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<title>URL Rewriting Guide</title>
<summary>
<note>
<p>Originally written by<br />
<cite>Ralf S. Engelschall <rse@apache.org></cite><br />
December 1997</p>
</note>
<p>This document supplements the <module>mod_rewrite</module>
It describes how one can use Apache's <module>mod_rewrite</module>
to solve typical URL-based problems webmasters are usually confronted
with in practice. I give detailed descriptions on how to
solve each problem by configuring URL rewriting rulesets.</p>
</summary>
<section id="ToC1">
<title>Introduction to <code>mod_rewrite</code></title>
<p>The Apache module <module>mod_rewrite</module> is a killer
one, i.e. it is a really sophisticated module which provides
a powerful way to do URL manipulations. With it you can nearly
do all types of URL manipulations you ever dreamed about.
The price you have to pay is to accept complexity, because
<module>mod_rewrite</module>'s major drawback is that it is
not easy to understand and use for the beginner. And even
Apache experts sometimes discover new aspects where
<module>mod_rewrite</module> can help.</p>
<p>In other words: With <module>mod_rewrite</module> you either
shoot yourself in the foot the first time and never use it again
or love it for the rest of your life because of its power.
This paper tries to give you a few initial success events to
avoid the first case by presenting already invented solutions
to you.</p>
</section>
<section id="ToC2">
<title>Practical Solutions</title>
<p>Here come a lot of practical solutions I've either invented
myself or collected from other peoples solutions in the past.
Feel free to learn the black magic of URL rewriting from
these examples.</p>
<note type="warning">ATTENTION: Depending on your server-configuration
it can be necessary to slightly change the examples for your
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. It
avoid problems.</note>
</section>
<section id="url">
<title>URL Layout</title>
<section>
<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
<example><pre>
RewriteRule ^/<strong>~</strong>([^/]+)/?(.*) /<strong>u</strong>/$1/$2 [<strong>R</strong>]
RewriteRule ^/([uge])/(<strong>[^/]+</strong>)$ /$1/$2<strong>/</strong> [<strong>R</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Canonical Hostnames</title>
<dl>
<dt>Description:</dt>
<dd>...</dd>
<dt>Solution:</dt>
<dd>
<example><pre>
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/(.*) http://fully.qualified.domain.name/$1 [L,R]
</pre></example>
</dd>
</dl>
</section>
<section>
<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, it is
perhaps just one entity of a lot of data pools. For instance at
the Intranet) etc. Now because the data of the <directive module="core"
to make sure that all inlined images and other stuff inside this
data pool work for subsequent requests.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We just redirect the URL <code>/</code> to
actually trivial with <module>mod_rewrite</module>, only.
Because the typical old mechanisms of URL <em>Aliases</em>
(as provides by <module>mod_alias</module> and friends)
only used <em>prefix</em> matching. With this you cannot
do such a redirection because the <directive module="core"
>DocumentRoot</directive> is a prefix of all URLs. With
<module>mod_rewrite</module> it is really trivial:</p>
<example><pre>
RewriteEngine on
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Trailing Slash Problem</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Every webmaster can sing a song about the problem of
the trailing slash on URLs referencing directories. If they
are missing, the server dumps an error, because if you say
then the server searches for a <em>file</em> named
<code>foo</code>. And because this file is a directory it
complains. Actually it tries to fix it itself in most of
the cases, but sometimes this mechanism need to be emulated
by you. For instance after you have done a lot of
complicated URL rewritings to CGI scripts etc.</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
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>The crazy and lazy can even do the following in the
top-level <code>.htaccess</code> file of their homedir.
But notice that this creates some processing
overhead.</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteCond %{REQUEST_FILENAME} <strong>-d</strong>
RewriteRule ^(.+<strong>[^/]</strong>)$ $1<strong>/</strong> [R]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Webcluster through Homogeneous URL Layout</title>
<dl>
<dt>Description:</dt>
<dd>
<p>We want to create a homogeneous and consistent URL
layout over all WWW servers on a Intranet webcluster, i.e.
all URLs (per definition server local and thus server
dependent!) become actually server <em>independent</em>!
