The linkcheck.php cronjob will check every external URL recorded in the system, and mark them as invalid if they can't be reached.

If an URL is marked as invalid by this cronjob, the has_content attribute for the matching attribute will return false. This is not very intuitive, and moreover, it's not documented.

This should imho be added to http://ez.no/doc/ez_publish/technical_manual/4_x/concepts_and_basics/content_management/url_storage as well as http://ez.no/doc/ez_publish/technical_manual/4_x/reference/datatypes/url.