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 <div class="toplang">
 <p><span>Available Languages: </span><a href="../en/vhosts/mass.html" title="English">&nbsp;en&nbsp;</a> |
 <a href="../ko/vhosts/mass.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
 </div>


     <p>This document describes how to efficiently serve an
     arbitrary number of virtual hosts with Apache 1.3.
     </p>

 </div>
 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#motivation">Motivation</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#overview">Overview</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#simple">Simple dynamic virtual hosts</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#homepages">A virtually hosted homepages system</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#combinations">Using more than
     one virtual hosting system on the same server</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#ipbased">More efficient IP-based virtual hosting</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#oldversion">Using older versions of Apache</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#simple.rewrite">Simple dynamic
     virtual hosts using <code>mod_rewrite</code></a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#homepages.rewrite">A
     homepages system using <code>mod_rewrite</code></a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#xtra-conf">Using a separate virtual
     host configuration file</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="motivation" id="motivation">Motivation</a></h2>

     <p>The techniques described here are of interest if your
     <code>httpd.conf</code> contains many
     <code>&lt;VirtualHost&gt;</code> sections that are
     substantially the same, for example:</p>

 <div class="example"><p><code>
 NameVirtualHost 111.22.33.44<br />
 &lt;VirtualHost 111.22.33.44&gt;<br />
 <span class="indent">
     ServerName                 www.customer-1.com<br />
     DocumentRoot        /www/hosts/www.customer-1.com/docs<br />
     ScriptAlias  /cgi-bin/  /www/hosts/www.customer-1.com/cgi-bin<br />
 </span>
 &lt;/VirtualHost&gt;<br />
 &lt;VirtualHost 111.22.33.44&gt;<br />
 <span class="indent">
     ServerName                 www.customer-2.com<br />
     DocumentRoot        /www/hosts/www.customer-2.com/docs<br />
     ScriptAlias  /cgi-bin/  /www/hosts/www.customer-2.com/cgi-bin<br />
 </span>
 &lt;/VirtualHost&gt;<br />
 # blah blah blah<br />
 &lt;VirtualHost 111.22.33.44&gt;<br />
 <span class="indent">
     ServerName                 www.customer-N.com<br />
     DocumentRoot        /www/hosts/www.customer-N.com/docs<br />
     ScriptAlias  /cgi-bin/  /www/hosts/www.customer-N.com/cgi-bin<br />
 </span>
 &lt;/VirtualHost&gt;
 </code></p></div>

     <p>The basic idea is to replace all of the static
     <code>&lt;VirtualHost&gt;</code> configuration with a mechanism
     that works it out dynamically. This has a number of
     advantages:</p>

     <ol>
       <li>Your configuration file is smaller so Apache starts
       faster and uses less memory.</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 very many
     virtual hosts then doing this is dubious anyway because it eats
     file descriptors. It is better to log to a pipe or a fifo and
     arrange for the process at the other end to distribute the logs
     to the customers (it can also accumulate statistics, etc.).</p>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <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
     is based on automatically inserting this information into the
     pathname of the file that is used to satisfy the request. This
     is done most easily using <code class="module"><a href="../mod/mod_vhost_alias.html">mod_vhost_alias</a></code>,
     but if you are using a version of Apache up to 1.3.6 then you
     must use <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
     Both of these modules are disabled by default; you must enable
     one of them when configuring and building Apache if you want to
     use this technique.</p>

     <p>A couple of things need to be `faked' to make the dynamic
     virtual host look like a normal one. The most important is the
     server name which is used by Apache 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
     comes from the contents of the <code>Host:</code> header in the
     request. With <code>UseCanonicalName DNS</code> it comes 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 Apache 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 `fake' is the document root (configured
     with <code>DocumentRoot</code> and available to CGIs via the
     <code>DOCUMENT_ROOT</code> environment variable). In a normal
     configuration this setting is used by the core module when
     mapping URIs to filenames, but when the server is configured to
     do dynamic virtual hosting that job is taken over by another
     module (either <code>mod_vhost_alias</code> or
     <code>mod_rewrite</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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="simple" id="simple">Simple dynamic virtual hosts</a></h2>

