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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="#cluster">Webcluster through Homogeneous URL Layout</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Homedirs</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#filereorg">Filesystem Reorganization</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#redirect404">Redirect Failing URLs To Other Webserver</a></li>
<li><img alt="" src="../images/down.gif" /> Archive Access Multiplexer</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>
</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><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="cluster" id="cluster">Webcluster through Homogeneous URL Layout</a></h2>

      

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

        <dd>
          <p>We want to create a homogeneous and consistent URL
          layout over all WWW servers on a Intranet webcluster, i.e.
          all URLs (per definition server local and thus server
          dependent!) become actually server <em>independent</em>!
          What we want is to give the WWW namespace a consistent
          server-independent layout: no URL should have to include
          any physically correct target server. The cluster itself
          should drive us automatically to the physical target
          host.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>First, the knowledge of the target servers come from
          (distributed) external maps which contain information
          where our users, groups and entities stay. The have the
          form</p>

<div class="example"><pre>
user1  server_of_user1
user2  server_of_user2
:      :
</pre></div>

          <p>We put them into files <code>map.xxx-to-host</code>.
          Second we need to instruct all servers to redirect URLs
          of the forms</p>

<div class="example"><pre>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</pre></div>

          <p>to</p>

<div class="example"><pre>
http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath
</pre></div>

          <p>when the URL is not locally valid to a server. The
          following ruleset does this for us by the help of the map
          files (assuming that server0 is a default server which
          will be used if a user has no entry in the map):</p>

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

RewriteMap      user-to-host   txt:/path/to/map.user-to-host
RewriteMap     group-to-host   txt:/path/to/map.group-to-host
RewriteMap    entity-to-host   txt:/path/to/map.entity-to-host

RewriteRule   ^/u/<strong>([^/]+)</strong>/?(.*)   http://<strong>${user-to-host:$1|server0}</strong>/u/$1/$2
RewriteRule   ^/g/<strong>([^/]+)</strong>/?(.*)  http://<strong>${group-to-host:$1|server0}</strong>/g/$1/$2
RewriteRule   ^/e/<strong>([^/]+)</strong>/?(.*) http://<strong>${entity-to-host:$1|server0}</strong>/e/$1/$2

RewriteRule   ^/([uge])/([^/]+)/?$          /$1/$2/.www/
RewriteRule   ^/([uge])/([^/]+)/([^.]+.+)   /$1/$2/.www/$3\
</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="structuredhomedirs" id="structuredhomedirs">Structured Homedirs</a></h2>

      

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

        <dd>
          <p>Some sites with thousands of users usually use a
          structured homedir layout, i.e. each homedir is in a
          subdirectory which begins for instance with the first
          character of the username. So, <code>/~foo/anypath</code>
          is <code>/home/<strong>f</strong>/foo/.www/anypath</code>
          while <code>/~bar/anypath</code> is
          <code>/home/<strong>b</strong>/bar/.www/anypath</code>.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We use the following ruleset to expand the tilde URLs
          into exactly the above layout.</p>

<div class="example"><pre>
RewriteEngine on
RewriteRule   ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)  /home/<strong>$2</strong>/$1/.www$3
</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="filereorg" id="filereorg">Filesystem Reorganization</a></h2>

      

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

        <dd>
          <p>This really is a hardcore example: a killer application
          which heavily uses per-directory
          <code>RewriteRules</code> to get a smooth look and feel
          on the Web while its data structure is never touched or
          adjusted. Background: <strong><em>net.sw</em></strong> is
          my archive of freely available Unix software packages,
          which I started to collect in 1992. It is both my hobby
          and job to to this, because while I'm studying computer
          science I have also worked for many years as a system and
          network administrator in my spare time. Every week I need
          some sort of software so I created a deep hierarchy of
          directories where I stored the packages:</p>

