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Apache > HTTP Server > Documentation > Version 2.4

Mapping URLs to Filesystem Locations

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This document explains how the Apache HTTP Server uses the URL of a request to determine the filesystem location from which to serve a file.

See also

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Related Modules and Directives

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DocumentRoot

In deciding what file to serve for a given request, httpd'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 DocumentRoot specified in your configuration files. Therefore, the files and directories underneath the DocumentRoot make up the basic document tree which will be visible from the web.

For example, if DocumentRoot were set to /var/www/html then a request for http://www.example.com/fish/guppies.html would result in the file /var/www/html/fish/guppies.html being served to the requesting client.

If a directory is requested (i.e. a path ending with /), the file served from that directory is defined by the DirectoryIndex directive. For example, if DocumentRoot were set as above, and you were to set:

DirectoryIndex index.html index.php

Then a request for http://www.example.com/fish/ will cause httpd to attempt to serve the file /var/www/html/fish/index.html. In the event that that file does not exist, it will next attempt to serve the file /var/www/html/fish/index.php.

If neither of these files existed, the next step is to attempt to provide a directory index, if mod_autoindex is loaded and configured to permit that.

httpd is also capable of Virtual Hosting, where the server receives requests for more than one host. In this case, a different DocumentRoot can be specified for each virtual host, or alternatively, the directives provided by the module mod_vhost_alias can be used to dynamically determine the appropriate place from which to serve content based on the requested IP address or hostname.

The DocumentRoot directive is set in your main server configuration file (apache2.conf) and, possibly, once per additional Virtual Host you create.

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Files Outside the DocumentRoot

There are frequently circumstances where it is necessary to allow web access to parts of the filesystem that are not strictly underneath the DocumentRoot. httpd offers several different ways to accomplish this. On Unix systems, symbolic links can bring other parts of the filesystem under the DocumentRoot. For security reasons, httpd will follow symbolic links only if the Options setting for the relevant directory includes FollowSymLinks or SymLinksIfOwnerMatch.

Alternatively, the Alias directive will map any part of the filesystem into the web space. For example, with

Alias "/docs" "/var/web"

the URL http://www.example.com/docs/dir/file.html will be served from /var/web/dir/file.html. The ScriptAlias directive works the same way, with the additional effect that all content located at the target path is treated as CGI scripts.

For situations where you require additional flexibility, you can use the AliasMatch and ScriptAliasMatch directives to do powerful regular expression based matching and substitution. For example,

ScriptAliasMatch "^/~([a-zA-Z0-9]+)/cgi-bin/(.+)"   "/home/$1/cgi-bin/$2"

will map a request to http://example.com/~user/cgi-bin/script.cgi to the path /home/user/cgi-bin/script.cgi and will treat the resulting file as a CGI script.

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User Directories

Traditionally on Unix systems, the home directory of a particular user can be referred to as ~user/. The module mod_userdir extends this idea to the web by allowing files under each user's home directory to be accessed using URLs such as the following.

http://www.example.com/~user/file.html

For security reasons, it is inappropriate to give direct access to a user's home directory from the web. Therefore, the UserDir directive specifies a directory underneath the user's home directory where web files are located. Using the default setting of Userdir public_html, the above URL maps to a file at a directory like /home/user/public_html/file.html where /home/user/ is the user's home directory as specified in /etc/passwd.

There are also several other forms of the Userdir directive which you can use on systems where /etc/passwd does not contain the location of the home directory.

Some people find the "~" symbol (which is often encoded on the web as %7e) 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 AliasMatch directive to achieve the desired effect. For example, to make http://www.example.com/upages/user/file.html map to /home/user/public_html/file.html, use the following AliasMatch directive:

AliasMatch "^/upages/([a-zA-Z0-9]+)(/(.*))?$"   "/home/$1/public_html/$3"
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URL Redirection

The configuration directives discussed in the above sections tell httpd 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 redirection and is implement