path: root/docs/manual/rewrite/rewrite_guide.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
<!-- $LastChangedRevision$ -->

<!--
 Licensed to the Apache Software Foundation (ASF) under one or more
 contributor license agreements.  See the NOTICE file distributed with
 this work for additional information regarding copyright ownership.
 The ASF licenses this file to You under the Apache License, Version 2.0
 (the "License"); you may not use this file except in compliance with
 the License.  You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

 Unless required by applicable law or agreed to in writing, software
 distributed under the License is distributed on an "AS IS" BASIS,
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
-->

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

  <title>URL Rewriting Guide</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 slightly change the examples for your
    situation, e.g. adding the <code>[PT]</code> flag when
    additionally using <module>mod_alias</module> and
    <module>mod_userdir</module>, 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.</note>

  </summary>
<seealso><a href="../mod/mod_rewrite.html">Module
documentation</a></seealso>
<seealso><a href="intro.html">mod_rewrite
introduction</a></seealso>
<seealso><a href="rewrite_guide_advanced.html">Advanced Rewrite Guide - advanced 
useful examples</a></seealso>
<seealso><a href="tech.html">Technical details</a></seealso>


<section id="canonicalurl">

<title>Canonical URLs</title>

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

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

   <dt>Solution:</dt>

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

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

    </section>

    <section id="moveddocroot">

      <title>Moved <code>DocumentRoot</code></title>

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

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

        <dt>Solution:</dt>

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

    <p>Note that this can also be handled using the <directive
    module="mod_alias">RedirectMatch</directive> directive:</p>

<example>
RedirectMatch ^/$ http://example.com/about/
</example>

<p>Note also that the example rewrites only the root URL. That is, it
rewrites a request for <code>http://example.com/</code>, but not a
request for <code>http://example.com/page.html</code>. If you have in 
fact changed your document root - that is, if <strong>all</strong> of 
your content is in fact in that subdirectory, it is greatly preferable 
to simply change your <directive module="core">DocumentRoot</directive>
directive, rather than rewriting URLs.</p>
</dd>
</dl>

    </section>

    <section id="trailingslash">

      <title>Trailing Slash Problem</title>

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

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

        <dt>Solution:</dt>

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

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

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

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

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

    </section>

    <section id="movehomedirs">

      <title>Move Homedirs to Different Webserver</title>

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

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

        <dt>Solution:</dt>

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

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

    </section>

    <section id="setenvvars">

      <title>Set Environment Variables According To URL Parts</title>

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

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

        <dt>Solution:</dt>

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

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

    </section>

    <section id="uservhosts">

      <title>Virtual Hosts Per User</title>

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

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

        <dt>Solution:</dt>

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

<example><pre>
RewriteEngine on
RewriteCond   %{<strong>HTTP_HOST</strong>}                 ^www\.<strong>([^.]+)</strong>\.host\.com$
RewriteRule   ^(.*) /home/<strong>%1</strong>$1
</pre></example>

<p>Parentheses used in a <directive
module="mod_rewrite">RewriteCond</directive> are captured into the
backreferences <code>%1</code>, <code>%2</code>, etc, while parentheses
used in <directive module="mod_rewrite">RewriteRule</directive> are
captured into the backreferences <code>$1</code>, <code>$2</code>,
etc.</p>
        </dd>
      </dl>

    </section>

    <section id="redirecthome">

      <title>Redirect Homedirs For Foreigners</title>

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

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

        <dt>Solution:</dt>

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

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

    </section>

    <section id="redirectanchors">

      <title>Redirecting Anchors</title>

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

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

        <dt>Solution:</dt>

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

    </section>

    <section id="time-dependent">

      <title>Time-Dependent Rewriting</title>

      <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 <module>mod_rewrite</module>?</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>

<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></example>

          <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>

    </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 webserver. 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="external-rewriting">

      <title>External Rewriting Engine</title>

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

        <dd>
          <p>A FAQ: How can we solve the FOO/BAR/QUUX/etc.
          problem? There seems no solution by the use of
          <module>mod_rewrite</module>...</p>
        </dd>

        <dt>Solution:</dt>

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

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

<example><pre>
#!/path/to/perl

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

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

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

    </section>

    <section id="cluster">

      <title>URL-based sharding accross multiple backends</title>

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

        <dd>
          <p>A common technique for distributing the burden of 
          server load or storage space is called "sharding". 
          When using this method, a front-end server will use the
          url to consistently "shard" users or objects to seperate
          backend servers.
        </dd>

        <dt>Solution:</dt>

        <dd>
          <p>A mapping is maintained, from users to target servers, in
          external maps  files. They look like:</p>

<example><pre>
user1  physical_host_of_user1
user2  physical_host_of_user2
:      :
</pre></example>

          <p>We put this into  a <code>map.users-to-hosts</code> file. The
            aim is the map;</p>

<example><pre>
/u/user1/anypath
</pre></example>

          <p>to</p>

<example><pre>
http://physical_host_of_user1/u/user/anypath
</pre></example>

          <p>thus every URL path need not be valid on every backend physical
          host. 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      users-to-hosts   txt:/path/to/map.users-to-hosts

RewriteRule   ^/u/<strong>([^/]+)</strong>/?(.*)   http://<strong>${users-to-hosts:$1|server0}</strong>/u/$1/$2
</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="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 [C]
# REQUEST_FILENAME usage below is correct in this per-server context example 
# because the rule that references REQUEST_FILENAME is chained to a rule that
# sets REQUEST_FILENAME. 
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 auxiliary
              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>
# This example is valid in per-directory context only
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>