URPF (Unicast Reverse Path Forwarding) is a feature used to combat IP spoofing in network and security devices. URPF checks whether the packet comes from the appropriate interface by comparing the source IP address of the packet with the routing table. URPF is valid for systems protected by a security/network device, not for packets coming from the Internet.When performing DDoS tests, a network/security device at the exit of the tested network may usually block the generated fake IP packets due to URPF (or a similar feature).