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<title>Apache 1.3 URL Rewriting Guide</title>
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<h3>Apache HTTP Server Version 1.3</h3>
<p><small><em>Is this the version you want? For more recent
versions, check our <a href="/docs/">documentation
index</a>.</em></small></p>
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<h1>Apache 1.3<br />
URL Rewriting Guide<br />
</h1>
<address>
Originally written by<br />
Ralf S. Engelschall <rse@apache.org><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 with which webmasters are often
confronted. We give detailed descriptions on how to
solve each problem by configuring URL rewriting rulesets.</p>
<h2><a id="ToC1" 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 do nearly 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.</p>
<h2><a id="ToC2" 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.
<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 in order to
avoid problems.</td>
</tr>
</table>
<h1>URL Layout</h1>
<h2>Canonical URLs</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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. Independent which URL the
user supplied with the request he should finally see the
canonical one only.</dd>
<dt><strong>Solution:</strong></dt>
<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>.
<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/$2<strong>/</strong> [<strong>R</strong>]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Canonical Hostnames</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>The goal of this rule is to force the use of a particular
hostname, in preference to other hostnames which may be used to
reach the same site. For example, if you wish to force the use
of <strong>www.example.com</strong> instead of
<strong>example.com</strong>, you might use a variant of the
following recipe.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
# For sites running on a port other than 80
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteCond %{SERVER_PORT} !^80$
RewriteRule ^/(.*) http://fully.qualified.domain.name:%{SERVER_PORT}/$1 [L,R]
# And for a site running on port 80
RewriteCond %{HTTP_HOST} !^fully\.qualified\.domain\.name [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule ^/(.*) http://fully.qualified.domain.name/$1 [L,R]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Moved DocumentRoot</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>Usually the DocumentRoot of the webserver directly
relates to the URL ``<code>/</code>''. But often this data
is not really of top-level priority, it is perhaps just one
entity of a lot of data pools. For instance at our Intranet
sites there are <code>/e/www/</code> (the homepage for
WWW), <code>/e/sww/</code> (the homepage for the Intranet)
etc. Now because the data of the DocumentRoot stays at
<code>/e/www/</code> we had to make sure that all inlined
images and other stuff inside this data pool work for
subsequent requests.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We just redirect the URL <code>/</code> to
<code>/e/www/</code>. While is seems trivial it is
actually trivial with mod_rewrite, only. Because the
typical old mechanisms of URL <em>Aliases</em> (as
provides by mod_alias and friends) only used
<em>prefix</em> matching. With this you cannot do such a
redirection because the DocumentRoot is a prefix of all
URLs. With mod_rewrite it is really trivial:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteRule <strong>^/$</strong> /e/www/ [<strong>R</strong>]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Trailing Slash Problem</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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>$</strong> foo<strong>/</strong> [<strong>R</strong>]
</pre>
</td>
</tr>
</table>
<p>The crazy and lazy can even do the following in the
top-level <code>.htaccess</code> file of their homedir.
But notice that this creates some processing
overhead.</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteBase /~quux/
RewriteCond %{REQUEST_FILENAME} <strong>-d</strong>
RewriteRule ^(.+<strong>[^/]</strong>)$ $1<strong>/</strong> [R]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Webcluster through Homogeneous URL Layout</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>We want to create a homogenous and consistent URL
layout over all WWW servers on a Intranet webcluster, i.e.
all URLs (per definition server local and thus server
dependent!) become actually server <em>independed</em>!
What we want is to give the WWW namespace a consistent
server-independend layout: no URL should have to include
any physically correct target server. The cluster itself
should drive us automatically to the physical target
host.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
First, the knowledge of the target servers come from
(distributed) external maps which contain information
where our users, groups and entities stay. The have the
form
<pre>
user1 server_of_user1
user2 server_of_user2
: :
</pre>
<p>We put them into files <code>map.xxx-to-host</code>.
