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<manualpage metafile="dns-caveats.xml.meta">

<title>Issues Regarding DNS and Apache HTTP Server</title>

<summary>

<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>

</summary>

<section id="example">

<title>A Simple Example</title>

<highlight language="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;

</highlight>

<p>In order for the server to function properly, it absolutely needs

to have two pieces of information about each virtual host: the

<directive module="core">ServerName</directive> 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>

<highlight language="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;

</highlight>

<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>

<highlight language="config">

&lt;VirtualHost 192.0.2.1&gt;

ServerName www.example.dom

ServerAdmin webgirl@example.dom

DocumentRoot "/www/example"

&lt;/VirtualHost&gt;

</highlight>

</section>

<section id="denial">

<title>Denial of Service</title>

<p>Consider this configuration snippet:</p>

<highlight language="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;

</highlight>

<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>

</section>

<section id="main">

<title>The "main server" Address</title>

<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 <program>httpd</program>

is running on. To get this address it uses either the global

<directive module="core">ServerName</directive>

(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

<module>mod_env</module> to control the environment. It's best

to consult the man pages or FAQs for your OS.</p>

</section>

<section id="tips">

<title>Tips to Avoid These Problems</title>

<ul>

<li>

use IP addresses in

<directive module="core">VirtualHost</directive>

</li>

<li>

use IP addresses in

<directive module="mpm_common">Listen</directive>

</li>

<li>

ensure all virtual hosts have an explicit

<directive module="core">ServerName</directive>

</li>

<li>create a <code>&lt;VirtualHost _default_:*&gt;</code>

server that has no pages to serve</li>

</ul>

</section>

</manualpage>