Name-based Virtual Host Support

This document describes when and how to use name-based virtual hosts.

Name-based vs. IP-based Virtual Hosts ¶

IP-based virtual hosts use the IP address of the connection to determine the correct virtual host to serve. Therefore you need to have a separate IP address for each host.

With name-based virtual hosting, the server relies on the client to report the hostname as part of the HTTP headers. Using this technique, many different hosts can share the same IP address.

Name-based virtual hosting is usually simpler, since you need only configure your DNS server to map each hostname to the correct IP address and then configure the Apache HTTP Server to recognize the different hostnames. Name-based virtual hosting also eases the demand for scarce IP addresses. Therefore you should use name-based virtual hosting unless you are using equipment that explicitly demands IP-based hosting. Historical reasons for IP-based virtual hosting based on client support are no longer applicable to a general-purpose web server.

Name-based virtual hosting builds off of the IP-based virtual host selection algorithm, meaning that comparisons of ther requested hostname to configured hostnames occurs only within the set of virtual hosts that share the most specific IP:PORT based match at the network layer.

How the server selects the proper name-based virtual host ¶

It is important to recognize that the first step in name-based virtual host resolution is IP-based resolution. Name-based virtual host resolution only chooses the most appropriate name-based virtual host after narrowing down the candidates to the best (most specific) IP-based match. Using a wildcard (*) for the IP address in all of the VirtualHost directives makes this IP-based mapping irrelevant.

When a request arrives, the server will find the best (most specific) matching <VirtualHost> argument based on the IP address and port used by the request. If there is more than one virtual host containing this best-match address and port combination, Apache will further compare the ServerName and ServerAlias directives to the server name present in the request.

If you omit the ServerName directive from any name-based virtual host, the server will default to a fully qualified domain name (FQDN) derived from the system hostname. This implicitly set server name can lead to counter-intuitive virtual host matching and is discouraged.

The default name-based vhost for an IP and port combination

If no matching ServerName or ServerAlias is found in the set of virtual hosts containing the most specific matching IP address and port combination, then the first listed virtual host that matches that will be used.

Using Name-based Virtual Hosts ¶

Related Modules	Related Directives
`core`	`DocumentRoot` `ServerAlias` `ServerName` `<VirtualHost>`

The first step is to create a <VirtualHost> block for each different host that you would like to serve. Inside each <VirtualHost> block, you will need at minimum a ServerName directive to designate which host is served and a DocumentRoot directive to show where in the filesystem the content for that host lives.

Main host goes away

Any request that doesn't match an existing <VirtualHost> is handled by the global server configuration, regardless of the hostname or ServerName.

When you add a name-based virtual host to an existing server, and the virtual host arguments match preexisting IP and port combinations, requests will now be handled by an explicit virtual host. In this case, it's usually wise to create a default virtual host with a ServerName matching that of the base server. New domains on the same interface and port, but requiring separate configurations, can then be added as subsequent (non-default) virtual hosts.

ServerName inheritance

It is best to always explicitly list a ServerName in every name-based virtual host.

If a VirtualHost doesn't specify a ServerName, a server name will be inherited from the base server configuration. If no server name was specified globally, one is detected at startup through reverse DNS resolution of the first listening address. In either case, this inherited server name will influence name-based virtual host resolution, so it is best to always explicitly list a ServerName in every name-based virtual host.

For example, suppose that you are serving the domain www.example.com and you wish to add the virtual host other.example.com, which points at the same IP address. Then you simply add the following to httpd.conf:

<VirtualHost *:80>
    # This first-listed virtual host is also the default for *:80
    ServerName www.example.com
    ServerAlias example.com
    DocumentRoot "/www/domain"
</VirtualHost>

<VirtualHost *:80>
    ServerName other.example.com
    DocumentRoot "/www/otherdomain"
</VirtualHost>

You can alternatively specify an explicit IP address in place of the * in <VirtualHost> directives. For example, you might want to do this in order to run some name-based virtual hosts on one IP address, and either IP-based, or another set of name-based virtual hosts on another address.

Many servers want to be accessible by more than one name. This is possible with the ServerAlias directive, placed inside the <VirtualHost> section. For example in the first <VirtualHost> block above, the ServerAlias directive indicates that the listed names are other names which people can use to see that same web site:

ServerAlias example.com *.example.com

then requests for all hosts in the example.com domain will be served by the www.example.com virtual host. The wildcard characters * and ? can be used to match names. Of course, you can't just make up names and place them in ServerName or ServerAlias. You must first have your DNS server properly configured to map those names to an IP address associated with your server.

Typically, IP addresses of local interfaces or the wildcard '*' are specified within the <virtualhost> tag and hostnames are specified only in ServerName or ServerAlias. When a hostname is specified within the virtualhost tag, its addresses are resolved and added for IP based virtual host resolution and the hostname:port combination is also checked prior to ServerName and ServerAlias checks for name-based virtual host resolution. The use of hostnames within the virtualhost tag is relatively obscure, error prone, and should generally be avoided.

Name-based virtual hosts for the best-matching set of <virtualhost>s are processed in the order they appear in the configuration. The first matching ServerName or ServerAlias is used, with no different precedence for wildcards (nor for ServerName vs. ServerAlias).

Finally, you can fine-tune the configuration of the virtual hosts by placing other directives inside the <VirtualHost> containers. Most directives can be placed in these containers and will then change the configuration only of the relevant virtual host. To find out if a particular directive is allowed, check the Context of the directive. Configuration directives set in the main server context (outside any <VirtualHost> container) will be used only if they are not overridden by the virtual host settings.