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<title>URL Rewriting Guide - Apache HTTP Server</title>
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    <p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
    <a href="../mod/mod_rewrite.html">reference documentation</a>.
    It describes how one can use Apache's <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
    to solve typical URL-based problems with which webmasters are
    commonony confronted. We give detailed descriptions on how to
    solve each problem by configuring URL rewriting rulesets.</p>

    <div class="warning">ATTENTION: Depending on your server configuration
    it may be necessary to slightly change the examples for your
    situation, e.g. adding the <code>[PT]</code> flag when
    additionally using <code class="module"><a href="../mod/mod_alias.html">mod_alias</a></code> and
    <code class="module"><a href="../mod/mod_userdir.html">mod_userdir</a></code>, etc. Or rewriting a ruleset
    to fit in <code>.htaccess</code> context instead
    of per-server context. Always try to understand what a
    particular ruleset really does before you use it. This
    avoids many problems.</div>

  </div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#canonicalurl">Canonical URLs</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#canonicalhost">Canonical Hostnames</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#moveddocroot">Moved <code>DocumentRoot</code></a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#trailingslash">Trailing Slash Problem</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#movehomedirs">Move Homedirs to Different Webserver</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#multipledirs">Search pages in more than one directory</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables According To URL Parts</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#uservhosts">Virtual User Hosts</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#redirecthome">Redirect Homedirs For Foreigners</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
<li><img alt="" src="../images/down.gif" /> Time-Dependent Rewriting</li>
<li><img alt="" src="../images/down.gif" /> Backward Compatibility for YYYY to XXXX migration</li>
<li><img alt="" src="../images/down.gif" /> <a href="#content">Content Handling</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#access">Access Restriction</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#other">Other</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_advanced.html">Practical solutions to
advanced problems</a></li><li><a href="rewrite_tech.html">Technical details</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="canonicalurl" id="canonicalurl">Canonical URLs</a></h2>



<dl>
 <dt>Description:</dt>

   <dd>
     <p>On some webservers there are more than one URL for a
     resource. Usually there are canonical URLs (which should be
     actually used and distributed) and those which are just
     shortcuts, internal ones, etc. Independent of which URL the
     user supplied with the request he should finally see the
     canonical one only.</p>
   </dd>

   <dt>Solution:</dt>

     <dd>
       <p>We do an external HTTP redirect for all non-canonical
       URLs to fix them in the location view of the Browser and
       for all subsequent requests. In the example ruleset below
       we replace <code>/~user</code> by the canonical
       <code>/u/user</code> and fix a missing trailing slash for
       <code>/u/user</code>.</p>

<div class="example"><pre>
RewriteRule   ^/<strong>~</strong>([^/]+)/?(.*)    /<strong>u</strong>/$1/$2  [<strong>R</strong>]
RewriteRule   ^/([uge])/(<strong>[^/]+</strong>)$  /$1/$2<strong>/</strong>   [<strong>R</strong>]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="canonicalhost" id="canonicalhost">Canonical Hostnames</a></h2>

      <dl>
        <dt>Description:</dt>

        <dd>The goal of this rule is to force the use of a particular
        hostname, in preference to other hostnames which may be used to
        reach the same site. For example, if you wish to force the use
        of <strong>www.example.com</strong> instead of
        <strong>example.com</strong>, you might use a variant of the
        following recipe.</dd>

        <dt>Solution:</dt>

        <dd>
<p>For sites running on a port other than 80:</p>
<div class="example"><pre>
RewriteCond %{HTTP_HOST}   !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST}   !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*)         http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
</pre></div>

<p>And for a site running on port 80</p>
<div class="example"><pre>
RewriteCond %{HTTP_HOST}   !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST}   !^$
RewriteRule ^/(.*)         http://fully.qualified.domain.name/$1 [L,R]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="moveddocroot" id="moveddocroot">Moved <code>DocumentRoot</code></a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
<p>Usually the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>
of the webserver directly relates to the URL "<code>/</code>".
But often this data is not really of top-level priority. For example,
you may wish for visitors, on first entering a site, to go to a
particular subdirectory <code>/about/</code>. This may be accomplished
using the following ruleset:</p>
</dd>

        <dt>Solution:</dt>

        <dd>
          <p>We redirect the URL <code>/</code> to
          <code>/about/</code>:
          </p>
         
<div class="example"><pre>
RewriteEngine on
RewriteRule   <strong>^/$</strong>  /about/  [<strong>R</strong>]
</pre></div>

    <p>Note that this can also be handled using the <code class="directive"><a href="../mod/mod_alias.html#redirectmatch">RedirectMatch</a></code> directive:</p>

