mod_ext_filter - Apache HTTP Server Version 2.4

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Apache HTTP Server Version 2.4

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Apache Module mod_ext_filter

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Description:Pass the response body through an external program before
delivery to the client

    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

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




Bugfix checklisthttpd changelogKnown issuesReport a bugSee also



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


    Implementing a content encoding filter
      Note: this gzip example is just for the purposes of illustration.
      Please refer to mod_deflate for a practical

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


    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 \

<Location "/">
    # core directive to cause the slowdown filter to
    # be run several times on output
    SetOutputFilter slowdown;slowdown;slowdown


    Using sed to replace text in the response
      # mod_ext_filter directive to define a filter which
# replaces text in the response
ExtFilterDefine fixtext mode=output intype=text/html \
    cmd="/bin/sed s/verdana/arial/g"

<Location "/">
    # core directive to cause the fixtext filter to
    # be run on output
    SetOutputFilter fixtext

You can do the same thing using mod_substitute
without invoking an external process.


    Tracing another filter
      # Trace the data read and written by mod_deflate
# for a particular client (IP
# experiencing compression problems.
# This filter will trace what goes into mod_deflate.
ExtFilterDefine tracebefore \
    cmd="/bin/ /tmp/tracebefore" \

# This filter will trace what goes after mod_deflate.
# Note that without the ftype parameter, the default
# filter type of AP_FTYPE_RESOURCE would cause the
# filter to be placed *before* mod_deflate in the filter
# chain.  Giving it a numeric value slightly higher than
# AP_FTYPE_CONTENT_SET will ensure that it is placed
# after mod_deflate.
ExtFilterDefine traceafter \
    cmd="/bin/ /tmp/traceafter" \
    EnableEnv=trace_this_client ftype=21

<Directory "/usr/local/docs">
    SetEnvIf Remote_Addr trace_this_client
    SetOutputFilter tracebefore;deflate;traceafter

      Here is the filter which traces the data:#!/usr/local/bin/perl -w
use strict;

open(SAVE, ">$ARGV[0]")
    or die "can't open $ARGV[0]: $?";

while (<STDIN>) {
    print SAVE $_;
    print $_;



ExtFilterDefine Directive

Description:Define an external filter
Syntax:ExtFilterDefine filtername parameters
Context:server config

    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


      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 (e.g., cmd="/bin/mypgm
      arg1 arg2".) Normal shell quoting is
      not necessary since the program is run directly, bypassing the shell.
      Program arguments are blank-delimited. A backslash can be used to
      escape blanks which should be part of a program argument. Any
      backslashes which are part of the argument must be escaped with
      backslash themselves.  In addition to the standard CGI environment
      variables, DOCUMENT_URI, DOCUMENT_PATH_INFO, and
      QUERY_STRING_UNESCAPED will also be set for the program.


      Use mode=output (the default) for filters which
      process the response.  Use mode=input for filters
      which process the request.  mode=input is available
      in Apache 2.1 and later.


      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


      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


      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.


      This parameter specifies the numeric value for filter type
      that the filter should be registered as.  The default value,
      AP_FTYPE_RESOURCE, is sufficient in most cases.  If the filter
      needs to operate at a different point in the filter chain than
      resource filters, then this parameter will be necessary.  See
      the AP_FTYPE_foo definitions in util_filter.h for appropriate


      This parameter specifies the name of an environment variable
      which, if set, will disable the filter.


      This parameter specifies the name of an environment variable
      which must be set, or the filter will be disabled.

ExtFilterOptions Directive

Description:Configure mod_ext_filter options
Syntax:ExtFilterOptions option [option] ...
Default:ExtFilterOptions NoLogStderr

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

      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.

      Determines how to proceed if the external filter program
      cannot be started.  With abort (the default value)
      the request will be aborted.  With remove, the
      filter is removed and the request continues without it.

    ExtFilterOptions LogStderr

    Messages written to the filter's standard error will be stored
    in the Apache error log.

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