Network Working Group Y. Gilad Internet-Draft Hebrew University of Jerusalem Intended status: Best Current Practice S. Goldberg Expires: May 6, 2021 Boston University K. Sriram USA NIST J. Snijders NTT B. Maddison Workonline Communications November 2, 2020 The Use of Maxlength in the RPKI draft-ietf-sidrops-rpkimaxlen-05 Abstract This document recommends ways to reduce forged-origin hijack attack surface by prudently limiting the set of IP prefixes that are included in a Route Origin Authorization (ROA). One recommendation is to avoid using the maxLength attribute in ROAs except in some specific cases. The recommendations complement and extend those in RFC 7115. The document also discusses creation of ROAs for facilitating the use of Distributed Denial of Service (DDoS) mitigation services. Considerations related to ROAs and origin validation in the context of destination-based Remote Triggered Black Hole (RTBH) filtering are also highlighted. 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 https://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 6, 2021. Gilad, et al. Expires May 6, 2021 [Page 1] Internet-Draft RPKI maxLength November 2020 Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Documentation Prefixes . . . . . . . . . . . . . . . . . 4 2. Suggested Reading . . . . . . . . . . . . . . . . . . . . . . 4 3. Forged-Origin Subprefix Hijack . . . . . . . . . . . . . . . 4 4. Measurements of Today's RPKI . . . . . . . . . . . . . . . . 6 5. Recommendations about Minimal ROAs and Maxlength . . . . . . 7 5.1. Creation of ROAs Facilitating DDoS Mitigation Service . . 7 6. ROAs and Origin Validation for RTBH Filtering Scenario . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction The RPKI [RFC6480] uses Route Origin Authorizations (ROAs) to create a cryptographically verifiable mapping from an IP prefix to a set of autonomous systems (ASes) that are authorized to originate that prefix. Each ROA contains a set of IP prefixes and an AS number of an AS authorized to originate all the IP prefixes in the set [RFC6482]. The ROA is cryptographically signed by the party that holds a certificate for the set of IP prefixes. The ROA format also supports a maxLength attribute. According to [RFC6482], "When present, the maxLength specifies the maximum length of the IP address prefix that the AS is authorized to advertise." Thus, rather than requiring the ROA to list each prefix the AS is authorized to originate, the maxLength attribute provides a shorthand that authorizes an AS to originate a set of IP prefixes. Gilad, et al. Expires May 6, 2021 [Page 2] Internet-Draft RPKI maxLength November 2020 However, measurements of current RPKI deployments have found that use of the maxLength in ROAs tends to lead to security problems. Specifically, measurements have shown that 84% of the prefixes specified in ROAs that use the maxLength attribute, are vulnerable to a forged-origin subprefix hijack [HARMFUL]. The forged-origin prefix or subprefix hijack involves inserting the legitimate AS (as specified in the ROA) as the origin AS in the AS_PATH, and can be launched against any IP prefix/subprefix that has a ROA. Consider a prefix/subprefix that has a ROA but is unused, i.e., not announced in BGP by a legitimate AS. A forged-origin hijack involving such a prefix/subprefix can propagate widely throughout the Internet. On the other hand, if the prefix/subprefix were announced by the legitimate AS, then the propagation of the forged-origin hijack is somewhat limited because of its increased AS_PATH length relative to the legitimate announcement. Of course, forged-origin hijacks are harmful in both cases but the extent of harm is greater for unannounced prefixes/subprefixes. For this reason, this document recommends that, whenever possible, operators SHOULD use "minimal ROAs" that authorize only those IP prefixes that are actually originated in BGP, and no other prefixes. Further, it recommends ways to reduce forged-origin attack surface by prudently limiting the address space that is included in Route Origin Authorizations (ROAs). One recommendation is to avoid using the maxLength attribute in ROAs except in some specific cases. The recommendations complement and extend those in [RFC7115]. The document also discusses creation of ROAs for facilitating