From
blackhole(4):
Quote:
In the UDP instance, enabling blackhole behaviour turns off the sending
of an ICMP port unreachable message in response to a UDP datagram which
arrives on a port where there is no socket listening. It must be noted
that this behaviour will prevent remote systems from running
traceroute(8) to a system.
The blackhole behaviour is useful to slow down anyone who is port scan-
ning a system, attempting to detect vulnerable services on a system. It
could potentially also slow down someone who is attempting a denial of
service attack.
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Might be worth exploring in your case.