(Applies to Debian 9.0+ or Ubuntu 16.04+) Once cryptsetup is installed, configured and working, we install dropbear-initramfs

apt install busybox dropbear-initramfs

edit /etc/dropbear-initramfs/config

DROPBEAR_OPTIONS="-p 2322 -s -j -k -I 60"

edit /etc/dropbear-initramfs/authorized_keys add public key

no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="/bin/cryptroot-unlock" ssh-rsa AAAAxxxxPUBLIC_KEYxxxx `

edit /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT="ip=192.168.0.5::192.168.0.254:255.255.255.0 quiet"

update-initramfs -u