<div class="example"><pre>
drwxrwxr-x   2 netsw  users    512 Aug  3 18:39 Audio/
drwxrwxr-x   2 netsw  users    512 Jul  9 14:37 Benchmark/
drwxrwxr-x  12 netsw  users    512 Jul  9 00:34 Crypto/
drwxrwxr-x   5 netsw  users    512 Jul  9 00:41 Database/
drwxrwxr-x   4 netsw  users    512 Jul 30 19:25 Dicts/
drwxrwxr-x  10 netsw  users    512 Jul  9 01:54 Graphic/
drwxrwxr-x   5 netsw  users    512 Jul  9 01:58 Hackers/
drwxrwxr-x   8 netsw  users    512 Jul  9 03:19 InfoSys/
drwxrwxr-x   3 netsw  users    512 Jul  9 03:21 Math/
drwxrwxr-x   3 netsw  users    512 Jul  9 03:24 Misc/
drwxrwxr-x   9 netsw  users    512 Aug  1 16:33 Network/
drwxrwxr-x   2 netsw  users    512 Jul  9 05:53 Office/
drwxrwxr-x   7 netsw  users    512 Jul  9 09:24 SoftEng/
drwxrwxr-x   7 netsw  users    512 Jul  9 12:17 System/
drwxrwxr-x  12 netsw  users    512 Aug  3 20:15 Typesetting/
drwxrwxr-x  10 netsw  users    512 Jul  9 14:08 X11/
</pre></div>

          <p>In July 1996 I decided to make this archive public to
          the world via a nice Web interface. "Nice" means that I
          wanted to offer an interface where you can browse
          directly through the archive hierarchy. And "nice" means
          that I didn't wanted to change anything inside this
          hierarchy - not even by putting some CGI scripts at the
          top of it. Why? Because the above structure should be
          later accessible via FTP as well, and I didn't want any
          Web or CGI stuff to be there.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>The solution has two parts: The first is a set of CGI
          scripts which create all the pages at all directory
          levels on-the-fly. I put them under
          <code>/e/netsw/.www/</code> as follows:</p>

<div class="example"><pre>
-rw-r--r--   1 netsw  users    1318 Aug  1 18:10 .wwwacl
drwxr-xr-x  18 netsw  users     512 Aug  5 15:51 DATA/
-rw-rw-rw-   1 netsw  users  372982 Aug  5 16:35 LOGFILE
-rw-r--r--   1 netsw  users     659 Aug  4 09:27 TODO
-rw-r--r--   1 netsw  users    5697 Aug  1 18:01 netsw-about.html
-rwxr-xr-x   1 netsw  users     579 Aug  2 10:33 netsw-access.pl
-rwxr-xr-x   1 netsw  users    1532 Aug  1 17:35 netsw-changes.cgi
-rwxr-xr-x   1 netsw  users    2866 Aug  5 14:49 netsw-home.cgi
drwxr-xr-x   2 netsw  users     512 Jul  8 23:47 netsw-img/
-rwxr-xr-x   1 netsw  users   24050 Aug  5 15:49 netsw-lsdir.cgi
-rwxr-xr-x   1 netsw  users    1589 Aug  3 18:43 netsw-search.cgi
-rwxr-xr-x   1 netsw  users    1885 Aug  1 17:41 netsw-tree.cgi
-rw-r--r--   1 netsw  users     234 Jul 30 16:35 netsw-unlimit.lst
</pre></div>

          <p>The <code>DATA/</code> subdirectory holds the above
          directory structure, i.e. the real
          <strong><em>net.sw</em></strong> stuff and gets
          automatically updated via <code>rdist</code> from time to
          time. The second part of the problem remains: how to link
          these two structures together into one smooth-looking URL
          tree? We want to hide the <code>DATA/</code> directory
          from the user while running the appropriate CGI scripts
          for the various URLs. Here is the solution: first I put
          the following into the per-directory configuration file
          in the <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>
          of the server to rewrite the announced URL
          <code>/net.sw/</code> to the internal path
          <code>/e/netsw</code>:</p>

