The Use of Maxlength in the RPKI
draft-ietf-sidrops-rpkimaxlen-01

Network Working Group                                           Y. Gilad
Internet-Draft                                               S. Goldberg
Intended status: Best Current Practice                 Boston University
Expires: April 25, 2019                                        K. Sriram
                                                                USA NIST
                                                             J. Snijders
                                                                     NTT
                                                             B. Maddison
                                               Workonline Communications
                                                        October 22, 2018

                    The Use of Maxlength in the RPKI
                    draft-ietf-sidrops-rpkimaxlen-01

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

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   carefully, as they describe your rights and restrictions with respect
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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.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 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].
   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.

   However, measurements of current RPKI deployments have found that use
   of the maxLength in ROAs tends to lead to security problems.
   Specifically, as of June 2017, 84% of the prefixes specified in ROAs
   that use the maxLength attribute, are vulnerable to a forged-origin
   subprefix hijack [HARMFUL].  The forged-origin subprefix hijack, as
   described below, can be launched against any IP prefix that is
   authorized in ROA but is not originated in BGP.  The impact of such
   an attack is the same as that of a subprefix hijack in the absence of
   ROA-based protection.

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   For this reason, this document recommends that, whenever possible,
   operators SHOULD use "minimal ROAs" that include only those IP
   prefixes that are actually originated in BGP, and no other prefixes.
   Operators SHOULD also avoid using the maxLength attribute in their
   ROAS whenever possible.  One ideal place to implement these
   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.

   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.

   This best current 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].

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 [RFC8205].

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 [RFC8205]
   (or any similar method to validate the truthfulness of the BGP
   AS_PATH attribute) is not deployed.

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

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   Consider the IP prefix 168.122.0.0/16 which is allocated to an
   organization that also operates AS 64496.  In BGP, AS 64496
   originates 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 64496 to originate these two IP prefixes.  That is,
   the ROA should be

      ROA:(168.122.0.0/16,168.122.225.0/24, 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].

   Now suppose an attacking AS 64511 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 64511 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 64511 is a longer prefix
   than 168.122.0.0/16 originated by AS 64496.

   However, the minimal ROA renders AS 64511'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
   64511 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 the "minimal ROA" was replaced with a "loose ROA"
   that used maxLength as a shorthand for set of IP prefixes that AS
   64496 is authorized to originate.  The "loose ROA" would be:

      ROA:(168.122.0.0/16-24, AS 64496)

   This "loose ROA" authorizes AS 64496 to originate any subprefix of
   168.122.0.0/16, up to length /24.  That is, AS 64496 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 64496 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 64511 sends a BGP announcement "168.122.0.0/24: AS
      64511, AS 64496", falsely claiming that AS 64511 is a neighbor of
      AS 64496 and falsely claiming that AS 64496 originates the IP

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      prefix 168.122.0.0/24.  In fact, the IP prefix 168.122.0.0/24 is
      not originated by AS 64496.

   The hijacker's BGP announcement is valid according to the RPKI, since
   the ROA (168.122.0.0/16-24, AS 64496) authorizes AS 64496 to
   originate BGP routes for 168.122.0.0/24.  Because AS 64496 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 always preferred over the legitimate route to
   168.122.0.0/16 originated by AS 64496.  Thus, the hijacker's route
   propagates through the Internet, the traffic destined for IP
   addresses in 168.122.0.0/24 will be delivered to the hijacker.

   The forged origin *subprefix* hijack would have failed if the
   "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 64511 sends a BGP announcement "168.122.0.0/16: 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.  With a forged-origin
   *prefix* hijack, AS 64496 legitimately originates 168.122.0.0/16 in
   BGP, so the hijacker AS 64511 is not presenting the *only* route to
   168.122.0.0/16.  Moreover, the path originated by AS 64511 is one hop
   longer than the path originated by the legitimate origin AS 64496.
   As discussed in [LSG16], 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.

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

4.  Measurements of Today's RPKI

   Network measurements from June 1, 2017 show that 12% of the IP
   prefixes authorized in ROAs have a maxLength longer than their prefix
   length.  The vast majority of these (84%) of these are vulnerable to
   forged-origin subprefix hijacks.  Even large providers are vulnerable
   to these attacks.  See [GSG17] for details.

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

5.  Use Minimal ROAs without Maxlength

   Operators SHOULD avoid using the maxLength attribute in their ROAs.

   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.  When a Minimal ROA Cannot Be Used?

   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.

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

   Consider the following scenario prior to deployment of RPKI.  Suppose
   AS 64496 announced 168.122.0.0/16 and has a contract with a DDoS
   mitigation service provider that holds AS 64500.  Further, assume
   that the DDoS mitigation service contract applies to all IP addresses
   covered by 168.122.0.0/22.  When a DDoS attack is detected and
   reported by AS 64496, AS 64500 immediately originates 168.122.0.0/22,
   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 a /22 prefix that is longer than
   than AS 64496's /16 prefix, and so all the traffic (destined to
   addresses in 168.122.0.0/22) that normally goes to AS 64496 goes to
   AS 64500 instead.

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   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 traffic-scrubbing scheme would
   not work.  That is, if AS 64500 originates the /22 prefix in BGP
   during a DDoS attack, the announcement would be invalid [RFC6811].

   Therefore, the RPKI should have two ROAs: one for AS 64496 and one
   for AS 64500.

      ROA:(168.122.0.0/16,168.122.225.0/24, AS 64496)

      ROA:(168.122.0.0/22, 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 168.122.0.0/22 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 "168.122.0.0/22: AS 64511, AS 64500", falsely claiming
   that AS 64511 is a neighbor of AS 64500 and falsely claiming that AS
   64500 originates 168.122.0.0/22.)

   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
   168.122.0.0/24.  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:(168.122.0.0/16,168.122.225.0/24, AS 64496)

      ROA:(168.122.0.0/22-24, AS 64500)

   The second ROA uses the maxLength attribute because it is designed to
   explicitly enable AS 64500 to originate *any* /24 subprefix of
   168.122.0.0/22.

   As before, the second ROA is also not "minimal" because it contains
   prefixes that are not originated by anyone in BGP during normal
   operations.  As before, all IP addresses in 168.122.0.0/22 are
   vulnerable to a forged-origin subprefix hijack during normal
   operations, when the /22 prefix is not originated.

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   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 168.122.0.0/24 during a DDoS attack in that space,
   it also makes the *other* /24 prefixes covered by the /22 prefix
   (i.e., 168.122.1.0/24, 168.122.2.0/24, 168.122.3.0/24) vulnerable to
   a forged-origin subprefix attacks.

6.  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).

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,
              <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>.

7.2.  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/>.

   [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>.

   [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

   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

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   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.co.za

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