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+<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.4</p>
+<img alt="" src="./images/feather.png" /></div>
+<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="./images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="./">Version 2.4</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
+<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</div>
+
+ <p>This page could be summarized with the statement: don't
+ configure Apache HTTP Server in such a way that it relies on DNS resolution
+ for parsing of the configuration files. If httpd requires DNS
+ resolution to parse the configuration files then your server
+ may be subject to reliability problems (ie. it might not start up),
+ or denial and theft of service attacks (including virtual hosts able
+ to steal hits from other virtual hosts).</p>
+ </div>
+<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
+<li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
+</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="example" id="example">A Simple Example</a></h2>
+
+
+ <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
+&lt;VirtualHost www.example.dom&gt;
+ ServerAdmin webgirl@example.dom
+ DocumentRoot "/www/example"
+&lt;/VirtualHost&gt;</pre>
+
+
+ <p>In order for the server to function properly, it absolutely needs
+ to have two pieces of information about each virtual host: the
+ <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
+ IP address that the server will bind and respond to. The above
+ example does not include the IP address, so httpd must use DNS
+ to find the address of <code>www.example.dom</code>. If for some
+ reason DNS is not available at the time your server is parsing
+ its config file, then this virtual host <strong>will not be
+ configured</strong>. It won't be able to respond to any hits
+ to this virtual host.</p>
+
+ <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
+ Then consider this configuration snippet:</p>
+
+ <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
+&lt;VirtualHost 192.0.2.1&gt;
+ ServerAdmin webgirl@example.dom
+ DocumentRoot "/www/example"
+&lt;/VirtualHost&gt;</pre>
+
+
+ <p>This time httpd needs to use reverse DNS to find the
+ <code>ServerName</code> for this virtualhost. If that reverse
+ lookup fails then it will partially disable the virtualhost.
+ If the virtual host is name-based then it will effectively be
+ totally disabled, but if it is IP-based then it will mostly
+ work. However, if httpd should ever have to generate a full
+ URL for the server which includes the server name (such as when a
+ Redirect is issued), then it will fail to generate a valid URL.</p>
+
+ <p>Here is a snippet that avoids both of these problems:</p>
+
+ <pre class="prettyprint lang-config">&lt;VirtualHost 192.0.2.1&gt;
+ ServerName www.example.dom
+ ServerAdmin webgirl@example.dom
+ DocumentRoot "/www/example"
+&lt;/VirtualHost&gt;</pre>
+
+ </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="denial" id="denial">Denial of Service</a></h2>
+
+
+ <p>Consider this configuration snippet:</p>
+
+ <pre class="prettyprint lang-config">&lt;VirtualHost www.example1.dom&gt;
+ ServerAdmin webgirl@example1.dom
+ DocumentRoot "/www/example1"
+&lt;/VirtualHost&gt;
+&lt;VirtualHost www.example2.dom&gt;
+ ServerAdmin webguy@example2.dom
+ DocumentRoot "/www/example2"
+&lt;/VirtualHost&gt;</pre>
+
+
+ <p>Suppose that you've assigned 192.0.2.1 to
+ <code>www.example1.dom</code> and 192.0.2.2 to
+ <code>www.example2.dom</code>. Furthermore, suppose that
+ <code>example1.dom</code> has control of their own DNS. With this
+ config you have put <code>example1.dom</code> into a position where
+ they can steal all traffic destined to <code>example2.dom</code>. To
+ do so, all they have to do is set <code>www.example1.dom</code> to
+ 192.0.2.2. Since they control their own DNS you can't stop them
+ from pointing the <code>www.example1.dom</code> record wherever they
+ wish.</p>
+
+ <p>Requests coming in to 192.0.2.2 (including all those where
+ users typed in URLs of the form
+ <code>http://www.example2.dom/whatever</code>) will all be served by
+ the <code>example1.dom</code> virtual host. To better understand why
+ this happens requires a more in-depth discussion of how httpd
+ matches up incoming requests with the virtual host that will
+ serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
+ </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="main" id="main">The "main server" Address</a></h2>
+
+
+ <p><a href="vhosts/name-based.html">Name-based
+ virtual host support</a> requires httpd to know
+ the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
+ is running on. To get this address it uses either the global
+ <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
+ (if present) or calls the C function <code>gethostname</code>
+ (which should return the same as typing "hostname" at the
+ command prompt). Then it performs a DNS lookup on this address.
+ At present there is no way to avoid this lookup.</p>
+
+ <p>If you fear that this lookup might fail because your DNS
+ server is down then you can insert the hostname in
+ <code>/etc/hosts</code> (where you probably already have it so
+ that the machine can boot properly). Then ensure that your
+ machine is configured to use <code>/etc/hosts</code> in the
+ event that DNS fails. Depending on what OS you are using this
+ might be accomplished by editing <code>/etc/resolv.conf</code>,
+ or maybe <code>/etc/nsswitch.conf</code>.</p>
+
+ <p>If your server doesn't have to perform DNS for any other
+ reason then you might be able to get away with running httpd
+ with the <code>HOSTRESORDER</code> environment variable set to
+ "local". This all depends on what OS and resolver libraries you
+ are using. It also affects CGIs unless you use
+ <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
+ to consult the man pages or FAQs for your OS.</p>
+ </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
+
+
+ <ul>
+ <li>
+ use IP addresses in
+ <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
+ </li>
+
+ <li>
+ use IP addresses in
+ <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
+ </li>
+
+ <li>
+ ensure all virtual hosts have an explicit
+ <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
+ </li>
+
+ <li>create a <code>&lt;VirtualHost _default_:*&gt;</code>
+ server that has no pages to serve</li>
+ </ul>
+ </div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
+<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese">&nbsp;ja&nbsp;</a> |
+<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</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&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed 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 Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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