<div class="example"><pre>
RewriteRule  ^net.sw$       net.sw/        [R]
RewriteRule  ^net.sw/(.*)$  e/netsw/$1
</pre></div>

          <p>The first rule is for requests which miss the trailing
          slash! The second rule does the real thing. And then
          comes the killer configuration which stays in the
          per-directory config file
          <code>/e/netsw/.www/.wwwacl</code>:</p>

<div class="example"><pre>
Options       ExecCGI FollowSymLinks Includes MultiViews

RewriteEngine on

#  we are reached via /net.sw/ prefix
RewriteBase   /net.sw/

#  first we rewrite the root dir to
#  the handling cgi script
RewriteRule   ^$                       netsw-home.cgi     [L]
RewriteRule   ^index\.html$            netsw-home.cgi     [L]

#  strip out the subdirs when
#  the browser requests us from perdir pages
RewriteRule   ^.+/(netsw-[^/]+/.+)$    $1                 [L]

#  and now break the rewriting for local files
RewriteRule   ^netsw-home\.cgi.*       -                  [L]
RewriteRule   ^netsw-changes\.cgi.*    -                  [L]
RewriteRule   ^netsw-search\.cgi.*     -                  [L]
RewriteRule   ^netsw-tree\.cgi$        -                  [L]
RewriteRule   ^netsw-about\.html$      -                  [L]
RewriteRule   ^netsw-img/.*$           -                  [L]

#  anything else is a subdir which gets handled
#  by another cgi script
RewriteRule   !^netsw-lsdir\.cgi.*     -                  [C]
RewriteRule   (.*)                     netsw-lsdir.cgi/$1
</pre></div>

          <p>Some hints for interpretation:</p>

          <ol>
            <li>Notice the <code>L</code> (last) flag and no
            substitution field ('<code>-</code>') in the forth part</li>

            <li>Notice the <code>!</code> (not) character and
            the <code>C</code> (chain) flag at the first rule
            in the last part</li>

            <li>Notice the catch-all pattern in the last rule</li>
          </ol>
        </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="redirect404" id="redirect404">Redirect Failing URLs To Other Webserver</a></h2>

      

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

        <dd>
          <p>A typical FAQ about URL rewriting is how to redirect
          failing requests on webserver A to webserver B. Usually
          this is done via <code class="directive"><a href="../mod/core.html#errordocument">ErrorDocument</a></code> CGI-scripts in Perl, but
          there is also a <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code> solution.
          But notice that this performs more poorly than using an
          <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>

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

      

      <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>

    </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>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>
        </dd>
      </dl>

    

    <h3>Dynamic Mirror</h3>

      

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

        <dd>
          <p>Assume there are nice webpages on remote hosts we want
          to bring into our namespace. For FTP servers we would use
          the <code>mirror</code> program which actually maintains an
          explicit up-to-date copy of the remote data on the local
          machine. For a webserver we could use the program
          <code>webcopy</code> which acts similar via HTTP. But both
          techniques have one major drawback: The local copy is
          always just as up-to-date as often we run the program. It
          would be much better if the mirror is not a static one we
          have to establish explicitly. Instead we want a dynamic
          mirror with data which gets updated automatically when
          there is need (updated data on the remote host).</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>To provide this feature we map the remote webpage or even
          the complete remote webarea to our namespace by the use
          of the <dfn>Proxy Throughput</dfn> feature
          (flag <code>[P]</code>):</p>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<strong>hotsheet/</strong>(.*)$  <strong>http://www.tstimpreso.com/hotsheet/</strong>$1  [<strong>P</strong>]
</pre></div>

<div class="example"><pre>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<strong>usa-news\.html</strong>$   <strong>http://www.quux-corp.com/news/index.html</strong>  [<strong>P</strong>]
</pre></div>
        </dd>
      </dl>

    

    <h3>Reverse Dynamic Mirror</h3>

      