the use of Distributed Denial of Service (DDoS) mitigation services. Considerations related to ROAs and origin validation in the context of destination-based Remote Triggered Black Hole (RTBH) filtering are also highlighted. One ideal place to implement the ROA related recommendations is in the user interfaces for configuring ROAs. Thus, this document further recommends that designers and/or providers of such user interfaces SHOULD provide warnings to draw the user's attention to the risks of using the maxLength attribute. Best current practices described in this document require no changes to the RPKI specification and will not increase the number of signed ROAs in the RPKI, because ROAs already support lists of IP prefixes [RFC6482]. 1.1. Requirements 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 [RFC2119]. Gilad, et al. Expires May 6, 2021 [Page 3] Internet-Draft RPKI maxLength November 2020 1.2. Documentation Prefixes The documentation prefixes recommended in [RFC5737] are insufficient for use as example prefixes in this document. Therefore, this document uses [RFC1918] address space for constructing example prefixes. 2. Suggested Reading It is assumed that the reader understands BGP [RFC4271], RPKI [RFC6480], Route Origin Authorizations (ROAs) [RFC6482], RPKI-based Prefix Validation [RFC6811], and BGPsec [RFC8205]. 3. Forged-Origin Subprefix Hijack A detailed description and discussion of forged-origin subprefix hijacks are presented here, especially considering the case when the subprefix is not announced in BGP. The forged-origin subprefix hijack is relevant to a scenario in which: (1) the RPKI [RFC6480] is deployed, and (2) routers use RPKI origin validation to drop invalid routes [RFC6811], but (3) BGPsec [RFC8205] (or any similar method to validate the truthfulness of the BGP AS_PATH attribute) is not deployed. Note that this set of assumptions accurately describes a substantial, and growing, number of large Internet networks at the time writing. The forged-origin subprefix hijack [RFC7115] [GCHSS] is described here using a running example. Consider the IP prefix 192.168.0.0/16 which is allocated to an organization that also operates AS 64496. In BGP, AS 64496 originates the IP prefix 192.168.0.0/16 as well as its subprefix 192.168.225.0/24. Therefore, the RPKI should contain a ROA authorizing AS 64496 to originate these two IP prefixes. Suppose, however, the organization issues and publishes a ROA including a maxLength value of 24: ROA:(192.168.0.0/16-24, AS 64496) We refer to the above as a "loose ROA" since it authorizes AS 64496 to originate any subprefix of 192.168.0.0/16 up to and including Gilad, et al. Expires May 6, 2021 [Page 4] Internet-Draft RPKI maxLength November 2020 length /24, rather than only those prefixes that are intended to be announced in BGP. Because AS 64496 only originates two prefixes in BGP: 192.168.0.0/16 and 192.168.255.0/24, all other prefixes authorized by the "loose ROA" (for instance, 192.168.0.0/24), are vulnerable to the following forged-origin subprefix hijack [RFC7115] [GCHSS]: The hijacker AS 64511 sends a BGP announcement "192.168.0.0/24: AS 64511, AS 64496", falsely claiming that AS 64511 is a neighbor of AS 64496 and falsely claiming that AS 64496 originates the IP prefix 192.168.0.0/24. In fact, the IP prefix 192.168.0.0/24 is not originated by AS 64496. The hijacker's BGP announcement is valid according to the RPKI, since the ROA (192.168.0.0/16-24, AS 64496) authorizes AS 64496 to originate BGP routes for 192.168.0.0/24. Because AS 64496 does not actually originate a route for 192.168.0.0/24, the hijacker's route is the *only* route to the 192.168.0.0/24. Longest-prefix-match routing ensures that the hijacker's route to the subprefix 192.168.0.0/24 is always preferred over the legitimate route to 192.168.0.0/16 originated by AS 64496. Thus, the hijacker's route propagates through the Internet and the traffic destined for IP addresses in 192.168.0.0/24 will be delivered to the hijacker. The forged-origin *subprefix* hijack would have failed if a "minimal ROA" described below was used instead of the "loose ROA". In this example, a "minimal ROA" would be: ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496) This ROA is "minimal" because it includes only those IP prefixes that AS 