Function TestInjectLoopback

wgengine/netstack/netstack_test.go:1687–1749

TestInjectLoopback verifies that the inject goroutine delivers self-addressed packets back into gVisor (via DeliverLoopback) instead of sending them to WireGuard outbound. This is a regression test for a bug where self-dial packets were sent to WireGuard and silently dropped.

(t *testing.T)

// WireGuard outbound. This is a regression test for a bug where self-dial
// packets were sent to WireGuard and silently dropped.
func TestInjectLoopback(t *testing.T) {
	selfIP4 := netip.MustParseAddr("100.64.1.2")

	ns := makeNetstack(t, func(impl *Impl) {
		impl.ProcessLocalIPs = true
		impl.atomicIsLocalIPFunc.Store(func(addr netip.Addr) bool {
			return addr == selfIP4
		})
	})

	// Register gVisor's NIC address so the stack accepts and routes
	// packets for this IP.
	protocolAddr := tcpip.ProtocolAddress{
		Protocol:          header.IPv4ProtocolNumber,
		AddressWithPrefix: tcpip.AddrFrom4(selfIP4.As4()).WithPrefix(),
	}
	if err := ns.ipstack.AddProtocolAddress(nicID, protocolAddr, stack.AddressProperties{}); err != nil {
		t.Fatalf("AddProtocolAddress: %v", err)
	}

	// Bind a UDP socket on the gVisor stack to receive the loopback packet.
	pc, err := gonet.DialUDP(ns.ipstack, &tcpip.FullAddress{
		NIC:  nicID,
		Addr: tcpip.AddrFrom4(selfIP4.As4()),
		Port: 8081,
	}, nil, header.IPv4ProtocolNumber)
	if err != nil {
		t.Fatalf("DialUDP: %v", err)
	}
	defer pc.Close()

	// Build a valid self-addressed UDP packet from raw bytes and wrap it
	// in a gVisor PacketBuffer with headers already pushed, as gVisor's
	// outbound path produces.
	payload := []byte("loopback test")
	raw := udp4raw(t, selfIP4, selfIP4, 12345, 8081, payload)

	pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
		ReserveHeaderBytes: header.IPv4MinimumSize + header.UDPMinimumSize,
		Payload:            buffer.MakeWithData(payload),
	})
	copy(pkt.TransportHeader().Push(header.UDPMinimumSize),
		raw[header.IPv4MinimumSize:header.IPv4MinimumSize+header.UDPMinimumSize])
	pkt.TransportProtocolNumber = header.UDPProtocolNumber
	copy(pkt.NetworkHeader().Push(header.IPv4MinimumSize), raw[:header.IPv4MinimumSize])
	pkt.NetworkProtocolNumber = header.IPv4ProtocolNumber

	if err := ns.linkEP.q.Write(pkt); err != nil {
		t.Fatalf("queue.Write: %v", err)
	}

	// The inject goroutine should detect the self-addressed packet via
	// isSelfDst and deliver it back into gVisor via DeliverLoopback.
	pc.SetReadDeadline(time.Now().Add(5 * time.Second))
	buf := make([]byte, 256)
	n, _, err := pc.ReadFrom(buf)
	if err != nil {
		t.Fatalf("ReadFrom: %v (self-addressed packet was not looped back)", err)