<div class="example"><p><code>
RedirectMatch ^/$ http://example.com/e/www/
</code></p></div>
</dd>
</dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="trailingslash" id="trailingslash">Trailing Slash Problem</a></h2>

      

      <dl>
        <dt>Description:</dt>

    <dd><p>The vast majority of "trailing slash" problems can be dealt
    with using the techniques discussed in the <a href="http://httpd.apache.org/docs/misc/FAQ-E.html#set-servername">FAQ
    entry</a>. However, occasionally, there is a need to use mod_rewrite
    to handle a case where a missing trailing slash causes a URL to
    fail. This can happen, for example, after a series of complex
    rewrite rules.</p>
    </dd>

        <dt>Solution:</dt>

        <dd>
          <p>The solution to this subtle problem is to let the server
          add the trailing slash automatically. To do this
          correctly we have to use an external redirect, so the
          browser correctly requests subsequent images etc. If we
          only did a internal rewrite, this would only work for the
          directory page, but would go wrong when any images are
          included into this page with relative URLs, because the
          browser would request an in-lined object. For instance, a
          request for <code>image.gif</code> in
          <code>/~quux/foo/index.html</code> would become
          <code>/~quux/image.gif</code> without the external
          redirect!</p>

          <p>So, to do this trick we write:</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^foo<strong>$</strong>  foo<strong>/</strong>  [<strong>R</strong>]
</pre></div>

   <p>Alternately, you can put the following in a
   top-level <code>.htaccess</code> file in the content directory.
   But note that this creates some processing overhead.</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteCond    %{REQUEST_FILENAME}  <strong>-d</strong>
RewriteRule    ^(.+<strong>[^/]</strong>)$           $1<strong>/</strong>  [R]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="movehomedirs" id="movehomedirs">Move Homedirs to Different Webserver</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Many webmasters have asked for a solution to the
          following situation: They wanted to redirect just all
          homedirs on a webserver to another webserver. They usually
          need such things when establishing a newer webserver which
          will replace the old one over time.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>The solution is trivial with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
          On the old webserver we just redirect all
          <code>/~user/anypath</code> URLs to
          <code>http://newserver/~user/anypath</code>.</p>

<div class="example"><pre>
RewriteEngine on
RewriteRule   ^/~(.+)  http://<strong>newserver</strong>/~$1  [R,L]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="multipledirs" id="multipledirs">Search pages in more than one directory</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Sometimes it is necessary to let the webserver search
          for pages in more than one directory. Here MultiViews or
          other techniques cannot help.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We program a explicit ruleset which searches for the
          files in the directories.</p>

<div class="example"><pre>
RewriteEngine on

#   first try to find it in custom/...
#   ...and if found stop and be happy:
RewriteCond         /your/docroot/<strong>dir1</strong>/%{REQUEST_FILENAME}  -f
RewriteRule  ^(.+)  /your/docroot/<strong>dir1</strong>/$1  [L]

#   second try to find it in pub/...
#   ...and if found stop and be happy:
RewriteCond         /your/docroot/<strong>dir2</strong>/%{REQUEST_FILENAME}  -f
RewriteRule  ^(.+)  /your/docroot/<strong>dir2</strong>/$1  [L]

#   else go on for other Alias or ScriptAlias directives,
#   etc.
RewriteRule   ^(.+)  -  [PT]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="setenvvars" id="setenvvars">Set Environment Variables According To URL Parts</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Perhaps you want to keep status information between
          requests and use the URL to encode it. But you don't want
          to use a CGI wrapper for all pages just to strip out this
          information.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We use a rewrite rule to strip out the status information
          and remember it via an environment variable which can be
          later dereferenced from within XSSI or CGI. This way a
          URL <code>/foo/S=java/bar/</code> gets translated to
          <code>/foo/bar/</code> and the environment variable named
          <code>STATUS</code> is set to the value "java".</p>

<div class="example"><pre>
RewriteEngine on
RewriteRule   ^(.*)/<strong>S=([^/]+)</strong>/(.*)    $1/$3 [E=<strong>STATUS:$2</strong>]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="uservhosts" id="uservhosts">Virtual User Hosts</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Assume that you want to provide
          <code>www.<strong>username</strong>.host.domain.com</code>
          for the homepage of username via just DNS A records to the
          same machine and without any virtualhosts on this
          machine.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>For HTTP/1.0 requests there is no solution, but for
          HTTP/1.1 requests which contain a Host: HTTP header we
          can use the following ruleset to rewrite
          <code>http://www.username.host.com/anypath</code>
          internally to <code>/home/username/anypath</code>:</p>