Second we need to instruct all servers to redirect URLs
of the forms</p>
<pre>
/u/user/anypath
/g/group/anypath
/e/entity/anypath
</pre>
<p>to</p>
<pre>
http://physical-host/u/user/anypath
http://physical-host/g/group/anypath
http://physical-host/e/entity/anypath
</pre>
<p>when the URL is not locally valid to a server. The
following ruleset does this for us by the help of the map
files (assuming that server0 is a default server which
will be used if a user has no entry in the map):</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteMap user-to-host txt:/path/to/map.user-to-host
RewriteMap group-to-host txt:/path/to/map.group-to-host
RewriteMap entity-to-host txt:/path/to/map.entity-to-host
RewriteRule ^/u/<strong>([^/]+)</strong>/?(.*) http://<strong>${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>
</dd>
</dl>
<h2>Move Homedirs to Different Webserver</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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>.
<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>
</dd>
</dl>
<h2>Structured Homedirs</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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>.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We use the following ruleset to expand the tilde URLs
into exactly the above layout.
<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>
</dd>
</dl>
<h2>Filesystem Reorganisation</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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:
<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>
</dd>
<dt><strong>Solution:</strong></dt>
<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:
<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:</p>
<ol>
<li>Notice the L (last) flag and no substitution field
('-') in the forth part</li>
<li>Notice the ! (not) character and the C (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>
<h2>NCSA imagemap to Apache mod_imap</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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>.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We use a global rule to remove the prefix on-the-fly for
all requests:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteRule ^/cgi-bin/imagemap(.*) $1 [PT]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Search pages in more than one directory</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>Sometimes it is neccessary to let the webserver search
for pages in more than one directory. Here MultiViews or
other techniques cannot help.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We program a explicit ruleset which searches for the
files in the directories.
<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>
</dd>
</dl>
<h2>Set Environment Variables According To URL Parts</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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".
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteRule ^(.*)/<strong>S=([^/]+)</strong>/(.*) $1/$3 [E=<strong>STATUS:$2</strong>]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Virtual User Hosts</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>Assume that you want to provide
<code>www.<strong>username</strong>.host.domain.com</code>
for the homepage of username via just DNS A records to the
same machine and without any virtualhosts on this
machine.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
For HTTP/1.0 requests there is no solution, but for
HTTP/1.1 requests which contain a Host: HTTP header we
can use the following ruleset to rewrite
<code>http://www.username.host.com/anypath</code>
internally to <code>/home/username/anypath</code>:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteCond %{<strong>HTTP_HOST</strong>} ^www\.<strong>[^.]+</strong>\.host\.com$
RewriteRule ^(.+) %{HTTP_HOST}$1 [C]
RewriteRule ^www\.<strong>([^.]+)</strong>\.host\.com(.*) /home/<strong>$1</strong>$2
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Redirect Homedirs For Foreigners</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>We want to redirect homedir URLs to another webserver
<code>www.somewhere.com</code> when the requesting user
does not stay in the local domain
<code>ourdomain.com</code>. This is sometimes used in
virtual host contexts.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
Just a rewrite condition:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteCond %{REMOTE_HOST} <strong>!^.+\.ourdomain\.com$</strong>
RewriteRule ^(/~.+) http://www.somewhere.com/$1 [R,L]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Redirect Failing URLs To Other Webserver</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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!</dd>
<dt><strong>Solution:</strong></dt>
<dd>
The first solution has the best performance but less
flexibility and is less error safe:
<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.</p>
</dd>
</dl>
<h2>Extended Redirection</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<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):
<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>
#!/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: $url\n";
print "Content-type: text/html\n";
print "\n";
print "<html>\n";
print "<head>\n";
print "<title>302 Moved Temporarily (EXTENDED)</title>\n";
print "</head>\n";
print "<body>\n";
print "<h1>Moved Temporarily (EXTENDED)</h1>\n";
print "The document has moved <a HREF=\"$url\">here</a>.<p>\n";
print "</body>\n";
print "</html>\n";
##EOF##
</pre>
</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.</p>
</dd>
</dl>
<h2>Archive Access Multiplexer</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<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.
