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Internet Engineering Task Force (IETF)                           F. GontRequest for Comments: 6980                        SI6 Networks / UTN-FRHUpdates:3971,4861                                          August 2013Category: Standards TrackISSN: 2070-1721Security Implications of IPv6 Fragmentation with IPv6 Neighbor DiscoveryAbstract   This document analyzes the security implications of employing IPv6   fragmentation with Neighbor Discovery (ND) messages.  It updatesRFC4861 such that use of the IPv6 Fragmentation Header is forbidden in   all Neighbor Discovery messages, thus allowing for simple and   effective countermeasures for Neighbor Discovery attacks.  Finally,   it discusses the security implications of using IPv6 fragmentation   with SEcure Neighbor Discovery (SEND) and formally updatesRFC 3971   to provide advice regarding how the aforementioned security   implications can be mitigated.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6980.Gont                         Standards Track                    [Page 1]

RFC 6980                ND and IPv6 Fragmentation            August 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   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 Contents1. Introduction ....................................................22. Traditional Neighbor Discovery and IPv6 Fragmentation ...........43. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation .........5   4. Rationale for Forbidding IPv6 Fragmentation in Neighbor      Discovery .......................................................65. Specification ...................................................66. Operational Advice ..............................................77. Security Considerations .........................................78. Acknowledgements ................................................89. References ......................................................89.1. Normative References .......................................89.2. Informative References .....................................9Appendix A. Message Size When Carrying Certificates ...............101.  Introduction   The Neighbor Discovery Protocol (NDP) is specified inRFC 4861   [RFC4861].  It is used by both hosts and routers.  Its functions   include Neighbor Discovery (ND), Router Discovery (RD), address   autoconfiguration, address resolution, Neighbor Unreachability   Detection (NUD), Duplicate Address Detection (DAD), and redirection.   Many of the possible attacks against the Neighbor Discovery Protocol   are discussed in detail in [RFC3756].  In order to mitigate the   aforementioned possible attacks, SEcure Neighbor Discovery (SEND) was   standardized.  SEND employs a number of mechanisms to certify the   origin of Neighbor Discovery packets and the authority of routers,   and to protect Neighbor Discovery packets from being the subject of   modification or replay attacks.Gont                         Standards Track                    [Page 2]

RFC 6980                ND and IPv6 Fragmentation            August 2013   However, a number of factors, such as the high administrative   overhead of deploying trust anchors and the unavailability of SEND   implementations for many widely deployed operating systems, make SEND   hard to deploy [Gont-DPSC].  Thus, in many general scenarios, it may   be necessary and/or convenient to use other mitigation techniques for   NDP-based attacks.  The following mitigations are currently available   for NDP attacks:   o  Static Access Control Lists (ACLs) in switches   o  Layer-2 filtering of Neighbor Discovery packets (such as RA-Guard      [RFC6105])   o  Neighbor Discovery monitoring tools (e.g., NDPMon [NDPMon] and      ramond [ramond])   o  Intrusion Prevention Systems (IPS)   IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique   for attack vectors based on ICMPv6 Router Advertisement (RA)   messages.  It is meant to block attack packets at a layer-2 device   before the attack packets actually reach the target nodes.  [RFC6104]   describes the problem statement of "Rogue IPv6 Router   Advertisements", and [RFC6105] specifies the "IPv6 Router   Advertisement Guard" functionality.   Tools such as NDPMon [NDPMon] and ramond [ramond] aim to monitor   Neighbor Discovery traffic in the hopes of detecting possible attacks   when there are discrepancies between the information advertised in   Neighbor Discovery packets and the information stored on a local   database.   Some Intrusion Prevention Systems (IPS) can mitigate Neighbor   Discovery attacks.  We recommend that Intrusion Prevention Systems   implement mitigations for NDP attacks.   IPv6 fragmentation introduces a key challenge for these mitigation or   monitoring techniques, since it is trivial for an attacker to conceal   his attack by fragmenting his packets into multiple fragments.  This   may limit or even eliminate the effectiveness of the aforementioned   mitigation or monitoring techniques.  Recent work [CPNI-IPv6]   indicates that current implementations of the aforementioned   mitigations for NDP attacks can be trivially evaded.  For example, as   noted in [RA-GUARD], current RA-Guard implementations can be   trivially evaded by fragmenting the attack packets into multiple   fragments, such that the layer-2 device cannot find all the necessary   information to perform packet filtering in the same packet.  While   Neighbor Discovery monitoring tools could (in theory) implement IPv6Gont                         Standards Track                    [Page 3]

