1<?xml version="1.0" encoding="ISO-8859-1"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- 4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 5 This file is generated from xml source: DO NOT EDIT 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 --> 8<title>Apache mod_rewrite Technical Details - Apache HTTP Server</title> 9<link href="/style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 10<link href="/style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 11<link href="/style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="/style/css/prettify.css" /> 12<script src="/style/scripts/prettify.js" type="text/javascript"> 13</script> 14 15<link href="/images/favicon.ico" rel="shortcut icon" /></head> 16<body id="manual-page"><div id="page-header"> 17<p class="menu"><a href="/mod/">Modules</a> | <a href="/mod/directives.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> 18<p class="apache">Apache HTTP Server Version 2.2</p> 19<img alt="" src="/images/feather.gif" /></div> 20<div class="up"><a href="./"><img title="<-" alt="<-" src="/images/left.gif" /></a></div> 21<div id="path"> 22<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 2.2</a> > <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Apache mod_rewrite Technical Details</h1> 23<div class="toplang"> 24<p><span>Available Languages: </span><a href="/en/rewrite/tech.html" title="English"> en </a> | 25<a href="/fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a></p> 26</div> 27 28<p>This document discusses some of the technical details of mod_rewrite 29and URL matching.</p> 30</div> 31<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#Internal">Internal Processing</a></li> 32<li><img alt="" src="/images/down.gif" /> <a href="#InternalAPI">API Phases</a></li> 33<li><img alt="" src="/images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li> 34</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 and tricks</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li></ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 35<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 36<div class="section"> 37<h2><a name="Internal" id="Internal">Internal Processing</a></h2> 38 39 <p>The internal processing of this module is very complex but 40 needs to be explained once even to the average user to avoid 41 common mistakes and to let you exploit its full 42 functionality.</p> 43</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 44<div class="section"> 45<h2><a name="InternalAPI" id="InternalAPI">API Phases</a></h2> 46 47 <p>First you have to understand that when Apache processes a 48 HTTP request it does this in phases. A hook for each of these 49 phases is provided by the Apache API. Mod_rewrite uses two of 50 these hooks: the URL-to-filename translation hook which is 51 used after the HTTP request has been read but before any 52 authorization starts and the Fixup hook which is triggered 53 after the authorization phases and after the per-directory 54 config files (<code>.htaccess</code>) have been read, but 55 before the content handler is activated.</p> 56 57 <p>So, after a request comes in and Apache has determined the 58 corresponding server (or virtual server) the rewriting engine 59 starts processing of all mod_rewrite directives from the 60 per-server configuration in the URL-to-filename phase. A few 61 steps later when the final data directories are found, the 62 per-directory configuration directives of mod_rewrite are 63 triggered in the Fixup phase. In both situations mod_rewrite 64 rewrites URLs either to new URLs or to filenames, although 65 there is no obvious distinction between them. This is a usage 66 of the API which was not intended to be this way when the API 67 was designed, but as of Apache 1.x this is the only way 68 mod_rewrite can operate. To make this point more clear 69 remember the following two points:</p> 70 71 <ol> 72 <li>Although mod_rewrite rewrites URLs to URLs, URLs to 73 filenames and even filenames to filenames, the API 74 currently provides only a URL-to-filename hook. In Apache 75 2.0 the two missing hooks will be added to make the 76 processing more clear. But this point has no drawbacks for 77 the user, it is just a fact which should be remembered: 78 Apache does more in the URL-to-filename hook than the API 79 intends for it.</li> 80 81 <li> 82 Unbelievably mod_rewrite provides URL manipulations in 83 per-directory context, <em>i.e.</em>, within 84 <code>.htaccess</code> files, although these are reached 85 a very long time after the URLs have been translated to 86 filenames. It has to be this way because 87 <code>.htaccess</code> files live in the filesystem, so 88 processing has already reached this stage. In other 89 words: According to the API phases at this time it is too 90 late for any URL manipulations. To overcome this chicken 91 and egg problem mod_rewrite uses a trick: When you 92 manipulate a URL/filename in per-directory context 93 mod_rewrite first rewrites the filename back to its 94 corresponding URL (which is usually impossible, but see 95 the <code>RewriteBase</code> directive below for the 96 trick to achieve this) and then initiates a new internal 97 sub-request with the new URL. This restarts processing of 98 the API phases. 99 100 <p>Again mod_rewrite tries hard to make this complicated 101 step totally transparent to the user, but you should 102 remember here: While URL manipulations in per-server 103 context are really fast and efficient, per-directory 104 rewrites are slow and inefficient due to this chicken and 105 egg problem. But on the other hand this is the only way 106 mod_rewrite can provide (locally restricted) URL 107 manipulations to the average user.</p> 108 </li> 109 </ol> 110 111 <p>Don't forget these two points!</p> 112</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 113<div class="section"> 114<h2><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a></h2> 115 116 <p>Now when mod_rewrite is triggered in these two API phases, it 117 reads the configured rulesets from its configuration 118 structure (which itself was either created on startup for 119 per-server context or during the directory walk of the Apache 120 kernel for per-directory context). Then the URL rewriting 121 engine is started with the contained ruleset (one or more 122 rules together with their conditions). The operation of the 123 URL rewriting engine itself is exactly the same for both 124 configuration contexts. Only the final result processing is 125 different. </p> 126 127 <p>The order of rules in the ruleset is important because the 128 rewriting engine processes them in a special (and not very 129 obvious) order. The rule is this: The rewriting engine loops 130 through the ruleset rule by rule (<code class="directive"><a href="/mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives) and 131 when a particular rule matches it optionally loops through 132 existing corresponding conditions (<code>RewriteCond</code> 133 directives). For historical reasons the conditions are given 134 first, and so the control flow is a little bit long-winded. See 135 Figure 1 for more details.</p> 136<p class="figure"> 137 <img src="/images/rewrite_rule_flow.png" alt="Flow of RewriteRule and RewriteCond matching" /><br /> 138 <dfn>Figure 1:</dfn>The control flow through the rewriting ruleset 139</p> 140 <p>As you can see, first the URL is matched against the 141 <em>Pattern</em> of each rule. When it fails mod_rewrite 142 immediately stops processing this rule and continues with the 143 next rule. If the <em>Pattern</em> matches, mod_rewrite looks 144 for corresponding rule conditions. If none are present, it 145 just substitutes the URL with a new value which is 146 constructed from the string <em>Substitution</em> and goes on 147 with its rule-looping. But if conditions exist, it starts an 148 inner loop for processing them in the order that they are 149 listed. For conditions the logic is different: we don't match 150 a pattern against the current URL. Instead we first create a 151 string <em>TestString</em> by expanding variables, 152 back-references, map lookups, <em>etc.</em> and then we try 153 to match <em>CondPattern</em> against it. If the pattern 154 doesn't match, the complete set of conditions and the 155 corresponding rule fails. If the pattern matches, then the 156 next condition is processed until no more conditions are 157 available. If all conditions match, processing is continued 158 with the substitution of the URL with 159 <em>Substitution</em>.</p> 160 161</div></div> 162<div class="bottomlang"> 163<p><span>Available Languages: </span><a href="/en/rewrite/tech.html" title="English"> en </a> | 164<a href="/fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a></p> 165</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 again by our moderators if they are either implemented or considered invalid/off-topic. 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