Confirmed on the operator's NAS. The earlier advice (insert an ACCEPT rule
ahead of DSM's chain) does not hold: DSM reloads its firewall periodically,
flushing and rebuilding the chains. You can watch FORWARD_FIREWALL's DROP
counter reset to zero and the inserted rule vanish with it, having matched no
packets. That is also why the symptom looked intermittent — 60/40 success on
repeated connections, with RELATED,ESTABLISHED keeping existing flows alive.
The fix is one level down: net.bridge.bridge-nf-call-iptables=0 takes
same-bridge traffic out of iptables entirely, so DSM's reloading cannot affect
it. Persisted with a Boot-up Triggered Task.
Section 8 now also answers "is that safe", accurately: the flag affects only
packets bridged within one network. Traffic between different Docker networks
is routed, so DOCKER-ISOLATION still applies; inbound traffic to published
ports is still routed and still firewalled; INPUT, which protects the NAS
itself, is untouched. The one real caveat is that it is host-wide, so VMs
sharing a bridge lose same-bridge filtering too.
The migrate failure message points at the sysctl now.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nux3LCu4zP9nih4BRhPyHw
lukasandClaude Opus 5
cc63efa968