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10 <title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server Version 2.5</title>
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20 <p class="apache">Apache HTTP Server Version 2.5</p>
21 <img alt="" src="./images/feather.png" /></div>
22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
24 <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 2.5</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
26 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
27 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
28 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
29 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
30 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
33 <p>This page could be summarized with the statement: don't
34 configure Apache HTTP Server in such a way that it relies on DNS resolution
35 for parsing of the configuration files. If httpd requires DNS
36 resolution to parse the configuration files then your server
37 may be subject to reliability problems (ie. it might not start up),
38 or denial and theft of service attacks (including virtual hosts able
39 to steal hits from other virtual hosts).</p>
41 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
42 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
43 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
44 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
45 </ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
46 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
48 <h2><a name="example" id="example">A Simple Example</a></h2>
51 <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
52 <VirtualHost www.example.dom>
53 ServerAdmin webgirl@example.dom
54 DocumentRoot "/www/example"
55 </VirtualHost></pre>
58 <p>In order for the server to function properly, it absolutely needs
59 to have two pieces of information about each virtual host: the
60 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
61 IP address that the server will bind and respond to. The above
62 example does not include the IP address, so httpd must use DNS
63 to find the address of <code>www.example.dom</code>. If for some
64 reason DNS is not available at the time your server is parsing
65 its config file, then this virtual host <strong>will not be
66 configured</strong>. It won't be able to respond to any hits
67 to this virtual host.</p>
69 <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
70 Then consider this configuration snippet:</p>
72 <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
73 <VirtualHost 192.0.2.1>
74 ServerAdmin webgirl@example.dom
75 DocumentRoot "/www/example"
76 </VirtualHost></pre>
79 <p>This time httpd needs to use reverse DNS to find the
80 <code>ServerName</code> for this virtualhost. If that reverse
81 lookup fails then it will partially disable the virtualhost.
82 If the virtual host is name-based then it will effectively be
83 totally disabled, but if it is IP-based then it will mostly
84 work. However, if httpd should ever have to generate a full
85 URL for the server which includes the server name (such as when a
86 Redirect is issued), then it will fail to generate a valid URL.</p>
88 <p>Here is a snippet that avoids both of these problems:</p>
90 <pre class="prettyprint lang-config"><VirtualHost 192.0.2.1>
91 ServerName www.example.dom
92 ServerAdmin webgirl@example.dom
93 DocumentRoot "/www/example"
94 </VirtualHost></pre>
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>Consider this configuration snippet:</p>
103 <pre class="prettyprint lang-config"><VirtualHost www.example1.dom>
104 ServerAdmin webgirl@example1.dom
105 DocumentRoot "/www/example1"
107 <VirtualHost www.example2.dom>
108 ServerAdmin webguy@example2.dom
109 DocumentRoot "/www/example2"
110 </VirtualHost></pre>
113 <p>Suppose that you've assigned 192.0.2.1 to
114 <code>www.example1.dom</code> and 192.0.2.2 to
115 <code>www.example2.dom</code>. Furthermore, suppose that
116 <code>example1.dom</code> has control of their own DNS. With this
117 config you have put <code>example1.dom</code> into a position where
118 they can steal all traffic destined to <code>example2.dom</code>. To
119 do so, all they have to do is set <code>www.example1.dom</code> to
120 192.0.2.2. Since they control their own DNS you can't stop them
121 from pointing the <code>www.example1.dom</code> record wherever they
124 <p>Requests coming in to 192.0.2.2 (including all those where
125 users typed in URLs of the form
126 <code>http://www.example2.dom/whatever</code>) will all be served by
127 the <code>example1.dom</code> virtual host. To better understand why
128 this happens requires a more in-depth discussion of how httpd
129 matches up incoming requests with the virtual host that will
130 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
131 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
132 <div class="section">
133 <h2><a name="main" id="main">The "main server" Address</a></h2>
136 <p><a href="vhosts/name-based.html">Name-based
137 virtual host support</a> requires httpd to know
138 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
139 is running on. To get this address it uses either the global
140 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
141 (if present) or calls the C function <code>gethostname</code>
142 (which should return the same as typing "hostname" at the
143 command prompt). Then it performs a DNS lookup on this address.
144 At present there is no way to avoid this lookup.</p>
146 <p>If you fear that this lookup might fail because your DNS
147 server is down then you can insert the hostname in
148 <code>/etc/hosts</code> (where you probably already have it so
149 that the machine can boot properly). Then ensure that your
150 machine is configured to use <code>/etc/hosts</code> in the
151 event that DNS fails. Depending on what OS you are using this
152 might be accomplished by editing <code>/etc/resolv.conf</code>,
153 or maybe <code>/etc/nsswitch.conf</code>.</p>
155 <p>If your server doesn't have to perform DNS for any other
156 reason then you might be able to get away with running httpd
157 with the <code>HOSTRESORDER</code> environment variable set to
158 "local". This all depends on what OS and resolver libraries you
159 are using. It also affects CGIs unless you use
160 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
161 to consult the man pages or FAQs for your OS.</p>
162 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
163 <div class="section">
164 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
170 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
175 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
179 ensure all virtual hosts have an explicit
180 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
183 <li>create a <code><VirtualHost _default_:*></code>
184 server that has no pages to serve</li>
187 <div class="bottomlang">
188 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
189 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
190 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
191 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
192 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
193 </div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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