The VirtualBox Guest Additions are a set of drivers and applications to be deployed on a virtual machine to have better performance and enable features such as folder sharing. While it's possible to include the Guest Additions directly in the box, not all the boxes you'll find have it, and even when they do, they can be outdated very quickly.
The solution is to automatically deploy the VirtualBox Guest Additions on demand, through a plugin.
The downside to using this plugin is that the Vagrant box may now take longer to boot, as it may need to download and install the right guest additions for the box.
To step through this recipe, you will need the following: