Serial number
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.