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  Apache HTTP Server Version 1.3
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<H1>
Apache 1.3<BR>
URL Rewriting Guide<BR>
</H1>

<ADDRESS>Originally written by<BR>
Ralf S. Engelschall &lt;rse@apache.org&gt<BR>
December 1997</ADDRESS>

</DIV>

<P>
This document supplements the mod_rewrite <A
HREF="../mod/mod_rewrite.html">reference documentation</A>. It describes
how one can use Apache's mod_rewrite 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.

<H2><A name="ToC1">Introduction to mod_rewrite</A></H2>

The Apache module mod_rewrite 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
mod_rewrite'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
mod_rewrite can help.
<P>
In other words: With mod_rewrite 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.

<H2><A name="ToC2">Practical Solutions</A></H2>

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>
<TABLE BGCOLOR="#FFE0E0" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD>
ATTENTION: Depending on your server-configuration it can be necessary to
slightly change the examples for your situation, e.g. adding the [PT] flag
when additionally using mod_alias and mod_userdir, 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.
</TD></TR></TABLE>

<H1>URL Layout</H1>

<P>
<H2>Canonical URLs</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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.  Independed which URL the user
supplied with the request he should finally see the canonical one only.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule   ^/<STRONG>~</STRONG>([^/]+)/?(.*)    /<STRONG>u</STRONG>/$1/$2  [<STRONG>R</STRONG>]
RewriteRule   ^/([uge])/(<STRONG>[^/]+</STRONG>)$  /$1/$2<STRONG>/</STRONG>   [<STRONG>R</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Canonical Hostnames</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
...

<P>
<DT><STRONG>Solution:</STRONG>
<DD>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Moved DocumentRoot</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Usually the DocumentRoot 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 our Intranet
sites there are <CODE>/e/www/</CODE> (the homepage for WWW), <CODE>/e/sww/</CODE> (the
homepage for the Intranet) etc. Now because the data of the DocumentRoot stays
at <CODE>/e/www/</CODE> we had to make sure that all inlined images and other
stuff inside this data pool work for subsequent requests. 

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We just redirect the URL <CODE>/</CODE> to <CODE>/e/www/</CODE>.  While is seems
trivial it is actually trivial with mod_rewrite, only.  Because the typical
old mechanisms of URL <EM>Aliases</EM> (as provides by mod_alias and friends)
only used <EM>prefix</EM> matching. With this you cannot do such a redirection
because the DocumentRoot is a prefix of all URLs. With mod_rewrite it is
really trivial:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteRule   <STRONG>^/$</STRONG>  /e/www/  [<STRONG>R</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Trailing Slash Problem</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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 <CODE>/~quux/foo</CODE> instead of
<CODE>/~quux/foo/</CODE> then the server searches for a <EM>file</EM> named
<CODE>foo</CODE>. And because this file is a directory it complains. Actually
is tries to fix it themself 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>
<DT><STRONG>Solution:</STRONG>
<DD>
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>
So, to do this trick we write:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^foo<STRONG>$</STRONG>  foo<STRONG>/</STRONG>  [<STRONG>R</STRONG>]
</PRE></TD></TR></TABLE>

<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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteCond    %{REQUEST_FILENAME}  <STRONG>-d</STRONG>
RewriteRule    ^(.+<STRONG>[^/]</STRONG>)$           $1<STRONG>/</STRONG>  [R]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Webcluster through Homogeneous URL Layout</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
We want to create a homogenous 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>independed</EM>!  What we want is
to give the WWW namespace a consistent server-independend 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>
<DT><STRONG>Solution:</STRONG>
<DD>
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><PRE>
user1  server_of_user1
user2  server_of_user2
:      :
</PRE><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><PRE>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</PRE><P>

to

<P><PRE>
http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath
</PRE><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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Move Homedirs to Different Webserver</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
A lot of webmaster aksed 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>
<DT><STRONG>Solution:</STRONG>
<DD>
The solution is trivial with mod_rewrite. On the old webserver we just
redirect all <CODE>/~user/anypath</CODE> URLs to
<CODE>http://newserver/~user/anypath</CODE>.

