NXDOMAIN
NXDOMAIN means the queried DNS name does not exist. It is different from an existing name that simply has no record of the requested type.
Responses, Caching And Transport
DNS response codes, cache behaviour, EDNS, UDP, TCP, and encrypted DNS transport.
SERVFAIL
SERVFAIL means a DNS server or resolver could not complete the query because something failed. It can be caused by broken authoritative servers, DNSSEC validation problems, timeouts, or resolver issues.
Negative Caching
Negative caching is the caching of negative answers, such as NXDOMAIN. This means a missing or recently fixed record may continue to appear missing until the cached negative response expires.
Resolver Cache
A resolver cache stores DNS answers until their TTL expires. Cached answers reduce lookup time and DNS load, but can also make recent DNS changes appear delayed.
EDNS
EDNS is an extension mechanism for DNS. It allows larger responses and extra features beyond the original DNS protocol limits.
Truncated Response
A truncated response means a DNS reply was too large to fit in the UDP response size being used. Resolvers may retry over TCP or use EDNS to allow larger responses.
DNS Over TCP
DNS commonly uses UDP, but it can also use TCP. TCP is used for zone transfers and may be used when responses are too large for UDP.