Opsec Working Group K. Sriram
Internet-Draft NIST
Intended status: Best Current Practice D. Montgomery
Expires: November 4, 2017 US NIST
May 3, 2017
Enhanced Feasible-Path Unicast Reverse Path Filtering
draft-sriram-opsec-urpf-improvements-01
Abstract
This document identifies a need for improvement of the unicast
Reverse Path Filtering techniques (uRPF) [BCP84] for source address
validation (SAV) [BCP38]. The strict uRPF is inflexible about
directionality, the loose uRPF is oblivious to directionality, and
the current feasible-path uRPF attempts to strike a balance between
the two [BCP84]. However, as shown in this draft, the existing
feasible-path uRPF still has short comings. This document proposes
an enhanced feasible-path uRPF technique, which aims to be more
flexible (in a meaningful way) about directionality than the
feasible-path uRPF. It is expected to alleviate ISPs' concerns about
the possibility of disrupting service for their customers, and
encourage greater deployment of uRPF.
Status of This Memo
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Review of Existing Source Address Validation Techniques . . . 3
2.1. SAV using Access Control List . . . . . . . . . . . . . . 3
2.2. SAV using Strict Unicast Reverse Path Filtering . . . . . 4
2.3. SAV using Feasible-Path Unicast Reverse Path Filtering . 5
2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 6
3. Proposed New Technique: SAV using Enhanced Feasible-Path uRPF 6
3.1. Description of the Method . . . . . . . . . . . . . . . . 6
3.2. Operational Recommendations . . . . . . . . . . . . . . . 8
3.3. Customer Cone Consideration . . . . . . . . . . . . . . . 9
3.4. Implementation Consideration . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. Informative References . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This internet draft identifies a need for improvement of the unicast
Reverse Path Filtering techniques (uRPF) [RFC2827] for source address
validation (SAV) [RFC3704]. The strict uRPF is inflexible about
directionality, the loose uRPF is oblivious to directionality, and
the current feasible-path uRPF attempts to strike a balance between
the two [RFC3704]. However, as shown in this draft, the existing
feasible-path uRPF still has short comings. Even with the feasible-
path uRPF, ISPs are often apprehensive that they may be denying
customers' data packets with legitimate source addresses. This
document proposes an enhanced feasible-path uRPF technique, which
aims to be more flexible (in a meaningful way) about directionality
than the feasible-path uRPF. It is based on the principle that if
BGP updates for multiple prefixes with the same origin AS were
received on different interfaces (at an edge router), then data
packets with source addresses in any of those prefixes are allowed to
be received on any of those interfaces. This technique is expected
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to add greater operational logic and efficacy to uRPF, and alleviate
ISPs' concerns about the possibility of disrupting service for their
customers. It should encourage greater deployment of uRPF to realize
its DDoS prevention benefits network wide.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Review of Existing Source Address Validation Techniques
There are various existing techniques for deterrence against DDoS
attacks with spoofed addresses [RFC2827] [RFC3704]. There are also
some techniques used for prevention of reflection-amplification
attacks [RRL] [TA14-017A], which are used in achieving greater impact
in DDoS attacks. Employing a combination of these preventive
techniques in enterprise and ISP border routers, DNS servers,
broadband and wireless access networks, and data centers provides the
necessary protections against DDoS attacks.
Source address validation (SAV) is performed in network edge devices
such as border routers, Cable Modem Termination Systems (CMTS),
Digital Subscriber Line Access Multiplexers (DSLAM), and Packet Data
Network (PDN) gateways in mobile networks. Ingress Access Control
List (ACL) and unicast Reverse Path Filtering (uRPF) are techniques
employed for implementing SAV [RFC2827] [RFC3704] [ISOC].
2.1. SAV using Access Control List
Ingress/egress Access Control Lists (ACLs) are maintained which list
acceptable (or alternatively, unacceptable) prefixes for the source
addresses in the incoming/outgoing Internet Protocol (IP) packets.
Any packet with a source address that does not match the filter is
dropped. The ACLs for the ingress/egress filters need to be
maintained to keep them up to date. Hence, this method may be
operationally difficult or infeasible in dynamic environments such as
when a customer network is multihomed, has address space allocations
from multiple ISPs, or dynamically varies its BGP announcements (i.e.
routing) for traffic engineering purposes.
