Add dns client

dns/rcode.go

@@ -0,0 +1,33 @@
+package dns
+
+import F "github.com/sagernet/sing/common/format"
+
+const (
+	RCodeSuccess        RCodeError = 0 // NoError
+	RCodeFormatError    RCodeError = 1 // FormErr
+	RCodeServerFailure  RCodeError = 2 // ServFail
+	RCodeNameError      RCodeError = 3 // NXDomain
+	RCodeNotImplemented RCodeError = 4 // NotImp
+	RCodeRefused        RCodeError = 5 // Refused
+)
+
+type RCodeError uint16
+
+func (e RCodeError) Error() string {
+	switch e {
+	case RCodeSuccess:
+		return "success"
+	case RCodeFormatError:
+		return "format error"
+	case RCodeServerFailure:
+		return "server failure"
+	case RCodeNameError:
+		return "name error"
+	case RCodeNotImplemented:
+		return "not implemented"
+	case RCodeRefused:
+		return "refused"
+	default:
+		return F.ToString("unknown error: ", uint16(e))
+	}
+}