apache配置文件之-Configuration Sections

The <IfDefine> directive encloses directives that will only be applied if an appropriate parameter is defined on the httpd command line. For example, with the following configuration, all requests will be redirected to another site only if the server is started using httpd -DClosedForNow:

The <IfModule> directive is very similar, except it encloses directives that will only be applied if a particular module is available in the server. The module must either be statically compiled in the server, or it must be dynamically compiled and its LoadModule line must be earlier in the configuration file. This directive should only be used if you need your configuration file to work whether or not certain modules are installed. It should not be used to enclose directives that you want to work all the time, because it can suppress useful error messages about missing modules.

<IfModule>标志也非常类似，只是他是在一个特定的module在服务器中有效时才被应用。这个module要么被静态编译到服务器中，要么被动态编译并且它的LoadModule行必须事前已经在配置文件中。该标志应该仅在你需要确认在你的配置文件中指定modules是否被安装时使用，而不应该用在服务器的运行时间里。因为他会禁止掉有用的关于丢失modules的错误信息。

In the following example, the MimeMagicFiles directive will be applied only if mod_mime_magic is available.

The <IfVersion> directive is very similar to <IfDefine> and <IfModule>, except it encloses directives that will only be applied if a particular version of the server is executing. This module is designed for the use in test suites and large networks which have to deal with different httpd versions and different configurations.

<IfDefine>, <IfModule>, and the <IfVersion> can apply negative conditions by preceding their test with "!". Also, these sections can be nested to achieve more complex restrictions.