64496 originates in BGP, but no other IP prefixes [RFC6907]. The "minimal ROA" renders AS 64511's BGP announcement invalid, because: (1) this ROA "covers" the attacker's announcement (since 192.168.0.0/24 is a subprefix of 192.168.0.0/16), and (2) there is no ROA "matching" the attacker's announcement (i.e., there is no ROA for AS 64511 and IP prefix 192.168.0.0/24) [RFC6811]. Gilad, et al. Expires May 6, 2021 [Page 5] Internet-Draft RPKI maxLength November 2020 If routers ignore invalid BGP announcements, the minimal ROA above ensures that the subprefix hijack will fail. Thus, if a "minimal ROA" had been used, the attacker would be forced to launch a forged-origin *prefix* hijack in order to attract traffic, as follows: The hijacker AS 64511 sends a BGP announcement "192.168.0.0/16: AS 64511, AS 64496", falsely claiming that AS 64511 is a neighbor of AS 64496. This forged-origin *prefix* hijack is significantly less damaging than the forged-origin *subprefix* hijack: AS 64496 legitimately originates 192.168.0.0/16 in BGP, so the hijacker AS 64511 is not presenting the *only* route to 192.168.0.0/16. Moreover, the path originated by AS 64511 is one hop longer than the path originated by the legitimate origin AS 64496. This means that the hijacker will attract less traffic than he would have in the forged-origin *subprefix* hijack, where the hijacker presents the *only* route to the hijacked subprefix [LSG16]. In summary, a forged-origin subprefix hijack has the same impact as a regular subprefix hijack, despite the increased AS_PATH length of the illegitimate route. A forged-origin *subprefix* hijack is also more damaging than forged-origin *prefix* hijack. 4. Measurements of Today's RPKI Network measurements have shown that 12% of the IP prefixes authorized in ROAs have a maxLength longer than their prefix length. The vast majority of these (84%) are vulnerable to forged-origin subprefix hijacks. These subprefixes are not announced in BGP by the legitimate AS. Even large providers are vulnerable to these attacks. See [GSG17] for details. These measurements suggest that operators commonly misconfigure the maxLength attribute, and unwittingly open themselves up to forged- origin subprefix hijacks. That is, they are exposing a much larger attack surface for forged-origin hijacks than necessary. Gilad, et al. Expires May 6, 2021 [Page 6] Internet-Draft RPKI maxLength November 2020 5. Recommendations about Minimal ROAs and Maxlength Operators SHOULD avoid using the maxLength attribute in their ROAs except in some special cases. One such exception may be when all more specific prefixes permitted by the maxLength are actually announced by the AS in the ROA. Another exception for use of maxLength is when (a) the maxLength is substantially larger compared to the specified prefix length in the ROA, and (b) a large number of more specific prefixes in that range is announced by the AS in the ROA. This case should occur rarely in practice (if at all). Operator discretion is necessary in this case. Operators SHOULD use "minimal ROAs" whenever possible. A minimal ROA contains only those IP prefixes that are actually originated by an AS in BGP, and no other IP prefixes. (See Section 3 for an example.) This practice requires no changes to the RPKI specification and will not increase the number of signed ROAs in the RPKI, because ROAs already support lists of IP prefixes [RFC6482]. See also [GSG17] for further discussion of why this practice will have minimal impact on the performance of the RPKI ecosystem. 5.1. Creation of ROAs Facilitating DDoS Mitigation Service Sometimes, it is not possible to use a "minimal ROA", because an operator wants to issue a ROA that includes an IP prefix that is sometimes (but not always) originated in BGP. In this case, the ROA SHOULD include (1) the set of IP prefixes that are always originated in BGP, and (2) the set IP prefixes that are sometimes, but not always, originated in BGP. The ROA SHOULD NOT include any IP prefixes that the operator knows will not be originated in BGP. Whenever possible, the ROA SHOULD also avoid the use of the maxLength attribute. The running example is now extended to illustrate one situation where it is not possible to issue a minimal ROA. Consider the following scenario prior to deployment of RPKI. Suppose