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Versions: (draft-yossigi-rpkimaxlen) 00 01 02
03 04 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2021.
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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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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Documentation Prefixes . . . . . . . . . . . . . . . . . 4
2. Suggested Reading . . . . . . . . . . . . . . . . . . . . . . 4
3. Forged-Origin Subprefix Hijack . . . . . . . . . . . . . . . 4
4. Measurements of Today's RPKI . . . . . . . . . . . . . . . . 6
5. Recommendations about Minimal ROAs and Maxlength . . . . . . 7
5.1. Creation of ROAs Facilitating DDoS Mitigation Service . . 7
6. ROAs and Origin Validation for RTBH Filtering Scenario . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The RPKI [RFC6480] uses Route Origin Authorizations (ROAs) to create
a cryptographically verifiable mapping from an IP prefix to a set of
autonomous systems (ASes) that are authorized to originate that
prefix. Each ROA contains a set of IP prefixes and an AS number of
an AS authorized to originate all the IP prefixes in the set
[RFC6482]. The ROA is cryptographically signed by the party that
holds a certificate for the set of IP prefixes.
The ROA format also supports a maxLength attribute. According to
[RFC6482], "When present, the maxLength specifies the maximum length
of the IP address prefix that the AS is authorized to advertise."
Thus, rather than requiring the ROA to list each prefix the AS is
authorized to originate, the maxLength attribute provides a shorthand
that authorizes an AS to originate a set of IP prefixes.
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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
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