A booter attack doesn't "hack" your server — it drowns the network link in front of it. Understanding which layer is under attack tells you how to stop it.
Layer 3/4 vs Layer 7
Volumetric L3/4 floods are absorbed upstream before they reach your machine. L7 attacks mimic real players joining and pinging — those need protocol-aware filtering, which is exactly what a whitelist can't give you.