Reducing NXDOMAIN Responses

An NXDOMAIN (nonexistent) response is returned when UltraDNS receives a query for a record that doesn't exist in the authoritative zone under management.  Excessive NXDOMAIN responses can occur by accident (misconfiguration) or can be malicious (scanning a zone).  Since the queries that result in NXDOMAIN responses are for records that don't exist in the zone, UltraDNS needs to create a response and synthesize a TTL value for the response.  The synthesized TTL facilitates negative caching and will indicate to the recursive resolver how long it should hold on to this NXDOMAIN response.  The TTL for an NXDOMAIN response is standardized in RFC 2308 (sections 3, 4, and 5) as follows:

...The TTL of this record is set from the minimum of the MINIMUM field
of the SOA record and the TTL of the SOA itself, and indicates how long a 
resolver may cache the negative answer.

The SOA record for a domain can be updated in the UltraDNS portal by navigating to Domains → [select the domain you want to update] → Properties and selecting the Edit button for the SOA record:

According to RFC, a query for a nonexistent record will be given a 3600 second TTL.  This should help mitigate elevated NXDOMAIN responses that are the result of a misconfiguration.