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<parentdocument href="./">Virtual Hosts</parentdocument>

<title>Dynamically Configured Mass Virtual Hosting</title>

<summary>

<p>This document describes how to efficiently serve an

arbitrary number of virtual hosts with the Apache httpd webserver. A

<a href="/rewrite/mass.html">separate document</a> discusses using

<module>mod_rewrite</module> to create dynamic mass virtual hosts.

</p>

</summary>

<section id="motivation"><title>Motivation</title>

<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>

<example>

<pre>

NameVirtualHost 111.22.33.44

&lt;VirtualHost 111.22.33.44&gt;

ServerName www.customer-1.com

DocumentRoot /www/hosts/www.customer-1.com/docs

ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin

&lt;/VirtualHost&gt;

&lt;VirtualHost 111.22.33.44&gt;

ServerName www.customer-2.com

DocumentRoot /www/hosts/www.customer-2.com/docs

ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin

&lt;/VirtualHost&gt;

&lt;VirtualHost 111.22.33.44&gt;

ServerName www.customer-N.com

DocumentRoot /www/hosts/www.customer-N.com/docs

ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin

&lt;/VirtualHost&gt;

</pre>

</example>

<p>We wish to replace these multiple

<code>&lt;VirtualHost&gt;</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>.

It is better to <a href="/logs.html#piped">log to a pipe or a fifo</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

be found in the <a

href="/programs/other.html#split-logfile">split-logfile</a>

utility.</p>

</section>

<section id="overview"><title>Overview</title>

<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 <module>mod_vhost_alias</module>

with Apache 2.0. Alternatively,

<a href="/rewrite/vhosts.html"><module>mod_rewrite</module> 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 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 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 <directive

module="core">UseCanonicalName</directive>

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 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 determine 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 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>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>

<!-- TODO: Is that true? I was pretty sure that mod_vhost_alias did

</section>

<section id="simple"><title>Dynamic Virtual Hosts with

mod_vhost_alias</title>

<p>This extract from <code>httpd.conf</code> implements the

virtual host arrangement outlined in the <a

href="#motivation">Motivation</a> section above

using <code>mod_vhost_alias</code>.</p>

<example>

# 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 />

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

</example>

<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>

<p>See the <module>mod_vhost_alias</module> documentation for more usage

examples.</p>

</section>

<section id="homepages"><title>A Virtually Hosted Homepages System</title>

<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, for example, 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>

<example>

# 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 />

</example>

<p>There are examples of more complicated

<code>VirtualDocumentRoot</code> settings in the

<module>mod_vhost_alias</module> documentation.</p>

</section>

<section id="combinations"><title>Using Multiple Virtual

Hosting Systems on the Same Server</title>

<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 general customers' homepages,

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>

<example>

UseCanonicalName Off<br />

<br />

LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon<br />

<br />

&lt;Directory /www/commercial&gt;<br />

<indent>

Options FollowSymLinks<br />

AllowOverride All<br />

</indent>

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

<br />

&lt;Directory /www/homepages&gt;<br />

<indent>

Options FollowSymLinks<br />

AllowOverride None<br />

</indent>

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

<br />

&lt;VirtualHost 111.22.33.44&gt;<br />

<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 />

</indent>

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

<br />

&lt;VirtualHost 111.22.33.45&gt;<br />

<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 />

</indent>

&lt;/VirtualHost&gt;

</example>

<note>

<title>Note</title>

<p>If the first VirtualHost block does <em>not</em> include a

<directive module="core">ServerName</directive> 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 (<code>ServerName

none.example.com</code>) can be added to get around this

behaviour.</p>

</note>

</section>

<section id="ipbased"><title>More Efficient IP-Based Virtual Hosting</title>

<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>

<example>

# 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 />

</example>

</section>

<section id="rewrite"><title>Mass virtual hosts with

mod_rewrite</title>

<p>

Mass virtual hosting may also be accomplished using

<module>mod_rewrite</module>, either using simple <directive

module="mod_rewrite">RewriteRule</directive> directives, or using more

complicated techniques such as storing the vhost definitions externally

and accessing them via <directive

module="mod_rewrite">RewriteMap</directive>. These techniques are

discussed in the <a href="/rewrite/vhosts.html">rewrite

documentation</a>.</p>

</section>

</manualpage>