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<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">&nbsp;en&nbsp;</a> |
<a href="../fr/mod/mod_filter.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
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<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"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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 Apache HTTP Server 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><h3 class="directives">Directives</h3>
<ul id="toc">
<li><img alt="" src="../images/down.gif" /> <a href="#addoutputfilterbytype">AddOutputFilterByType</a></li>
<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>Bugfix checklist</h3><ul class="seealso"><li><a href="https://www.apache.org/dist/httpd/CHANGES_2.4">httpd changelog</a></li><li><a href="https://bz.apache.org/bugzilla/buglist.cgi?bug_status=__open__&amp;list_id=144532&amp;product=Apache%20httpd-2&amp;query_format=specific&amp;order=changeddate%20DESC%2Cpriority%2Cbug_severity&amp;component=mod_filter">Known issues</a></li><li><a href="https://bz.apache.org/bugzilla/enter_bug.cgi?product=Apache%20httpd-2&amp;component=mod_filter">Report a bug</a></li></ul><h3>See also</h3>
<ul class="seealso">
<li><a href="#comments_section">Comments</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 complex boolean
    <a href="../expr.html">expressions</a> This generalises the limited
    flexibility offered by <code class="directive">AddOutputFilterByType</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="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 (&amp;&amp; / ||)
    and brackets. The details of the expression syntax are described in
    the <a href="../expr.html">ap_expr documentation</a>.</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 Apache HTTP Server 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 support substring matches.  They 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 replacing <code class="directive">AddOutputFilterByType</code>
    <pre class="prettyprint lang-config">FilterDeclare SSI
FilterProvider SSI INCLUDES "%{CONTENT_TYPE} =~ m|^text/html|"
FilterChain SSI</pre>

    </dd>

    <dt>Server side Includes (SSI)</dt>
    <dd>The same as the above but dispatching on handler (classic
    SSI behaviour; .shtml files get processed).
    <pre class="prettyprint lang-config">FilterProvider SSI INCLUDES "%{HANDLER} = 'server-parsed'"
FilterChain SSI</pre>

    </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.
    <pre class="prettyprint lang-config">FilterDeclare gzip CONTENT_SET
FilterProvider gzip inflate "%{req:Accept-Encoding} !~ /gzip/"
FilterChain gzip</pre>

    </dd>

    <dt>Image Downsampling</dt>
    <dd>Suppose we want to downsample all web images, and have filters
    for GIF, JPEG and PNG.
    <pre class="prettyprint lang-config">FilterProvider unpack jpeg_unpack "%{CONTENT_TYPE} = 'image/jpeg'"
FilterProvider unpack gif_unpack  "%{CONTENT_TYPE} = 'image/gif'"
FilterProvider unpack png_unpack  "%{CONTENT_TYPE} = 'image/png'"

FilterProvider downsample downsample_filter "%{CONTENT_TYPE} = m|^image/(jpeg|gif|png)|"
FilterProtocol downsample "change=yes"

FilterProvider repack jpeg_pack "%{CONTENT_TYPE} = 'image/jpeg'"
FilterProvider repack gif_pack  "%{CONTENT_TYPE} = 'image/gif'"
FilterProvider repack png_pack  "%{CONTENT_TYPE} = 'image/png'"
&lt;Location "/image-filter"&gt;
    FilterChain unpack downsample repack
&lt;/Location&gt;</pre>

    </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="AddOutputFilterByType" id="AddOutputFilterByType">AddOutputFilterByType</a> <a name="addoutputfilterbytype" id="addoutputfilterbytype">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>assigns an output filter to a particular media-type</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>AddOutputFilterByType <var>filter</var>[;<var>filter</var>...]
<var>media-type</var> [<var>media-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>FileInfo</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>
<tr><th><a href="directive-dict.html#Compatibility">Compatibility:</a></th><td>Had severe limitations before
being moved to <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code> in version 2.3.7</td></tr>
</table>
    <p>This directive activates a particular output <a href="../filter.html">filter</a> for a request depending on the
    response <a class="glossarylink" href="../glossary.html#media-type" title="see glossary">media-type</a>.</p>

    <p>The following example uses the <code>DEFLATE</code> filter, which
    is provided by <code class="module"><a href="../mod/mod_deflate.html">mod_deflate</a></code>. It will compress all
    output (either static or dynamic) which is labeled as
    <code>text/html</code> or <code>text/plain</code> before it is sent
    to the client.</p>

    <pre class="prettyprint lang-config">AddOutputFilterByType DEFLATE text/html text/plain</pre>


    <p>If you want the content to be processed by more than one filter, their
    names have to be separated by semicolons. It's also possible to use one
    <code class="directive">AddOutputFilterByType</code> directive for each of
    these filters.</p>

    <p>The configuration below causes all script output labeled as
    <code>text/html</code> to be processed at first by the
    <code>INCLUDES</code> filter and then by the <code>DEFLATE</code>
    filter.</p>

    <pre class="prettyprint lang-config">&lt;Location "/cgi-bin/"&gt;
    Options Includes
    AddOutputFilterByType INCLUDES;DEFLATE text/html
&lt;/Location&gt;</pre>



<h3>See also</h3>
<ul>
<li><code class="directive"><a href="../mod/mod_mime.html#addoutputfilter">AddOutputFilter</a></code></li>
<li><code class="directive"><a href="../mod/core.html#setoutputfilter">SetOutputFilter</a></code></li>
<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="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>Flags specified with this directive are merged with the flags 
    that underlying providers may have registerd with 
    <code class="module"><a href="../mod/mod_filter.html">mod_filter</a></code>. For example, a filter may internally specify
    the equivalent of <code>change=yes</code>, but a particular 
    configuration of the module can override with <code>change=no</code>.
    </p>

    <p><var>proto-flags</var> is one or more of</p>

    <dl>
    <dt><code>change=yes|no</code></dt>
    <dd>Specifies whether the filter changes the content, including possibly 
    the content length.  The "no" argument is supported in 2.4.7 and later.</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>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<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> is an
    <a href="../expr.html">ap_expr</a>.</p>


<h3>See also</h3>
<ul>
<li><a href="../expr.html">Expressions in Apache HTTP Server</a>,
for a complete reference and examples.</li>
<li><code class="module"><a href="../mod/mod_include.html">mod_include</a></code></li>
</ul>
</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>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/mod/mod_filter.html" title="English">&nbsp;en&nbsp;</a> |
<a href="../fr/mod/mod_filter.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a></p>
</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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