ingress filtering

A method [R2827] for countering attacks that use packets with false IP source addresses, by blocking such packets at the boundary between connected networks.

Tutorial: Suppose network A of an internet service provider (ISP) includes a filtering router that is connected to customer network B, and an attacker in B at IP source address "foo" attempts to send packets with false source address "bar" into A. The false address may be either fixed or randomly changing, and it may either be unreachable or be a forged address that legitimately exists within either B or some other network C. In ingress filtering, the ISP's router blocks all inbound packet that arrive from B with a source address that is not within the range of legitimately advertised addresses for B. This method does not prevent all attacks that can originate from B, but the actual source of such attacks can be more easily traced because the originating network is known.