Every .dk domain with a nameserver whose address has the PTR name 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.9.2.0.0.1.0.4.0.0.9.1.0.0.6.2.bc.googleusercontent.com. Data comes from our own scanners, which re-resolve every .dk domain in the zone roughly once a week.