apache/docs/manual/rewrite/avoid.xml

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

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

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

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

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

<title>When not to use mod_rewrite</title>

<summary>

<p>This document supplements the <module>mod_rewrite</module> 
<a href="../mod/mod_rewrite.html">reference documentation</a>. It describes
perhaps one of the most important concepts about mod_rewrite - namely,
when to avoid using it.</p>

<p>mod_rewrite should be considered a last resort, when other
alternatives are found wanting. Using it when there are simpler
alternatives leads to configurations which are confusing, fragile, and
hard to maintain. Understanding what other alternatives are available is
a very important step towards mod_rewrite mastery.</p>

<p>Note that many of these examples won't work unchanged in your
particular server configuration, so it's important that you understand
them, rather than merely cutting and pasting the examples into your
configuration.</p>

<p>The most common situation in which <module>mod_rewrite</module> is
the right tool is when the very best solution requires access to the
server configuration files, and you don't have that access. Some
configuration directives are only available in the server configuration
file. So if you are in a hosting situation where you only have .htaccess
files to work with, you may need to resort to
<module>mod_rewrite</module>.</p>

</summary>
<seealso><a href="../mod/mod_rewrite.html">Module documentation</a></seealso>
<seealso><a href="intro.html">mod_rewrite introduction</a></seealso>
<seealso><a href="remapping.html">Redirection and remapping</a></seealso>
<seealso><a href="access.html">Controlling access</a></seealso>
<seealso><a href="vhosts.html">Virtual hosts</a></seealso>
<seealso><a href="proxy.html">Proxying</a></seealso>
<seealso><a href="rewritemap.html">Using RewriteMap</a></seealso>
<seealso><a href="advanced.html">Advanced techniques and tricks</a></seealso>
<!--<seealso><a href="avoid.html">When not to use mod_rewrite</a></seealso>-->

<section id="redirect">
<title>Simple Redirection</title>

<p><module>mod_alias</module> provides the <directive
module="mod_alias">Redirect</directive> and <directive
module="mod_alias">RedirectMatch</directive> directives, which provide a
means to redirect one URL to another. This kind of simple redirection of
one URL, or a class of URLs, to somewhere else, should be accomplished
using these directives rather than <directive
module="mod_rewrite">RewriteRule</directive>. <code>RedirectMatch</code>
allows you to include a regular expression in your redirection criteria,
providing many of the benefits of using <code>RewriteRule</code>.</p>

<p>A common use for <code>RewriteRule</code> is to redirect an entire
class of URLs. For example, all URLs in the <code>/one</code> directory
must be redirected to <code>http://one.example.com/</code>, or perhaps
all <code>http</code> requests must be redirected to
<code>https</code>.</p>

<p>These situations are better handled by the <code>Redirect</code>
directive. Remember that <code>Redirect</code> preserves path
information. That is to say, a redirect for a URL <code>/one</code> will
also redirect all URLs under that, such as <code>/one/two.html</code>
and <code>/one/three/four.html</code>.</p>

<p>To redirect URLs under <code>/one</code> to
<code>http://one.example.com</code>, do the following:</p>

<example>
Redirect /one/ http://one.example.com/
</example>

<p>To redirect <code>http</code> URLs to <code>https</code>, do the
following:</p>

<example>
&lt;VirtualHost *:80&gt;
ServerName www.example.com<br />
Redirect / https://www.example.com/<br />
&lt;/VirtualHost &gt;
<br />
&lt;VirtualHost *:443&gt;
ServerName www.example.com<br />
<br />
# ... SSL configuration goes here<br />
&lt;/VirtualHost &gt;
</example>

<p>The use of <code>RewriteRule</code> to perform this task may be
appropriate if there are other <code>RewriteRule</code> directives in
the same scope. This is because, when there are <code>Redirect</code>
and <code>RewriteRule</code> directives in the same scope, the
<code>RewriteRule</code> directives will run first, regardless of the
order of appearance in the configuration file.</p>

<p>In the case of the <em>http-to-https</em> redirection, the use of
<code>RewriteRule</code> would be appropriate if you don't have access
to the main server configuration file, and are obliged to perform this
task in a <code>.htaccess</code> file instead.</p>

