RSS enclosures are a way of attaching multimedia content to RSS 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 file data is not embedded into the feed (unless a data URL is used). 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 2000 , was an important prerequisite for the emergence of podcasting, perhaps the most common use of the feature as of 2012[update]. 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://example.com/file.mp3" length="123456789" type="audio/mpeg" />
It is recommended that only one <enclosure> element is included per <item>.
The RSS <enclosure> has similarities to:
- the SMIL <prefetch> element,
- the HTML <link> element with rel="prefetch".
- the HTTP Link header with rel="prefetch". (See RFC 2068 section 22.214.171.124.)
- the Atom <link> element with rel="enclosure"