improve address finding

main.go

@@ -88,14 +88,26 @@ func doPing(host string, count int, interval time.Duration) (*ping.Statistics, e
 	if err != nil {
 		return nil, err
 	}
-	addr, err := nif.Addrs()
+	addrs, err := nif.Addrs()
 	if err != nil {
 		return nil, err
 	}
-	if len(addr) == 0 {
+	if len(addrs) == 0 {
 		return nil, fmt.Errorf("interface %s has no addressed assigned", conf.UploadInterface)
 	}
-	pinger.Source = addr[0].(*net.IPNet).IP.String()
+
+	for _, addr := range addrs {
+		if addr.(*net.IPNet).IP.To4() == nil {
+			continue
+		} else {
+			pinger.Source = addr.(*net.IPNet).IP.String()
+			break
+		}
+	}
+
+	if pinger.Source == "" {
+		return nil, fmt.Errorf("interface %s has no suitable addressed assigned", conf.UploadInterface)
+	}
 
 	err = pinger.Run()
 	if err != nil {