fix(net/netmon): skip RTM_MISS route messages on Darwin (#113)

On Darwin, the AF_ROUTE socket delivers RTM_MISS on every failed
route lookup. When a STUN probe targets an IPv6 address with no
route (common on machines without global v6 connectivity), each
probe generates an RTM_MISS that netmon treats as a LinkChange.
The LinkChange triggers ReSTUN, which fires another probe, creating
a self-sustaining loop (~1.3 events/s vs the normal ~1/30s cadence).

RTM_MISS is emitted from the route lookup path, not the table
mutation path. Route withdrawals always emit RTM_DELETE before any
subsequent lookup can miss, so RTM_MISS is never the leading signal
for a real topology change. Every mature BSD route-socket consumer
(bird, dhcpcd, frr) ignores it for table-change purposes.

Author: ethanndickson

net/netmon/netmon_darwin.go

@@ -132,6 +132,18 @@ func (m *darwinRouteMon) skipInterfaceAddrMessage(msg *route.InterfaceAddrMessag
 }
 
 func (m *darwinRouteMon) skipRouteMessage(msg *route.RouteMessage) bool {
+	// RTM_MISS fires on every failed route lookup (no matching
+	// entry in the routing table). It scales with traffic volume,
+	// not network-state changes, and is never the leading signal
+	// for a topology change — route withdrawals emit RTM_DELETE
+	// synchronously before any subsequent lookup can miss.
+	// Letting these through causes netmon to report spurious
+	// LinkChange events, which trigger a ReSTUN/netcheck loop
+	// when the destination is unreachable (e.g. STUN probes to
+	// an IPv6 address with no route).
+	if msg.Type == unix.RTM_MISS {
+		return true
+	}
 	if ip := ipOfAddr(addrType(msg.Addrs, unix.RTAX_DST)); ip.IsLinkLocalUnicast() {
 		// Skip those like:
 		// dst = fe80::b476:66ff:fe30:c8f6%15