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<title>Advanced Techniques with mod_rewrite - Apache HTTP Server Version 2.5</title>
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<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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</script>
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<link href="../images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/quickreference.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.5</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.5</a> > <a href="./">Rewrite</a></div><div id="page-content"><div id="preamble"><h1>Advanced Techniques with mod_rewrite</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
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</div>
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<p>This document supplements the <code class="module"><a href="../mod/mod_rewrite.html">mod_rewrite</a></code>
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<a href="../mod/mod_rewrite.html">reference documentation</a>. It provides
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a few advanced techniques using mod_rewrite.</p>
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<div class="warning">Note that many of these examples won't work unchanged in your
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particular server configuration, so it's important that you understand
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them, rather than merely cutting and pasting the examples into your
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configuration.</div>
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</div>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#sharding">URL-based sharding across multiple backends</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#on-the-fly-content">On-the-fly Content-Regeneration</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#load-balancing">Load Balancing</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#structuredhomedirs">Structured Userdirs</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#redirectanchors">Redirecting Anchors</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#time-dependent">Time-Dependent Rewriting</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#setenvvars">Set Environment Variables Based On URL Parts</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="../mod/mod_rewrite.html">Module documentation</a></li><li><a href="intro.html">mod_rewrite introduction</a></li><li><a href="remapping.html">Redirection and remapping</a></li><li><a href="access.html">Controlling access</a></li><li><a href="vhosts.html">Virtual hosts</a></li><li><a href="proxy.html">Proxying</a></li><li><a href="rewritemap.html">Using RewriteMap</a></li><li><a href="avoid.html">When not to use mod_rewrite</a></li><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="sharding" id="sharding">URL-based sharding across multiple backends</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>A common technique for distributing the burden of
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server load or storage space is called "sharding".
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When using this method, a front-end server will use the
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url to consistently "shard" users or objects to separate
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backend servers.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>A mapping is maintained, from users to target servers, in
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external map files. They look like:</p>
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<div class="example"><p><code>
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user1 physical_host_of_user1<br />
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user2 physical_host_of_user2<br />
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: :
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</code></p></div>
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<p>We put this into a <code>map.users-to-hosts</code> file. The
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aim is to map;</p>
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<div class="example"><p><code>
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/u/user1/anypath
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</code></p></div>
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<p>to</p>
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<div class="example"><p><code>
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http://physical_host_of_user1/u/user/anypath
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</code></p></div>
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<p>thus every URL path need not be valid on every backend physical
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host. The following ruleset does this for us with the help of the map
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files assuming that server0 is a default server which will be used if
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a user has no entry in the map:</p>
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<pre class="prettyprint lang-config">RewriteEngine on
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RewriteMap users-to-hosts "txt:/path/to/map.users-to-hosts"
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RewriteRule "^/u/([^/]+)/?(.*)" "http://${users-to-hosts:$1|server0}/u/$1/$2"</pre>
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</dd>
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</dl>
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<p>See the <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code>
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documentation for more discussion of the syntax of this directive.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="on-the-fly-content" id="on-the-fly-content">On-the-fly Content-Regeneration</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>We wish to dynamically generate content, but store it
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statically once it is generated. This rule will check for the
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existence of the static file, and if it's not there, generate
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it. The static files can be removed periodically, if desired (say,
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via cron) and will be regenerated on demand.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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This is done via the following ruleset:
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<pre class="prettyprint lang-config"># This example is valid in per-directory context only
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RewriteCond "%{REQUEST_URI}" !-U
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RewriteRule "^(.+)\.html$" "/regenerate_page.cgi" [PT,L]</pre>
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<p>The <code>-U</code> operator determines whether the test string
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(in this case, <code>REQUEST_URI</code>) is a valid URL. It does
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this via a subrequest. In the event that this subrequest fails -
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that is, the requested resource doesn't exist - this rule invokes
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the CGI program <code>/regenerate_page.cgi</code>, which generates
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the requested resource and saves it into the document directory, so
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that the next time it is requested, a static copy can be served.</p>
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<p>In this way, documents that are infrequently updated can be served in
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static form. if documents need to be refreshed, they can be deleted
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from the document directory, and they will then be regenerated the
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next time they are requested.</p>
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</dd>
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</dl>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="load-balancing" id="load-balancing">Load Balancing</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>We wish to randomly distribute load across several servers
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using mod_rewrite.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>We'll use <code class="directive"><a href="../mod/mod_rewrite.html#rewritemap">RewriteMap</a></code> and a list of servers
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to accomplish this.</p>
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<pre class="prettyprint lang-config">RewriteEngine on
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RewriteMap lb "rnd:/path/to/serverlist.txt"