     <p>This extract from <code>httpd.conf</code> implements the
     virtual host arrangement outlined in the <a href="#motivation">Motivation</a> section above, but in a
     generic fashion using <code>mod_vhost_alias</code>.</p>

 <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 />
 LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
 CustomLog logs/access_log vcommon<br />
 <br />
 # include the server name in the filenames used to satisfy requests<br />
 VirtualDocumentRoot /www/hosts/%0/docs<br />
 VirtualScriptAlias  /www/hosts/%0/cgi-bin
 </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.</p>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="homepages" id="homepages">A virtually hosted homepages system</a></h2>

     <p>This is an adjustment of the above system tailored for an
     ISP's homepages server. Using a slightly more complicated
     configuration we can select substrings of the server name to
     use in the filename so that e.g. the documents for
     <code>www.user.isp.com</code> are found in
     <code>/home/user/</code>. It uses a single <code>cgi-bin</code>
     directory instead of one per virtual host.</p>

 <div class="example"><p><code>
 # all the preliminary stuff is the same as above, then<br />
 <br />
 # include part of the server name in the filenames<br />
 VirtualDocumentRoot /www/hosts/%2/docs<br />
 <br />
 # single cgi-bin directory<br />
 ScriptAlias  /cgi-bin/  /www/std-cgi/<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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="combinations" id="combinations">Using more than
     one virtual hosting system on the same server</a></h2>

     <p>With more complicated setups you can use Apache's normal
     <code>&lt;VirtualHost&gt;</code> directives to control the
     scope of the various virtual hosting configurations. For
     example, you could have one IP address for homepages customers
     and another for commercial customers with the following setup.
     This can of course be combined with conventional
     <code>&lt;VirtualHost&gt;</code> configuration sections.</p>

 <div class="example"><p><code>
 UseCanonicalName Off<br />
 <br />
 LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
 <br />
 &lt;Directory /www/commercial&gt;<br />
 <span class="indent">
     Options FollowSymLinks<br />
     AllowOverride All<br />
 </span>
 &lt;/Directory&gt;<br />
 <br />
 &lt;Directory /www/homepages&gt;<br />
 <span class="indent">
     Options FollowSymLinks<br />
     AllowOverride None<br />
 </span>
 &lt;/Directory&gt;<br />
 <br />
 &lt;VirtualHost 111.22.33.44&gt;<br />
 <span class="indent">
     ServerName www.commercial.isp.com<br />
     <br />
     CustomLog logs/access_log.commercial vcommon<br />
     <br />
     VirtualDocumentRoot /www/commercial/%0/docs<br />
     VirtualScriptAlias  /www/commercial/%0/cgi-bin<br />
 </span>
 &lt;/VirtualHost&gt;<br />
 <br />
 &lt;VirtualHost 111.22.33.45&gt;<br />
 <span class="indent">
     ServerName www.homepages.isp.com<br />
     <br />
     CustomLog logs/access_log.homepages vcommon<br />
     <br />
     VirtualDocumentRoot /www/homepages/%0/docs<br />
     ScriptAlias         /cgi-bin/ /www/std-cgi/<br />
 </span>
 &lt;/VirtualHost&gt;
 </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="ipbased" id="ipbased">More efficient IP-based virtual hosting</a></h2>

     <p>After <a href="#simple">the first example</a> I noted that
     it is easy to turn it into an IP-based virtual hosting setup.
     Unfortunately that configuration is not very efficient because
     it requires a DNS lookup for every request. This can be avoided
     by laying out the filesystem according to the IP addresses
     themselves rather than the corresponding names and changing the
     logging similarly. Apache will then usually not need to work
     out the server name and so incur a DNS lookup.</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 />
 CustomLog logs/access_log vcommon<br />
 <br />
 # include the IP address in the filenames<br />
 VirtualDocumentRootIP /www/hosts/%0/docs<br />
 VirtualScriptAliasIP  /www/hosts/%0/cgi-bin<br />
 </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="oldversion" id="oldversion">Using older versions of Apache</a></h2>

