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

<summary>

<p>This page could be summarized with the statement: don't

configure Apache in such a way that it relies on DNS resolution

for parsing of the configuration files. If Apache requires DNS

resolution to parse the configuration files then your server

may be subject to reliability problems (ie. it might not boot),

or denial and theft of service attacks (including users able

to steal hits from other users).</p>

</summary>

<section id="example">

<title>A Simple Example</title>

<example>

&lt;VirtualHost www.abc.dom&gt; <br />

ServerAdmin webgirl@abc.dom <br />

DocumentRoot /www/abc <br />

&lt;/VirtualHost&gt;

</example>

<p>In order for Apache 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 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>

<example>

&lt;VirtualHost 10.0.0.1&gt; <br />

ServerAdmin webgirl@abc.dom <br />

DocumentRoot /www/abc <br />

&lt;/VirtualHost&gt;

</example>

<p>This time 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>

<example>

&lt;VirtualHost 10.0.0.1&gt; <br />

ServerName www.abc.dom <br />

ServerAdmin webgirl@abc.dom <br />

DocumentRoot /www/abc <br />

&lt;/VirtualHost&gt;

</example>

</section>

<section id="denial">

<title>Denial of Service</title>

<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, 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>

<example>

&lt;VirtualHost www.abc.dom&gt; <br />

&nbsp;&nbsp;ServerAdmin webgirl@abc.dom <br />

&nbsp;&nbsp;DocumentRoot /www/abc <br />

&lt;/VirtualHost&gt; <br />

<br />

&lt;VirtualHost www.def.dom&gt; <br />

&nbsp;&nbsp;ServerAdmin webguy@def.dom <br />

&nbsp;&nbsp;DocumentRoot /www/def <br />

&lt;/VirtualHost&gt;

</example>

<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.dom</code> has control of their own DNS. With this

config you have put <code>def.dom</code> into a position where

they can steal all traffic destined to <code>abc.dom</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.dom</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.dom</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>

</section>

<section id="main">

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

<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

<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 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

<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>

<section id="appendix">

<title>Appendix: Future Directions</title>

<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 case,

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.</p>

</section>

</manualpage>