Firewalld zones/policies for forwarding (Docker bridge → gre-ashburn),
iptables for Docker-specific rules (DNAT, DOCKER-USER, mangle, SNAT).
Both coexist at different netfilter priorities.
See docs/postmortem-ashburn-relay-outbound.md for root cause analysis.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>