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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta name="generator" content="HTML Tidy, see www.w3.org" />
<title>Mapping URLs to Filesystem Locations - Apache HTTP
Server</title>
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
vlink="#000080" alink="#FF0000">
<!--#include virtual="header.html" -->
<h1 align="center">Mapping URLs to Filesystem Locations</h1>
<p>This document explains how Apache uses the URL of a request
to determine the filesystem location from which to serve a
file.</p>
<ul>
<li><a href="#documentroot">DocumentRoot</a></li>
<li><a href="#outside">Files Outside the
DocumentRoot</a></li>
<li><a href="#user">User Directories</a></li>
<li><a href="#redirect">URL Redirection</a></li>
<li><a href="#rewrite">Rewrite Engine</a></li>
<li><a href="#notfound">File Not Found</a></li>
</ul>
<hr />
<table border="1">
<tr>
<td valign="top"><strong>Related Modules</strong><br />
<br />
<a href="mod/mod_alias.html">mod_alias</a><br />
<a href="mod/mod_rewrite.html">mod_rewrite</a><br />
<a href="mod/mod_userdir.html">mod_userdir</a><br />
<a href="mod/mod_speling.html">mod_speling</a><br />
<a
href="mod/mod_vhost_alias.html">mod_vhost_alias</a><br />
</td>
<td valign="top"><strong>Related Directives</strong><br />
<br />
<a href="mod/mod_alias.html#alias">Alias</a><br />
<a
href="mod/mod_alias.html#aliasmatch">AliasMatch</a><br />
<a
href="mod/mod_speling.html#checkspelling">CheckSpelling</a><br />
<a
href="mod/core.html#documentroot">DocumentRoot</a><br />
<a
href="mod/core.html#errordocument">ErrorDocument</a><br />
<a href="mod/core.html#options">Options</a><br />
<a href="mod/mod_alias.html#redirect">Redirect</a><br />
<a
href="mod/mod_alias.html#redirectmatch">RedirectMatch</a><br />
<a
href="mod/mod_rewrite.html#RewriteCond">RewriteCond</a><br />
<a
href="mod/mod_rewrite.html#RewriteRule">RewriteRule</a><br />
<a
href="mod/mod_alias.html#scriptalias">ScriptAlias</a><br />
<a
href="mod/mod_alias.html#scriptaliasmatch">ScriptAliasMatch</a><br />
<a href="mod/mod_userdir.html#userdir">UserDir</a><br />
</td>
</tr>
</table>
<h2><a id="documentroot"
name="documentroot">DocumentRoot</a></h2>
<p>In deciding what file to serve for a given request, Apache's
default behavior is to take the URL-Path for the request (the
part of the URL following the hostname and port) and add it to
the end of the <a
href="mod/core.html#documentroot">DocumentRoot</a> specified in
your configuration files. Therefore, the files and directories
underneath the <code>DocumentRoot</code> make up the basic
document tree which will be visible from the web.</p>
<p>Apache is also capable of <a href="vhosts/">Virtual
Hosting</a>, where the server receives requests for more than
one host. In this case, a different <code>DocumentRoot</code>
can be specified for each virtual host, or alternatively, the
directives provided by the module <a
href="mod/mod_vhost_alias.html">mod_vhost_alias</a> can be used
to dynamically determine the appropriate place from which to
serve content based on the requested IP address or
hostname.</p>
<h2><a id="outside" name="outside">Files Outside the
DocumentRoot</a></h2>
<p>There are frequently circumstances where it is necessary to
allow web access to parts of the filesystem that are not
strictly underneath the <a
href="mod/core.html#documentroot">DocumentRoot</a>. Apache
offers several different ways to accomplish this. On Unix
systems, symbolic links can bring other parts of the filesystem
under the <code>DocumentRoot</code>. For security reasons,
Apache will follow symbolic links only if the <a
href="mod/core.html#options">Options</a> setting for the
relevant directory includes <code>FollowSymLinks</code> or
<code>SymLinksIfOwnerMatch</code>.</p>
<p>Alternatively, the <a
href="mod/mod_alias.html#alias">Alias</a> directive will map
any part of the filesystem into the web space. For example,
with</p>
<blockquote>
<code>Alias /docs /var/web/</code>
</blockquote>
<p>the URL
<code>http://www.example.com/docs/dir/file.html</code> will be
served from <code>/var/web/dir/file.html</code>. The <a
href="mod/mod_alias.html#scriptalias">ScriptAlias</a> directive
works the same way, with the additional effect that all content
located at the target path is treated as CGI scripts.</p>
<p>For situations where you require additional flexibility, you
can use the <a
href="mod/mod_alias.html#aliasmatch">AliasMatch</a> and <a
href="mod/mod_alias.html#scriptaliasmatch">ScriptAliasMatch</a>
directives to do powerful regular-expression based matching and
substitution. For example,</p>
<blockquote>
<code>ScriptAliasMatch ^/~([^/]*)/cgi-bin/(.*)
/home/$1/cgi-bin/$2</code>
</blockquote>
<p>will map a request to
<code>http://example.com/~user/cgi-bin/script.cgi</code> to the
path <code>/home/user/cgi-bin/script.cgi</code> and will treat
the resulting file as a CGI script.</p>
<h2><a id="user" name="user">User Directories</a></h2>
<p>Traditionally on Unix systems, the home directory of a
particular <em>user</em> can be referred to as
<code>~user/</code>. The module <a
href="mod/mod_userdir.html">mod_userdir</a> extends this idea
to the web by allowing files under each user's home directory
to be accessed using URLs such as the following.</p>
<blockquote>
<code>http://www.example.com/~user/file.html</code>
</blockquote>
<p>For security reasons, it is inappropriate to give direct
access to a user's home directory from the web. Therefore, the
<a href="mod/mod_userdir.html#userdir">UserDir</a> directive
specifies a directory underneath the user's home directory
