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    process different content-types through different filters, even

    when the content-type is not known in advance (e.g. in a proxy).</p>

    <p>mod_filter works by introducing indirection into the filter

    chain.  Instead of inserting filters in the chain, we insert

    <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 mod_filter; no change to existing filter modules is required

    (although it may be possible to simplify them).</p>

    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>

    flexibility available for server admins to allow the chain to be

    configured dynamically.</p>

    <p>mod_filter 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

    <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

    alt="[This image shows the mod_filter model]"/><br />

    <dfn>Figure 2:</dfn> The <module>mod_filter</module> model</p>

    <p>mod_filter works by introducing indirection into the filter

    chain.  Instead of inserting filters in the chain, we insert

    <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 mod_filter; no change to existing filter modules is required

    (although it may be possible to simplify them).  There can be

    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>

</section>

<section id="config"><title>Configuring the Chain</title>

    <p>There are three stages to configuring a filter chain with mod_filter.

    For details of the directives, see below.</p>

    <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>FilterDeclare</directive> directive declares a filter,

    assigning it a name and a dispatch criterion.</dd>

    <dd>The <directive module="mod_filter">FilterDeclare</directive> directive

    declares a filter, assigning it a name and a dispatch criterion.</dd>

    <dt>Register Providers</dt>

    <dd>The <directive>FilterProvider</directive> directive registers a

    provider with a filter. The filter must have been registered with

    <directive>FilterDeclare</directive>. The provider must have been

    <dd>The <directive module="mod_filter">FilterProvider</directive>

    directive registers a provider with a filter. The filter must have

    been registered with <directive module="mod_filter"

    >FilterDeclare</directive>. The provider must have been

    registered with <code>ap_register_output_filter</code> by some module.

    The final argument to <directive>FilterProvider</directive> is a match

    string, that will be checked against the filter's dispatch criterion

    to determine whether to run this provider.</dd>

    The final argument to <directive module="mod_filter"

    >FilterProvider</directive> is a match string, that will be checked

    against the filter's dispatch criterion to determine whether to run

    this provider.</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>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>

    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="examples"><title>Examples</title>

    <dl>

    <dt>Server side Includes (SSI)</dt>

    <dd>A simple case of using mod_filter in place of

    <dd>A simple case of using <module>mod_filter</module> in place of

    <directive module="core">AddOutputFilterByType</directive>

    <example>

      FilterDeclare SSI Content-Type<br/>

    <li>Filters may make responses uncacheable.</li>

    </ul>

    <p>mod_filter 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>FilterProtocol</directive>

    implements some of this functionality, but there are no API calls yet.</p>

    <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, but there are no API calls yet.</p>

    <p>At the same time, mod_filter 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>FilterProtocol</directive> directives), mod_filter

    <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>

</section>

<name>FilterDeclare</name>

<description>Declare a smart filter</description>

<syntax>FilterDeclare <var>filter-name</var> [req|resp|env]=<var>dispatch</var>

<var>[type]</var>

</syntax>

    <var>[type]</var></syntax>

<contextlist><context>server config</context><context>virtual host</context>

<context>directory</context><context>.htaccess</context></contextlist>

<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 <code>filter-name</code>

    for use in <directive>FilterProvider</directive>,

    <directive>FilterChain</directive> and

    <directive>FilterProtocol</directive> directives.</p>

    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 second 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 "handler", which causes mod_filter to dispatch on the handler.</p>

    word <code>handler</code>, which causes <module>mod_filter</module>

    to dispatch on the content handler.</p>

    <p>The final (optional) argument

    is the type of filter, and takes values of <var>ap_filter_type</var>

    - namely <var>RESOURCE</var> (the default), <var>CONTENT_SET</var>,

    <var>PROTOCOL</var>, <var>TRANSCODE</var>, <var>CONNECTION</var>

    or <var>NETWORK</var>.</p>

    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>

    <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> in the <directive>FilterDeclare</directive>

    directive that declared <var>filter-name</var>.</p>

    as <var>dispatch</var> in the <directive module="mod_filter"

    >FilterDeclare</directive> directive that declared

    <var>filter-name</var>.</p>

    <p><var>filter-name</var> must have been declared with

    <directive>FilterDeclare</directive>.  <var>provider-name</var> must have

    been registered by loading a module that registers the name with

    <directive module="mod_filter">FilterDeclare</directive>.

