GRE

GRE is a tunneling protocol that wraps one network protocol inside another, letting traffic cross networks that could not otherwise carry it. GRE itself provides no encryption or authentication, which is why it is often paired with IPsec for site-to-site VPNs. Attackers abuse GRE tunnels to exfiltrate data past firewalls and to launder large DDoS floods, so unexpected GRE traffic is worth investigating.