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This document explains how Apache uses the URL of a request to determine the filesystem location from which to serve a file.
| Related Modules mod_alias mod_rewrite mod_userdir mod_speling mod_vhost_alias |
Related Directives Alias AliasMatch CheckSpelling DocumentRoot ErrorDocument Options Redirect RedirectMatch RewriteCond RewriteRule ScriptAlias ScriptAliasMatch UserDir |
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 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.
Apache 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.
There are frequently circumstances where it is necessary to
allow web access to parts of the filesystem that are not
strictly underneath the DocumentRoot. Apache
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,
Apache 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 ^/~([^/]*)/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.
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/([^/]*)/?(.*)
/home/$1/public_html/$2
The configuration directives discussed in the above sections
tell Apache to get content from a specific place in the
filesystem and return it to the client. Sometimes, it is
desirable instead to inform the client that the requested
content is located at a different URL, and instruct the client
to make a new request with the new URL. This is called
redirection and is implemented by the Redirect directive. For
example, if the contents of the directory /foo/
under the DocumentRoot are moved to the new
directory /bar/, you can instruct clients to
request the content at the new location as follows:
Redirect permanent /foo/
http://www.example.com/bar/
This will redirect any URL-Path starting in
/foo/ to the same URL path on the
www.example.com server with /bar/
substituted for /foo/. You can redirect clients to
any server, not only the origin server.
Apache also provides a RedirectMatch directive for more complicated rewriting problems. For example, to redirect requests for the site home page to a different site, but leave all other requests alone, use the following configuration:
RedirectMatch permanent ^/$
http://www.example.com/startpage.html
Alternatively, to temporarily redirect all pages on a site to one particular page, use the following:
RedirectMatch temp .*
http://www.example.com/startpage.html
When even more powerful substitution is required, the rewriting engine provided by mod_rewrite 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 URL Rewriting Guide.
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 URL redirection 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.
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 mod_speling (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.
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.
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 ErrorDocument directive and can be customized in a flexible manner as discussed in the Custom error responses and International Server Error Responses documents.
