originating from any other prefix can be summarily discarded instead
of sending it to an ISP.
5. Security Considerations
Ingress filtering is typically performed to ensure that traffic
arriving on one network interface legitimately comes from a computer
residing on a network reachable through that interface.
The closer to the actual source ingress filtering is performed, the
more effective it is. One could wish that the first hop router would
ensure that traffic being sourced from its neighboring end system was
correctly addressed; a router further away can only ensure that it is
possible that there is such a system within the indicated prefix.
Therefore, ingress filtering should be done at multiple levels, with
different level of granularity.
It bears to keep in mind that while one goal of ingress filtering is
to make attacks traceable, it is impossible to know whether the
particular attacker "somewhere in the Internet" is being ingress
filtered or not. Therefore, one can only guess whether the source
addresses have been spoofed or not: in any case, getting a possible
lead -- e.g., to contact a potential source to ask whether they’re
observing an attack or not -- is still valuable, and more so when the
ingress filtering gets more and more widely deployed.
In consequence, every administrative domain should try to ensure a
sufficient level of ingress filtering on its borders.
Security properties and applicability of different ingress filtering
types differ a lot.
o Ingress Access Lists require typically manual maintenance, but are
the most bulletproof when done properly; typically, ingress access
lists are best fit between the edge and the ISP when the
configuration is not too dynamic if strict RPF is not an option,
between ISPs if the number of used prefixes is low, or as an
additional layer of protection.
o Strict RPF check is a very easy and sure way to implement ingress
filtering. It is typically fit between the edge network and the
ISP. In many cases, a simple strict RPF can be augmented by
operational procedures in the case of asymmetric traffic patterns,
or the feasible RPF technique to also account for other
alternative paths.
o Feasible Path RPF check is an extension of Strict RPF. It is
suitable in all the scenarios where Strict RPF is, but multihomed
or asymmetric scenarios in particular. However, one must remember
that Feasible RPF assumes the consistent origination and
propagation of routing information to work; the implications of
this must be understood especially if a prefix advertisement
passes through third parties.
o Loose RPF primarily filters out unrouted prefixes such as Martian
addresses. It can be applied in the upstream interfaces to reduce
the size of DoS attacks with unrouted source addresses. In the
downstream interfaces it can only be used as a contract
verification, that the other network has performed at least some
ingress filtering.
When weighing the tradeoffs of different ingress filtering
mechanisms, the security properties of a more relaxed approach should
be carefully considered before applying it. Especially when applied
by an ISP towards an edge network, there don’t seem to be many
reasons why a stricter form of ingress filtering would not be
appropriate.
6. Conclusions and Future Work
This memo describes ingress filtering techniques in general and the
options for multihomed networks in particular.
It is important for ISPs to implement ingress filtering to prevent
spoofed addresses being used, both to curtail DoS attacks and to make
them more traceable, and to protect their own infrastructure. This
memo describes mechanisms that could be used to achieve that effect,
and the tradeoffs of those mechanisms.
To summarize:
o Ingress filtering should always be done between the ISP and a
single-homed edge network.
o Ingress filtering with Feasible RPF or similar Strict RPF
techniques could almost always be applied between the ISP and
multi-homed edge networks as well.
o Both the ISPs and edge networks should verify that their own
addresses are not being used in source addresses in the packets
coming from outside their network.
o Some form of ingress filtering is also reasonable between ISPs,
especially if the number of prefixes is low.
This memo will lower the bar for the adoption of ingress filtering
especially in the scenarios like asymmetric/multihomed networks where
the general belief has been that ingress filtering is difficult to
implement.
One can identify multiple areas where additional work would be
useful:
o Specify the mechanisms in more detail: there is some variance
between implementations e.g., on whether traffic to multicast
destination addresses will always pass the Strict RPF filter or
not. By formally specifying the mechanisms the implementations
might get harmonized.
o Study and specify Routing Information Base (RIB) -based RPF
mechanisms, e.g., Feasible Path RPF, in more detail. In
particular, consider under which assumptions these mechanisms work
as intended and where they don’t.
o Write a more generic note on the ingress filtering mechanisms than
this memo, after the taxonomy and the details or the mechanisms
(points above) have been fleshed out.
o Consider the more complex case where a network has connectivity
with different properties (e.g., peers and upstreams), and wants
to ensure that traffic sourced with a peer’s address should not be
accepted from the upstream.
7. Acknowledgements
Rob Austein, Barry Greene, Christoph Reichert, Daniel Senie, Pedro
Roque, and Iljitsch van Beijnum reviewed this document and helped in
improving it. Thomas Narten, Ted Hardie, and Russ Housley provided
good feedback which boosted the document in its final stages.
8. References
8.1. Normative References
[1] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
8.2. Informative References
[2] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.
[3] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
[4] Bates, T. and Y. Rekhter, "Scalable Support for Multi-homed
Multi-provider Connectivity", RFC 2260, January 1998.
[5] Hagino, J. and H. Snyder, "IPv6 Multihoming Support at Site Exit
Routers", RFC 3178, October 2001.
9. Authors’ Addresses
Fred Baker
Cisco Systems
Santa Barbara, CA 93117
US
EMail: fred@cisco.com
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
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