What we want is to give the WWW namespace a consistent
server-independent layout: no URL should have to include
any physically correct target server. The cluster itself
should drive us automatically to the physical target
host.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>First, the knowledge of the target servers come from
(distributed) external maps which contain information
where our users, groups and entities stay. The have the
form</p>
<example><pre>
user1 server_of_user1
user2 server_of_user2
: :
</pre></example>
Second we need to instruct all servers to redirect URLs
of the forms</p>
<example><pre>
</pre></example>
<p>to</p>
<example><pre>
</pre></example>
<p>when the URL is not locally valid to a server. The
following ruleset does this for us by the help of the map
files (assuming that server0 is a default server which
will be used if a user has no entry in the map):</p>
<example><pre>
RewriteEngine on
RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${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
</pre></example>
</dd>
</dl>
</section>
<section>
<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
<example><pre>
RewriteEngine on
RewriteRule ^/~(.+) http://<strong>newserver</strong>/~$1 [R,L]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Structured Homedirs</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Some sites with thousands of users usually use a
structured homedir layout, i.e. each homedir is in a
subdirectory which begins for instance with the first
</dd>
<dt>Solution:</dt>
<dd>
<p>We use the following ruleset to expand the tilde URLs
into exactly the above layout.</p>
<example><pre>
RewriteEngine on
</pre></example>
</dd>
</dl>
</section>
<section>
<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
my archive of freely available Unix software packages,
which I started to collect in 1992. It is both my hobby
and job to to this, because while I'm studying computer
science I have also worked for many years as a system and
network administrator in my spare time. Every week I need
some sort of software so I created a deep hierarchy of
directories where I stored the packages:</p>
<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 wanted to change anything inside this
hierarchy - not even by putting some CGI scripts at the
top of it. Why? Because the above structure should be
later accessible via FTP as well, and I didn't want any
Web or CGI stuff to be there.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>The solution has two parts: The first is a set of CGI
scripts which create all the pages at all directory
levels on-the-fly. I put them under
<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, i.e. the real
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 announced URL
<example><pre>
</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
<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 forth part</li>
<li>Notice the <code>!</code> (not) character and
the <code>C</code> (chain) flag at the first rule
in the last part</li>
<li>Notice the catch-all pattern in the last rule</li>
</ol>
</dd>
</dl>
</section>
<section>
<title>NCSA imagemap to Apache <code>mod_imap</code></title>
<dl>
<dt>Description:</dt>
<dd>
<p>When switching from the NCSA webserver to the more
modern Apache webserver a lot of people want a smooth
transition. So they want pages which use their old NCSA
<code>imagemap</code> program to work under Apache with the
modern <module>mod_imap</module>. The problem is that there
are a lot of hyperlinks around which reference the
<code>imagemap</code> program via
Apache this has to read just
</dd>
<dt>Solution:</dt>
<dd>
<p>We use a global rule to remove the prefix on-the-fly for
all requests:</p>
<example><pre>
RewriteEngine on
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Search pages in more than one directory</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Sometimes it is necessary to let the webserver search
for pages in more than one directory. Here MultiViews or
other techniques cannot help.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We program a explicit ruleset which searches for the
