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This document explains the method in which Apache determines what filesystem location to serve a file from based on the URL of a request.
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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 first single slash) 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 which are not strictly
underneath the DocumentRoot.
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 DocumentRoot. For security reasons,
symbolic links will only be followed if the Options setting for the relevant
directory includes FollowSymLinks or
SymLinksIfOwnerMatch.
Alternatively, the Alias directive can be used to 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 additional flexibility is required, the AliasMatch and ScriptAliasMatch directives can 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.htmlFor security reasons, it would be inappropriate to give direct
access to a user's home directory from the web. Therefore, the UserDir directive is used to
specify a directory underneath the user's home directory where web
files will be located. Using the default setting of Userdir
public_html, the above URL would look for a file at a directory
like /home/user/public_html/file.html where the
/home/user/ is the user's home directory as specified in
/etc/passwd.
There are also several other forms of the Userdir
directive which can be used on systems where /etc/passwd
cannot be used to find 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, the following
AliasMatch directive can be used.
AliasMatch ^/upages/([^/]*)/?(.*) /home/$1/public_html/$2
The configuration directives discussed in the above sections are
used to tell Apache to get content from a specific place in the
filesystem and return it to the client. Sometimes, it is desirable
instead to inform the client that the content being requested is
located at an different URL, and instruct the client to make a new
request with the new URL. This is referred to as redirection
and is implemented by the Redirect directive. For example,
if the contents of the directory /foo/ under the
DocumentRoot have been moved to the new directory
/bar/, clients can instructed to request the content at
the new location as follows.
Redirect permanent
/foo/ http://www.example.com/bar/This will redirect any URL-Path starting in /foo/ to
the same URL path on the www.example.com server with
/bar/ substituted for /foo/. Note that
clients can be redirected to any server, not only the origin
server.
Apache also provides a RedirectMatch directive which can be used for more complicated rewriting problems. For example, to redirect requests for the site home page to a different site, but leave all other requests alone, the following configuration can be used.
RedirectMatch permanent ^/$ http://www.example.com/startpage.html
Alternatively, to temporarily redirect all pages on a site to one particular page, the following configuration is useful.
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 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 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 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.
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.
