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<title>mod_ext_filter - Apache HTTP Server</title>
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<div id="preamble"><h1>Apache Module mod_ext_filter</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_ext_filter.html" title="English"> en </a> |
<a href="../ja/mod/mod_ext_filter.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="../ko/mod/mod_ext_filter.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Pass the response body through an external program before
delivery to the client</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>ext_filter_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_ext_filter.c</td></tr></table>
<h3>Summary</h3>
<p><code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code> presents a simple and familiar
programming model for <a href="../filter.html">filters</a>. 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:</p>
<ul>
<li>the programming model is much simpler</li>
<li>any programming/scripting language can be used, provided
that it allows the program to read from standard input and
write to standard output</li>
<li>existing programs can be used unmodified as Apache
filters</li>
</ul>
<p>Even when the performance characteristics are not suitable
for production use, <code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code> can be used as
a prototype environment for filters.</p>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#extfilterdefine">ExtFilterDefine</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#extfilteroptions">ExtFilterOptions</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#examples">Examples</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><a href="../filter.html">Filters</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="examples" id="examples">Examples</a></h2>
<h3>Generating HTML from some other type of response</h3>
<div class="example"><p><code>
# mod_ext_filter directive to define a filter<br />
# to HTML-ize text/c files using the external<br />
# program /usr/bin/enscript, with the type of<br />
# the result set to text/html<br />
ExtFilterDefine c-to-html mode=output \<br />
<span class="indent">
intype=text/c outtype=text/html \<br />
cmd="/usr/bin/enscript --color -W html -Ec -o - -"<br />
</span>
<br />
<Directory "/export/home/trawick/apacheinst/htdocs/c"><br />
<span class="indent">
# core directive to cause the new filter to<br />
# be run on output<br />
SetOutputFilter c-to-html<br />
<br />
# mod_mime directive to set the type of .c<br />
# files to text/c<br />
AddType text/c .c<br />
<br />
# mod_ext_filter directive to set the debug<br />
# level just high enough to see a log message<br />
# per request showing the configuration in force<br />
ExtFilterOptions DebugLevel=1<br />
</span>
</Directory>
</code></p></div>
<h3>Implementing a content encoding filter</h3>
<p>Note: this gzip example is just for the purposes of illustration.
Please refer to <code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code> for a practical
implementation.</p>
<div class="example"><p><code>
# mod_ext_filter directive to define the external filter<br />
ExtFilterDefine gzip mode=output cmd=/bin/gzip<br />
<br />
<Location /gzipped><br />
<span class="indent">
# core directive to cause the gzip filter to be<br />
# run on output<br />
SetOutputFilter gzip<br />
<br />
# mod_header directive to add<br />
# "Content-Encoding: gzip" header field<br />
Header set Content-Encoding gzip<br />
</span>
</Location>
</code></p></div>
<h3>Slowing down the server</h3>
<div class="example"><p><code>
# mod_ext_filter directive to define a filter<br />
# which runs everything through cat; cat doesn't<br />
# modify anything; it just introduces extra pathlength<br />
# and consumes more resources<br />
ExtFilterDefine slowdown mode=output cmd=/bin/cat \<br />
<span class="indent">
preservescontentlength<br />
</span>
<br />
<Location /><br />
<span class="indent">
# core directive to cause the slowdown filter to<br />
# be run several times on output<br />
#<br />
SetOutputFilter slowdown;slowdown;slowdown<br />
</span>
</Location>
</code></p></div>
<h3>Using sed to replace text in the response</h3>
<div class="example"><p><code>
# mod_ext_filter directive to define a filter which<br />
# replaces text in the response<br />
#<br />
ExtFilterDefine fixtext mode=output intype=text/html \<br />
<span class="indent">
cmd="/bin/sed s/verdana/arial/g"<br />
</span>
<br />
<Location /><br />
<span class="indent">
# core directive to cause the fixtext filter to<br />
# be run on output<br />
SetOutputFilter fixtext<br />
</span>
</Location>
</code></p></div>
<h3>Tracing another filter</h3>
<div class="example"><p><code>
# Trace the data read and written by mod_deflate<br />
# for a particular client (IP 192.168.1.31)<br />
# experiencing compression problems.<br />
# This filter will trace what goes into mod_deflate.<br />
ExtFilterDefine tracebefore \<br />
<span class="indent">
cmd="/bin/tracefilter.pl /tmp/tracebefore" \<br />
EnableEnv=trace_this_client<br />
</span>
<br />
# This filter will trace what goes after mod_deflate.<br />
# Note that without the ftype parameter, the default<br />
# filter type of AP_FTYPE_RESOURCE would cause the<br />
# filter to be placed *before* mod_deflate in the filter<br />
# chain. Giving it a numeric value slightly higher than<br />
# AP_FTYPE_CONTENT_SET will ensure that it is placed<br />
# after mod_deflate.<br />
ExtFilterDefine traceafter \<br />
<span class="indent">
cmd="/bin/tracefilter.pl /tmp/traceafter" \<br />
EnableEnv=trace_this_client ftype=21<br />
</span>
<br />
<Directory /usr/local/docs><br />
<span class="indent">
SetEnvIf Remote_Addr 192.168.1.31 trace_this_client<br />
SetOutputFilter tracebefore;deflate;traceafter<br />
</span>
</Directory>
</code></p></div>
<div class="example"><h3>Here is the filter which traces the data:</h3><p><code>
#!/usr/local/bin/perl -w<br />
use strict;<br />
<br />
open(SAVE, ">$ARGV[0]")<br />
<span class="indent">
or die "can't open $ARGV[0]: $?";<br />
</span>
<br />
while (<STDIN>) {<br />
<span class="indent">
print SAVE $_;<br />
print $_;<br />
</span>
}<br />
<br />
close(SAVE);
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="ExtFilterDefine" id="ExtFilterDefine">ExtFilterDefine</a> <a name="extfilterdefine" id="extfilterdefine">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Define an external filter</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ExtFilterDefine <var>filtername</var> <var>parameters</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_ext_filter</td></tr>
</table>
<p>The <code class="directive">ExtFilterDefine</code> directive defines the
characteristics of an external filter, including the program to
run and its arguments.</p>
<p><var>filtername</var> specifies the name of the filter being
defined. This name can then be used in <code class="directive"><a href="../mod/core.html#setoutputfilter">SetOutputFilter</a></code>
directives. It must be unique among all registered filters.
