Filters (Apache: The Definitive Guide, 3rd Edition)

6.6. Filters

Apache v2 introduced a new mechanism called a "Filter", together with a reworking of Multiviews. The documentation says:

A filter is a process which is applied to data that is sent or received by the server. Data sent by clients to the server is processed by input filters while data sent by the server to the client is processed by output filters. Multiple filters can be applied to the data, and the order of the filters can be explicitly specified.

Filters are used internally by Apache to perform functions such as chunking and byte-range request handling. In addition, modules can provide filters which are selectable using run-time configuration directives. The set of filters which apply to data can be manipulated with the SetInputFilter and SetOutputFilter directives.

The only configurable filter currently included with the Apache distribution is the INCLUDES filter which is provided by mod_include to process output for Server Side Includes. There is also an experimental module called mod_ext_filter which allows for external programs to be defined as filters.

There is a demonstration filter that changes text to uppercase. In .../site.filter/htdocs we have two files, 1.txt and 1.html, which have the same contents:

HULLO WORLD FROM site.filter

The Config file is as follows:

User webuser
Group webgroup

Listen 80
ServerName my586

AddOutputFilter CaseFilter html
DocumentRoot /usr/www/APACHE3/site.filter/htdocs

If we visit the site, we are offered a directory. If we choose 1.txt, we see the contents as shown earlier. If we choose 1.html, we find it has been through the filter and is now all uppercase:

HULLO WORLD FROM SITE.FILTER

The Directives are as follows:

AddInputFilter

AddInputFilter filter[;filter...] extension [extension ...]
directory, files, location, .htaccess
AddInputFilter is only available in Apache 2.0.26 and later.

AddInputFilter maps the filename extensions extension to the filter or filters that will process client requests and POST input when they are received by the server. This is in addition to any filters defined elsewhere, including the SetInputFilter directive. This mapping is merged over any already in force, overriding any mappings that already exist for the same extension.

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content. Both the filter and extension arguments are case insensitive, and the extension may be specified with or without a leading dot.

AddOutputFilter

AddOutputFilter filter[;filter...] extension [extension ...]
directory, files, location, .htaccess
AddOutputFilter is only available in Apache 2.0.26 and later.

The AddOutputFilter directive maps the filename extensions extension to the filters that will process responses from the server before they are sent to the client. This is in addition to any filters defined elsewhere, including the SetOutputFilter directive. This mapping is merged over any already in force, overriding any mappings that already exist for the same extension. For example, the following configuration will process all .shtml files for server-side includes.

  AddOutputFilter INCLUDES shtml

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content. Both the filter and extension arguments are case insensitive, and the extension may be specified with or without a leading dot.

SetInputFilter

SetInputFilter filter[;filter...] 
Server config, virtual host, directory, .htaccess

The SetInputFilter directive sets the filter or filters that will process client requests and POST input when they are received by the server. This is in addition to any filters defined elsewhere, including the AddInputFilter directive.

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content.

SetOutputFilter

SetOutputFilter filter [filter] ... 
Server config, virtual host, directory, .htaccess

The SetOutputFilter directive sets the filters that will process responses from the server before they are sent to the client. This is in addition to any filters defined elsewhere, including the AddOutputFilter directive.

For example, the following configuration will process all files in the /www/data/ directory for server-side includes:

<Directory /www/data/>
SetOutputFilter INCLUDES
</Directory>

If more than one filter is specified, they must be separated by semicolons in the order in which they should process the content.

RemoveInputFilter

RemoveInputFilter extension [extension] ...
directory, .htaccess
RemoveInputFilter is only available in Apache 2.0.26 and later.

The RemoveInputFilter directive removes any input filter associations for files with the given extensions. This allows .htaccess files in subdirectories to undo any associations inherited from parent directories or the server config files.

The extension argument is case insensitive and can be specified with or without a leading dot.

RemoveOutputFilter

RemoveOutputFilter extension [extension] ...
directory, .htaccess
RemoveOutputFilter is only available in Apache 2.0.26 and later.

The RemoveOutputFilter directive removes any output filter associations for files with the given extensions. This allows .htaccess files in subdirectories to undo any associations inherited from parent directories or the server config files.

The extension argument is case insensitive and can be specified with or without a leading dot.