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then drops nothing, nothing re-arms, and the withdrawn service stays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
Christian Glombek
15458a89f1