First published in Apache Week issue 31 (6th September 1996). Last update 20th September 1998

Picking an IP address
There are two types of virtual hosts: IP-based and non-IP-based. The former is where each virtual host has its own IP address. You will need a new IP address for each virtual host you want to set up, either from your existing allocation or by obtaining more from your service provider. Once you have extra IP addresses, you tell your machine to handle them. On some operating systems, you can give a single ethernet interface multiple addresses (typically with an ifconfig alias command). On other systems you will have to have a different physical interface for each IP address (typically by buying extra ethernet cards).

IP addresses are a resource that costs money and are increasingly difficult to get hold of, so modern browsers can now also use 'non-IP' virtual hosts. This lets you use the same IP address for multiple host names. When the server receives an incoming Web connection it does not know the hostname what was used in the URL, however the new HTTP/1.1 specification adds a facility where the browser must tell the server the hostname it is using, on the Host: header. If an older browser connects to a non-IP virtual host, it will not send the Host: header, so the server will have to respond with a list of possible virtual hosts. Apache provides some help for configuring a site for both old and new browsers.

Picking a Hostname and Updating the DNS
Having selected an IP address, the next stage is to update the DNS so that browsers can convert the hostname into the right address. The DNS is the system that every machine connected to the Internet uses to find the IP address of host names. If your hostname is not in the DNS, no-one will be able to connect to your server (except by the unfriendly IP address).

If the virtual host name you are going to use is under your existing domain, you can just add the record into your own DNS server. If the virtual host name is in someone else's domain, you will need to get them to add it to their DNS server files. In some cases, you will want to use a domain not yet used on the internet, in which case you will have to apply for the domain name from the InterNIC and set up the primary and secondary DNS servers for it, before adding the entry for your virtual host.

In any of these cases, the entry you need to add to the DNS is an address record (an A record) pointing at the appropriate IP address. For example, say you want the domain www.my-dom.com to access your host with IP address 10.1.2.3: you will need to add the following line to the DNS zone file for my-dom.com:

  www         A     10.1.2.3

How Apache Handles Virtual Hosts
Configuring Apache for virtual hosts is a two stage process. Firstly, it needs to be told which IP addresses (and ports) to listen to for incoming web connections. By default Apache listens to port 80 on all IP addresses of the local machine, and this is often sufficient. If you have a more complex requirement, such as listening on various port numbers, or only to specific IP addresses, the BindAddress or Listen directives can be used.

Secondly, having accepted an incoming web connection, the server needs to be configured to handle the request differently depending on what virtual host it was addressed to. This usually involves configuring Apache to use a different DocumentRoot.

Telling Apache Which Addresses to Listen To
If you are happy for Apache to listen to all local IP addresses on the port specified by the Port directive, you can skip this section. However there are some cases where you will want to use the directives explained here:

• If you have many IP addresses on the machine but only want to run a web server on some of them
• If one or more of your virtual hosts is on a different port
• If you want to run multiple copies of the Apache server serving different virtual hosts

  Listen 10.1.2.3:80
  Listen 10.1.2.4:8000

Configuring the Virtual Hosts
Having got Apache to listen to the appropriate IP addresses and ports, the final stage is to configure the server to behave differently for requests on each of the different addresses. This is done using <VirtualHost> sections in the configuration files, normally in httpd.conf.

A typical (but minimal) virtual host configuration looks like this:

  <VirtualHost 10.1.2.3>
  DocumentRoot /www/vhost1
  ServerName  www.my-dom.com
  </VirtualHost>

VirtualHost sections for non-IP Virtual Hosts
Non-IP virtual hosts are configured in a very similar way. The IP address that the requests will arrive on is given in the <VirtualHost> directive, and the host name is put in the ServerName directive. The difference is that there will (usually) be more than one <VirtualHost> section handling the same IP address. In order for Apache to know whether a request arriving on a particular IP address is supposed to be a name-based requests, the NameVirtualHost directive is used to tell Apache the IP addresses for name-based requests. A virtual host can handle more than one non-IP hostname by using the ServerAlias directive, in addition to the ServerName.