RFC 6980                ND and IPv6 Fragmentation            August 2013   fragment reassembly, this is usually an arms-race with the attacker   (an attacker can generate lots of forged fragments to "confuse" the   monitoring tools), and therefore the aforementioned tools are   unreliable for the detection of such attacks.Section 2 analyzes the use of IPv6 fragmentation in traditional   Neighbor Discovery.Section 3 analyzes the use of IPv6 fragmentation   in SEcure Neighbor Discovery (SEND).Section 4 provides the   rationale for forbidding the use of IPv6 fragmentation with Neighbor   Discovery.Section 5 formally updatesRFC 4861 such that the use of   the IPv6 Fragment Header with traditional Neighbor Discovery is   forbidden, and also formally updatesRFC 3971 by providing advice on   the use of IPv6 fragmentation with SEND.Section 6 provides   operational advice about interoperability problems arising from the   use of IPv6 fragmentation with Neighbor Discovery.   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 inRFC 2119 [RFC2119].2.  Traditional Neighbor Discovery and IPv6 Fragmentation   The only potential use case for IPv6 fragmentation with traditional   (i.e., non-SEND) IPv6 Neighbor Discovery would be that in which a   Router Advertisement must include a large number of options (Prefix   Information Options, Route Information Options, etc.).  However, this   could still be achieved without employing fragmentation, by splitting   the aforementioned information into multiple Router Advertisement   messages.      Some Neighbor Discovery implementations are known to silently      ignore Router Advertisement messages that employ fragmentation.      Therefore, splitting the necessary information into multiple RA      messages (rather than sending a large RA message that is      fragmented into multiple IPv6 fragments) is probably desirable      even from an interoperability point of view.   Thus, avoiding the use of IPv6 fragmentation in traditional Neighbor   Discovery would greatly simplify and improve the effectiveness of   monitoring and filtering Neighbor Discovery traffic and would also   prevent interoperability problems with those implementations that do   not support fragmentation in Neighbor Discovery messages.Gont                         Standards Track                    [Page 4]

RFC 6980                ND and IPv6 Fragmentation            August 20133.  SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation   SEND packets typically carry more information than traditional   Neighbor Discovery packets: for example, they include additional   options such as the Cryptographically Generated Address (CGA) option   and the RSA signature option.   When SEND nodes employ any of the Neighbor Discovery messages   specified in [RFC4861], the situation is roughly the same: if more   information than would fit in a non-fragmented Neighbor Discovery   packet needs to be sent, it should be split into multiple Neighbor   Discovery messages (such that IPv6 fragmentation is avoided).   However, Certification Path Advertisement (CPA) messages (specified   in [RFC3971]) pose a different situation, since the Certificate   Option they include typically contains much more information than any   other Neighbor Discovery option.  For example,Appendix C of   [RFC3971] reports Certification Path Advertisement messages from 1050   to 1066 bytes on an Ethernet link layer.  Since the size of CPA   messages could potentially exceed the MTU of the local link,Section 5 recommends that fragmented CPA messages be processed   normally, but discourages the use of keys that would result in   fragmented CPA messages.   It should be noted that relying on fragmentation opens the door to a   variety of IPv6 fragmentation-based attacks against SEND.  In   particular, if an attacker is located on the same broadcast domain as   the victim host and Certification Path Advertisement messages employ   IPv6 fragmentation, it would be trivial for the attacker to forge   IPv6 fragments such that they result in "Fragment ID collisions",   causing both the attack fragments and the legitimate fragments to be   discarded by the victim node.  This would eventually cause   Authorization Delegation Discovery (Section 6 of [RFC3971]) to fail,   thus leading the host to (depending on local configuration) either   fall back to unsecured mode or reject the corresponding Router   Advertisement messages (possibly resulting in a denial of service).Gont                         Standards Track                    [Page 5]

RFC 6980                ND and IPv6 Fragmentation            August 20134.  Rationale for Forbidding IPv6 Fragmentation in Neighbor Discovery   A number of considerations should be made regarding the use of IPv6   fragmentation with Neighbor Discovery:   o  A significant number of existing implementations already silently      drop fragmented ND messages, so the use of IPv6 fragmentation may      hamper interoperability among IPv6 implementations.   o  Although it is possible to build an ND message that needs to be      fragmented, such packets are unlikely to exist in the real world      because of the large number of options that would be required for      the resulting packet to exceed the minimum IPv6 MTU of      1280 octets.   o  If an ND message was so large as to need fragmentation, there is      an option to distribute the same information amongst more than one      message, each of which is small enough to not need fragmentation.   Thus, forbidding the use of IPv6 fragmentation with Neighbor   Discovery normalizes existing behavior and sets the expectations of   all implementations to the existing lowest common denominator.5.  Specification   Nodes MUST NOT employ IPv6 fragmentation for sending any of the   following Neighbor Discovery and SEcure Neighbor Discovery messages:   o  Neighbor Solicitation   o  Neighbor Advertisement   o  Router Solicitation   o  Router Advertisement   o  Redirect   o  Certification Path Solicitation   Nodes SHOULD NOT employ IPv6 fragmentation for sending the following   messages (seeSection 6.4.2 of [RFC3971]):   o  Certification Path AdvertisementGont                         Standards Track                    [Page 6]

RFC 6980                ND and IPv6 Fragmentation            August 2013   Nodes MUST silently ignore the following Neighbor Discovery and   SEcure Neighbor Discovery messages if the packets carrying them   include an IPv6 Fragmentation Header:   o  Neighbor Solicitation   o  Neighbor Advertisement   o  Router Solicitation   o  Router Advertisement   o  Redirect   o  Certification Path Solicitation   Nodes SHOULD normally process the following messages when the packets   carrying them include an IPv6 Fragmentation Header:   o  Certification Path Advertisement   SEND nodes SHOULD NOT employ keys that would result in fragmented CPA   messages.6.  Operational Advice   An operator detecting that Neighbor Discovery traffic is being   silently dropped should find whether the corresponding Neighbor   Discovery messages are employing IPv6 fragmentation.  If they are, it   is likely that the devices receiving such packets are silently   dropping them merely because they employ IPv6 fragmentation.  In such   a case, an operator should check whether the sending device has an   option to prevent fragmentation of ND messages, and/or see whether it   is possible to reduce the options carried on such messages.  We note   that solving this (unlikely) problem might require a software upgrade   to a version that does not employ IPv6 fragmentation with Neighbor   Discovery.7.  Security Considerations   The IPv6 Fragmentation Header can be leveraged to circumvent network   monitoring tools and current implementations of mechanisms such as   RA-Guard [RA-GUARD].  By updating the relevant specifications such   that the IPv6 Fragment Header is not allowed in any Neighbor   Discovery messages except Certification Path Advertisement messages,   protection of local nodes against Neighbor Discovery attacks, as well   as the monitoring of Neighbor Discovery traffic, are greatly   simplified.Gont                         Standards Track                    [Page 7]

