![[APACHE DOCUMENTATION]](../images/sub.gif) 
 
      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 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.
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.
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.
 
    