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4.4. Dynamically Configured Virtual Hosting

An even neater method of managing Virtual Hosting is provided by mod_vhost_alias, which lets you define a single boilerplate configuration and then fills in the details at service time from the IP address and or the Host header in the HTTP request.

All the directives in this module interpolate a string into a pathname. The interpolated string (called the "name") may be either the server name (see the UseCanonicalName directive for details on how this is determined) or the IP address of the virtual host on the server in dotted-quad format (xxx.xxx.xxx.xxx).

The interpolation is controlled by a mantra, %<code-letter>, which is replaced by some value you supply in the Config file. It's not unlike the controls for logging — see Chapter 10.

These are the possible formats:

%%
Insert a literal %.

%p
Insert the port number of the virtual host.

%N.M
Insert (part of ) the name. N and M are numbers, used to specify substrings of the name. N selects from the dot-separated components of the name, and M selects characters within whatever N has selected. M is optional and defaults to zero if it isn't present. The dot must be present if and only if M is present. If we are trying to parse sales.butterthlies.com, the interpretation of N is as follows:

0
The whole name: sales.butterthlies.com

1
The first part: sales

2
The second part: butterthlies

-1
The last part: com

-2
The penultimate part: butterthlies

2+
The second and all subsequent parts: butterthlies.com

-2+
The penultimate and all preceding parts: www.butterthlies

1+ and -1+
The same as 0: sales.butterthlies.com

If N or M is greater than the number of parts available, a single underscore is interpolated.

4.4.1. Examples

For simple name-based virtual hosts, you might use the following directives in your server-configuration file: 

UseCanonicalName Off
VirtualDocumentRoot /usr/local/apache/vhosts/%0

A request for http://www.example.com/directory/file.html will be satisfied by the file /usr/local/apache/vhosts/www.example.com/directory/file.html.

On .../site.dynamic we have implemented a version of the familiar Buttterthlies site, with a password-protected salesperson's department. The first Config file, .../conf/httpd1.conf, is as follows: 

User webuser
Group webgroup

ServerName my586

UseCanonicalName Off
VirtualDocumentRoot /usr/www/APACHE3/site.dynamic/htdocs/%0 
<Directory /usr/www/APACHE3/site.dynamic/htdocs/sales.butterthlies.com>
AuthType Basic
AuthName Darkness
AuthUserFile /usr/www/APACHE3/ok_users/sales
AuthGroupFile /usr/www/APACHE3/ok_users/groups
Require group cleaners
</Directory>

Launch it with go 1; it responds nicely to http://www.butterthlies.com and http://sales.butterthlies.com.

There is an equivalent VirtualScriptAlias directive, but it insists on URLs containing ../cgi-bin/... — for instance, www.butterthlies.com/cgi-bin/mycgi. In view of the reputed horror some search engines have for "cgi-bin", you might prefer not to use it and to keep "cgi-bin" out of your URLs with this: 

ScriptAliasMatch /(.*) /usr/www/APACHE3/cgi-bin/handler/$1

The effect should be that any visitor to <http://yourURL>/fredwill call the script .../cgi-bin/handler and pass "fred" to it in the PATH_INFO Environment variable.

If you have a very large number of virtual hosts, it's a good idea to arrange the files to reduce the size of the vhosts directory. To do this, you might use the following in your configuration file: 

UseCanonicalName Off
VirtualDocumentRoot /usr/local/apache/vhosts/%3+/%2.1/%2.2/%2.3/%2

A request for http://www.example.isp.com/directory/file.html will be satisfied by the file /usr/local/apache/vhosts/isp.com/e/x/a/example/directory/file.html (because isp.com matches to %3+, e matches to %2.1 — the first character of the second part of the URL example, and so on). The point is that most OSes are very slow if you have thousands of subdirectories in a single directory: this scheme spreads them out.

A more even spread of files can often be achieved by selecting from the end of the name, for example: 

VirtualDocumentRoot /usr/local/apache/vhosts/%3+/%2.-1/%2.-2/%2.-3/%2

The example request would come from /usr/local/apache/vhosts/isp.com/e/l/p/example/directory/file.html. Alternatively, you might use: 

VirtualDocumentRoot /usr/local/apache/vhosts/%3+/%2.1/%2.2/%2.3/%2.4+

The example request would come from /usr/local/apache/vhosts/isp.com/e/x/a/mple/directory/file.html.

For IP-based virtual hosting you might use the following in your configuration file: 

UseCanonicalName DNS
VirtualDocumentRootIP /usr/local/apache/vhosts/%1/%2/%3/%4/docs
VirtualScriptAliasIP /usr/local/apache/vhosts/%1/%2/%3/%4/cgi-bin

A request for http://www.example.isp.com/directory/file.html would be satisfied by the file /usr/local/apache/vhosts/10/20/30/40/docs/directory/file.html if the IP address of www.example.com were 10.20.30.40. A request for http://www.example.isp.com/cgi-bin/script.pl would be satisfied by executing the program /usr/local/apache/vhosts/10/20/30/40/cgi-bin/script.pl.

If you want to include the . character in a VirtualDocumentRoot directive, but it clashes with a % directive, you can work around the problem in the following way: 

VirtualDocumentRoot /usr/local/apache/vhosts/%2.0.%3.0

A request for http://www.example.isp.com/directory/file.html will be satisfied by the file /usr/local/apache/vhosts/example.isp/directory/file.html.

The LogFormat directives %V and %A are useful in conjunction with this module. See Chapter 10.

VirtualDocumentRoot
VirtualDocumentRoot interpolated-directory
Default: None
Server config, virtual host
Compatibility: VirtualDocumentRoot is only available in 1.3.7 and later.

The VirtualDocumentRoot directive allows you to determine where Apache will find your documents based on the value of the server name. The result of expanding interpolated-directory is used as the root of the document tree in a similar manner to the DocumentRoot directive's argument. If interpolated-directory is none, then VirtualDocumentRoot is turned off. This directive cannot be used in the same context as VirtualDocumentRootIP.

VirtualDocumentRootIP
VirtualDocumentRootIP interpolated-directory
Default: None
Server config, virtual host

The VirtualDocumentRootIP directive is like the VirtualDocumentRoot directive, except that it uses the IP address of the server end of the connection instead of the server name.

VirtualScriptAlias
VirtualScriptAlias interpolated-directory
Default: None
Server config, virtual host

The VirtualScriptAlias directive allows you to determine where Apache will find CGI scripts in a manner similar to how VirtualDocumentRoot does for other documents. It matches requests for URIs starting /cgi-bin/, much like the following: 

ScriptAlias /cgi-bin/ ...
VirtualScriptAliasIP
VirtualScriptAliasIP interpolated-directoryDefault: NoneServer config, virtual host

The VirtualScriptAliasIP directive is like the VirtualScriptAlias directive, except that it uses the IP address of the server end of the connection instead of the server name.

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