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

        <dd>...</dd>

        <dt>Solution:</dt>

        <dd>
<div class="example"><pre>
RewriteEngine on
RewriteCond   /mirror/of/remotesite/$1           -U
RewriteRule   ^http://www\.remotesite\.com/(.*)$ /mirror/of/remotesite/$1
</pre></div>
        </dd>
      </dl>

    

    <h3>Retrieve Missing Data from Intranet</h3>

      

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

        <dd>
          <p>This is a tricky way of virtually running a corporate
          (external) Internet webserver
          (<code>www.quux-corp.dom</code>), while actually keeping
          and maintaining its data on a (internal) Intranet webserver
          (<code>www2.quux-corp.dom</code>) which is protected by a
          firewall. The trick is that on the external webserver we
          retrieve the requested data on-the-fly from the internal
          one.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>First, we have to make sure that our firewall still
          protects the internal webserver and that only the
          external webserver is allowed to retrieve data from it.
          For a packet-filtering firewall we could for instance
          configure a firewall ruleset like the following:</p>

<div class="example"><pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port &gt;1024 --&gt; Host www2.quux-corp.dom Port <strong>80</strong>
<strong>DENY</strong>  Host *                 Port *     --&gt; Host www2.quux-corp.dom Port <strong>80</strong>
</pre></div>

          <p>Just adjust it to your actual configuration syntax.
          Now we can establish the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
          rules which request the missing data in the background
          through the proxy throughput feature:</p>

<div class="example"><pre>
RewriteRule ^/~([^/]+)/?(.*)          /home/$1/.www/$2
RewriteCond %{REQUEST_FILENAME}       <strong>!-f</strong>
RewriteCond %{REQUEST_FILENAME}       <strong>!-d</strong>
RewriteRule ^/home/([^/]+)/.www/?(.*) http://<strong>www2</strong>.quux-corp.dom/~$1/pub/$2 [<strong>P</strong>]
</pre></div>
        </dd>
      </dl>

    

    <h3>Load Balancing</h3>

      

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

        <dd>
          <p>Suppose we want to load balance the traffic to
          <code>www.foo.com</code> over <code>www[0-5].foo.com</code>
          (a total of 6 servers). How can this be done?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>There are a lot of possible solutions for this problem.
          We will discuss first a commonly known DNS-based variant
          and then the special one with <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>:</p>

          <ol>
            <li>
              <strong>DNS Round-Robin</strong>

              <p>The simplest method for load-balancing is to use
              the DNS round-robin feature of <code>BIND</code>.
              Here you just configure <code>www[0-9].foo.com</code>
              as usual in your DNS with A(address) records, e.g.</p>

<div class="example"><pre>
www0   IN  A       1.2.3.1
www1   IN  A       1.2.3.2
www2   IN  A       1.2.3.3
www3   IN  A       1.2.3.4
www4   IN  A       1.2.3.5
www5   IN  A       1.2.3.6
</pre></div>

              <p>Then you additionally add the following entry:</p>

<div class="example"><pre>
www   IN  A       1.2.3.1
www   IN  A       1.2.3.2
www   IN  A       1.2.3.3
www   IN  A       1.2.3.4
www   IN  A       1.2.3.5
</pre></div>

              <p>Now when <code>www.foo.com</code> gets
              resolved, <code>BIND</code> gives out <code>www0-www5</code>
              - but in a slightly permutated/rotated order every time.
              This way the clients are spread over the various
              servers. But notice that this not a perfect load
              balancing scheme, because DNS resolve information
              gets cached by the other nameservers on the net, so
              once a client has resolved <code>www.foo.com</code>
              to a particular <code>wwwN.foo.com</code>, all
              subsequent requests also go to this particular name
              <code>wwwN.foo.com</code>. But the final result is
              ok, because the total sum of the requests are really
              spread over the various webservers.</p>
            </li>

            <li>
              <strong>DNS Load-Balancing</strong>

              <p>A sophisticated DNS-based method for
              load-balancing is to use the program
              <code>lbnamed</code> which can be found at <a href="http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html">
              http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html</a>.
              It is a Perl 5 program in conjunction with auxilliary
              tools which provides a real load-balancing for
              DNS.</p>
            </li>