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteRule   ^/~(.+)  http://<STRONG>newserver</STRONG>/~$1  [R,L]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Structured Homedirs</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Some sites with thousend of users usually use a structured homedir layout,
i.e.  each homedir is in a subdirectory which begins for instance with the
first character of the username. So, <CODE>/~foo/anypath</CODE> is
<CODE>/home/<STRONG>f</STRONG>/foo/.www/anypath</CODE> while <CODE>/~bar/anypath</CODE> is
<CODE>/home/<STRONG>b</STRONG>/bar/.www/anypath</CODE>.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We use the following ruleset to expand the tilde URLs into exactly the above
layout.

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteRule   ^/~(<STRONG>([a-z])</STRONG>[a-z0-9]+)(.*)  /home/<STRONG>$2</STRONG>/$1/.www$3
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Filesystem Reorganisation</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
This really is a hardcore example: a killer application which heavily uses
per-directory <CODE>RewriteRules</CODE> to get a smooth look and feel on the Web
while its data structure is never touched or adjusted.

Background: <STRONG><EM>net.sw</EM></STRONG> is my archive of freely available Unix
software packages, which I started to collect in 1992. It is both my hobby and
job to to this, because while I'm studying computer science I have also worked
for many years as a system and network administrator in my spare time. Every
week I need some sort of software so I created a deep hierarchy of
directories where I stored the packages: 

<P><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><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>
<DT><STRONG>Solution:</STRONG>
<DD>
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><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><P>

The <CODE>DATA/</CODE> subdirectory holds the above directory structure, i.e.  the
real <STRONG><EM>net.sw</EM></STRONG> stuff and gets automatically updated via
<CODE>rdist</CODE> from time to time. 

The second part of the problem remains: how to link these two structures
together into one smooth-looking URL tree? We want to hide the <CODE>DATA/</CODE>
directory from the user while running the appropriate CGI scripts for the
various URLs. 

Here is the solution: first I put the following into the per-directory
configuration file in the Document Root of the server to rewrite the announced
URL <CODE>/net.sw/</CODE> to the internal path <CODE>/e/netsw</CODE>:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule  ^net.sw$       net.sw/        [R]
RewriteRule  ^net.sw/(.*)$  e/netsw/$1
</PRE></TD></TR></TABLE>

<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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>
Some hints for interpretation:
    <ol>
    <li> Notice the L (last) flag and no substitution field ('-') in the
         forth part
    <li> Notice the ! (not) character and the C (chain) flag
         at the first rule in the last part
    <li> Notice the catch-all pattern in the last rule
    </ol>

</DL>

<P>
<H2>NCSA imagemap to Apache mod_imap</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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
<CODE>mod_imap</CODE>. The problem is that there are a lot of
hyperlinks around which reference the <CODE>imagemap</CODE> program via
<CODE>/cgi-bin/imagemap/path/to/page.map</CODE>. Under Apache this
has to read just <CODE>/path/to/page.map</CODE>.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We use a global rule to remove the prefix on-the-fly for all requests:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteRule    ^/cgi-bin/imagemap(.*)  $1  [PT]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Search pages in more than one directory</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Sometimes it is neccessary to let the webserver search for pages in more than
one directory. Here MultiViews or other techniques cannot help.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We program a explicit ruleset which searches for the files in the directories.

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on

#   first try to find it in custom/...
#   ...and if found stop and be happy:
RewriteCond         /your/docroot/<STRONG>dir1</STRONG>/%{REQUEST_FILENAME}  -f
RewriteRule  ^(.+)  /your/docroot/<STRONG>dir1</STRONG>/$1  [L]

#   second try to find it in pub/...
#   ...and if found stop and be happy:
RewriteCond         /your/docroot/<STRONG>dir2</STRONG>/%{REQUEST_FILENAME}  -f
RewriteRule  ^(.+)  /your/docroot/<STRONG>dir2</STRONG>/$1  [L]

#   else go on for other Alias or ScriptAlias directives,
#   etc.
RewriteRule   ^(.+)  -  [PT]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Set Environment Variables According To URL Parts</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteRule   ^(.*)/<STRONG>S=([^/]+)</STRONG>/(.*)    $1/$3 [E=<STRONG>STATUS:$2</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Virtual User Hosts</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Redirect Homedirs For Foreigners</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
Just a rewrite condition:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteCond   %{REMOTE_HOST}  <STRONG>!^.+\.ourdomain\.com$</STRONG>
RewriteRule   ^(/~.+)         http://www.somewhere.com/$1 [R,L]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Redirect Failing URLs To Other Webserver</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
A typical FAQ about URL rewriting is how to redirect failing requests on
webserver A to webserver B.  Usually this is done via ErrorDocument
CGI-scripts in Perl, but there is also a mod_rewrite solution. But notice that
this is less performant than using a ErrorDocument CGI-script!

