Apache HTTP Server Version 2.1

Apache > HTTP Server > Documentation > Version 2.1

# Filters

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This document describes the use of filters in Apache.

## Filters

Related Modules | Related Directives
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- `mod_deflate`
- `mod_ext_filter`
- `mod_include`

- `AddInputFilter`
- `AddOutputFilter`
- `RemoveInputFilter`
- `RemoveOutputFilter`
- `ExtFilterDefine`
- `ExtFilterOptions`
- `SetInputFilter`
- `SetOutputFilter`

A filter is a process that 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 that are selectable using run-time configuration directives. The set of filters that apply to data can be manipulated with the `SetInputFilter`, `SetOutputFilter`, `AddInputFilter`, `AddOutputFilter`, `RemoveInputFilter`, and `RemoveOutputFilter` directives.

The following user-selectable filters are currently provided with the Apache HTTP Server distribution.

INCLUDES
  Server-Side Includes processing by `mod_include`
DEFLATE
  Compress output before sending it to the client using `mod_deflate`

In addition, the module `mod_ext_filter` allows for external programs to be defined as filters.

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