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Networks receive packets from other networks. Normally a packet will contain the IP address of the computer that originally sent it. This allows devices in the receiving network to know where it came from, allowing a reply to be routed back (amongst other things), except when IP addresses are used through a proxy or a spoofed IP address, which does not pinpoint a specific user within that pool of users.
A sender IP address can be faked ('spoofed'), characterising a spoofing attack. This disguises the origin of packets sent, for example in a denial-of-service attack. The same holds true for proxies, although in a different manner than "IP Spoofing."
One potential solution involves implementing the use of intermediate internet gateways (i.e., those servers connecting disparate networks along the path followed by any given packet) filtering or denying any packet deemed to be illegitimate. The gateway processing the packet might simply ignore the packet completely, or where possible, it might send a packet back to the sender relaying a message that the illegitimate packet has been denied. HIPS products are one example of technical engineering applications that help to identify, prevent and/or deter unwanted, unsuspected and/or suspicious events and intrusions.
Any router that implements ingress filtering checks the source IP field of IP packets it receives, and drops packets if the packets don't have an IP address in the IP address block, to which the interface is connected. This may not be possible if the end host is multi-homed (terminology clarification needed) and also sends transit network traffic.
In ingress filtering, packets coming into the network are filtered if the network sending it should not send packets from the originating IP address(es). If the end host is a stub network or host, the router needs to filter all IP packets that have, as the source IP, private addresses (RFC 1918), bogon addresses or addresses that do not have the same network address as the interface.
Network ingress filtering is a packet filtering technique used by many Internet service providers to try to prevent source address spoofing of Internet traffic, and thus indirectly combat various types of net abuse by making Internet traffic traceable to its source.
Network ingress filtering is a "good neighbor" policy which relies on cooperation between ISPs for their mutual benefit.
BCP 84 recommends that upstream providers of IP connectivity filter packets entering their networks from downstream customers, and discard any packets which have a source address which is not allocated to that customer.
There are many possible ways of implementing this policy; one common mechanism is to enable reverse path forwarding on links to customers, which will indirectly apply this policy based on the provider's route filtering of their customers' route announcements.
As of 2012, one report suggests that, contrary to general opinion about the lack of BCP38 deployment, some 80% of the Internet (by various measures) were already applying anti-spoofing packet filtering in their networks.
- Robert Gezelter (1995) Security on the Internet Chapter 23 in Hutt, Bosworth, and Hoytt (1995) "Computer Security Handbook, Third Edition", Wiley, section 23.6(b), pp 23-12, et seq.
- Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing. IETF. May 2000. BCP 38. RFC 2827. https://tools.ietf.org/html/rfc2827. Retrieved 18 Feb 2014.
- Ingress Filtering for Multihomed Networks. IETF. March 2004. BCP 84. RFC 3704. https://tools.ietf.org/html/rfc3704. Retrieved 18 Feb 2014.
- Barry Greene (June 11, 2012). "Everyone should be deploying BCP 38! Wait, they are …". senki.org.