Typically, the egress ACLs in access aggregation devices (e.g. CMTS,
DSLAM) permit source addresses only from the address spaces
(prefixes) that are associated with the interface on which the
customer network is connected. Ingress ACLs are typically deployed
on border routers, and drop ingress packets when the source address
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is spoofed (i.e. belongs to obviously disallowed prefix blocks, RFC
1918 prefixes, or provider's own prefixes).
2.2. SAV using Strict Unicast Reverse Path Filtering
In the strict unicast Reverse Path Filtering (uRPF) method, an
ingress packet on an interface at the border router is accepted only
if the Forwarding Information Base (FIB) contains a prefix that
encompasses the source address and packet forwarding for that prefix
points to said interface. In other words, the best path for routing
to that source address (if it were used as a destination address)
should point to said interface. It is well known that this method
has limitations when a network or autonomous system is multi-homed
and there is asymmetric routing of packets. Asymmetric routing
occurs (see Figure 1) when a customer AS announces one prefix (P1) to
one transit provider (ISP-a) and a different prefix (P2) to another
transit provider (ISP-b), but routes data packets with source
addresses in the second prefix (P2) to the first transit provider
(ISP-a) or vice versa.
+------------+ ---- P1[AS2 AS1] ---> +------------+
| AS2(ISP-a) | <----P2[AS3 AS1] ---- | AS3(ISP-b)|
+------------+ +------------+
/\ /\
\ /
\ /
\ /
P1[AS1]\ /P2[AS1]
\ /
+-----------------------+
| AS1(customer) |
+-----------------------+
P1, P2 (prefixes originated)
Consider data packets received at AS2
(1) from AS1 with source address in P2, or
(2) from AS3 that originated from AS1
with source address in P1:
* Strict uRPF fails
* Feasible-path uRPF fails
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 1: Scenario 1 for illustration of efficacy of uRPF schemes.
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2.3. SAV using Feasible-Path Unicast Reverse Path Filtering
The feasible-path uRPF helps partially overcome the problem
identified with the strict uRPF in the multi-homing case. The
feasible-path uRPF is similar to the strict uRPF, but the difference
is that instead of inserting one best route in the FIB (or an
equivalent RPF table), alternative routes are also added there. This
method relies on announcements for the same prefixes (albeit some may
be prepended to effect lower preference) propagating to all the
routers performing feasible-path uRPF check. So in the multi-homing
scenario, if the customer AS announces routes for both prefixes (P1,
P2) to both transit providers (with suitable prepends if needed for
traffic engineering), then the feasible-path uRPF method works (see
Figure 2). It should be mentioned that the feasible-path uRPF works
in this scenario only if customer route is preferred at AS2 and AS3
over the shorter path.
+------------+ routes for P1, P2 +-----------+
| AS2(ISP-a) |<-------------------->| AS3(ISP-b)|
+------------+ (p2p) +-----------+
/\ /\
\ /
P1[AS1]\ /P2[AS1]
\ /
P2[AS1 AS1 AS1]\ /P1[AS1 AS1 AS1]
\ /
+-----------------------+
| AS1(customer) |
+-----------------------+
P1, P2 (prefixes originated)
Consider data packets received at AS2 via AS3
that originated from AS1 and have source address in P1:
* Feasible-path uRPF works (if customer route preferred
at AS3 over shorter path)
* Feasible-path uRPF fails (if shorter path preferred
at AS3 over customer route)
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 2: Scenario 2 for illustration of efficacy of uRPF schemes.
However, the feasible-path uRPF method has limitations as well. One
form of limitation naturally occurs when the recommendation of
propagating the same prefixes to all routers is not heeded. Another
form of limitation can be described as follows. In Scenario 2
(described above, illustrated in Figure 2), it is possible that the
second transit provider (ISP-b or AS3) does not propagate the
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prepended route for prefix P1 to the first transit provider (ISP-a or
AS2). This is because AS3's decision policy permits giving priority
to a shorter route to prefix P1 via a peer (AS2) over a longer route
learned directly from the customer (AS1). In such a scenario, AS3
would not send any route announcement for prefix P1 to AS2. Then a
data packet with source address in prefix P1 that originates from AS1
and traverses via AS3 to AS2 will get dropped at AS2.
