# Module mod_ext_filter

This module provides the ability to pass the response body through an external program before delivering to the client.

Status: Experimental
Source File: mod_ext_filter.c
Module Identifier: ext_filter_module

## Summary

This is an experimental module and should be used with care. Test your `mod_ext_filter` configuration carefully to ensure that it performs the desired function. You may wish to review XXX for background on the Apache filtering model.

`mod_ext_filter` presents a simple and familiar programming model for filters. With this module, a program which reads from stdin and writes to stdout (i.e., a Unix-style filter command) can be a filter for Apache. This filtering mechanism is much slower than using a filter which is specially written for the Apache API and runs inside of the Apache server process, but it does have the following benefits:

- the programming model is much simpler
- any programming/scripting language can be used, provided that it allows the program to read from standard input and write to standard output
- existing programs can be used unmodified as Apache filters

Even when the performance characteristics are not suitable for production use, `mod_ext_filter` can be used as a prototype environment for filters.

## Directives

- ExtFilterDefine
- ExtFilterOptions

## Examples

### Generating HTML from some other type of response

        # mod_ext_filter directive to define a filter to HTML-ize text/c files
        # using the external program /usr/bin/enscript, with the type of the
        # result set to text/html
        ExtFilterDefine c-to-html mode=output intype=text/c outtype=text/html \
                        cmd="/usr/bin/enscript --color -W html -Ec -o - -"

        <Directory "/export/home/trawick/apacheinst/htdocs/c">

        # core directive to cause the new filter to be run on output
        SetOutputFilter c-to-heml

        # mod_mime directive to set the type of .c files to text/c
        AddType text/c .c

        # mod_ext_filter directive to set the debug level just high
        # enough to see a log message per request showing the configuration
        # in force
        ExtFilterOptions DebugLevel=1

        </Directory>

### Implementing a content encoding filter

      # mod_ext_filter directive to define the external filter
      ExtFilterDefine gzip mode=output cmd=/bin/gzip

      <Location /gzipped>

      # core directive to cause the gzip filter to be run on output
      SetOutputFilter gzip

      # mod_header directive to add "Content-Encoding: gzip" header field
      Header set Content-Encoding gzip

      </Location>

### Slowing down the server

      # mod_ext_filter directive to define a filter which runs everything
      # through cat; cat doesn't modify anything; it just introduces extra
      # pathlength and consumes more resources
      ExtFilterDefine slowdown mode=output cmd=/bin/cat preservescontentlength

      <Location />

      # core directive to cause the slowdown filter to be run several times on
      # output
      SetOutputFilter slowdown slowdown slowdown

      </Location>

## ExtFilterDefine

Syntax: ExtFilterDefine filtername parameters
Default: None
Context: server
Override: none
Status: Experimental
Module: mod_ext_filter

The `ExtFilterDefine` directive defines the characteristics of an external filter, including the program to run and its arguments.

filtername specifies the name of the filter being defined. This name can then be used in SetOutputFilter directives. It must be unique among all registered filters. At the present time, no error is reported by the register-filter API, so a problem with duplicate names isn't reported to the user.

Subsequent parameters can appear in any order and define the external command to run and certain other characteristics. The only required parameter is cmd=. These parameters are:

cmd=cmdline
  The `cmd=` keyword allows you to specify the external command to run. If there are arguments after the program name, the command line should be surrounded in quotation marks.
mode=mode
  mode should be output for now (the default). In the future, mode=input will be used to specify a filter for request bodies.
intype=imt
  This parameter specifies the internet media type (i.e., MIME type) of documents which should be filtered. By default, all documents are filtered. If `intype=` is specified, the filter will be disabled for documents of other types.
outtype=imt
  This parameter specifies the internet media type (i.e., MIME type) of filtered documents. It is useful when the filter changes the internet media type as part of the filtering operation. By default, the internet media type is unchanged.
PreservesContentLength
  The `PreservesContentLength` keyword specifies that the filter preserves the content length. This is not the default, as most filters change the content length. In the event that the filter doesn't modify the length, this keyword should be specified.

## ExtFilterOptions

Syntax: ExtFilterOptions option [option] ...
Default: DebugLevel=0 NoLogStderr
Context: directory
Override: none
Status: Experimental
Module: mod_ext_filter

The `ExtFilterOptions` directive specifies special processing options for `mod_ext_filter`. Option can be one of

DebugLevel=n
  The `DebugLevel` keyword allows you to specify the level of debug messages generated by `mod_ext_filter`. By default, no debug messages are generated. This is equivalent to `DebugLevel=0`. With higher numbers, more debug messages are generated, and server performance will be degraded. The actual meanings of the numeric values are described with the definitions of the DBGLVL_ constants near the beginning of `mod_ext_filter.c`.

  Note: The core directive LogLevel should be used to cause debug messages to be stored in the Apache error log.

LogStderr | NoLogStderr
  The `LogStderr` keyword specifies that messages written to standard error by the external filter program will be saved in the Apache error log. `NoLogStderr` disables this feature.

Example:

        ExtFilterOptions  LogStderr DebugLevel=0

Messages written to the filter's standard error will be stored in the Apache error log. No debug messages will be generated by `mod_ext_filter`.