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* plugin/cache: add prefer_positive stale policy Add an opt-in serve_stale_policy that prefers an eligible success-cache answer over denial-cache entries while serve_stale is enabled. Preserve the existing ncache-first behavior when the policy is absent. Also classify SOA-backed CNAME NODATA responses in the cache so incomplete answers cannot be selected as positive stale responses. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 25da81ab-92dd-4663-b480-efd6262090c6 Signed-off-by: Nitin Nizhawan <nnizhawan@microsoft.com> * plugin/cache: retain last-known-good positive answers Keep an answering success-cache item reachable when a later NOERROR or referral response overwrites the visible cache key without answering the question. This lets prefer_positive survive empty responses, referrals, and additional-only data while leaving policy-off lookup behavior unchanged. Return the exact accepted verify refresh item instead of re-reading an ambiguous cache key, avoiding expired TTL wraparound for uncacheable replies. Add regression coverage for non-answer refreshes, NODATA, SERVFAIL, NOTIMP, stale-window expiry, and bounded verify reply shaping. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 25da81ab-92dd-4663-b480-efd6262090c6 Signed-off-by: Nitin Nizhawan <nnizhawan@microsoft.com> * plugin/cache: validate preferred stale answers Reject truncated, DNSSEC-expired, mismatched-class, unrelated ANY, and ambiguous CNAME refreshes before replacing a stale last-known-good answer. Precompute answer eligibility when cache items are created so prefer_positive hits avoid repeated CNAME walks. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 25da81ab-92dd-4663-b480-efd6262090c6 Signed-off-by: Nitin Nizhawan <nnizhawan@microsoft.com> --------- Signed-off-by: Nitin Nizhawan <nnizhawan@microsoft.com> Co-authored-by: Nitin Nizhawan <nnizhawan@microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 25da81ab-92dd-4663-b480-efd6262090c6