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<?xml version="1.0" encoding="ISO-8859-1"?>
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<title>mod_filter - Apache HTTP Server</title>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Modules</a></div>
<div id="page-content">
<div id="preamble"><h1>Apache Module mod_filter</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/mod/mod_filter.html" title="English"> en </a></p>
</div>
<table class="module"><tr><th><a href="module-dict.html#Description">Description:</a></th><td>Context-sensitive smart filter configuration module</td></tr>
<tr><th><a href="module-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="module-dict.html#ModuleIdentifier">Module Identifier:</a></th><td>filter_module</td></tr>
<tr><th><a href="module-dict.html#SourceFile">Source File:</a></th><td>mod_filter.c</td></tr>
<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Version 2.1 and later</td></tr></table>
<h3>Summary</h3>
<p>This module enables smart, context-sensitive configuration of
output content filters. For example, apache can be configured to
process different content-types through different filters, even
when the content-type is not known in advance (e.g. in a proxy).</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; no change to existing filter modules is
required (although it may be possible to simplify them).</p>
</div>
<div id="quickview"><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#filterchain">FilterChain</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#filterdeclare">FilterDeclare</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#filterprotocol">FilterProtocol</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#filterprovider">FilterProvider</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#filtertrace">FilterTrace</a></li>
</ul>
<h3>Topics</h3>
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#smart">Smart Filtering</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#terms">Filter Declarations, Providers and Chains</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#config">Configuring the Chain</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#errordocs">Filtering and Response Status</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#upgrade">Upgrading from HTTPD 2.2 Configuration</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#examples">Examples</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#protocol">Protocol Handling</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="smart" id="smart">Smart Filtering</a></h2>
<p>In the traditional filtering model, filters are inserted unconditionally
using <code class="directive"><a href="../mod/mod_mime.html#addoutputfilter">AddOutputFilter</a></code> and family.
Each filter then needs to determine whether to run, and there is little
flexibility available for server admins to allow the chain to be
configured dynamically.</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> by contrast gives server administrators a
great deal of flexibility in configuring the filter chain. In fact,
filters can be inserted based on any Request Header, Response Header
or Environment Variable. This generalises the limited flexibility offered
by <code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code>, and fixes
it to work correctly with dynamic content, regardless of the
content generator. The ability to dispatch based on Environment
Variables offers the full flexibility of configuration with
<code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> to anyone who needs it.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="terms" id="terms">Filter Declarations, Providers and Chains</a></h2>
<p class="figure">
<img src="../images/mod_filter_old.gif" width="160" height="310" alt="[This image displays the traditional filter model]" /><br />
<dfn>Figure 1:</dfn> The traditional filter model</p>
<p>In the traditional model, output filters are a simple chain
from the content generator (handler) to the client. This works well
provided the filter chain can be correctly configured, but presents
problems when the filters need to be configured dynamically based on
the outcome of the handler.</p>
<p class="figure">
<img src="../images/mod_filter_new.gif" width="423" height="331" alt="[This image shows the mod_filter model]" /><br />
<dfn>Figure 2:</dfn> The <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> model</p>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> works by introducing indirection into
the filter chain. Instead of inserting filters in the chain, we insert
a filter harness which in turn dispatches conditionally
to a filter provider. Any content filter may be used as a provider
to <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>; no change to existing filter modules
is required (although it may be possible to simplify them). There can be
multiple providers for one filter, but no more than one provider will
run for any single request.</p>
<p>A filter chain comprises any number of instances of the filter
harness, each of which may have any number of providers. A special
case is that of a single provider with unconditional dispatch: this
is equivalent to inserting the provider filter directly into the chain.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="config" id="config">Configuring the Chain</a></h2>
<p>There are three stages to configuring a filter chain with
<code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>. For details of the directives, see below.</p>
<dl>
<dt>Declare Filters</dt>
<dd>The <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code> directive
declares a filter, assigning it a name and filter type. Required
only if the filter is not the default type AP_FTYPE_RESOURCE.</dd>
<dt>Register Providers</dt>
<dd>The <code class="directive"><a href="#filterprovider">FilterProvider</a></code>
directive registers a provider with a filter. The filter may have
been declared with <code class="directive"><a href="#filterdeclare">FilterDeclare</a></code>; if not, FilterProvider will implicitly
declare it with the default type AP_FTYPE_RESOURCE. The provider
must have been
registered with <code>ap_register_output_filter</code> by some module.