            <li>
              <strong>Proxy Throughput Round-Robin</strong>

              <p>In this variant we use <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
              and its proxy throughput feature. First we dedicate
              <code>www0.foo.com</code> to be actually
              <code>www.foo.com</code> by using a single</p>

<div class="example"><pre>
www    IN  CNAME   www0.foo.com.
</pre></div>

              <p>entry in the DNS. Then we convert
              <code>www0.foo.com</code> to a proxy-only server,
              i.e. we configure this machine so all arriving URLs
              are just pushed through the internal proxy to one of
              the 5 other servers (<code>www1-www5</code>). To
              accomplish this we first establish a ruleset which
              contacts a load balancing script <code>lb.pl</code>
              for all URLs.</p>

<div class="example"><pre>
RewriteEngine on
RewriteMap    lb      prg:/path/to/lb.pl
RewriteRule   ^/(.+)$ ${lb:$1}           [P,L]
</pre></div>

              <p>Then we write <code>lb.pl</code>:</p>

<div class="example"><pre>
#!/path/to/perl
##
##  lb.pl -- load balancing script
##

$| = 1;

$name   = "www";     # the hostname base
$first  = 1;         # the first server (not 0 here, because 0 is myself)
$last   = 5;         # the last server in the round-robin
$domain = "foo.dom"; # the domainname

$cnt = 0;
while (&lt;STDIN&gt;) {
    $cnt = (($cnt+1) % ($last+1-$first));
    $server = sprintf("%s%d.%s", $name, $cnt+$first, $domain);
    print "http://$server/$_";
}

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

              <div class="note">A last notice: Why is this useful? Seems like
              <code>www0.foo.com</code> still is overloaded? The
              answer is yes, it is overloaded, but with plain proxy
              throughput requests, only! All SSI, CGI, ePerl, etc.
              processing is completely done on the other machines.
              This is the essential point.</div>
            </li>

            <li>
              <strong>Hardware/TCP Round-Robin</strong>

              <p>There is a hardware solution available, too. Cisco
              has a beast called LocalDirector which does a load
              balancing at the TCP/IP level. Actually this is some
              sort of a circuit level gateway in front of a
              webcluster. If you have enough money and really need
              a solution with high performance, use this one.</p>
            </li>
          </ol>
        </dd>
      </dl>

    

    <h3>New MIME-type, New Service</h3>

      

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

        <dd>
          <p>On the net there are a lot of nifty CGI programs. But
          their usage is usually boring, so a lot of webmaster
          don't use them. Even Apache's Action handler feature for
          MIME-types is only appropriate when the CGI programs
          don't need special URLs (actually <code>PATH_INFO</code>
          and <code>QUERY_STRINGS</code>) as their input. First,
          let us configure a new file type with extension
          <code>.scgi</code> (for secure CGI) which will be processed
          by the popular <code>cgiwrap</code> program. The problem
          here is that for instance we use a Homogeneous URL Layout
          (see above) a file inside the user homedirs has the URL
          <code>/u/user/foo/bar.scgi</code>. But
          <code>cgiwrap</code> needs the URL in the form
          <code>/~user/foo/bar.scgi/</code>. The following rule
          solves the problem:</p>

<div class="example"><pre>
RewriteRule ^/[uge]/<strong>([^/]+)</strong>/\.www/(.+)\.scgi(.*) ...
... /internal/cgi/user/cgiwrap/~<strong>$1</strong>/$2.scgi$3  [NS,<strong>T=application/x-http-cgi</strong>]
</pre></div>

          <p>Or assume we have some more nifty programs:
          <code>wwwlog</code> (which displays the
          <code>access.log</code> for a URL subtree and
          <code>wwwidx</code> (which runs Glimpse on a URL
          subtree). We have to provide the URL area to these
          programs so they know on which area they have to act on.
          But usually this ugly, because they are all the times
          still requested from that areas, i.e. typically we would
          run the <code>swwidx</code> program from within
          <code>/u/user/foo/</code> via hyperlink to</p>

