Web Design & Development Guide
RSS enclosures are a way of attaching
multimedia content to
feeds by providing the URL of a file associated with an entry, such
as an MP3 file to a music recommendation or a photo to a diary entry.
Unlike e-mail attachments, enclosures are merely hyperlinks to files,
the actual data is not embedded into the feed. Support and implementation among
aggregators varies: if the software understands the specified file
format, it may automatically download and display the content, otherwise provide a link to it or
silently ignore it.
The addition of enclosures to RSS, as first implemented by Dave Winer in late
, was an important prerequisite for the emergence of
arguably the most common use of the feature
as of 2006.
In podcasts and related technologies enclosures are not merely attachments to
entries, but provide the main content of a feed.
In RSS 2.0, the syntax for the <enclosure> tag, an optional child of the
<item> element, is as follows:
<enclosure url="http://domain.com/file.mp3" length="123456789" type="audio/mpeg" />
where the value of the url attribute is a
URL of a file,
length is its size in
bytes, and type
its mime type.
There may only be a single <enclosure> per <item>.
The RSS <enclosure> has similarities to:
SMIL <prefetch> element,
- the HTML
<link> element with rel="prefetch".
- the HTTP
Link header with rel="prefetch". (See
RFC 2068 section 18.104.22.168.)
Atom <link> element with rel="enclosure"