The rotating privacy address was not itself the packet-loss mechanism:
the 2026-07-13 discrimination experiment (4 flows, dual-ended capture)
showed the loss keyed to hbc's exact 5-tuple — upstream per-flow state
poisoned when the flow was created during boot/apply address churn, kept
alive forever by the 10s heartbeat cadence. Fresh flows to the same
host/port were clean; restarting hbc fixed it instantly. use_tempaddr=0
stays: it removes one source of the churn that poisons newborn flows.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NuPg8vz6FmDUj4SgEJ525C
Root cause of the flow-selective IPv6 heartbeat loss investigated
2026-07-12/13: Ubuntu ships /usr/lib/sysctl.d/55-ipv6-privacy.conf with
use_tempaddr=2 system-wide. Nothing on wlan0 overrode it before this
morning's netplan fix, so the interface carried a rotating privacy
address alongside the stable one. hbc's long-lived flow eventually
straddled a temp-address deprecation event mid-flight, which looked
exactly like random upstream packet loss (in-transit, IPv6-only,
flow-selective) and cost a day of packet captures before the cause
turned out to be this default fighting the WAN config instead.
The netplan/NM ipv6.ip6-privacy=false fix already forces this per
connection, but that's a timing-dependent override racing a package
default. This makes it explicit and permanent: 99- loads after (and
wins over) 55- the next time sysctl --system runs.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Andreas WredeandClaude Fable 5