This indicates an attack attempt to exploit a Denial of Service vulnerability in ISC DHCP.
The vulnerability is due to an error when vulnerable module handles a crafted DNS request. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
ISC DHCP Server 4.0.x
ISC DHCP Server 4.1-ESV through 4.1-ESV-R12
ISC DHCP Server 4.1.x
ISC DHCP Server 4.2.x
ISC DHCP Server 4.3.0 through 4.3.3
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.