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