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10 <title>Apache mod_rewrite Technical Details - Apache HTTP Server Version 2.5</title>
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20 <p class="apache">Apache HTTP Server Version 2.5</p>
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24 <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>
26 <p><span>Available Languages: </span><a href="../en/rewrite/tech.html" title="English"> en </a> |
27 <a href="../fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
30 <p>This document discusses some of the technical details of mod_rewrite
33 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#InternalAPI">API Phases</a></li>
34 <li><img alt="" src="../images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li>
35 </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>
36 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
38 <h2><a name="InternalAPI" id="InternalAPI">API Phases</a> <a title="Permanent link" href="#InternalAPI" class="permalink">¶</a></h2>
40 <p>The Apache HTTP Server handles requests in several phases. At
41 each of these phases, one or more modules may be called upon to
42 handle that portion of the request lifecycle. Phases include things
43 like URL-to-filename translation, authentication, authorization,
44 content, and logging. (This is not an exhaustive list.)</p>
46 <p>mod_rewrite acts in two of these phases (or "hooks", as they are
47 often called) to influence how URLs may be rewritten.</p>
49 <p>First, it uses the URL-to-filename translation hook, which occurs
50 after the HTTP request has been read, but before any authorization
51 starts. Secondly, it uses the Fixup hook, which is after the
52 authorization phases, and after per-directory configuration files
53 (<code>.htaccess</code> files) have been read, but before the
54 content handler is called.</p>
56 <p>After a request comes in and a corresponding server or
57 virtual host has been determined, the rewriting engine starts
58 processing any <code>mod_rewrite</code> directives appearing in the
59 per-server configuration. (i.e., in the main server configuration file
60 and <code class="directive"><a href="../mod/core.html#virtualhost"><Virtualhost></a></code>
61 sections.) This happens in the URL-to-filename phase.</p>
63 <p>A few steps later, once the final data directories have been found,
64 the per-directory configuration directives (<code>.htaccess</code>
65 files and <code class="directive"><a href="../mod/core.html#directory"><Directory></a></code> blocks) are applied. This
66 happens in the Fixup phase.</p>
68 <p>In each of these cases, mod_rewrite rewrites the
69 <code>REQUEST_URI</code> either to a new URL, or to a filename.</p>
71 <p>In per-directory context (i.e., within <code>.htaccess</code> files
72 and <code>Directory</code> blocks), these rules are being applied
73 after a URL has already been translated to a filename. Because of
74 this, the URL-path that mod_rewrite initially compares <code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives against
75 is the full filesystem path to the translated filename with the current
76 directories path (including a trailing slash) removed from the front.</p>
78 <p> To illustrate: If rules are in /var/www/foo/.htaccess and a request
79 for /foo/bar/baz is being processed, an expression like ^bar/baz$ would
82 <p> If a substitution is made in per-directory context, a new internal
83 subrequest is issued with the new URL, which restarts processing of the
84 request phases. If the substitution is a relative path, the <code class="directive"><a href="../mod/mod_rewrite.html#rewritebase">RewriteBase</a></code> directive
85 determines the URL-path prefix prepended to the substitution.
86 In per-directory context, care must be taken to
87 create rules which will eventually (in some future "round" of per-directory
88 rewrite processing) not perform a substitution to avoid looping.
89 (See <a href="http://wiki.apache.org/httpd/RewriteLooping">RewriteLooping</a>
90 for further discussion of this problem.)</p>
92 <p>Because of this further manipulation of the URL in per-directory
93 context, you'll need to take care to craft your rewrite rules
94 differently in that context. In particular, remember that the
95 leading directory path will be stripped off of the URL that your
96 rewrite rules will see. Consider the examples below for further
99 <table class="bordered">
102 <th>Location of rule</th>
107 <td>VirtualHost section</td>
108 <td>RewriteRule "^/images/(.+)\.jpg" "/images/$1.gif"</td>
112 <td>.htaccess file in document root</td>
113 <td>RewriteRule "^images/(.+)\.jpg" "images/$1.gif"</td>
117 <td>.htaccess file in images directory</td>
118 <td>RewriteRule "^(.+)\.jpg" "$1.gif"</td>
123 <p>For even more insight into how mod_rewrite manipulates URLs in
124 different contexts, you should consult the <a href="../mod/mod_rewrite.html#logging">log entries</a> made during
127 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
128 <div class="section">
129 <h2><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a> <a title="Permanent link" href="#InternalRuleset" class="permalink">¶</a></h2>
131 <p>Now when mod_rewrite is triggered in these two API phases, it
132 reads the configured rulesets from its configuration
133 structure (which itself was either created on startup for
134 per-server context or during the directory walk of the Apache
135 kernel for per-directory context). Then the URL rewriting
136 engine is started with the contained ruleset (one or more
137 rules together with their conditions). The operation of the
138 URL rewriting engine itself is exactly the same for both
139 configuration contexts. Only the final result processing is
142 <p>The order of rules in the ruleset is important because the
143 rewriting engine processes them in a special (and not very
144 obvious) order. The rule is this: The rewriting engine loops
145 through the ruleset rule by rule (<code class="directive"><a href="../mod/mod_rewrite.html#rewriterule">RewriteRule</a></code> directives) and
146 when a particular rule matches it optionally loops through
147 existing corresponding conditions (<code>RewriteCond</code>
148 directives). For historical reasons the conditions are given
149 first, and so the control flow is a little bit long-winded. See
150 Figure 1 for more details.</p>
152 <img src="../images/rewrite_process_uri.png" alt="Flow of RewriteRule and RewriteCond matching" /><br />
153 <dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
155 <p>First the URL is matched against the
156 <em>Pattern</em> of each rule. If it fails, mod_rewrite
157 immediately stops processing this rule, and continues with the
158 next rule. If the <em>Pattern</em> matches, mod_rewrite looks
159 for corresponding rule conditions (RewriteCond directives,
160 appearing immediately above the RewriteRule in the configuration).
161 If none are present, it substitutes the URL with a new value, which is
162 constructed from the string <em>Substitution</em>, and goes on
163 with its rule-looping. But if conditions exist, it starts an
164 inner loop for processing them in the order that they are
165 listed. For conditions, the logic is different: we don't match
166 a pattern against the current URL. Instead we first create a
167 string <em>TestString</em> by expanding variables,
168 back-references, map lookups, <em>etc.</em> and then we try
169 to match <em>CondPattern</em> against it. If the pattern
170 doesn't match, the complete set of conditions and the
171 corresponding rule fails. If the pattern matches, then the
172 next condition is processed until no more conditions are
173 available. If all conditions match, processing is continued
174 with the substitution of the URL with
175 <em>Substitution</em>.</p>
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179 <p><span>Available Languages: </span><a href="../en/rewrite/tech.html" title="English"> en </a> |
180 <a href="../fr/rewrite/tech.html" hreflang="fr" rel="alternate" title="Français"> fr </a></p>
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