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+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
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+ This file is generated from xml source: DO NOT EDIT
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+ --><title>Issues Regarding DNS and Apache - Apache HTTP Server</title><link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /><link href="./images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div id="page-header"><p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p><p class="apache">Apache HTTP Server Version 2.0</p><img alt="" src="./images/feather.gif" /></div><div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="./">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1>
+ <p>This page could be summarized with the statement: don't require Apache
+ to use DNS for any parsing of the configuration files. 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>
+ </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><li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</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>
+
+
+ <div class="example"><p><code>
+ <VirtualHost www.abc.dom> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </code></p></div>
+
+ <p>In order for Apache 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 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>
+
+ <div class="example"><p><code>
+ <VirtualHost 10.0.0.1> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </code></p></div>
+
+ <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>
+
+ <div class="example"><p><code>
+ <VirtualHost 10.0.0.1> <br />
+ ServerName www.abc.dom <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </code></p></div>
+ </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>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>
+
+ <div class="example"><p><code>
+ <VirtualHost www.abc.dom> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost> <br />
+ <br />
+ <VirtualHost www.def.dom> <br />
+ ServerAdmin webguy@def.dom <br />
+ DocumentRoot /www/def <br />
+ </VirtualHost>
+ </code></p></div>
+
+ <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>
+ </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>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 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 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
+ <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 class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div><div class="section"><h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
+
+
+ <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.</p>
+ </div></div><div id="footer"><p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div></body></html>
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-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
- "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
-
-<html xmlns="http://www.w3.org/1999/xhtml">
- <head>
- <meta name="generator" content="HTML Tidy, see www.w3.org" />
-
- <title>Issues Regarding DNS and Apache</title>
- </head>
- <!-- Background white, links blue (unvisited), navy (visited), red (active) -->
-
- <body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
- vlink="#000080" alink="#FF0000">
- <!--#include virtual="header.html" -->
-
- <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>
-
+<?xml version="1.0" encoding="UTF-8" ?>
+<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
+<?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
+
+<manualpage>
+ <relativepath href="." />
+
+ <title>Issues Regarding DNS and Apache</title>
+
+ <summary>
+ <p>This page could be summarized with the statement: don't require Apache
+ to use DNS for any parsing of the configuration files. 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>
+ </summary>
+
+ <section id="example">
+ <title>A Simple Example</title>
+
+ <example>
+ <VirtualHost www.abc.dom> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </example>
+
<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>
+ <directive module="core">ServerName</directive>
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
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>
-
+
+ <example>
+ <VirtualHost 10.0.0.1> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </example>
+
<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
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>
-
+
+ <example>
+ <VirtualHost 10.0.0.1> <br />
+ ServerName www.abc.dom <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost>
+ </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
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>
-
+
+ <example>
+ <VirtualHost www.abc.dom> <br />
+ ServerAdmin webgirl@abc.dom <br />
+ DocumentRoot /www/abc <br />
+ </VirtualHost> <br />
+ <br />
+ <VirtualHost www.def.dom> <br />
+ ServerAdmin webguy@def.dom <br />
+ DocumentRoot /www/def <br />
+ </VirtualHost>
+ </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
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
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>
-
+ </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 <code>ServerName</code>
+ 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
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>
-
+ 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 <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>
+ 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><VirtualHost _default_:*></code>
server that has no pages to serve</li>
</ul>
-
- <h3>Appendix: Future Directions</h3>
-
+ </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
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
configuration. But as of March 1997 these features have not
been deployed widely enough to be put into use on critical
webservers.</p>
- <!--#include virtual="footer.html" -->
- </body>
-</html>
-
+ </section>
+</manualpage>
\ No newline at end of file
projects / apache / commitdiff