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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<title>Apache mod_rewrite Technical Details - Apache HTTP Server Version 2.5</title>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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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 2.5</a> > <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Apache mod_rewrite Technical Details</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="../en/rewrite/tech.html" title="English"> en </a> |
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<a href="../fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
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</div>
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<p>This document discusses some of the technical details of mod_rewrite
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and URL matching.</p>
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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#InternalAPI">API Phases</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li>
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</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><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="InternalAPI" id="InternalAPI">API Phases</a> <a title="Permanent link" href="#InternalAPI" class="permalink">¶</a></h2>
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<p>The Apache HTTP Server handles requests in several phases. At
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each of these phases, one or more modules may be called upon to
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handle that portion of the request lifecycle. Phases include things
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like URL-to-filename translation, authentication, authorization,
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content, and logging. (This is not an exhaustive list.)</p>
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<p>mod_rewrite acts in two of these phases (or "hooks", as they are
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often called) to influence how URLs may be rewritten.</p>
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<p>First, it uses the URL-to-filename translation hook, which occurs
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after the HTTP request has been read, but before any authorization
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starts. Secondly, it uses the Fixup hook, which is after the
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authorization phases, and after per-directory configuration files
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(<code>.htaccess</code> files) have been read, but before the
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content handler is called.</p>
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<p>After a request comes in and a corresponding server or
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virtual host has been determined, the rewriting engine starts
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processing any <code>mod_rewrite</code> directives appearing in the
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per-server configuration. (i.e., in the main server configuration file
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and <code class="directive"><a href="../mod/core.html#virtualhost"><Virtualhost></a></code>
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sections.) This happens in the URL-to-filename phase.</p>
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<p>A few steps later, once the final data directories have been found,
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the per-directory configuration directives (<code>.htaccess</code>
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files and <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> blocks) are applied. This
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happens in the Fixup phase.</p>
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<p>In each of these cases, mod_rewrite rewrites the
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<code>REQUEST_URI</code> either to a new URL, or to a filename.</p>
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<p>In per-directory context (i.e., within <code>.htaccess</code> files
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and <code>Directory</code> blocks), these rules are being applied
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after a URL has already been translated to a filename. Because of
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this, the URL-path that mod_rewrite initially compares <code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives against
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is the full filesystem path to the translated filename with the current
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directories path (including a trailing slash) removed from the front.</p>
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<p> To illustrate: If rules are in /var/www/foo/.htaccess and a request
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for /foo/bar/baz is being processed, an expression like ^bar/baz$ would
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match.</p>
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<p> If a substitution is made in per-directory context, a new internal
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subrequest is issued with the new URL, which restarts processing of the
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request phases. If the substitution is a relative path, the <code class="directive"><a href="../mod/mod_rewrite.html#rewritebase">RewriteBase</a></code> directive
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determines the URL-path prefix prepended to the substitution.
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In per-directory context, care must be taken to
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create rules which will eventually (in some future "round" of per-directory
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rewrite processing) not perform a substitution to avoid looping.
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(See <a href="http://wiki.apache.org/httpd/RewriteLooping">RewriteLooping</a>
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for further discussion of this problem.)</p>
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<p>Because of this further manipulation of the URL in per-directory
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context, you'll need to take care to craft your rewrite rules
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differently in that context. In particular, remember that the
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leading directory path will be stripped off of the URL that your
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rewrite rules will see. Consider the examples below for further
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clarification.</p>
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<table class="bordered">
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<tr>
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<th>Location of rule</th>
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<th>Rule</th>
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</tr>
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<tr>
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<td>VirtualHost section</td>
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<td>RewriteRule "^/images/(.+)\.jpg" "/images/$1.gif"</td>
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</tr>
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<tr>
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<td>.htaccess file in document root</td>
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<td>RewriteRule "^images/(.+)\.jpg" "images/$1.gif"</td>
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</tr>
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<tr>
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<td>.htaccess file in images directory</td>
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<td>RewriteRule "^(.+)\.jpg" "$1.gif"</td>
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</tr>
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</table>
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<p>For even more insight into how mod_rewrite manipulates URLs in
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different contexts, you should consult the <a href="../mod/mod_rewrite.html#logging">log entries</a> made during
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rewriting.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a> <a title="Permanent link" href="#InternalRuleset" class="permalink">¶</a></h2>
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<p>Now when mod_rewrite is triggered in these two API phases, it
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reads the configured rulesets from its configuration
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structure (which itself was either created on startup for
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per-server context or during the directory walk of the Apache
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kernel for per-directory context). Then the URL rewriting
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engine is started with the contained ruleset (one or more
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rules together with their conditions). The operation of the
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URL rewriting engine itself is exactly the same for both
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configuration contexts. Only the final result processing is
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different.</p>
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<p>The order of rules in the ruleset is important because the
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rewriting engine processes them in a special (and not very
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obvious) order. The rule is this: The rewriting engine loops
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through the ruleset rule by rule (<code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives) and
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when a particular rule matches it optionally loops through
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existing corresponding conditions (<code>RewriteCond</code>
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directives). For historical reasons the conditions are given
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first, and so the control flow is a little bit long-winded. See
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Figure 1 for more details.</p>
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<p class="figure">
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<img src="../images/rewrite_process_uri.png" alt="Flow of RewriteRule and RewriteCond matching" /><br />
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<dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
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</p>
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<p>First the URL is matched against the
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<em>Pattern</em> of each rule. If it fails, mod_rewrite
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immediately stops processing this rule, and continues with the
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next rule. If the <em>Pattern</em> matches, mod_rewrite looks
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for corresponding rule conditions (RewriteCond directives,
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appearing immediately above the RewriteRule in the configuration).
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If none are present, it substitutes the URL with a new value, which is
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constructed from the string <em>Substitution</em>, and goes on
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with its rule-looping. But if conditions exist, it starts an
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inner loop for processing them in the order that they are
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listed. For conditions, the logic is different: we don't match
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a pattern against the current URL. Instead we first create a
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string <em>TestString</em> by expanding variables,
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back-references, map lookups, <em>etc.</em> and then we try
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to match <em>CondPattern</em> against it. If the pattern
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doesn't match, the complete set of conditions and the
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corresponding rule fails. If the pattern matches, then the
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next condition is processed until no more conditions are
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available. If all conditions match, processing is continued
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with the substitution of the URL with
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<em>Substitution</em>.</p>
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</div></div>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/rewrite/tech.html" title="English"> en </a> |
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<a href="../fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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