Allow sending of broadcast packets w/o route.

Currently sending a broadcast packet (for DHCP, e.g.) requires a "default
route" of the format "0.0.0.0/0 via 0.0.0.0 <intf>". There is no good reason
for this and on devices with several ports this creates a rather akward route
table with lots of such default routes (which defeats the purpose of a default
route).

PiperOrigin-RevId: 224378769
Change-Id: Icd7ec8a206eb08083cff9a837f6f9ab231c73a19

pkg/tcpip/stack/BUILD

@@ -40,6 +40,7 @@ go_test(
         ":stack",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/link/channel",
         "//pkg/waiter",
     ],

pkg/tcpip/stack/stack.go

@@ -716,12 +716,29 @@ func (s *Stack) GetMainNICAddress(id tcpip.NICID, protocol tcpip.NetworkProtocol
 	return "", tcpip.Subnet{}, tcpip.ErrUnknownNICID
 }
 
+func (s *Stack) getRefEP(nic *NIC, localAddr tcpip.Address, netProto tcpip.NetworkProtocolNumber) (ref *referencedNetworkEndpoint) {
+	if len(localAddr) == 0 {
+		return nic.primaryEndpoint(netProto)
+	}
+	return nic.findEndpoint(netProto, localAddr, CanBePrimaryEndpoint)
+}
+
 // FindRoute creates a route to the given destination address, leaving through
 // the given nic and local address (if provided).
 func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, netProto tcpip.NetworkProtocolNumber) (Route, *tcpip.Error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
+	// We don't require a route in the table to send a broadcast out on a NIC.
+	if id != 0 && remoteAddr == header.IPv4Broadcast {
+		if nic, ok := s.nics[id]; ok {
+			if ref := s.getRefEP(nic, localAddr, netProto); ref != nil {
+				return makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref), nil
+			}
+		}
+		return Route{}, tcpip.ErrNoRoute
+	}
+
 	for i := range s.routeTable {
 		if (id != 0 && id != s.routeTable[i].NIC) || (len(remoteAddr) != 0 && !s.routeTable[i].Match(remoteAddr)) {
 			continue
@@ -732,12 +749,7 @@ func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, n
 			continue
 		}
 
-		var ref *referencedNetworkEndpoint
-		if len(localAddr) != 0 {
-			ref = nic.findEndpoint(netProto, localAddr, CanBePrimaryEndpoint)
-		} else {
-			ref = nic.primaryEndpoint(netProto)
-		}
+		ref := s.getRefEP(nic, localAddr, netProto)
 		if ref == nil {
 			continue
 		}

pkg/tcpip/stack/stack_test.go

@@ -26,6 +26,7 @@ import (
 
 	"gvisor.googlesource.com/gvisor/pkg/tcpip"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/buffer"
+	"gvisor.googlesource.com/gvisor/pkg/tcpip/header"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/link/channel"
 	"gvisor.googlesource.com/gvisor/pkg/tcpip/stack"
 )
@@ -643,6 +644,42 @@ func TestAddressSpoofing(t *testing.T) {
 	}
 }
 
+func TestBroadcastNeedsNoRoute(t *testing.T) {
+	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
+
+	id, _ := channel.New(10, defaultMTU, "")
+	if err := s.CreateNIC(1, id); err != nil {
+		t.Fatalf("CreateNIC failed: %v", err)
+	}
+	s.SetRouteTable([]tcpip.Route{})
+
+	// If there is no endpoint, it won't work.
+	if _, err := s.FindRoute(1, header.IPv4Any, header.IPv4Broadcast, fakeNetNumber); err != tcpip.ErrNoRoute {
+		t.Fatalf("got FindRoute(1, %v, %v, %v) = %v, want = %v", header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, err, tcpip.ErrNoRoute)
+	}
+
+	if err := s.AddAddress(1, fakeNetNumber, header.IPv4Any); err != nil {
+		t.Fatalf("AddAddress(%v, %v) failed: %v", fakeNetNumber, header.IPv4Any, err)
+	}
+	r, err := s.FindRoute(1, header.IPv4Any, header.IPv4Broadcast, fakeNetNumber)
+	if err != nil {
+		t.Fatalf("FindRoute(1, %v, %v, %v) failed: %v", header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, err)
+	}
+
+	if r.LocalAddress != header.IPv4Any {
+		t.Errorf("Bad local address: got %v, want = %v", r.LocalAddress, header.IPv4Any)
+	}
+
+	if r.RemoteAddress != header.IPv4Broadcast {
+		t.Errorf("Bad remote address: got %v, want = %v", r.RemoteAddress, header.IPv4Broadcast)
+	}
+
+	// If the NIC doesn't exist, it won't work.
+	if _, err := s.FindRoute(2, header.IPv4Any, header.IPv4Broadcast, fakeNetNumber); err != tcpip.ErrNoRoute {
+		t.Fatalf("got FindRoute(2, %v, %v, %v) = %v want = %v", header.IPv4Any, header.IPv4Broadcast, fakeNetNumber, err, tcpip.ErrNoRoute)
+	}
+}
+
 // Set the subnet, then check that packet is delivered.
 func TestSubnetAcceptsMatchingPacket(t *testing.T) {
 	s := stack.New([]string{"fakeNet"}, nil, stack.Options{})
@@ -849,7 +886,7 @@ func TestGetMainNICAddressAddPrimaryNonPrimary(t *testing.T) {
 								if !ok {
 									t.Fatalf("GetMainNICAddress: got address = %v, wanted any in {%v}", gotAddress, primaryAddrAdded)
 								}
-								if expectedSubnet != gotSubnet {
+								if gotSubnet != expectedSubnet {
 									t.Fatalf("GetMainNICAddress: got subnet = %v, wanted %v", gotSubnet, expectedSubnet)
 								}
 							}