This indicates an attempt to exploit a Denial of Service vulnerability in Multiple DNS Server applications.
The vulnerability is due to a validation error when the vulnerable software handles a maliciously crafted DNS query. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
PowerDNS PowerDNS Recursor 3.6.1 and eralier
Unbound 1.5.0 and eralier
BIND 9.0.x through 9.8.x
BIND 9.9.0 through 9.9.6
BIND 9.10.0 through 9.10.1
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendors