# Filters

Related Modules mod_ext_filter mod_include | Related Directives AddInputFilter AddOutputFilter 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.

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.