[APACHE DOCUMENTATION]

Apache HTTP Server Version 1.3

Apache name-based Virtual Host Support

See Also: IP-based Virtual Host Support

Name-based vs. IP-based virtual hosts

Early versions of HTTP (like many other protocols, e.g. FTP) required a different IP address for each virtual host on the server. On some platforms this can limit the number of virtual hosts you can run, and because there are concerns about the availability of IP addresses it is strongly discouraged by the registraries (ARIN, RIPE, and APNIC).

The HTTP/1.1 protocol, and a common extension to HTTP/1.0, includes 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 old IP-address-per-hostname method.

The benefits of using the name-based virtual hosts is a practically unlimited number of servers, ease of configuration and use, and it requires no additional hardware or software. The main disadvantage is that the client must support this part of the protocol. Almost all browsers do, but there are still tiny numbers of very old browsers in use which do not. This can cause problems, although a possible solution is addressed below.

Using name-based virtual hosts

Using name-based virtual hosts is quite easy, and superficially looks like the old method. The notable difference between IP-based and name-based virtual host configuration is the NameVirtualHost directive which specifies an IP address that should be used as a target for name-based virtual hosts.

For example, suppose that both www.domain.tld and www.otherdomain.tld point at the IP address 111.22.33.44. Then you simply add to one of the Apache configuration files (most likely httpd.conf or srm.conf) code similar to the following:

    NameVirtualHost 111.22.33.44

    <VirtualHost 111.22.33.44>
    ServerName www.domain.tld
    DocumentRoot /www/domain
    </VirtualHost>

    <VirtualHost 111.22.33.44>
    ServerName www.otherdomain.tld
    DocumentRoot /www/otherdomain
    </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 names www.domain.tld and www.otherdomain.tld are pointing to the IP address 111.22.33.44

Note: When you specify an IP address in a NameVirtualHost directive then requests to that IP address will only ever be served by matching <VirtualHost>s. The "main server" will never be served from the specified IP address. If you start to use virtual hosts you should stop using the "main server" as an independent server and rather use it as a place for configuration directives that are common for all your virtual hosts. In other words, you should add a <VirtualHost> section for every server (hostname) you want to maintain on your server.

In Apache 1.3.13 and later you can specify the NameVirtualHost IP address as the wildcard * which matches any IP address not covered by more specific virtual host directive(s). This is useful for configuring a server whose IP address you do not know in advance, e.g. because it has a dynamically configured IP address or because it is part of a load-balanced cluster in which every machine shares the same configuration file.

Additionally, many servers may wish to be accessible by more than one name. For example, the example server might want to be accessible as domain.tld, or www2.domain.tld, assuming the IP addresses pointed to the same server. In fact, one might want it so that all addresses at domain.tld were picked up by the server. This is possible with the ServerAlias directive, placed inside the <VirtualHost> section. For example:

    ServerAlias domain.tld *.domain.tld

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.foobar" then you will need to add ServerAlias www www.foobar. 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. The ServerAlias directive is generally a way to have different hostnames pointing to the same virtual host.

Compatibility with Older Browsers

As mentioned earlier, there are still some clients in use who do not send the required data for the name-based virtual hosts to work properly. These clients will always be sent the pages from the first virtual host listed for that IP address (the primary name-based virtual host).

There is a possible workaround with the ServerPath directive, albeit a slightly cumbersome one:

Example configuration:

    NameVirtualHost 111.22.33.44

    <VirtualHost 111.22.33.44>
    ServerName www.domain.tld
    ServerPath /domain
    DocumentRoot /web/domain
    </VirtualHost>

What does this mean? It means that a request for any URI beginning with "/domain" will be served from the virtual host www.domain.tld This means that the pages can be accessed as http://www.domain.tld/domain/ for all clients, although clients sending a Host: header can also access it as http://www.domain.tld/.

In order to make this work, put a link on your primary virtual host's page to http://www.domain.tld/domain/ 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 /domain/ (e.g., "http://www.domain.tld/domain/misc/file.html" or "/domain/misc/file.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.

See also: ServerPath configuration example

Apache HTTP Server Version 1.3

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