files in the directories.</p>
<example><pre>
RewriteEngine on
# first try to find it in custom/...
# ...and if found stop and be happy:
# second try to find it in pub/...
# ...and if found stop and be happy:
# else go on for other Alias or ScriptAlias directives,
# etc.
RewriteRule ^(.+) - [PT]
</pre></example>
</dd>
</dl>
</section>
<section>
<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
<code>STATUS</code> is set to the value "java".</p>
<example><pre>
RewriteEngine on
RewriteRule ^(.*)/<strong>S=([^/]+)</strong>/(.*) $1/$3 [E=<strong>STATUS:$2</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Virtual User Hosts</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Assume that you want to provide
for the homepage of username via just DNS A records to the
same machine and without any virtualhosts on this
machine.</p>
</dd>
<dt>Solution:</dt>
<dd>
can use the following ruleset to rewrite
<example><pre>
RewriteEngine on
RewriteCond %{<strong>HTTP_HOST</strong>} ^www\.<strong>[^.]+</strong>\.host\.com$
RewriteRule ^(.+) %{HTTP_HOST}$1 [C]
RewriteRule ^www\.<strong>([^.]+)</strong>\.host\.com(.*) /home/<strong>$1</strong>$2
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Redirect Homedirs For Foreigners</title>
<dl>
<dt>Description:</dt>
<dd>
<p>We want to redirect homedir URLs to another webserver
does not stay in the local domain
virtual host contexts.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>Just a rewrite condition:</p>
<example><pre>
RewriteEngine on
RewriteCond %{REMOTE_HOST} <strong>!^.+\.ourdomain\.com$</strong>
RewriteRule ^(/~.+) http://www.somewhere.com/$1 [R,L]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Redirect Failing URLs To Other Webserver</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 notice 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 error safe:</p>
<example><pre>
RewriteEngine on
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 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 a safe way. But it does a performance impact on
the webserver, because for every request there is one
more internal subrequest. So, if your webserver runs on a
powerful CPU, use this one. If it is a slow machine, use
the first approach or better a <directive module="core"
>ErrorDocument</directive> CGI-script.</p>
</dd>
</dl>
</section>
<section>
<title>Extended Redirection</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Sometimes we need more control (concerning the
character escaping mechanism) of URLs on redirects.
Usually the Apache kernels URL escape function also
You cannot use this directly on redirects with
<module>mod_rewrite</module> because the
<code>uri_escape()</code> function of Apache
would also escape the hash character.
How can we redirect to such a URL?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We have to use a kludge by the use of a NPH-CGI script
which does the redirect itself. Because here no escaping
is done (NPH=non-parseable headers). First we introduce a
new URL scheme <code>xredirect:</code> by the following
per-server config-line (should be one of the last rewrite
rules):</p>
<example><pre>
[T=application/x-httpd-cgi,L]
</pre></example>
<p>This forces all URLs prefixed with
<code>xredirect:</code> to be piped through the
just looks like:</p>
<example><pre>
##
## Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved.
##
$| = 1;
$url = $ENV{'PATH_INFO'};
print "Server: $ENV{'SERVER_SOFTWARE'}\n";
print "Location: $url\n";
print "\n";
print "<html>\n";
print "<head>\n";
print "<title>302 Moved Temporarily (EXTENDED)</title>\n";
print "</head>\n";
print "<body>\n";
print "<h1>Moved Temporarily (EXTENDED)</h1>\n";
print "The document has moved <a HREF=\"$url\">here</a>.<p>\n";
print "</body>\n";
print "</html>\n";
##EOF##
</pre></example>
<p>This provides you with the functionality to do
redirects to all URL schemes, i.e. including the one
which are not directly accepted by <module>mod_rewrite</module>.
For instance you can now also redirect to
<code>news:newsgroup</code> via</p>
<example><pre>
RewriteRule ^anyurl xredirect:news:newsgroup
</pre></example>
<note>Notice: You have not to put <code>[R]</code> or
<code>[R,L]</code> to the above rule because the
<code>xredirect:</code> need to be expanded later
by our special "pipe through" rule above.</note>
</dd>
</dl>
</section>
<section>
<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>?
