If a user mistyped a host name, we might see this type of query. C.ISI.EDU would answer it with:
+---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE, AA, RCODE=NE | +---------------------------------------------------+ Question | QNAME=SIR-NIC.ARPA., QCLASS=IN, QTYPE=A | +---------------------------------------------------+ Answer | <empty> | +---------------------------------------------------+ Authority | . SOA SRI-NIC.ARPA. HOSTMASTER.SRI-NIC.ARPA. | | 870611 1800 300 604800 86400 | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+
This response states that the name does not exist. This condition is signalled in the response code (RCODE) section of the header.
The SOA RR in the authority section is the optional negative caching information which allows the resolver using this response to assume that the name will not exist for the SOA MINIMUM (86400) seconds.