argus caught it: binding the socket to the mesh address does not open
the port. NixOS's firewall is default-deny and filters in netfilter,
before a packet reaches a bound socket — the bind chooses which
address accepts connections, not whether packets arrive. As shipped
the receiver was unreachable.
swarm.nix already shows the pattern for exactly this situation: it
opens the mesh's UDP port explicitly right after bringing the
interface up.
Interface-scoped to wg-hive rather than host-wide, so the option's
"reachable exactly by mesh peers" claim is actually true. A global
allowedTCPPorts would open the port on every interface including the
public NIC, leaving only the socket's bind address between the
internet and a root btrfs receive.
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