[BCP16]    Best Current Practice 16,
              <https://www.rfc-editor.org/info/bcp16>.
              At the time of writing, this BCP comprises the following:

              Elz, R., Bush, R., Bradner, S., and M. Patton, "Selection
              and Operation of Secondary DNS Servers", BCP 16, RFC 2182,
              DOI 10.17487/RFC2182, July 1997,
              <https://www.rfc-editor.org/info/rfc2182>.