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<title>Dynamically Configured Mass Virtual Hosting - Apache HTTP Server</title>
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<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 2.3</p>
<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/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Virtual Hosts</a></div><div id="page-content"><div id="preamble"><h1>Dynamically Configured Mass Virtual Hosting</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="/en/vhosts/mass.html" title="English"> en </a> |
</div>
<p>This document describes how to efficiently serve an
arbitrary number of virtual hosts with the Apache HTTP Server. A
<code class="module"><a href="/mod/mod_rewrite.html">mod_rewrite</a></code> to create dynamic mass virtual hosts.
</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#motivation">Motivation</a></li>
mod_vhost_alias</a></li>
<li><img alt="" src="/images/down.gif" /> <a href="#homepages">Simplified Dynamic Virtual Hosts</a></li>
Hosting Systems on the Same Server</a></li>
<li><img alt="" src="/images/down.gif" /> <a href="#ipbased">More Efficient IP-Based Virtual Hosting</a></li>
mod_rewrite</a></li>
</ul></div>
<div class="section">
<h2><a name="motivation" id="motivation">Motivation</a></h2>
<p>The techniques described here are of interest if your
<code><VirtualHost></code> sections that are
substantially the same, for example:</p>
<div class="example"><pre>
<VirtualHost 111.22.33.44>
ServerName customer-1.example.com
</VirtualHost>
<VirtualHost 111.22.33.44>
ServerName customer-2.example.com
</VirtualHost>
<VirtualHost 111.22.33.44>
ServerName customer-N.example.com
</VirtualHost>
</pre></div>
<p>We wish to replace these multiple
<code><VirtualHost></code> blocks with a mechanism
that works them out dynamically. This has a number of
advantages:</p>
<ol>
<li>Your configuration file is smaller, so Apache starts
more quickly and uses less memory. Perhaps more importantly, the
smaller configuration is easier to maintain, and leaves less room
for errors.</li>
<li>Adding virtual hosts is simply a matter of creating the
appropriate directories in the filesystem and entries in the
DNS - you don't need to reconfigure or restart Apache.</li>
</ol>
<p>The main disadvantage is that you cannot have a different log file for
each virtual host; however, if you have many virtual hosts, doing
this can be a bad idea anyway, because of the <a href="fd-limits.html">number of file descriptors needed</a>.
and arrange for the process at the other end to split up the log
files into one per virtual host. One example of such a process can
utility.</p>
<div class="section">
<h2><a name="overview" id="overview">Overview</a></h2>
<p>A virtual host is defined by two pieces of information: its
IP address, and the contents of the <code>Host:</code> header
in the HTTP request. The dynamic mass virtual hosting technique
used here is based on automatically inserting this information into the
pathname of the file that is used to satisfy the request. This
can be most easily done by using <code class="module"><a href="/mod/mod_vhost_alias.html">mod_vhost_alias</a></code>
with Apache httpd. Alternatively,
<a href="/rewrite/vhosts.html">mod_rewrite can
be used</a>.</p>
<p>Both of these modules are disabled by default; you must enable
one of them when configuring and building Apache httpd if you want to
use this technique.</p>
<p>A couple of things need to be determined from the request in
order to make the dynamic
virtual host look like a normal one. The most important is the
server name, which is used by the server to generate
self-referential URLs etc. It is configured with the
<code>ServerName</code> directive, and it is available to CGIs
via the <code>SERVER_NAME</code> environment variable. The
actual value used at run time is controlled by the <code class="directive"><a href="/mod/core.html#usecanonicalname">UseCanonicalName</a></code>
setting. With <code>UseCanonicalName Off</code>, the server name
is taken from the contents of the <code>Host:</code> header in the
request. With <code>UseCanonicalName DNS</code>, it is taken from a
reverse DNS lookup of the virtual host's IP address. The former
setting is used for name-based dynamic virtual hosting, and the
latter is used for IP-based hosting. If httpd cannot work out
the server name because there is no <code>Host:</code> header,
or the DNS lookup fails, then the value configured with
<code>ServerName</code> is used instead.</p>
<p>The other thing to determine is the document root (configured
with <code>DocumentRoot</code> and available to CGI scripts via the
<code>DOCUMENT_ROOT</code> environment variable). In a normal
configuration, this is used by the core module when
mapping URIs to filenames, but when the server is configured to
do dynamic virtual hosting, that job must be taken over by another
module (either <code class="module"><a href="/mod/mod_vhost_alias.html">mod_vhost_alias</a></code> or
<code class="module"><a href="/mod/mod_rewrite.html">mod_rewrite</a></code>), which has a different way of doing
the mapping. Neither of these modules is responsible for
setting the <code>DOCUMENT_ROOT</code> environment variable so
if any CGIs or SSI documents make use of it, they will get a
misleading value.</p>
<div class="section">
