Add new option for preferring of new IP

config/config.go

 	SecretModeTLS
 )
 
+type PreferIP uint8
+
+const (
+	PreferIPv4 PreferIP = iota
+	PreferIPv6
+)
+
 const SimpleSecretLength = 16
 
 type OptionType uint8
 	OptionTypeDebug OptionType = iota
 	OptionTypeVerbose
 
+	OptionTypePreferIP
+
 	OptionTypeBind
 	OptionTypePublicIPv4
 	OptionTypePublicIPv6
 	Debug      bool       `json:"debug"`
 	Verbose    bool       `json:"verbose"`
 	SecretMode SecretMode `json:"secret_mode"`
+	PreferIP   PreferIP   `json:"prefer_ip"`
 
 	Secret []byte `json:"secret"`
 	AdTag  []byte `json:"adtag"`
 			C.Debug = opt.Value.(bool)
 		case OptionTypeVerbose:
 			C.Verbose = opt.Value.(bool)
+		case OptionTypePreferIP:
+			value := opt.Value.(string)
+			switch value {
+			case "ipv4":
+				C.PreferIP = PreferIPv4
+			case "ipv6":
+				C.PreferIP = PreferIPv6
+			default:
+				return fmt.Errorf("incorrect direct IP mode %s", value)
+			}
 		case OptionTypeBind:
 			C.Bind = opt.Value.(*net.TCPAddr)
 		case OptionTypePublicIPv4:

main.go

 		Short('v').
 		Envar("MTG_VERBOSE").
 		Bool()
+	runPreferIP = runCommand.Flag("prefer-ip",
+		"Prefer this IP protocol if possible. Valid options are 'ipv4' and 'ipv6'").
+		Envar("MTG_PREFER_DIRECT_IP").
+		Default("ipv6").
+		Enum("ipv4", "ipv6")
 	runBind = runCommand.Flag("bind",
 		"Host:Port to bind proxy to.").
 		Short('b').
 		err := config.Init(
 			config.Opt{Option: config.OptionTypeDebug, Value: *runDebug},
 			config.Opt{Option: config.OptionTypeVerbose, Value: *runVerbose},
+			config.Opt{Option: config.OptionTypePreferIP, Value: *runPreferIP},
 			config.Opt{Option: config.OptionTypeBind, Value: *runBind},
 			config.Opt{Option: config.OptionTypePublicIPv4, Value: *runPublicIPv4},
 			config.Opt{Option: config.OptionTypePublicIPv6, Value: *runPublicIPv6},