This indicates an attack attempt against Denial of Service vulnerability exists in Nginx.
The vulnerability is due to an error when the vulnerable software handles maliciously crafted DNS requests. A remote attacker can exploit this to crash the target vulnerable system, via a crafted request.
nginx HTTP Server 0.6.18 through 1.9.9
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.