<div class="example"><pre>
/internal/cgi/user/swwidx?i=/u/user/foo/
</pre></div>

          <p>which is ugly. Because we have to hard-code
          <strong>both</strong> the location of the area
          <strong>and</strong> the location of the CGI inside the
          hyperlink. When we have to reorganize the area, we spend a
          lot of time changing the various hyperlinks.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>The solution here is to provide a special new URL format
          which automatically leads to the proper CGI invocation.
          We configure the following:</p>

<div class="example"><pre>
RewriteRule   ^/([uge])/([^/]+)(/?.*)/\*  /internal/cgi/user/wwwidx?i=/$1/$2$3/
RewriteRule   ^/([uge])/([^/]+)(/?.*):log /internal/cgi/user/wwwlog?f=/$1/$2$3
</pre></div>

          <p>Now the hyperlink to search at
          <code>/u/user/foo/</code> reads only</p>

<div class="example"><pre>
HREF="*"
</pre></div>

          <p>which internally gets automatically transformed to</p>

<div class="example"><pre>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</pre></div>

          <p>The same approach leads to an invocation for the
          access log CGI program when the hyperlink
          <code>:log</code> gets used.</p>
        </dd>
      </dl>

    

    <h3>On-the-fly Content-Regeneration</h3>

      

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

        <dd>
          <p>Here comes a really esoteric feature: Dynamically
          generated but statically served pages, i.e. pages should be
          delivered as pure static pages (read from the filesystem
          and just passed through), but they have to be generated
          dynamically by the webserver if missing. This way you can
          have CGI-generated pages which are statically served unless
          one (or a cronjob) removes the static contents. Then the
          contents gets refreshed.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          This is done via the following ruleset:

<div class="example"><pre>
RewriteCond %{REQUEST_FILENAME}   <strong>!-s</strong>
RewriteRule ^page\.<strong>html</strong>$          page.<strong>cgi</strong>   [T=application/x-httpd-cgi,L]
</pre></div>

          <p>Here a request to <code>page.html</code> leads to a
          internal run of a corresponding <code>page.cgi</code> if
          <code>page.html</code> is still missing or has filesize
          null. The trick here is that <code>page.cgi</code> is a
          usual CGI script which (additionally to its <code>STDOUT</code>)
          writes its output to the file <code>page.html</code>.
          Once it was run, the server sends out the data of
          <code>page.html</code>. When the webmaster wants to force
          a refresh the contents, he just removes
          <code>page.html</code> (usually done by a cronjob).</p>
        </dd>
      </dl>

    

    <h3>Document With Autorefresh</h3>

      

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

        <dd>
          <p>Wouldn't it be nice while creating a complex webpage if
          the webbrowser would automatically refresh the page every
          time we write a new version from within our editor?
          Impossible?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>No! We just combine the MIME multipart feature, the
          webserver NPH feature and the URL manipulation power of
          <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>. First, we establish a new
          URL feature: Adding just <code>:refresh</code> to any
          URL causes this to be refreshed every time it gets
          updated on the filesystem.</p>

<div class="example"><pre>
RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1
</pre></div>

          <p>Now when we reference the URL</p>

<div class="example"><pre>
/u/foo/bar/page.html:refresh
</pre></div>

          <p>this leads to the internal invocation of the URL</p>

<div class="example"><pre>
/internal/cgi/apache/nph-refresh?f=/u/foo/bar/page.html
</pre></div>

          <p>The only missing part is the NPH-CGI script. Although
          one would usually say "left as an exercise to the reader"
          ;-) I will provide this, too.</p>

<div class="example"><pre>
#!/sw/bin/perl
##
##  nph-refresh -- NPH/CGI script for auto refreshing pages
##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;

