How does URPF guard against source address attack?
URPF has two modes:
(1) Strict: the inbound interface (activate URPF interface) of the packet must be consistent with forwarding interface of checked matching routing table with source address as destination address.
(2) Loose: find the route entries corresponded to the source address.
Configuration: in the interface view, ip urpf strict to enable URPF.
Note: URPF is applied to the upstream ingress of a router.
General route forwarding: when the router receive the packets, it gets the destination address, and then check the routing table(by longest-match); according to the results searched, it processes the packets (forward/discard)
Unicast Reverse Path Forwarding (URPF) functions to guard against network attacks based on source address. When the router enabling URPF receives packets, it gets the source address first and then takes it as the destination to search in the routing table and make sure if it contains the route entries related to the source address. If yes, the packets will be forwarded; if not, the packet will be regarded as illegal and discarded directly.