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
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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. 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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