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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><!--%hypertext --><!-- mod_rewrite.html --><!-- Documentation for the mod_rewrite Apache module --><html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta name="generator" content="HTML Tidy, see www.w3.org" /> <title>Apache module mod_rewrite</title> </head> <!-- Background white, links blue (unvisited), navy (visited), red (active) --> <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#000080" alink="#FF0000"> <blockquote> <!-- page indentation --> <div align="CENTER"> <img src="../images/sub.gif" alt="[APACHE DOCUMENTATION]" /> <h3>Apache HTTP Server Version 1.3</h3> <p><small><em>Is this the version you want? For more recent versions, check our <a href="/docs/">documentation index</a>.</em></small></p> </div> <br /> <h1 align="CENTER">Module mod_rewrite<br /> URL Rewriting Engine</h1> <p>This module provides a rule-based rewriting engine to rewrite requested URLs on the fly.</p> <p><a href="module-dict.html#Status" rel="Help"><strong>Status:</strong></a> Extension<br /> <a href="module-dict.html#SourceFile" rel="Help"><strong>Source File:</strong></a> mod_rewrite.c<br /> <a href="module-dict.html#ModuleIdentifier" rel="Help"><strong>Module Identifier:</strong></a> rewrite_module<br /> <a href="module-dict.html#Compatibility" rel="Help"><strong>Compatibility:</strong></a> Available in Apache 1.2 and later.</p> <hr noshade="noshade" size="1" /> <br /> <h2>Summary</h2> <blockquote> <blockquote> <blockquote> <em>``The great thing about mod_rewrite is it gives you all the configurability and flexibility of Sendmail. The downside to mod_rewrite is that it gives you all the configurability and flexibility of Sendmail.''</em> <div align="RIGHT"> -- Brian Behlendorf<br /> Apache Group </div> </blockquote> </blockquote> </blockquote> <blockquote> <blockquote> <blockquote> <em>`` Despite the tons of examples and docs, mod_rewrite is voodoo. Damned cool voodoo, but still voodoo. ''</em> <div align="RIGHT"> -- Brian Moore<br /> bem@news.cmc.net </div> </blockquote> </blockquote> </blockquote> Welcome to mod_rewrite, the Swiss Army Knife of URL manipulation! <p>This module uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly. It supports an unlimited number of rules and an unlimited number of attached rule conditions for each rule to provide a really flexible and powerful URL manipulation mechanism. The URL manipulations can depend on various tests, for instance server variables, environment variables, HTTP headers, time stamps and even external database lookups in various formats can be used to achieve a really granular URL matching.</p> <p>This module operates on the full URLs (including the path-info part) both in per-server context (<code>httpd.conf</code>) and per-directory context (<code>.htaccess</code>) and can even generate query-string parts on result. The rewritten result can lead to internal sub-processing, external request redirection or even to an internal proxy throughput.</p> <p>But all this functionality and flexibility has its drawback: complexity. So don't expect to understand this entire module in just one day.</p> <p>This module was invented and originally written in April 1996<br /> and gifted exclusively to the The Apache Group in July 1997 by</p> <blockquote> <a href="http://www.engelschall.com/"><code>Ralf S. Engelschall</code></a><br /> <a href="mailto:rse@engelschall.com"><code>rse@engelschall.com</code></a><br /> <a href="http://www.engelschall.com/"><code>www.engelschall.com</code></a> </blockquote> <hr noshade="noshade" size="1" /> <h2>Table Of Contents</h2> <p><strong>Internal Processing</strong></p> <ul> <li><a href="#InternalAPI">API Phases</a></li> <li><a href="#InternalRuleset">Ruleset Processing</a></li> <li><a href="#InternalBackRefs">Regex Back-Reference Availability</a></li> </ul> <p><strong>Configuration Directives</strong></p> <ul> <li><a href="#RewriteEngine">RewriteEngine</a></li> <li><a href="#RewriteOptions">RewriteOptions</a></li> <li><a href="#RewriteLog">RewriteLog</a></li> <li><a href="#RewriteLogLevel">RewriteLogLevel</a></li> <li><a href="#RewriteLock">RewriteLock</a></li> <li><a href="#RewriteMap">RewriteMap</a></li> <li><a href="#RewriteBase">RewriteBase</a></li> <li><a href="#RewriteCond">RewriteCond</a></li> <li><a href="#RewriteRule">RewriteRule</a></li> </ul> <strong>Miscellaneous</strong> <ul> <li><a href="#EnvVar">Environment Variables</a></li> <li><a href="#Solutions">Practical Solutions</a></li> </ul> <hr noshade="noshade" size="1" /> <center> <h1><a id="Internal" name="Internal">Internal Processing</a></h1> </center> <hr noshade="noshade" size="1" /> <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> <h2><a id="InternalAPI" name="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> <h2><a id="InternalRuleset" name="InternalRuleset">Ruleset Processing</a></h2> 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>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>RewriteRule</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> <div align="CENTER"> <table cellspacing="0" cellpadding="2" border="0"> <tr> <td bgcolor="#CCCCCC"><img src="../images/mod_rewrite_fig1.gif" width="428" height="385" alt="[Needs graphics capability to display]" /></td> </tr> <tr> <td align="CENTER"><strong>Figure 1:</strong> The control flow through the rewriting ruleset</td> </tr> </table> </div> <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> <h2><a id="quoting" name="quoting">Quoting Special Characters</a></h2> <p>As of Apache 1.3.20, special characters in <i>TestString</i> and <i>Substitution</i> strings can be escaped (that is, treated as normal characters without their usual special meaning) by prefixing them with a slosh ('\') character. In other words, you can include an actual dollar-sign character in a <i>Substitution</i> string by using '<code>\$</code>'; this keeps mod_rewrite from trying to treat it as a backreference.</p> <h2><a id="InternalBackRefs" name="InternalBackRefs">Regex Back-Reference Availability</a></h2> One important thing here has to be remembered: Whenever you use parentheses in <em>Pattern</em> or in one of the <em>CondPattern</em>, back-references are internally created which can be used with the strings <code>$N</code> and <code>%N</code> (see below). 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