Disable auto-generation of IPv6 link-local addresses for loopback NICs

Test: Test that an IPv6 link-local address is not auto-generated for loopback
NICs, even when it is enabled for non-loopback NICS.
PiperOrigin-RevId: 288519591

pkg/tcpip/stack/nic.go

@@ -174,7 +174,8 @@ func (n *NIC) enable() *tcpip.Error {
 		return err
 	}
 
-	if !n.stack.autoGenIPv6LinkLocal {
+	// Do not auto-generate an IPv6 link-local address for loopback devices.
+	if !n.stack.autoGenIPv6LinkLocal || n.loopback {
 		return nil
 	}
 

pkg/tcpip/stack/stack.go

@@ -444,8 +444,8 @@ type Stack struct {
 	ndpConfigs NDPConfigurations
 
 	// autoGenIPv6LinkLocal determines whether or not the stack will attempt
-	// to auto-generate an IPv6 link-local address for newly enabled NICs.
-	// See the AutoGenIPv6LinkLocal field of Options for more details.
+	// to auto-generate an IPv6 link-local address for newly enabled non-loopback
+	// NICs. See the AutoGenIPv6LinkLocal field of Options for more details.
 	autoGenIPv6LinkLocal bool
 
 	// ndpDisp is the NDP event dispatcher that is used to send the netstack
@@ -496,13 +496,15 @@ type Options struct {
 	// before assigning an address to a NIC.
 	NDPConfigs NDPConfigurations
 
-	// AutoGenIPv6LinkLocal determins whether or not the stack will attempt
-	// to auto-generate an IPv6 link-local address for newly enabled NICs.
+	// AutoGenIPv6LinkLocal determines whether or not the stack will attempt to
+	// auto-generate an IPv6 link-local address for newly enabled non-loopback
+	// NICs.
+	//
 	// Note, setting this to true does not mean that a link-local address
-	// will be assigned right away, or at all. If Duplicate Address
-	// Detection is enabled, an address will only be assigned if it
-	// successfully resolves. If it fails, no further attempt will be made
-	// to auto-generate an IPv6 link-local address.
+	// will be assigned right away, or at all. If Duplicate Address Detection
+	// is enabled, an address will only be assigned if it successfully resolves.
+	// If it fails, no further attempt will be made to auto-generate an IPv6
+	// link-local address.
 	//
 	// The generated link-local address will follow RFC 4291 Appendix A
 	// guidelines.

pkg/tcpip/stack/stack_test.go

@@ -2121,6 +2121,55 @@ func TestNICAutoGenAddrWithOpaque(t *testing.T) {
 	}
 }
 
+// TestNoLinkLocalAutoGenForLoopbackNIC tests that IPv6 link-local addresses are
+// not auto-generated for loopback NICs.
+func TestNoLinkLocalAutoGenForLoopbackNIC(t *testing.T) {
+	const nicID = 1
+	const nicName = "nicName"
+
+	tests := []struct {
+		name          string
+		opaqueIIDOpts stack.OpaqueInterfaceIdentifierOptions
+	}{
+		{
+			name:          "IID From MAC",
+			opaqueIIDOpts: stack.OpaqueInterfaceIdentifierOptions{},
+		},
+		{
+			name: "Opaque IID",
+			opaqueIIDOpts: stack.OpaqueInterfaceIdentifierOptions{
+				NICNameFromID: func(_ tcpip.NICID, nicName string) string {
+					return nicName
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			opts := stack.Options{
+				NetworkProtocols:     []stack.NetworkProtocol{ipv6.NewProtocol()},
+				AutoGenIPv6LinkLocal: true,
+				OpaqueIIDOpts:        test.opaqueIIDOpts,
+			}
+
+			e := channel.New(0, 1280, linkAddr1)
+			s := stack.New(opts)
+			if err := s.CreateNamedLoopbackNIC(nicID, nicName, e); err != nil {
+				t.Fatalf("CreateNamedLoopbackNIC(%d, %q, _) = %s", nicID, nicName, err)
+			}
+
+			addr, err := s.GetMainNICAddress(nicID, header.IPv6ProtocolNumber)
+			if err != nil {
+				t.Fatalf("stack.GetMainNICAddress(%d, _) err = %s", nicID, err)
+			}
+			if want := (tcpip.AddressWithPrefix{}); addr != want {
+				t.Errorf("got stack.GetMainNICAddress(%d, _) = %s, want = %s", nicID, addr, want)
+			}
+		})
+	}
+}
+
 // TestNICAutoGenAddrDoesDAD tests that the successful auto-generation of IPv6
 // link-local addresses will only be assigned after the DAD process resolves.
 func TestNICAutoGenAddrDoesDAD(t *testing.T) {