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<br />
<h1 align="CENTER">Module mod_rewrite<br />
URL Rewriting Engine</h1>
<p>This module provides a rule-based rewriting engine to
rewrite requested URLs on the fly.</p>
<p><a href="module-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="module-dict.html#SourceFile"
rel="Help"><strong>Source File:</strong></a>
mod_rewrite.c<br />
<a href="module-dict.html#ModuleIdentifier"
rel="Help"><strong>Module Identifier:</strong></a>
rewrite_module<br />
<a href="module-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Available in
Apache 1.2 and later.</p>
<hr noshade="noshade" size="1" />
<br />
<h2>Summary</h2>
<blockquote>
<blockquote>
<blockquote>
<em>``The great thing about mod_rewrite is it gives you
all the configurability and flexibility of Sendmail.
The downside to mod_rewrite is that it gives you all
the configurability and flexibility of Sendmail.''</em>
<div align="RIGHT">
-- Brian Behlendorf<br />
Apache Group
</div>
</blockquote>
</blockquote>
</blockquote>
<blockquote>
<blockquote>
<blockquote>
<em>`` Despite the tons of examples and docs,
mod_rewrite is voodoo. Damned cool voodoo, but still
voodoo. ''</em>
<div align="RIGHT">
-- Brian Moore<br />
bem@news.cmc.net
</div>
</blockquote>
</blockquote>
</blockquote>
Welcome to mod_rewrite, the Swiss Army Knife of URL
manipulation!
<p>This module uses a rule-based rewriting engine (based on a
regular-expression parser) to rewrite requested URLs on the
fly. It supports an unlimited number of rules and an
unlimited number of attached rule conditions for each rule to
provide a really flexible and powerful URL manipulation
mechanism. The URL manipulations can depend on various tests,
for instance server variables, environment variables, HTTP
headers, time stamps and even external database lookups in
various formats can be used to achieve a really granular URL
matching.</p>
<p>This module operates on the full URLs (including the
path-info part) both in per-server context
(<code>httpd.conf</code>) and per-directory context
(<code>.htaccess</code>) and can even generate query-string
parts on result. The rewritten result can lead to internal
sub-processing, external request redirection or even to an
internal proxy throughput.</p>
<p>But all this functionality and flexibility has its
drawback: complexity. So don't expect to understand this
entire module in just one day.</p>
<p>This module was invented and originally written in April
1996<br />
and gifted exclusively to the The Apache Group in July 1997
by</p>
<blockquote>
<a href="http://www.engelschall.com/"><code>Ralf S.
Engelschall</code></a><br />
<a
href="mailto:rse@engelschall.com"><code>rse@engelschall.com</code></a><br />
<a
href="http://www.engelschall.com/"><code>www.engelschall.com</code></a>
</blockquote>
<hr noshade="noshade" size="1" />
<h2>Table Of Contents</h2>
<p><strong>Internal Processing</strong></p>
<ul>
<li><a href="#InternalAPI">API Phases</a></li>
<li><a href="#InternalRuleset">Ruleset Processing</a></li>
<li><a href="#InternalBackRefs">Regex Back-Reference
Availability</a></li>
</ul>
<p><strong>Configuration Directives</strong></p>
<ul>
<li><a href="#RewriteEngine">RewriteEngine</a></li>
<li><a href="#RewriteOptions">RewriteOptions</a></li>
<li><a href="#RewriteLog">RewriteLog</a></li>
<li><a href="#RewriteLogLevel">RewriteLogLevel</a></li>
<li><a href="#RewriteLock">RewriteLock</a></li>
<li><a href="#RewriteMap">RewriteMap</a></li>
<li><a href="#RewriteBase">RewriteBase</a></li>
<li><a href="#RewriteCond">RewriteCond</a></li>
<li><a href="#RewriteRule">RewriteRule</a></li>
</ul>
<strong>Miscellaneous</strong>
<ul>
<li><a href="#EnvVar">Environment Variables</a></li>
<li><a href="#Solutions">Practical Solutions</a></li>
</ul>
<hr noshade="noshade" size="1" />
<center>
<h1><a id="Internal" name="Internal">Internal
Processing</a></h1>
</center>
<hr noshade="noshade" size="1" />
<p>The internal processing of this module is very complex but
needs to be explained once even to the average user to avoid
common mistakes and to let you exploit its full
functionality.</p>
<h2><a id="InternalAPI" name="InternalAPI">API
Phases</a></h2>
<p>First you have to understand that when Apache processes a
HTTP request it does this in phases. A hook for each of these
phases is provided by the Apache API. Mod_rewrite uses two of
these hooks: the URL-to-filename translation hook which is
used after the HTTP request has been read but before any
authorization starts and the Fixup hook which is triggered
after the authorization phases and after the per-directory
config files (<code>.htaccess</code>) have been read, but
before the content handler is activated.</p>
<p>So, after a request comes in and Apache has determined the
corresponding server (or virtual server) the rewriting engine
starts processing of all mod_rewrite directives from the
per-server configuration in the URL-to-filename phase. A few
steps later when the final data directories are found, the
per-directory configuration directives of mod_rewrite are
triggered in the Fixup phase. In both situations mod_rewrite
rewrites URLs either to new URLs or to filenames, although
there is no obvious distinction between them. This is a usage
of the API which was not intended to be this way when the API
was designed, but as of Apache 1.x this is the only way
mod_rewrite can operate. To make this point more clear
remember the following two points:</p>
<ol>
<li>Although mod_rewrite rewrites URLs to URLs, URLs to
filenames and even filenames to filenames, the API
currently provides only a URL-to-filename hook. In Apache
2.0 the two missing hooks will be added to make the
processing more clear. But this point has no drawbacks for
the user, it is just a fact which should be remembered:
Apache does more in the URL-to-filename hook than the API
intends for it.</li>
<li>
Unbelievably mod_rewrite provides URL manipulations in
per-directory context, <em>i.e.</em>, within
<code>.htaccess</code> files, although these are reached
a very long time after the URLs have been translated to
filenames. It has to be this way because
<code>.htaccess</code> files live in the filesystem, so
processing has already reached this stage. In other
words: According to the API phases at this time it is too
late for any URL manipulations. To overcome this chicken
and egg problem mod_rewrite uses a trick: When you
manipulate a URL/filename in per-directory context
mod_rewrite first rewrites the filename back to its
corresponding URL (which is usually impossible, but see
the <code>RewriteBase</code> directive below for the
trick to achieve this) and then initiates a new internal
sub-request with the new URL. This restarts processing of
the API phases.
<p>Again mod_rewrite tries hard to make this complicated
step totally transparent to the user, but you should
remember here: While URL manipulations in per-server
context are really fast and efficient, per-directory
rewrites are slow and inefficient due to this chicken and
egg problem. But on the other hand this is the only way
mod_rewrite can provide (locally restricted) URL
manipulations to the average user.</p>
</li>
</ol>
<p>Don't forget these two points!</p>
<h2><a id="InternalRuleset" name="InternalRuleset">Ruleset
Processing</a></h2>
Now when mod_rewrite is triggered in these two API phases, it
reads the configured rulesets from its configuration
structure (which itself was either created on startup for
per-server context or during the directory walk of the Apache
kernel for per-directory context). Then the URL rewriting
engine is started with the contained ruleset (one or more
rules together with their conditions). The operation of the
URL rewriting engine itself is exactly the same for both
configuration contexts. Only the final result processing is
different.