This does a redirect to one of several FTP servers around
the world which carry a CPAN mirror and is approximately
near the location of the requesting client. Actually this
can be called an FTP access multiplexing service. While
CPAN runs via CGI scripts, how can a similar approach
implemented via <module>mod_rewrite</module>?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>First we notice that from 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
RewriteRule ^/CxAN/(.*) %{REMOTE_HOST}::$1 [C]
RewriteRule ^.+\.<strong>([a-zA-Z]+)</strong>::(.*)$ ${multiplex:<strong>$1</strong>|ftp.default.dom}$2 [R,L]
</pre></example>
<example><pre>
##
## map.cxan -- Multiplexing Map for CxAN
##
:
##EOF##
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Time-Dependent Rewriting</title>
<dl>
<dt>Description:</dt>
<dd>
<p>When tricks like time-dependent content should happen a
lot of webmasters still use CGI scripts which do for
instance redirects to specialized pages. How can it be done
via <module>mod_rewrite</module>?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>There are a lot of variables named <code>TIME_xxx</code>
for rewrite conditions. In conjunction with the special
lexicographic comparison patterns <code><STRING</code>,
<code>>STRING</code> and <code>=STRING</code> we can
do time-dependent redirects:</p>
<example><pre>
RewriteEngine on
RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700
RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900
RewriteRule ^foo\.html$ foo.day.html
RewriteRule ^foo\.html$ foo.night.html
</pre></example>
<code>07:00-19:00</code> and at the remaining time the
feature for a homepage...</p>
</dd>
</dl>
</section>
<section>
<title>Backward Compatibility for YYYY to XXXX migration</title>
<dl>
<dt>Description:</dt>
<dd>
<p>How can we make URLs backward compatible (still
bunch of <code>.html</code> files to <code>.phtml</code>?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We just rewrite the name to its basename and test for
existence of the new extension. If it exists, we take
that name, else we rewrite the URL to its original state.</p>
<example><pre>
# backward compatibility ruleset for
# rewriting document.html to document.phtml
# when and only when document.phtml exists
# but no longer document.html
RewriteEngine on
RewriteBase /~quux/
# parse out basename, but remember the fact
RewriteRule ^(.*)\.html$ $1 [C,E=WasHTML:yes]
# rewrite to document.phtml if exists
RewriteCond %{REQUEST_FILENAME}.phtml -f
RewriteRule ^(.*)$ $1.phtml [S=1]
# else reverse the previous basename cutout
RewriteCond %{ENV:WasHTML} ^yes$
RewriteRule ^(.*)$ $1.html
</pre></example>
</dd>
</dl>
</section>
</section>
<section id="content">
<title>Content Handling</title>
<section>
<title>From Old to New (intern)</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Assume we have recently renamed the page
to provide the old URL for backward compatibility. Actually
we want that users of the old URL even not recognize that
the pages was renamed.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We rewrite the old URL to the new one internally via the
following rule:</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html
</pre></example>
</dd>
</dl>
</section>
<section>
<title>From Old to New (extern)</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Assume again that we have recently renamed the page
to provide the old URL for backward compatibility. But this
time we want that the users of the old URL get hinted to
the new one, i.e. their browsers Location field should
change, too.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We force a HTTP redirect to the new URL which leads to a
change of the browsers and thus the users view:</p>
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html [<strong>R</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<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 a maximum version for the
latest Netscape variants, a minimum version for the Lynx
browsers and a average feature version for all others.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We cannot use content negotiation because the browsers do
not provide their type in that form. Instead we have to
act on the HTTP header "User-Agent". The following condig
does the following: If the HTTP header "User-Agent"
rewriting stops. If the browser is "Lynx" or "Mozilla" of
This is done by 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>
<title>Dynamic Mirror</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Assume there are nice webpages on remote hosts we want
to bring into our namespace. For FTP servers we would use
the <code>mirror</code> program which actually maintains an
explicit up-to-date copy of the remote data on the local
machine. For a webserver we could use the program
<code>webcopy</code> which acts similar via HTTP. But both
techniques have one major drawback: The local copy is
always just as up-to-date as often we run the program. It
would be much better if the mirror is not a static one we
have to establish explicitly. Instead we want a dynamic
mirror with data which gets updated automatically when
there is need (updated data on the remote host).</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <dfn>Proxy Throughput</dfn> feature
(flag <code>[P]</code>):</p>
<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>
<title>Reverse Dynamic Mirror</title>
<dl>
<dt>Description:</dt>
<dd>...</dd>
<dt>Solution:</dt>
<dd>
<example><pre>
RewriteEngine on
</pre></example>
</dd>
</dl>
</section>
<section>
<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 webserver
and maintaining its data on a (internal) Intranet webserver
firewall. The trick is that on the external webserver we
retrieve the requested data on-the-fly from the internal
one.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>First, we have to make sure that our firewall still
protects the internal webserver and that only the
external webserver is allowed to retrieve data from it.
For a packet-filtering firewall we could for instance
configure a firewall ruleset like the following:</p>
<example><pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port >1024 --> 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>
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>
<title>Load Balancing</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Suppose we want to load balance the traffic to
(a total of 6 servers). How can this be done?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>There are a lot of possible solutions for this problem.