     <p>The examples above rely on <code>mod_vhost_alias</code>
     which appeared after version 1.3.6. If you are using a version
     of Apache without <code>mod_vhost_alias</code> then you can
     implement this technique with <code>mod_rewrite</code> as
     illustrated below, but only for Host:-header-based virtual
     hosts.</p>

     <p>In addition there are some things to beware of with logging.
     Apache 1.3.6 is the first version to include the
     <code>%V</code> log format directive; in versions 1.3.0 - 1.3.3
     the <code>%v</code> option did what <code>%V</code> does;
     version 1.3.4 has no equivalent. In all these versions of
     Apache the <code>UseCanonicalName</code> directive can appear
     in <code>.htaccess</code> files which means that customers can
     cause the wrong thing to be logged. Therefore the best thing to
     do is use the <code>%{Host}i</code> directive which logs the
     <code>Host:</code> header directly; note that this may include
     <code>:port</code> on the end which is not the case for
     <code>%V</code>.</p>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="simple.rewrite" id="simple.rewrite">Simple dynamic
     virtual hosts using <code>mod_rewrite</code></a></h2>

     <p>This extract from <code>httpd.conf</code> does the same
     thing as <a href="#simple">the first example</a>. The first
     half is very similar to the corresponding part above but with
     some changes for backward compatibility and to make the
     <code>mod_rewrite</code> part work properly; the second half
     configures <code>mod_rewrite</code> to do the actual work.</p>

     <p>There are a couple of especially tricky bits: By default,
     <code>mod_rewrite</code> runs before the other URI translation
     modules (<code>mod_alias</code> etc.) so if they are used then
     <code>mod_rewrite</code> must be configured to accommodate
     them. Also, some magic must be performed to do a
     per-dynamic-virtual-host equivalent of
     <code>ScriptAlias</code>.</p>

 <div class="example"><p><code>
 # get the server name from the Host: header<br />
 UseCanonicalName Off<br />
 <br />
 # splittable logs<br />
 LogFormat "%{Host}i %h %l %u %t \"%r\" %s %b" vcommon<br />
 CustomLog logs/access_log vcommon<br />
 <br />
 &lt;Directory /www/hosts&gt;<br />
 <span class="indent">
     # ExecCGI is needed here because we can't force<br />
     # CGI execution in the way that ScriptAlias does<br />
     Options FollowSymLinks ExecCGI<br />
 </span>
 &lt;/Directory&gt;<br />
 <br />
 # now for the hard bit<br />
 <br />
 RewriteEngine On<br />
 <br />
 # a ServerName derived from a Host: header may be any case at all<br />
 RewriteMap  lowercase  int:tolower<br />
 <br />
 ## deal with normal documents first:<br />
 # allow Alias /icons/ to work - repeat for other aliases<br />
 RewriteCond  %{REQUEST_URI}  !^/icons/<br />
 # allow CGIs to work<br />
 RewriteCond  %{REQUEST_URI}  !^/cgi-bin/<br />
 # do the magic<br />
 RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1<br />
 <br />
 ## and now deal with CGIs - we have to force a MIME type<br />
 RewriteCond  %{REQUEST_URI}  ^/cgi-bin/<br />
 RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1  [T=application/x-httpd-cgi]<br />
 <br />
 # that's it!
 </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="homepages.rewrite" id="homepages.rewrite">A
     homepages system using <code>mod_rewrite</code></a></h2>

     <p>This does the same thing as <a href="#homepages">the second
     example</a>.</p>

