Installing VirtualBox Guest Additions in a Debian 10 (Buster) guest using PPA

VirtualBox will not be in the Debian 10 (Buster) distribution, not even in backports.

You can use the Oracle PPA for Buster to install VirtualBox, but it doesn’t contain the guest utils

Fortunately, Lucas Nussbaum has set up a PPA with an unofficial backport, that includes them.

Instructions:

1. Add the cryptographic signature of the PPA:
   wget -O - https://db.debian.org/fetchkey.cgi?fingerprint=FEDEC1CB337BCF509F43C2243914B532F4DFBE99 | apt-key add
2. Add the folllowing text to /etc/apt/sources.list.d/VirtualBox_Lucas.list :
   # Lucas Nussbaum's unofficial VirtualBox backport
deb https://people.debian.org/~lucas/virtualbox-buster/ ./
3. Update package list and install the guest additions:
   apt update
apt install virtualbox-guest-utils virtualbox-guest-x11
4. Restart the guest OS.
5. If automatic window resizing doesn’t work after restarting, run the following command:
   VBoxClient --vmsvga
6. If that makes it work, add the command to your autostart.