</section>

<section id="alias"><title>URL Aliasing</title>
<p>The <directive module="mod_alias">Alias</directive> directive
provides mapping from a URI to a directory - usually a directory outside
of your <directive module="core">DocumentRoot</directive>. Although it
is possible to perform this mapping with <code>mod_rewrite</code>,
<code>Alias</code> is the preferred method, for reasons of simplicity
and performance.</p>

<example><title>Using Alias</title>
Alias /cats /var/www/virtualhosts/felines/htdocs
</example>

<p>
The use of <code>mod_rewrite</code> to perform this mapping may be
appropriate when you do not have access to the server configuration
files. Alias may only be used in server or virtualhost context, and not
in a <code>.htaccess</code> file.
</p>

<p>Symbolic links would be another way to accomplish the same thing, if
you have <code>Options FollowSymLinks</code> enabled on your
server.</p>
</section>

<section id="vhosts"><title>Virtual Hosting</title>
<p>Although it is possible to handle <a href="vhosts.html">virtual hosts
with mod_rewrite</a>, it is seldom the right way. Creating individual
&lt;VirtualHost&gt; blocks is almost always the right way to go. In the
event that you have an enormous number of virtual hosts, consider using
<module>mod_vhost_alias</module> to create these hosts automatically.</p>

<p>Third-party modules such as <a href="link_needed">mod_macro</a> are
also useful for creating a large number of virtual hosts dynamically.</p>

<p>Using <module>mod_rewrite</module> for vitualhost creation may be
appropriate if you are using a hosting service that does not provide
you access to the server configuration files, and you are therefore
restricted to configuration using <code>.htaccess</code> files.</p>

<p>See the <a href="vhosts.html">virtual hosts with mod_rewrite</a>
document for more details on how you might accomplish this if it still
seems like the right approach.</p>

</section>

<section id="proxy"><title>Simple Proxying</title>

<p><code>RewriteRule</code> provides the <a
href="flags.html#flag_p">[P]</a> flag to pass rewritten URIs through
<module>mod_proxy</module>.</p>

<example>
RewriteRule ^/?images(.*) http://imageserver.local/images$1 [P]
</example>

<p>However, in many cases, when there is no actual pattern matching
meeded, as in the example shown above, the <directive
module="mod_proxy">ProxyPass</directive> directive is a better choice.
The example here could be rendered as:</p>

<example>
ProxyPass /images/ http://imageserver.local/images/
</example>

<p>Note that whether you use <directive
module="mod_rewrite">RewriteRule</directive> or <directive
module="mod_proxy">ProxyPass</directive>, you'll still need to use the
<directive module="mod_proxy">ProxyPassReverse</directive> directive to
catch redirects issued from the back-end server:</p>

<example>
ProxyPassReverse /images/ http://imageserver.local/images/
</example>

<p>You may need to use <code>RewriteRule</code> instead when there are
other <code>RewriteRule</code>s in effect in the same scope, as a
<code>RewriteRule</code> will usually take effect before a
<code>ProxyPass</code>, and so may preempt what you're trying to
accomplish.</p>

</section>

<section id="setenv"><title>Environment Variable Testing</title>

<p><module>mod_rewrite</module> is frequently used to take a particular
action based on the presence or absense of a particular environment
variable or request header. This can be done more efficiently using the
<directive module="core" type="section">If</directive>.</p>

<p>Consider, for example, the common scenario where
<directive>RewriteRule</directive> is used to enforce a canonical
hostname, such as <code>www.example.com</code> instead of
<code>example.com</code>. This can be done using the <directive
module="core" type="section">If</directive> direct, as shown here:</p>

<example>
&lt;If "$req{Host} = 'example.com'"&gt;<br />
RedirectMatch (.*) http://www.example.com$1
&lt;/If&gt;
</example>

<p>This technique can be used to take actions based on any request
header, response header, or environment variable, replacing
<module>mod_rewrite</module> in many common scenarios.</p>

</section>

</manualpage>