The final argument to <code class="directive"><a href="#filterprovider">FilterProvider</a></code> is an expression: the provider will be
selected to run for a request if and only if the expression evaluates
to true. The expression may evaluate HTTP request or response
headers, environment variables, or the Handler used by this request.
Unlike earlier versions, mod_filter now supports complex expressions
involving multiple criteria with AND / OR logic (&& / ||)
and brackets.</dd>
<dt>Configure the Chain</dt>
<dd>The above directives build components of a smart filter chain,
but do not configure it to run. The <code class="directive"><a href="#filterchain">FilterChain</a></code> directive builds a filter chain from smart
filters declared, offering the flexibility to insert filters at the
beginning or end of the chain, remove a filter, or clear the chain.</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="errordocs" id="errordocs">Filtering and Response Status</a></h2>
<p>mod_filter normally only runs filters on responses with
HTTP status 200 (OK). If you want to filter documents with
other response statuses, you can set the <var>filter-errordocs</var>
environment variable, and it will work on all responses
regardless of status. To refine this further, you can use
expression conditions with <code class="directive">FilterProvider</code>.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="upgrade" id="upgrade">Upgrading from HTTPD 2.2 Configuration</a></h2>
<p>The <code class="directive"><a href="#filterprovider">FilterProvider</a></code>
directive has changed from HTTPD 2.2: the <var>match</var> and
<var>dispatch</var> arguments are replaced with a single but
more versatile <var>expression</var>. In general, you can convert
a match/dispatch pair to the two sides of an expression, using
something like:</p>
<div class="example"><p><code>"dispatch = match"</code></p></div>
<p>The Request headers, Response headers and Environment variables
are now interpreted from syntax <var>$req{foo}</var>,
<var>$resp{foo}</var> and <var>$env{foo}</var> respectively.
The variables <var>$handler</var> and <var>$Content-Type</var>
are also supported.</p>
<p>Note that the match no longer supports integer comparisons
or substring matches. The latter can be replaced by regular
expression matches.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="examples" id="examples">Examples</a></h2>
<dl>
<dt>Server side Includes (SSI)</dt>
<dd>A simple case of using <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> in place of
<code class="directive"><a href="../mod/core.html#addoutputfilterbytype">AddOutputFilterByType</a></code>
<div class="example"><p><code>
FilterDeclare SSI<br />
FilterProvider SSI INCLUDES "$resp{Content-Type} = /^text\/html/"<br />
FilterChain SSI
</code></p></div>
</dd>
<dt>Server side Includes (SSI)</dt>
<dd>The same as the above but dispatching on handler (classic
SSI behaviour; .shtml files get processed).
<div class="example"><p><code>
FilterProvider SSI INCLUDES "Handler = server-parsed"<br />
FilterChain SSI
</code></p></div>
</dd>
<dt>Emulating mod_gzip with mod_deflate</dt>
<dd>Insert INFLATE filter only if "gzip" is NOT in the
Accept-Encoding header. This filter runs with ftype CONTENT_SET.
<div class="example"><p><code>
FilterDeclare gzip CONTENT_SET<br />
FilterProvider gzip inflate "$req{Accept-Encoding} != /gzip/"<br />
FilterChain gzip
</code></p></div>
</dd>
<dt>Image Downsampling</dt>
<dd>Suppose we want to downsample all web images, and have filters
for GIF, JPEG and PNG.
