FreeBSD has certain variables in the kernel that can be modified by the super-user with sysctl. The net.inet.tcp.blackhole and net.inet.udp.blackhole values determine whether the machine should send RSTs to SYNs sent to closed ports (TCP) and whether it should send ICMP error messages to UDP packets sent to closed ports (UDP). Changing this parameters allows a system administrator to make the port scanner's life (or actually the attacker who runs it) a lot more difficult since sending back RST and SYN packets governs how port scanners know whether a port is open or closed.
Credit:
The information has been provided by Wizdumb.
First, these changes will modify your OS fingerprint, and NMap will not recognize your OS. Only SYN scans work (and they work really slowly) - UDP scans will get slower and slower, FIN, NUL, and XMAS scans will report all TCP ports as filtered or closed, and OSScans whines for ages, and then prints each line of the OS fingerprint 2 or 3 times, or only print a fragment of the print.