 <div class="example"><p><code>
 RewriteEngine on<br />
 <br />
 RewriteMap   lowercase  int:tolower<br />
 <br />
 # allow CGIs to work<br />
 RewriteCond  %{REQUEST_URI}  !^/cgi-bin/<br />
 <br />
 # check the hostname is right so that the RewriteRule works<br />
 RewriteCond  ${lowercase:%{SERVER_NAME}}  ^www\.[a-z-]+\.isp\.com$<br />
 <br />
 # concatenate the virtual host name onto the start of the URI<br />
 # the [C] means do the next rewrite on the result of this one<br />
 RewriteRule  ^(.+)  ${lowercase:%{SERVER_NAME}}$1  [C]<br />
 <br />
 # now create the real file name<br />
 RewriteRule  ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2<br />
 <br />
 # define the global CGI directory<br />
 ScriptAlias  /cgi-bin/  /www/std-cgi/
 </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="xtra-conf" id="xtra-conf">Using a separate virtual
     host configuration file</a></h2>

     <p>This arrangement uses more advanced <code>mod_rewrite</code>
     features to get the translation from virtual host to document
     root from a separate configuration file. This provides more
     flexibility but requires more complicated configuration.</p>

     <p>The <code>vhost.map</code> file contains something like
     this:</p>

 <div class="example"><p><code>
 www.customer-1.com  /www/customers/1<br />
 www.customer-2.com  /www/customers/2<br />
 # ...<br />
 www.customer-N.com  /www/customers/N<br />
 </code></p></div>

     <p>The <code>http.conf</code> contains this:</p>

 <div class="example"><p><code>
 RewriteEngine on<br />
 <br />
 RewriteMap   lowercase  int:tolower<br />
 <br />
 # define the map file<br />
 RewriteMap   vhost      txt:/www/conf/vhost.map<br />
 <br />
 # deal with aliases as above<br />
 RewriteCond  %{REQUEST_URI}               !^/icons/<br />
 RewriteCond  %{REQUEST_URI}               !^/cgi-bin/<br />
 RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$<br />
 # this does the file-based remap<br />
 RewriteCond  ${vhost:%1}                  ^(/.*)$<br />
 RewriteRule  ^/(.*)$                      %1/docs/$1<br />
 <br />
 RewriteCond  %{REQUEST_URI}               ^/cgi-bin/<br />
 RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$<br />
 RewriteCond  ${vhost:%1}                  ^(/.*)$<br />
 RewriteRule  ^/(.*)$                      %1/cgi-bin/$1
 </code></p></div>

 </div></div>
 <div class="bottomlang">
 <p><span>Available Languages: </span><a href="../en/vhosts/mass.html" title="English">&nbsp;en&nbsp;</a> |
 <a href="../ko/vhosts/mass.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
 </div><div id="footer">
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	<img alt="" src="../images/feather.gif" /></div>
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	<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 2.1</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> \|
	<a href="../ko/vhosts/mass.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
	</div>


	<p>This document describes how to efficiently serve an
	arbitrary number of virtual hosts with Apache 1.3.
	</p>

	</div>
	<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#motivation">Motivation</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#overview">Overview</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#simple">Simple dynamic virtual hosts</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#homepages">A virtually hosted homepages system</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#combinations">Using more than
	one virtual hosting system on the same server</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#ipbased">More efficient IP-based virtual hosting</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#oldversion">Using older versions of Apache</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#simple.rewrite">Simple dynamic
	virtual hosts using <code>mod_rewrite</code></a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#homepages.rewrite">A
	homepages system using <code>mod_rewrite</code></a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#xtra-conf">Using a separate virtual
	host configuration file</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="motivation" id="motivation">Motivation</a></h2>

	<p>The techniques described here are of interest if your
	<code>httpd.conf</code> contains many
	<code><VirtualHost></code> sections that are
	substantially the same, for example:</p>