We will discuss first a commonly known DNS-based variant
and then the special one with <module>mod_rewrite</module>:</p>
<ol>
<li>
<strong>DNS Round-Robin</strong>
<p>The simplest method for load-balancing is to use
the DNS round-robin feature of <code>BIND</code>.
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 entry:</p>
<example><pre>
www IN CNAME www0.foo.com.
IN CNAME www1.foo.com.
IN CNAME www2.foo.com.
IN CNAME www3.foo.com.
IN CNAME www4.foo.com.
IN CNAME www5.foo.com.
IN CNAME www6.foo.com.
</pre></example>
<p>Notice that this seems wrong, but is actually an
intended feature of <code>BIND</code> and can be used
resolved, <code>BIND</code> gives out <code>www0-www6</code>
- but in a slightly permutated/rotated order every time.
This way the clients are spread over the various
servers. But notice that this not a perfect load
balancing scheme, because DNS resolve information
gets cached by the other nameservers on the net, so
subsequent requests also go to this particular name
ok, because the total sum of the requests are really
spread over the various webservers.</p>
</li>
<li>
<strong>DNS Load-Balancing</strong>
<p>A sophisticated DNS-based method for
load-balancing is to use the program
<code>lbnamed</code> which can be found at <a
It is a Perl 5 program in conjunction with auxilliary
tools which provides a real load-balancing for
DNS.</p>
</li>
<li>
<strong>Proxy Throughput Round-Robin</strong>
<p>In this variant we use <module>mod_rewrite</module>
and its proxy throughput feature. First we dedicate
<example><pre>
www IN CNAME www0.foo.com.
</pre></example>
<p>entry in the DNS. Then we convert
i.e. we configure this machine so all arriving URLs
are just pushed through the internal proxy to one of
the 5 other servers (<code>www1-www5</code>). To
accomplish this we first establish a ruleset which
for all URLs.</p>
<example><pre>
RewriteEngine on
RewriteRule ^/(.+)$ ${lb:$1} [P,L]
</pre></example>
<example><pre>
##
## 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/$_";
}
##EOF##
</pre></example>
<note>A last notice: Why is this useful? Seems like
answer is yes, it is overloaded, but with plain proxy
throughput requests, only! All SSI, CGI, ePerl, etc.
processing is completely done on the other machines.
This is the essential point.</note>
</li>
<li>
<p>There is a hardware solution available, too. Cisco
has a beast called LocalDirector which does a load
sort of a circuit level gateway in front of a
webcluster. If you have enough money and really need
a solution with high performance, use this one.</p>
</li>
</ol>
</dd>
</dl>
</section>
<section>
<title>Reverse Proxy</title>
<dl>
<dt>Description:</dt>
<dd>...</dd>
<dt>Solution:</dt>
<dd>
<example><pre>
##
## apache-rproxy.conf -- Apache configuration for Reverse Proxy Usage
##
# server type
ServerType standalone
Listen 8000
MinSpareServers 16
StartServers 16
MaxSpareServers 16
MaxClients 16
MaxRequestsPerChild 100
# server operation parameters
KeepAlive on
MaxKeepAliveRequests 100
KeepAliveTimeout 15
Timeout 400
IdentityCheck off
HostnameLookups off
# paths to runtime files
# unused paths
ServerRoot /tmp
DocumentRoot /tmp
CacheRoot /tmp
# speed up and secure processing
<Directory />
Options -FollowSymLinks -SymLinksIfOwnerMatch
AllowOverride None
</Directory>
# the status page for monitoring the reverse proxy
<Location /apache-rproxy-status>
SetHandler server-status
</Location>
# enable the URL rewriting engine
RewriteEngine on
RewriteLogLevel 0
# define a rewriting map with value-lists where
# mod_rewrite randomly chooses a particular value
# make sure the status page is handled locally
# and make sure no one uses our proxy except ourself
RewriteRule ^/apache-rproxy-status.* - [L]
RewriteRule ^(http|ftp)://.* - [F]
# now choose the possible servers for particular URL types
RewriteRule ^/(.*\.(cgi|shtml))$ to://${server:dynamic}/$1 [S=1]
RewriteRule ^/(.*)$ to://${server:static}/$1
# and delegate the generated URL by passing it
# through the proxy module
RewriteRule ^to://([^/]+)/(.*) http://$1/$2 [E=SERVER:$1,P,L]