where web files are located. Using the default setting of
<code>Userdir public_html</code>, the above URL maps to a file
at a directory like
<code>/home/user/public_html/file.html</code> where
<code>/home/user/</code> is the user's home directory as
specified in <code>/etc/passwd</code>.</p>
<p>There are also several other forms of the
<code>Userdir</code> directive which you can use on systems
where <code>/etc/passwd</code> does not contain the location of
the home directory.</p>
<p>Some people find the "~" symbol (which is often encoded on
the web as <code>%7e</code>) to be awkward and prefer to use an
alternate string to represent user directories. This
functionality is not supported by mod_userdir. However, if
users' home directories are structured in a regular way, then
it is possible to use the <a
href="mod/mod_alias.html#aliasmatch">AliasMatch</a> directive
to achieve the desired effect. For example, to make
<code>http://www.example.com/upages/user/file.html</code> map
to <code>/home/user/public_html/file.html</code>, use the
following <code>AliasMatch</code> directive:</p>
<blockquote>
<code>AliasMatch ^/upages/([^/]*)/?(.*)
/home/$1/public_html/$2</code>
</blockquote>
<h2><a id="redirect" name="redirect">URL Redirection</a></h2>
<p>The configuration directives discussed in the above sections
tell Apache to get content from a specific place in the
filesystem and return it to the client. Sometimes, it is
desirable instead to inform the client that the requested
content is located at a different URL, and instruct the client
to make a new request with the new URL. This is called
<em>redirection</em> and is implemented by the <a
href="mod/mod_alias.html#redirect">Redirect</a> directive. For
example, if the contents of the directory <code>/foo/</code>
under the <code>DocumentRoot</code> are moved to the new
directory <code>/bar/</code>, you can instruct clients to
request the content at the new location as follows:</p>
<blockquote>
<code>Redirect permanent /foo/
http://www.example.com/bar/</code>
</blockquote>
<p>This will redirect any URL-Path starting in
<code>/foo/</code> to the same URL path on the
<code>www.example.com</code> server with <code>/bar/</code>
substituted for <code>/foo/</code>. You can redirect clients to
any server, not only the origin server.</p>
<p>Apache also provides a <a
href="mod/mod_alias.html#redirectmatch">RedirectMatch</a>
directive for more complicated rewriting problems. For example,
to redirect requests for the site home page to a different
site, but leave all other requests alone, use the following
configuration:</p>
<blockquote>
<code>RedirectMatch permanent ^/$
http://www.example.com/startpage.html</code>
</blockquote>
<p>Alternatively, to temporarily redirect all pages on a site
to one particular page, use the following:</p>
<blockquote>
<code>RedirectMatch temp .*
http://www.example.com/startpage.html</code>
</blockquote>
<h2><a id="rewrite" name="rewrite">Rewriting Engine</a></h2>
<p>When even more powerful substitution is required, the
rewriting engine provided by <a
href="mod/mod_rewrite.html">mod_rewrite</a> can be useful. The
directives provided by this module use characteristics of the
request such as browser type or source IP address in deciding
from where to serve content. In addition, mod_rewrite can use
external database files or programs to determine how to handle
a request. Many practical examples employing mod_rewrite are
discussed in the <a href="misc/rewriteguide.html">URL Rewriting
Guide</a>.</p>
<h2><a id="notfound" name="notfound">File Not Found</a></h2>
<p>Inevitably, URLs will be requested for which no matching
file can be found in the filesystem. This can happen for
several reasons. In some cases, it can be a result of moving
documents from one location to another. In this case, it is
best to use <a href="#redirect">URL redirection</a> to inform
clients of the new location of the resource. In this way, you
can assure that old bookmarks and links will continue to work,
even though the resource is at a new location.</p>
<p>Another common cause of "File Not Found" errors is
accidental mistyping of URLs, either directly in the browser,
or in HTML links. Apache provides the module <a
href="mod/mod_speling.html">mod_speling</a> (sic) to help with
this problem. When this module is activated, it will intercept
"File Not Found" errors and look for a resource with a similar
filename. If one such file is found, mod_speling will send an
HTTP redirect to the client informing it of the correct
location. If several "close" files are found, a list of
available alternatives will be presented to the client.</p>
<p>An especially useful feature of mod_speling, is that it will
compare filenames without respect to case. This can help
systems where users are unaware of the case-sensitive nature of
URLs and the unix filesystem. But using mod_speling for
anything more than the occasional URL correction can place
additional load on the server, since each "incorrect" request
is followed by a URL redirection and a new request from the
client.</p>
<p>If all attempts to locate the content fail, Apache returns
an error page with HTTP status code 404 (file not found). The
appearance of this page is controlled with the <a
href="mod/core.html#errordocument">ErrorDocument</a> directive
and can be customized in a flexible manner as discussed in the
<a href="custom-error.html">Custom error responses</a> and <a
href="misc/custom_errordocs.html">International Server Error
Responses</a> documents.</p>
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</body>
</html>
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