fix: cap TTL to 10s in legacy unicast responses - #504
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Problem: RFC 6762 §6.7 requires the records in a legacy unicast response to carry a TTL no greater than 10 seconds, since legacy resolvers cache them without the mDNS cache-coherency mechanisms. Responses went out with the records' true TTLs (120s for host records, 4500s for PTR/TXT). Fix: Fold the TTL cap into the existing per-record pass that clears cache-flush bits, renaming clear_cache_flush_bits() to update_records_for_legacy_unicast(). Add the LEGACY_UNICAST_MAX_TTL constant and assert the cap in test_legacy_unicast_response. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Fixes: #496 , the second problem.
Implement this part in RFC 6762 §6.7:
The resource record TTL given in a legacy unicast response SHOULD NOT be greater than ten seconds, even if the true TTL of the Multicast DNS resource record is higher. This is because Multicast DNS responders that fully participate in the protocol use the cache coherency mechanisms described in Section 10, "Resource Record TTL Values and Cache Coherency", to update and invalidate stale data. Were unicast responses sent to legacy resolvers to use the same high TTLs, these legacy resolvers, which do not implement these cache coherency mechanisms, could retain stale cached resource record data long after it is no longer valid.