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.