AS 64496 announced 192.168.0.0/16 and has a contract with a Distributed Denial of Service (DDoS) mitigation service provider that holds AS 64500. Further, assume that the DDoS mitigation service contract applies to all IP addresses covered by 192.168.0.0/22. When a DDoS attack is detected and reported by AS 64496, AS 64500 immediately originates 192.168.0.0/22, thus attracting all the DDoS traffic to itself. The traffic is scrubbed at AS 64500 and then sent back to AS 64496 over a backhaul data link. Notice that, during a DDoS attack, the DDoS mitigation service provider AS 64500 originates Gilad, et al. Expires May 6, 2021 [Page 7] Internet-Draft RPKI maxLength November 2020 a /22 prefix that is longer than AS 64496's /16 prefix, and so all the traffic (destined to addresses in 192.168.0.0/22) that normally goes to AS 64496 goes to AS 64500 instead. First, suppose the RPKI only had the minimal ROA for AS 64496, as described in Section 3. But if there is no ROA authorizing AS 64500 to announce the /22 prefix, then the DDoS mitigation (and traffic scrubbing) scheme would not work. That is, if AS 64500 originates the /22 prefix in BGP during DDoS attacks, the announcement would be invalid [RFC6811]. Therefore, the RPKI should have two ROAs: one for AS 64496 and one for AS 64500. ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496) ROA:(192.168.0.0/22, AS 64500) Neither ROA uses the maxLength attribute. But the second ROA is not "minimal" because it contains a /22 prefix that is not originated by anyone in BGP during normal operations. The /22 prefix is only originated by AS 64500 as part of its DDoS mitigation service during a DDoS attack. Notice, however, that this scheme does not come without risks. Namely, all IP addresses in 192.168.0.0/22 are vulnerable to a forged-origin subprefix hijack during normal operations, when the /22 prefix is not originated. (The hijacker AS 64511 would send the BGP announcement "192.168.0.0/22: AS 64511, AS 64500", falsely claiming that AS 64511 is a neighbor of AS 64500 and falsely claiming that AS 64500 originates 192.168.0.0/22.) In some situations, the DDoS mitigation service at AS 64500 might want to limit the amount of DDoS traffic that it attracts and scrubs. Suppose that a DDoS attack only targets IP addresses in 192.168.0.0/24. Then, the DDoS mitigation service at AS 64500 only wants to attract the traffic designated for the /24 prefix that is under attack, but not the entire /22 prefix. To allow for this, the RPKI should have two ROAs: one for AS 64496 and one for AS 64500. ROA:(192.168.0.0/16, 192.168.225.0/24, AS 64496) ROA:(192.168.0.0/22-24, AS 64500) The second ROA uses the maxLength attribute because it is designed to explicitly enable AS 64500 to originate *any* /24 subprefix of 192.168.0.0/22. Gilad, et al. Expires May 6, 2021 [Page 8] Internet-Draft RPKI maxLength November 2020 As before, the second ROA is not "minimal" because it contains prefixes that are not originated by anyone in BGP during normal operations. As before, all IP addresses in 192.168.0.0/22 are vulnerable to a forged-origin subprefix hijack during normal operations, when the /22 prefix is not originated. The use of maxLength in this second ROA also comes with an additional risk. While it permits the DDoS mitigation service at AS 64500 to originate prefix 192.168.0.0/24 during a DDoS attack in that space, it also makes the *other* /24 prefixes covered by the /22 prefix (i.e., 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24) vulnerable to a forged-origin subprefix attacks. There is another entirely different way of managing ROAs for DDoS mitigation service. In this scheme, ROAs are not pre-created for the DDoS mitigation service but are created on the fly when the DDoS mitigation service request is made. Further, the BGP announcements for actuating the DDoS mitigation service will not be made until the ROAs propagate fully through the RPKI system. Hence, there would be a latency involved in DDoS mitigation service going into effect. This method would be effective only if the latency is guaranteed to be within some acceptable limit. This calls for mechanisms to be in place for RPKI data propagation to occur very fast. Thus, this scheme of managing ROAs for DDoS mitigation service helps with eliminating the attack surface for prefixes requiring this service. However, the viability of this scheme depends on future work related to achieving fast ROA propagation in the global RPKI system. 