An unhandled packet is a packet that does not match any policy rule. By default, the XTM device always denies unhandled packets. From
Auto-block source of packets not handled
Select to automatically block the source of unhandled packets. The XTM device adds the IP address that sent the packet to the temporary Blocked Sites list.
The "Auto-block source of packets not handled" option does not apply to broadcast traffic that is dropped as unhandled.
Send an error message to clients whose connections are disabled
Select to send a TCP reset or ICMP error back to the client when the XTM device receives an unhandled packet.
About Blocked Sites
About Default Packet Handling Options