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<title>URL Rewriting Guide - Advanced topics - Apache HTTP Server</title>
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<p class="menu"><a href="../mod/index.html">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/index.html">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.2</p>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../index.html">Version 2.2</a> > <a href="./index.html">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>URL Rewriting Guide - Advanced topics</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../../en/rewrite/rewrite_guide_advanced.html" title="English"> en </a></p>
</div>
<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
commonly 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 adjust the examples for your
situation, e.g., adding the <code>[PT]</code> flag if
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 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.</div>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../../images/down.gif" /> <a href="#cluster">Web Cluster with Consistent URL Space</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 Another Web Server</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#archive-access-multiplexer">Archive Access Multiplexer</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#browser-dependent-content">Browser Dependent Content</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#dynamic-mirror">Dynamic Mirror</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#reverse-dynamic-mirror">Reverse Dynamic Mirror</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#retrieve-missing-data">Retrieve Missing Data from Intranet</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#new-mime-type">New MIME-type, New Service</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#autorefresh">Document With Autorefresh</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#mass-virtual-hosting">Mass Virtual Hosting</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#host-deny">Host Deny</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#proxy-deny">Proxy Deny</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#special-authentication">Special Authentication Variant</a></li>
<li><img alt="" src="../../images/down.gif" /> <a href="#referer-deflector">Referer-based Deflector</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">Rewrite Guide - useful
examples</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">Web Cluster with Consistent URL Space</a></h2>
<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>
<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 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>
<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 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>
<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 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>
<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 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>
<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, <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 <code class="directive"><a href="../mod/core.html#documentroot">DocumentRoot</a></code>
of the server to rewrite the public URL path
<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 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>
</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 Another Web Server</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 note 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 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 a 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 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 <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><a name="archive-access-multiplexer" id="archive-access-multiplexer">Archive Access Multiplexer</a></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>?
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 <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>First we notice that as of 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}$2 [R,L]
</pre></div>
<div class="example"><pre>
##
## map.cxan -- Multiplexing Map for CxAN
##
de ftp://ftp.cxan.de/CxAN/
uk ftp://ftp.cxan.uk/CxAN/
com ftp://ftp.cxan.com/CxAN/
:
##EOF##
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="browser-dependent-content" id="browser-dependent-content">Browser Dependent Content</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>At least for important top-level pages it is sometimes
necessary to provide the optimum of browser dependent
content, i.e., one has to provide one version for
current browsers, a different version for the Lynx and text-mode
browsers, and another for other browsers.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We cannot use content negotiation because the browsers do
not provide their type in that form. Instead we have to
act on the HTTP header "User-Agent". The following config
does the following: If the HTTP header "User-Agent"
begins with "Mozilla/3", the page <code>foo.html</code>
is rewritten to <code>foo.NS.html</code> and the
rewriting stops. If the browser is "Lynx" or "Mozilla" of
version 1 or 2, the URL becomes <code>foo.20.html</code>.
All other browsers receive page <code>foo.32.html</code>.
This is done with the following ruleset:</p>
<div class="example"><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>Mozilla/3</strong>.*
RewriteRule ^foo\.html$ 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="dynamic-mirror" id="dynamic-mirror">Dynamic Mirror</a></h2>
<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>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="reverse-dynamic-mirror" id="reverse-dynamic-mirror">Reverse Dynamic Mirror</a></h2>
<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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="retrieve-missing-data" id="retrieve-missing-data">Retrieve Missing Data from Intranet</a></h2>
<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>
<div class="example"><pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port >1024 --> Host www2.quux-corp.dom Port <strong>80</strong>
<strong>DENY</strong> Host * Port * --> 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
<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 <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
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>
<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 entries:</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.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 <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
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>
<div class="example"><pre>
www IN CNAME www0.example.com.
