tstest/natlab: add dual stack with blackholed IPv4

This reproduces the bug report from
https://github.com/tailscale/tailscale/issues/13346

It does not yet fix it.

Updates #13346

Change-Id: Ia5af7b0481a64a37efe259c798facdda6d9da618
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/nat/nat_test.go

@@ -115,6 +115,17 @@ func easyAnd6(c *vnet.Config) *vnet.Node {
 		vnet.EasyNAT))
 }
 
+func v6AndBlackholedIPv4(c *vnet.Config) *vnet.Node {
+	n := c.NumNodes() + 1
+	nw := c.AddNetwork(
+		fmt.Sprintf("2.%d.%d.%d", n, n, n), // public IP
+		fmt.Sprintf("192.168.%d.1/24", n),
+		v6cidr(n),
+		vnet.EasyNAT)
+	nw.SetBlackholedIPv4(true)
+	return c.AddNode(nw)
+}
+
 func just6(c *vnet.Config) *vnet.Node {
 	n := c.NumNodes() + 1
 	return c.AddNode(c.AddNetwork(v6cidr(n))) // public IPv6 prefix
@@ -482,6 +493,20 @@ func TestSingleJustIPv6(t *testing.T) {
 	nt.runTest(just6)
 }
 
+var knownBroken = flag.Bool("known-broken", false, "run known-broken tests")
+
+// TestSingleDualStackButBrokenIPv4 tests a dual-stack node with broken
+// (blackholed) IPv4.
+//
+// See https://github.com/tailscale/tailscale/issues/13346
+func TestSingleDualBrokenIPv4(t *testing.T) {
+	if !*knownBroken {
+		t.Skip("skipping known-broken test; set --known-broken to run; see https://github.com/tailscale/tailscale/issues/13346")
+	}
+	nt := newNatTest(t)
+	nt.runTest(v6AndBlackholedIPv4)
+}
+
 func TestJustIPv6(t *testing.T) {
 	nt := newNatTest(t)
 	nt.runTest(just6, just6)

tstest/natlab/vnet/conf.go

@@ -272,9 +272,10 @@ type Network struct {
 
 	wanIP6 netip.Prefix // global unicast router in host bits; CIDR is /64 delegated to LAN
 
-	wanIP4 netip.Addr // IPv4 WAN IP, if any
+	wanIP4    netip.Addr // IPv4 WAN IP, if any
-	lanIP4 netip.Prefix
+	lanIP4    netip.Prefix
-	nodes  []*Node
+	nodes     []*Node
+	breakWAN4 bool // whether to break WAN IPv4 connectivity
 
 	svcs set.Set[NetworkService]
 
@@ -282,6 +283,12 @@ type Network struct {
 	err error // carried error
 }
 
+// SetBlackholedIPv4 sets whether the network should blackhole all IPv4 traffic
+// out to the Internet. (DHCP etc continues to work on the LAN.)
+func (n *Network) SetBlackholedIPv4(v bool) {
+	n.breakWAN4 = v
+}
+
 func (n *Network) CanV4() bool {
 	return n.lanIP4.IsValid() || n.wanIP4.IsValid()
 }
@@ -353,6 +360,7 @@ func (s *Server) initFromConfig(c *Config) error {
 			v6:         conf.wanIP6.IsValid(),
 			wanIP4:     conf.wanIP4,
 			lanIP4:     conf.lanIP4,
+			breakWAN4:  conf.breakWAN4,
 			nodesByIP4: map[netip.Addr]*node{},
 			nodesByMAC: map[MAC]*node{},
 			logf:       logger.WithPrefix(s.logf, fmt.Sprintf("[net-%v] ", conf.mac)),

tstest/natlab/vnet/vnet.go

@@ -514,6 +514,7 @@ type network struct {
 	wanIP6         netip.Prefix         // router's WAN IPv6, if any, as a /64.
 	wanIP4         netip.Addr           // router's LAN IPv4, if any
 	lanIP4         netip.Prefix         // router's LAN IP + CIDR (e.g. 192.168.2.1/24)
+	breakWAN4      bool                 // break WAN IPv4 connectivity
 	nodesByIP4     map[netip.Addr]*node // by LAN IPv4
 	nodesByMAC     map[MAC]*node
 	logf           func(format string, args ...any)
@@ -1104,6 +1105,10 @@ func (n *network) HandleUDPPacket(p UDPPacket) {
 		Length:         len(buf),
 		InterfaceIndex: n.wanInterfaceID,
 	}, buf)
+	if p.Dst.Addr().Is4() && n.breakWAN4 {
+		// Blackhole the packet.
+		return
+	}
 	dst := n.doNATIn(p.Src, p.Dst)
 	if !dst.IsValid() {
 		n.logf("Warning: NAT dropped packet; no mapping for %v=>%v", p.Src, p.Dst)
@@ -1239,6 +1244,10 @@ func (n *network) HandleEthernetPacketForRouter(ep EthernetPacket) {
 	}
 
 	if toForward && n.s.shouldInterceptTCP(packet) {
+		if flow.dst.Is4() && n.breakWAN4 {
+			// Blackhole the packet.
+			return
+		}
 		var base *layers.BaseLayer
 		proto := header.IPv4ProtocolNumber
 		if v4, ok := packet.Layer(layers.LayerTypeIPv4).(*layers.IPv4); ok {
@@ -1324,6 +1333,10 @@ func (n *network) handleUDPPacketForRouter(ep EthernetPacket, udp *layers.UDP, t
 	}
 
 	if toForward {
+		if dstIP.Is4() && n.breakWAN4 {
+			// Blackhole the packet.
+			return
+		}
 		src := netip.AddrPortFrom(srcIP, uint16(udp.SrcPort))
 		dst := netip.AddrPortFrom(dstIP, uint16(udp.DstPort))
 		buf, err := n.serializedUDPPacket(src, dst, udp.Payload, nil)