# and make really sure all other stuff is forbidden
# when it should survive the above rules...
RewriteRule .* - [F]
# enable the Proxy module without caching
ProxyRequests on
NoCache *
# setup URL reverse mapping for redirect reponses
ProxyPassReverse / http://www1.foo.dom/
ProxyPassReverse / http://www2.foo.dom/
ProxyPassReverse / http://www3.foo.dom/
ProxyPassReverse / http://www4.foo.dom/
ProxyPassReverse / http://www5.foo.dom/
ProxyPassReverse / http://www6.foo.dom/
</pre></example>
<example><pre>
##
##
# list of backend servers which serve static
# pages (HTML files and Images, etc.)
# list of backend servers which serve dynamically
# generated page (CGI programs or mod_perl scripts)
dynamic www5.foo.dom|www6.foo.dom
</pre></example>
</dd>
</dl>
</section>
<section>
<title>New MIME-type, New Service</title>
<dl>
<dt>Description:</dt>
<dd>
<p>On the net there are a lot of nifty CGI programs. But
their usage is usually boring, so a lot of webmaster
don't use them. Even Apache's Action handler feature for
MIME-types is only appropriate when the CGI programs
don't need special URLs (actually <code>PATH_INFO</code>
and <code>QUERY_STRINGS</code>) as their input. First,
let us configure a new file type with extension
<code>.scgi</code> (for secure CGI) which will be processed
by the popular <code>cgiwrap</code> program. The problem
here is that for instance we use a Homogeneous URL Layout
(see above) a file inside the user homedirs has the URL
<code>cgiwrap</code> needs the URL in the form
solves the problem:</p>
<example><pre>
RewriteRule ^/[uge]/<strong>([^/]+)</strong>/\.www/(.+)\.scgi(.*) ...
... /internal/cgi/user/cgiwrap/~<strong>$1</strong>/$2.scgi$3 [NS,<strong>T=application/x-http-cgi</strong>]
</pre></example>
<p>Or assume we have some more nifty programs:
<code>wwwlog</code> (which displays the
<code>wwwidx</code> (which runs Glimpse on a URL
subtree). We have to provide the URL area to these
programs so they know on which area they have to act on.
But usually this ugly, because they are all the times
still requested from that areas, i.e. typically we would
run the <code>swwidx</code> program from within
<example><pre>
</pre></example>
<p>which is ugly. Because we have to hard-code
<strong>both</strong> the location of the area
<strong>and</strong> the location of the CGI inside the
hyperlink. When we have to reorganize the area, we spend a
lot of time changing the various hyperlinks.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>The solution here is to provide a special new URL format
which automatically leads to the proper CGI invocation.
We configure the following:</p>
<example><pre>
</pre></example>
<p>Now the hyperlink to search at
<example><pre>
HREF="*"
</pre></example>
<p>which internally gets automatically transformed to</p>
<example><pre>
</pre></example>
<p>The same approach leads to an invocation for the
access log CGI program when the hyperlink
<code>:log</code> gets used.</p>
</dd>
</dl>
</section>
<section>
<title>From Static to Dynamic</title>
<dl>
<dt>Description:</dt>
<dd>
<p>How can we transform a static page
</dd>
<dt>Solution:</dt>
<dd>
<p>We just rewrite the URL to the CGI-script and force the
correct MIME-type so it gets really run as a CGI-script.
internally leads to the invocation of
<example><pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo\.<strong>html</strong>$ foo.<strong>cgi</strong> [T=<strong>application/x-httpd-cgi</strong>]
</pre></example>
</dd>
</dl>
</section>
<section>
<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 webserver if missing. This way you can
have CGI-generated pages which are statically served unless
one (or a cronjob) removes the static contents. Then the
contents gets refreshed.</p>
</dd>
<dt>Solution:</dt>
<dd>
This is done via the following ruleset:
<example><pre>
RewriteCond %{REQUEST_FILENAME} <strong>!-s</strong>
</pre></example>
usual CGI script which (additionally to its <code>STDOUT</code>)
Once it was run, the server sends out the data of
a refresh the contents, he just removes
</dd>
</dl>
</section>
<section>
<title>Document With Autorefresh</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Wouldn't it be nice while creating a complex webpage if
the webbrowser would automatically refresh the page every
time we write a new version from within our editor?
