This OS fingerprinting probe tests to determine if the remote host echoes
						back the IP 'DF' (Don't Fragment) bit in a response packet. An attacker
						sends a UDP datagram with the DF bit set to a closed port on the remote host
						to observe whether the 'DF' bit is set in the response packet. Some
						operating systems will echo the bit in the ICMP error message while others
						will zero out the bit in the response packet.