CKEDITOR.on( 'dialogDefinition', function( ev )
{
// Take the dialog name and its definition from the event data.
var dialogName = ev.data.name;
var dialogDefinition = ev.data.definition;

// Check if the definition is from the dialog window you are interested in (the "Link" dialog window).
if ( dialogName == 'link' )
{
// Get a reference to the "Link Advanced" tab.
var advTab = dialogDefinition.getContents( 'advanced' );
var relField = advTab.get( 'advRel' );
relField['default'] = 'nofollow';
}
});