Christian Glombek
15458a89f1
StaleRetentionSec= is a resilience feature for unicast DNS: when the configured servers are unreachable, answering from an expired cache entry beats answering nothing at all. That rationale does not carry over to the link-local protocols, where there is no configured server that could be unreachable -- peers simply come and go, and a record's TTL is the only thing bounding how long a departed peer is still answered for. For mDNS the mismatch goes further. RFC 6762 treats record expiry as a presence signal -- a host or service whose records lapse is to be considered gone (section 10.1 goodbye packets even force this early by publishing TTL=0), and the section 5.2 cache maintenance scheme exists precisely so that expiry happens on time. Retaining expired mDNS records for StaleRetentionSec= contradicts that: vanished peers linger in the cache for the whole retention window, are served to clients as if still present, and DNS-SD browsers do not see their services disappear when the records' TTL lapses. Beyond lingering entries, retention also swallows goodbyes outright, as noted in review of the goodbye re-arm fix [1]: dns_cache_prune() and dns_cache_expiry_in_one_second() key on the retention-extended "until", so with StaleRetentionSec= set a goodbye's TTL=1 rewrite lands a full retention window out -- the one-second prune of RFC 6762 section 10.1 then drops nothing, nothing re-arms, and the withdrawn service stays listed for the whole window. LLMNR has no goodbye mechanism, so only the lingering half applies to it, but it does apply: dns_transaction_cache_answer() passes the retention through for LLMNR replies just like for unicast DNS, and the stale-serving gate in the transaction code does not look at the protocol, so a departed LLMNR peer keeps being answered from cache for the whole window as well. Zero the retention centrally in dns_cache_put() for everything but unicast DNS, which covers both put paths: transaction replies and the mDNS packets cached directly by on_mdns_packet(). Add tests pinning both directions, including the goodbye rewrite of an existing mDNS entry. [1] https://github.com/systemd/systemd/pull/43259#discussion_r3809488076
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