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
The first solution has the best performance but less flexibility and is less
error safe:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteCond   /your/docroot/%{REQUEST_FILENAME} <STRONG>!-f</STRONG>
RewriteRule   ^(.+)                             http://<STRONG>webserverB</STRONG>.dom/$1
</PRE></TD></TR></TABLE>

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

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteCond   %{REQUEST_URI} <STRONG>!-U</STRONG>
RewriteRule   ^(.+)          http://<STRONG>webserverB</STRONG>.dom/$1
</PRE></TD></TR></TABLE>

<P>
This uses the URL look-ahead feature of mod_rewrite. 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 ErrorDocument
CGI-script.

</DL>

<P>
<H2>Extended Redirection</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Sometimes we need more control (concerning the character escaping mechanism)
of URLs on redirects. Usually the Apache kernels URL escape function also
escapes anchors, i.e. URLs like "url#anchor". You cannot use this directly on
redirects with mod_rewrite because the uri_escape() function of Apache would
also escape the hash character. How can we redirect to such a URL?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule ^xredirect:(.+) /path/to/nph-xredirect.cgi/$1 \
            [T=application/x-httpd-cgi,L]
</PRE></TD></TR></TABLE>

<P>
This forces all URLs prefixed with <CODE>xredirect:</CODE> to be piped through the
<CODE>nph-xredirect.cgi</CODE> program. And this program just looks like:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
<PRE>
#!/path/to/perl
##
##  nph-xredirect.cgi -- NPH/CGI script for extended redirects
##  Copyright (c) 1997 Ralf S. Engelschall, All Rights Reserved. 
##

$| = 1;
$url = $ENV{'PATH_INFO'};

print "HTTP/1.0 302 Moved Temporarily\n";
print "Server: $ENV{'SERVER_SOFTWARE'}\n";
print "Location: $url\n";
print "Content-type: text/html\n";
print "\n";
print "&lt;html&gt;\n";
print "&lt;head&gt;\n";
print "&lt;title&gt;302 Moved Temporarily (EXTENDED)&lt;/title&gt;\n";
print "&lt;/head&gt;\n";
print "&lt;body&gt;\n";
print "&lt;h1&gt;Moved Temporarily (EXTENDED)&lt;/h1&gt;\n";
print "The document has moved &lt;a HREF=\"$url\"&gt;here&lt;/a&gt;.&lt;p&gt;\n";
print "&lt;/body&gt;\n";
print "&lt;/html&gt;\n";

##EOF##
</PRE>
</PRE></TD></TR></TABLE>

<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 mod_rewrite. For
instance you can now also redirect to <CODE>news:newsgroup</CODE> via

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule ^anyurl  xredirect:news:newsgroup
</PRE></TD></TR></TABLE>

<P>
Notice: You have not to put [R] or [R,L] to the above rule because the
<CODE>xredirect:</CODE> need to be expanded later by our special "pipe through"
rule above.

</DL>

<P>
<H2>Archive Access Multiplexer</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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 mod_rewrite?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
First we notice that from version 3.0.0 mod_rewrite can also use the "ftp:"
scheme on redirects. And second, the location approximation can be done by a
rewritemap 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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Time-Dependend Rewriting</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
When tricks like time-dependend 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 mod_rewrite?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
There are a lot of variables named <CODE>TIME_xxx</CODE> for rewrite conditions.
In conjunction with the special lexicographic comparison patterns &lt;STRING,
&gt;STRING and =STRING we can do time-dependend redirects:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>
This provides the content of <CODE>foo.day.html</CODE> under the URL
<CODE>foo.html</CODE> from 07:00-19:00 and at the remaining time the contents of
<CODE>foo.night.html</CODE>. Just a nice feature for a homepage...