Impossible?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>No! We just combine the MIME multipart feature, the
webserver NPH feature and the URL manipulation power of
<module>mod_rewrite</module>. First, we establish a new
URL feature: Adding just <code>:refresh</code> to any
URL causes this to be refreshed every time it gets
updated on the filesystem.</p>
<example><pre>
</pre></example>
<p>Now when we reference the URL</p>
<example><pre>
</pre></example>
<p>this leads to the internal invocation of the URL</p>
<example><pre>
</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>
##
## 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 = '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 "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
exit(0);
}
if (! -f $QS_f) {
print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
exit(0);
}
sub print_http_headers_multipart_begin {
$bound = "ThisRandomString12345";
print "Content-type: multipart/x-mixed-replace;boundary=$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-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($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Mass Virtual Hosting</title>
<dl>
<dt>Description:</dt>
<dd>
<p>The <directive type="section" module="core"
>VirtualHost</directive> feature of Apache is nice
and works great when you just have a few dozens
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts to provide this feature is not the best
choice.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <dfn>Proxy Throughput</dfn> feature (flag <code>[P]</code>):</p>
<example><pre>
##
## vhost.map
##
:
</pre></example>
<example><pre>
##
## httpd.conf
##
:
# use the canonical hostname on redirects, etc.
UseCanonicalName on
:
# add the virtual host in front of the CLF-format
:
# enable the rewriting engine in the main server
RewriteEngine on
# define two maps: one for fixing the URL and one which defines
# the available virtual hosts with their corresponding
# DocumentRoot.
RewriteMap lowercase int:tolower
# Now do the actual virtual host mapping
# via a huge and complicated single rule:
#
# 1. make sure we don't map for common locations
RewriteCond %{REQUEST_URL} !^/commonurl1/.*
RewriteCond %{REQUEST_URL} !^/commonurl2/.*
:
RewriteCond %{REQUEST_URL} !^/commonurlN/.*
#
# 2. make sure we have a Host header, because
# currently our approach only supports
# virtual hosting through this header
RewriteCond %{HTTP_HOST} !^$
#
# 3. lowercase the hostname
RewriteCond ${lowercase:%{HTTP_HOST}|NONE} ^(.+)$
#
# 4. lookup this hostname in vhost.map and
# remember it only when it is a path
# (and not "NONE" from above)
RewriteCond ${vhost:%1} ^(/.*)$
#
# 5. finally we can map the URL to its docroot location
# and remember the virtual host for logging puposes
RewriteRule ^/(.*)$ %1/$1 [E=VHOST:${lowercase:%{HTTP_HOST}}]
:
</pre></example>
</dd>
</dl>
</section>
</section>
<section id="access">
<title>Access Restriction</title>
<section>
<title>Blocking of Robots</title>
<dl>
<dt>Description:</dt>
<dd>
<p>How can we block a really annoying robot from
retrieving pages of a specific webarea? A
"Robot Exclusion Protocol" is typically not enough to get
rid of such a robot.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We use a ruleset which forbids the URLs of the webarea
directory indexed area where the robot traversal would
create big server load). We have to make sure that we
forbid access only to the particular robot, i.e. just
forbidding the host where the robot runs is not enough.