<em>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.</em></p>
<p>Subsequent parameters can appear in any order and define the
external command to run and certain other characteristics. The
only required parameter is <code>cmd=</code>. These parameters
are:</p>
<dl>
<dt><code>cmd=<var>cmdline</var></code></dt>
<dd>The <code>cmd=</code> 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 (<em>e.g.</em>, <code>cmd="<var>/bin/mypgm</var>
<var>arg1</var> <var>arg2</var>"</code>.) 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.</dd>
<dt><code>mode=<var>mode</var></code></dt>
<dd>Use <code>mode=output</code> (the default) for filters which
process the response. Use <code>mode=input</code> for filters
which process the request. <code>mode=input</code> is available
in Apache 2.1 and later.</dd>
<dt><code>intype=<var>imt</var></code></dt>
<dd>This parameter specifies the internet media type (<em>i.e.</em>,
MIME type) of documents which should be filtered. By default,
all documents are filtered. If <code>intype=</code> is
specified, the filter will be disabled for documents of other
types.</dd>
<dt><code>outtype=<var>imt</var></code></dt>
<dd>This parameter specifies the internet media type (<em>i.e.</em>,
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.</dd>
<dt><code>PreservesContentLength</code></dt>
<dd>The <code>PreservesContentLength</code> 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.</dd>
<dt><code>ftype=<var>filtertype</var></code></dt>
<dd>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
values.</dd>
<dt><code>disableenv=<var>env</var></code></dt>
<dd>This parameter specifies the name of an environment variable
which, if set, will disable the filter.</dd>
<dt><code>enableenv=<var>env</var></code></dt>
<dd>This parameter specifies the name of an environment variable
which must be set, or the filter will be disabled.</dd>
</dl>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="ExtFilterOptions" id="ExtFilterOptions">ExtFilterOptions</a> <a name="extfilteroptions" id="extfilteroptions">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure <code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code> options</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>ExtFilterOptions <var>option</var> [<var>option</var>] ...</code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>ExtFilterOptions DebugLevel=0 NoLogStderr</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>directory</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_ext_filter</td></tr>
</table>
<p>The <code class="directive">ExtFilterOptions</code> directive specifies
special processing options for <code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code>.
<var>Option</var> can be one of</p>
<dl>
<dt><code>DebugLevel=<var>n</var></code></dt>
<dd>
The <code>DebugLevel</code> keyword allows you to specify
the level of debug messages generated by
<code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code>. By default, no debug messages
are generated. This is equivalent to
<code>DebugLevel=0</code>. 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 <code>mod_ext_filter.c</code>.
<p>Note: The core directive <code class="directive"><a href="../mod/core.html#loglevel">LogLevel</a></code> should be used to cause debug messages to
be stored in the Apache error log.</p>
</dd>
<dt><code>LogStderr | NoLogStderr</code></dt>
<dd>The <code>LogStderr</code> keyword specifies that
messages written to standard error by the external filter
program will be saved in the Apache error log.
<code>NoLogStderr</code> disables this feature.</dd>
<dt><code>Onfail=[abort|remove]</code> (new in httpd version 2.2.12).</dt>
<dd>Determines how to proceed if the external filter program
cannot be started. With <code>abort</code> (the default value)
the request will be aborted. With <code>remove</code>, the
filter is removed and the request continues without it.</dd>
</dl>
<div class="example"><h3>Example</h3><p><code>
ExtFilterOptions LogStderr DebugLevel=0
</code></p></div>
<p>Messages written to the filter's standard error will be stored
in the Apache error log. No debug messages will be generated by
<code class="module"><a href="../mod/mod_ext_filter.html">mod_ext_filter</a></code>. </p>
</div>
</div>
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