diff --git a/install_functions.sh b/install_functions.sh
index f85f58d685320e9d5d8d926d53d46458e955380f..f7031921115d7eb696a0b3696353268392e56de6 100644
--- a/install_functions.sh
+++ b/install_functions.sh
@@ -120,6 +120,8 @@ install_mono_complete() {
 }
 
 install_virtualbox() {
+    # extension pack downloads: https://www.virtualbox.org/wiki/Downloads, probably under older releases
+    # guest additions downloads: https://download.virtualbox.org/virtualbox, in case vbox auto download does not work
     $apt_install_cmd virtualbox
     $add_user_to_group_cmd vboxusers $username
 }