2.4. SAV using Loose Unicast Reverse Path Filtering
In the loose unicast Reverse Path Filtering (uRPF) method, an ingress
packet at the border router is accepted only if the FIB has one or
more prefixes that encompass the source address. That is, a packet
is dropped if no route exists in the FIB for the source address.
Loose uRPF sacrifices directionality. In most cases, this method is
not useful for prevention of address spoofing. It only drops packets
if the spoofed address is non-routable (e.g. RFC 1918, unallocated,
allocated but currently not routed).
3. Proposed New Technique: SAV using Enhanced Feasible-Path uRPF
3.1. Description of the Method
Enhanced feasible-path uRPF adds greater operational logic and
efficacy to existing uRPF methods discussed in Section 2. It can be
best explained with an example. Let us say, a border router of ISP-A
has in its Adj-RIB-in the set of prefixes {Q1, Q2, Q3} each of which
has AS-x as its origin and AS-x belongs in ISP-A's customer cone.
Further, the border router received a route for prefix Q1 over a
customer facing interface, while it learned routes for prefixes Q2
and Q3 from a lateral peer and an upstream transit provider,
respectively. All these prefixes passed route filtering and/or
origin validation (i.e. the origin AS-x is deemed legitimate). In
this example scenario, the enhanced feasible-path uRPF method allows
source addresses to belong in {Q1, Q2, Q3} on any of the three
specific interfaces in question (customer, peer, provider) on which
the three routes were learned.
Thus, enhanced feasible-path uRPF defines feasible paths in a more
generalized but precise way (as compared to feasible-path uRPF). In
the above example, routes for prefixes Q2 and Q3 were not received on
a customer facing interface at the border router, yet data packets
with source addresses in Q2 or Q3 are accepted by the router if they
come in on the same customer interface on which the route for prefix
Q1 was received (based on these prefix routes having the same origin
AS).
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Looking back at Scenarios 1 and 2 (Figure 1 and Figure 2), the
enhanced feasible-path uRPF provides comparable or better performance
than the other uRPF methods for those scenarios. Scenario 3
(Figure 3) further illustrates the enhanced feasible-path uRPF method
with a more concrete example. In this scenario, the focus is on
operation of the feasible-path uRPF at ISP4 (AS4). ISP4 learns a
route for prefix P1 via a customer-to-provider (C2P) interface from
customer ISP2 (AS2). This route for P1 has origin AS1. ISP4 also
learns a route for P2 via another C2P interface from customer ISP3
(AS3). Additionally, AS4 learns an alternate route for P2 via a
peer-to-peer (p2p) interface from ISP5 (AS5). Both routes for P2
have the same origin AS (i.e. AS1) as does the route for P1.
Applying the principle of enhanced feasible-path uRPF, given the
commonality of the origin AS across the above-mentioned routes for P1
and P2, AS4 permits the SA in data packets to belong in P1 or P2 on
any of the three interfaces (from AS2, AS3, and AS5).
+----------+ P2[AS5 AS1] +------------+
| AS4(ISP4)|<---------------| AS5(ISP5) |
+----------+ (p2p) +------------+
/\ /\ /\
/ \ /
P1[AS2 AS1]/ \P2[AS3 AS1] /
(C2P)/ \(C2P) /
/ \ /
+----------+ +----------+ /
| AS2(ISP2)| | AS3(ISP3)| /
+----------+ +----------+ /
/\ /\ /
\ / /
P1[AS1]\ /P2[AS1] /P2[AS1]
(C2P)\ /(C2P) /(C2P)
\ / /
+----------------+ /
| AS1(customer) |/
+----------------+
P1, P2 (prefixes originated)
Consider that data packets (sourced from AS1)
may be received at AS4 with source address
in P1 or P2 via any of the neighbors (AS2, AS3, AS5):
* Feasible-path uRPF fails
* Loose uRPF works (but not desirable)
* Enhanced Feasible-path uRPF works best
Figure 3: Scenario 3 for illustration of efficacy of uRPF schemes.
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Based on the above, it can be possibly rationalized that the proposed
enhanced feasible-path uRPF method would help alleviate ISP concerns
about possible service disruption for their customers and encourage
greater adoption of uRPF.
3.2. Operational Recommendations
The following operational recommendations if followed will make
robust the desired operation of the enhanced feasible-path uRPF
proposed here.
