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<title>Apache mod_rewrite Technical Details - Apache HTTP Server</title>
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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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Apache mod_rewrite Technical Details</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/rewrite/rewrite_tech.html" title="English"> en </a></p>
</div>
<p>This document discusses some of the technical details of mod_rewrite
and URL matching.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#Internal">Internal Processing</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#InternalAPI">API Phases</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#InternalRuleset">Ruleset Processing</a></li>
</ul><h3>See also</h3><ul class="seealso"><li><a href="../mod/mod_rewrite.html">Module
documentation</a></li><li><a href="rewrite_intro.html">mod_rewrite
introduction</a></li><li><a href="rewrite_guide.html">Practical solutions to common
problems</a></li></ul></div>
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<div class="section">
<h2><a name="Internal" id="Internal">Internal Processing</a></h2>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="InternalAPI" id="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>
</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/mod_rewrite_fig1.gif" width="428" height="385" alt="[Needs graphics capability to display]" /><br />
<dfn>Figure 1:</dfn>The control flow through the rewriting ruleset
</p>
<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>
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