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

The Use of Maxlength in the RPKI


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

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 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]:

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 which is allocated to an organization that also operates AS 111. In BGP, AS 111 announces the IP prefix as well as its subprefix Therefore, the RPKI should contain a ROA authorizing AS 111 to originate these two IP prefixes. That is, the ROA should be [RFC6907]

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

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

In the absence of the minimal ROA above, AS 666 could intercept traffic for the addresses in This is because routers perform a longest-prefix match when deciding where to forward IP packets, and originated by AS 666 is a longer prefix than 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 is a subprefix of, and (2) there is no ROA "matching" the attacker's announcement (there is no ROA for AS 666 and IP prefix [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:

This ROA authorizes AS 111 to originate any subprefix of, up to length /24. That is, AS 111 could originate as well as all of,, ..., but not

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

The hijacker's BGP announcement is valid according the RPKI, since the ROA (, AS 111) authorizes AS 111 to originate BGP routes for Becaue AS 111 does not actually originate a route for, the hijacker's route is the *only* route to the Longest-prefix-match routing ensures that the hijacker's route to the subprefix is always preferred over the legitimate route to announced by AS 111. Thus, if the hijacker's route propagates through the Internet, the hijacker will intercept traffic destined for IP addresses in

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:

Notice, however, that this hijack is significantly less effective for the hijacker, since AS 111 is actually originating in BGP. In contrast to the forged-origin subprefix hijack, with this hijack AS 666 is not presenting the *only* route to 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.

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 and, 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.

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 (except those in 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 ` AS 666, AS 222'', falsely claiming that AS 666 is a neighbor of AS 222 and falsely claiming that AS 222 originates

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, then the following ROAs suffice:

Now, fewer IP addresses (namely, only those addresses in 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.
[RFC4271] Rekhter, Y., Li, T. and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480, February 2012.
[RFC6482] Lepinski, M., Kent, S. and D. Kong, "A Profile for Route Origin Authorizations (ROAs)", RFC 6482, DOI 10.17487/RFC6482, February 2012.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R. and R. Austein, "BGP Prefix Origin Validation", RFC 6811, DOI 10.17487/RFC6811, January 2013.

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.
[LSG16] Lychev, R., Shapira, M. and S. Goldberg, "Rethinking Security for Internet Routing", in Communications of the ACM, October 2016.
[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.
[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.
[RFC7115] Bush, R., "Origin Validation Operation Based on the Resource Public Key Infrastructure (RPKI)", BCP 185, RFC 7115, DOI 10.17487/RFC7115, January 2014.
[I-D.ietf-sidr-bgpsec-protocol] Lepinski, M. and K. Sriram, "BGPsec Protocol Specification", Internet-Draft draft-ietf-sidr-bgpsec-protocol-22, January 2017.

Authors' Addresses

Yossi Gilad Boston University 111 Cummington St, MCS135 Boston, MA 02215 USA EMail:
Sharon Goldberg Boston University 111 Cummington St, MCS135 Boston, MA 02215 USA EMail:
Kotikalapudi Sriram NIST 100 Bureau Drive Gaithersburg, MD 20899 USA EMail: