Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 6980 SI6 Networks / UTN-FRH
Updates: 3971, 4861 August 2013
Category: Standards Track
Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery
This document analyzes the security implications of employing IPv6
fragmentation with Neighbor Discovery (ND) messages. It updates RFC
4861 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 updates RFC 3971
to provide advice regarding how the aforementioned security
implications can be mitigated.
Status of This Memo
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Table of Contents
1. Introduction ....................................................22. Traditional Neighbor Discovery and IPv6 Fragmentation ...........43. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation .........54. 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 in RFC 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.
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
o Neighbor Discovery monitoring tools (e.g., NDPMon [NDPMon] and
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
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 IPv6
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 updates RFC 4861 such that the use of
the IPv6 Fragment Header with traditional Neighbor Discovery is
forbidden, and also formally updates RFC 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 in RFC 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
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.
3. 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).
4. 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
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.
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 Certification Path Solicitation
Nodes SHOULD NOT employ IPv6 fragmentation for sending the following
messages (see Section 6.4.2 of [RFC3971]):
o Certification Path Advertisement
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 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
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
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
As noted in Section 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.
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 in Appendix 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).
9.1. 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,
[RFC6494] Gagliano, R., Krishnan, S., and A. Kukec, "Certificate
Profile and Certificate Management for SEcure Neighbor
Discovery (SEND)", RFC 6494, February 2012.
Appendix A. Message Size When Carrying Certificates
This section aims at estimating the size of normal Certification Path
Considering a Certification Path Advertisement (CPA) such as that of
Appendix 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
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.
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