Explain how caching is used in the Domain Name System (DNS) during an iterative query. Specify what information is cached at the local host, the local DNS, the root DNS, the Top Level Domain (TLD) DNS and the authoritative DNS. Be sure to include only cached information not all information stored.
Try an answer before revealing the guidance below.
Key Concepts
- Iterative query
- Local DNS server
- Referral
- NS and A records
- TTL
Answer Approach
- Trace which server receives which answer or referral in an iterative lookup.
- A server can only cache what it receives in a reply.
- Separate cached data from zone data a server stores authoritatively.
- Go through each of the five places named in the question.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Local host: caches the final hostname-to-IP mapping returned by its local DNS server. Local DNS: caches everything it learns while iterating — the referral to the TLD servers (NS records and their addresses), the referral to the authoritative servers, and the final A record — each for its TTL, so later queries can skip the root and often the TLD. Root DNS: caches nothing from this query; it only answers from its own data with a referral. TLD DNS: likewise caches nothing; it returns a referral from its own zone data. Authoritative DNS: caches nothing; it answers from its own records. In an iterative query only the requester side receives answers, so only the local host and local DNS cache.
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