draft-ietf-sidrops-rpkimaxlen-05 - The Use of Maxlength in the RPKI
Author: Snijders, Job

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    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
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       Internet-Drafts are working documents of the Internet Engineering
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       This Internet-Draft will expire on May 6, 2021.

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    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
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       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.

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       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].

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    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

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       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].

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       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.

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    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

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       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.

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       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].)

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    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,
                  <https://www.rfc-editor.org/info/rfc1918>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [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,
                  <https://www.rfc-editor.org/info/rfc4271>.

       [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
                  Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
                  February 2012, <https://www.rfc-editor.org/info/rfc6480>.

       [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
                  Origin Authorizations (ROAs)", RFC 6482,
                  DOI 10.17487/RFC6482, February 2012,
                  <https://www.rfc-editor.org/info/rfc6482>.

       [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
                  Austein, "BGP Prefix Origin Validation", RFC 6811,
                  DOI 10.17487/RFC6811, January 2013,
                  <https://www.rfc-editor.org/info/rfc6811>.

    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,
                  <https://eprint.iacr.org/2016/1010.pdf>.

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       [GSG17]    Gilad, Y., Sagga, O., and S. Goldberg, "Maxlength
                  Considered Harmful to the RPKI", in ACM CoNEXT 2017,
                  December 2017, <https://eprint.iacr.org/2016/1015.pdf>.

       [HARMFUL]  Gilad, Y., Sagga, O., and S. Goldberg, "MaxLength
                  Considered Harmful to the RPKI", 2017,
                  <https://eprint.iacr.org/2016/1015.pdf>.

       [LSG16]    Lychev, R., Shapira, M., and S. Goldberg, "Rethinking
                  Security for Internet Routing", in Communications of the
                  ACM, October 2016, <http://cacm.acm.org/
                  magazines/2016/10/207763-rethinking-security-for-internet-
                  routing/>.

       [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,
                  <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
                  NIST.SP.800-189.pdf>.

       [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,
                  <https://www.rfc-editor.org/info/rfc6907>.

       [RFC7115]  Bush, R., "Origin Validation Operation Based on the
                  Resource Public Key Infrastructure (RPKI)", BCP 185,
                  RFC 7115, DOI 10.17487/RFC7115, January 2014,
                  <https://www.rfc-editor.org/info/rfc7115>.

       [RFC7999]  King, T., Dietzel, C., Snijders, J., Doering, G., and G.
                  Hankins, "BLACKHOLE Community", RFC 7999,
                  DOI 10.17487/RFC7999, October 2016,
                  <https://www.rfc-editor.org/info/rfc7999>.

       [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
                  Specification", RFC 8205, DOI 10.17487/RFC8205, September
                  2017, <https://www.rfc-editor.org/info/rfc8205>.

    Authors' Addresses

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       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]

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