The library's primary DNS server is powered off. Clients still receive authoritative answers for the library zone from the server that received a copy of the zone by zone transfer. Which server is answering?
Select an answer to reveal the explanation.
Short Explanation
The original phone book is off the hook, and a photocopy is still answering. You are hearing the secondary DNS server, the one that received the zone by zone transfer and can still answer while the primary is down. A cached guess, a stub that only lists the name servers, or the primary that is powered off is not that copy.
Full Explanation
A secondary DNS server holds the zone it received from the primary by zone transfer and answers authoritatively while the primary is off. That is the server the stem describes. A recursive resolver may answer from cache. Cached data is not a zone received by transfer, and calling the cache authoritative does not make it the secondary. A stub server is the tempting twin because it is also a DNS role that knows about the zone. It holds the list of name servers, and it does not answer the library records from a transferred copy. The primary is powered off, so it cannot be the server still answering. Exam caveat: a secondary that cannot refresh will eventually expire the zone. This outage assumes the transferred copy is still inside its valid lifetime. Operational check: query the library zone against the secondary, confirm the response is authoritative, and confirm the primary does not answer while it is powered off. Record the reading before the change and the reading after it so the next shift can see the symptom actually moved.