Opsec Working Group K. Sriram Internet-Draft NIST Intended status: Best Current Practice D. Montgomery Expires: May 4, 2017 US NIST October 31, 2016 Enhanced Feasible-Path Unicast Reverse Path Filtering draft-sriram-opsec-urpf-improvements-00 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 4, 2017. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. Sriram & Montgomery Expires May 4, 2017 [Page 1] Internet-Draft Enhanced FP-uRPF October 2016 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 . 4 2.4. SAV using Loose Unicast Reverse Path Filtering . . . . . 6 3. Proposed New Technique: SAV using Enhanced Feasible-Path uRPF 6 3.1. Customer Cone Consideration . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Informative References . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 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, then data packets with source addresses in any of those prefixes may be received on any of those interfaces. This flexibility is expected to add greater intelligence and accuracy to uRPF operation, 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. Sriram & Montgomery Expires May 4, 2017 [Page 2] Internet-Draft Enhanced FP-uRPF October 2016 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 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 is spoofed (i.e. belongs to obviously disallowed prefix blocks, RFC 1918 prefixes, or provider's own prefixes). Sriram & Montgomery Expires May 4, 2017 [Page 3] Internet-Draft Enhanced FP-uRPF October 2016 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 packet received at AS2 via AS2 or 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. 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 Sriram & Montgomery Expires May 4, 2017 [Page 4] Internet-Draft Enhanced FP-uRPF October 2016 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 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 packet received at AS2 via AS3 that originated from AS1 with source address in P1: * Feasible-path uRPF works exi(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) does not propagate the prepended route for the first prefix (P1) to the first transit provider (ISP1). This is because the second transit provider's (ISP-b's) decision policy permits giving priority to a shorter route to the first prefix (P1) via the first provider (ISP-a) over a longer route learned directly from the customer AS (AS1). In such a scenario, the second transit provider (ISP-b) would not send any route announcement for the first prefix (P1) to first transit provider (ISP-a). Then a data packet with source address in the first prefix (P1) that traverses Sriram & Montgomery Expires May 4, 2017 [Page 5] Internet-Draft Enhanced FP-uRPF October 2016 via the second transit provider (ISP-b) will get dropped at the first transit provider (ISP-a). 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 Enhanced feasible-path uRPF adds greater flexibility and accuracy to uRPF operation than the 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). Scenario 3 (Figure 3) further illustrates the enhanced feasible-path uRPF method with a more concrete example. In this example, we focus 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 Sriram & Montgomery Expires May 4, 2017 [Page 6] Internet-Draft Enhanced FP-uRPF October 2016 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 may be received at AS4 with source address in P1 or P2 from 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. The proposed enhanced feasible-path uRPF method works best (when compared to existing uRPF method) in several realistic scenarios (Scenarios 1, 2, and 3 in Figures 1, 2, and 3, respectively). This should help alleviate ISP concerns about possible service disruption for their customers and encourage greater adoption of uRPF. 3.1. 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 interfaces directly connecting to the top layer of ASes in a customer cone. Let P = {P1, Sriram & Montgomery Expires May 4, 2017 [Page 7] Internet-Draft Enhanced FP-uRPF October 2016 P2, ..., Pm} represent the set of all prefixes for which routes have been received over the interfaces in I. Then, over all interfaces in I, permit data packets with SA in any of the prefixes in P. It should be emphasized that, in spite of the flexibilities incorporated into uRPF, a multi-homed customer should be always advised to advertise their routes to each of its upstream ISPs. When a customer AS is known to be single-homed stub, then strict uRPF should be used and would serve well. 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. Informative References [ISOC] Vixie (Ed.), P., "Addressing the challenge of IP spoofing", ISOC report , September 2015, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March 2004, . [RRL] "Response Rate Limiting in the Domain Name System", Redbarn blog , . [TA14-017A] "UDP-Based Amplification Attacks", US-CERT alert TA14-017A , January 2014, . Sriram & Montgomery Expires May 4, 2017 [Page 8] Internet-Draft Enhanced FP-uRPF October 2016 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 Sriram & Montgomery Expires May 4, 2017 [Page 9]