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<h3>Apache HTTP Server Version 1.3</h3>
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<h1 align="CENTER">Apache non-IP Virtual Hosts</h1>
<strong>See Also:</strong> <a href="virtual-host.html">Virtual
Host Support</a>
<hr />
<h2>What is a Virtual Host</h2>
<p>The "Virtual Host" refers to the practice of maintaining
more than one server on one machine, as differentiated by their
apparent hostname. For example, it is often desirable for
companies sharing a web server to have their own domains, with
web servers accessible as <code>www.company1.com</code> and
<code>www.company2.com</code>, without requiring the user to
know any extra path information.</p>
<p>Apache was one of the first servers to support virtual hosts
right out of the box, but since the base <code>HTTP</code>
(HyperText Transport Protocol) standard does not allow any
method for the server to determine the hostname it is being
addressed as, Apache's virtual host support has required a
separate IP address for each server. Documentation on using
this approach (which still works very well) <a
href="virtual-host.html">is available</a>.</p>
<p>While the approach described above works, with the available
IP address space growing smaller, and the number of domains
increasing, it is not the most elegant solution, and is hard to
implement on some machines. The <code>HTTP/1.1</code> protocol
contains a method for the server to identify what name it is
being addressed as. Apache 1.1 and later support this approach
as well as the traditional IP-address-per-hostname method.</p>
<p>The benefits of using the new virtual host support is a
practically unlimited number of servers, ease of configuration
and use, and requires no additional hardware or software. The
main disadvantage is that the user's browser must support this
part of the protocol. The latest versions of many browsers
(including Netscape Navigator 2.0 and later) do, but many
browsers, especially older ones, do not. This can cause
problems, although a possible solution is addressed below.</p>
<h2>Using non-IP Virtual Hosts</h2>
<p>Using the new virtual hosts is quite easy, and superficially
looks like the old method. You simply add to one of the Apache
configuration files (most likely <code>httpd.conf</code> or
<code>srm.conf</code>) code similar to the following:</p>
<pre>
<VirtualHost www.apache.org>
ServerName www.apache.org
DocumentRoot /usr/web/apache
</VirtualHost>
</pre>
<p>Of course, any additional directives can (and should) be
placed into the <code><VirtualHost></code> section. To
make this work, all that is needed is to make sure that the
<code>www.apache.org</code> DNS entry points to the same IP
address as the main server. Optionally, you could simply use
that IP address in the <VirtualHost> entry.</p>
<p>Additionally, many servers may wish to be accessible by more
than one name. For example, the Apache server might want to be
accessible as <code>apache.org</code>, or
<code>ftp.apache.org</code>, assuming the IP addresses pointed
to the same server. In fact, one might want it so that all
addresses at <code>apache.org</code> were picked up by the
server. This is possible with the <code>ServerAlias</code>
directive, placed inside the <VirtualHost> section. For
example:</p>
<pre>
ServerAlias apache.org *.apache.org
</pre>
<p>Note that you can use <code>*</code> and <code>?</code> as
wild-card characters.</p>
<p>You also might need ServerAlias if you are serving local
users who do not always include the domain name. For example,
if local users are familiar with typing "www" or "www.physics"
then you will need to add <code>ServerAlias www
www.physics</code>. It isn't possible for the server to know
what domain the client uses for their name resolution because
the client doesn't provide that information in the request.</p>
<h2>Security Considerations</h2>
Apache allows all virtual hosts to be made accessible via the
<code>Host:</code> header through all IP interfaces, even those
which are configured to use different IP interfaces. For
example, if the configuration for <code>www.foo.com</code>
contained a virtual host section for <code>www.bar.com</code>,
and <code>www.bar.com</code> was a separate IP interface, such
that non-<code>Host:</code>-header-supporting browsers can use
it, as before with Apache 1.0. If a request is made to
<code>www.foo.com</code> and the request includes the header
<code>Host: www.bar.com</code>, a page from
<code>www.bar.com</code> will be sent.
<p>This is a security concern if you are controlling access to
a particular server based on IP-layer controls, such as from
within a firewall or router. Let's say <code>www.bar.com</code>
in the above example was instead an intra-net server called
<code>private.foo.com</code>, and the router used by foo.com
only let internal users access <code>private.foo.com</code>.
Obviously, <code>Host:</code> header functionality now allows
someone who has access to <code>www.foo.com</code> to get
<code>private.foo.com</code>, if they send a <code>Host:
private.foo.com</code> header. It is important to note that
this condition exists only if you only implement this policy at
the IP layer - all security controls used by Apache
(<em>i.e.</em>, <a href="../mod/mod_access.html">Allow, Deny
from,</a> <em>etc.</em>) are consistently respected.</p>
<h2>Compatibility with Older Browsers</h2>
<p>As mentioned earlier, a majority of browsers do not send the
required data for the new virtual hosts to work properly. These
browsers will always be sent to the main server's pages. There
is a workaround, albeit a slightly cumbersome one:</p>
<p>To continue the <code>www.apache.org</code> example (Note:
Apache's web server does not actually function in this manner),
we might use the new <code>ServerPath</code> directive in the
<code>www.apache.org</code> virtual host, for example:</p>
<pre>
ServerPath /apache
</pre>
<p>What does this mean? It means that a request for any file
beginning with "<code>/apache</code>" will be looked for in the
Apache docs. This means that the pages can be accessed as
<code>http://www.apache.org/apache/</code> for all browsers,
although new browsers can also access it as
<code>http://www.apache.org/</code>.</p>
<p>In order to make this work, put a link on your main server's
page to <code>http://www.apache.org/apache/</code> (Note: Do
not use <code>http://www.apache.org/</code> - this would create
an endless loop). Then, in the virtual host's pages, be sure to
use either purely relative links (<em>e.g.</em>,
"<code>file.html</code>" or "<code>../icons/image.gif</code>"
or links containing the prefacing <code>/apache/</code>
(<em>e.g.</em>,
"<code>http://www.apache.org/apache/file.html</code>" or
"<code>/apache/docs/1.1/index.html</code>").</p>
<p>This requires a bit of discipline, but adherence to these
guidelines will, for the most part, ensure that your pages will
work with all browsers, new and old. When a new browser
contacts <code>http://www.apache.org/</code>, they will be
directly taken to the Apache pages. Older browsers will be able
to click on the link from the main server, go to
<code>http://www.apache.org/apache/</code>, and then access the
pages.</p>
<hr />
<h3 align="CENTER">Apache HTTP Server Version 1.3</h3>
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