This would block users from this host, too. We accomplish
this by also matching the User-Agent HTTP header
information.</p>
<example><pre>
RewriteCond %{HTTP_USER_AGENT} ^<strong>NameOfBadRobot</strong>.*
RewriteCond %{REMOTE_ADDR} ^<strong>123\.45\.67\.[8-9]</strong>$
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Blocked Inline-Images</title>
<dl>
<dt>Description:</dt>
<dd>
some pages with inlined GIF graphics. These graphics are
nice, so others directly incorporate them via hyperlinks to
their pages. We don't like this practice because it adds
useless traffic to our server.</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>While we cannot 100% protect the images from inclusion,
we can at least restrict the cases where the browser
sends a HTTP Referer header.</p>
<example><pre>
RewriteCond %{HTTP_REFERER} <strong>!^$</strong>
RewriteCond %{HTTP_REFERER} !^http://www.quux-corp.de/~quux/.*$ [NC]
RewriteRule <strong>.*\.gif$</strong> - [F]
</pre></example>
<example><pre>
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !.*/foo-with-gif\.html$
RewriteRule <strong>^inlined-in-foo\.gif$</strong> - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<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 >= 1.3b6:</p>
<example><pre>
RewriteEngine on
RewriteCond ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND} !=NOT-FOUND [OR]
RewriteCond ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^/.* - [F]
</pre></example>
<p>For Apache <= 1.3b6:</p>
<example><pre>
RewriteEngine on
RewriteRule ^/(.*)$ ${hosts-deny:%{REMOTE_HOST}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND}/$1
RewriteRule !^NOT-FOUND/.* - [F]
RewriteRule ^NOT-FOUND/(.*)$ /$1
</pre></example>
<example><pre>
##
## hosts.deny
##
## ATTENTION! This is a map, not a list, even when we treat it as such.
## dummy value "-" must be present for each entry.
##
193.102.180.41 -
192.76.162.40 -
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Proxy Deny</title>
<dl>
<dt>Description:</dt>
<dd>
<p>How can we forbid a certain host or even a user of a
special host from using the Apache proxy?</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We first have to make sure <module>mod_rewrite</module>
is below(!) <module>mod_proxy</module> in the Configuration
file when compiling the Apache webserver. This way it gets
called <em>before</em> <module>mod_proxy</module>. Then we
configure the following for a host-dependent deny...</p>
<example><pre>
RewriteCond %{REMOTE_HOST} <strong>^badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></example>
<example><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>^badguy@badhost\.mydomain\.com$</strong>
RewriteRule !^http://[^/.]\.mydomain.com.* - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<title>Special Authentication Variant</title>
<dl>
<dt>Description:</dt>
<dd>
<p>Sometimes a very special authentication is needed, for
instance a authentication which checks for a set of
explicitly configured users. Only these should receive
access and without explicit prompting (which would occur
when using the Basic Auth via <module>mod_auth_basic</module>).</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>We use a list of rewrite conditions to exclude all except
our friends:</p>
<example><pre>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST} <strong>!^friend1@client1.quux-corp\.com$</strong>
RewriteRule ^/~quux/only-for-friends/ - [F]
</pre></example>
</dd>
</dl>
</section>
<section>
<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>
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}} ^-$
RewriteRule ^.* %{HTTP_REFERER} [R,L]
RewriteCond %{HTTP_REFERER} !=""
RewriteCond ${deflector:%{HTTP_REFERER}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^.* ${deflector:%{HTTP_REFERER}} [R,L]
</pre></example>
<p>... in conjunction with a corresponding rewrite
map:</p>
<example><pre>
##
##
</pre></example>
<p>This automatically redirects the request back to the
referring page (when "<code>-</code>" is used as the value
in the map) or to a specific URL (when an URL is specified
in the map as the second argument).</p>
</dd>
</dl>
</section>
</section>
<section id="other">
<title>Other</title>
<section>
<title>External Rewriting Engine</title>
<dl>
<dt>Description:</dt>
<dd>
problem? There seems no solution by the use of
<module>mod_rewrite</module>...</p>
</dd>
<dt>Solution:</dt>
<dd>
<p>Use an external <directive module="mod_rewrite"
>RewriteMap</directive>, i.e. a program which acts
like a <directive module="mod_rewrite"
>RewriteMap</directive>. It is run once on startup of Apache
receives the requested URLs on <code>STDIN</code> and has
to put the resulting (usually rewritten) URL on
<code>STDOUT</code> (same order!).</p>
<example><pre>
RewriteEngine on
RewriteRule ^/~quux/(.*)$ /~quux/<strong>${quux-map:$1}</strong>
</pre></example>
<example><pre>
# 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 (<>) {
s|^foo/|bar/|;
print $_;
}
</pre></example>
<p>This is a demonstration-only example and just rewrites
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>
</manualpage>