filter
Click on the red underlined text to get to the source
... create problems, e.g., with multihoming. This document describes
the current ingress filtering operational mechanisms, examines
generic issues related to ingress filtering and delves into the
...
... the current ingress filtering operational mechanisms, examines
generic issues related to ingress filtering and delves into the
effects on multihoming in particular.
...
... source
address not legitimately in use by the customer network. The
filtering includes but is in no way limited to the traffic whose
source address ...
... 240.0.0.0/4.
The reasoning behind the ingress filtering procedure is that
Distributed Denial of Service Attacks frequently spoof other systems'
...
... traffic;
in this case, the attacked sites can protect themselves by proper
filtering, by verifying that their prefixes are not used in the
source addresses ...
... edge network operators who 1) would
like to learn more of ingress filtering methods in general, or 2) are
already using ingress filtering ...
... ingress filtering methods in general, or 2) are
already using ingress filtering to some degree but who would like to
expand its use and want to avoid the pitfalls of ingress filtering in
...
... already using ingress filtering to some degree but who would like to
expand its use and want to avoid the pitfalls of ingress filtering in
the multihomed/asymmetric scenarios.
...
... the multihomed/asymmetric scenarios.
In section 2, several different ways to implement ingress filtering
are described and examined in the generic context. In section 3,
...
... are described and examined in the generic context. In section 3,
some clarifications on the applicability of ingress filtering methods
are made. In section 4, ingress filtering ...
... ingress filtering methods
are made. In section 4, ingress filtering is analyzed in detail from
the multihoming perspective. In section 5, conclusions and potential
...
... Different Ways to Implement Ingress Filtering ...
...
This section serves as an introduction to different operational
techniques used to implement ingress filtering as of writing this
memo. The mechanisms are described and analyzed in general terms,
and multihoming ...
...
An Ingress Access List is a filter that checks the source address of
every message received on a network interface ...
... acceptable prefixes, dropping any packet that does not match the
filter. While this is by no means the only way to implement an
ingress filter, it is the one proposed by RFC 2827 ...
... filter. While this is by no means the only way to implement an
ingress filter, it is the one proposed by RFC 2827 [1], and in some
...
... prefixes changes (e.g., as a result of multihoming) might lead to
discarding the packets if they do not pass the ingress filter.
Naturally, this problem is not limited to Ingress Access Lists -- it
...
...
Naturally, this problem is not limited to Ingress Access Lists -- it
is inherent to Ingress Filtering when the ingress filter is not
complete. However, usually Ingress Access Lists are more difficult
...
... Naturally, this problem is not limited to Ingress Access Lists -- it
is inherent to Ingress Filtering when the ingress filter is not
complete. However, usually Ingress Access Lists are more difficult
to maintain than the other mechanisms, and having an outdated list
...
... Reverse Path Forwarding (Strict RPF) is a simple way to
implement an ingress filter. It is conceptually identical to using
access lists for ingress filtering, with the exception that the
...
... implement an ingress filter. It is conceptually identical to using
access lists for ingress filtering, with the exception that the
access list is dynamic. This may also be used to avoid duplicate
configuration (e.g., maintaining both static routes or BGP ...
... BGP prefix-
list filters and interface access-lists). The procedure is that the
source address ...
... prefixes using BGP. It makes for a simple, cheap, fast, and dynamic
filter.
But Strict Reverse Path Forwarding ...
... prefixes are not being advertised or not being accepted by the ISP
under its policy, the effect is the same as ingress filtering using
an incomplete access list: some legitimate traffic is filtered for
...
... traffic is filtered for
lack of a route in the filtering router's Forwarding Information
Base.
...
... method assumes that there is no strict
RPF filtering between the primary and secondary edge routers; in
particular, when applied to multihoming ...
... traffic legitimately sourced from that network,
in most cases, rendering the mechanism useless as an ingress
filtering mechanism.
Also, many ISPs ...
... One case where Loose RPF might fit well could be an ISP filtering
packets from its upstream providers, to get rid of packets with
...
... verification: the other network is presumably certifying
that it has provided appropriate ingress filtering rules, so the
network doing the filtering ...
... ingress filtering rules, so the
network doing the filtering need only verify the fact and react if
any packets which would show a breach in the contract are detected.
Of course, this mechanism would only show if the source addresses ...
... Clarifying the Applicability of Ingress Filtering ...
...
What may not be readily apparent is that ingress filtering is not
applied only at the "last-mile" interface between the ISP ...
... interface between the ISP and the end
user. It's perfectly fine, and recommended, to also perform ingress
filtering at the edges of ISPs where appropriate, at the routers ...
... Ingress Filtering at Multiple Levels ...
...
Because of wider deployment of ingress filtering, the issue is
recursive. Ingress filtering has to work everywhere where it's used,
...
