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If you have implemented many virtual hosts on your server that have basically the same configuration, you can use a technique called dynamic virtual hosting to have these virtual hosts generated dynamically. The code for implementing your virtual hosts becomes much smaller, and as a result, your server accesses them faster. Adding yet more virtual hosts becomes a simple matter of creating appropriate directories and adding entries for them in the DNS server.

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To make dynamic virtual hosting work, the server uses commands in the mod_vhost_alias module (supported in Apache version 1.3.6 and up) to rewrite both the server name and the document root to those of the appropriate virtual server (for older Apache versions before 1.3.6, you use the mod_rewrite module). Dynamic virtual hosting can be either name-based or IP-based. In either case, you have to set the UseCanonicalName directive in such a way as to allow the server to use the virtual host name instead of the server's own name. For namebased hosting, you simply turn off UseCanonicalName. This allows your server to obtain the hostname from the host header of the user request. For IP-based hosting, you set the UseCanonicalName directive to DNS. This allows the server to look up the host in the DNS server.

You then have to enable the server to locate the different document root directories and CGI bin directories for your various virtual hosts. You use the VirtualDocumentRoot directive to specify the template for virtual hosts' directories. For example, if you place the different host directories in the /var/www/hosts directory, then you could set the VirtualDocumentRoot directive accordingly.

The %0 will be replaced with the virtual host's name when that virtual host is accessed. It is important that you create the dynamic virtual host's directory using that host's name. For example, for a dynamic virtual host called www.mygolf.org, you would create a directory named /var/www/hosts/www.mygolf.org. Then create subdirectories for the document root and CGI programs as in /var/www/hosts/www.mygolf.org/html. For the CGI directory, use the VirtualScriptAlias directive to specify the CGI subdirectory you use.

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A simple example of name-based dynamic virtual hosting directives follows:

UseCanonicalName Off VirtualDocumentRoot /var/www/hosts/%0/html VirtualScriptAlias /var/www/hosts/%0/cgi-bin

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The mod_vhots_alias module supports various interpolated strings, each beginning with a % symbol and followed by a number. As you have seen, %0 references the entire Web address. %1 references only the first segment, %2 references the second, %-1 references the last part, and %2+ references from the second part on. For example, if you only want to use the second part of a Web address for the directory name, you would use the following directives:

In this case, a request made for www.mygolf.com/html/mypage would use only the second part of the Web address. This would be "mygolf" in www.mygolf.com, and would evaluate to

If you used %2+ instead, as in /var/www/hosts/%2/html, then the request for www.mygolf.com/html/mypage would evaluate to

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The same method works for IP addresses, where %1 references the first IP address segment, %2 references the second, and so on. A simple example of dynamic virtual hosting is shown here:

UseCanonicalName Off NameVirtualHost 192.168.1.5 <VirtualHost 192.168.1.5> ServerName www.mygolf.com ServerAdmin webmaster@mail.mygolf.com VirtualDocumentRoot /var/www/hosts/%0/html VirtualScriptAlias /var/www/hosts/%0/cgi-bin ... </VirtualHost>

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To implement IP-based dynamic virtual hosting instead, set the UseCanonicalName to DNS instead of Off.

UseCanonicalName DNS VirtualDocumentRoot /var/www/hosts/%0/html VirtualScriptAlias /var/www/hosts/%0/cgi-bin

One drawback of dynamic virtual hosting is that you can only set up one log for all your hosts. However, you can create your own shell program to simply cut out the entries for the different hosts in that log.

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