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<manualpage metafile="rewrite_guide_advanced.xml.meta">
  <parentdocument href="./">Rewrite</parentdocument>

  <title>URL Rewriting Guide - Advanced topics</title>

  <summary>

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

    <note type="warning">ATTENTION: Depending on your server configuration
    it may be necessary to adjust the examples for your
    situation, e.g., adding the <code>[PT]</code> flag if
    using <module>mod_alias</module> and
    <module>mod_userdir</module>, etc. Or rewriting a ruleset
    to work 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.</note>

  </summary>
<seealso><a href="../mod/mod_rewrite.html">Module
documentation</a></seealso>
<seealso><a href="rewrite_intro.html">mod_rewrite
introduction</a></seealso>
<seealso><a href="rewrite_guide.html">Rewrite Guide - useful
examples</a></seealso>
<seealso><a href="rewrite_tech.html">Technical details</a></seealso>


    <section id="cluster">

      <title>Web Cluster with Consistent URL Space</title>

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

        <dd>
          <p>We want to create a homogeneous and consistent URL
          layout across all WWW servers on an Intranet web cluster, i.e.,
          all URLs (by definition server-local and thus
          server-dependent!) become server <em>independent</em>!
          What we want is to give the WWW namespace a single consistent
          layout: no URL should refer to
          any particular target server. The cluster itself
          should connect users automatically to a physical target
          host as needed, invisibly.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>First, the knowledge of the target servers comes from
          (distributed) external maps which contain information on
          where our users, groups, and entities reside. They have the
          form:</p>

<example><pre>
user1  server_of_user1
user2  server_of_user2
:      :
</pre></example>

          <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>

<example><pre>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</pre></example>

          <p>to</p>

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

          <p>when any URL path need not be valid on every server. The
          following ruleset does this for us with 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>

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

    </section>

    <section id="structuredhomedirs">

      <title>Structured Homedirs</title>

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

        <dd>
          <p>Some sites with thousands of users use a
          structured homedir layout, <em>i.e.</em> 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 the above layout.</p>

<example><pre>
RewriteEngine on
RewriteRule   ^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)  /home/<strong>$2</strong>/$1/.www$3
</pre></example>
        </dd>
      </dl>

    </section>

    <section id="filereorg">

      <title>Filesystem Reorganization</title>

      <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 do 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>

<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></example>

          <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 want to change anything inside this
          hierarchy - not even by putting some CGI scripts at the
          top of it. Why? Because the above structure should later be
          accessible via FTP as well, and I didn't want any
          Web or CGI stuff mixed in 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>

<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></example>

          <p>The <code>DATA/</code> subdirectory holds the above
          directory structure, <em>i.e.</em> 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 <directive module="core">DocumentRoot</directive>
          of the server to rewrite the public URL path
          <code>/net.sw/</code> to the internal path
          <code>/e/netsw</code>:</p>

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

          <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>

<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></example>

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

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

    </section>

    <section id="redirect404">

      <title>Redirect Failing URLs to Another Web Server</title>

      <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 <directive module="core"
          >ErrorDocument</directive> CGI scripts in Perl, but
          there is also a <module>mod_rewrite</module> solution.
          But note that this performs more poorly than using an
          <directive module="core">ErrorDocument</directive>
          CGI script!</p>
        </dd>

        <dt>Solution:</dt>

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

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

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

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

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

    </section>

   <section id="archive-access-multiplexer">

      <title>Archive Access Multiplexer</title>

      <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>?
          CPAN automatically redirects browsers to one of many FTP
          servers around the world (generally one near the requesting
          client); each server carries a full CPAN mirror. This is
          effectively an FTP access multiplexing service.
          CPAN runs via CGI scripts, but how could a similar approach
          be implemented via <module>mod_rewrite</module>?</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>First we notice that as of version 3.0.0,
          <module>mod_rewrite</module> can
          also use the "<code>ftp:</code>" scheme on redirects.
          And second, the location approximation can be done by a
          <directive module="mod_rewrite">RewriteMap</directive>
          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>

<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></example>

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

    </section>

   <section id="browser-dependent-content">

      <title>Browser Dependent Content</title>

      <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 one version for
          current browsers, a different version for the Lynx and text-mode
          browsers, and another for other browsers.</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 config
          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 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 with the following ruleset:</p>

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

    </section>

    <section id="dynamic-mirror">

      <title>Dynamic Mirror</title>

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

        <dd>
          <p>Assume there are nice web pages 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 web server we could use the program
          <code>webcopy</code> which runs via HTTP. But both
          techniques have a major drawback: The local copy is
          always only as up-to-date as the last time we ran the program. It
          would be much better if the mirror was not a static one we
          have to establish explicitly. Instead we want a dynamic
          mirror with data which gets updated automatically
          as needed on the remote host(s).</p>
        </dd>

        <dt>Solution:</dt>

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

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

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

    </section>

    <section id="reverse-dynamic-mirror">

      <title>Reverse Dynamic Mirror</title>

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

        <dd>...</dd>

        <dt>Solution:</dt>

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

    </section>

    <section id="retrieve-missing-data">

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

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

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

        <dt>Solution:</dt>

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

<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></example>

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

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

    </section>

    <section id="load-balancing">

      <title>Load Balancing</title>

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

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

        <dt>Solution:</dt>

        <dd>
          <p>There are many possible solutions for this problem.
          We will first discuss a common DNS-based method,
          and then one based on <module>mod_rewrite</module>:</p>

