Internet DRAFT - draft-ietf-6man-nd-extension-headers
draft-ietf-6man-nd-extension-headers
IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Updates: 3971, 4861 (if approved) June 3, 2013
Intended status: Standards Track
Expires: December 5, 2013
Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery
draft-ietf-6man-nd-extension-headers-05
Abstract
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 counter-measures 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 prevented.
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 http://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 December 5, 2013.
Copyright Notice
Copyright (c) 2013 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Gont Expires December 5, 2013 [Page 1]
Internet-Draft ND and IPv6 Fragmentation June 2013
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 Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Traditional Neighbor Discovery and IPv6 Fragmentation . . . . 5
3. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation . . . 6
4. Rationale for Forbidding IPv6 Fragmentation in Neighbor
Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Operational Advice . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Message Size When Carrying Certificates . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Gont Expires December 5, 2013 [Page 2]
Internet-Draft ND and IPv6 Fragmentation June 2013
1. 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, the 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-DEEPSEC2011]. 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., such as NDPMon
[NDPMon], 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 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 at monitoring
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.
Gont Expires December 5, 2013 [Page 3]
Internet-Draft ND and IPv6 Fragmentation June 2013
Some Intrusion Prevention Systems (IPS) can mitigate Neighbor
Discovery attacks. We recommend that Intrusion Prevention Systems
(IPS) implement mitigations for NDP attacks.
A key challenge that these mitigation or monitoring techniques face
is that introduced by IPv6 fragmentation, 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
[I-D.ietf-v6ops-ra-guard-implementation], 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 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 use of the
IPv6 Fragment Header with traditional Neighbor Discovery is
forbidden, and also formally updates RFC 3971 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].
Gont Expires December 5, 2013 [Page 4]
Internet-Draft ND and IPv6 Fragmentation June 2013
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 Expires December 5, 2013 [Page 5]
Internet-Draft ND and IPv6 Fragmentation June 2013
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 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 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 normally processed, 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 the
Authorization Delegation Discovery (Section 6 of [RFC3971]) to fail,
thus leading the host to fall back (depending on local configuration)
either to unsecured mode, or to reject the corresponding Router
Advertisement messages (possibly resulting in a Denial of Service).
Gont Expires December 5, 2013 [Page 6]
Internet-Draft ND and IPv6 Fragmentation June 2013
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 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.
Gont Expires December 5, 2013 [Page 7]
Internet-Draft ND and IPv6 Fragmentation June 2013
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 (see Section 6.4.2 of [RFC3971]):
o Certification Path Advertisement messages
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.
Gont Expires December 5, 2013 [Page 8]
Internet-Draft ND and IPv6 Fragmentation June 2013
6. Operational Advice
An operator detecting that Neighbor Discovery traffic is being
silently dropped should find whether the corresponding Neighbor
Discovery 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 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 need a software upgrade to
a version that does not employ IPv6 fragmentation with Neighbor
Discovery.
Gont Expires December 5, 2013 [Page 9]
Internet-Draft ND and IPv6 Fragmentation June 2013
7. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
Gont Expires December 5, 2013 [Page 10]
Internet-Draft ND and IPv6 Fragmentation June 2013
8. Security Considerations
The IPv6 Fragmentation Header can be leveraged to circumvent network
monitoring tools and current implementations of mechanisms such as
RA-Guard [I-D.ietf-v6ops-ra-guard-implementation]. By updating the
relevant specifications such that the IPv6 Fragment Header is not
allowed in any Neighbor Discovery messages except "Certification Path
Advertisement", protection of local nodes against Neighbor Discovery
attacks, and monitoring of Neighbor Discovery traffic is greatly
simplified.
[I-D.ietf-v6ops-ra-guard-implementation] discusses an improvement to
the RA-Guard mechanism that can mitigate Neighbor Discovery attacks
that employ IPv6 Fragmentation. However, it should be noted that
unless [RFC4861] is updated (as proposed in this document), Neighbor
Discovery monitoring tools (such as NDPMon [NDPMon], and ramond
[ramond]) would remain unreliable and trivial to circumvent by a
skilled attacker.
As noted in Section 3, 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.
Gont Expires December 5, 2013 [Page 11]
Internet-Draft ND and IPv6 Fragmentation June 2013
9. 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 like to thank Roque Gagliano, who contributed the
information regarding messages sizes in Appendix A.
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). The author would like to thank the
UK CPNI, for their continued support.
Gont Expires December 5, 2013 [Page 12]
Internet-Draft ND and IPv6 Fragmentation June 2013
10. References
10.1. 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.
10.2. Informative References
[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.
[NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",
<http://ndpmon.sourceforge.net/>.
[ramond] "ramond", <http://ramond.sourceforge.net/>.
[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)",
draft-ietf-v6ops-ra-guard-implementation-07 (work in
progress), November 2012.
[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-DEEPSEC2011]
Gont Expires December 5, 2013 [Page 13]
Internet-Draft ND and IPv6 Fragmentation June 2013
Gont, "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>.
Gont Expires December 5, 2013 [Page 14]
Internet-Draft ND and IPv6 Fragmentation June 2013
Appendix 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 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 to 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 (i.e. 4096 bits) were required in the future, a
larger MTU size might be required to to convey such information in
Neighbor Discovery packets without the need to employ fragmentation.
Gont Expires December 5, 2013 [Page 15]
Internet-Draft ND and IPv6 Fragmentation June 2013
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.com
Gont Expires December 5, 2013 [Page 16]
