net/netmon: skip RTM_MISS route messages on darwin (#20050)

macOS 26.4 emits RTM_MISS on the routing socket for every failed route
lookup. skipRouteMessage never inspected the message type, so each miss
woke the monitor as a link change and triggered a netcheck. On networks
without an IPv6 default route the netcheck's IPv6 DERP probes fail and
emit more RTM_MISS messages, sustaining the loop indefinitely: netchecks
run at roughly 40x the intended rate, with sustained probe traffic and
corresponding CPU and battery cost.

RTM_MISS scales with traffic volume, not network state, and is never
the leading signal for a topology change: route withdrawals emit
RTM_DELETE synchronously before any subsequent lookup can miss, so
ignoring it loses no signal. Other routing daemons (bird, dhcpcd, frr)
ignore it as well.

Same fix as coder/tailscale@e956a95074.

Fixes #19324

Signed-off-by: Doug Bryant <dougbryant@anthropic.com>

net/netmon/netmon_darwin.go

@@ -150,6 +150,17 @@ 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
+	// link changes, which trigger a re-STUN/netcheck loop when a probe
+	// destination is unreachable (e.g. IPv6 DERP probes on a network with
+	// no IPv6 default route), as each failed probe emits another RTM_MISS.
+	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

net/netmon/netmon_darwin_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"golang.org/x/net/route"
+	"golang.org/x/sys/unix"
 )
 
 func TestIssue1416RIB(t *testing.T) {
@@ -25,3 +26,31 @@ func TestIssue1416RIB(t *testing.T) {
 	}
 	t.Logf("Got: %#v", msgs)
 }
+
+func TestSkipRouteMessage(t *testing.T) {
+	m := &darwinRouteMon{logf: t.Logf}
+	dst := &route.Inet6Addr{IP: [16]byte{0x26, 0x07}} // 2607:: (global unicast)
+	tests := []struct {
+		name string
+		msg  *route.RouteMessage
+		want bool
+	}{
+		{
+			name: "rtm_miss",
+			msg:  &route.RouteMessage{Type: unix.RTM_MISS, Addrs: []route.Addr{unix.RTAX_DST: dst}},
+			want: true,
+		},
+		{
+			name: "rtm_add",
+			msg:  &route.RouteMessage{Type: unix.RTM_ADD, Addrs: []route.Addr{unix.RTAX_DST: dst}},
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := m.skipRouteMessage(tt.msg); got != tt.want {
+				t.Errorf("skipRouteMessage = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}