<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}$2 [R,L]
</pre>
</td>
</tr>
</table>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
##
## map.cxan -- Multiplexing Map for CxAN
##
de ftp://ftp.cxan.de/CxAN/
uk ftp://ftp.cxan.uk/CxAN/
com ftp://ftp.cxan.com/CxAN/
:
##EOF##
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Time-Dependend Rewriting</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>When tricks like time-dependend content should happen a
lot of webmasters still use CGI scripts which do for
instance redirects to specialized pages. How can it be done
via mod_rewrite?</dd>
<dt><strong>Solution:</strong></dt>
<dd>
There are a lot of variables named <code>TIME_xxx</code>
for rewrite conditions. In conjunction with the special
lexicographic comparison patterns <STRING, >STRING
and =STRING we can do time-dependend redirects:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteCond %{TIME_HOUR}%{TIME_MIN} >0700
RewriteCond %{TIME_HOUR}%{TIME_MIN} <1900
RewriteRule ^foo\.html$ foo.day.html
RewriteRule ^foo\.html$ foo.night.html
</pre>
</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...</p>
</dd>
</dl>
<h2>Backward Compatibility for YYYY to XXXX migration</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<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.
<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 ^(.*)$ $1.html
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h1>Content Handling</h1>
<h2>From Old to New (intern)</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>Assume we have recently renamed the page
<code>foo.html</code> to <code>bar.html</code> and now want
to provide the old URL for backward compatibility. Actually
we want that users of the old URL even not recognize that
the pages was renamed.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We rewrite the old URL to the new one internally via the
following rule:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>foo</strong>\.html$ <strong>bar</strong>.html
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>From Old to New (extern)</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>Assume again that we have recently renamed the page
<code>foo.html</code> to <code>bar.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.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We force a HTTP redirect to the new URL which leads to a
change of the browsers and thus the users view:
<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>
</dd>
</dl>
<h2>Browser Dependend Content</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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:
<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>
</dd>
</dl>
<h2>Dynamic Mirror</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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).</dd>
<dt><strong>Solution:</strong></dt>
<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]):
<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>$1 [<strong>P</strong>]
</pre>
</td>
</tr>
</table>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^<strong>usa-news\.html</strong>$ <strong>http://www.quux-corp.com/news/index.html</strong> [<strong>P</strong>]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Reverse Dynamic Mirror</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
<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>
</dd>
</dl>
<h2>Retrieve Missing Data from Intranet</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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:
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
<strong>ALLOW</strong> Host www.quux-corp.dom Port >1024 --> Host www2.quux-corp.dom Port <strong>80</strong>
<strong>DENY</strong> Host * Port * --> Host www2.quux-corp.dom Port <strong>80</strong>
</pre>
</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>
</dd>
</dl>
<h2>Load Balancing</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<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>
<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>
<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 (<STDIN>) {
$cnt = (($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>
<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.</p>
</li>
</ol>
</dd>
</dl>
<h2>New MIME-type, New Service</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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:
<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>
</dd>
<dt><strong>Solution:</strong></dt>
<dd>
The solution here is to provide a special new URL format
which automatically leads to the proper CGI invocation.
We configure the following:
<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$3
</pre>
</td>
</tr>
</table>
<p>Now the hyperlink to search at
<code>/u/user/foo/</code> reads only</p>
<pre>
HREF="*"
</pre>
<p>which internally gets automatically transformed to</p>
<pre>
/internal/cgi/user/wwwidx?i=/u/user/foo/
</pre>
<p>The same approach leads to an invocation for the
access log CGI program when the hyperlink
<code>:log</code> gets used.</p>
</dd>
</dl>
<h2>From Static to Dynamic</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>How can we transform a static page
<code>foo.html</code> into a dynamic variant
<code>foo.cgi</code> in a seamless way, i.e. without notice
by the browser/user.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We just rewrite the URL to the CGI-script and force the
correct MIME-type so it gets really run as a CGI-script.
This way a request to <code>/~quux/foo.html</code>
internally leads to the invokation of
<code>/~quux/foo.cgi</code>.
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo\.<strong>html</strong>$ foo.<strong>cgi</strong> [T=<strong>application/x-httpd-cgi</strong>]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>On-the-fly Content-Regeneration</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
This is done via the following ruleset:
<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).</p>
</dd>
</dl>
<h2>Document With Autorefresh</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<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.
<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(/&/, $ENV{'QUERY_STRING'});
foreach $pair (@pairs) {
($name, $value) = split(/=/, $pair);
$name =~ tr/A-Z/a-z/;
$name = 'QS_' . $name;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
eval "\$$name = \"$value\"";
}
$QS_s = 1 if ($QS_s eq '');
$QS_n = 3600 if ($QS_n eq '');
if ($QS_f eq '') {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: No file given\n";
exit(0);
}
if (! -f $QS_f) {
print "HTTP/1.0 200 OK\n";
print "Content-type: text/html\n\n";
print "&lt;b&gt;ERROR&lt;/b&gt;: File $QS_f not found\n";
exit(0);
}
sub print_http_headers_multipart_begin {
print "HTTP/1.0 200 OK\n";
$bound = "ThisRandomString12345";
print "Content-type: multipart/x-mixed-replace;boundary=$bound\n";
&print_http_headers_multipart_next;
}
sub print_http_headers_multipart_next {
print "\n--$bound\n";
}
sub print_http_headers_multipart_end {
print "\n--$bound--\n";
}
sub displayhtml {
local($buffer) = @_;
$len = length($buffer);
print "Content-type: text/html\n";
print "Content-length: $len\n\n";
print $buffer;
}
sub readfile {
local($file) = @_;
local(*FP, $size, $buffer, $bytes);
($x, $x, $x, $x, $x, $x, $x, $size) = stat($file);
$size = sprintf("%d", $size);
open(FP, "&lt;$file");
$bytes = sysread(FP, $buffer, $size);
close(FP);
return $buffer;
}
$buffer = &readfile($QS_f);
&print_http_headers_multipart_begin;
&displayhtml($buffer);
sub mystat {
local($file) = $_[0];
local($time);
($x, $x, $x, $x, $x, $x, $x, $x, $x, $mtime) = stat($file);
return $mtime;
}
$mtimeL = &mystat($QS_f);
$mtime = $mtime;
for ($n = 0; $n &lt; $QS_n; $n++) {
while (1) {
$mtime = &mystat($QS_f);
if ($mtime ne $mtimeL) {
$mtimeL = $mtime;
sleep(2);
$buffer = &readfile($QS_f);
&print_http_headers_multipart_next;
&displayhtml($buffer);
sleep(5);
$mtimeL = &mystat($QS_f);
last;
}
sleep($QS_s);
}
}
&print_http_headers_multipart_end;
exit(0);
##EOF##
</pre>
</dd>
</dl>
<h2>Mass Virtual Hosting</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>The <code><VirtualHost></code> feature of Apache
is nice and works great when you just have a few dozens
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts to provide this feature is not the best
choice.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the <i>Proxy Throughput</i> feature (flag [P]):
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
##
## vhost.map
##
www.vhost1.dom:80 /path/to/docroot/vhost1
www.vhost2.dom:80 /path/to/docroot/vhost2
:
www.vhostN.dom:80 /path/to/docroot/vhostN
</pre>
</td>
</tr>
</table>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
##
## httpd.conf
##
:
# use the canonical hostname on redirects, etc.
UseCanonicalName on
:
# add the virtual host in front of the CLF-format
CustomLog /path/to/access_log "%{VHOST}e %h %l %u %t \"%r\" %>s %b"
:
# enable the rewriting engine in the main server
RewriteEngine on
# define two maps: one for fixing the URL and one which defines
# the available virtual hosts with their corresponding
# DocumentRoot.
RewriteMap lowercase int:tolower
RewriteMap vhost txt:/path/to/vhost.map
# Now do the actual virtual host mapping
# via a huge and complicated single rule:
#
# 1. make sure we don't map for common locations
RewriteCond %{REQUEST_URI} !^/commonurl1/.*
RewriteCond %{REQUEST_URI} !^/commonurl2/.*
:
RewriteCond %{REQUEST_URI} !^/commonurlN/.*
#
# 2. make sure we have a Host header, because
# currently our approach only supports
# virtual hosting through this header
RewriteCond %{HTTP_HOST} !^$
#
# 3. lowercase the hostname
RewriteCond ${lowercase:%{HTTP_HOST}|NONE} ^(.+)$
#
# 4. lookup this hostname in vhost.map and
# remember it only when it is a path
# (and not "NONE" from above)
RewriteCond ${vhost:%1} ^(/.*)$
#
# 5. finally we can map the URL to its docroot location
# and remember the virtual host for logging puposes
RewriteRule ^/(.*)$ %1/$1 [E=VHOST:${lowercase:%{HTTP_HOST}}]
:
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h1>Access Restriction</h1>
<h2>Blocking of Robots</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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.
<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>
</dd>
</dl>
<h2>Blocked Inline-Images</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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.</dd>
<dt><strong>Solution:</strong></dt>
<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.
<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$</strong> - [F]
</pre>
</td>
</tr>
</table>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !.*/foo-with-gif\.html$
RewriteRule <strong>^inlined-in-foo\.gif$</strong> - [F]
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Host Deny</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>How can we forbid a list of externally configured hosts
from using our server?</dd>
<dt><strong>Solution:</strong></dt>
<dd>
For Apache >= 1.3b6:
<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 ${hosts-deny:%{REMOTE_ADDR}|NOT-FOUND} !=NOT-FOUND
RewriteRule ^/.* - [F]
</pre>
</td>
</tr>
</table>
<p>For Apache <= 1.3b6:</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteMap hosts-deny txt:/path/to/hosts.deny
RewriteRule ^/(.*)$ ${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>
<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>
</dd>
</dl>
<h2>URL-Restricted Proxy</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>How can we restrict the proxy to allow access to a
configurable set of internet sites only? The site list is
extracted from a prepared bookmarks file.</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We first have to make sure mod_rewrite is below(!)
mod_proxy in the <code>Configuration</code> file when
compiling the Apache webserver (or in the
<code>AddModule</code> list of <code>httpd.conf</code> in
the case of dynamically loaded modules), as it must get
called <em>_before_</em> mod_proxy.
<p>For simplicity, we generate the site list as a
textfile map (but see the <a
href="../mod/mod_rewrite.html#RewriteMap">mod_rewrite
documentation</a> for a conversion script to DBM format).
A typical Netscape bookmarks file can be converted to a
list of sites with a shell script like this:</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
#!/bin/sh
cat ${1:-~/.netscape/bookmarks.html} |
tr -d '\015' | tr '[A-Z]' '[a-z]' | grep href=\" |
sed -e '/href="file:/d;' -e '/href="news:/d;' \
-e 's|^.*href="[^:]*://\([^:/"]*\).*$|\1 OK|;' \
-e '/href="/s|^.*href="\([^:/"]*\).*$|\1 OK|;' |
sort -u
</pre>
</td>
</tr>
</table>
<p>We redirect the resulting output into a text file
called <code>goodsites.txt</code>. It now looks similar
to this:</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
www.apache.org OK
xml.apache.org OK
jakarta.apache.org OK
perl.apache.org OK
...
