Moved some of the gorey details out of the module reference doc and into

<tr><th><a href="module-dict.html#Compatibility">Compatibility:</a></th><td>Available in Apache 1.3 and later</td></tr></table>

<h3>Summary</h3>

      <blockquote>

            <p>``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.''</p>

            <p class="cite">-- <cite>Brian Behlendorf</cite><br />

            Apache Group</p>

      </blockquote>

      <blockquote>

            <p>`` Despite the tons of examples and docs,

            mod_rewrite is voodoo. Damned cool voodoo, but still

            voodoo. ''</p>

            <p class="cite">-- <cite>Brian Moore</cite><br />

            bem@news.cmc.net</p>

      </blockquote>

      <p>Welcome to mod_rewrite, the Swiss Army Knife of URL

      manipulation!</p>

      <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

      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 and gifted exclusively to the The Apache Group in July 1997

      by</p>

      <p class="indent">

        <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>

      </p>

      <p>Further details, discussion, and examples, are provided in the

      <a href="../rewrite/">detailed mod_rewrite documentation</a>.</p>

</div>

<div id="quickview"><h3 class="directives">Directives</h3>

<ul id="toc">

</ul>

<h3>Topics</h3>

<ul id="topics">

<li><img alt="" src="../images/down.gif" /> <a href="#Internal">Internal Processing</a></li>

<li><img alt="" src="../images/down.gif" /> <a href="#quoting">Quoting Special Characters</a></li>

<li><img alt="" src="../images/down.gif" /> <a href="#InternalBackRefs">Regex Back-Reference Availability</a></li>

<li><img alt="" src="../images/down.gif" /> <a href="#EnvVar">Environment Variables</a></li>

<li><img alt="" src="../images/down.gif" /> <a href="#Solutions">Practical Solutions</a></li>

</ul></div>

<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>

<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>

<h3><a name="InternalAPI" id="InternalAPI">API Phases</a></h3>

      <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>

<h3><a name="InternalRuleset" id="InternalRuleset">Ruleset Processing</a></h3>

      <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="#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>

<h3><a name="quoting" id="quoting">Quoting Special Characters</a></h3>

<h2><a name="quoting" id="quoting">Quoting Special Characters</a></h2>

      <p>As of Apache 1.3.20, special characters in

      <em>TestString</em> and <em>Substitution</em> strings can be

      dollar-sign character in a <em>Substitution</em> string by

      using '<code>\$</code>'; this keeps mod_rewrite from trying

      to treat it as a backreference.</p>

<h3><a name="InternalBackRefs" id="InternalBackRefs">Regex Back-Reference Availability</a></h3>

</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>

<div class="section">

<h2><a name="InternalBackRefs" id="InternalBackRefs">Regex Back-Reference Availability</a></h2>

      <p>One important thing here has to be remembered: Whenever you

      use parentheses in <em>Pattern</em> or in one of the

      reading the following documentation of the available

      directives.</p>

</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>

<div class="section">

<h2><a name="EnvVar" id="EnvVar">Environment Variables</a></h2>

<div class="section">

<h2><a name="Solutions" id="Solutions">Practical Solutions</a></h2>

      <p>We also have an <a href="../misc/rewriteguide.html">URL

      Rewriting Guide</a> available, which provides a collection of

      practical solutions for URL-based problems. There you can

      find real-life rulesets and additional information about

      mod_rewrite.</p>

    <p>For numerous examples of common, and not-so-common, uses for

    mod_rewrite, see the <a href="../rewrite/rewrite_guide.html">Rewrite

    Guide</a>, and the <a href="../rewrite/rewrite_guide_advanced.html">Advanced Rewrite

    Guide</a> documents.</p>

</div>

<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>

<div class="directive-section"><h2><a name="RewriteBase" id="RewriteBase">RewriteBase</a> <a name="rewritebase" id="rewritebase">Directive</a></h2>

<compatibility>Available in Apache 1.3 and later</compatibility>

<summary>

      <blockquote>

            <p>``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.''</p>

            <p class="cite">-- <cite>Brian Behlendorf</cite><br />

            Apache Group</p>

      </blockquote>

      <blockquote>

            <p>`` Despite the tons of examples and docs,

            mod_rewrite is voodoo. Damned cool voodoo, but still

            voodoo. ''</p>

            <p class="cite">-- <cite>Brian Moore</cite><br />

            bem@news.cmc.net</p>

      </blockquote>

      <p>Welcome to mod_rewrite, the Swiss Army Knife of URL

      manipulation!</p>

      <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

      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 and gifted exclusively to the The Apache Group in July 1997

      by</p>

      <p class="indent">

        <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>

      </p>

      <p>Further details, discussion, and examples, are provided in the

      <a href="../rewrite/">detailed mod_rewrite documentation</a>.</p>

</summary>

<section id="Internal"><title>Internal Processing</title>

      <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>

<section id="InternalAPI"><title>API Phases</title>

      <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>

</section>

<section id="InternalRuleset"><title>Ruleset Processing</title>

      <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 (<directive

      module="mod_rewrite">RewriteRule</directive> 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>

</section>

<section id="quoting"><title>Quoting Special Characters</title>

      <p>As of Apache 1.3.20, special characters in

      directives.</p>

</section>

</section>

<section id="EnvVar"><title>Environment Variables</title>

<section id="Solutions"><title>Practical Solutions</title>

      <p>We also have an <a href="../misc/rewriteguide.html">URL

      Rewriting Guide</a> available, which provides a collection of

      practical solutions for URL-based problems. There you can

      find real-life rulesets and additional information about

      mod_rewrite.</p>

</section>

    <p>For numerous examples of common, and not-so-common, uses for

    mod_rewrite, see the <a href="../rewrite/rewrite_guide.html">Rewrite

    Guide</a>, and the <a

    href="../rewrite/rewrite_guide_advanced.html">Advanced Rewrite

    Guide</a> documents.</p>

</section>

<directivesynopsis>

<name>RewriteEngine</name>