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<title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server Version 2.4</title>
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<p class="apache">Apache HTTP Server Version 2.4</p>
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<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 2.4</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"> en </a> |
<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Franais"> fr </a> |
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<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"><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><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<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
<VirtualHost www.example.dom>
ServerAdmin webgirl@example.dom
DocumentRoot "/www/example"
</VirtualHost></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
<VirtualHost 192.0.2.1>
ServerAdmin webgirl@example.dom
DocumentRoot "/www/example"
</VirtualHost></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"><VirtualHost 192.0.2.1>
ServerName www.example.dom
ServerAdmin webgirl@example.dom
DocumentRoot "/www/example"
</VirtualHost></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"><VirtualHost www.example1.dom>
ServerAdmin webgirl@example1.dom
DocumentRoot "/www/example1"
</VirtualHost>
<VirtualHost www.example2.dom>
ServerAdmin webguy@example2.dom
DocumentRoot "/www/example2"
</VirtualHost></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><VirtualHost _default_:*></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"> en </a> |
<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Franais"> fr </a> |
<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Trke"> tr </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&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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