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Versions: 00 01 02 draft-ietf-sidrops-rpkimaxlen

Network Working Group                                           Y. Gilad
Internet-Draft                                               S. Goldberg
Intended status: Best Current Practice                 Boston University
Expires: September 14, 2017                                    K. Sriram
                                                                    NIST
                                                          March 13, 2017


                    The Use of Maxlength in the RPKI
                      draft-yossigi-rpkimaxlen-00

Abstract

   This document recommends that operators avoid using the maxLength
   attribute when issuing Route Origin Authorizations (ROAs) in the
   Resource Public Key Infrastructure (RPKI).  These recommendations
   complement those in [RFC7115].

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
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   This Internet-Draft will expire on September 14, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   include Simplified BSD License text as described in Section 4.e of




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   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
   2.  Suggested Reading . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Forged Origin Subprefix Hijack  . . . . . . . . . . . . . . .   3
   4.  Measurements of Today's RPKI  . . . . . . . . . . . . . . . .   5
   5.  Use Minimal ROAs without Maxlength  . . . . . . . . . . . . .   6
     5.1.  When a Minimal ROA Cannot Be Used?  . . . . . . . . . . .   6
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The RPKI [RFC6480] uses Route Origin Authorizations (ROAs) to create
   a trusted mapping from an IP prefix to a set of autonomous systems
   (ASes) that are authorized to originate this prefix.  Each ROA
   contains a set of IP prefixes, and an AS number of an AS authorized
   originate all the IP prefixes in the set [RFC6482].  Each ROA is
   cryptographically signed by the party that is authorized to allocate
   the set of IP prefixes.

   The RPKI 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 explictly list each prefix the
   AS is authorized to originate, the maxLength attribute provides a
   shorthand that authorizes an AS to announce a set of IP prefixes.

   However, measurements of current RPKI deployments have found that use
   of the maxLength in ROAs tends to lead to security problems.
   Specifically, as of September 2016, 89% of the prefixes specified in
   ROAs that use the maxLength attribute, are vulnerable to a forged-
   origin subprefix hijack.  The forged-origin subprefix hijack affects
   any IP prefix that is authorized in ROA but is not announced in BGP.
   The impact of such an attack is the same as standard subprefix hijack
   on an IP prefix that is unprotected by a ROA in the RPKI.

   For this reason, this document recommends that operators avoid using
   the maxLength attribute in their ROAs as a best current practice.
   Instead, ROAs should be consist of explicit lists of the IP prefixes
   that an AS is authorized to announce, without using the maxLength



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   attribute.  Whenever possible, this ROA should also be "miminal", in
   that it includes only the list of IP prefixes that are actually
   originated in BGP.  The recommendations in this document clarify and
   extend the following recommendation from [RFC7115]:

      One advantage of minimal ROA length is that the forged origin
      attack does not work for sub-prefixes that are not covered by
      overly long max length.  For example, if, instead of
      10.0.0.0/16-24, one issues 10.0.0.0/16 and 10.0.42.0/24, a forged
      origin attack cannot succeed against 10.0.666.0/24.  They must
      attack the whole /16, which is more likely to be noticed because
      of its size.

   These recommendations 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].

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].

2.  Suggested Reading

   It is assumed that the reader understands BGP [RFC4271], the RPKI
   [RFC6480] Route Origin Authorizations (ROAs) [RFC6482], RPKI-based
   Prefix Validation [RFC6811], and BGPSEC
   [I-D.ietf-sidr-bgpsec-protocol].

3.  Forged Origin Subprefix Hijack

   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
   [I-D.ietf-sidr-bgpsec-protocol] is not deployed.

   We describe the forged-origin subprefix hijack [RFC7115] [GCHSS]
   using a running example.

   Consider the IP prefix 168.122.0.0/16 which is allocated to an
   organization that also operates AS 111.  In BGP, AS 111 announces the
   IP prefix 168.122.0.0/16 as well as its subprefix 168.122.225.0/24.
   Therefore, the RPKI should contain a ROA authorizing AS 111 to
   originate these two IP prefixes.  That is, the ROA should be

      ROA:(168.122.0.0/16,168.122.225.0/24, AS 111)




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   This ROA is "minimal" because it includes only those two prefixes
   that are actually originated by AS 111 in BGP.  [RFC6907]

   Now suppose an attacking AS 666 originates a BGP announcement for a
   subprefix 168.122.0.0/24.  This is a standard "subprefix hijack".

