_get_dnat_container_ips() used a concatenating docker inspect format that
produced "invalid IP" when containers had multiple network attachments.
The old ensure_postup_dnat appended rather than replacing, so each update
call added a broken duplicate set of rules causing iptables to fail on
startup and tear down wg0 entirely.
Fix _get_dnat_container_ips to use a space separator in the format string
and validate each token as a real IP before accepting it.
Rewrite ensure_postup_dnat with _is_dnat_rule() helper: strips every
managed DNAT/FORWARD rule (any IP, port 53/80) on semicolon-split and
appends a single correct set — fully idempotent regardless of prior state.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
roof
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