When an HTTP client (generally a web browser) requests a URL that points to a directory structure instead of an actual web page within the directory structure, the web server will generally serve a default page, which is often referred to as a main or "index" page.
A common filename for such a page is index.
html
, but most modern HTTP servers offer a configurable list of filenames that the server can use as an index. If a server is configured to support
server-side scripting, the list will usually include entries allowing dynamic content to be used as the index page (e.g. index.
cgi
, index.
pl
, index.
php
, index.
shtml
, index.
jsp
, default.
asp
) even though it may be more appropriate to still specify the HTML output (index.html.php
or index.html.aspx
), as this should not be taken for granted. An example is the popular
open source web server
Apache, where the list of filenames is controlled by the DirectoryIndex
[1] directive in the main server configuration file or in the
configuration file for that directory. It is possible to not use file extensions at all, and be neutral to content delivery methods, and set the server to automatically pick the best file through
content negotiation.
If the server is unable to find a file with any of the names listed in its configuration, it may either return an error (usually
403 Index Listing Forbidden or
404 Not Found) or generate its own
index page listing the files in the directory. Usually this option, often named autoindex
, is also configurable.
[2]
<!DOCTYPE html>
<html>
<head> <title>My First Web Page</title> </head> <body>
This is a paragraph of text.
</body>
</html>
A scheme where web server serves a default file on per-subdirectory basis has been supported as early as
NCSA HTTPd 0.3beta (22 April 1993),
[3] which defaults to serve index.html
file in the directory.
[3]
[4] This scheme has been then adopted by
CERN HTTPd since at least 2.17beta (5 April 1994), whose default supports Welcome.html
and welcome.html
in addition to the NCSA-originated index.html
.
[5]
Later web servers typically support this default file scheme in one form or another; this is usually configurable, with index.html
being one of the default file names.
[6]
[7]
[8]
In some cases, the home page of a website can be a menu of language options for large sites that use geotargeting. It is also possible to avoid this step, for example, by using content negotiation.
In cases where no known index.*
file exists within a given directory, the web server may be configured to provide an automatically generated listing of the files within the directory instead. With the Apache web server, for example, this behavior is provided by the mod_autoindex module
[9] and controlled by the Options +Indexes
directive
[10] in the web server
configuration files. These automated directory listings are sometimes a security risk because they enumerate sensitive files which may not be intended for public access, in a process known as a directory indexing attack.
[11] Such a security misconfiguration
[12] may also assist in other attacks, such as a path or
directory traversal attack.
[13]
When accessing a directory, the various available index methods may also have a different impact on usage of OS resources ( RAM, CPU time, etc.) and thus on web server performances.
Proceeding from fastest to slowest method, here is the list:
index.html
, etc.;index.php
;index.cgi
.