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<modulesynopsis metafile="mod_filter.xml.meta">

<name>mod_filter</name>
<description>Context-sensitive smart filter configuration module</description>
<status>Base</status>
<sourcefile>mod_filter.c</sourcefile>
<identifier>filter_module</identifier>
<compatibility>Version 2.1 and later</compatibility>

<summary>
    <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><module>mod_filter</module> 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 <module>mod_filter</module>; no change to existing filter modules is
    required (although it may be possible to simplify them).</p>
</summary>

<section id="smart"><title>Smart Filtering</title>
    <p>In the traditional filtering model, filters are inserted unconditionally
    using <directive module="mod_mime">AddOutputFilter</directive> 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><module>mod_filter</module> 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 <directive module="core">AddOutputFilterByType</directive>, 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
    <module>mod_rewrite</module> to anyone who needs it.</p>
</section>

<section id="terms"><title>Filter Declarations, Providers and Chains</title>
    <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 <module>mod_filter</module> model</p>

    <p><module>mod_filter</module> 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 <module>mod_filter</module>; 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>
</section>

<section id="config"><title>Configuring the Chain</title>
    <p>There are three stages to configuring a filter chain with
    <module>mod_filter</module>. For details of the directives, see below.</p>

    <dl>
    <dt>Declare Filters</dt>
    <dd>The <directive module="mod_filter">FilterDeclare</directive> 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 <directive module="mod_filter">FilterProvider</directive>
    directive registers a provider with a filter. The filter may have
    been declared with <directive module="mod_filter"
    >FilterDeclare</directive>; 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 remaining arguments to <directive module="mod_filter"
    >FilterProvider</directive> are a dispatch criterion and a match string.
    The former may be an HTTP request or response header, an environment
    variable, or the Handler used by this request.  The latter is matched
    to it for each request, to determine whether this provider will be
    used to implement the filter for this request.</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 <directive module="mod_filter"
    >FilterChain</directive> 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>
</section>
<section id="errordocs"><title>Filtering and Response Status</title>
    <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 <directive>FilterProvider</directive>.</p>
</section>

<section id="examples"><title>Examples</title>
    <dl>
    <dt>Server side Includes (SSI)</dt>
    <dd>A simple case of using <module>mod_filter</module> in place of
    <directive module="core">AddOutputFilterByType</directive>
    <example>
      FilterDeclare SSI<br/>
      FilterProvider SSI INCLUDES resp=Content-Type $text/html<br/>
      FilterChain SSI
    </example>
    </dd>

    <dt>Server side Includes (SSI)</dt>
    <dd>The same as the above but dispatching on handler (classic
    SSI behaviour; .shtml files get processed).
    <example>
      FilterProvider SSI INCLUDES Handler server-parsed<br/>
      FilterChain SSI
    </example>
    </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.
    <example>
      FilterDeclare gzip CONTENT_SET<br/>
      FilterProvider gzip inflate req=Accept-Encoding !$gzip<br/>
      FilterChain gzip
    </example>
    </dd>

    <dt>Image Downsampling</dt>
    <dd>Suppose we want to downsample all web images, and have filters
    for GIF, JPEG and PNG.
    <example>
      FilterProvider unpack jpeg_unpack Content-Type $image/jpeg<br/>
      FilterProvider unpack gif_unpack Content-Type $image/gif<br/>
      FilterProvider unpack png_unpack Content-Type $image/png<br/>
      <br />
      FilterProvider downsample downsample_filter Content-Type $image<br/>
      FilterProtocol downsample "change=yes"<br/>
      <br />
      FilterProvider repack jpeg_pack Content-Type $image/jpeg<br/>
      FilterProvider repack gif_pack Content-Type $image/gif<br/>
      FilterProvider repack png_pack Content-Type $image/png<br/>
      &lt;Location /image-filter&gt;<br/>
      <indent>
        FilterChain unpack downsample repack<br/>
      </indent>
      &lt;/Location&gt;
    </example>
    </dd>
    </dl>
</section>

<section id="protocol"><title>Protocol Handling</title>
    <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><module>mod_filter</module> 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
    <directive module="mod_filter">FilterProtocol</directive> 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, <module>mod_filter</module> should not interfere
    with a filter that wants to handle all aspects of the protocol.  By
    default (i.e. in the absence of any <directive module="mod_filter"
    >FilterProtocol</directive> directives), <module>mod_filter</module>
    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>
</section>

