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5123f066275e7d29f5779cb6c9303659184ec0acwrowe<title>Apache mod_rewrite Technical Details - Apache HTTP Server</title>
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5123f066275e7d29f5779cb6c9303659184ec0acwrowe<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>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<p class="apache">Apache HTTP Server Version 2.3</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<img alt="" src="/images/feather.gif" /></div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="/images/left.gif" /></a></div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div id="path">
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Apache mod_rewrite Technical Details</h1>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div class="toplang">
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<p><span>Available Languages: </span><a href="/en/rewrite/tech.html" title="English">&nbsp;en&nbsp;</a> |
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<a href="/fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Fran�ais">&nbsp;fr&nbsp;</a></p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe</div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<p>This document discusses some of the technical details of mod_rewrite
5123f066275e7d29f5779cb6c9303659184ec0acwroweand URL matching.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe</div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#InternalAPI">API Phases</a></li>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<li><img alt="" src="/images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe</ul><h3>See also</h3><ul class="seealso"><li><a href="/mod/mod_rewrite.html">Module documentation</a></li><li><a href="intro.html">mod_rewrite introduction</a></li><li><a href="remapping.html">Redirection and remapping</a></li><li><a href="access.html">Controlling access</a></li><li><a href="vhosts.html">Virtual hosts</a></li><li><a href="proxy.html">Proxying</a></li><li><a href="rewritemap.html">Using RewriteMap</a></li><li><a href="advanced.html">Advanced techniques</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul></div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<div class="section">
5123f066275e7d29f5779cb6c9303659184ec0acwrowe<h2><a name="InternalAPI" id="InternalAPI">API Phases</a></h2>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>The Apache HTTP Server handles requests in several phases. At
5123f066275e7d29f5779cb6c9303659184ec0acwrowe each of these phases, one or more modules may be called upon to
5123f066275e7d29f5779cb6c9303659184ec0acwrowe handle that portion of the request lifecycle. Phases include things
5123f066275e7d29f5779cb6c9303659184ec0acwrowe like URL-to-filename translation, authentication, authorization,
5123f066275e7d29f5779cb6c9303659184ec0acwrowe content, and logging. (This is not an exhaustive list.)</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>mod_rewrite acts in two of these phases (or "hooks", as they are
5123f066275e7d29f5779cb6c9303659184ec0acwrowe often called) to influence how URLs may be rewritten.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>First, it uses the URL-to-filename translation hook, which occurs
5123f066275e7d29f5779cb6c9303659184ec0acwrowe after the HTTP request has been read, but before any authorization
5123f066275e7d29f5779cb6c9303659184ec0acwrowe starts. Secondly, it uses the Fixup hook, which is after the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe authorization phases, and after per-directory configuration files
5123f066275e7d29f5779cb6c9303659184ec0acwrowe (<code>.htaccess</code> files) have been read, but before the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe content handler is called.</p>
37ad54b8fd2611b7a4f2b269eec3d27ed784a25dwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>So, after a request comes in and a corresponding server or
5123f066275e7d29f5779cb6c9303659184ec0acwrowe virtual host has been determined, the rewriting engine starts
5123f066275e7d29f5779cb6c9303659184ec0acwrowe processing any <code>mod_rewrite</code> directives appearing in the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe per-server configuration. (i.e., in the main server configuration file
5ac28f3fe2417368757f29cf381338357605fd52wrowe and <code class="directive"><a href="/mod/core.html#virtualhost">&lt;Virtualhost&gt;</a></code>
5ac28f3fe2417368757f29cf381338357605fd52wrowe sections.) This happens in the URL-to-filename phase.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>A few steps later, once the final data directories have been found,