<div class="example"><p><code>
FilterProvider unpack jpeg_unpack "$resp{Content-Type} = image/jpeg"<br />
FilterProvider unpack gif_unpack "$resp{Content-Type} = image/gif"<br />
FilterProvider unpack png_unpack "$resp{Content-Type} = image/png"<br />
<br />
FilterProvider downsample downsample_filter "$resp{Content-Type} = /image\/(jpeg|gif|png)/"<br />
FilterProtocol downsample "change=yes"<br />
<br />
FilterProvider repack jpeg_pack "$resp{Content-Type} = image/jpeg"<br />
FilterProvider repack gif_pack "$resp{Content-Type} = image/gif"<br />
FilterProvider repack png_pack "$resp{Content-Type} = image/png"<br />
<Location /image-filter><br />
<span class="indent">
FilterChain unpack downsample repack<br />
</span>
</Location>
</code></p></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="protocol" id="protocol">Protocol Handling</a></h2>
<p>Historically, each filter is responsible for ensuring that whatever
changes it makes are correctly represented in the HTTP response headers,
and that it does not run when it would make an illegal change. This
imposes a burden on filter authors to re-implement some common
functionality in every filter:</p>
<ul>
<li>Many filters will change the content, invalidating existing content
tags, checksums, hashes, and lengths.</li>
<li>Filters that require an entire, unbroken response in input need to
ensure they don't get byteranges from a backend.</li>
<li>Filters that transform output in a filter need to ensure they don't
violate a <code>Cache-Control: no-transform</code> header from the
backend.</li>
<li>Filters may make responses uncacheable.</li>
</ul>
<p><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> aims to offer generic handling of these
details of filter implementation, reducing the complexity required of
content filter modules. This is work-in-progress; the
<code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> implements
some of this functionality for back-compatibility with Apache 2.0
modules. For httpd 2.1 and later, the
<code>ap_register_output_filter_protocol</code> and
<code>ap_filter_protocol</code> API enables filter modules to
declare their own behaviour.</p>
<p>At the same time, <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> should not interfere
with a filter that wants to handle all aspects of the protocol. By
default (i.e. in the absence of any <code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> directives), <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>
will leave the headers untouched.</p>
<p>At the time of writing, this feature is largely untested,
as modules in common use are designed to work with 2.0.
Modules using it should test it carefully.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="FilterChain" id="FilterChain">FilterChain</a> <a name="filterchain" id="filterchain">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Configure the filter chain</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>FilterChain [+=-@!]<var>filter-name</var> <var>...</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>Options</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_filter</td></tr>
</table>
<p>This configures an actual filter chain, from declared filters.
<code class="directive">FilterChain</code> takes any number of arguments,
each optionally preceded with a single-character control that
determines what to do:</p>
<dl>
<dt><code>+<var>filter-name</var></code></dt>
<dd>Add <var>filter-name</var> to the end of the filter chain</dd>
<dt><code>@<var>filter-name</var></code></dt>
<dd>Insert <var>filter-name</var> at the start of the filter chain</dd>
<dt><code>-<var>filter-name</var></code></dt>
<dd>Remove <var>filter-name</var> from the filter chain</dd>
<dt><code>=<var>filter-name</var></code></dt>
<dd>Empty the filter chain and insert <var>filter-name</var></dd>
<dt><code>!</code></dt>
<dd>Empty the filter chain</dd>
<dt><code><var>filter-name</var></code></dt>
<dd>Equivalent to <code>+<var>filter-name</var></code></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="FilterDeclare" id="FilterDeclare">FilterDeclare</a> <a name="filterdeclare" id="filterdeclare">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Declare a smart filter</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>FilterDeclare <var>filter-name</var> <var>[type]</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>Options</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_filter</td></tr>
</table>
<p>This directive declares an output filter together with a
header or environment variable that will determine runtime
configuration. The first argument is a <var>filter-name</var>
for use in <code class="directive"><a href="#filterprovider">FilterProvider</a></code>,
<code class="directive"><a href="#filterchain">FilterChain</a></code> and
<code class="directive"><a href="#filterprotocol">FilterProtocol</a></code> directives.</p>
<p>The final (optional) argument
is the type of filter, and takes values of <code>ap_filter_type</code>
- namely <code>RESOURCE</code> (the default), <code>CONTENT_SET</code>,
<code>PROTOCOL</code>, <code>TRANSCODE</code>, <code>CONNECTION</code>
or <code>NETWORK</code>.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="FilterProtocol" id="FilterProtocol">FilterProtocol</a> <a name="filterprotocol" id="filterprotocol">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Deal with correct HTTP protocol handling</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>FilterProtocol <var>filter-name</var> [<var>provider-name</var>]
<var>proto-flags</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>Options</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_filter</td></tr>
</table>
<p>This directs <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> to deal with ensuring the
filter doesn't run when it shouldn't, and that the HTTP response
headers are correctly set taking into account the effects of the
filter.</p>
<p>There are two forms of this directive. With three arguments, it
applies specifically to a <var>filter-name</var> and a
<var>provider-name</var> for that filter.