</pre></div>
<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>
<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 (<STDIN>) {
$cnt = (($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.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.</div>
</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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="new-mime-type" id="new-mime-type">New MIME-type, New Service</a></h2>
<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>
<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 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>
<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, 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$3
</pre></div>
<p>Now the hyperlink to search at
<code>/u/user/foo/</code> reads only</p>
<div class="example"><pre>
HREF="*"
</pre></div>
<p>which internally gets automatically transformed to</p>
<div class="example"><pre>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</pre></div>
<p>The same approach leads to an invocation for the
access log CGI program when the hyperlink
<code>:log</code> gets used.</p>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>Here comes a really esoteric feature: Dynamically
generated but statically served pages, i.e., pages should be
delivered as pure static pages (read from the filesystem
and just passed through), but they have to be generated
dynamically by the web server if missing. This way you can
have CGI-generated pages which are statically served unless an
admin (or a <code>cron</code> job) removes the static contents. Then the
contents gets refreshed.</p>
</dd>
<dt>Solution:</dt>
<dd>
This is done via the following ruleset:
<div class="example"><pre>
RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong>
RewriteRule ^page\.<strong>html</strong>$ page.<strong>cgi</strong> [T=application/x-httpd-cgi,L]
</pre></div>
<p>Here a request 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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="autorefresh" id="autorefresh">Document With Autorefresh</a></h2>
<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
<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 the 'page' to be refreshed every time it is
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(/&/, $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 "&lt;b&gt;ERROR&lt;/b&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 "&lt;b&gt;ERROR&lt;/b&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";
&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", $size);
open(FP, "&lt;$file");
$bytes = sysread(FP, $buffer, $size);
close(FP);
return $buffer;
}
$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);
sub mystat {
local($file) = $_[0];
local($time);
($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
return $mtime;
}
$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &lt; $QS_n; $n++) {
while (1) {
$mtime = &mystat($QS_f);
if ($mtime ne $mtimeL) {
$mtimeL = $mtime;
sleep(2);
$buffer = &readfile($QS_f);
&print_http_headers_multipart_next;
&displayhtml($buffer);
sleep(5);
$mtimeL = &mystat($QS_f);
last;
}
sleep($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="mass-virtual-hosting" id="mass-virtual-hosting">Mass Virtual Hosting</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>The <code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code> feature of Apache is nice
and works great when you just have a few dozen
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts, this feature is suboptimal.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>To provide this feature we map the remote web page or even
the complete remote web area to our namespace using the
<dfn>Proxy Throughput</dfn> feature (flag <code>[P]</code>):</p>
<div class="example"><pre>
##
## vhost.map
##
www.vhost1.dom:80 /path/to/docroot/vhost1
www.vhost2.dom:80 /path/to/docroot/vhost2
:
www.vhostN.dom:80 /path/to/docroot/vhostN
</pre></div>
<div class="example"><pre>
##
## apache2.conf
##
:
# use the canonical hostname on redirects, etc.
UseCanonicalName on
:
# add the virtual host in front of the CLF-format
CustomLog /path/to/access_log "%{VHOST}e %h %l %u %t \"%r\" %>s %b"
:
# enable the rewriting engine in the main server
RewriteEngine on
# define two maps: one for fixing the URL and one which defines
# the available virtual hosts with their corresponding
# DocumentRoot.
RewriteMap lowercase int:tolower
RewriteMap vhost txt:/path/to/vhost.map
# Now do the actual virtual host mapping
# via a huge and complicated single rule:
#
# 1. make sure we don't map for common locations
RewriteCond %{REQUEST_URI} !^/commonurl1/.*
RewriteCond %{REQUEST_URI} !^/commonurl2/.*
:
RewriteCond %{REQUEST_URI} !^/commonurlN/.*
#
# 2. make sure we have a Host header, because
# currently our approach only supports
# virtual hosting through this header
RewriteCond %{HTTP_HOST} !^$
#
# 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></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="host-deny" id="host-deny">Host Deny</a></h2>
<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 >= 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 ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^/.* - [F]
</pre></div>
<p>For Apache <= 1.3b6:</p>
<div class="example"><pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteRule ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ /$1
</pre></div>
<div class="example"><pre>
##
## hosts.deny
##
## ATTENTION! This is a map, not a list, even when we treat it as such.
## mod_rewrite parses it for key/value pairs, so at least a
## dummy value "-" must be present for each entry.
##
193.102.180.41 -
bsdti1.sdm.de -
192.76.162.40 -
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="proxy-deny" id="proxy-deny">Proxy Deny</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>How can we forbid a certain host or even a user of a
special host from using the Apache proxy?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We first have to make sure <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
is below(!) <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code> in the Configuration
file when compiling the Apache web server. This way it gets
called <em>before</em> <code class="module"><a href="../mod/mod_proxy.html">mod_proxy</a></code>. Then we
configure the following for a host-dependent deny...</p>
<div class="example"><pre>
RewriteCond %{REMOTE_HOST} <strong>^badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></div>
<p>...and this one for a user@host-dependent deny:</p>
<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>^badguy@badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></div>
</dd>
</dl>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="special-authentication" id="special-authentication">Special Authentication Variant</a></h2>
<dl>
<dt>Description:</dt>
<dd>
<p>Sometimes very special authentication is needed, for
instance authentication which checks for a set of
explicitly configured users. Only these should receive
access and without explicit prompting (which would occur
when using Basic Auth via <code class="module"><a href="../mod/mod_auth_basic.html">mod_auth_basic</a></code>).</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We use a list of rewrite conditions to exclude all except
our friends:</p>
<div class="example"><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend1@client1.quux-corp\.com$</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>
</div><div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif" /></a></div>
<div class="section">
<h2><a name="referer-deflector" id="referer-deflector">Referer-based Deflector</a></h2>
<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>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../../en/rewrite/rewrite_guide_advanced.html" title="English"> en </a></p>
</div><div id="footer">
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