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          <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code>
          CGI-script!</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>The first solution has the best performance but less
          flexibility, and is less error safe:</p>

<div class="example"><pre>
RewriteEngine on
RewriteCond   /your/docroot/%{REQUEST_FILENAME} <strong>!-f</strong>
RewriteRule   ^(.+)                             http://<strong>webserverB</strong>.dom/$1
</pre></div>

          <p>The problem here is that this will only work for pages
          inside the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>. While you can add more
          Conditions (for instance to also handle homedirs, etc.)
          there is better variant:</p>

<div class="example"><pre>
RewriteEngine on
RewriteCond   %{REQUEST_URI} <strong>!-U</strong>
RewriteRule   ^(.+)          http://<strong>webserverB</strong>.dom/$1
</pre></div>

          <p>This uses the URL look-ahead feature of <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
          The result is that this will work for all types of URLs
          and is a safe way. But it does a performance impact on
          the webserver, because for every request there is one
          more internal subrequest. So, if your webserver runs on a
          powerful CPU, use this one. If it is a slow machine, use
          the first approach or better a <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> CGI-script.</p>
        </dd>
      </dl>

    

    <h3>Extended Redirection</h3>

      

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

        <dd>
          <p>Sometimes we need more control (concerning the
          character escaping mechanism) of URLs on redirects.
          Usually the Apache kernels URL escape function also
          escapes anchors, i.e. URLs like "<code>url#anchor</code>".
          You cannot use this directly on redirects with
          <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> because the
          <code>uri_escape()</code> function of Apache
          would also escape the hash character.
          How can we redirect to such a URL?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We have to use a kludge by the use of a NPH-CGI script
          which does the redirect itself. Because here no escaping
          is done (NPH=non-parseable headers). First we introduce a
          new URL scheme <code>xredirect:</code> by the following
          per-server config-line (should be one of the last rewrite
          rules):</p>

<div class="example"><pre>
RewriteRule ^xredirect:(.+) /path/to/nph-xredirect.cgi/$1 \
            [T=application/x-httpd-cgi,L]
</pre></div>

          <p>This forces all URLs prefixed with
          <code>xredirect:</code> to be piped through the
          <code>nph-xredirect.cgi</code> program. And this program
          just looks like:</p>

<div class="example"><pre>
#!/path/to/perl
##
##  nph-xredirect.cgi -- NPH/CGI script for extended redirects
##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##

$| = 1;
$url = $ENV{'PATH_INFO'};

print "HTTP/1.0 302 Moved Temporarily\n";
print "Server: $ENV{'SERVER_SOFTWARE'}\n";
print "Location: $url\n";
print "Content-type: text/html\n";
print "\n";
print "&lt;html&gt;\n";
print "&lt;head&gt;\n";
print "&lt;title&gt;302 Moved Temporarily (EXTENDED)&lt;/title&gt;\n";
print "&lt;/head&gt;\n";
print "&lt;body&gt;\n";
print "&lt;h1&gt;Moved Temporarily (EXTENDED)&lt;/h1&gt;\n";
print "The document has moved &lt;a HREF=\"$url\"&gt;here&lt;/a&gt;.&lt;p&gt;\n";
print "&lt;/body&gt;\n";
print "&lt;/html&gt;\n";

##EOF##
</pre></div>

          <p>This provides you with the functionality to do
          redirects to all URL schemes, i.e. including the one
          which are not directly accepted by <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>.
          For instance you can now also redirect to
          <code>news:newsgroup</code> via</p>

<div class="example"><pre>
RewriteRule ^anyurl  xredirect:news:newsgroup
</pre></div>

          <div class="note">Notice: You have not to put <code>[R]</code> or
          <code>[R,L]</code> to the above rule because the
          <code>xredirect:</code> need to be expanded later
          by our special "pipe through" rule above.</div>
        </dd>
      </dl>

    

    <h3>Archive Access Multiplexer</h3>

      

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

        <dd>
          <p>Do you know the great CPAN (Comprehensive Perl Archive
          Network) under <a href="http://www.perl.com/CPAN">http://www.perl.com/CPAN</a>?
          This does a redirect to one of several FTP servers around
          the world which carry a CPAN mirror and is approximately
          near the location of the requesting client. Actually this
          can be called an FTP access multiplexing service. While
          CPAN runs via CGI scripts, how can a similar approach
          implemented via <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>First we notice that from version 3.0.0
          <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> can
          also use the "<code>ftp:</code>" scheme on redirects.
          And second, the location approximation can be done by a
          <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
          over the top-level domain of the client.
          With a tricky chained ruleset we can use this top-level
          domain as a key to our multiplexing map.</p>

<div class="example"><pre>
RewriteEngine on
RewriteMap    multiplex                txt:/path/to/map.cxan
RewriteRule   ^/CxAN/(.*)              %{REMOTE_HOST}::$1                 [C]
RewriteRule   ^.+\.<strong>([a-zA-Z]+)</strong>::(.*)$  ${multiplex:<strong>$1</strong>|ftp.default.dom}$2  [R,L]
</pre></div>

<div class="example"><pre>
##
##  map.cxan -- Multiplexing Map for CxAN
##

de        ftp://ftp.cxan.de/CxAN/
uk        ftp://ftp.cxan.uk/CxAN/
com       ftp://ftp.cxan.com/CxAN/
 :
##EOF##
</pre></div>
        </dd>
      </dl>

    

    <h3>Time-Dependent Rewriting</h3>

      

      <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>

    

    <h3>Backward Compatibility for YYYY to XXXX migration</h3>

      

      <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>Browser Dependent Content</h3>

      

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

        <dd>
          <p>At least for important top-level pages it is sometimes
          necessary to provide the optimum of browser dependent
          content, i.e. one has to provide a maximum version for the
          latest Netscape variants, a minimum version for the Lynx
          browsers and a average feature version for all others.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We cannot use content negotiation because the browsers do
          not provide their type in that form. Instead we have to
          act on the HTTP header "User-Agent". The following condig
          does the following: If the HTTP header "User-Agent"
          begins with "Mozilla/3", the page <code>foo.html</code>
          is rewritten to <code>foo.NS.html</code> and and the
          rewriting stops. If the browser is "Lynx" or "Mozilla" of
          version 1 or 2 the URL becomes <code>foo.20.html</code>.
          All other browsers receive page <code>foo.32.html</code>.
          This is done by the following ruleset:</p>

<div class="example"><pre>
RewriteCond %{HTTP_USER_AGENT}  ^<strong>Mozilla/3</strong>.*
RewriteRule ^foo\.html$         foo.<strong>NS</strong>.html          [<strong>L</strong>]

RewriteCond %{HTTP_USER_AGENT}  ^<strong>Lynx/</strong>.*         [OR]
RewriteCond %{HTTP_USER_AGENT}  ^<strong>Mozilla/[12]</strong>.*
RewriteRule ^foo\.html$         foo.<strong>20</strong>.html          [<strong>L</strong>]

RewriteRule ^foo\.html$         foo.<strong>32</strong>.html          [<strong>L</strong>]
</pre></div>