# Filters

Related Modules mod_ext_filter mod_include | Related Directives ExtFilterDefine ExtFilterOptions SetInputFilter SetOutputFilter

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. In addition, since filters apply to all content, they allow for flexible manipulation of data such as processing the output of CGI scripts for Server Side Includes.

The set of filters which apply to data can be manipulated with the `SetInputFilter` and `SetOutputFilter` directives.

The only filter currently included with the Apache distribution is the `INCLUDE` 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.