<h2><a name="simple" id="simple">Dynamic Virtual Hosts with
mod_vhost_alias</a></h2>
virtual host arrangement outlined in the <a href="#motivation">Motivation</a> section above
<div class="example"><p><code>
# get the server name from the Host: header<br />
UseCanonicalName Off<br />
<br />
# this log format can be split per-virtual-host based on the first field<br />
# using the split-logfile utility.<br />
LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
<br />
# include the server name in the filenames used to satisfy requests<br />
</code></p></div>
<p>This configuration can be changed into an IP-based virtual
hosting solution by just turning <code>UseCanonicalName
Off</code> into <code>UseCanonicalName DNS</code>. The server
name that is inserted into the filename is then derived from
the IP address of the virtual host. The variable <code>%0</code>
references the requested servername, as indicated in the
<code>Host:</code> header.</p>
<p>See the <code class="module"><a href="/mod/mod_vhost_alias.html">mod_vhost_alias</a></code> documentation for more usage
examples.</p>
<div class="section">
<h2><a name="homepages" id="homepages">Simplified Dynamic Virtual Hosts</a></h2>
<p>This is an adjustment of the above system, tailored for an
ISP's web hosting server. Using <code>%2</code>,
we can select substrings of the server name to
use in the filename so that, for example, the documents for
directory instead of one per virtual host.</p>
<div class="example"><p><code>
UseCanonicalName Off<br />
<br />
LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
<br />
# include part of the server name in the filenames<br />
<br />
# single cgi-bin directory<br />
</code></p></div>
<p>There are examples of more complicated
<code>VirtualDocumentRoot</code> settings in the
<code class="module"><a href="/mod/mod_vhost_alias.html">mod_vhost_alias</a></code> documentation.</p>
<div class="section">
<h2><a name="combinations" id="combinations">Using Multiple Virtual
Hosting Systems on the Same Server</a></h2>
<p>With more complicated setups, you can use httpd's normal
<code><VirtualHost></code> directives to control the
scope of the various virtual hosting configurations. For
example, you could have one IP address for general customers' homepages,
and another for commercial customers, with the following setup.
This can be combined with conventional
<code><VirtualHost></code> configuration sections, as shown
below.</p>
<div class="example"><p><code>
UseCanonicalName Off<br />
<br />
LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
<br />
<span class="indent">
Options FollowSymLinks<br />
AllowOverride All<br />
</span>
</Directory><br />
<br />
<span class="indent">
Options FollowSymLinks<br />
AllowOverride None<br />
</span>
</Directory><br />
<br />
<VirtualHost 111.22.33.44><br />
<span class="indent">
ServerName www.commercial.example.com<br />
<br />
<br />
</span>
</VirtualHost><br />
<br />
<VirtualHost 111.22.33.45><br />
<span class="indent">
ServerName www.homepages.example.com<br />
<br />
<br />
</span>
</VirtualHost>
</code></p></div>
<div class="note">
<h3>Note</h3>
<p>If the first VirtualHost block does <em>not</em> include a
<code class="directive"><a href="/mod/core.html#servername">ServerName</a></code> directive, the reverse
DNS of the relevant IP will be used instead.
If this is not the server name you
wish to use, a bogus entry (eg. <code>ServerName
none.example.com</code>) can be added to get around this
behaviour.</p>
</div>
<div class="section">
<h2><a name="ipbased" id="ipbased">More Efficient IP-Based Virtual Hosting</a></h2>
<p>The configuration changes suggested to turn <a href="#simple">the first
example</a> into an IP-based virtual hosting setup result in
a rather inefficient setup. A new DNS lookup is required for every
request. To avoid this overhead, the filesystem can be arranged to
correspond to the IP addresses, instead of to the host names, thereby
negating the need for a DNS lookup. Logging will also have to be adjusted
to fit this system.</p>
<div class="example"><p><code>
# get the server name from the reverse DNS of the IP address<br />
UseCanonicalName DNS<br />
<br />
# include the IP address in the logs so they may be split<br />
LogFormat "%A %h %l %u %t \"%r\" %s %b" vcommon<br />
<br />
# include the IP address in the filenames<br />
</code></p></div>
<div class="section">
<h2><a name="rewrite" id="rewrite">Mass virtual hosts with
mod_rewrite</a></h2>
<p>
Mass virtual hosting may also be accomplished using
<code class="module"><a href="/mod/mod_rewrite.html">mod_rewrite</a></code>, either using simple <code class="directive"><a href="/mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives, or using more
complicated techniques such as storing the vhost definitions externally
and accessing them via <code class="directive"><a href="/mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>. These techniques are
discussed in the <a href="/rewrite/vhosts.html">rewrite
documentation</a>.</p>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="/en/vhosts/mass.html" title="English"> en </a> |
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<p class="apache">Copyright 2011 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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