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findings: v6.broken claimed a cause it had no evidence for

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A phone reported "IPv6 is configured but not working" while loading an IPv6-only
site over TCP perfectly well. The finding fired on one signal - ICMPv6 echo
getting no reply - at HIGH confidence. ICMPv6 echo is widely filtered on
networks where IPv6 works, so the two cases are indistinguishable from where the
app stands, and it was picking one.

Same class of error as the multi-homed downstream-loss bug: a confident
measurement of something that was not happening. Now v6.no_icmp_reply, low
severity, medium confidence, naming both explanations. Still reported, because
filtered ICMPv6 breaks Path MTU Discovery - large packets vanish instead of
being reported as too big - which is a fault in its own right.

Corroborating with a real IPv6 connection would separate the two properly, but
needs a target, which runs into the hardcoded-deployment issue already open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
mrambossek
2026-08-01 20:16:37 +02:00
co-authored by Claude Fable 5
parent b5c8dda9a2
commit fec374abf5
4 changed files with 56 additions and 11 deletions
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docs/findings-registry.md
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@@ -89,9 +89,16 @@ rolled up under *connectivity* instead — the third occurrence of rule 1 being
| code | severity | means | rules out |
|---|---|---|---|
| `v6.broken` | medium | IPv6 is configured on this network but does not work. | Absence of IPv6: it is provisioned, it simply fails. |
| `v6.no_icmp_reply` | low | IPv6 is configured but ICMPv6 echo gets no reply. | Nothing on its own: IPv6 may work fine with ICMP filtered. |
| `v6.not_offered` | info | This network does not offer IPv6. | — |
`v6.no_icmp_reply` was `v6.broken` until a phone reported it while loading an IPv6-only site over
TCP perfectly well. The only evidence behind it is ICMPv6 echo, which is widely filtered on
networks where IPv6 works — so the finding now states what was observed and names both
explanations instead of choosing one. It is still worth reporting: filtered ICMPv6 breaks Path MTU
Discovery. Corroborating it with a real IPv6 connection would let the two cases be separated, and
is the proper fix.
`v6.not_offered` is **info and must stay info**. Most networks still do not offer IPv6 and that is
not a fault; reporting it as a warning lights a yellow verdict on a healthy network, which teaches
people to ignore the light — the one thing a diagnostic must never do.