In order to keep the GuestAdditions in sync with your VirtualBox (VB) version, I am using dotless-de’s vbguest Vagrant plugin. See their documentation for usage.
If the configuration of a local file path for the ISO file does not work with the following error (maybe Windows specific):
Downloading VirtualBox Guest Additions ISO from C:/Coding/vm/dev/Vagrantfile/VBoxGuestAdditions_5.1.0_RC1.iso ==> myVm: Checking for guest additions in VM... An error occurred while downloading the remote file. The error message, if any, is reproduced below. Please fix this error and try again. Protocol "C" not supported or disabled in libcurl
Running vbguest as a command should fix it, providing it the iso file as a cli option:
vagrant vbguest --iso your/path/to/VBoxGuestAdditions_5.1.0_RC1.iso --do install
If you get an error:
VirtualBox Guest Additions installer You appear to have a version of the VBoxGuestAdditions software on your system which was installed from a different source or using a different type of installer. If you installed it from a package from your Linux distribution or if it is a default part of the system then we strongly recommend that you cancel this installation and remove it properly before installing this version. If this is simply an older or a damaged installation you may safely proceed. Do you wish to continue anyway? [yes or no] Cancelling installation. An error occurred during installation of VirtualBox Guest Additions 5.1.0. Some functionality may not work as intended. In most cases it is OK that the "Window System drivers" installation failed. Redirecting to /bin/systemctl start vboxadd.service Failed to start vboxadd.service: Unit not found.
You might have a “VBoxGuestAdditions-x.x.x” folder in your /opt directory on the guest system, indicating that VBoxGuestAdditions had been installed before and not with the vbguest Plugin. For me, the folder was empty. Remove the folder and rerun vbguest.
See these links for more troubleshooting info:
Get your VBoxGuestAdditions.iso from Oracle’s download page.