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220 lines
10 KiB
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220 lines
10 KiB
XML
<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="request.xml.meta">
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<parentdocument href="./">Developer Documentation</parentdocument>
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<title>Request Processing in the Apache HTTP Server 2.x</title>
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<summary>
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<note type="warning"><title>Warning</title>
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<p>Warning - this is a first (fast) draft that needs further
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revision!</p>
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</note>
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<p>Several changes in 2.0 and above affect the internal request
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processing mechanics. Module authors need to be aware of these
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changes so they may take advantage of the optimizations and
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security enhancements.</p>
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<p>The first major change is to the subrequest and redirect
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mechanisms. There were a number of different code paths in
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the Apache HTTP Server 1.3 to attempt to optimize subrequest
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or redirect behavior. As patches were introduced to 2.0, these
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optimizations (and the server behavior) were quickly broken due
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to this duplication of code. All duplicate code has been folded
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back into <code>ap_process_request_internal()</code> to prevent
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the code from falling out of sync again.</p>
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<p>This means that much of the existing code was 'unoptimized'.
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It is the Apache HTTP Project's first goal to create a robust
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and correct implementation of the HTTP server RFC. Additional
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goals include security, scalability and optimization. New
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methods were sought to optimize the server (beyond the
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performance of 1.3) without introducing fragile or
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insecure code.</p>
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</summary>
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<section id="processing"><title>The Request Processing Cycle</title>
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<p>All requests pass through <code>ap_process_request_internal()</code>
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in <code>request.c</code>, including subrequests and redirects. If a module
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doesn't pass generated requests through this code, the author is cautioned
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that the module may be broken by future changes to request
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processing.</p>
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<p>To streamline requests, the module author can take advantage
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of the hooks offered to drop out of the request cycle early, or
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to bypass core hooks which are irrelevant (and costly in
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terms of CPU.)</p>
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</section>
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<section id="parsing"><title>The Request Parsing Phase</title>
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<section id="unescape"><title>Unescapes the URL</title>
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<p>The request's <code>parsed_uri</code> path is unescaped, once and only
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once, at the beginning of internal request processing.</p>
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<p>This step is bypassed if the proxyreq flag is set, or the
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<code>parsed_uri.path</code> element is unset. The module has no further
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control of this one-time unescape operation, either failing to
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unescape or multiply unescaping the URL leads to security
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repercussions.</p>
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</section>
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<section id="strip"><title>Strips Parent and This Elements from the
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URI</title>
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<p>All <code>/../</code> and <code>/./</code> elements are
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removed by <code>ap_getparents()</code>. This helps to ensure
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the path is (nearly) absolute before the request processing
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continues.</p>
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<p>This step cannot be bypassed.</p>
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</section>
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<section id="inital-location-walk"><title>Initial URI Location Walk</title>
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<p>Every request is subject to an
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<code>ap_location_walk()</code> call. This ensures that
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<directive type="section" module="core">Location</directive> sections
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are consistently enforced for all requests. If the request is an internal
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redirect or a sub-request, it may borrow some or all of the processing
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from the previous or parent request's ap_location_walk, so this step
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is generally very efficient after processing the main request.</p>
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</section>
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<section id="translate_name"><title>translate_name</title>
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<p>Modules can determine the file name, or alter the given URI
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in this step. For example, <module>mod_vhost_alias</module> will
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translate the URI's path into the configured virtual host,
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<module>mod_alias</module> will translate the path to an alias path,
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and if the request falls back on the core, the <directive module="core"
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>DocumentRoot</directive> is prepended to the request resource.</p>
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<p>If all modules <code>DECLINE</code> this phase, an error 500 is
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returned to the browser, and a "couldn't translate name" error is logged
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automatically.</p>
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</section>
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<section id="map_to_storage"><title>Hook: map_to_storage</title>
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<p>After the file or correct URI was determined, the
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appropriate per-dir configurations are merged together. For
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example, <module>mod_proxy</module> compares and merges the appropriate
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<directive module="mod_proxy" type="section">Proxy</directive> sections.
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If the URI is nothing more than a local (non-proxy) <code>TRACE</code>
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request, the core handles the request and returns <code>DONE</code>.
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If no module answers this hook with <code>OK</code> or <code>DONE</code>,
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the core will run the request filename against the <directive
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module="core" type="section">Directory</directive> and <directive
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module="core" type="section">Files</directive> sections. If the request
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'filename' isn't an absolute, legal filename, a note is set for
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later termination.</p>
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</section>
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<section id="location-walk"><title>URI Location Walk</title>
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<p>Every request is hardened by a second
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<code>ap_location_walk()</code> call. This reassures that a
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translated request is still subjected to the configured
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<directive module="core" type="section">Location</directive> sections.
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The request again borrows some or all of the processing from its previous
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<code>location_walk</code> above, so this step is almost always very
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efficient unless the translated URI mapped to a substantially different
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path or Virtual Host.</p>
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</section>
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<section id="header_parser"><title>Hook: header_parser</title>
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<p>The main request then parses the client's headers. This
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prepares the remaining request processing steps to better serve
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the client's request.</p>
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</section>
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</section>
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<section id="security"><title>The Security Phase</title>
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<p>Needs Documentation. Code is:</p>
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<highlight language="c">
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if ((access_status = ap_run_access_checker(r)) != 0) {
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return decl_die(access_status, "check access", r);
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}
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if ((access_status = ap_run_check_user_id(r)) != 0) {
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return decl_die(access_status, "check user", r);
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}
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if ((access_status = ap_run_auth_checker(r)) != 0) {
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return decl_die(access_status, "check authorization", r);
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}
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</highlight>
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</section>
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<section id="preparation"><title>The Preparation Phase</title>
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<section id="type_checker"><title>Hook: type_checker</title>
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<p>The modules have an opportunity to test the URI or filename
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against the target resource, and set mime information for the
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request. Both <module>mod_mime</module> and
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<module>mod_mime_magic</module> use this phase to compare the file
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name or contents against the administrator's configuration and set the
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content type, language, character set and request handler. Some modules
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may set up their filters or other request handling parameters at this
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time.</p>
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<p>If all modules <code>DECLINE</code> this phase, an error 500 is
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returned to the browser, and a "couldn't find types" error is logged
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automatically.</p>
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</section>
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<section id="fixups"><title>Hook: fixups</title>
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<p>Many modules are 'trounced' by some phase above. The fixups
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phase is used by modules to 'reassert' their ownership or force
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the request's fields to their appropriate values. It isn't
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always the cleanest mechanism, but occasionally it's the only
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option.</p>
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</section>
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</section>
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<section id="handler"><title>The Handler Phase</title>
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<p>This phase is <strong>not</strong> part of the processing in
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<code>ap_process_request_internal()</code>. Many
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modules prepare one or more subrequests prior to creating any
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content at all. After the core, or a module calls
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<code>ap_process_request_internal()</code> it then calls
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<code>ap_invoke_handler()</code> to generate the request.</p>
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<section id="insert_filter"><title>Hook: insert_filter</title>
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<p>Modules that transform the content in some way can insert
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their values and override existing filters, such that if the
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user configured a more advanced filter out-of-order, then the
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module can move its order as need be. There is no result code,
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so actions in this hook better be trusted to always succeed.</p>
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</section>
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<section id="hook_handler"><title>Hook: handler</title>
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<p>The module finally has a chance to serve the request in its
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handler hook. Note that not every prepared request is sent to
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the handler hook. Many modules, such as <module>mod_autoindex</module>,
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will create subrequests for a given URI, and then never serve the
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subrequest, but simply lists it for the user. Remember not to
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put required teardown from the hooks above into this module,
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but register pool cleanups against the request pool to free
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resources as required.</p>
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</section>
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</section>
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</manualpage>
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