[net] only listen to tcp4 address

Signed-off-by: Leo Chen <leo@harmony.one>

api/client/service/server.go

@@ -43,7 +43,7 @@ func (s *Server) GetFreeToken(ctx context.Context, request *proto.GetFreeTokenRe
 func (s *Server) Start(ip, port string) (*grpc.Server, error) {
 	// TODO(minhdoan): Currently not using ip. Fix it later.
 	addr := net.JoinHostPort("", port)
-	lis, err := net.Listen("tcp", addr)
+	lis, err := net.Listen("tcp4", addr)
 	if err != nil {
 		log.Fatalf("failed to listen: %v", err)
 	}

api/proto/message/server.go

@@ -81,7 +81,7 @@ func (s *Server) Process(ctx context.Context, message *Message) (*Response, erro
 // Start starts the Server on given ip and port.
 func (s *Server) Start() (*grpc.Server, error) {
 	addr := net.JoinHostPort(IP, Port)
-	lis, err := net.Listen("tcp", addr)
+	lis, err := net.Listen("tcp4", addr)
 	if err != nil {
 		log.Fatalf("failed to listen: %v", err)
 	}

api/service/syncing/downloader/server.go

@@ -43,7 +43,7 @@ func (s *Server) Query(ctx context.Context, request *pb.DownloaderRequest) (*pb.
 // Start starts the Server on given ip and port.
 func (s *Server) Start(ip, port string) (*grpc.Server, error) {
 	addr := net.JoinHostPort("", port)
-	lis, err := net.Listen("tcp", addr)
+	lis, err := net.Listen("tcp4", addr)
 	if err != nil {
 		log.Fatalf("[SYNC] failed to listen: %v", err)
 	}