<directivesynopsis>
<name>FilterDeclare</name>
<description>Declare a smart filter</description>
<syntax>FilterDeclare <var>filter-name</var> <var>[type]</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>Options</override>

<usage>
    <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 <directive module="mod_filter">FilterProvider</directive>,
    <directive module="mod_filter">FilterChain</directive> and
    <directive module="mod_filter">FilterProtocol</directive> 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>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>FilterProvider</name>
<description>Register a content filter</description>
<syntax>FilterProvider <var>filter-name</var> <var>provider-name</var>
 [req|resp|env]=<var>dispatch</var> <var>match</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>Options</override>

<usage>
    <p>This directive registers a <em>provider</em> for the smart filter.
    The provider will be called if and only if the <var>match</var> declared
    here matches the value of the header or environment variable declared
    as <var>dispatch</var>.</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>.
<!-- Placeholder; this is totally broken as of now
     , or declared with
    <directive module="mod_filter">FilterDeclare</directive>
    (or implicitly with another
    <directive module="mod_filter">FilterProvider</directive>).
-->
    </p>

    <p>The <var>dispatch</var> argument is a string with optional
    <code>req=</code>, <code>resp=</code> or <code>env=</code> prefix
    causing it to dispatch on (respectively) the request header, response
    header, or environment variable named.  In the absence of a
    prefix, it defaults to a response header.  A special case is the
    word <code>handler</code>, which causes <module>mod_filter</module>
    to dispatch on the content handler.</p>

    <p>The <var>match</var> argument specifies a match that will be applied to
    the filter's <var>dispatch</var> criterion.  The <var>match</var> may be
    a string match (exact match or substring), a <glossary ref="regex"
    >regex</glossary>, an integer (greater, lessthan or equals), or
    unconditional.  The first characters of the <var>match</var> argument
    determines this:</p>

    <p><strong>First</strong>, if the first character is an exclamation mark
    (<code>!</code>), this reverses the rule, so the provider will be used
    if and only if the match <em>fails</em>.</p>

    <p><strong>Second</strong>, it interprets the first character excluding
    any leading <code>!</code> as follows:</p>

    <table style="zebra" border="yes">
    <tr><th>Character</th><th>Description</th></tr>
    <tr><td><em>(none)</em></td><td>exact match</td></tr>
    <tr><td><code>$</code></td><td>substring match</td></tr>
    <tr><td><code>/</code></td><td>regex match (delimited by a second <code>/</code>)</td></tr>
    <tr><td><code>=</code></td><td>integer equality</td></tr>
    <tr><td><code>&lt;</code></td><td>integer less-than</td></tr>
    <tr><td><code>&lt;=</code></td><td>integer less-than or equal</td></tr>
    <tr><td><code>&gt;</code></td><td>integer greater-than</td></tr>
    <tr><td><code>&gt;=</code></td><td>integer greater-than or equal</td></tr>
    <tr><td><code>*</code></td><td>Unconditional match</td></tr>
    </table>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>FilterChain</name>
<description>Configure the filter chain</description>
<syntax>FilterChain [+=-@!]<var>filter-name</var> <var>...</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>Options</override>

<usage>
    <p>This configures an actual filter chain, from declared filters.
    <directive>FilterChain</directive> 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>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>FilterProtocol</name>
<description>Deal with correct HTTP protocol handling</description>
<syntax>FilterProtocol <var>filter-name</var> [<var>provider-name</var>]
    <var>proto-flags</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context><context>.htaccess</context></contextlist>
<override>Options</override>

<usage>
    <p>This directs <module>mod_filter</module> 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>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>FilterTrace</name>
<description>Get debug/diagnostic information from
    <module>mod_filter</module></description>
<syntax>FilterTrace <var>filter-name</var> <var>level</var></syntax>
<contextlist><context>server config</context><context>virtual host</context>
<context>directory</context></contextlist>

<usage>
    <p>This directive generates debug information from
    <module>mod_filter</module>.
    It is designed to help test and debug providers (filter modules), although
    it may also help with <module>mod_filter</module> 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><module>mod_filter</module> 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>
</usage>
</directivesynopsis>

</modulesynopsis>