    <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>The <var>match</var> argument specifies a match that will be applied to

    the filter's <var>dispatch</var> criterion.  The match may be a string

    match (exact match or substring), a regexp, an integer (greater, lessthan

    or equals), or unconditional.  The first characters of the <var>match</var>

    argument determines this:</p>

    the filter's <var>dispatch</var> criterion.  The <var>match</var> may be

    a string match (exact match or substring), a regexp, 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

    <strong>!</strong>, this reverses the rule, so the provider will be used

    (<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 ! as follows:</p>

    <dl>

    <dt>default</dt>

    <dd>exact match</dd>

    <dt>$</dt>

    <dd>substring match</dd>

    <dt>/</dt>

    <dd>regexp match</dd>

    <dt>=</dt>

    <dd>integer equality</dd>

    <dt>&lt;</dt>

    <dd>integer less-than</dd>

    <dt>&gt;</dt>

    <dd>integer greater-than</dd>

    <dt>*</dt>

    <dd>Unconditional match</dd>

    </dl>

    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>regexp match</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>&gt;</code></td><td>integer greater-than</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>)+</syntax>

<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>

    determines what to do:</p>

    <dl>

    <dt>+filter-name</dt>

    <dd>Add filter-name to the end of the filter chain</dd>

    <dt><code>+<var>filter-name</var></code></dt>

    <dd>Add <var>filter-name</var> to the end of the filter chain</dd>

    <dt>@filter-name</dt>

    <dd>Insert filter-name at the start 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>-filter-name</dt>

    <dd>Remove filter-name from the filter chain</dd>

    <dt><code>-<var>filter-name</var></code></dt>

    <dd>Remove <var>filter-name</var> from the filter chain</dd>

    <dt>=filter-name</dt>

    <dd>Empty the filter chain and insert filter-name</dd>

    <dt><code>=<var>filter-name</var></code></dt>

    <dd>Empty the filter chain and insert <var>filter-name</var></dd>

    <dt>!</dt>

    <dt><code>!</code></dt>

    <dd>Empty the filter chain</dd>

    <dt>filter-name</dt>

    <dd>Equivalent to +filter-name</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 filter-name [provider-name] "proto-flags"</syntax>

<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>

<usage>

    <p>This directs mod_filter 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>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 filter-name and a provider for that filter.

    With two arguments it applies to a filter-name whenever the filter runs

    <em>any</em> provider.</p>

    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>change=yes</dt>

    <dt><code>change=yes</code></dt>

    <dd>The filter changes the content, including possibly the content

    length</dd>

    <dt>change=1:1</dt>

    <dt><code>change=1:1</code></dt>

    <dd>The filter changes the content, but will not change the content

    length</dd>

    <dt>byteranges=no</dt>

    <dt><code>byteranges=no</code></dt>

    <dd>The filter cannot work on byteranges and requires complete input</dd>

    <dt>proxy=no</dt>

    <dt><code>proxy=no</code></dt>

    <dd>The filter should not run in a proxy context</dd>

    <dt>proxy=transform</dt>

    <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>cache=no</dt>

    <dt><code>cache=no</code></dt>

    <dd>The filter renders the output uncacheable (eg by introducing randomised

    content changes)</dd>

    </dl>

<directivesynopsis>

<name>FilterTrace</name>

<description>Get debug/diagnostic information from mod_filter</description>

<syntax>FilterTrace filter-name level</syntax>

<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><context>.htaccess</context></contextlist>

<usage>

    <p>This directive generates debug information from mod_filter.

    <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 mod_filter itself.</p>

    it may also help with <module>mod_filter</module> itself.</p>

    <p>The debug output depends on the level set:</p>

    <p>The debug output depends on the <var>level</var> set:</p>

    <dl>

    <dt>0 (default)</dt>

    <dt><code>0</code> (default)</dt>

    <dd>No debug information is generated.</dd>

    <dt>1</dt>

    <dd>mod_filter 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

    <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>2 (not yet implemented)</dt>

    <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>