Apache: The Definitive Guide, 3rd Edition

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