With two arguments it applies to a <var>filter-name</var> whenever the
filter runs <em>any</em> provider.</p>
<p><var>proto-flags</var> is one or more of</p>
<dl>
<dt><code>change=yes</code></dt>
<dd>The filter changes the content, including possibly the content
length</dd>
<dt><code>change=1:1</code></dt>
<dd>The filter changes the content, but will not change the content
length</dd>
<dt><code>byteranges=no</code></dt>
<dd>The filter cannot work on byteranges and requires complete input</dd>
<dt><code>proxy=no</code></dt>
<dd>The filter should not run in a proxy context</dd>
<dt><code>proxy=transform</code></dt>
<dd>The filter transforms the response in a manner incompatible with
the HTTP <code>Cache-Control: no-transform</code> header.</dd>
<dt><code>cache=no</code></dt>
<dd>The filter renders the output uncacheable (eg by introducing randomised
content changes)</dd>
</dl>
</div>
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<div class="directive-section"><h2><a name="FilterProvider" id="FilterProvider">FilterProvider</a> <a name="filterprovider" id="filterprovider">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Register a content filter</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>FilterProvider <var>filter-name</var> <var>provider-name</var>
<var>expression</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory, .htaccess</td></tr>
<tr><th><a href="directive-dict.html#Override">Override:</a></th><td>Options</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_filter</td></tr>
</table>
<p>This directive registers a <em>provider</em> for the smart filter.
The provider will be called if and only if the <var>expression</var>
declared evaluates to true when the harness is first called.</p>
<p>
<var>provider-name</var> must have been registered by loading
a module that registers the name with
<code>ap_register_output_filter</code>.
</p>
<p><var>expression</var> can be any of the following:</p>
<dl>
<dt><code><var>string</var></code></dt>
<dd>true if <var>string</var> is not empty</dd>
<dt><code><var>string1</var> = <var>string2</var><br />
<var>string1</var> == <var>string2</var><br />
<var>string1</var> != <var>string2</var></code></dt>
<dd><p>Compare <var>string1</var> with <var>string2</var>. If
<var>string2</var> has the form <code>/<var>string2</var>/</code>
then it is treated as a regular expression. Regular expressions are
implemented by the <a href="http://www.pcre.org">PCRE</a> engine and
have the same syntax as those in <a href="http://www.perl.com">perl
5</a>. Note that <code>==</code> is just an alias for <code>=</code>
and behaves exactly the same way.</p>
</dd>
<dt><code><var>string1</var> < <var>string2</var><br />
<var>string1</var> <= <var>string2</var><br />
<var>string1</var> > <var>string2</var><br />
<var>string1</var> >= <var>string2</var></code></dt>
<dd>Compare <var>string1</var> with <var>string2</var>. Note, that
strings are compared <em>literally</em> (using
<code>strcmp(3)</code>). Therefore the string "100" is less than
"20".</dd>
<dt><code>( <var>expression</var> )</code></dt>
<dd>true if <var>expression</var> is true</dd>
<dt><code>! <var>expression</var></code></dt>
<dd>true if <var>expression</var> is false</dd>
<dt><code><var>expression1</var> &&
<var>expression2</var></code></dt>
<dd>true if both <var>expression1</var> and
<var>expression2</var> are true</dd>
<dt><code><var>expression1</var> ||
<var>expression2</var></code></dt>
<dd>true if either <var>expression1</var> or
<var>expression2</var> is true</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="FilterTrace" id="FilterTrace">FilterTrace</a> <a name="filtertrace" id="filtertrace">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Get debug/diagnostic information from
<code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code></td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>FilterTrace <var>filter-name</var> <var>level</var></code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host, directory</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Base</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_filter</td></tr>
</table>
<p>This directive generates debug information from
<code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>.
It is designed to help test and debug providers (filter modules), although
it may also help with <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> itself.</p>
<p>The debug output depends on the <var>level</var> set:</p>
<dl>
<dt><code>0</code> (default)</dt>
<dd>No debug information is generated.</dd>
<dt><code>1</code></dt>
<dd><code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> will record buckets and brigades
passing through the filter to the error log, before the provider has
processed them. This is similar to the information generated by
<a href="http://apache.webthing.com/mod_diagnostics/">mod_diagnostics</a>.
</dd>
<dt><code>2</code> (not yet implemented)</dt>
<dd>Will dump the full data passing through to a tempfile before the
provider. <strong>For single-user debug only</strong>; this will not
support concurrent hits.</dd>
</dl>
</div>
</div>
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