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From Joshua Slive <sl...@commerce.ubc.ca>
Subject urlmapping - take 2
Date Fri, 16 Feb 2001 03:20:22 GMT
Here's a second draft of the url mapping document.  I will commit this if
nobody has feedback, but I would greatly appreciate any comments or
suggestions.

It is also available at
http://garibaldi.commerce.ubc.ca:8080/ap13/htdocs/manual/urlmapping.html

Another document that is needed as a followup to this is a summary of
directory handling (mod_dir, mod_autoindex).


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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 <code>DocumentRoot</code> 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 <code>DocumentRoot</code>.  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 <code>Options</code> setting
for the relevant directory includes <code>FollowSymLinks</code> or
<code>SymLinksIfOwnerMatch</code>.</p>

<p>Alternatively, the <code>Alias</code> directive can be used to map
any part of the filesystem into the web space.  For example,
<blockquote><code>
       Alias /docs /var/web/docs
</blockquote></code>
will cause Apache to look in <code>/var/web/docs</code> for
any request made with a URL-Path starting in <code>/docs</code>.
The <code>ScriptAlias</code> 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
<code>AliasMatch</code> and <code>ScriptAliasMatch</code> 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
mod_userdir extends this idea to the web by allowing files under each
user's home directory to be accessed using URLs like</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
<code>UserDir</code> 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
<code>AliasMatch</code> 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 <code>Redirect</code> 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 <code>/foo/</code> directory of the <code>www.example.com</code>
server.  Note that clients can be redirected to any server, not
only the origin server.</p>

<p>Apache also provides a <code>RedirectMatch</code> 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 ^/$ 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="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 <code>ErrorDocument</code> 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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