	<div class="example"><p><code>
	NameVirtualHost 111.22.33.44<br />
	<VirtualHost 111.22.33.44><br />
	<span class="indent">
	ServerName www.customer-1.com<br />
	DocumentRoot /www/hosts/www.customer-1.com/docs<br />
	ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin<br />
	</span>
	</VirtualHost><br />
	<VirtualHost 111.22.33.44><br />
	<span class="indent">
	ServerName www.customer-2.com<br />
	DocumentRoot /www/hosts/www.customer-2.com/docs<br />
	ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin<br />
	</span>
	</VirtualHost><br />
	# blah blah blah<br />
	<VirtualHost 111.22.33.44><br />
	<span class="indent">
	ServerName www.customer-N.com<br />
	DocumentRoot /www/hosts/www.customer-N.com/docs<br />
	ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin<br />
	</span>
	</VirtualHost>
	</code></p></div>

	<p>The basic idea is to replace all of the static
	<code><VirtualHost></code> configuration with a mechanism
	that works it out dynamically. This has a number of
	advantages:</p>

	<ol>
	<li>Your configuration file is smaller so Apache starts
	faster and uses less memory.</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 very many
	virtual hosts then doing this is dubious anyway because it eats
	file descriptors. It is better to log to a pipe or a fifo and
	arrange for the process at the other end to distribute the logs
	to the customers (it can also accumulate statistics, etc.).</p>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<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
	is based on automatically inserting this information into the
	pathname of the file that is used to satisfy the request. This
	is done most easily using <code class="module"><a href="../mod/mod_vhost_alias.html">mod_vhost_alias</a></code>,
	but if you are using a version of Apache up to 1.3.6 then you
	must use <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
	Both of these modules are disabled by default; you must enable
	one of them when configuring and building Apache if you want to
	use this technique.</p>

	<p>A couple of things need to be `faked' to make the dynamic
	virtual host look like a normal one. The most important is the
	server name which is used by Apache 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
	comes from the contents of the <code>Host:</code> header in the
	request. With <code>UseCanonicalName DNS</code> it comes 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 Apache 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 `fake' is the document root (configured
	with <code>DocumentRoot</code> and available to CGIs via the
	<code>DOCUMENT_ROOT</code> environment variable). In a normal
	configuration this setting is used by the core module when
	mapping URIs to filenames, but when the server is configured to
	do dynamic virtual hosting that job is taken over by another
	module (either <code>mod_vhost_alias</code> or
	<code>mod_rewrite</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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="simple" id="simple">Simple dynamic virtual hosts</a></h2>

	<p>This extract from <code>httpd.conf</code> implements the
	virtual host arrangement outlined in the <a href="#motivation">Motivation</a> section above, but in a
	generic fashion using <code>mod_vhost_alias</code>.</p>

	<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 />
	LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
	CustomLog logs/access_log vcommon<br />
	<br />
	# include the server name in the filenames used to satisfy requests<br />
	VirtualDocumentRoot /www/hosts/%0/docs<br />
	VirtualScriptAlias /www/hosts/%0/cgi-bin
	</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.</p>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="homepages" id="homepages">A virtually hosted homepages system</a></h2>

	<p>This is an adjustment of the above system tailored for an
	ISP's homepages server. Using a slightly more complicated
	configuration we can select substrings of the server name to
	use in the filename so that e.g. the documents for
	<code>www.user.isp.com</code> are found in
	<code>/home/user/</code>. It uses a single <code>cgi-bin</code>
	directory instead of one per virtual host.</p>

	<div class="example"><p><code>
	# all the preliminary stuff is the same as above, then<br />
	<br />
	# include part of the server name in the filenames<br />
	VirtualDocumentRoot /www/hosts/%2/docs<br />
	<br />
	# single cgi-bin directory<br />
	ScriptAlias /cgi-bin/ /www/std-cgi/<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><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="combinations" id="combinations">Using more than
	one virtual hosting system on the same server</a></h2>