</DL>

<P>
<H2>Backward Compatibility for YYYY to XXXX migration</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
How can we make URLs backward compatible (still existing virtually) after
migrating document.YYYY to document.XXXX, e.g. after translating a bunch of
.html files to .phtml?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<H1>Content Handling</H1>

<P>
<H2>From Old to New (intern)</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Assume we have recently renamed the page <CODE>bar.html</CODE> to
<CODE>foo.html</CODE> and now want 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>
<DT><STRONG>Solution:</STRONG>
<DD>
We rewrite the old URL to the new one internally via the following rule:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<STRONG>foo</STRONG>\.html$  <STRONG>bar</STRONG>.html
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>From Old to New (extern)</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Assume again that we have recently renamed the page <CODE>bar.html</CODE> to
<CODE>foo.html</CODE> and now want 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>
<DT><STRONG>Solution:</STRONG>
<DD>
We force a HTTP redirect to the new URL which leads to a change of the
browsers and thus the users view:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<STRONG>foo</STRONG>\.html$  <STRONG>bar</STRONG>.html  [<STRONG>R</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Browser Dependend Content</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
At least for important top-level pages it is sometimes necesarry to provide
the optimum of browser dependend 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>
<DT><STRONG>Solution:</STRONG>
<DD>
We cannot use content negotiation because the browsers do not provide their
type in that form. Instead we have to act on the HTTP header "User-Agent".
The following condig does the following: If the HTTP header "User-Agent"
begins with "Mozilla/3", the page <CODE>foo.html</CODE> is rewritten to
<CODE>foo.NS.html</CODE> and and the rewriting stops.  If the browser is "Lynx" or
"Mozilla" of version 1 or 2 the URL becomes <CODE>foo.20.html</CODE>.  All other
browsers receive page <CODE>foo.32.html</CODE>. This is done by the following
ruleset:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Dynamic Mirror</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
To provide this feature we map the remote webpage or even the complete remote
webarea to our namespace by the use of the <I>Proxy Throughput</I> feature
(flag [P]):

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^<STRONG>hotsheet/</STRONG>(.*)$  <STRONG>http://www.tstimpreso.com/hotsheet/</STRONG>$1  [<STRONG>P</STRONG>]
</PRE></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Reverse Dynamic Mirror</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
...

<P>
<DT><STRONG>Solution:</STRONG>
<DD>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteCond   /mirror/of/remotesite/$1           -U 
RewriteRule   ^http://www\.remotesite\.com/(.*)$ /mirror/of/remotesite/$1
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Retrieve Missing Data from Intranet</H2>
<P>

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

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

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

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Load Balancing</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Suppose we want to load balance the traffic to <CODE>www.foo.com</CODE> over
<CODE>www[0-5].foo.com</CODE> (a total of 6 servers). How can this be done?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
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 mod_rewrite:

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

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

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>
Then you additionally add the following entry:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>
Notice that this seems wrong, but is actually an intended feature of BIND and
can be used in this way. However, now when <CODE>www.foo.com</CODE> gets resolved,
BIND 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 once a client
has resolved <CODE>www.foo.com</CODE> to a particular <CODE>wwwN.foo.com</CODE>, all
subsequent requests also go to this particular name <CODE>wwwN.foo.com</CODE>. But
the final result is ok, because the total sum of the requests are really
spread over the various webservers.

<P>
<li><STRONG>DNS Load-Balancing</STRONG>

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

<P>
<li><STRONG>Proxy Throughput Round-Robin</STRONG>

<P>
In this variant we use mod_rewrite and its proxy throughput feature.  First we
dedicate <CODE>www0.foo.com</CODE> to be actually <CODE>www.foo.com</CODE> by using a
single

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
www    IN  CNAME   www0.foo.com.
</PRE></TD></TR></TABLE>

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

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteMap    lb      prg:/path/to/lb.pl
RewriteRule   ^/(.+)$ ${lb:$1}           [P,L]
</PRE></TD></TR></TABLE>

<P>
Then we write <CODE>lb.pl</CODE>:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

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

<P>
<li><STRONG>Hardware/TCP Round-Robin</STRONG>

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

</ol>

</DL>

<P>
<H2>Reverse Proxy</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
...

<P>
<DT><STRONG>Solution:</STRONG>
<DD>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
##
##  apache-rproxy.conf -- Apache configuration for Reverse Proxy Usage
##

#   server type
ServerType           standalone
Port                 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
PidFile              /path/to/apache-rproxy.pid
LockFile             /path/to/apache-rproxy.lock
ErrorLog             /path/to/apache-rproxy.elog
CustomLog            /path/to/apache-rproxy.dlog "%{%v/%T}t %h -&gt; %{SERVER}e URL: %U"

#   unused paths
ServerRoot           /tmp
DocumentRoot         /tmp
CacheRoot            /tmp
RewriteLog           /dev/null
TransferLog          /dev/null
TypesConfig          /dev/null
AccessConfig         /dev/null
ResourceConfig       /dev/null

