==== ASCII/Stretch ====
  # apt-get update
  # apt-get upgrade
  # apt-get dist-upgrade
  # reboot
  # apt-get install linux-headers-$(uname -r)
  - insert guest additions CD
  # mkdir /tmp/vboxsucks
  # mount /dev/sr0 /tmp/vboxsucks
  # cp /tmp/vboxsucks/VBoxLinuxAdditions.run ~
  # umount /dev/sr0
  # cd ~
  # ./VBoxLinuxAdditions.run

==== Jessie ====
Insert additions disk via VBox and then:
  # cd /tmp
  # mount /dev/cdrom /media/cdrom0
  # cp /media/cdrom0/VBoxLinuxAdditions.run /tmp
  # eject /media/cdrom0
  # apt-get install linux-headers-`uname -r` dkms build-essential
  # ./VBoxLinuxAdditions.run
  # reboot