<p>The order of rules in the ruleset is important because the
rewriting engine processes them in a special (and not very
obvious) order. The rule is this: The rewriting engine loops
through the ruleset rule by rule (<code>RewriteRule</code>
directives) and when a particular rule matches it optionally
loops through existing corresponding conditions
(<code>RewriteCond</code> directives). For historical reasons
the conditions are given first, and so the control flow is a
little bit long-winded. See Figure 1 for more details.</p>
<div align="CENTER">
<table cellspacing="0" cellpadding="2" border="0">
<tr>
<td bgcolor="#CCCCCC"><img
src="../images/mod_rewrite_fig1.gif" width="428"
height="385"
alt="[Needs graphics capability to display]" /></td>
</tr>
<tr>
<td align="CENTER"><strong>Figure 1:</strong> The
control flow through the rewriting ruleset</td>
</tr>
</table>
</div>
<p>As you can see, first the URL is matched against the
<em>Pattern</em> of each rule. When it fails mod_rewrite
immediately stops processing this rule and continues with the
next rule. If the <em>Pattern</em> matches, mod_rewrite looks
for corresponding rule conditions. If none are present, it
just substitutes the URL with a new value which is
constructed from the string <em>Substitution</em> and goes on
with its rule-looping. But if conditions exist, it starts an
inner loop for processing them in the order that they are
listed. For conditions the logic is different: we don't match
a pattern against the current URL. Instead we first create a
string <em>TestString</em> by expanding variables,
back-references, map lookups, <em>etc.</em> and then we try
to match <em>CondPattern</em> against it. If the pattern
doesn't match, the complete set of conditions and the
corresponding rule fails. If the pattern matches, then the
next condition is processed until no more conditions are
available. If all conditions match, processing is continued
with the substitution of the URL with
<em>Substitution</em>.</p>
<h2><a id="quoting" name="quoting">Quoting Special
Characters</a></h2>
<p>As of Apache 1.3.20, special characters in
<i>TestString</i> and <i>Substitution</i> strings can be
escaped (that is, treated as normal characters without their
usual special meaning) by prefixing them with a slosh ('\')
character. In other words, you can include an actual
dollar-sign character in a <i>Substitution</i> string by
using '<code>\$</code>'; this keeps mod_rewrite from trying
to treat it as a backreference.</p>
<h2><a id="InternalBackRefs" name="InternalBackRefs">Regex
Back-Reference Availability</a></h2>
One important thing here has to be remembered: Whenever you
use parentheses in <em>Pattern</em> or in one of the
<em>CondPattern</em>, back-references are internally created
which can be used with the strings <code>$N</code> and
<code>%N</code> (see below). These are available for creating
the strings <em>Substitution</em> and <em>TestString</em>.
Figure 2 shows to which locations the back-references are
transfered for expansion.
<div align="CENTER">
<table cellspacing="0" cellpadding="2" border="0">
<tr>
<td bgcolor="#CCCCCC"><img
src="../images/mod_rewrite_fig2.gif" width="381"
height="179"
alt="[Needs graphics capability to display]" /></td>
</tr>
<tr>
<td align="CENTER"><strong>Figure 2:</strong> The
back-reference flow through a rule</td>
</tr>
</table>
</div>
<p>We know this was a crash course on mod_rewrite's internal
processing. But you will benefit from this knowledge when
reading the following documentation of the available
directives.</p>
<hr noshade="noshade" size="1" />
<center>
<h1><a id="Configuration"
name="Configuration">Configuration Directives</a></h1>
</center>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteEngine"
name="RewriteEngine">RewriteEngine</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteEngine
on|off<br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <code>RewriteEngine
off</code><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host, directory, .htaccess<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> FileInfo<br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.2<br />
<p>The <code>RewriteEngine</code> directive enables or
disables the runtime rewriting engine. If it is set to
<code>off</code> this module does no runtime processing at
all. It does not even update the <code>SCRIPT_URx</code>
environment variables.</p>
<p>Use this directive to disable the module instead of
commenting out all the <code>RewriteRule</code>
directives!</p>
<p>Note that, by default, rewrite configurations are not
inherited. This means that you need to have a
<code>RewriteEngine on</code> directive for each virtual host
in which you wish to use it.</p>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteOptions"
name="RewriteOptions">RewriteOptions</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteOptions
<em>Option</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>None</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host, directory, .htaccess<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> FileInfo<br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.2<br />
<p>The <code>RewriteOptions</code> directive sets some
special options for the current per-server or per-directory
configuration. The <em>Option</em> strings can be one of the
following:</p>
<ul>
<li>'<strong><code>inherit</code></strong>'<br />
This forces the current configuration to inherit the
configuration of the parent. In per-virtual-server context
this means that the maps, conditions and rules of the main
server are inherited. In per-directory context this means
that conditions and rules of the parent directory's
<code>.htaccess</code> configuration are inherited.</li>
</ul>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteLog" name="RewriteLog">RewriteLog</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteLog
<em>file-path</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>None</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> <em>Not
applicable</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.2<br />
<p>The <code>RewriteLog</code> directive sets the name of the
file to which the server logs any rewriting actions it
performs. If the name does not begin with a slash
('<code>/</code>') then it is assumed to be relative to the
<em>Server Root</em>. The directive should occur only once
per server config.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Note</strong>: To disable the logging of
rewriting actions it is not recommended to set
<em>Filename</em> to <code>/dev/null</code>, because
although the rewriting engine does not then output to a
logfile it still creates the logfile output internally.
<strong>This will slow down the server with no advantage
to the administrator!</strong> To disable logging either
remove or comment out the <code>RewriteLog</code>
directive or use <code>RewriteLogLevel 0</code>!</td>
</tr>
</table>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Security</strong>: See the <a
href="../misc/security_tips.html">Apache Security
Tips</a> document for details on why your security could
be compromised if the directory where logfiles are stored
is writable by anyone other than the user that starts the
server.</td>
</tr>
</table>
<p><strong>Example:</strong></p>
<blockquote>
<pre>
RewriteLog "/usr/local/var/apache/logs/rewrite.log"
</pre>
</blockquote>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteLogLevel"
name="RewriteLogLevel">RewriteLogLevel</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteLogLevel
<em>Level</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a>
<code>RewriteLogLevel 0</code><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> <em>Not
applicable</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.2<br />
<p>The <code>RewriteLogLevel</code> directive sets the
verbosity level of the rewriting logfile. The default level 0
means no logging, while 9 or more means that practically all
actions are logged.</p>
<p>To disable the logging of rewriting actions simply set
<em>Level</em> to 0. This disables all rewrite action
logs.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Notice:</strong> Using a high value for
<em>Level</em> will slow down your Apache server
dramatically! Use the rewriting logfile at a
<em>Level</em> greater than 2 only for debugging!</td>
</tr>
</table>
<p><strong>Example:</strong></p>
<blockquote>
<pre>
RewriteLogLevel 3
</pre>
</blockquote>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteLock"
name="RewriteLock">RewriteLock</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteLock
<em>file-path</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>None</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> <em>Not
applicable</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.3<br />
<p>This directive sets the filename for a synchronization
lockfile which mod_rewrite needs to communicate with
<samp>RewriteMap</samp> <em>programs</em>. Set this lockfile
to a local path (not on a NFS-mounted device) when you want
to use a rewriting map-program. It is not required for other
types of rewriting maps.</p>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteMap" name="RewriteMap">RewriteMap</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteMap
<em>MapName</em> <em>MapType</em>:<em>MapSource</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> not used per
default<br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a> <em>Not
applicable</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache 1.2
(partially), Apache 1.3<br />
<p>The <code>RewriteMap</code> directive defines a
<em>Rewriting Map</em> which can be used inside rule
substitution strings by the mapping-functions to
insert/substitute fields through a key lookup. The source of
this lookup can be of various types.</p>
<p>The <a id="mapfunc" name="mapfunc"><em>MapName</em></a> is
the name of the map and will be used to specify a
mapping-function for the substitution strings of a rewriting
rule via one of the following constructs:</p>
<blockquote>
<strong><code>${</code> <em>MapName</em> <code>:</code>
<em>LookupKey</em> <code>}</code><br />
<code>${</code> <em>MapName</em> <code>:</code>
<em>LookupKey</em> <code>|</code> <em>DefaultValue</em>
<code>}</code></strong>
</blockquote>
When such a construct occurs the map <em>MapName</em> is
consulted and the key <em>LookupKey</em> is looked-up. If the
key is found, the map-function construct is substituted by
<em>SubstValue</em>. If the key is not found then it is
substituted by <em>DefaultValue</em> or by the empty string
if no <em>DefaultValue</em> was specified.
<p>The following combinations for <em>MapType</em> and
<em>MapSource</em> can be used:</p>
<ul>
<li>
<strong>Standard Plain Text</strong><br />
MapType: <code>txt</code>, MapSource: Unix filesystem
path to valid regular file
<p>This is the standard rewriting map feature where the
<em>MapSource</em> is a plain ASCII file containing
either blank lines, comment lines (starting with a '#'
character) or pairs like the following - one per
line.</p>
<blockquote>
<strong><em>MatchingKey</em>
<em>SubstValue</em></strong>
</blockquote>
<p>Example:</p>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
##
## map.txt -- rewriting map
##
Ralf.S.Engelschall rse # Bastard Operator From Hell
Mr.Joe.Average joe # Mr. Average
</pre>
</td>
</tr>
</table>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
RewriteMap real-to-user txt:/path/to/file/map.txt
</pre>
</td>
</tr>
</table>
</li>
<li>
<strong>Randomized Plain Text</strong><br />
MapType: <code>rnd</code>, MapSource: Unix filesystem
path to valid regular file
<p>This is identical to the Standard Plain Text variant
above but with a special post-processing feature: After
looking up a value it is parsed according to contained
``<code>|</code>'' characters which have the meaning of
``or''. In other words they indicate a set of
alternatives from which the actual returned value is
chosen randomly. Although this sounds crazy and useless,
it was actually designed for load balancing in a reverse
proxy situation where the looked up values are server
names. Example:</p>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
##
## map.txt -- rewriting map
##
static www1|www2|www3|www4
dynamic www5|www6
</pre>
</td>
</tr>
</table>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
RewriteMap servers rnd:/path/to/file/map.txt
</pre>
</td>
</tr>
</table>
</li>
<li>
<strong>Hash File</strong><br />
MapType: <code>dbm</code>, MapSource: Unix filesystem
path to valid regular file
<p>Here the source is a binary NDBM format file
containing the same contents as a <em>Plain Text</em>
format file, but in a special representation which is
optimized for really fast lookups. You can create such a
file with any NDBM tool or with the following Perl
script:</p>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
#!/path/to/bin/perl
##
## txt2dbm -- convert txt map to dbm format
##
($txtmap, $dbmmap) = @ARGV;
open(TXT, "&lt;$txtmap");
dbmopen(%DB, $dbmmap, 0644);
while (&lt;TXT&gt;) {
next if (m|^s*#.*| or m|^s*$|);
$DB{$1} = $2 if (m|^\s*(\S+)\s+(\S+)$|);
}
dbmclose(%DB);
close(TXT)
</pre>
</td>
</tr>
</table>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
$ txt2dbm map.txt map.db
</pre>
</td>
</tr>
</table>
</li>
<li>
<strong>Internal Function</strong><br />
MapType: <code>int</code>, MapSource: Internal Apache
function
<p>Here the source is an internal Apache function.
Currently you cannot create your own, but the following
functions already exists:</p>
<ul>
<li><strong>toupper</strong>:<br />
Converts the looked up key to all upper case.</li>
<li><strong>tolower</strong>:<br />
Converts the looked up key to all lower case.</li>
<li><strong>escape</strong>:<br />
Translates special characters in the looked up key to
hex-encodings.</li>
<li><strong>unescape</strong>:<br />
Translates hex-encodings in the looked up key back to
special characters.</li>
</ul>
</li>
<li>
<strong>External Rewriting Program</strong><br />
MapType: <code>prg</code>, MapSource: Unix filesystem
path to valid regular file
<p>Here the source is a program, not a map file. To
create it you can use the language of your choice, but
the result has to be a executable (<em>i.e.</em>, either
object-code or a script with the magic cookie trick
'<code>#!/path/to/interpreter</code>' as the first
line).</p>
<p>This program is started once at startup of the Apache
servers and then communicates with the rewriting engine
over its <code>stdin</code> and <code>stdout</code>
file-handles. For each map-function lookup it will
receive the key to lookup as a newline-terminated string
on <code>stdin</code>. It then has to give back the
looked-up value as a newline-terminated string on
<code>stdout</code> or the four-character string
``<code>NULL</code>'' if it fails (<em>i.e.</em>, there
is no corresponding value for the given key). A trivial
program which will implement a 1:1 map (<em>i.e.</em>,
key == value) could be:</p>
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
#!/usr/bin/perl
$| = 1;
while (&lt;STDIN&gt;) {
# ...put here any transformations or lookups...
print $_;
}
</pre>
</td>
</tr>
</table>
<p>But be very careful:<br />
</p>
<ol>
<li>``<em>Keep it simple, stupid</em>'' (KISS), because
if this program hangs it will hang the Apache server
when the rule occurs.</li>
<li>Avoid one common mistake: never do buffered I/O on
<code>stdout</code>! This will cause a deadloop! Hence
the ``<code>$|=1</code>'' in the above example...</li>
<li>Use the <samp>RewriteLock</samp> directive to
define a lockfile mod_rewrite can use to synchronize
the communication to the program. By default no such
synchronization takes place.</li>
</ol>
</li>
</ul>
The <code>RewriteMap</code> directive can occur more than
once. For each mapping-function use one
<code>RewriteMap</code> directive to declare its rewriting
mapfile. While you cannot <strong>declare</strong> a map in
per-directory context it is of course possible to
<strong>use</strong> this map in per-directory context.
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Note:</strong> For plain text and DBM format
files the looked-up keys are cached in-core until the
<code>mtime</code> of the mapfile changes or the server
does a restart. This way you can have map-functions in
rules which are used for <strong>every</strong> request.
This is no problem, because the external lookup only
happens once!</td>
</tr>
</table>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteBase"
name="RewriteBase">RewriteBase</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteBase
<em>URL-path</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>default is the
physical directory path</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> directory,
.htaccess<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a>
<em>FileInfo</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache
1.2<br />
<p>The <code>RewriteBase</code> directive explicitly sets the
base URL for per-directory rewrites. As you will see below,
<code>RewriteRule</code> can be used in per-directory config
files (<code>.htaccess</code>). There it will act locally,
<em>i.e.</em>, the local directory prefix is stripped at this
stage of processing and your rewriting rules act only on the
remainder. At the end it is automatically added back to the
path.</p>
<p>When a substitution occurs for a new URL, this module has
to re-inject the URL into the server processing. To be able
to do this it needs to know what the corresponding URL-prefix
or URL-base is. By default this prefix is the corresponding
filepath itself. <strong>But at most websites URLs are NOT
directly related to physical filename paths, so this
assumption will usually be wrong!</strong> There you have to
use the <code>RewriteBase</code> directive to specify the
correct URL-prefix.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Notice:</strong> If your webserver's URLs are
<strong>not</strong> directly related to physical file
paths, you have to use <code>RewriteBase</code> in every
<code>.htaccess</code> files where you want to use
<code>RewriteRule</code> directives.</td>
</tr>
</table>
<p><strong>Example:</strong></p>
<blockquote>
Assume the following per-directory config file:
<table border="0" cellspacing="1" cellpadding="5"
bgcolor="#F0F0F0">
<tr>
<td>
<pre>
#
# /abc/def/.htaccess -- per-dir config file for directory /abc/def
# Remember: /abc/def is the physical path of /xyz, <em>i.e.</em>, the server
# has a 'Alias /xyz /abc/def' directive <em>e.g.</em>
#
RewriteEngine On
# let the server know that we were reached via /xyz and not
# via the physical path prefix /abc/def
RewriteBase /xyz
# now the rewriting rules
RewriteRule ^oldstuff\.html$ newstuff.html
</pre>
</td>
</tr>
</table>
<p>In the above example, a request to
<code>/xyz/oldstuff.html</code> gets correctly rewritten to
the physical file <code>/abc/def/newstuff.html</code>.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td>
<font size="-1"><strong>Note - For Apache
hackers:</strong><br />
The following list gives detailed information about
the internal processing steps:</font>
<pre>
<font size="-1">Request:
/xyz/oldstuff.html
Internal Processing:
/xyz/oldstuff.html -&gt; /abc/def/oldstuff.html (per-server Alias)
/abc/def/oldstuff.html -&gt; /abc/def/newstuff.html (per-dir RewriteRule)
/abc/def/newstuff.html -&gt; /xyz/newstuff.html (per-dir RewriteBase)
/xyz/newstuff.html -&gt; /abc/def/newstuff.html (per-server Alias)
Result:
/abc/def/newstuff.html
</font>
</pre>
<font size="-1">This seems very complicated but is
the correct Apache internal processing, because the
per-directory rewriting comes too late in the
process. So, when it occurs the (rewritten) request
has to be re-injected into the Apache kernel! BUT:
While this seems like a serious overhead, it really
isn't, because this re-injection happens fully
internally to the Apache server and the same
procedure is used by many other operations inside
Apache. So, you can be sure the design and
implementation is correct.</font>
</td>
</tr>
</table>
</blockquote>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteCond"
name="RewriteCond">RewriteCond</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteCond
<em>TestString</em> <em>CondPattern</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>None</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host, directory, .htaccess<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a>
<em>FileInfo</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache 1.2
(partially), Apache 1.3<br />
<p>The <code>RewriteCond</code> directive defines a rule
condition. Precede a <code>RewriteRule</code> directive with
one or more <code>RewriteCond</code> directives. The
following rewriting rule is only used if its pattern matches
the current state of the URI <strong>and</strong> if these
additional conditions apply too.</p>
<p><em>TestString</em> is a string which can contains the
following expanded constructs in addition to plain text:</p>
<ul>
<li>
<strong>RewriteRule backreferences</strong>: These are
backreferences of the form
<blockquote>
<strong><code>$N</code></strong>
</blockquote>
(0 &lt;= N &lt;= 9) which provide access to the grouped
parts (parenthesis!) of the pattern from the
corresponding <code>RewriteRule</code> directive (the one
following the current bunch of <code>RewriteCond</code>
directives).
</li>
<li>
<strong>RewriteCond backreferences</strong>: These are
backreferences of the form
<blockquote>
<strong><code>%N</code></strong>
</blockquote>
(1 &lt;= N &lt;= 9) which provide access to the grouped
parts (parentheses!) of the pattern from the last matched
<code>RewriteCond</code> directive in the current bunch
of conditions.
</li>
<li>
<strong>RewriteMap expansions</strong>: These are
expansions of the form
<blockquote>
<strong><code>${mapname:key|default}</code></strong>
</blockquote>
See <a href="#mapfunc">the documentation for
RewriteMap</a> for more details.
</li>
<li>
<strong>Server-Variables</strong>: These are variables of
the form
<blockquote>
<strong><code>%{</code> <em>NAME_OF_VARIABLE</em>
<code>}</code></strong>
</blockquote>
where <em>NAME_OF_VARIABLE</em> can be a string taken
from the following list:
<table bgcolor="#F0F0F0" cellspacing="0" cellpadding="5">
<tr>
<td valign="TOP">
<strong>HTTP headers:</strong>
<p><font size="-1">HTTP_USER_AGENT<br />
HTTP_REFERER<br />
HTTP_COOKIE<br />
HTTP_FORWARDED<br />
HTTP_HOST<br />
HTTP_PROXY_CONNECTION<br />
HTTP_ACCEPT<br />
</font></p>
</td>
<td valign="TOP">
<strong>connection &amp; request:</strong>
<p><font size="-1">REMOTE_ADDR<br />
REMOTE_HOST<br />
REMOTE_USER<br />
REMOTE_IDENT<br />
REQUEST_METHOD<br />
SCRIPT_FILENAME<br />
PATH_INFO<br />
QUERY_STRING<br />
AUTH_TYPE<br />
</font></p>
</td>
</tr>
<tr>
<td valign="TOP">
<strong>server internals:</strong>
<p><font size="-1">DOCUMENT_ROOT<br />
SERVER_ADMIN<br />
SERVER_NAME<br />
SERVER_ADDR<br />
SERVER_PORT<br />
SERVER_PROTOCOL<br />
SERVER_SOFTWARE<br />
</font></p>
</td>
<td valign="TOP">
<strong>system stuff:</strong>
<p><font size="-1">TIME_YEAR<br />
TIME_MON<br />
TIME_DAY<br />
TIME_HOUR<br />
TIME_MIN<br />
TIME_SEC<br />
TIME_WDAY<br />
TIME<br />
</font></p>
</td>
<td valign="TOP">
<strong>specials:</strong>
<p><font size="-1">API_VERSION<br />
THE_REQUEST<br />
REQUEST_URI<br />
REQUEST_FILENAME<br />
IS_SUBREQ<br />
</font></p>
</td>
</tr>
</table>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td>
<p><strong>Notice:</strong> These variables all
correspond to the similarly named HTTP
MIME-headers, C variables of the Apache server or
<code>struct tm</code> fields of the Unix system.
Most are documented elsewhere in the Manual or in
the CGI specification. Those that are special to
mod_rewrite include:</p>
<dl>
<dt><code>IS_SUBREQ</code></dt>
<dd>Will contain the text "true" if the request
currently being processed is a sub-request,
"false" otherwise. Sub-requests may be generated
by modules that need to resolve additional files
or URIs in order to complete their tasks.</dd>
<dt><code>API_VERSION</code></dt>
<dd>This is the version of the Apache module API
(the internal interface between server and
module) in the current httpd build, as defined in
include/ap_mmn.h. The module API version
corresponds to the version of Apache in use (in
the release version of Apache 1.3.14, for
instance, it is 19990320:10), but is mainly of
interest to module authors.</dd>
<dt><code>THE_REQUEST</code></dt>
<dd>The full HTTP request line sent by the
browser to the server (e.g., "<code>GET
/index.html HTTP/1.1</code>"). This does not
include any additional headers sent by the
browser.</dd>
<dt><code>REQUEST_URI</code></dt>
<dd>The resource requested in the HTTP request
line. (In the example above, this would be
"/index.html".)</dd>
<dt><code>REQUEST_FILENAME</code></dt>
<dd>The full local filesystem path to the file or
script matching the request.</dd>
</dl>
</td>
</tr>
</table>
</li>
</ul>
<p>Special Notes:</p>
<ol>
<li>The variables SCRIPT_FILENAME and REQUEST_FILENAME
contain the same value, <em>i.e.</em>, the value of the
<code>filename</code> field of the internal
<code>request_rec</code> structure of the Apache server.
The first name is just the commonly known CGI variable name
while the second is the consistent counterpart to
REQUEST_URI (which contains the value of the
<code>uri</code> field of <code>request_rec</code>).</li>
<li>There is the special format:
<code>%{ENV:variable}</code> where <em>variable</em> can be
any environment variable. This is looked-up via internal
Apache structures and (if not found there) via
<code>getenv()</code> from the Apache server process.</li>
<li>There is the special format:
<code>%{HTTP:header}</code> where <em>header</em> can be
any HTTP MIME-header name. This is looked-up from the HTTP
request. Example: <code>%{HTTP:Proxy-Connection}</code> is
the value of the HTTP header
``<code>Proxy-Connection:</code>''.</li>
<li>There is the special format
<code>%{LA-U:variable}</code> for look-aheads which perform
an internal (URL-based) sub-request to determine the final
value of <em>variable</em>. Use this when you want to use a
variable for rewriting which is actually set later in an
API phase and thus is not available at the current stage.
