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<title>Issues Regarding DNS and Apache</title>
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<h3>Apache HTTP Server Version 1.3</h3>
<p><small><em>Is this the version you want? For more recent
versions, check our <a href="/docs/">documentation
index</a>.</em></small></p>
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<h1 align="CENTER">Issues Regarding DNS and Apache</h1>
<p>This page could be summarized with the statement: <em>don't
require Apache to use DNS for any parsing of the configuration
files</em>. If Apache has to use DNS to parse the configuration
files then your server may be subject to reliability problems
(it might not boot), or denial and theft of service attacks
(including users able to steal hits from other users).</p>
<h3>A Simple Example</h3>
Consider this configuration snippet:
<blockquote>
<pre>
<VirtualHost www.abc.dom>
ServerAdmin webgirl@abc.dom
DocumentRoot /www/abc
</VirtualHost>
</pre>
</blockquote>
<p>In order for Apache to function properly it absolutely needs
to have two pieces of information about each virtual host: the
<a href="mod/core.html#servername"><code>ServerName</code></a>
and at least one IP address that the server responds to. This
example does not include the IP address, so Apache must use DNS
to find the address of <code>www.abc.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 (prior to Apache version 1.2 the server would
not even boot).</p>
<p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
Then consider this configuration snippet:</p>
<blockquote>
<pre>
<VirtualHost 10.0.0.1>
ServerAdmin webgirl@abc.dom
DocumentRoot /www/abc
</VirtualHost>
</pre>
</blockquote>
<p>Now Apache 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
(prior to Apache version 1.2 the server would not even boot).
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 Apache should ever have to generate a full URL
for the server which includes the server name then it will fail
to generate a valid URL.</p>
<p>Here is a snippet that avoids both of these problems.</p>
<blockquote>
<pre>
<VirtualHost 10.0.0.1>
ServerName www.abc.dom
ServerAdmin webgirl@abc.dom
DocumentRoot /www/abc
</VirtualHost>
</pre>
</blockquote>
<h3>Denial of Service</h3>
<p>There are (at least) two forms that denial of service can
come in. If you are running a version of Apache prior to
version 1.2 then your server will not even boot if one of the
two DNS lookups mentioned above fails for any of your virtual
hosts. In some cases this DNS lookup may not even be under your
control. For example, if <code>abc.dom</code> is one of your
customers and they control their own DNS then they can force
your (pre-1.2) server to fail while booting simply by deleting
the <code>www.abc.dom</code> record.</p>
<p>Another form is far more insidious. Consider this
configuration snippet:</p>
<blockquote>
<pre>
<VirtualHost www.abc.dom>
ServerAdmin webgirl@abc.dom
DocumentRoot /www/abc
</VirtualHost>
</pre>
</blockquote>
<blockquote>
<pre>
<VirtualHost www.def.dom>
ServerAdmin webguy@def.dom
DocumentRoot /www/def
</VirtualHost>
</pre>
</blockquote>
<p>Suppose that you've assigned 10.0.0.1 to
<code>www.abc.dom</code> and 10.0.0.2 to
<code>www.def.dom</code>. Furthermore, suppose that
<code>def.com</code> has control of their own DNS. With this
config you have put <code>def.com</code> into a position where
they can steal all traffic destined to <code>abc.com</code>. To
do so, all they have to do is set <code>www.def.dom</code> to
10.0.0.1. Since they control their own DNS you can't stop them
from pointing the <code>www.def.com</code> record wherever they
wish.</p>
<p>Requests coming in to 10.0.0.1 (including all those where
users typed in URLs of the form
<code>http://www.abc.dom/whatever</code>) will all be served by
the <code>def.com</code> virtual host. To better understand why
this happens requires a more in-depth discussion of how Apache
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>
<h3>The "main server" Address</h3>
<p>The addition of <a href="vhosts/name-based.html">name-based
virtual host support</a> in Apache 1.1 requires Apache to know
the IP address(es) of the host that httpd is running on. To get
this address it uses either the global <code>ServerName</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 Apache
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 <a
href="mod/mod_env.html"><code>mod_env</code></a> to control the
environment. It's best to consult the man pages or FAQs for
your OS.</p>
<h3><a id="tips" name="tips">Tips to Avoid these
problems</a></h3>
<ul>
<li>use IP addresses in <code><VirtualHost></code></li>
<li>use IP addresses in <code>Listen</code></li>
<li>use IP addresses in <code>BindAddress</code></li>
<li>ensure all virtual hosts have an explicit
<code>ServerName</code></li>
<li>create a <code><VirtualHost _default_:*></code>
server that has no pages to serve</li>
</ul>
<h3>Appendix: Future Directions</h3>
<p>The situation regarding DNS is highly undesirable. For
Apache 1.2 we've attempted to make the server at least continue
booting in the event of failed DNS, but it might not be the
best we can do. In any event requiring the use of explicit IP
addresses in configuration files is highly undesirable in
today's Internet where renumbering is a necessity.</p>
<p>A possible work around to the theft of service attack
described above would be to perform a reverse DNS lookup on the
IP address returned by the forward lookup and compare the two
names. In the event of a mismatch the virtualhost would be
disabled. This would require reverse DNS to be configured
properly (which is something that most admins are familiar with
because of the common use of "double-reverse" DNS lookups by
FTP servers and TCP wrappers).</p>
<p>In any event it doesn't seem possible to reliably boot a
virtual-hosted web server when DNS has failed unless IP
addresses are used. Partial solutions such as disabling
portions of the configuration might be worse than not booting
at all depending on what the webserver is supposed to
accomplish.</p>
<p>As HTTP/1.1 is deployed and browsers and proxies start
issuing the <code>Host</code> header it will become possible to
avoid the use of IP-based virtual hosts entirely. In this event
a webserver has no requirement to do DNS lookups during
configuration. But as of March 1997 these features have not
been deployed widely enough to be put into use on critical
webservers. <hr />
<h3 align="CENTER">Apache HTTP Server</h3>
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