#   speed up and secure processing
&lt;Directory /&gt;
Options -FollowSymLinks -SymLinksIfOwnerMatch
AllowOverwrite None
&lt;/Directory&gt;

#   the status page for monitoring the reverse proxy
&lt;Location /rproxy-status&gt;
SetHandler server-status
&lt;/Location&gt;

#   enable the URL rewriting engine
RewriteEngine        on
RewriteLogLevel      0

#   define a rewriting map with value-lists where
#   mod_rewrite randomly chooses a particular value
RewriteMap     server  rnd:/path/to/apache-rproxy.conf-servers

#   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></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
##
##  apache-rproxy.conf-servers -- Apache/mod_rewrite selection table
##

#   list of backend servers which serve static
#   pages (HTML files and Images, etc.)
static    www1.foo.dom|www2.foo.dom|www3.foo.dom|www4.foo.dom

#   list of backend servers which serve dynamically 
#   generated page (CGI programs or mod_perl scripts)
dynamic   www5.foo.dom|www6.foo.dom
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>New MIME-type, New Service</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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 PATH_INFO and QUERY_STRINGS) as their input. 

First, let us configure a new file type with extension <CODE>.scgi</CODE>
(for secure CGI) which will be processed by the popular <CODE>cgiwrap</CODE>
program. The problem here is that for instance we use a Homogeneous URL Layout
(see above) a file inside the user homedirs has the URL
<CODE>/u/user/foo/bar.scgi</CODE>. But <CODE>cgiwrap</CODE> needs the URL in the form
<CODE>/~user/foo/bar.scgi/</CODE>. The following rule solves the problem:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

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

<P><PRE>
/internal/cgi/user/swwidx?i=/u/user/foo/
</PRE><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
reorganise or area, we spend a lot of time changing the various hyperlinks.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
The solution here is to provide a special new URL format which automatically
leads to the proper CGI invocation. We configure the following:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule   ^/([uge])/([^/]+)(/?.*)/\*  /internal/cgi/user/wwwidx?i=/$1/$2$3/
RewriteRule   ^/([uge])/([^/]+)(/?.*):log /internal/cgi/user/wwwlog?f=/$1/$2$3
</PRE></TD></TR></TABLE>

<P>
Now the hyperlink to search at <CODE>/u/user/foo/</CODE> reads only

<P><PRE>
HREF="*"
</PRE><P>

which internally gets automatically transformed to 

<P><PRE>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</PRE><P>

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

</DL>

<P>
<H2>From Static to Dynamic</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
How can we transform a static page <CODE>foo.html</CODE> into a dynamic variant
<CODE>foo.cgi</CODE> in a seemless way, i.e.  without notice by the browser/user.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We just rewrite the URL to the CGI-script and force the correct MIME-type so
it gets really run as a CGI-script. This way a request to
<CODE>/~quux/foo.html</CODE> internally leads to the invokation of
<CODE>/~quux/foo.cgi</CODE>.

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine  on
RewriteBase    /~quux/
RewriteRule    ^foo\.<STRONG>html</STRONG>$  foo.<STRONG>cgi</STRONG>  [T=<STRONG>application/x-httpd-cgi</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>On-the-fly Content-Regeneration</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
This is done via the following ruleset:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{REQUEST_FILENAME}   <STRONG>!-s</STRONG>
RewriteRule ^page\.<STRONG>html</STRONG>$          page.<STRONG>cgi</STRONG>   [T=application/x-httpd-cgi,L]
</PRE></TD></TR></TABLE>

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

</DL>

<P>
<H2>Document With Autorefresh</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
No! We just combine the MIME multipart feature, the webserver NPH feature and
the URL manipulation power of mod_rewrite. 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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteRule   ^(/[uge]/[^/]+/?.*):refresh  /internal/cgi/apache/nph-refresh?f=$1
</PRE></TD></TR></TABLE>

<P>
Now when we reference the URL

<P><PRE>
/u/foo/bar/page.html:refresh
</PRE><P>

this leads to the internal invocation of the URL

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

</DL>

<P>
<H2>Mass Virtual Hosting</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
The <CODE>&lt;VirtualHost&gt;</CODE> feature of Apache is nice and works great
when you just have a few dozens virtual hosts. But when you are an ISP and
have hundreds of virtual hosts to provide this feature is not the best choice.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
To provide this feature we map the remote webpage or even the complete remote
webarea to our namespace by the use of the <I>Proxy Throughput</I> feature
(flag [P]):

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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_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></TD></TR></TABLE>

</DL>

<H1>Access Restriction</H1>

<P>
<H2>Blocking of Robots</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
How can we block a really annoying robot from retrieving pages of a specific
webarea? A <CODE>/robots.txt</CODE> file containing entries of the "Robot
Exclusion Protocol" is typically not enough to get rid of such a robot.

