[BCP123]   Best Current Practice 123,
              <https://www.rfc-editor.org/info/bcp123>.
              At the time of writing, this BCP comprises the following:

              Larson, M. and P. Barber, "Observed DNS Resolution
              Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
              October 2006, <https://www.rfc-editor.org/info/rfc4697>.