	<p>With more complicated setups you can use Apache'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 homepages customers
	and another for commercial customers with the following setup.
	This can of course be combined with conventional
	<code><VirtualHost></code> configuration sections.</p>

	<div class="example"><p><code>
	UseCanonicalName Off<br />
	<br />
	LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />
	<br />
	<Directory /www/commercial><br />
	<span class="indent">
	Options FollowSymLinks<br />
	AllowOverride All<br />
	</span>
	</Directory><br />
	<br />
	<Directory /www/homepages><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.isp.com<br />
	<br />
	CustomLog logs/access_log.commercial vcommon<br />
	<br />
	VirtualDocumentRoot /www/commercial/%0/docs<br />
	VirtualScriptAlias /www/commercial/%0/cgi-bin<br />
	</span>
	</VirtualHost><br />
	<br />
	<VirtualHost 111.22.33.45><br />
	<span class="indent">
	ServerName www.homepages.isp.com<br />
	<br />
	CustomLog logs/access_log.homepages vcommon<br />
	<br />
	VirtualDocumentRoot /www/homepages/%0/docs<br />
	ScriptAlias /cgi-bin/ /www/std-cgi/<br />
	</span>
	</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="ipbased" id="ipbased">More efficient IP-based virtual hosting</a></h2>

	<p>After <a href="#simple">the first example</a> I noted that
	it is easy to turn it into an IP-based virtual hosting setup.
	Unfortunately that configuration is not very efficient because
	it requires a DNS lookup for every request. This can be avoided
	by laying out the filesystem according to the IP addresses
	themselves rather than the corresponding names and changing the
	logging similarly. Apache will then usually not need to work
	out the server name and so incur a DNS lookup.</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 />
	CustomLog logs/access_log vcommon<br />
	<br />
	# include the IP address in the filenames<br />
	VirtualDocumentRootIP /www/hosts/%0/docs<br />
	VirtualScriptAliasIP /www/hosts/%0/cgi-bin<br />
	</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="oldversion" id="oldversion">Using older versions of Apache</a></h2>

	<p>The examples above rely on <code>mod_vhost_alias</code>
	which appeared after version 1.3.6. If you are using a version
	of Apache without <code>mod_vhost_alias</code> then you can
	implement this technique with <code>mod_rewrite</code> as
	illustrated below, but only for Host:-header-based virtual
	hosts.</p>

	<p>In addition there are some things to beware of with logging.
	Apache 1.3.6 is the first version to include the
	<code>%V</code> log format directive; in versions 1.3.0 - 1.3.3
	the <code>%v</code> option did what <code>%V</code> does;
	version 1.3.4 has no equivalent. In all these versions of
	Apache the <code>UseCanonicalName</code> directive can appear
	in <code>.htaccess</code> files which means that customers can
	cause the wrong thing to be logged. Therefore the best thing to
	do is use the <code>%{Host}i</code> directive which logs the
	<code>Host:</code> header directly; note that this may include
	<code>:port</code> on the end which is not the case for
	<code>%V</code>.</p>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="simple.rewrite" id="simple.rewrite">Simple dynamic
	virtual hosts using <code>mod_rewrite</code></a></h2>

	<p>This extract from <code>httpd.conf</code> does the same
	thing as <a href="#simple">the first example</a>. The first
	half is very similar to the corresponding part above but with
	some changes for backward compatibility and to make the
	<code>mod_rewrite</code> part work properly; the second half
	configures <code>mod_rewrite</code> to do the actual work.</p>

	<p>There are a couple of especially tricky bits: By default,
	<code>mod_rewrite</code> runs before the other URI translation
	modules (<code>mod_alias</code> etc.) so if they are used then
	<code>mod_rewrite</code> must be configured to accommodate
	them. Also, some magic must be performed to do a
	per-dynamic-virtual-host equivalent of
	<code>ScriptAlias</code>.</p>

