Apache non-IP Virtual Hosts

What is a Virtual Host

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 www.company1.com and www.company2.com, without requiring the user to know any extra path information.

Apache was one of the first servers to support virtual hosts right out of the box, but since the base HTTP (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) is available.

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 HTTP/1.1 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.

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.

Using non-IP Virtual Hosts

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 httpd.conf or srm.conf) code similar to the following:


    <VirtualHost www.apache.org>
    ServerName www.apache.org
    DocumentRoot /usr/web/apache
    </VirtualHost>

Of course, any additional directives can (and should) be placed into the <VirtualHost> section. To make this work, all that is needed is to make sure that the www.apache.org DNS entry points to the same IP address as the main server. Optionally, you could simply use that IP address in the <VirtualHost> entry.

Additionally, many servers may wish to be accessible by more than one name. For example, the Apache server might want to be accessible as apache.org, or ftp.apache.org, assuming the IP addresses pointed to the same server. In fact, one might want it so that all addresses at apache.org were picked up by the server. This is possible with the ServerAlias directive, placed inside the <VirtualHost> section. For example:


    ServerAlias apache.org *.apache.org

Note that you can use * and ? as wild-card characters.

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 ServerAlias www www.physics. 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.

Security Considerations

Apache allows all virtual hosts to be made accessible via the Host: header through all IP interfaces, even those which are configured to use different IP interfaces. For example, if the configuration for www.foo.com contained a virtual host section for www.bar.com, and www.bar.com was a separate IP interface, such that non-Host:-header-supporting browsers can use it, as before with Apache 1.0. If a request is made to www.foo.com and the request includes the header Host: www.bar.com, a page from www.bar.com will be sent.

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 www.bar.com in the above example was instead an intra-net server called private.foo.com, and the router used by foo.com only let internal users access private.foo.com. Obviously, Host: header functionality now allows someone who has access to www.foo.com to get private.foo.com, if they send a Host: private.foo.com 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 (i.e., allow, deny from, etc.) are consistently respected.

Compatibility with Older Browsers

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:

To continue the www.apache.org example (Note: Apache's web server does not actually function in this manner), we might use the new ServerPath directive in the www.apache.org virtual host, for example: ServerPath /apache

What does this mean? It means that a request for any file beginning with "/apache" will be looked for in the Apache docs. This means that the pages can be accessed as http://www.apache.org/apache/ for all browsers, although new browsers can also access it as http://www.apache.org/.

In order to make this work, put a link on your main server's page to http://www.apache.org/apache/ (Note: Do not use http://www.apache.org/ - this would create an endless loop). Then, in the virtual host's pages, be sure to use either purely relative links (e.g. "file.html" or "../icons/image.gif" or links containing the prefacing /apache/ (e.g. "http://www.apache.org/apache/file.html" or "/apache/docs/1.1/index.html").

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 http://www.apache.org/, 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 http://www.apache.org/apache/, and then access the pages.