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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<TITLE>Mapping URLs to Filesystem Locations - Apache HTTP Server</TITLE>
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<h1 align="center">Mapping URLs to Filesystem Locations</h1>
<p>This document explains the method in which Apache determines
what filesystem location to serve a file from based on the
URL of a request.</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 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 first single slash) 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 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 which 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 be used to bring other parts of the
filesystem under the <code>DocumentRoot</code>. For security reasons,
symbolic links will only be followed 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 can be used to map any part of the filesystem into the web
space. For example, with</p>
<blockquote><code>Alias /docs /var/web/
</blockquote></code>
<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 additional flexibility is required, the <a
href="mod/mod_alias.html#aliasmatch">AliasMatch</a> and <a
href="mod/mod_alias.html#scriptaliasmatch">ScriptAliasMatch</a>
directives can do powerful <a
href="misc/FAQ.html#regex">regular-expression</a> 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 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 would be 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 is used to
specify a directory underneath the user's home directory where web
files will be located. Using the default setting of <code>Userdir
public_html</code>, the above URL would look for a file at a directory
like <code>/home/user/public_html/file.html</code> where the
</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 can be used on systems where <code>/etc/passwd</code>
cannot be used to find 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>, the following
<code>AliasMatch</code> directive can be used.</p>
<blockquote><code>
AliasMatch ^/upages/([^/]*)/?(.*) /home/$1/public_html/$2
</code></blockquote>
<h2><a name="redirect">URL Redirection</a></h2>
<p>The configuration directives discussed in the above sections are
used to tell Apache to get content from a specific place in the
filesystem and return it to the client. Sometimes, it is desirable
instead to inform the client that the content being requested is
located at an different URL, and instruct the client to make a new
request with the new URL. This is referred to as <em>redirection</em>
and is implemented by the <a
href="mod/mod_alias.html#redirect">Redirect</a> directive. For example,
if the contents of the directory <code>/foo/</code> under the
<code>DocumentRoot</code> have been moved to the new directory
<code>/bar/</code>, clients can instructed to request the content at
the new location as follows.</p>
<blockquote><code>Redirect permanent
/foo/ http://www.example.com/bar/</code></blockquote>
<p>This will redirect any URL-Path starting in <code>/foo/</code> to
the same URL path on the <code>www.example.com</code> server with
<code>/bar/</code> substituted for <code>/foo/</code>. Note that
clients can be redirected to any server, not only the origin
server.</p>
<p>Apache also provides a <a
href="mod/mod_alias.html#redirectmatch">RedirectMatch</a> directive
which can be used for more complicated rewriting problems. For
example, to redirect requests for the site home page to a different
site, but leave all other requests alone, the following configuration
can be used.</p>
<blockquote><code>
RedirectMatch permanent ^/$ http://www.example.com/startpage.html
</code></blockquote>
<p>Alternatively, to temporarily redirect all pages on a site to one
particular page, the following configuration is useful.</p>
<blockquote><code>
RedirectMatch temp .* http://www.example.com/startpage.html
</code></blockquote>
<h2><a 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 can 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 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">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 be useful for
systems where users are unaware of the case-sensitive nature of URLs
and the unix filesystem. However, using mod_speling for anything more
than the occasional URL correction can lead to 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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