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 malicious requests. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
ISC DHCP Server 4.1.x before 4.1-ESV-R13
ISC DHCP Server 4.2.x
ISC DHCP Server 4.3.x before 4.3.4
Denial of Service: Remote attackers can crash vulnerable systems or services
Apply the most recent upgrade or patch from the vendor.