... deployment of ingress filtering, the issue is
recursive. Ingress filtering has to work everywhere where it's used,
not just between the first two parties. That is, if a user
negotiates a special ingress filtering ...
... Ingress filtering has to work everywhere where it's used,
not just between the first two parties. That is, if a user
negotiates a special ingress filtering arrangement with his ISP, he
should also ensure (or make sure the ISP ...
... upstream and peering links, if
ingress filtering is being used there -- or will get used, at some
point in the future; similarly with the upstream ISPs ...
... In consequence, manual models which do not automatically propagate
the information to every party where the packets would go and where
ingress filtering might be applied have only limited generic
usefulness.
...
... Ingress Filtering to Protect Your Own Infrastructure ...
...
Another feature stemming from wider deployment of ingress filtering
may not be readily apparent. The routers and other ISP ...
... systems.
However, unless ingress filtering (or at least, a limited subset of
it) has been deployed at every border (towards the customers, peers
...
... of the infrastructure gear.
Therefore, by deploying ingress filtering, one does not just help the
Internet as a whole, but protects against several classes ...
... Ingress Filtering on Peering Links ...
... downstream"
or "upstream" ingress filtering.
However, it's important to note that with mixed upstream ...
... links may have different properties
(e.g., relating to contracts, trust, viability of the ingress
filtering mechanisms, etc.). In the most typical case, just using an
ingress filtering mechanism towards a peer (e.g., Strict RPF ...
... trust, viability of the ingress
filtering mechanisms, etc.). In the most typical case, just using an
ingress filtering mechanism towards a peer (e.g., Strict RPF) works
just fine as long as the routing ...
... just fine as long as the routing between the peers is kept reasonably
symmetric. It might even be considered useful to be able to filter
out source addresses coming from an upstream ...
... Solutions to Ingress Filtering with Multihoming ...
...
One can imagine a number of approaches to working around the
limitations of ingress filters for multihomed networks. Options
include:
...
...
4. On some interfaces, weaken ingress filtering by using an
appropriate form of loose RPF check, as described in Section 4.1.
...
... 5. Ensure, by BGP or by contract, that each ISP's ingress filter is
complete, as described in Section 4.2.
...
... traffic to their ISPs that
will in fact pass the ingress filter, as described in Section
4.3.
...
...
Next, we now look at the viable ways for dealing with the side-
effects of ingress filters.
...
...
Where asymmetric routing is preferred or is unavoidable, ingress
filtering may be difficult to deploy using a mechanism such as strict
RPF which requires the paths to be symmetrical. In many cases, using
...
... operational methods or feasible RPF may ensure the ingress filter is
complete, like described below. Failing that, the only real options
are to not perform ingress filtering ...
... filter is
complete, like described below. Failing that, the only real options
are to not perform ingress filtering, use a manual access-list
(possibly in addition to some other mechanisms), or to using some
form of Loose RPF ...
... RPF check.
Failing to provide any ingress filter at all essentially trusts the
downstream network ...
... network and the ISP, since it loses the directionality of the
test. This argues in favor of either using a complete filter in the
upstream network ...
... There are a number of techniques which make it easier to ensure the
ISP's ingress filter is complete. Feasible RPF and Strict RPF with
...
... TACACS, or
Diameter, the ingress filtering can be the most easily ensured and
kept up-to-date with Strict RPF or Ingress Access Lists generated
...
... addressing and
whose ISPs implement ingress filters (which they should do), the
third option is to route traffic ...
...
Ingress filtering is typically performed to ensure that traffic
arriving on one network interface ...
... 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
...
... 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.
...
... 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
...
... 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 ...
... In consequence, every administrative domain should try to ensure a
sufficient level of ingress filtering on its borders.
Security properties ...
...
Security properties and applicability of different ingress filtering
types differ a lot.
...
...
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
...
... verification, that the other network has performed at least some
ingress filtering.
When weighing the tradeoffs of different ingress filtering ...
... ingress filtering.
When weighing the tradeoffs of different ingress filtering
mechanisms, the security properties of a more relaxed approach should
...
... edge network, there don't seem to be many
reasons why a stricter form of ingress filtering would not be
appropriate.
...
...
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 ...
... network.
o Some form of ingress filtering is also reasonable between ISPs,
especially if the number of prefixes ...
... prefixes is low.
This memo will lower the bar for the adoption of ingress filtering
especially in the scenarios like asymmetric/multihomed networks where
...
... especially in the scenarios like asymmetric/multihomed networks where
the general belief has been that ingress filtering is difficult to
implement.
...
... destination addresses will always pass the Strict RPF filter or
not. By formally specifying the mechanisms the implementations
might get harmonized.
...
... 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
...
... Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing ...