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254 lines
11 KiB
Plaintext
Executable File
254 lines
11 KiB
Plaintext
Executable File
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<TITLE>Mapping URLs to Filesystem Locations - Apache HTTP Server</TITLE>
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</HEAD>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<BODY
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#000080"
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ALINK="#FF0000"
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>
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<!--#include virtual="header.html" -->
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<h1 align="center">Mapping URLs to Filesystem Locations</h1>
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<p>This document explains the method in which Apache determines
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what filesystem location to serve a file from based on the
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URL of a request.</p>
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<ul>
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<li><a href="#documentroot">DocumentRoot</a></li>
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<li><a href="#outside">Files Outside the DocumentRoot</a></li>
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<li><a href="#user">User Directories</a></li>
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<li><a href="#redirect">URL Redirection</a></li>
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<li><a href="#rewrite">Rewrite Engine</a></li>
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<li><a href="#notfound">File Not Found</a></li>
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</ul>
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<hr>
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<table border="1">
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<tr><td valign="top">
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<strong>Related Modules</strong><br><br>
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<a href="mod/mod_alias.html">mod_alias</a><br>
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<a href="mod/mod_rewrite.html">mod_rewrite</a><br>
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<a href="mod/mod_userdir.html">mod_userdir</a><br>
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<a href="mod/mod_speling.html">mod_speling</a><br>
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<a href="mod/mod_vhost_alias.html">mod_vhost_alias</a><br>
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</td><td valign="top">
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<strong>Related Directives</strong><br><br>
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<A HREF="mod/mod_alias.html#alias">Alias</A><br>
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<A HREF="mod/mod_alias.html#aliasmatch">AliasMatch</A><br>
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<A HREF="mod/mod_speling.html#checkspelling">CheckSpelling</A><br>
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<A HREF="mod/core.html#documentroot">DocumentRoot</A><br>
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<A HREF="mod/core.html#errordocument">ErrorDocument</A><br>
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<a href="mod/core.html#options">Options</a><br>
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<A HREF="mod/mod_alias.html#redirect">Redirect</A><br>
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<A HREF="mod/mod_alias.html#redirectmatch">RedirectMatch</A><br>
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<A HREF="mod/mod_rewrite.html#RewriteCond">RewriteCond</A><br>
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<A HREF="mod/mod_rewrite.html#RewriteRule">RewriteRule</A><br>
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<A HREF="mod/mod_alias.html#scriptalias">ScriptAlias</A><br>
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<A HREF="mod/mod_alias.html#scriptaliasmatch">ScriptAliasMatch</A><br>
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<A HREF="mod/mod_userdir.html#userdir">UserDir</A><br>
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</td></tr></table>
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<h2><a name="documentroot">DocumentRoot</a></h2>
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<p>In deciding what file to serve for a given request, Apache's
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default behavior is to take the URL-Path for the request (the part of
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the URL following the first single slash) and add it to the end of the
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<a href="mod/core.html#documentroot">DocumentRoot</a> specified in
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your configuration files. Therefore, the files and directories
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underneath the <code>DocumentRoot</code> make up the basic document
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tree which will be visible from the web.</p>
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<p>Apache is also capable of <a href="vhosts/">Virtual Hosting</a>,
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where the server receives requests for more than one host. In this
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case, a different <code>DocumentRoot</code> can be specified for each
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virtual host, or alternatively, the directives provided by the module
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<a href="mod/mod_vhost_alias.html">mod_vhost_alias</a> can be used to
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dynamically determine the appropriate place from which to serve
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content based on the requested IP address or hostname.</p>
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<h2><a name="outside">Files Outside the DocumentRoot</a></h2>
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<p>There are frequently circumstances where it is necessary to allow
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web access to parts of the filesystem which are not strictly
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underneath the <a href="mod/core.html#documentroot">DocumentRoot</a>.
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Apache offers several different ways to accomplish this. On Unix
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systems, symbolic links can be used to bring other parts of the
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filesystem under the <code>DocumentRoot</code>. For security reasons,
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symbolic links will only be followed if the <a
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href="mod/core.html#options">Options</a> setting for the relevant
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directory includes <code>FollowSymLinks</code> or
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<code>SymLinksIfOwnerMatch</code>.</p>
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<p>Alternatively, the <a href="mod/mod_alias.html#alias">Alias</a>
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directive can be used to map any part of the filesystem into the web
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space. For example, with</p>
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<blockquote><code>Alias /docs /var/web/
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</blockquote></code>
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<p>the URL <code>http://www.example.com/docs/dir/file.html</code> will
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be served from <code>/var/web/dir/file.html</code>. The <a
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href="mod/mod_alias.html#scriptalias">ScriptAlias</a> directive works
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the same way, with the additional effect that all content located at
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the target path is treated as CGI scripts.</p>
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<p>For situations where additional flexibility is required, the <a
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href="mod/mod_alias.html#aliasmatch">AliasMatch</a> and <a
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href="mod/mod_alias.html#scriptaliasmatch">ScriptAliasMatch</a>
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directives can do powerful <a
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href="misc/FAQ.html#regex">regular-expression</a> based matching and
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substitution. For example,</p>
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<blockquote><code> ScriptAliasMatch ^/~([^/]*)/cgi-bin/(.*)
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/home/$1/cgi-bin/$2 </code></blockquote>
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<p>will map a request to
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<code>http://example.com/~user/cgi-bin/script.cgi</code> to the path
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<code>/home/user/cgi-bin/script.cgi</code> and will treat the
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resulting file as a CGI script.</p>
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<h2><a name="user">User Directories</a></h2>
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<p>Traditionally on Unix systems, the home directory of a particular
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<em>user</em> can be referred to as <code>~user/</code>. The module
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<a href="mod/mod_userdir.html">mod_userdir</a> extends this idea to
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the web by allowing files under each user's home directory to be
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accessed using URLs such as the following.</p>
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<blockquote><code>http://www.example.com/~user/file.html</code></blockquote>
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<p>For security reasons, it would be inappropriate to give direct
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access to a user's home directory from the web. Therefore, the <a
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href="mod/mod_userdir.html#userdir">UserDir</a> directive is used to
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specify a directory underneath the user's home directory where web
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files will be located. Using the default setting of <code>Userdir
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public_html</code>, the above URL would look for a file at a directory
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like <code>/home/user/public_html/file.html</code> where the
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</code>/home/user/</code> is the user's home directory as specified in
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<code>/etc/passwd</code>.</p>
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<p>There are also several other forms of the <code>Userdir</code>
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directive which can be used on systems where <code>/etc/passwd</code>
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cannot be used to find the location of the home directory.</p>
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<p>Some people find the "~" symbol (which is often encoded on the web
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as <code>%7e</code>) to be awkward and prefer to use an alternate
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string to represent user directories. This functionality is not
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supported by mod_userdir. However, if users' home directories are
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structured in a regular way, then it is possible to use the <a
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href="mod/mod_alias.html#aliasmatch">AliasMatch</a> directive to
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achieve the desired effect. For example, to make
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<code>http://www.example.com/upages/user/file.html</code> map to
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<code>/home/user/public_html/file.html</code>, the following
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<code>AliasMatch</code> directive can be used.</p>
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<blockquote><code>
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AliasMatch ^/upages/([^/]*)/?(.*) /home/$1/public_html/$2
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</code></blockquote>
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<h2><a name="redirect">URL Redirection</a></h2>
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<p>The configuration directives discussed in the above sections are
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used to tell Apache to get content from a specific place in the
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filesystem and return it to the client. Sometimes, it is desirable
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instead to inform the client that the content being requested is
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located at an different URL, and instruct the client to make a new
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request with the new URL. This is referred to as <em>redirection</em>
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and is implemented by the <a
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href="mod/mod_alias.html#redirect">Redirect</a> directive. For example,
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if the contents of the directory <code>/foo/</code> under the
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<code>DocumentRoot</code> have been moved to the new directory
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<code>/bar/</code>, clients can instructed to request the content at
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the new location as follows.</p>
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<blockquote><code>Redirect permanent
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/foo/ http://www.example.com/bar/</code></blockquote>
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<p>This will redirect any URL-Path starting in <code>/foo/</code> to
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the same URL path on the <code>www.example.com</code> server with
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<code>/bar/</code> substituted for <code>/foo/</code>. Note that
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clients can be redirected to any server, not only the origin
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server.</p>
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<p>Apache also provides a <a
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href="mod/mod_alias.html#redirectmatch">RedirectMatch</a> directive
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which can be used for more complicated rewriting problems. For
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example, to redirect requests for the site home page to a different
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site, but leave all other requests alone, the following configuration
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can be used.</p>
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<blockquote><code>
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RedirectMatch permanent ^/$ http://www.example.com/startpage.html
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</code></blockquote>
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<p>Alternatively, to temporarily redirect all pages on a site to one
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particular page, the following configuration is useful.</p>
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<blockquote><code>
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RedirectMatch temp .* http://www.example.com/startpage.html
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</code></blockquote>
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<h2><a name="rewrite">Rewriting Engine</a></h2>
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<p>When even more powerful substitution is required, the rewriting
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engine provided by <a href="mod/mod_rewrite.html">mod_rewrite</a> can
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be useful. The directives provided by this module can use
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characteristics of the request such as browser type or source IP
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address in deciding from where to serve content. In addition,
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mod_rewrite can use external database files or programs to determine
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how to handle a request. Many practical examples employing
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mod_rewrite are discussed in the <a href="misc/rewriteguide.html">URL
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Rewriting Guide</a>.</p>
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<h2><a name="notfound">File Not Found</a></h2>
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<p>Inevitably, URLs will be requested for which no matching file can
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be found in the filesystem. This can happen for several reasons. In
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some cases, it can be a result of moving documents from one location
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to another. In this case, it is best to use <a href="#redirect">URL
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redirection</a> to inform clients of the new location of the resource.
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In this way, you can assure that old bookmarks and links will continue
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to work, even though the resource is at a new location.</p>
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<p>Another common cause of "File Not Found" errors is accidental
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mistyping of URLs, either directly in the browser, or in HTML links.
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Apache provides the module <a href="mod/mod_speling">mod_speling</a>
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(sic) to help with this problem. When this module is activated, it
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will intercept "File Not Found" errors and look for a resource with a
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similar filename. If one such file is found, mod_speling will send an
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HTTP redirect to the client informing it of the correct location. If
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several "close" files are found, a list of available alternatives will
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be presented to the client.</p>
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<p>An especially useful feature of mod_speling, is that it will
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compare filenames without respect to case. This can be useful for
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systems where users are unaware of the case-sensitive nature of URLs
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and the unix filesystem. However, using mod_speling for anything more
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than the occasional URL correction can lead to additional load on the
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server, since each "incorrect" request is followed by a URL
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redirection and a new request from the client.</p>
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<p>If all attempts to locate the content fail, Apache returns an error
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page with HTTP status code 404 (file not found). The appearance of
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this page is controlled with the <a
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href="mod/core.html#errordocument">ErrorDocument</a> directive and can
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be customized in a flexible manner as discussed in the <a
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href="custom-error.html">Custom error responses</a> and <a
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href="misc/custom_errordocs.html">International Server Error
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Responses</a> documents.</p>
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<!--#include virtual="footer.html" -->
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</BODY>
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</HTML>
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