Security audit findings NET-04 and Codex #15 ("forged mDNS candidates
can monopolize discovery slots").
Discovery admits at most 64 active or cooling-down candidates, keyed by
claimed peer ID and full socket address. Both keys are attacker chosen:
one LAN host can advertise 64 distinct peer IDs on 64 ports within
milliseconds, fill every slot, and repeat the burst every 5 seconds so
that every genuinely new peer is dropped with "recent-attempt limit is
full". The audit proposed FIFO eviction instead, but that would let the
same flood evict legitimate candidates; the real asymmetry is that a
host can mint identities and ports cheaply but cannot mint IP addresses
without also answering QUIC on them.
Admission now additionally refuses a candidate when its source IP
already accounts for MAX_DISCOVERY_CANDIDATES_PER_SOURCE_IP (8)
entries across the active set and the unexpired cooldown list. A
flooding host can therefore occupy at most 8 of the 64 slots; other
hosts are unaffected. Eight is generous for the legitimate case of a
few peer instances on one machine.
Test plan: `just test`. The new unit test fills one IP's budget,
verifies other IPs are still admitted, and verifies the budget is
released after the cooldown. The pre-existing active-cap test now
spreads its 64 candidates over distinct hosts.
Claude-Session: https://claude.ai/code/session_017C3Nbgwpdm3YNwZhhFLHwg
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