Technical Note

Later sections override earlier ones, however each module is responsible - for interpreting what form this override takes. A later configuration section - with directives from a given module might cause a conceptual "merge" of some - directives, all directives, or a complete replacement of the modules - configuration with the module defaults and directives explicitly listed in - the later context.

Relationship between modules and configuration sections +

One question that often arises after reading how configuration sections are + merged is related to how and when directives of specific modules like mod_rewrite + are processed. The answer is not trivial and needs a bit of background. + Each httpd module manages its own configuration, and each of its directives in httpd.conf specify one piece + of configuration in a particular context. httpd does not execute a command as it is read.

At runtime, the core of httpd iterates over the defined configuration sections in the order + described above to determine which ones apply to the current request. When the first section matches, + it is considered the current configuration for this request. If a subsequent section matches too, + then each module with a directive in either of the sections is given a chance to merge its configuration between the two sections. The result is a third configuration, and the process goes on until all the configuration sections + are evaluated.

After the above step, the "real" processing of the HTTP request begins: each module has a chance to run + and perform whatever tasks they like. They can retrieve their own final merged configuration from the core + of the httpd to determine how they should act.

An example can help to visualize the whole process. The following configuration uses the + Header directive of mod_headers to set + a specific HTTP header. What value will httpd set in the CustomHeaderName header for a request to + /example/index.html ? +

+ + +<Directory "/"> + Header set CustomHeaderName one + <FilesMatch ".*"> + Header set CustomHeaderName three + </FilesMatch> +</Directory> + +<Directory "/example"> + Header set CustomHeaderName two +</Directory> + + +

Directory "/" matches and an initial configuration to set the CustomHeaderName header with the value one is created.
Directory "/example" matches, and since mod_headers specifies in its code to override in case of a merge, a new configuration is created to set the CustomHeaderName header with the value two.
FilesMatch ".*" matches and another merge opportunity arises, causing the CustomHeaderName header to be set with the value three.
Eventually during the next steps of the HTTP request processing mod_headers will be called and it will receive the configuration to set the CustomHeaderName header with the value three. mod_headers normally uses this configuration to perfom its job, namely setting the foo header. This does not mean that a module can't perform a more complex action like discarding directives because not needed or deprecated, etc..

+ +

This is true for .htaccess too since they have the same priority as Directory in the merge order. The important concept to understand is that configuration sections like Directory and FilesMatch are not comparable to module specific directives like Header or RewriteRule because they operate on different levels. +