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+<title>Apache mod_rewrite Technical Details - Apache HTTP Server</title>
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+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.0</p>
+<img alt="" src="../images/feather.gif" /></div>
+<div class="up"><a href="./index.html"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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+<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.0</a></div><div id="page-content"><div id="preamble"><h1>Apache mod_rewrite Technical Details</h1>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/rewrite/rewrite_tech.html" title="English"> en </a></p>
+</div>
+
+<p>This document discusses some of the technical details of mod_rewrite
+and URL matching.</p>
+</div>
+<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#Internal">Internal Processing</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#InternalAPI">API Phases</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li>
+</ul><h3>See also</h3><ul class="seealso"><li><a href="../mod/mod_rewrite.html">Module
+documentation</a></li><li><a href="rewrite_intro.html">mod_rewrite
+introduction</a></li><li><a href="rewrite_guide.html">Practical solutions to common
+problems</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="Internal" id="Internal">Internal Processing</a></h2>
+
+ <p>The internal processing of this module is very complex but
+ needs to be explained once even to the average user to avoid
+ common mistakes and to let you exploit its full
+ functionality.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="InternalAPI" id="InternalAPI">API Phases</a></h2>
+
+ <p>First you have to understand that when Apache processes a
+ HTTP request it does this in phases. A hook for each of these
+ phases is provided by the Apache API. Mod_rewrite uses two of
+ these hooks: the URL-to-filename translation hook which is
+ used after the HTTP request has been read but before any
+ authorization starts and the Fixup hook which is triggered
+ after the authorization phases and after the per-directory
+ config files (<code>.htaccess</code>) have been read, but
+ before the content handler is activated.</p>
+
+ <p>So, after a request comes in and Apache has determined the
+ corresponding server (or virtual server) the rewriting engine
+ starts processing of all mod_rewrite directives from the
+ per-server configuration in the URL-to-filename phase. A few
+ steps later when the final data directories are found, the
+ per-directory configuration directives of mod_rewrite are
+ triggered in the Fixup phase. In both situations mod_rewrite
+ rewrites URLs either to new URLs or to filenames, although
+ there is no obvious distinction between them. This is a usage
+ of the API which was not intended to be this way when the API
+ was designed, but as of Apache 1.x this is the only way
+ mod_rewrite can operate. To make this point more clear
+ remember the following two points:</p>
+
+ <ol>
+ <li>Although mod_rewrite rewrites URLs to URLs, URLs to
+ filenames and even filenames to filenames, the API
+ currently provides only a URL-to-filename hook. In Apache
+ 2.0 the two missing hooks will be added to make the
+ processing more clear. But this point has no drawbacks for
+ the user, it is just a fact which should be remembered:
+ Apache does more in the URL-to-filename hook than the API
+ intends for it.</li>
+
+ <li>
+ Unbelievably mod_rewrite provides URL manipulations in
+ per-directory context, <em>i.e.</em>, within
+ <code>.htaccess</code> files, although these are reached
+ a very long time after the URLs have been translated to
+ filenames. It has to be this way because
+ <code>.htaccess</code> files live in the filesystem, so
+ processing has already reached this stage. In other
+ words: According to the API phases at this time it is too
+ late for any URL manipulations. To overcome this chicken
+ and egg problem mod_rewrite uses a trick: When you
+ manipulate a URL/filename in per-directory context
+ mod_rewrite first rewrites the filename back to its
+ corresponding URL (which is usually impossible, but see
+ the <code>RewriteBase</code> directive below for the
+ trick to achieve this) and then initiates a new internal
+ sub-request with the new URL. This restarts processing of
+ the API phases.
+
+ <p>Again mod_rewrite tries hard to make this complicated
+ step totally transparent to the user, but you should
+ remember here: While URL manipulations in per-server
+ context are really fast and efficient, per-directory
+ rewrites are slow and inefficient due to this chicken and
+ egg problem. But on the other hand this is the only way
+ mod_rewrite can provide (locally restricted) URL
+ manipulations to the average user.</p>
+ </li>
+ </ol>
+
+ <p>Don't forget these two points!</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a></h2>
+
+ <p>Now when mod_rewrite is triggered in these two API phases, it
+ reads the configured rulesets from its configuration
+ structure (which itself was either created on startup for
+ per-server context or during the directory walk of the Apache
+ kernel for per-directory context). Then the URL rewriting
+ engine is started with the contained ruleset (one or more
+ rules together with their conditions). The operation of the
+ URL rewriting engine itself is exactly the same for both
+ configuration contexts. Only the final result processing is
+ different. </p>
+
+ <p>The order of rules in the ruleset is important because the
+ rewriting engine processes them in a special (and not very
+ obvious) order. The rule is this: The rewriting engine loops
+ through the ruleset rule by rule (<code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives) and
+ when a particular rule matches it optionally loops through
+ existing corresponding conditions (<code>RewriteCond</code>
+ directives). For historical reasons the conditions are given
+ first, and so the control flow is a little bit long-winded. See
+ Figure 1 for more details.</p>
+<p class="figure">
+ <img src="../images/mod_rewrite_fig1.gif" width="428" height="385" alt="[Needs graphics capability to display]" /><br />
+ <dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
+</p>
+ <p>As you can see, first the URL is matched against the
+ <em>Pattern</em> of each rule. When it fails mod_rewrite
+ immediately stops processing this rule and continues with the
+ next rule. If the <em>Pattern</em> matches, mod_rewrite looks
+ for corresponding rule conditions. If none are present, it
+ just substitutes the URL with a new value which is
+ constructed from the string <em>Substitution</em> and goes on
+ with its rule-looping. But if conditions exist, it starts an
+ inner loop for processing them in the order that they are
+ listed. For conditions the logic is different: we don't match
+ a pattern against the current URL. Instead we first create a
+ string <em>TestString</em> by expanding variables,
+ back-references, map lookups, <em>etc.</em> and then we try
+ to match <em>CondPattern</em> against it. If the pattern
+ doesn't match, the complete set of conditions and the
+ corresponding rule fails. If the pattern matches, then the
+ next condition is processed until no more conditions are
+ available. If all conditions match, processing is continued
+ with the substitution of the URL with
+ <em>Substitution</em>.</p>
+
+</div></div>
+<div class="bottomlang">
+<p><span>Available Languages: </span><a href="../en/rewrite/rewrite_tech.html" title="English"> en </a></p>
+</div><div id="footer">
+<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
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