</pre>
</td>
</tr>
</table>
<p>We reference this site file within the configuration
for the <code>VirtualHost</code> which is responsible for
serving as a proxy (often not port 80, but 81, 8080 or
8008).</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
<VirtualHost *:8008>
...
RewriteEngine On
# Either use the (plaintext) allow list from goodsites.txt
RewriteMap ProxyAllow txt:/usr/local/apache/conf/goodsites.txt
# Or, for faster access, convert it to a DBM database:
#RewriteMap ProxyAllow dbm:/usr/local/apache/conf/goodsites
# Match lowercased hostnames
RewriteMap lowercase int:tolower
# Here we go:
# 1) first lowercase the site name and strip off a :port suffix
RewriteCond ${lowercase:%{HTTP_HOST}} ^([^:]*).*$
# 2) next look it up in the map file.
# "%1" refers to the previous regex.
# If the result is "OK", proxy access is granted.
RewriteCond ${ProxyAllow:%1|DENY} !^OK$ [NC]
# 3) Disallow proxy requests if the site was _not_ tagged "OK":
RewriteRule ^proxy: - [F]
...
</VirtualHost>
</pre>
</td>
</tr>
</table>
</dd>
</dl>
<h2>Proxy Deny</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>How can we forbid a certain host or even a user of a
special host from using the Apache proxy?</dd>
<dt><strong>Solution:</strong></dt>
<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
<em>_before_</em> mod_proxy. Then we configure the
following for a host-dependend deny...
<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>
</dd>
</dl>
<h2>Special Authentication Variant</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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).</dd>
<dt><strong>Solution:</strong></dt>
<dd>
We use a list of rewrite conditions to exclude all except
our friends:
<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>
</dd>
</dl>
<h2>Referer-based Deflector</h2>
<dl>
<dt><strong>Description:</strong></dt>
<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?</dd>
<dt><strong>Solution:</strong></dt>
<dd>
Use the following really tricky ruleset...
<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 ^.* ${deflector:%{HTTP_REFERER}} [R,L]
</pre>
</td>
</tr>
</table>
<p>... in conjunction with a corresponding rewrite
map:</p>
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
##
## deflector.map
##
http://www.badguys.com/bad/index.html -
http://www.badguys.com/bad/index2.html -
http://www.badguys.com/bad/index3.html http://somewhere.com/
</pre>
</td>
</tr>
</table>
<p>This automatically redirects the request back to the
referring page (when "-" is used as the value in the map)
or to a specific URL (when an URL is specified in the map
as the second argument).</p>
</dd>
</dl>
<h1>Other</h1>
<h2>External Rewriting Engine</h2>
<dl>
<dt><strong>Description:</strong></dt>
<dd>A FAQ: How can we solve the FOO/BAR/QUUX/etc. problem?
There seems no solution by the use of mod_rewrite...</dd>
<dt><strong>Solution:</strong></dt>
<dd>
Use an external rewrite map, i.e. a program which acts
like a rewrite map. It is run once on startup of Apache
receives the requested URLs on STDIN and has to put the
resulting (usually rewritten) URL on STDOUT (same
order!).
<table bgcolor="#E0E5F5" border="0" cellspacing="0"
cellpadding="5">
<tr>
<td>
<pre>
RewriteEngine on
RewriteMap quux-map <strong>prg:</strong>/path/to/map.quux.pl
RewriteRule ^/~quux/(.*)$ /~quux/<strong>${quux-map:$1}</strong>
</pre>
</td>
</tr>
</table>
<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 (<>) {
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.</p>
</dd>
</dl>
<hr />
<h3 align="CENTER">Apache HTTP Server Version 1.3</h3>
<a href="./"><img src="../images/index.gif" alt="Index" /></a>
<a href="../"><img src="../images/home.gif" alt="Home" /></a>
</blockquote>
</body>
</html>