For instance when you want to rewrite according to the
<code>REMOTE_USER</code> variable from within the
per-server context (<code>httpd.conf</code> file) you have
to use <code>%{LA-U:REMOTE_USER}</code> because this
variable is set by the authorization phases which come
<em>after</em> the URL translation phase where mod_rewrite
operates. On the other hand, because mod_rewrite implements
its per-directory context (<code>.htaccess</code> file) via
the Fixup phase of the API and because the authorization
phases come <em>before</em> this phase, you just can use
<code>%{REMOTE_USER}</code> there.</li>
<li>There is the special format:
<code>%{LA-F:variable}</code> which performs an internal
(filename-based) sub-request to determine the final value
of <em>variable</em>. Most of the time this is the same as
LA-U above.</li>
</ol>
<p><em>CondPattern</em> is the condition pattern,
<em>i.e.</em>, a regular expression which is applied to the
current instance of the <em>TestString</em>, <em>i.e.</em>,
<em>TestString</em> is evaluated and then matched against
<em>CondPattern</em>.</p>
<p><strong>Remember:</strong> <em>CondPattern</em> is a
standard <em>Extended Regular Expression</em> with some
additions:</p>
<ol>
<li>You can prefix the pattern string with a
'<code>!</code>' character (exclamation mark) to specify a
<strong>non</strong>-matching pattern.</li>
<li>
There are some special variants of <em>CondPatterns</em>.
Instead of real regular expression strings you can also
use one of the following:
<ul>
<li>'<strong>&lt;CondPattern</strong>' (is lexically
lower)<br />
Treats the <em>CondPattern</em> as a plain string and
compares it lexically to <em>TestString</em>. True if
<em>TestString</em> is lexically lower than
<em>CondPattern</em>.</li>
<li>'<strong>&gt;CondPattern</strong>' (is lexically
greater)<br />
Treats the <em>CondPattern</em> as a plain string and
compares it lexically to <em>TestString</em>. True if
<em>TestString</em> is lexically greater than
<em>CondPattern</em>.</li>
<li>'<strong>=CondPattern</strong>' (is lexically
equal)<br />
Treats the <em>CondPattern</em> as a plain string and
compares it lexically to <em>TestString</em>. True if
<em>TestString</em> is lexically equal to
<em>CondPattern</em>, i.e the two strings are exactly
equal (character by character). If <em>CondPattern</em>
is just <samp>""</samp> (two quotation marks) this
compares <em>TestString</em> to the empty string.</li>
<li>'<strong>-d</strong>' (is
<strong>d</strong>irectory)<br />
Treats the <em>TestString</em> as a pathname and tests
if it exists and is a directory.</li>
<li>'<strong>-f</strong>' (is regular
<strong>f</strong>ile)<br />
Treats the <em>TestString</em> as a pathname and tests
if it exists and is a regular file.</li>
<li>'<strong>-s</strong>' (is regular file with
<strong>s</strong>ize)<br />
Treats the <em>TestString</em> as a pathname and tests
if it exists and is a regular file with size greater
than zero.</li>
<li>'<strong>-l</strong>' (is symbolic
<strong>l</strong>ink)<br />
Treats the <em>TestString</em> as a pathname and tests
if it exists and is a symbolic link.</li>
<li>'<strong>-F</strong>' (is existing file via
subrequest)<br />
Checks if <em>TestString</em> is a valid file and
accessible via all the server's currently-configured
access controls for that path. This uses an internal
subrequest to determine the check, so use it with care
because it decreases your servers performance!</li>
<li>'<strong>-U</strong>' (is existing URL via
subrequest)<br />
Checks if <em>TestString</em> is a valid URL and
accessible via all the server's currently-configured
access controls for that path. This uses an internal
subrequest to determine the check, so use it with care
because it decreases your server's performance!</li>
</ul>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Notice:</strong> All of these tests can
also be prefixed by an exclamation mark ('!') to
negate their meaning.</td>
</tr>
</table>
</li>
</ol>
<p>Additionally you can set special flags for
<em>CondPattern</em> by appending</p>
<blockquote>
<strong><code>[</code><em>flags</em><code>]</code></strong>
</blockquote>
as the third argument to the <code>RewriteCond</code>
directive. <em>Flags</em> is a comma-separated list of the
following flags:
<ul>
<li>'<strong><code>nocase|NC</code></strong>'
(<strong>n</strong>o <strong>c</strong>ase)<br />
This makes the test case-insensitive, <em>i.e.</em>, there
is no difference between 'A-Z' and 'a-z' both in the
expanded <em>TestString</em> and the <em>CondPattern</em>.
This flag is effective only for comparisons between
<em>TestString</em> and <em>CondPattern</em>. It has no
effect on filesystem and subrequest checks.</li>
<li>
'<strong><code>ornext|OR</code></strong>'
(<strong>or</strong> next condition)<br />
Use this to combine rule conditions with a local OR
instead of the implicit AND. Typical example:
<blockquote>
<pre>
RewriteCond %{REMOTE_HOST} ^host1.* [OR]
RewriteCond %{REMOTE_HOST} ^host2.* [OR]
RewriteCond %{REMOTE_HOST} ^host3.*
RewriteRule ...some special stuff for any of these hosts...
</pre>
</blockquote>
Without this flag you would have to write the cond/rule
three times.
</li>
</ul>
<p><strong>Example:</strong></p>
<blockquote>
To rewrite the Homepage of a site according to the
``<code>User-Agent:</code>'' header of the request, you can
use the following:
<blockquote>
<pre>
RewriteCond %{HTTP_USER_AGENT} ^Mozilla.*
RewriteRule ^/$ /homepage.max.html [L]
RewriteCond %{HTTP_USER_AGENT} ^Lynx.*
RewriteRule ^/$ /homepage.min.html [L]
RewriteRule ^/$ /homepage.std.html [L]
</pre>
</blockquote>
Interpretation: If you use Netscape Navigator as your
browser (which identifies itself as 'Mozilla'), then you
get the max homepage, which includes Frames, <em>etc.</em>
If you use the Lynx browser (which is Terminal-based), then
you get the min homepage, which contains no images, no
tables, <em>etc.</em> If you use any other browser you get
the standard homepage.
</blockquote>
<hr noshade="noshade" size="1" />
<h3><a id="RewriteRule"
name="RewriteRule">RewriteRule</a></h3>
<a href="directive-dict.html#Syntax"
rel="Help"><strong>Syntax:</strong></a> RewriteRule
<em>Pattern</em> <em>Substitution</em><br />
<a href="directive-dict.html#Default"
rel="Help"><strong>Default:</strong></a> <em>None</em><br />
<a href="directive-dict.html#Context"
rel="Help"><strong>Context:</strong></a> server config,
virtual host, directory, .htaccess<br />
<a href="directive-dict.html#Override"
rel="Help"><strong>Override:</strong></a>
<em>FileInfo</em><br />
<a href="directive-dict.html#Status"
rel="Help"><strong>Status:</strong></a> Extension<br />
<a href="directive-dict.html#Module"
rel="Help"><strong>Module:</strong></a> mod_rewrite.c<br />
<a href="directive-dict.html#Compatibility"
rel="Help"><strong>Compatibility:</strong></a> Apache 1.2
(partially), Apache 1.3<br />
<p>The <code>RewriteRule</code> directive is the real
rewriting workhorse. The directive can occur more than once.
Each directive then defines one single rewriting rule. The
<strong>definition order</strong> of these rules is
<strong>important</strong>, because this order is used when
applying the rules at run-time.</p>
<p><a id="patterns" name="patterns"><em>Pattern</em></a> can
be (for Apache 1.1.x a System V8 and for Apache 1.2.x and
later a POSIX) <a id="regexp" name="regexp">regular
expression</a> which gets applied to the current URL. Here
``current'' means the value of the URL when this rule gets
applied. This may not be the originally requested URL,
because any number of rules may already have matched and made
alterations to it.</p>
<p>Some hints about the syntax of regular expressions:</p>
<table bgcolor="#F0F0F0" cellspacing="0" cellpadding="5">
<tr>
<td valign="TOP">
<pre>
<strong>Text:</strong>
<strong><code>.</code></strong> Any single character
<strong><code>[</code></strong>chars<strong><code>]</code></strong> Character class: One of chars
<strong><code>[^</code></strong>chars<strong><code>]</code></strong> Character class: None of chars
text1<strong><code>|</code></strong>text2 Alternative: text1 or text2
<strong>Quantifiers:</strong>
<strong><code>?</code></strong> 0 or 1 of the preceding text
<strong><code>*</code></strong> 0 or N of the preceding text (N &gt; 0)
<strong><code>+</code></strong> 1 or N of the preceding text (N &gt; 1)
<strong>Grouping:</strong>
<strong><code>(</code></strong>text<strong><code>)</code></strong> Grouping of text
(either to set the borders of an alternative or
for making backreferences where the <strong>N</strong>th group can
be used on the RHS of a RewriteRule with <code>$</code><strong>N</strong>)
<strong>Anchors:</strong>
<strong><code>^</code></strong> Start of line anchor
<strong><code>$</code></strong> End of line anchor
<strong>Escaping:</strong>
<strong><code>\</code></strong>char escape that particular char
(for instance to specify the chars "<code>.[]()</code>" <em>etc.</em>)
</pre>
</td>
</tr>
</table>
<p>For more information about regular expressions either have
a look at your local regex(3) manpage or its
<code>src/regex/regex.3</code> copy in the Apache 1.3
distribution. If you are interested in more detailed
information about regular expressions and their variants
(POSIX regex, Perl regex, <em>etc.</em>) have a look at the
following dedicated book on this topic:</p>
<blockquote>
<em>Mastering Regular Expressions</em><br />
Jeffrey E.F. Friedl<br />
Nutshell Handbook Series<br />
O'Reilly &amp; Associates, Inc. 1997<br />
ISBN 1-56592-257-3<br />
</blockquote>
<p>Additionally in mod_rewrite the NOT character
('<code>!</code>') is a possible pattern prefix. This gives
you the ability to negate a pattern; to say, for instance:
``<em>if the current URL does <strong>NOT</strong> match this
pattern</em>''. This can be used for exceptional cases, where
it is easier to match the negative pattern, or as a last
default rule.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Notice:</strong> When using the NOT character
to negate a pattern you cannot have grouped wildcard
parts in the pattern. This is impossible because when the
pattern does NOT match, there are no contents for the
groups. In consequence, if negated patterns are used, you
cannot use <code>$N</code> in the substitution
string!</td>
</tr>
</table>
<p><a id="rhs" name="rhs"><em>Substitution</em></a> of a
rewriting rule is the string which is substituted for (or
replaces) the original URL for which <em>Pattern</em>
matched. Beside plain text you can use</p>
<ol>
<li>back-references <code>$N</code> to the RewriteRule
pattern</li>
<li>back-references <code>%N</code> to the last matched
RewriteCond pattern</li>
<li>server-variables as in rule condition test-strings
(<code>%{VARNAME}</code>)</li>
<li><a href="#mapfunc">mapping-function</a> calls
(<code>${mapname:key|default}</code>)</li>
</ol>
Back-references are <code>$</code><strong>N</strong>
(<strong>N</strong>=0..9) identifiers which will be replaced
by the contents of the <strong>N</strong>th group of the
matched <em>Pattern</em>. The server-variables are the same
as for the <em>TestString</em> of a <code>RewriteCond</code>
directive. The mapping-functions come from the
<code>RewriteMap</code> directive and are explained there.
These three types of variables are expanded in the order of
the above list.
<p>As already mentioned above, all the rewriting rules are
applied to the <em>Substitution</em> (in the order of
definition in the config file). The URL is <strong>completely
replaced</strong> by the <em>Substitution</em> and the
rewriting process goes on until there are no more rules
unless explicitly terminated by a
<code><strong>L</strong></code> flag - see below.</p>
<p>There is a special substitution string named
'<code>-</code>' which means: <strong>NO
substitution</strong>! Sounds silly? No, it is useful to
provide rewriting rules which <strong>only</strong> match
some URLs but do no substitution, <em>e.g.</em>, in
conjunction with the <strong>C</strong> (chain) flag to be
able to have more than one pattern to be applied before a
substitution occurs.</p>
<p>One more note: You can even create URLs in the
substitution string containing a query string part. Just use
a question mark inside the substitution string to indicate
that the following stuff should be re-injected into the
QUERY_STRING. When you want to erase an existing query
string, end the substitution string with just the question
mark.</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Note</strong>: There is a special feature:
When you prefix a substitution field with
<code>http://</code><em>thishost</em>[<em>:thisport</em>]
then <strong>mod_rewrite</strong> automatically strips it
out. This auto-reduction on implicit external redirect
URLs is a useful and important feature when used in
combination with a mapping-function which generates the
hostname part. Have a look at the first example in the
example section below to understand this.</td>
</tr>
</table>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Remember:</strong> An unconditional external
redirect to your own server will not work with the prefix
<code>http://thishost</code> because of this feature. To
achieve such a self-redirect, you have to use the
<strong>R</strong>-flag (see below).</td>
</tr>
</table>
<p>Additionally you can set special flags for
<em>Substitution</em> by appending</p>
<blockquote>
<strong><code>[</code><em>flags</em><code>]</code></strong>
</blockquote>
as the third argument to the <code>RewriteRule</code>
directive. <em>Flags</em> is a comma-separated list of the
following flags:
<ul>
<li>
'<strong><code>redirect|R</code>
[=<em>code</em>]</strong>' (force <a id="redirect"
name="redirect"><strong>r</strong>edirect</a>)<br />
Prefix <em>Substitution</em> with
<code>http://thishost[:thisport]/</code> (which makes the
new URL a URI) to force a external redirection. If no
<em>code</em> is given a HTTP response of 302 (MOVED
TEMPORARILY) is used. If you want to use other response
codes in the range 300-400 just specify them as a number
or use one of the following symbolic names:
<code>temp</code> (default), <code>permanent</code>,
<code>seeother</code>. Use it for rules which should
canonicalize the URL and give it back to the client,
<em>e.g.</em>, translate ``<code>/~</code>'' into
``<code>/u/</code>'' or always append a slash to
<code>/u/</code><em>user</em>, etc.<br />
<p><strong>Note:</strong> When you use this flag, make
sure that the substitution field is a valid URL! If not,
you are redirecting to an invalid location! And remember
that this flag itself only prefixes the URL with
<code>http://thishost[:thisport]/</code>, rewriting
continues. Usually you also want to stop and do the
redirection immediately. To stop the rewriting you also
have to provide the 'L' flag.</p>
</li>
<li>'<strong><code>forbidden|F</code></strong>' (force URL
to be <strong>f</strong>orbidden)<br />
This forces the current URL to be forbidden,
<em>i.e.</em>, it immediately sends back a HTTP response of
403 (FORBIDDEN). Use this flag in conjunction with
appropriate RewriteConds to conditionally block some
URLs.</li>
<li>'<strong><code>gone|G</code></strong>' (force URL to be
<strong>g</strong>one)<br />
This forces the current URL to be gone, <em>i.e.</em>, it
immediately sends back a HTTP response of 410 (GONE). Use
this flag to mark pages which no longer exist as gone.</li>
<li>
'<strong><code>proxy|P</code></strong>' (force
<strong>p</strong>roxy)<br />
This flag forces the substitution part to be internally
forced as a proxy request and immediately (<em>i.e.</em>,
rewriting rule processing stops here) put through the <a
href="mod_proxy.html">proxy module</a>. You have to make
sure that the substitution string is a valid URI
(<em>e.g.</em>, typically starting with
<code>http://</code><em>hostname</em>) which can be
handled by the Apache proxy module. If not you get an
error from the proxy module. Use this flag to achieve a
more powerful implementation of the <a
href="mod_proxy.html#proxypass">ProxyPass</a> directive,
to map some remote stuff into the namespace of the local
server.