6. ROAs and Origin Validation for RTBH Filtering Scenario Considerations related to ROAs and origin validation [RFC6811] for the case of destination-based Remote Triggered Black Hole (RTBH) filtering are addressed here. In RTBH, highly specific prefixes (greater than /24 in IPv4 and greater than /48 in IPv6; possibly even /32 (IPv4) and /128 (IPv6)) are announced in BGP. These announcements are tagged with a BLACKHOLE Community [RFC7999]. It is obviously not desirable to use large maxLength or include any such highly specific prefixes in the ROAs to accommodate destination-based RTBH filtering. Therefore, operators SHOULD accommodate this scenario by accepting BGP announcements tagged with BLACKHOLE Community only if the following conditions are met: (1) the announcement is received on a BGP session on which there is agreement to honor BLACKHOLE Community, and (2) the prefix in the announcement is covered by a ROA that has an AS number matching with the AS number of the peer on that BGP session. (Some helpful discussion can be found in Section 5.5 in [NIST-800-189].) Gilad, et al. Expires May 6, 2021 [Page 9] Internet-Draft RPKI maxLength November 2020 7. Acknowledgments The authors would like to thank the following people for their review and contributions to this document: Omar Sagga (Boston University) and Aris Lambrianidis (AMS-IX). Thanks are also due to Matthias Waehlisch for comments and suggestions. 8. References 8.1. Normative References [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, . [RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480, February 2012, . [RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route Origin Authorizations (ROAs)", RFC 6482, DOI 10.17487/RFC6482, February 2012, . [RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R. Austein, "BGP Prefix Origin Validation", RFC 6811, DOI 10.17487/RFC6811, January 2013, . 8.2. Informative References [GCHSS] Gilad, Y., Cohen, A., Herzberg, A., Schapira, M., and H. Shulman, "Are We There Yet? On RPKI's Deployment and Security", in NDSS 2017, February 2017, . Gilad, et al. Expires May 6, 2021 [Page 10] Internet-Draft RPKI maxLength November 2020 [GSG17] Gilad, Y., Sagga, O., and S. Goldberg, "Maxlength Considered Harmful to the RPKI", in ACM CoNEXT 2017, December 2017, . [HARMFUL] Gilad, Y., Sagga, O., and S. Goldberg, "MaxLength Considered Harmful to the RPKI", 2017, . [LSG16] Lychev, R., Shapira, M., and S. Goldberg, "Rethinking Security for Internet Routing", in Communications of the ACM, October 2016, . [NIST-800-189] Sriram, K. and D. Montgomery, "Resilient Interdomain Traffic Exchange: BGP Security and DDoS Mitigation", NIST Special Publication, NIST SP 800-189, December 2019, . [RFC6907] Manderson, T., Sriram, K., and R. White, "Use Cases and Interpretations of Resource Public Key Infrastructure (RPKI) Objects for Issuers and Relying Parties", RFC 6907, DOI 10.17487/RFC6907, March 2013, . [RFC7115] Bush, R., "Origin Validation Operation Based on the Resource Public Key Infrastructure (RPKI)", BCP 185, RFC 7115, DOI 10.17487/RFC7115, January 2014, . [RFC7999] King, T., Dietzel, C., Snijders, J., Doering, G., and G. Hankins, "BLACKHOLE Community", RFC 7999, DOI 10.17487/RFC7999, October 2016, . [RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol Specification", RFC 8205, DOI 10.17487/RFC8205, September 2017, . Authors' Addresses Gilad, et al. Expires May 6, 2021 [Page 11] Internet-Draft RPKI maxLength November 2020 Yossi Gilad Hebrew University of Jerusalem Rothburg Family Buildings, Edmond J. Safra Campus Jerusalem 9190416 Israel EMail: yossigi@cs.huji.ac.il Sharon Goldberg Boston University 111 Cummington St, MCS135 Boston, MA 02215 USA EMail: goldbe@cs.bu.edu Kotikalapudi Sriram USA National Institute of Standards and Technology 100 Bureau Drive Gaithersburg, MD 20899 USA EMail: kotikalapudi.sriram@nist.gov Job Snijders NTT Communications Theodorus Majofskistraat 100 Amsterdam 1065 SZ The Netherlands EMail: job@ntt.net Ben Maddison Workonline Communications 30 Waterkant St Cape Town 8001 South Africa EMail: benm@workonline.africa Gilad, et al. Expires May 6, 2021 [Page 12]