this happens requires a more in-depth discussion of how httpd
133 matches up incoming requests with the virtual host that will
134 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
135 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
136 <div class="section">
137 <h2><a name="main" id="main">The "main server" Address</a></h2>
140 <p><a href="vhosts/name-based.html">Name-based
141 virtual host support</a> requires httpd to know
142 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
143 is running on. To get this address it uses either the global
144 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
145 (if present) or calls the C function <code>gethostname</code>
146 (which should return the same as typing "hostname" at the
147 command prompt). Then it performs a DNS lookup on this address.
148 At present there is no way to avoid this lookup.</p>
150 <p>If you fear that this lookup might fail because your DNS
151 server is down then you can insert the hostname in
152 <code>/etc/hosts</code> (where you probably already have it so
153 that the machine can boot properly). Then ensure that your
154 machine is configured to use <code>/etc/hosts</code> in the
155 event that DNS fails. Depending on what OS you are using this
156 might be accomplished by editing <code>/etc/resolv.conf</code>,
157 or maybe <code>/etc/nsswitch.conf</code>.</p>
159 <p>If your server doesn't have to perform DNS for any other
160 reason then you might be able to get away with running httpd
161 with the <code>HOSTRESORDER</code> environment variable set to
162 "local". This all depends on what OS and resolver libraries you
163 are using. It also affects CGIs unless you use
164 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
165 to consult the man pages or FAQs for your OS.</p>
166 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
167 <div class="section">
168 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
174 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
179 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
183 ensure all virtual hosts have an explicit
184 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
187 <li>create a <code><VirtualHost _default_:*></code>
188 server that has no pages to serve</li>
191 <div class="bottomlang">
192 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
193 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
194 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
195 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
196 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
197 </div><div id="footer">
198 <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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