file; if request method is not known or if there is some other unacceptable condition (e.g. resource not found, internal server error, etc.) then an error response is sent. Serve static content If a web server program is capable of serving static content and it has been configured to do so, then it is able to send file content whenever a request message has a valid URL path matching (after URL mapping, URL translation and URL redirection) that of an existing file under the root directory of a website and file has attributes which match those required by internal rules of web server program. That kind of content is called static because usually it is not changed by the web server when it is sent to clients and because it remains the same until it is modified (file modification) by some program. NOTE: when serving static content only, a web server program usually does not change file contents of served websites (as they are only read and never written) and so it suffices to support only these HTTP methods: OPTIONS HEAD GET Response of static file content can be sped up by a file cache. Directory index files If a web server program receives a client request message with an URL whose path matches one of an existing directory and that directory is accessible and serving directory index file(s) is enabled then a web server program may try to serve the first of known (or configured) static index file names (a regular file) found in that directory; if no index file is found or other conditions are not met then an error message is returned. Most used names for static index files are: index.html, index.htm and Default.htm. Regular files If a web server program receives a client request message with an URL whose path matches the file name of an existing file and that file is accessible by web server program and its attributes match internal rules of web server program, then web server program can send that file to client. Usually, for security reasons, most web server programs are pre-configured to serve only regular files or to avoid to use special file types like device files, along with symbolic links or hard links to them. The aim is to avoid undesirable side effects when serving static web resources. Serve dynamic content If a web server program is capable of