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		<TITLE>Dynamically configured mass virtual hosting</TITLE>
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		<H1 ALIGN="CENTER">Dynamically configured mass virtual hosting</H1>

		<P>This document describes how to efficiently serve an arbitrary number
		of virtual hosts with Apache 1.3. Some familiarity with
		<A HREF="../mod/mod_rewrite.html"><CODE>mod_rewrite</CODE></A> is
		useful.</P>

		<!--

		Written by Tony Finch (fanf@demon.net) (dot@dotat.at).

		Some examples were derived from Ralf S. Engleschall's document
		http://www.engelschall.com/pw/apache/rewriteguide/

		Some suggestions were made by Brian Behlendorf.

		-->

		<H2><A NAME="contents">Contents:</A></H2>

		<UL>
		<LI><A HREF="#motivation">Motivation</A>
		<LI><A HREF="#overview">Overview of the technique</A>
		<LI><A HREF="#simple">Simple name-based dynamic virtual hosts</A>
		<LI><A HREF="#homepages">A virtually hosted homepages system</A>
		<LI><A HREF="#xtra-conf">Using a separate virtual host configuration file</A>
		<LI><A HREF="#combinations">Using more than one virtual hosting system on the same server</A>
		</UL>

		<HR><H2><A NAME="motivation">Motivation</A></H2>

		<P>The techniques described here are of interest if your
		<CODE>httpd.conf</CODE> contains hundreds of
		<CODE><VirtualHost></CODE> sections that are substantially the
		same, for example:
		<PRE>
		NameVirtualHost 111.22.33.44
		<VirtualHost 111.22.33.44>
		ServerName www.customer-1.com
		DocumentRoot /www/hosts/www.customer-1.com/docs
		ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin
		</VirtualHost>
		<VirtualHost 111.22.33.44>
		ServerName www.customer-2.com
		DocumentRoot /www/hosts/www.customer-2.com/docs
		ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin
		</VirtualHost>
		# blah blah blah
		<VirtualHost 111.22.33.44>
		ServerName www.customer-N.com
		DocumentRoot /www/hosts/www.customer-N.com/docs
		ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin
		</VirtualHost>
		</PRE>
		</P>

		<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:
		<OL>
		<LI>Your configuration file is smaller so Apache starts faster and
		uses less memory.
		<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.
		</OL>
		</P>

		<P>The main disadvantage is that you cannot have a different log file
		for each server; however if you have very many virtual hosts then
		doing this is dubious anyway because it eats file descriptors. It's
		better to log to a pipe or a fifo and arrange for the process at the
		other end to distribute the logs (and perhaps accumulate statistics,
		etc.). A <CODE>LogFormat</CODE> directive that includes
		<CODE>%v</CODE> for the virtual host makes it easy to do this.</P>


		<HR><H2><A NAME="overview">Overview of the technique</A></H2>

		<P>All of the dynamic virtual hosts will either be configured as part
		of the main server configuration, or within a
		<CODE><VirtualHost></CODE> section. For a simple (very uniform)
		setup, <CODE><VirtualHost></CODE> sections aren't needed at all.</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
		(configured with <CODE>ServerName</CODE> and available to CGIs via the
		<CODE>SERVER_NAME</CODE> environment variable). The way it is
		determined is controlled by the <CODE>UseCanonicalName</CODE>
		directive: with <CODE>UseCanonicalName off</CODE> the server name
		comes from the contents of the <CODE>Host:</CODE> header in the
		request. If there is no <CODE>Host:</CODE> header then the value
		configured with <CODE>ServerName</CODE> is used instead.</P>

		<P>The other one is the document root (configured with
		<CODE>DocumentRoot</CODE> and available to CGIs via the
		<CODE>DOCUMENT_ROOT</CODE> environment variable). This is used by the
		core module when mapping URIs to filenames, but in the context of
		dynamic virtual hosting its value only matters if any CGIs or SSI
		documents make use of the <CODE>DOCUMENT_ROOT</CODE> environment
		variable. This is an Apache extension to the CGI specification and as
		such shouldn't really be relied upon, especially because this
		technique breaks it: there isn't currently a way of setting
		<CODE>DOCUMENT_ROOT</CODE> dynamically.</P>

		<P>The meat of the mechanism works via Apache's URI-to-filename
		translation API phase. This is used by a number of modules:
		<A HREF="../mod/mod_rewrite.html"><CODE>mod_rewrite</CODE></A>,
		<A HREF="../mod/mod_alias.html"><CODE>mod_alias</CODE></A>,
		<A HREF="../mod/mod_userdir.html"><CODE>mod_userdir</CODE></A>,
		and <A HREF="../mod/core.html">the core module</A>.
		In the default configuration these modules are called in that order
		and given a chance to say that they know what the filename is. Most of
		these modules do it in a fairly simple fashion (e.g. the core module
		concatenates the document root and the URI) except for
		<CODE>mod_rewrite</CODE>, which provides enough functionality to do
		all sorts of sick and twisted things (like dynamic virtual hosting).
		Note that because of the order in which the modules are called, using
		a <CODE>mod_rewrite</CODE> configuration that matches any URI means
		that the other modules (particularly <CODE>mod_alias</CODE>) will
		cease to function. The examples below show how to deal with this.</P>

		<P><STRONG>The dynamic virtual hosting idea is very simple: use the
		server name as well as the URI to determine the corresponding
		filename.</STRONG></P>


		<HR><H2><A NAME="simple">Simple name-based 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.</P>