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We use a ruleset which forbids the URLs of the webarea
<CODE>/~quux/foo/arc/</CODE> (perhaps a very deep 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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{HTTP_USER_AGENT}   ^<STRONG>NameOfBadRobot</STRONG>.*      
RewriteCond %{REMOTE_ADDR}       ^<STRONG>123\.45\.67\.[8-9]</STRONG>$
RewriteRule ^<STRONG>/~quux/foo/arc/</STRONG>.+   -   [<STRONG>F</STRONG>]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Blocked Inline-Images</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
Assume we have under http://www.quux-corp.de/~quux/ 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>
<DT><STRONG>Solution:</STRONG>
<DD>
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><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{HTTP_REFERER} <STRONG>!^$</STRONG>                                  
RewriteCond %{HTTP_REFERER} !^http://www.quux-corp.de/~quux/.*$ [NC]
RewriteRule <STRONG>.*\.gif$</STRONG>        -                                    [F]
</PRE></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{HTTP_REFERER}         !^$                                  
RewriteCond %{HTTP_REFERER}         !.*/foo-with-gif\.html$
RewriteRule <STRONG>^inlined-in-foo\.gif$</STRONG>   -                        [F]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Host Deny</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
How can we forbid a list of externally configured hosts from using our server?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>

For Apache &gt;= 1.3b6:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE><P>

For Apache &lt;= 1.3b6:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Proxy Deny</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
How can we forbid a certain host or even a user of a special host from using
the Apache proxy?

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We first have to make sure mod_rewrite is below(!) mod_proxy in the
<CODE>Configuration</CODE> file when compiling the Apache webserver.  This way it
gets called _before_ mod_proxy. Then we configure the following for a
host-dependend deny...

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{REMOTE_HOST} <STRONG>^badhost\.mydomain\.com$</STRONG> 
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]
</PRE></TD></TR></TABLE>

<P>...and this one for a user@host-dependend deny:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteCond %{REMOTE_IDENT}@%{REMOTE_HOST}  <STRONG>^badguy@badhost\.mydomain\.com$</STRONG>
RewriteRule !^http://[^/.]\.mydomain.com.*  - [F]
</PRE></TD></TR></TABLE>

</DL>

<P>
<H2>Special Authentication Variant</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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 mod_access).

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
We use a list of rewrite conditions to exclude all except our friends:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

</DL>

<P>
<H2>Referer-based Deflector</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
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>
<DT><STRONG>Solution:</STRONG>
<DD>
Use the following really tricky ruleset...

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>...
in conjunction with a corresponding rewrite map:

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<P>
This automatically redirects the request back to the referring page (when "-"
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).

</DL>

<H1>Other</H1>

<P>
<H2>External Rewriting Engine</H2>
<P>

<DL>
<DT><STRONG>Description:</STRONG>
<DD>
A FAQ: How can we solve the FOO/BAR/QUUX/etc. problem? There seems no solution
by the use of mod_rewrite...

<P>
<DT><STRONG>Solution:</STRONG>
<DD>
Use an external rewrite map, i.e. a program which acts like a rewrite map.  It
is run once on startup of Apache receives the requested URLs on STDIN and has
to put the resulting (usually rewritten) URL on STDOUT (same order!).

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><PRE>
RewriteEngine on
RewriteMap    quux-map       <STRONG>prg:</STRONG>/path/to/map.quux.pl
RewriteRule   ^/~quux/(.*)$  /~quux/<STRONG>${quux-map:$1}</STRONG>
</PRE></TD></TR></TABLE>

<P><TABLE BGCOLOR="#E0E5F5" BORDER="0" CELLSPACING="0" CELLPADDING="5"><TR><TD><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></TD></TR></TABLE>

<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.

</DL>

<HR>

<H3 ALIGN="CENTER">
 Apache HTTP Server Version 1.3
</H3>

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