You may want to not send back the "unreachable port" ICMP messages that are the default behavior for ports on which there is nothing listening. The traditional

You may want to not send back the "unreachable port" ICMP messages that are the default behavior for ports on which there is nothing listening. The traditional configuration for a server or router is to block in the firewall all the unused ports, which means that ICMP responses will not be sent back, with the exception of the range of ports that are normally used by the "traceroute" utility. Unfortunately, there are also stupid people who block in the firewall even the traceroute ports, which makes more difficult the diagnosing of any network problem.