A flood of as many as 5 Million queries per second hit many of the Internet’s DNS Root Servers that act as the authoritative reference for mapping domain names to IP addresses. The attack, commonly known as Distributed Denial of Service (DDoS) attack, took place on two separate occasions. There is no indication of who or what was behind the large-scale DDoS attacks because the source IP addresses used in the attacks were very well distributed and randomized across the entire IPv4 address space.
Source: https://thehackernews.com/2015/12/dns-root-servers-ddos-attack.html