Recent DDoS attacks leverage open DNS resolvers
DNS reflection attacks use open DNS resolvers. In a DNS reflection or amplification attack, the attacker issues a request to an open DNS resolver for some large set of data and spoofs the source IP of the victim. The DNS server responds by sending a large amount of data back to the victim’s IP.
Read more here - http://blogs.cisco.com/security/chronology-of-a-ddos-spamhaus/