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233 lines
9.5 KiB
HTML
233 lines
9.5 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Request Processing in Apache 2.0</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1>Request Processing in Apache 2.0</h1>
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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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<p>Several changes in Apache 2.0 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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Apache 1.3 to attempt to optimize subrequest or redirect
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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_internal_request()</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 Apache 1.3) without introducing fragile or
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insecure code.</p>
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<h2>The Request Processing Cycle</h2>
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<p>All requests pass through
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<code>ap_process_request_internal()</code> in request.c,
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including subrequests and redirects. If a module doesn't pass
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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 Apache hooks which are irrelevant (and costly in
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terms of CPU.)</p>
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<h2>The Request Parsing Phase</h2>
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<h3>Unescapes the URL</h3>
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<p>The request's parsed_uri 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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parsed_uri.path 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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reprecussions.</p>
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<h3>Strips Parent and This Elements from the URI</h3>
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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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<h3>Initial URI Location Walk</h3>
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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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<Location > sections are consistently enforced for all
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requests. If the request is an internal redirect or a
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sub-request, it may borrow some or all of the processing from
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the previous or parent request's ap_location_walk, so this step
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is generally very efficient after processing the main
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request.</p>
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<h3>Hook: translate_name</h3>
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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, mod_vhost_alias will translate the
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URI's path into the configured virtual host, mod_alias will
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translate the path to an alias path, and if the request falls
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back on the core, the DocumentRoot is prepended to the request
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resource.</p>
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<p>If all modules DECLINE this phase, an error 500 is returned
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to the browser, and a "couldn't translate name" error is logged
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automatically.</p>
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<h3>Hook: map_to_storage</h3>
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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, mod_proxy compares and merges the appropriate
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<Proxy > sections. If the URI is nothing more than a
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local (non-proxy) TRACE request, the core handles the request
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and returns DONE. If no module answers this hook with OK or
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DONE, the core will run the request filename against the
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<Directory > and <Files > 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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<h3>Initial URI Location Walk</h3>
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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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<Location > sections. The request again borrows some or
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all of the processing from its previous location_walk above,
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so this step is almost always very efficient unless the
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translated URI mapped to a substantially different path or
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Virtual Host.</p>
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<h3>Hook: header_parser</h3>
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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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<h2>The Security Phase</h2>
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<p>Needs Documentation. Code is;</p>
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<pre>
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switch (ap_satisfies(r)) {
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case SATISFY_ALL:
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case SATISFY_NOSPEC:
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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 (ap_some_auth_required(r)) {
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if (((access_status = ap_run_check_user_id(r)) != 0) || !ap_auth_type(r)) {
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return decl_die(access_status, ap_auth_type(r)
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? "check user. No user file?"
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: "perform authentication. AuthType not set!", r);
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}
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if (((access_status = ap_run_auth_checker(r)) != 0) || !ap_auth_type(r)) {
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return decl_die(access_status, ap_auth_type(r)
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? "check access. No groups file?"
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: "perform authentication. AuthType not set!", r);
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}
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}
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break;
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case SATISFY_ANY:
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if (((access_status = ap_run_access_checker(r)) != 0) || !ap_auth_type(r)) {
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if (!ap_some_auth_required(r)) {
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return decl_die(access_status, ap_auth_type(r)
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? "check access"
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: "perform authentication. AuthType not set!", r);
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}
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if (((access_status = ap_run_check_user_id(r)) != 0) || !ap_auth_type(r)) {
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return decl_die(access_status, ap_auth_type(r)
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? "check user. No user file?"
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: "perform authentication. AuthType not set!", r);
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}
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if (((access_status = ap_run_auth_checker(r)) != 0) || !ap_auth_type(r)) {
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return decl_die(access_status, ap_auth_type(r)
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? "check access. No groups file?"
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: "perform authentication. AuthType not set!", r);
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}
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}
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break;
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}
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</pre>
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<h2>The Preparation Phase</h2>
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<h3>Hook: type_checker</h3>
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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 mod_mime and mod_mime_magic use this phase to
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compare the file name or contents against the administrator's
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configuration and set the content type, language, character set
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and request handler. Some modules may set up their filters or
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other request handling parameters at this time.</p>
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<p>If all modules DECLINE this phase, an error 500 is returned
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to the browser, and a "couldn't find types" error is logged
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automatically.</p>
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<h3>Hook: fixups</h3>
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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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<h2>The Handler Phase</h2>
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<p>This phase is <strong><em>not</em></strong> part of the
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processing in <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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<h3>Hook: insert_filter</h3>
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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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<h3>Hook: handler</h3>
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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 mod_autoindex, will
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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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<!--#include virtual="footer.html" -->
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</body>
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</html>
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