The process of virtually hosting multiple websites on a single IP address by using the requested name in the HTTP header. An example for Apache would be the following configuration in httpd.conf:

NameVirtualHost *

<VirtualHost *>
  ServerName example.com
  ServerAlias www.example.com
  ServerAdmin webmaster@example.com
  DocumentRoot /var/www/virtual/example.com
  CustomLog /var/log/apache/example.com-access.log combined
  ErrorLog /var/log/apache/example.com-error.log
</VirtualHost>

<VirtualHost *>
  ServerName example.net
  ServerAlias www.example.net
  ServerAdmin webmaster2@example.net
  DocumentRoot /var/www/virtual/example.net
  CustomLog /var/log/apache/example.net-access.log combined
  ErrorLog /var/log/apache/example.net-error.log
</VirtualHost>

If the server has more than one IP address and you want it to serve a particular site only one a specific one of these addresses, you can replace the asterisks in that configuration with a particular address and optionally a port, eg.

NameVirtualHost 12.34.56.78:80

<VirtualHost 12.34.56.78:80>
  ServerName example.com
  ...

The arguments of the NameVirtualHost and corresponding VirtualHost directives must match exactly. Of course, if your server has several IPs you probably want several NameVirtualHost directives too – one for every IP. You can then serve a different set of sites from each IP.

Note that you can also use things like “*:80” to serve a particular site only on a particular port, but on every IP of the server.

If you have a site that can be reached through several domains/hostnames, eg. with or without the www. and either .com or .co.nz, you might want to consider making one of them the canonical address. This is better for any number of reasons: it will make your referrer logs easier to process; it will make things more obvious for search engines (might improve your ranking!); people will use almost exclusively the canonical address to link to your site, making it easier to find out who is linking to you; and on and on. With Apache virtual hosting, you can do this easily; in the following example, www.example.co.nz was chosen as the canonical address:

NameVirtualHost *

<VirtualHost *>
  ServerName www.example.co.nz
  ServerAdmin webmaster@example.co.nz
  DocumentRoot /var/www/virtual/example.co.nz
  CustomLog /var/log/apache/example.co.nz-access.log combined
  ErrorLog /var/log/apache/example.co.nz-error.log
</VirtualHost>

<VirtualHost *>
  ServerName aliases.for.www.example.co.nz
  ServerAlias example.co.nz
  ServerAlias example.com
  ServerAlias www.example.com
  RedirectMatch 301 /?(.*) http://www.example.co.nz/$1
</VirtualHost>

With this configuration, any and every request for a page on one of the alias domains, eg. http://example.com/products/, will redirect the visitor the same page in the canonical domain, in this case http://www.example.co.nz/products/.

There are other ways to achieve this, such as using mod_rewrite, but none of them is as simple as this one.

The advantage with using the aliases.for.www.example.co.nz as the ServerName in the second VirtualHost is that when using the command apache2ctl -S you get the following output:

  port 80 namevhost www.example.co.nz
    (/etc/apache2/sites-enabled/ubersite.conf:3)
  port 80 namevhost aliases.for.www.example.co.nz
    (/etc/apache2/sites-enabled/ubersite.conf:11)

This allows you to easily identify the alias VirtualHosts.