		<P>The first half shows some other configuration options that are
		needed to make the <CODE>mod_rewrite</CODE> part work as expected; the
		second half uses <CODE>mod_rewrite</CODE> to do the actual work. Some
		care is taken to do a per-dynamic-virtual-host equivalent of
		<CODE>ScriptAlias</CODE>.</P>

		<PRE>
		# dynamic ServerName
		UseCanonicalName Off

		# splittable logs
		LogFormat "%v %h %l %u %t \"%r\" %s %b" vcommon
		CustomLog logs/access_log vcommon

		<Directory /www/hosts>
		# ExecCGI is needed here because we can't force
		# CGI execution in the way that ScriptAlias does
		Options FollowSymLinks ExecCGI
		</Directory>

		# now for the hard bit

		RewriteEngine On

		# a ServerName derived from a Host: header may be any case at all
		RewriteMap lowercase int:tolower

		## deal with normal documents first:
		# allow Alias /icons/ to work - repeat for other aliases
		RewriteCond %{REQUEST_URI} !^/icons/
		# allow CGIs to work
		RewriteCond %{REQUEST_URI} !^/cgi-bin/
		# do the magic
		RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1

		## and now deal with CGIs - we have to force a MIME type
		RewriteCond %{REQUEST_URI} ^/cgi-bin/
		RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]

		# that's it!
		</PRE>


		<HR><H2><A NAME="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 slightly more complicated rewriting rules we
		can select substrings of the server name to use in the filename so
		that e.g. the documents for <SAMP>www.user.isp.com</SAMP> are found in
		<CODE>/home/user/</CODE>. It uses a single <CODE>cgi-bin</CODE>
		directory instead of one per virtual host.</P>

		<PRE>
		RewriteEngine on

		RewriteMap lowercase int:tolower

		# allow CGIs to work
		RewriteCond %{REQUEST_URI} !^/cgi-bin/

		# check the hostname is right so that the RewriteRule works
		RewriteCond ${lowercase:%{HTTP_HOST}} ^www\.[a-z-]+\.isp\.com$

		# concatenate the virtual host name onto the start of the URI
		# the [C] means do the next rewrite on the result of this one
		RewriteRule ^(.+) ${lowercase:%{HTTP_HOST}}$1 [C]

		# now create the real file name
		RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2

		# define the global CGI directory
		ScriptAlias /cgi-bin/ /www/std-cgi/
		</PRE>


		<HR><H2><A NAME="xtra-conf">Using a separate virtual host configuration file</A></H2>

		<P>This arrangement uses a separate configuration file to specify the
		translation from virtual host to document root. This provides more
		flexibility but requires more configuration.</P>

		<P>The <CODE>vhost.map</CODE> file contains something like this:
		<PRE>
		www.customer-1.com /www/customers/1
		www.customer-2.com /www/customers/2
		# ...
		www.customer-N.com /www/customers/N
		</PRE>
		</P>

		<P>The <CODE>http.conf</CODE> contains this:
		<PRE>
		RewriteEngine on

		RewriteMap lowercase int:tolower

		# define the map file
		RewriteMap vhost txt:/www/conf/vhost.map

		# deal with aliases as above
		RewriteCond %{REQUEST_URI} !^/icons/
		RewriteCond %{REQUEST_URI} !^/cgi-bin/
		RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
		# this does the file-based remap
		RewriteCond ${vhost:%1} ^(/.*)$
		RewriteRule ^/(.*)$ %1/docs/$1

		RewriteCond %{REQUEST_URI} ^/cgi-bin/
		RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
		RewriteCond ${vhost:%1} ^(/.*)$
		RewriteRule ^/(.*)$ %1/cgi-bin/$1
		</PRE>
		</P>


		<HR><H2><A NAME="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 rewrite 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
		convential <CODE><VirtualHost></CODE> configuration
		sections.</P>

		<PRE>
		UseCanonicalName Off

		LogFormat "%v %h %l %u %t \"%r\" %s %b" vcommon
		CustomLog logs/access_log vcommon

		<Directory /www/commercial>
		Options FollowSymLinks ExecCGI
		AllowOverride All
		</Directory>

		<Directory /www/homepages>
		Options FollowSymLinks
		AllowOverride None
		</Directory>

		<VirtualHost 111.22.33.44>
		ServerName www.commercial.isp.com

		RewriteEngine On
		RewriteMap lowercase int:tolower

		RewriteCond %{REQUEST_URI} !^/icons/
		RewriteCond %{REQUEST_URI} !^/cgi-bin/
		RewriteRule ^/(.*)$ /www/commercial/${lowercase:%{SERVER_NAME}}/docs/$1

		RewriteCond %{REQUEST_URI} ^/cgi-bin/
		RewriteRule ^/(.*)$ /www/commercial/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]
		</VirtualHost>

		<VirtualHost 111.22.33.45>
		ServerName www.homepages.isp.com

		RewriteEngine on
		RewriteMap lowercase int:tolower

		RewriteCond %{REQUEST_URI} !^/cgi-bin/

		RewriteCond ${lowercase:%{HTTP_HOST}} ^www\.[a-z-]+\.isp\.com$
		RewriteRule ^(.+) ${lowercase:%{HTTP_HOST}}$1 [C]
		RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /www/homepages/$1/$2

		ScriptAlias /cgi-bin/ /www/std-cgi/
		</VirtualHost>
		</PRE>


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		Apache HTTP Server Version 1.3
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