Downloading Fedora

Q: Why don't the VirtualBox Guest Additions install properly on Fedora 23?

A: It seems that the VirtualBox Guest Additions don’t like the version of the X.Org Server software that comes with the Fedora 23 ISO. Before installing the additions, make sure to run:

sudo dnf update

This will get the latest version of X.Org Server (currently 1.18). After that, restart the VM. Then run:

sudo dnf install gcc kernel-devel-$(uname -r)

This command installs the gcc compiler and updated kernel headers, which the Guest Additions need in order to build themselves. $(uname -r) takes the output of the uname -r command, which returns the current kernel version number, and uses it to request to install the correct version for your system. Then try installing the Guest Additions again. You should see "Installing X.Org Server 1.18 modules [ OK ].” Restart the system again, and the full-screen feature should work.