   In the absence of the minimal ROA above, AS 666 could intercept
   traffic for the addresses in 168.122.0.0/24.  This is because routers
   perform a longest-prefix match when deciding where to forward IP
   packets, and 168.122.0.0/24 originated by AS 666 is a longer prefix
   than 168.122.0.0/16 originated by AS 111.

   However, the ROA above renders AS 666's BGP announcement invalid,
   because (1) this ROA "covers" the attacker's announcement (since
   168.122.0.0/24 is a subprefix of 168.122.0.0/16), and (2) there is no
   ROA "matching" the attacker's announcement (there is no ROA for AS
   666 and IP prefix 168.122.0.0/24) [RFC6811].  If routers ignore
   invalid BGP announcements, the minimal ROA above ensures that the
   subprefix hijack will fail.

   Now suppose that instead the ROA above was replaced with a "loose
   ROA" that used maxLength as a shorthand for set of IP prefixes that
   AS 111 is authorized to announce.  The ROA would be:

      ROA:(168.122.0.0/16-24, AS 111)

   This ROA authorizes AS 111 to originate any subprefix of
   168.122.0.0/16, up to length /24.  That is, AS 111 could originate
   168.122.225.0/24 as well as all of 168.122.0.0/17, 168.122.128.0/17,
   ..., 168.122.255.0/24 but not 168.122.0.0/25.

   However, AS 111 only originates two prefixes in BGP: 168.122.0.0/16
   and 168.122.255.0/24.  This means that all other prefixes authorized
   by the loose ROA (for instance, 168.122.0.0/24), are vulnerable to
   the following forged-origin subprefix hijack [RFC7115,[GCHSS]]:

      The hijacker AS 666 sends a BGP announcement "168.122.0.0/24: AS
      666, AS 111", falsely claiming that AS 666 is a neighbor of AS 111
      and falsely claiming that AS 111 originates the IP prefix
      168.122.0.0/24.  In fact, the IP prefix 168.122.0.0/24 is not
      announced by AS 111.

   The hijacker's BGP announcement is valid according the RPKI, since
   the ROA (168.122.0.0/16-24, AS 111) authorizes AS 111 to originate
   BGP routes for 168.122.0.0/24.  Becaue AS 111 does not actually
   originate a route for 168.122.0.0/24, the hijacker's route is the
   *only* route to the 168.122.0.0/24.  Longest-prefix-match routing
   ensures that the hijacker's route to the subprefix 168.122.0.0/24 is



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   always preferred over the legitimate route to 168.122.0.0/16
   announced by AS 111.  Thus, if the hijacker's route propagates
   through the Internet, the hijacker will intercept traffic destined
   for IP addresses in 168.122.0.0/24.

   The forged origin *subprefix* hijack would have failed if "minimal
   ROA" described above was used instead of the "loose ROA".  If the
   "minimal ROA" had been used instead, the attacker would be forced to
   launch a forged origin *prefix* hijack in order to attract traffic,
   as follows:

      The hijacker AS 666 sends a BGP announcement "168.122.0.0/16: AS
      666, AS 111", falsely claiming that AS 666 is a neighbor of AS
      111.

   Notice, however, that this hijack is significantly less effective for
   the hijacker, since AS 111 is actually originating 168.122.0.0/16 in
   BGP.  In contrast to the forged-origin subprefix hijack, with this
   hijack AS 666 is not presenting the *only* route to 168.122.0.0/16.
   Moreover, the path originated by AS 666 is one hop longer than the
   path originated by the legitimate origin AS 111.  As discussed in
   [LSG16], this means that the hijacker will attract less traffic than
   he would have in the forged origin *subprefix* hijack.

   In sum, a forged-origin subprefix hijack has exactly the same impact
   as a regular subprefix hijack.  A forged-origin subprefix hijack is
   also more damaging than than forged-origin prefix hijack.

   Any ROA with maxLength m longer than the prefix length p is
   vulnerable to a forged-origin subprefix hijack, unless every
   subprefix of prefix p of length m is legitimately announced in BGP.

4.  Measurements of Today's RPKI

   Network measurements from September 13, 2016 show that 16% of the IP
   prefixes authorized in ROAs have a maxLength longer than their prefix
   length.  The vast majority of these (89%) of these are vulnerable to
   forged-origin subprefix hijacks.  Even large providers are vulnerable
   to these attacks.  See [GSG16] for details.

   These measurements suggest that operators commonly misconfigure the
   maxLength attribute, and unwittingly open themselves up to forged-
   origin subprefix hijacks.








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5.  Use Minimal ROAs without Maxlength

   This document recommends that operators avoid using the maxLength
   attribute in their ROAs.

   Operators should use "minimal ROAs" whenever possible.  A minimal ROA
   enumerates the exact list of IP prefixes that are actually originated
   by an AS in BGP, as described in the running example of Section 3.

   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) announced in BGP.  In this case the ROA
   should still consist of an explicit list of IP prefixes, including
   those prefixes that are sometimes, but not always announced in BGP.
   The list of prefixes should still avoid the use of the maxLength
   attribute.

   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 [GSG16] for
   further discussion of why this practice will have minimal impact on
   the performance of the RPKI ecosystem.

5.1.  When a Minimal ROA Cannot Be Used?

   We now extend our running example to illustrate one situation where
   where it is not possible to issue a minimal ROA.

   Suppose AS 111 has a contract with a DDoS mitigation service provider
   that holds AS 222.  When a DDoS attack is detected, AS 222
   immediately originates 168.122.0.0/17 and 168.122.128.0/17, thus
   attracting all the DDoS traffic to itself.  The traffic is scrubbed
   at AS 222 and then and sent back to AS 111 over a backhaul data link.
   Notice that, during a DDoS attack, the DDoS mitigation service
   provider AS 222 originates two /17 prefixes that are longer than than
   AS 111's /16 prefix, and so all the traffic that normally goes to AS
   111 goes to AS 222 instead.

   First, suppose the RPKI only had the minimal ROA for AS 111, as
   described in Section 3.  But, if there is no ROA authorizing AS 222
   to announce the two /17 prefixes, then the traffic-scrubbing scheme
   would not work.  That is, if AS 222 originates the two /17 prefixes
   in BGP during a DDoS attack, the announcement would be invalid
   [RFC6811].

   Instead, the RPKI should have two ROAs: one for AS 111 and one for AS
   222.




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      ROA:(168.122.0.0/16,168.122.225.0/24, AS 111)

      ROA:(168.122.0.0/17,168.122.128.0/17, AS 222)

   Neither ROA uses the maxLength attribute.  But, the second ROA is not
   "minimal" because it contains two /17 prefixes that are not announced
   by anyone in BGP during normal operations.  These two /17 prefixes
   are only announced by AS 222 as part of its DDoS mitigation service
   during a DDoS attack.

   Notice, however, that this scheme does not come without risks.
   Namely, all of the IP addresses in 168.122.0.0/16 (except those in
   68.122.225.0/24) are vulnerable to a forged-origin subprefix hijack
   during normal operations, when the two /17 prefixes are not
   announced.  (The hijacker AS 666 would send the BGP announcement
   `168.122.0.0/17: AS 666, AS 222'', falsely claiming that AS 666 is a
   neighbor of AS 222 and falsely claiming that AS 222 originates
   168.122.0.0/17.)

   Thus, a better approach would be to limit the address space in the
   ROA for AS 222, so it includes only those IP addresses that must
   actively be protected by the DDoS mitigation service provider.  For
   instance, if DDoS protection is contracted only for those servers in
   AS 111 that have addresses in 168.122.0.0/23, then the following ROAs
   suffice:

      ROA:(168.122.0.0/16,168.122.225.0/24, AS 111)

      ROA:(168.122.0.0/23, AS 222)

   Now, fewer IP addresses (namely, only those addresses in
   168.122.0.0/23) are vulnerable to forged origin subprefix hijacks,
   and DDoS mitigation service could still protect these addresses
   during DDoS attacks.

6.  Contributors

   This document would not be possible without the work of Omar Sagga
   (Boston University).

7.  References

7.1.  Normative References

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



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   [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,
              <http://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, <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6811>.

7.2.  Informative References

   [GSG16]    Gilad, Y., Sagga, O., and S. Goldberg, "Maxlength
              Considered Harmful to the RPKI", in ePrint Cryptology
              Archive 2016/1015, February 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/>.

   [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>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7115>.




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   [I-D.ietf-sidr-bgpsec-protocol]
              Lepinski, M. and K. Sriram, "BGPsec Protocol
              Specification", draft-ietf-sidr-bgpsec-protocol-22 (work
              in progress), January 2017.

Authors' Addresses

   Yossi Gilad
   Boston University
   111 Cummington St, MCS135
   Boston, MA  02215
   USA

   EMail: yossigi@bu.edu


   Sharon Goldberg
   Boston University
   111 Cummington St, MCS135
   Boston, MA  02215
   USA

   EMail: goldbe@cs.bu.edu


   Kotikalapudi Sriram
   NIST
   100 Bureau Drive
   Gaithersburg, MD  20899
   USA

   EMail: kotikalapudi.sriram@nist.gov



















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