uRPF extends this idea by utilizing the knowledge all routers must have in their routing information base (RIB) or forwarding information base (FIB) to do their primary job, to help further restrict the possible source addresses that should be seen on an interface. Packets are only forwarded if they come from a router's best route to the source of a packet. Packets coming into an interface come from valid subnetworks, as indicated by the corresponding entry in the routing table are forwarded. Packets with source addresses that could "not" be reached via the input interface can be dropped without disruption to normal use, as they are probably from a misconfigured or malicious source.