Operator's framing, and it is a better argument than the terminology
correction that preceded it. Under v4, exposing a host needed two
affirmative acts -- a DNAT and an accept rule -- so missing either left
the host dark. There is no v4 misconfiguration that exposes an internal
host by accident. NAT was load-bearing security whether or not anyone
designed it that way.
v6 removes the first control entirely. The path exists inherently, so
the firewall is the only thing left, and the failure mode inverts from
fail-closed to fail-open. Rule-ordering slips, rulesets that silently
match only one address family, new VLANs added without policy, and
re-delegated prefixes unmatching address-literal rules all become
exposure events rather than no-ops.
Records the practical consequences: key rules on interface/zone rather
than address literals, treat enabling v6 on a segment as requiring
policy to exist first, and verify default-deny from off-net rather than
by reading the ruleset -- which is lesson 3's assert-the-effective-value
discipline applied to firewall policy.
Also corrects my own claim from the previous commit that the pending
firewall pass was 'smaller' than I had implied. It is not smaller, it is
different in kind.
vh
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