UDP Scan is a scanning used to detect which services are active on UDP.
Typically, the victim, if the door is open, will not send any response. If it is closed, it will send an ICMP type 3 code 3 (port unreachable) or type 3 code 13 (administratively prohibited). This package is used to actively reject a connection and is sent only if there are no personal firewalls on the host that block it and reach the scanner author only if there are no network firewalls that block the passage. For these reasons, UDP scanning is not a reliable technique, as if the attacker does not receive an answer, he can not be sure the door is open. The only certainty he has is that if he receives a reject ICMP then the door is closed.
Another disadvantage of this technique is that, to determine the status of a port, you have to wait for a timeout. So assuming you set a 1-second timeout, you need 65535 seconds (18.2 hours) to scan all ports on a host (65535).
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