The default configuration of ISC BIND before 9.4.1-P1, when configured as a caching name server, allows recursive queries and provides additional delegation information to arbitrary IP addresses, which allows remote attackers to cause a denial of service (traffic amplification) via DNS queries with spoofed source IP addresses.
http://www.us-cert.gov/reading_room/DNS-recursion121605.pdf
http://www.securityfocus.com/archive/1/426368/100/0/threaded