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RewriteRule "^/(.*)" "http://${lb:servers}/$1" [P,L]</pre>
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<p><code>serverlist.txt</code> will contain a list of the servers:</p>
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<div class="example"><p><code>
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## serverlist.txt<br />
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<br />
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servers one.example.com|two.example.com|three.example.com<br />
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</code></p></div>
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<p>If you want one particular server to get more of the load than the
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others, add it more times to the list.</p>
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</dd>
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<dt>Discussion</dt>
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<dd>
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<p>Apache comes with a load-balancing module -
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<code class="module"><a href="../mod/mod_proxy_balancer.html">mod_proxy_balancer</a></code> - which is far more flexible and
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featureful than anything you can cobble together using mod_rewrite.</p>
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</dd>
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</dl>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="structuredhomedirs" id="structuredhomedirs">Structured Userdirs</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>Some sites with thousands of users use a
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structured homedir layout, <em>i.e.</em> each homedir is in a
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subdirectory which begins (for instance) with the first
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character of the username. So, <code>/~larry/anypath</code>
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is <code>/home/<strong>l</strong>/larry/public_html/anypath</code>
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while <code>/~waldo/anypath</code> is
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<code>/home/<strong>w</strong>/waldo/public_html/anypath</code>.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>We use the following ruleset to expand the tilde URLs
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into the above layout.</p>
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<pre class="prettyprint lang-config">RewriteEngine on
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RewriteRule "^/~(<strong>([a-z])</strong>[a-z0-9]+)(.*)" "/home/<strong>$2</strong>/$1/public_html$3"</pre>
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</dd>
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</dl>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="redirectanchors" id="redirectanchors">Redirecting Anchors</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>By default, redirecting to an HTML anchor doesn't work,
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because mod_rewrite escapes the <code>#</code> character,
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turning it into <code>%23</code>. This, in turn, breaks the
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redirection.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>Use the <code>[NE]</code> flag on the
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<code>RewriteRule</code>. NE stands for No Escape.
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</p>
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</dd>
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<dt>Discussion:</dt>
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<dd>This technique will of course also work with other
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special characters that mod_rewrite, by default, URL-encodes.</dd>
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</dl>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="time-dependent" id="time-dependent">Time-Dependent Rewriting</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>We wish to use mod_rewrite to serve different content based on
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the time of day.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>There are a lot of variables named <code>TIME_xxx</code>
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for rewrite conditions. In conjunction with the special
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lexicographic comparison patterns <code><STRING</code>,
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<code>>STRING</code> and <code>=STRING</code> we can
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do time-dependent redirects:</p>
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<pre class="prettyprint lang-config">RewriteEngine on
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RewriteCond "%{TIME_HOUR}%{TIME_MIN}" >0700
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RewriteCond "%{TIME_HOUR}%{TIME_MIN}" <1900
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RewriteRule "^foo\.html$" "foo.day.html" [L]
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RewriteRule "^foo\.html$" "foo.night.html"</pre>
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<p>This provides the content of <code>foo.day.html</code>
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under the URL <code>foo.html</code> from
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<code>07:01-18:59</code> and at the remaining time the
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contents of <code>foo.night.html</code>.</p>
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<div class="warning"><code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code>, intermediate proxies
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and browsers may each cache responses and cause the either page to be
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shown outside of the time-window configured.
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<code class="module"><a href="../mod/mod_expires.html">mod_expires</a></code> may be used to control this
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effect. You are, of course, much better off simply serving the
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content dynamically, and customizing it based on the time of day.</div>
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</dd>
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</dl>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="setenvvars" id="setenvvars">Set Environment Variables Based On URL Parts</a></h2>
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<dl>
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<dt>Description:</dt>
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<dd>
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<p>At time, we want to maintain some kind of status when we
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perform a rewrite. For example, you want to make a note that
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you've done that rewrite, so that you can check later to see if a
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request can via that rewrite. One way to do this is by setting an
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environment variable.</p>
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</dd>
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<dt>Solution:</dt>
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<dd>
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<p>Use the [E] flag to set an environment variable.</p>
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<pre class="prettyprint lang-config">RewriteEngine on
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RewriteRule "^/horse/(.*)" "/pony/$1" [E=<strong>rewritten:1</strong>]</pre>
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<p>Later in your ruleset you might check for this environment
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variable using a RewriteCond:</p>
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<pre class="prettyprint lang-config">RewriteCond "%{ENV:rewritten}" =1</pre>
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<p>Note that environment variables do not survive an external
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redirect. You might consider using the [CO] flag to set a
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cookie.</p>
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</dd>
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</dl>
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</div></div>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/rewrite/advanced.html" title="English"> en </a></p>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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