Add opt to set default grpc scheme to passthrough

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

cli/cmd/root.go

@@ -36,6 +36,7 @@ import (
 	"os"
 
 	"code.fbi.h-da.de/danet/gosdn/cli/adapter"
+	"google.golang.org/grpc/resolver"
 
 	"github.com/google/uuid"
 	log "github.com/sirupsen/logrus"
@@ -45,6 +46,7 @@ import (
 
 var cfgFile string
 var verbose bool
+var gRPCPassthrough bool
 var loglevel string
 var grpcPort string
 var cliPnd string
@@ -85,6 +87,7 @@ func init() {
 	rootCmd.PersistentFlags().StringVar(&cfgFile, "config", "", "config file (./.gosdnc.toml)")
 	rootCmd.PersistentFlags().StringVarP(&loglevel, "log-level", "l", "", "log level 'debug' or 'trace'")
 	rootCmd.PersistentFlags().BoolVar(&verbose, "verbose", false, "show mne and sbi info")
+	rootCmd.PersistentFlags().BoolVar(&gRPCPassthrough, "grpcPassthrough", true, "set the default resolve scheme for grpc to passthrough, default is true")
 
 	rootCmd.Flags().StringVar(&grpcPort, "grpc-port", "55055", "port for gRPC NBI")
 }
@@ -147,6 +150,13 @@ func initConfig() {
 		cliPnd = uuid.New().String()
 	}
 
+	if gRPCPassthrough {
+		log.Info("Setting gRPC default resolver scheme to passthrough. No DNS queries are being made when doing a gRPC request.")
+		resolver.SetDefaultScheme("passthrough")
+	} else {
+		log.Info("gRPC default resolver scheme is not set to passthrough. This might cause issues with the gRPC connection when no real DNS server is available as each gRPC requests requires a DNS request.")
+	}
+
 	adapter, err := adapter.NewPndAdapter(cliPnd, viper.GetString("controllerAPIEndpoint"))
 	if err != nil {
 		log.Fatal(err)