usually instead employ the vastly superior distributed database replication mechanisms that the back ends themselves provide. Serial number comparisons Serial number comparisons are intended to use Serial Number Arithmetic as defined in RFC 1982. However, this was not clearly specified in RFC 1034, resulting in not all clients perform the serial number check, in the preamble, in the same way. Some clients check merely that the serial number supplied by the server is different from that known by the client, or non-zero. Other clients check that the serial number supplied by the server is within a given range of the serial number already known by the client. Yet other clients still perform the latter check and additionally check that the serial number supplied by the server is not zero. Multiple resource records Originally, in the actual data transfer each set of resource records for a single domain name and type was transferred in a separate response message from the server to the client. However, this is inefficient, and some DNS server software implemented optimizations, geared at allowing the response compression mechanism in the DNS protocol to reduce the total bandwidth requirements of data transfers, such as: performing "additional section processing" to include any "glue" resource record sets in the same response as an NS, SRV, or MX resource record set collecting all of the resource record sets relating to a single domain name together and sending them, if they fit, in a single response Some clients were written to expect only the original response format, and would fail to perform data transfer if such optimizations were employed. Several DNS server packages thus have a configuration setting allowing administrators to specify the use of "single answer format" responses for those clients that require it. Exposure of data The data contained in a DNS zone may be sensitive from an operational security aspect. This is because information such as server hostnames may become public knowledge, which can be used to discover information about an organization and even provide a larger attack surface. In June 2017 the registrar responsible for Russian top-level-domains accidentally enabled DNS zone transfers via AXFR which led to 5.6 million records being accidentally exposed. In 2008 a court in North Dakota, USA, ruled that performing a zone transfer as an unauthorized outsider to obtain information that was not publicly accessible constitutes a violation of North Dakota law. See also