5123f066275e7d29f5779cb6c9303659184ec0acwrowe the per-directory configuration directives (<code>.htaccess</code>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe files and <code class="directive"><a href="/mod/core.html#directory">&lt;Directory&gt;</a></code> blocks) are applied. This
5123f066275e7d29f5779cb6c9303659184ec0acwrowe happens in the Fixup phase.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>In each of these cases, mod_rewrite rewrites the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <code>REQUEST_URI</code> either to a new URL, or to a filename.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <p>In per-directory context (i.e., within <code>.htaccess</code> files
5123f066275e7d29f5779cb6c9303659184ec0acwrowe and <code>Directory</code> blocks), these rules are being applied
5123f066275e7d29f5779cb6c9303659184ec0acwrowe after a URL has already been translated to a filename. Because of
5123f066275e7d29f5779cb6c9303659184ec0acwrowe this, mod_rewrite temporarily translates the filename back into a URL,
5123f066275e7d29f5779cb6c9303659184ec0acwrowe by stripping off the directory path (including a trailing slash)
5ac28f3fe2417368757f29cf381338357605fd52wrowe before applying the rules. ) If a substitution is made, a new internal
e1ad80c048e29e968221817698529d73098f07a4wrowe subrequest is issued with the new URL, which restarts processing of the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe request phases. If the substution is a relative path, the <code class="directive"><a href="/mod/mod_rewrite.html#rewritebase">RewriteBase</a></code> directive
5123f066275e7d29f5779cb6c9303659184ec0acwrowe determines the URL-path prefix appended to the substitution.
5123f066275e7d29f5779cb6c9303659184ec0acwrowe In per-directory context, care must be taken to
37ad54b8fd2611b7a4f2b269eec3d27ed784a25dwrowe create rules which will eventually (in some future "round" of per-directory
5123f066275e7d29f5779cb6c9303659184ec0acwrowe rewrite processing) not perform a substitution to avoid looping.
5123f066275e7d29f5779cb6c9303659184ec0acwrowe (See <a href="http://wiki.apache.org/httpd/RewriteLooping">RewriteLooping</a>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe for further discussion of this problem.)</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5ac28f3fe2417368757f29cf381338357605fd52wrowe <p>Because of this further manipulation of the URL in per-directory
5ac28f3fe2417368757f29cf381338357605fd52wrowe context, you'll need to take care to craft your rewrite rules
5123f066275e7d29f5779cb6c9303659184ec0acwrowe differently in that context. In particular, remember that the
5123f066275e7d29f5779cb6c9303659184ec0acwrowe leading directory path will be stripped off of the URL that your
5123f066275e7d29f5779cb6c9303659184ec0acwrowe rewrite rules will see. Consider the examples below for further
5123f066275e7d29f5779cb6c9303659184ec0acwrowe clarification.</p>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <table class="bordered">
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <tr>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <th>Location of rule</th>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <th>Rule</th>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe </tr>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <tr>
8c8173f49dd7122e10636b3d20ae841551bd0b43wrowe <td>VirtualHost section</td>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe <td>RewriteRule ^/images/(.+)\.jpg /images/$1.gif</td>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe </tr>
5123f066275e7d29f5779cb6c9303659184ec0acwrowe
<tr>
<td>.htaccess file in document root</td>
<td>RewriteRule ^images/(.+)\.jpg images/$1.gif</td>
</tr>
<tr>
<td>.htaccess file in images directory</td>
<td>RewriteRule ^(.+)\.jpg $1.gif</td>
</tr>
</table>
<p>For even more insight into how mod_rewrite manipulates URLs in
different contexts, you should consult the <a href="/mod/mod_rewrite.html#logging">log entries</a> made during
rewriting.</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/rewrite_rule_flow.png" alt="Flow of RewriteRule and RewriteCond matching" /><br />
<dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
</p>
<p>First the URL is matched against the
<em>Pattern</em> of each rule. If 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 (RewriteCond directives,
appearing immediately above the RewriteRule in the configuration).
If none are present, it 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/tech.html" title="English">&nbsp;en&nbsp;</a> |
<a href="/fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Fran�ais">&nbsp;fr&nbsp;</a></p>
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<p class="apache">Copyright 2011 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>
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