The Use of Maxlength in the RPKI
draft-ietf-sidrops-rpkimaxlen-05

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

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Copyright Notice

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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]