#   split the QUERY_STRING variable
@pairs = split(/&amp;/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
    ($name, $value) = split(/=/, $pair);
    $name =~ tr/A-Z/a-z/;
    $name = 'QS_' . $name;
    $value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
    eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
    print "HTTP/1.0 200 OK\n";
    print "Content-type: text/html\n\n";
    print "&amp;lt;b&amp;gt;ERROR&amp;lt;/b&amp;gt;: No file given\n";
    exit(0);
}
if (! -f $QS_f) {
    print "HTTP/1.0 200 OK\n";
    print "Content-type: text/html\n\n";
    print "&amp;lt;b&amp;gt;ERROR&amp;lt;/b&amp;gt;: File $QS_f not found\n";
    exit(0);
}

sub print_http_headers_multipart_begin {
    print "HTTP/1.0 200 OK\n";
    $bound = "ThisRandomString12345";
    print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
    &amp;print_http_headers_multipart_next;
}

sub print_http_headers_multipart_next {
    print "\n--$bound\n";
}

sub print_http_headers_multipart_end {
    print "\n--$bound--\n";
}

sub displayhtml {
    local($buffer) = @_;
    $len = length($buffer);
    print "Content-type: text/html\n";
    print "Content-length: $len\n\n";
    print $buffer;
}

sub readfile {
    local($file) = @_;
    local(*FP, $size, $buffer, $bytes);
    ($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
    $size = sprintf("%d", $size);
    open(FP, "&amp;lt;$file");
    $bytes = sysread(FP, $buffer, $size);
    close(FP);
    return $buffer;
}

$buffer = &amp;readfile($QS_f);
&amp;print_http_headers_multipart_begin;
&amp;displayhtml($buffer);

sub mystat {
    local($file) = $_[0];
    local($time);

    ($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
    return $mtime;
}

$mtimeL = &amp;mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &amp;lt; $QS_n; $n++) {
    while (1) {
        $mtime = &amp;mystat($QS_f);
        if ($mtime ne $mtimeL) {
            $mtimeL = $mtime;
            sleep(2);
            $buffer = &amp;readfile($QS_f);
            &amp;print_http_headers_multipart_next;
            &amp;displayhtml($buffer);
            sleep(5);
            $mtimeL = &amp;mystat($QS_f);
            last;
        }
        sleep($QS_s);
    }
}

&amp;print_http_headers_multipart_end;

exit(0);

##EOF##
</pre></div>
        </dd>
      </dl>

    

    <h3>Mass Virtual Hosting</h3>

      

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

        <dd>
          <p>The <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code> feature of Apache is nice
          and works great when you just have a few dozens
          virtual hosts. But when you are an ISP and have hundreds of
          virtual hosts to provide this feature is not the best
          choice.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>To provide this feature we map the remote webpage or even
          the complete remote webarea to our namespace by the use
          of the <dfn>Proxy Throughput</dfn> feature (flag <code>[P]</code>):</p>

<div class="example"><pre>
##
##  vhost.map
##
www.vhost1.dom:80  /path/to/docroot/vhost1
www.vhost2.dom:80  /path/to/docroot/vhost2
     :
www.vhostN.dom:80  /path/to/docroot/vhostN
</pre></div>

<div class="example"><pre>
##
##  httpd.conf
##
    :
#   use the canonical hostname on redirects, etc.
UseCanonicalName on

    :
#   add the virtual host in front of the CLF-format
CustomLog  /path/to/access_log  "%{VHOST}e %h %l %u %t \"%r\" %&gt;s %b"
    :

#   enable the rewriting engine in the main server
RewriteEngine on

#   define two maps: one for fixing the URL and one which defines
#   the available virtual hosts with their corresponding
#   DocumentRoot.
RewriteMap    lowercase    int:tolower
RewriteMap    vhost        txt:/path/to/vhost.map

