This content is 20 years old. I don't routinely update old blog posts as they are only intended to represent a view at a particular point in time. Please be warned that the information here may be out of date.
When no default document is found on a web server, depending on the server configuration, users may be able to list the files in a given directory. For Apache servers, this may be prevented on a per-directory basis by adding add an IndexIgnore directive to an .htaccess file.
The syntax is:
IndexIgnore file [file] ...
For example, IndexIgnore * will prevent listing of all files, or alternatively, individual files may be specified.
Full details may be found in the Apache HTTP Server documentation.