For multi-homed stub AS:
o A multi-homed stub AS SHOULD announce at least one of its
origination prefixes to each transit provider AS.
For non-stub AS:
o A non-stub AS SHOULD announce at least one of its origination
prefixes to each transit provider AS.
o Additionally, from the routes it has learned from customers, a
non-stub AS SHOULD announce at least one route for each unique
{prefix, origin AS} pair to each transit provider AS.
(Note: It is worth noting that in the above recommendations if "at
least one" is replaced with "all", then even traditional feasible-
path uRPF will work as desired.)
Also, it should be observed that in the absence of ASes adhering the
above recommendations, the following type of example scenarios may be
constructed which pose a challenge for the enhanced feasible-path
uRPF (as well as for traditional feasible-path uRPF). In the
scenario illustrated in Figure 4, since routes for neither P1 nor P2
are propagated on the AS2-AS4 interface, the enhanced feasible-path
uRPF at AS4 will reject data packets received on that interface with
source addresses in P1 or P2. But this can be clearly avoided if the
above recommendations for stub and non-stub ASes are followed. In
this example, this would mean that the NO_EXPORT is avoided and
instead AS prepending is used (to depref routes) on the AS1-AS2
peering session.
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+----------+
| AS4(ISP4)|
+----------+
/\ /\
/ \ P1[AS3 AS1]
P1 and P2 not / \ P2[AS3 AS1]
propagated / \ (C2P)
(C2P) / \
+----------+ +----------+
| AS2(ISP2)| | AS3(ISP3)|
+----------+ +----------+
/\ /\
\ / P1[AS1]
P1[AS1] NO_EXPORT \ / P2[AS1]
P2[AS1] NO_EXPORT \ / (C2P)
(C2P) \ /
+----------------+
| AS1(customer) |
+----------------+
P1, P2 (prefixes originated)
Figure 4: Illustration of a challenging scenario.
3.3. Customer Cone Consideration
An additional degree of flexibility that can be incorporated in the
enhanced feasible-path uRPF can be described as follows. Let I =
{I1, I2, ..., In} represent the set of all directly-connected
customer interfaces at customer-facing edge routers in a transit
provider's AS. Let P = {P1, P2, ..., Pm} represent the set of all
prefixes for which routes have been received over the interfaces in
set I. Then, over all interfaces in the set I, the edge router
SHOULD permit ingress data packets with SA in any of the prefixes in
the set P.
3.4. Implementation Consideration
The existing RPF checks in edge routers take advantage of existing
line card implementations to perform the RPF functions. For
implementation of the proposed technique, the general necessary
feature would be to extend the line cards to take arbitrary RPF lists
that are not necessarily tied to the existing FIB contents. For
example, in the proposed method, the RPF lists are constructed by
applying a set of rules to all received BGP routes (not just those
selected as best path and installed in FIB).
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4. Security Considerations
This document offers a technique to improve the security features of
uRPF. The proposed technique does not warrant any additional
security considerations.
5. IANA Considerations
This document does not request new capabilities or attributes. It
does not create any new IANA registries.
6. Acknowledgements
The authors would like to thank Jeff Haas, Job Snijders, Marco
Marzetti, Marco d'Itri, Nick Hilliard, Gert Doering, Barry Greene,
and Joel Jaeggli for comments and suggestions.
7. Informative References
[ISOC] Vixie (Ed.), P., "Addressing the challenge of IP
spoofing", ISOC report , September 2015, <https://www.us-
cert.gov/ncas/alerts/TA14-017A>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <http://www.rfc-editor.org/info/rfc3704>.
[RRL] "Response Rate Limiting in the Domain Name System",
Redbarn blog , <http://www.redbarn.org/dns/ratelimits>.
[TA14-017A]
"UDP-Based Amplification Attacks", US-CERT alert
TA14-017A , January 2014, <https://www.us-
cert.gov/ncas/alerts/TA14-017A>.
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Authors' Addresses
Kotikalapudi Sriram
NIST
100 Bureau Drive
Gaithersburg MD 20899
USA
Email: ksriram@nist.gov
Doug Montgomery
US NIST
100 Bureau Drive
Gaithersburg MD 20899
USA
Email: dougm@nist.gov
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