<div class="example"><pre>
RewriteEngine on
RewriteCond   %{<strong>HTTP_HOST</strong>}                 ^www\.<strong>[^.]+</strong>\.host\.com$
RewriteRule   ^(.+)                        %{HTTP_HOST}$1          [C]
RewriteRule   ^www\.<strong>([^.]+)</strong>\.host\.com(.*) /home/<strong>$1</strong>$2
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="redirecthome" id="redirecthome">Redirect Homedirs For Foreigners</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>We want to redirect homedir URLs to another webserver
          <code>www.somewhere.com</code> when the requesting user
          does not stay in the local domain
          <code>ourdomain.com</code>. This is sometimes used in
          virtual host contexts.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>Just a rewrite condition:</p>

<div class="example"><pre>
RewriteEngine on
RewriteCond   %{REMOTE_HOST}  <strong>!^.+\.ourdomain\.com$</strong>
RewriteRule   ^(/~.+)         http://www.somewhere.com/$1 [R,L]
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
        <p>By default, redirecting to an HTML anchor doesn't work,
        because mod_rewrite escapes the <code>#</code> character,
        turning it into <code>%23</code>. This, in turn, breaks the
        redirection.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>Use the <code>[NE]</code> flag on the
          <code>RewriteRule</code>. NE stands for No Escape.
          </p>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2>Time-Dependent Rewriting</h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>When tricks like time-dependent content should happen a
          lot of webmasters still use CGI scripts which do for
          instance redirects to specialized pages. How can it be done
          via <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>There are a lot of variables named <code>TIME_xxx</code>
          for rewrite conditions. In conjunction with the special
          lexicographic comparison patterns <code>&lt;STRING</code>,
          <code>&gt;STRING</code> and <code>=STRING</code> we can
          do time-dependent redirects:</p>

<div class="example"><pre>
RewriteEngine on
RewriteCond   %{TIME_HOUR}%{TIME_MIN} &gt;0700
RewriteCond   %{TIME_HOUR}%{TIME_MIN} &lt;1900
RewriteRule   ^foo\.html$             foo.day.html
RewriteRule   ^foo\.html$             foo.night.html
</pre></div>

          <p>This provides the content of <code>foo.day.html</code>
          under the URL <code>foo.html</code> from
          <code>07:00-19:00</code> and at the remaining time the
          contents of <code>foo.night.html</code>. Just a nice
          feature for a homepage...</p>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2>Backward Compatibility for YYYY to XXXX migration</h2>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>How can we make URLs backward compatible (still
          existing virtually) after migrating <code>document.YYYY</code>
          to <code>document.XXXX</code>, e.g. after translating a
          bunch of <code>.html</code> files to <code>.phtml</code>?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We just rewrite the name to its basename and test for
          existence of the new extension. If it exists, we take
          that name, else we rewrite the URL to its original state.</p>


<div class="example"><pre>
#   backward compatibility ruleset for
#   rewriting document.html to document.phtml
#   when and only when document.phtml exists
#   but no longer document.html
RewriteEngine on
RewriteBase   /~quux/
#   parse out basename, but remember the fact
RewriteRule   ^(.*)\.html$              $1      [C,E=WasHTML:yes]
#   rewrite to document.phtml if exists
RewriteCond   %{REQUEST_FILENAME}.phtml -f
RewriteRule   ^(.*)$ $1.phtml                   [S=1]
#   else reverse the previous basename cutout
RewriteCond   %{ENV:WasHTML}            ^yes$
RewriteRule   ^(.*)$ $1.html
</pre></div>
        </dd>
      </dl>

    </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="content" id="content">Content Handling</a></h2>

    

    <h3>From Old to New (intern)</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Assume we have recently renamed the page
          <code>foo.html</code> to <code>bar.html</code> and now want
          to provide the old URL for backward compatibility. Actually
          we want that users of the old URL even not recognize that
          the pages was renamed.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We rewrite the old URL to the new one internally via the
          following rule:</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<strong>foo</strong>\.html$  <strong>bar</strong>.html
</pre></div>
        </dd>
      </dl>

    

    <h3>From Old to New (extern)</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Assume again that we have recently renamed the page
          <code>foo.html</code> to <code>bar.html</code> and now want
          to provide the old URL for backward compatibility. But this
          time we want that the users of the old URL get hinted to
          the new one, i.e. their browsers Location field should
          change, too.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We force a HTTP redirect to the new URL which leads to a
          change of the browsers and thus the users view:</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<strong>foo</strong>\.html$  <strong>bar</strong>.html  [<strong>R</strong>]
</pre></div>
        </dd>
      </dl>

    

    <h3>From Static to Dynamic</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>How can we transform a static page
          <code>foo.html</code> into a dynamic variant
          <code>foo.cgi</code> in a seamless way, i.e. without notice
          by the browser/user.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We just rewrite the URL to the CGI-script and force the
          correct MIME-type so it gets really run as a CGI-script.
          This way a request to <code>/~quux/foo.html</code>
          internally leads to the invocation of
          <code>/~quux/foo.cgi</code>.</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^foo\.<strong>html</strong>$  foo.<strong>cgi</strong>  [T=<strong>application/x-httpd-cgi</strong>]
</pre></div>
        </dd>
      </dl>

    
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="access" id="access">Access Restriction</a></h2>

    

    <h3>Blocking of Robots</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>How can we block a really annoying robot from
          retrieving pages of a specific webarea? A
          <code>/robots.txt</code> file containing entries of the
          "Robot Exclusion Protocol" is typically not enough to get
          rid of such a robot.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We use a ruleset which forbids the URLs of the webarea
          <code>/~quux/foo/arc/</code> (perhaps a very deep
          directory indexed area where the robot traversal would
          create big server load). We have to make sure that we
          forbid access only to the particular robot, i.e. just
          forbidding the host where the robot runs is not enough.
          This would block users from this host, too. We accomplish
          this by also matching the User-Agent HTTP header
          information.</p>

<div class="example"><pre>
RewriteCond %{HTTP_USER_AGENT}   ^<strong>NameOfBadRobot</strong>.*
RewriteCond %{REMOTE_ADDR}       ^<strong>123\.45\.67\.[8-9]</strong>$
RewriteRule ^<strong>/~quux/foo/arc/</strong>.+   -   [<strong>F</strong>]
</pre></div>
        </dd>
      </dl>

    

    <h3>Blocked Inline-Images</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>Assume we have under <code>http://www.quux-corp.de/~quux/</code>
          some pages with inlined GIF graphics. These graphics are
          nice, so others directly incorporate them via hyperlinks to
          their pages. We don't like this practice because it adds
          useless traffic to our server.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>While we cannot 100% protect the images from inclusion,
          we can at least restrict the cases where the browser
          sends a HTTP Referer header.</p>

<div class="example"><pre>
RewriteCond %{HTTP_REFERER} <strong>!^$</strong>
RewriteCond %{HTTP_REFERER} !^http://www.quux-corp.de/~quux/.*$ [NC]
RewriteRule <strong>.*\.gif$</strong>        -                                    [F]
</pre></div>

<div class="example"><pre>
RewriteCond %{HTTP_REFERER}         !^$
RewriteCond %{HTTP_REFERER}         !.*/foo-with-gif\.html$
RewriteRule <strong>^inlined-in-foo\.gif$</strong>   -                        [F]
</pre></div>
        </dd>
      </dl>

    

    <h3>Proxy Deny</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>How can we forbid a certain host or even a user of a
          special host from using the Apache proxy?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We first have to make sure <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
          is below(!) <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> in the Configuration
          file when compiling the Apache webserver. This way it gets
          called <em>before</em> <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. Then we
          configure the following for a host-dependent deny...</p>

<div class="example"><pre>
RewriteCond %{REMOTE_HOST} <strong>^badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]
</pre></div>

          <p>...and this one for a user@host-dependent deny:</p>

<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST}  <strong>^badguy@badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]
</pre></div>
        </dd>
      </dl>

    

  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="other" id="other">Other</a></h2>

    

    <h3>External Rewriting Engine</h3>

      

      <dl>
        <dt>Description:</dt>

        <dd>
          <p>A FAQ: How can we solve the FOO/BAR/QUUX/etc.
          problem? There seems no solution by the use of
          <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>...</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>Use an external <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>, i.e. a program which acts
          like a <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>. It is run once on startup of Apache
          receives the requested URLs on <code>STDIN</code> and has
          to put the resulting (usually rewritten) URL on
          <code>STDOUT</code> (same order!).</p>

<div class="example"><pre>
RewriteEngine on
RewriteMap    quux-map       <strong>prg:</strong>/path/to/map.quux.pl
RewriteRule   ^/~quux/(.*)$  /~quux/<strong>${quux-map:$1}</strong>
</pre></div>

<div class="example"><pre>
#!/path/to/perl

#   disable buffered I/O which would lead
#   to deadloops for the Apache server
$| = 1;

#   read URLs one per line from stdin and
#   generate substitution URL on stdout
while (&lt;&gt;) {
    s|^foo/|bar/|;
    print $_;
}
</pre></div>

          <p>This is a demonstration-only example and just rewrites
          all URLs <code>/~quux/foo/...</code> to
          <code>/~quux/bar/...</code>. Actually you can program
          whatever you like. But notice that while such maps can be
          <strong>used</strong> also by an average user, only the
          system administrator can <strong>define</strong> it.</p>
        </dd>
      </dl>

    

  </div></div>
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