<p>Notice: To use this functionality make sure you have
the proxy module compiled into your Apache server
program. If you don't know please check whether
<code>mod_proxy.c</code> is part of the ``<code>httpd
-l</code>'' output. If yes, this functionality is
available to mod_rewrite. If not, then you first have to
rebuild the ``<code>httpd</code>'' program with mod_proxy
enabled.</p>
</li>
<li>'<strong><code>last|L</code></strong>'
(<strong>l</strong>ast rule)<br />
Stop the rewriting process here and don't apply any more
rewriting rules. This corresponds to the Perl
<code>last</code> command or the <code>break</code> command
from the C language. Use this flag to prevent the currently
rewritten URL from being rewritten further by following
rules. For example, use it to rewrite the root-path URL
('<code>/</code>') to a real one, <em>e.g.</em>,
'<code>/e/www/</code>'.</li>
<li>'<strong><code>next|N</code></strong>'
(<strong>n</strong>ext round)<br />
Re-run the rewriting process (starting again with the
first rewriting rule). Here the URL to match is again not
the original URL but the URL from the last rewriting rule.
This corresponds to the Perl <code>next</code> command or
the <code>continue</code> command from the C language. Use
this flag to restart the rewriting process, <em>i.e.</em>,
to immediately go to the top of the loop.<br />
<strong>But be careful not to create an infinite
loop!</strong></li>
<li>'<strong><code>chain|C</code></strong>'
(<strong>c</strong>hained with next rule)<br />
This flag chains the current rule with the next rule
(which itself can be chained with the following rule,
<em>etc.</em>). This has the following effect: if a rule
matches, then processing continues as usual, <em>i.e.</em>,
the flag has no effect. If the rule does
<strong>not</strong> match, then all following chained
rules are skipped. For instance, use it to remove the
``<code>.www</code>'' part inside a per-directory rule set
when you let an external redirect happen (where the
``<code>.www</code>'' part should not to occur!).</li>
<li>
'<strong><code>type|T</code></strong>=<em>MIME-type</em>'
(force MIME <strong>t</strong>ype)<br />
Force the MIME-type of the target file to be
<em>MIME-type</em>. For instance, this can be used to
simulate the <code>mod_alias</code> directive
<code>ScriptAlias</code> which internally forces all files
inside the mapped directory to have a MIME type of
``<code>application/x-httpd-cgi</code>''.</li>
<li>
'<strong><code>nosubreq|NS</code></strong>' (used only if
<strong>n</strong>o internal
<strong>s</strong>ub-request)<br />
This flag forces the rewriting engine to skip a
rewriting rule if the current request is an internal
sub-request. For instance, sub-requests occur internally
in Apache when <code>mod_include</code> tries to find out
information about possible directory default files
(<code>index.xxx</code>). On sub-requests it is not
always useful and even sometimes causes a failure to if
the complete set of rules are applied. Use this flag to
exclude some rules.<br />
<p>Use the following rule for your decision: whenever you
prefix some URLs with CGI-scripts to force them to be
processed by the CGI-script, the chance is high that you
will run into problems (or even overhead) on
sub-requests. In these cases, use this flag.</p>
</li>
<li>'<strong><code>nocase|NC</code></strong>'
(<strong>n</strong>o <strong>c</strong>ase)<br />
This makes the <em>Pattern</em> case-insensitive,
<em>i.e.</em>, there is no difference between 'A-Z' and
'a-z' when <em>Pattern</em> is matched against the current
URL.</li>
<li>'<strong><code>qsappend|QSA</code></strong>'
(<strong>q</strong>uery <strong>s</strong>tring
<strong>a</strong>ppend)<br />
This flag forces the rewriting engine to append a query
string part in the substitution string to the existing one
instead of replacing it. Use this when you want to add more
data to the query string via a rewrite rule.</li>
<li>
'<strong><code>noescape|NE</code></strong>'
(<strong>n</strong>o URI <strong>e</strong>scaping of
output)<br />
This flag keeps mod_rewrite from applying the usual URI
escaping rules to the result of a rewrite. Ordinarily,
special characters (such as '%', '$', ';', and so on)
will be escaped into their hexcode equivalents ('%25',
'%24', and '%3B', respectively); this flag prevents this
from being done. This allows percent symbols to appear in
the output, as in
<pre>
RewriteRule /foo/(.*) /bar?arg=P1\%3d$1 [R,NE]
</pre>
which would turn '<code>/foo/zed</code>' into a safe
request for '<code>/bar?arg=P1=zed</code>'.
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Notice:</strong> The
<code>noescape</code> flag is only available with
Apache 1.3.20 and later versions.</td>
</tr>
</table>
</li>
<li>
'<strong><code>passthrough|PT</code></strong>'
(<strong>p</strong>ass <strong>t</strong>hrough to next
handler)<br />
This flag forces the rewriting engine to set the
<code>uri</code> field of the internal
<code>request_rec</code> structure to the value of the
<code>filename</code> field. This flag is just a hack to
be able to post-process the output of
<code>RewriteRule</code> directives by
<code>Alias</code>, <code>ScriptAlias</code>,
<code>Redirect</code>, <em>etc.</em> directives from
other URI-to-filename translators. A trivial example to
show the semantics: If you want to rewrite
<code>/abc</code> to <code>/def</code> via the rewriting
engine of <code>mod_rewrite</code> and then
<code>/def</code> to <code>/ghi</code> with
<code>mod_alias</code>:
<pre>
RewriteRule ^/abc(.*) /def$1 [PT]
Alias /def /ghi
</pre>
If you omit the <code>PT</code> flag then
<code>mod_rewrite</code> will do its job fine,
<em>i.e.</em>, it rewrites <code>uri=/abc/...</code> to
<code>filename=/def/...</code> as a full API-compliant
URI-to-filename translator should do. Then
<code>mod_alias</code> comes and tries to do a
URI-to-filename transition which will not work.
<p>Note: <strong>You have to use this flag if you want to
intermix directives of different modules which contain
URL-to-filename translators</strong>. The typical example
is the use of <code>mod_alias</code> and
<code>mod_rewrite</code>..</p>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><font size="-1"><strong>Note - For Apache
hackers:</strong><br />
If the current Apache API had a filename-to-filename
hook additionally to the URI-to-filename hook then we
wouldn't need this flag! But without such a hook this
flag is the only solution. The Apache Group has
discussed this problem and will add such a hook in
Apache version 2.0.</font> </td>
</tr>
</table>
</li>
<li>'<strong><code>skip|S</code></strong>=<em>num</em>'
(<strong>s</strong>kip next rule(s))<br />
This flag forces the rewriting engine to skip the next
<em>num</em> rules in sequence when the current rule
matches. Use this to make pseudo if-then-else constructs:
The last rule of the then-clause becomes
<code>skip=N</code> where N is the number of rules in the
else-clause. (This is <strong>not</strong> the same as the
'chain|C' flag!)</li>
<li>
'<strong><code>env|E=</code></strong><em>VAR</em>:<em>VAL</em>'
(set <strong>e</strong>nvironment variable)<br />
This forces an environment variable named <em>VAR</em> to
be set to the value <em>VAL</em>, where <em>VAL</em> can
contain regexp backreferences <code>$N</code> and
<code>%N</code> which will be expanded. You can use this
flag more than once to set more than one variable. The
variables can be later dereferenced in many situations, but
usually from within XSSI (via <code>&lt;!--#echo
var="VAR"--&gt;</code>) or CGI (<em>e.g.</em>
<code>$ENV{'VAR'}</code>). Additionally you can dereference
it in a following RewriteCond pattern via
<code>%{ENV:VAR}</code>. Use this to strip but remember
information from URLs.</li>
</ul>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td>
<strong>Note:</strong> Never forget that
<em>Pattern</em> is applied to a complete URL in
per-server configuration files. <strong>But in
per-directory configuration files, the per-directory
prefix (which always is the same for a specific
directory!) is automatically <em>removed</em> for the
pattern matching and automatically <em>added</em> after
the substitution has been done.</strong> This feature
is essential for many sorts of rewriting, because
without this prefix stripping you have to match the
parent directory which is not always possible.
<p>There is one exception: If a substitution string
starts with ``<code>http://</code>'' then the directory
prefix will <strong>not</strong> be added and an
external redirect or proxy throughput (if flag
<strong>P</strong> is used!) is forced!</p>
</td>
</tr>
</table>
<table width="70%" border="0" bgcolor="#E0E0F0"
cellspacing="0" cellpadding="10">
<tr>
<td><strong>Note:</strong> To enable the rewriting engine
for per-directory configuration files you need to set
``<code>RewriteEngine On</code>'' in these files
<strong>and</strong> ``<code>Options
FollowSymLinks</code>'' must be enabled. If your
administrator has disabled override of
<code>FollowSymLinks</code> for a user's directory, then
you cannot use the rewriting engine. This restriction is
needed for security reasons.</td>
</tr>
</table>
<p>Here are all possible substitution combinations and their
meanings:</p>
<p><strong>Inside per-server configuration
(<code>httpd.conf</code>)<br />
for request ``<code>GET
/somepath/pathinfo</code>'':</strong><br />
</p>
<table bgcolor="#F0F0F0" cellspacing="0" cellpadding="5">
<tr>
<td>
<pre>
<strong>Given Rule</strong> <strong>Resulting Substitution</strong>
---------------------------------------------- ----------------------------------
^/somepath(.*) otherpath$1 not supported, because invalid!
^/somepath(.*) otherpath$1 [R] not supported, because invalid!
^/somepath(.*) otherpath$1 [P] not supported, because invalid!
---------------------------------------------- ----------------------------------
^/somepath(.*) /otherpath$1 /otherpath/pathinfo
^/somepath(.*) /otherpath$1 [R] http://thishost/otherpath/pathinfo
via external redirection
^/somepath(.*) /otherpath$1 [P] not supported, because silly!
---------------------------------------------- ----------------------------------
^/somepath(.*) http://thishost/otherpath$1 /otherpath/pathinfo
^/somepath(.*) http://thishost/otherpath$1 [R] http://thishost/otherpath/pathinfo
via external redirection
^/somepath(.*) http://thishost/otherpath$1 [P] not supported, because silly!
---------------------------------------------- ----------------------------------
^/somepath(.*) http://otherhost/otherpath$1 http://otherhost/otherpath/pathinfo
via external redirection
^/somepath(.*) http://otherhost/otherpath$1 [R] http://otherhost/otherpath/pathinfo
via external redirection
(the [R] flag is redundant)
^/somepath(.*) http://otherhost/otherpath$1 [P] http://otherhost/otherpath/pathinfo
via internal proxy
</pre>
</td>
</tr>
</table>
<p><strong>Inside per-directory configuration for
<code>/somepath</code><br />
(<em>i.e.</em>, file <code>.htaccess</code> in dir
<code>/physical/path/to/somepath</code> containing
<code>RewriteBase /somepath</code>)<br />
for request ``<code>GET
/somepath/localpath/pathinfo</code>'':</strong><br />
</p>
<table bgcolor="#F0F0F0" cellspacing="0" cellpadding="5">
<tr>
<td>
<pre>
<strong>Given Rule</strong> <strong>Resulting Substitution</strong>
---------------------------------------------- ----------------------------------
^localpath(.*) otherpath$1 /somepath/otherpath/pathinfo
^localpath(.*) otherpath$1 [R] http://thishost/somepath/otherpath/pathinfo
via external redirection
^localpath(.*) otherpath$1 [P] not supported, because silly!
---------------------------------------------- ----------------------------------
^localpath(.*) /otherpath$1 /otherpath/pathinfo
^localpath(.*) /otherpath$1 [R] http://thishost/otherpath/pathinfo
via external redirection
^localpath(.*) /otherpath$1 [P] not supported, because silly!
---------------------------------------------- ----------------------------------
^localpath(.*) http://thishost/otherpath$1 /otherpath/pathinfo
^localpath(.*) http://thishost/otherpath$1 [R] http://thishost/otherpath/pathinfo
via external redirection
^localpath(.*) http://thishost/otherpath$1 [P] not supported, because silly!
---------------------------------------------- ----------------------------------
^localpath(.*) http://otherhost/otherpath$1 http://otherhost/otherpath/pathinfo
via external redirection
^localpath(.*) http://otherhost/otherpath$1 [R] http://otherhost/otherpath/pathinfo
via external redirection
(the [R] flag is redundant)
^localpath(.*) http://otherhost/otherpath$1 [P] http://otherhost/otherpath/pathinfo
via internal proxy
</pre>
</td>
</tr>
</table>
<p><strong>Example:</strong></p>
<blockquote>
We want to rewrite URLs of the form
<blockquote>
<code>/</code> <em>Language</em> <code>/~</code>
<em>Realname</em> <code>/.../</code> <em>File</em>
</blockquote>
into
<blockquote>
<code>/u/</code> <em>Username</em> <code>/.../</code>
<em>File</em> <code>.</code> <em>Language</em>
</blockquote>
<p>We take the rewrite mapfile from above and save it under
<code>/path/to/file/map.txt</code>. Then we only have to
add the following lines to the Apache server configuration
file:</p>
<blockquote>
<pre>
RewriteLog /path/to/file/rewrite.log
RewriteMap real-to-user txt:/path/to/file/map.txt
RewriteRule ^/([^/]+)/~([^/]+)/(.*)$ /u/${real-to-user:$2|nobody}/$3.$1
</pre>
</blockquote>
</blockquote>
<hr noshade="noshade" size="1" />
<center>
<h1><a id="Miscelleneous"
name="Miscelleneous">Miscellaneous</a></h1>
</center>
<hr noshade="noshade" size="1" />
<h2><a id="EnvVar" name="EnvVar">Environment
Variables</a></h2>
This module keeps track of two additional (non-standard)
CGI/SSI environment variables named <code>SCRIPT_URL</code>
and <code>SCRIPT_URI</code>. These contain the
<em>logical</em> Web-view to the current resource, while the
standard CGI/SSI variables <code>SCRIPT_NAME</code> and
<code>SCRIPT_FILENAME</code> contain the <em>physical</em>
System-view.
<p>Notice: These variables hold the URI/URL <em>as they were
initially requested</em>, <em>i.e.</em>, <em>before</em> any
rewriting. This is important because the rewriting process is
primarily used to rewrite logical URLs to physical
pathnames.</p>
<p><strong>Example:</strong></p>
<blockquote>
<pre>
SCRIPT_NAME=/sw/lib/w3s/tree/global/u/rse/.www/index.html
SCRIPT_FILENAME=/u/rse/.www/index.html
SCRIPT_URL=/u/rse/
SCRIPT_URI=http://en1.engelschall.com/u/rse/
</pre>
</blockquote>
<hr noshade="noshade" size="1" />
<h2><a id="Solutions" name="Solutions">Practical
Solutions</a></h2>
We also have an <a href="../misc/rewriteguide.html">URL
Rewriting Guide</a> available, which provides a collection of
practical solutions for URL-based problems. There you can
find real-life rulesets and additional information about
mod_rewrite. <!--#include virtual="footer.html" -->
</blockquote>
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</html>
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