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DNS Caching Lab: How TTLs Count Down and NXDOMAIN Responses Get Stored

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A hands-on Protocol Lab tutorial explains how DNS resolvers cache answers using Time-To-Live (TTL) values set by authoritative zones. The lab uses a five-node container topology to demonstrate three scenarios: a short-TTL record expiring in 60 seconds, a long-TTL record persisting for an hour, and a negative NXDOMAIN response being cached via an SOA record. When a client repeats a query just seconds later, the resolver serves the answer directly from cache with a reduced TTL, skipping any upstream lookup. Negative caching is governed by RFC 2308, where the SOA minimum field determines how long a 'no such domain' response is stored — 300 seconds in this example. The lab builds on a prerequisite recursive-resolution exercise and is estimated to take 50–65 minutes to complete.

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