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

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

<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></example>

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

<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></example>

              <p>Now when <code>www.example.com</code> gets
              resolved, <code>BIND</code> gives out <code>www0-www5</code>
              - but in a permutated (rotated) order every time.
              This way the clients are spread over the various
              servers. But notice that this is not a perfect load
              balancing scheme, because DNS resolutions are
              cached by clients and other nameservers, so
              once a client has resolved <code>www.example.com</code>
              to a particular <code>wwwN.example.com</code>, all its
              subsequent requests will continue to go to the same
              IP (and thus a single server), rather than being
              distributed across the other available servers. But the
              overall result is
              okay because the requests are collectively
              spread over the various web servers.</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/~riepel/lbnamed/">
              http://www.stanford.edu/~riepel/lbnamed/</a>.
              It is a Perl 5 program which, in conjunction with auxilliary
              tools, provides real load-balancing via
              DNS.</p>
            </li>

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

              <p>In this variant we use <module>mod_rewrite</module>
              and its proxy throughput feature. First we dedicate
              <code>www0.example.com</code> to be actually
              <code>www.example.com</code> by using a single</p>

<example><pre>
www    IN  CNAME   www0.example.com.
</pre></example>

              <p>entry in the DNS. Then we convert
              <code>www0.example.com</code> to a proxy-only server,
              i.e., we configure this machine so all arriving URLs
              are simply passed through its 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>

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

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

<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></example>

              <note>A last notice: Why is this useful? Seems like
              <code>www0.example.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 handled done on the other machines.
              For a complicated site, this may work well. The biggest
              risk here is that www0 is now a single point of failure --
              if it crashes, the other servers are inaccessible.</note>
            </li>

            <li>
              <strong>Dedicated Load Balancers</strong>

              <p>There are more sophisticated solutions, as well. Cisco,
              F5, and several other companies sell hardware load
              balancers (typically used in pairs for redundancy), which
              offer sophisticated load balancing and auto-failover
              features. There are software packages which offer similar
              features on commodity hardware, as well. If you have
              enough money or need, check these out. The <a
              href="http://vegan.net/lb/">lb-l mailing list</a> is a
              good place to research.</p>
            </li>
          </ol>
        </dd>
      </dl>

    </section>

    <section id="new-mime-type">

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

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

        <dd>
          <p>On the net there are many nifty CGI programs. But
          their usage is usually boring, so a lot of webmasters
          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 if we use a Homogeneous URL Layout
          (see above) a file inside the user homedirs might have a URL
          like <code>/u/user/foo/bar.scgi</code>, but
          <code>cgiwrap</code> needs URLs in the form
          <code>/~user/foo/bar.scgi/</code>. The following rule
          solves the problem:</p>

<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></example>

          <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 which area they are really working with.
          But usually this is complicated, because they may still be
          requested by the alternate URL form, i.e., typically we would
          run the <code>swwidx</code> program from within
          <code>/u/user/foo/</code> via hyperlink to</p>

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

          <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, 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>

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

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

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

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

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

          <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>

    </section>

    <section id="on-the-fly-content">

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

      <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 web server if missing. This way you can
          have CGI-generated pages which are statically served unless an
          admin (or a <code>cron</code> job) removes the static contents. Then the
          contents gets refreshed.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          This is done via the following ruleset:

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

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

    </section>

    <section id="autorefresh">

      <title>Document With Autorefresh</title>

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

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

        <dt>Solution:</dt>

        <dd>
          <p>No! We just combine the MIME multipart feature, the
          web server NPH feature, and the URL manipulation power of
          <module>mod_rewrite</module>. First, we establish a new
          URL feature: Adding just <code>:refresh</code> to any
          URL causes the 'page' to be refreshed every time it is
          updated on the filesystem.</p>

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

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

<example><pre>
/u/foo/bar/page.html:refresh
</pre></example>

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

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

          <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>

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

    </section>

    <section id="mass-virtual-hosting">

      <title>Mass Virtual Hosting</title>

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

        <dd>
          <p>The <directive type="section" module="core"
          >VirtualHost</directive> feature of Apache is nice
          and works great when you just have a few dozen
          virtual hosts. But when you are an ISP and have hundreds of
          virtual hosts, this feature is suboptimal.</p>
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>To provide this feature we map the remote web page or even
          the complete remote web area to our namespace using the
          <dfn>Proxy Throughput</dfn> feature (flag <code>[P]</code>):</p>

<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></example>

<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 purposes
RewriteRule   ^/(.*)$   %1/$1  [E=VHOST:${lowercase:%{HTTP_HOST}}]
    :
</pre></example>
        </dd>
      </dl>

    </section>

    <section id="host-deny">

      <title>Host Deny</title>

      <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>

<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></example>

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

<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></example>

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

    </section>

    <section id="proxy-deny">

      <title>Proxy Deny</title>

      <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 <module>mod_rewrite</module>
          is below(!) <module>mod_proxy</module> in the Configuration
          file when compiling the Apache web server. This way it gets
          called <em>before</em> <module>mod_proxy</module>. Then we
          configure the following for a host-dependent deny...</p>

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

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

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

    </section>

    <section id="special-authentication">

      <title>Special Authentication Variant</title>

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

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

        <dt>Solution:</dt>

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

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

    </section>

    <section id="referer-deflector">

      <title>Referer-based Deflector</title>

      <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>

<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></example>

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

<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></example>

          <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>

    </section>

</manualpage>