RFC 6980                ND and IPv6 Fragmentation            August 2013   As noted inSection 3, the use of SEND could potentially result in   fragmented Certification Path Advertisement messages, thus allowing   an attacker to employ IPv6 fragmentation-based attacks against such   messages.  Therefore, to the extent that is possible, such use of   fragmentation should be avoided.8.  Acknowledgements   The author would like to thank (in alphabetical order) Mikael   Abrahamsson, Ran Atkinson, Ron Bonica, Jean-Michel Combes, David   Farmer, Adrian Farrel, Stephen Farrell, Roque Gagliano, Brian   Haberman, Bob Hinden, Philip Homburg, Ray Hunter, Arturo Servin, Mark   Smith, and Martin Stiemerling for providing valuable comments on   earlier versions of this document.   The author would also like to thank Roque Gagliano for contributing   the information regarding message sizes inAppendix A, and Arturo   Servin for presenting this document at IETF 81.   Finally, the author would like to thank his brother, friend, and   colleague, Guillermo Gont, for his love and support.   This document resulted from the project "Security Assessment of the   Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by   Fernando Gont on behalf of the UK Centre for the Protection of   National Infrastructure (CPNI).9.  References9.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3971]   Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure               Neighbor Discovery (SEND)",RFC 3971, March 2005.   [RFC4861]   Narten, T., Nordmark, E., Simpson, W., and H. Soliman,               "Neighbor Discovery for IP version 6 (IPv6)",RFC 4861,               September 2007.   [RFC6494]   Gagliano, R., Krishnan, S., and A. Kukec, "Certificate               Profile and Certificate Management for SEcure Neighbor               Discovery (SEND)",RFC 6494, February 2012.Gont                         Standards Track                    [Page 8]

RFC 6980                ND and IPv6 Fragmentation            August 20139.2.  Informative References   [CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol               version 6 (IPv6)", UK Centre for the Protection of               National Infrastructure, (available on request).   [Gont-DPSC] Gont, F., "Results of a Security Assessment of the               Internet Protocol version 6 (IPv6)", DEEPSEC 2011               Conference, Vienna, Austria, November 2011,               <http://www.si6networks.com/presentations/deepsec2011/fgont-deepsec2011-ipv6-security.pdf>.   [NDPMon]    SourceForge, "NDPMon - IPv6 Neighbor Discovery Protocol               Monitor", July 2012, <http://ndpmon.sourceforge.net/>.   [RA-GUARD]  Gont, F., "Implementation Advice for IPv6 Router               Advertisement Guard (RA-Guard)", Work in Progress,               November 2012.   [ramond]    SourceForge, "ramond", January 2009,               <http://ramond.sourceforge.net/>.   [RFC3756]   Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor               Discovery (ND) Trust Models and Threats",RFC 3756,               May 2004.   [RFC6104]   Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement               Problem Statement",RFC 6104, February 2011.   [RFC6105]   Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and               J. Mohacsi, "IPv6 Router Advertisement Guard",RFC 6105,               February 2011.Gont                         Standards Track                    [Page 9]

RFC 6980                ND and IPv6 Fragmentation            August 2013Appendix A.  Message Size When Carrying Certificates   This section aims at estimating the size of normal Certification Path   Advertisement messages.   Considering a Certification Path Advertisement (CPA) such as that ofAppendix C of [RFC3971] (certification path length of 4, between 1   and 4 address prefix extensions, and a key length of 1024 bits), the   certificate lengths range between 864 and 888 bytes (and the   corresponding Ethernet packets from 1050 to 1066 bytes) [RFC3971].   Updating the aforementioned packet size to account for the larger   (2048 bits) keys required by [RFC6494] results in packet sizes   ranging from 1127 to 1238 bytes, which are smaller than the minimum   IPv6 MTU (1280 bytes) and much smaller than the ubiquitous Ethernet   MTU (1500 bytes).   However, we note that packet sizes may vary depending on a number of   factors, including:   o  the number of prefixes included in the certificate   o  the length of Fully Qualified Domain Names (FQDNs) in Trust Anchor      (TA) options [RFC3971] (if present)   If larger key sizes (e.g., 4096 bits) are required in the future, a   larger MTU size might be required to convey such information in   Neighbor Discovery packets without the need to employ fragmentation.Author's Address   Fernando Gont   SI6 Networks / UTN-FRH   Evaristo Carriego 2644   Haedo, Provincia de Buenos Aires  1706   Argentina   Phone: +54 11 4650 8472   EMail: fgont@si6networks.com   URI:http://www.si6networks.comGont                         Standards Track                   [Page 10]