#   Now do the actual virtual host mapping
#   via a huge and complicated single rule:
#
#   1. make sure we don't map for common locations
RewriteCond   %{REQUEST_URI}  !^/commonurl1/.*
RewriteCond   %{REQUEST_URI}  !^/commonurl2/.*
    :
RewriteCond   %{REQUEST_URI}  !^/commonurlN/.*
#
#   2. make sure we have a Host header, because
#      currently our approach only supports
#      virtual hosting through this header
RewriteCond   %{HTTP_HOST}  !^$
#
#   3. lowercase the hostname
RewriteCond   ${lowercase:%{HTTP_HOST}|NONE}  ^(.+)$
#
#   4. lookup this hostname in vhost.map and
#      remember it only when it is a path
#      (and not "NONE" from above)
RewriteCond   ${vhost:%1}  ^(/.*)$
#
#   5. finally we can map the URL to its docroot location
#      and remember the virtual host for logging puposes
RewriteRule   ^/(.*)$   %1/$1  [E=VHOST:${lowercase:%{HTTP_HOST}}]
    :
</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>Host Deny</h3>

      

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

        <dd>
          <p>How can we forbid a list of externally configured hosts
          from using our server?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>For Apache &gt;= 1.3b6:</p>

<div class="example"><pre>
RewriteEngine on
RewriteMap    hosts-deny  txt:/path/to/hosts.deny
RewriteCond   ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND} !=NOT-FOUND [OR]
RewriteCond   ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule   ^/.*  -  [F]
</pre></div>

          <p>For Apache &lt;= 1.3b6:</p>

<div class="example"><pre>
RewriteEngine on
RewriteMap    hosts-deny  txt:/path/to/hosts.deny
RewriteRule   ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule   !^NOT-FOUND/.* - [F]
RewriteRule   ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule   !^NOT-FOUND/.* - [F]
RewriteRule   ^NOT-FOUND/(.*)$ /$1
</pre></div>

<div class="example"><pre>
##
##  hosts.deny
##
##  ATTENTION! This is a map, not a list, even when we treat it as such.
##             mod_rewrite parses it for key/value pairs, so at least a
##             dummy value "-" must be present for each entry.
##

193.102.180.41 -
bsdti1.sdm.de  -
192.76.162.40  -
</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>

    

    <h3>Special Authentication Variant</h3>

      

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

        <dd>
          <p>Sometimes a very special authentication is needed, for
          instance a authentication which checks for a set of
          explicitly configured users. Only these should receive
          access and without explicit prompting (which would occur
          when using the Basic Auth via <code class="module"><a href="../mod/mod_auth.html">mod_auth</a></code>).</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>We use a list of rewrite conditions to exclude all except
          our friends:</p>

<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend1@client1.quux-corp\.com$</strong>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend2</strong>@client2.quux-corp\.com$
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend3</strong>@client3.quux-corp\.com$
RewriteRule ^/~quux/only-for-friends/      -                                 [F]
</pre></div>
        </dd>
      </dl>

    

    <h3>Referer-based Deflector</h3>

      

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

        <dd>
          <p>How can we program a flexible URL Deflector which acts
          on the "Referer" HTTP header and can be configured with as
          many referring pages as we like?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>Use the following really tricky ruleset...</p>

<div class="example"><pre>
RewriteMap  deflector txt:/path/to/deflector.map

RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}} ^-$
RewriteRule ^.* %{HTTP_REFERER} [R,L]

RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^.* ${deflector:%{HTTP_REFERER}} [R,L]
</pre></div>

          <p>... in conjunction with a corresponding rewrite
          map:</p>

<div class="example"><pre>
##
##  deflector.map
##

http://www.badguys.com/bad/index.html    -
http://www.badguys.com/bad/index2.html   -
http://www.badguys.com/bad/index3.html   http://somewhere.com/
</pre></div>

          <p>This automatically redirects the request back to the
          referring page (when "<code>-</code>" is used as the value
          in the map) or to a specific URL (when an URL is specified
          in the map as the second argument).</p>
        </dd>
      </dl>

    

  </div></div>
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<p><span>Available Languages: </span><a href="../en/rewrite/rewrite_guide_advanced.html" title="English">&nbsp;en&nbsp;</a></p>
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