	<div class="example"><p><code>
	# get the server name from the Host: header<br />
	UseCanonicalName Off<br />
	<br />
	# splittable logs<br />
	LogFormat "%{Host}i %h %l %u %t \"%r\" %s %b" vcommon<br />
	CustomLog logs/access_log vcommon<br />
	<br />
	<Directory /www/hosts><br />
	<span class="indent">
	# ExecCGI is needed here because we can't force<br />
	# CGI execution in the way that ScriptAlias does<br />
	Options FollowSymLinks ExecCGI<br />
	</span>
	</Directory><br />
	<br />
	# now for the hard bit<br />
	<br />
	RewriteEngine On<br />
	<br />
	# a ServerName derived from a Host: header may be any case at all<br />
	RewriteMap lowercase int:tolower<br />
	<br />
	## deal with normal documents first:<br />
	# allow Alias /icons/ to work - repeat for other aliases<br />
	RewriteCond %{REQUEST_URI} !^/icons/<br />
	# allow CGIs to work<br />
	RewriteCond %{REQUEST_URI} !^/cgi-bin/<br />
	# do the magic<br />
	RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1<br />
	<br />
	## and now deal with CGIs - we have to force a MIME type<br />
	RewriteCond %{REQUEST_URI} ^/cgi-bin/<br />
	RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]<br />
	<br />
	# that's it!
	</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="homepages.rewrite" id="homepages.rewrite">A
	homepages system using <code>mod_rewrite</code></a></h2>

	<p>This does the same thing as <a href="#homepages">the second
	example</a>.</p>

	<div class="example"><p><code>
	RewriteEngine on<br />
	<br />
	RewriteMap lowercase int:tolower<br />
	<br />
	# allow CGIs to work<br />
	RewriteCond %{REQUEST_URI} !^/cgi-bin/<br />
	<br />
	# check the hostname is right so that the RewriteRule works<br />
	RewriteCond ${lowercase:%{SERVER_NAME}} ^www\.[a-z-]+\.isp\.com$<br />
	<br />
	# concatenate the virtual host name onto the start of the URI<br />
	# the [C] means do the next rewrite on the result of this one<br />
	RewriteRule ^(.+) ${lowercase:%{SERVER_NAME}}$1 [C]<br />
	<br />
	# now create the real file name<br />
	RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2<br />
	<br />
	# define the global CGI directory<br />
	ScriptAlias /cgi-bin/ /www/std-cgi/
	</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="xtra-conf" id="xtra-conf">Using a separate virtual
	host configuration file</a></h2>

	<p>This arrangement uses more advanced <code>mod_rewrite</code>
	features to get the translation from virtual host to document
	root from a separate configuration file. This provides more
	flexibility but requires more complicated configuration.</p>

	<p>The <code>vhost.map</code> file contains something like
	this:</p>

	<div class="example"><p><code>
	www.customer-1.com /www/customers/1<br />
	www.customer-2.com /www/customers/2<br />
	# ...<br />
	www.customer-N.com /www/customers/N<br />
	</code></p></div>

	<p>The <code>http.conf</code> contains this:</p>

	<div class="example"><p><code>
	RewriteEngine on<br />
	<br />
	RewriteMap lowercase int:tolower<br />
	<br />
	# define the map file<br />
	RewriteMap vhost txt:/www/conf/vhost.map<br />
	<br />
	# deal with aliases as above<br />
	RewriteCond %{REQUEST_URI} !^/icons/<br />
	RewriteCond %{REQUEST_URI} !^/cgi-bin/<br />
	RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$<br />
	# this does the file-based remap<br />
	RewriteCond ${vhost:%1} ^(/.*)$<br />
	RewriteRule ^/(.*)$ %1/docs/$1<br />
	<br />
	RewriteCond %{REQUEST_URI} ^/cgi-bin/<br />
	RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$<br />
	RewriteCond ${vhost:%1} ^(/.*)$<br />
	RewriteRule ^/(.*)$ %1/cgi-bin/$1
	</code></p></div>

	</div></div>
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