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DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers

DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
Posted May 23, 2012
Authored by Fernando Gont

This document specifies a mechanism for protecting hosts connected to a broadcast network against rogue DHCPv6 servers. The aforementioned mechanism is based on DHCPv6 packet-filtering at the layer-2 device on which the packets are received. The aforementioned mechanism has been widely deployed in IPv4 networks ('DHCP snooping'), and hence it is desirable that similar functionality be provided for IPv6 networks.

tags | paper
MD5 | 56f4a21ac08ccb68e395b2dfbaf811cb

DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers

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Operational Security Capabilities for F. Gont
IP Network Infrastructure (opsec) SI6 Networks / UTN-FRH
Internet-Draft May 18, 2012
Intended status: BCP
Expires: November 19, 2012


DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
draft-gont-opsec-dhcpv6-shield-00

Abstract

This document specifies a mechanism for protecting hosts connected to
a broadcast network against rogue DHCPv6 servers. The aforementioned
mechanism is based on DHCPv6 packet-filtering at the layer-2 device
on which the packets are received. The aforementioned mechanism has
been widely deployed in IPv4 networks ('DHCP snooping'), and hence it
is desirable that similar functionality be provided for IPv6
networks.

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.

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 November 19, 2012.

Copyright Notice

Copyright (c) 2012 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



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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. DHCPv6-Shield Configuration . . . . . . . . . . . . . . . . . 4
3. DHCPv6-Shield Implementation Advice . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
































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1. Introduction

This document specifies a mechanism for protecting hosts connected to
a broadcast network against rogue DHCPv6 servers. This mechanism is
analogous to the RA-Guard mechanism [RFC6104] [RFC6105]
[I-D.ietf-v6ops-ra-guard-implementation] intended for protection
against rogue Router Advertisement messages.

The basic concept behind DHCPv6-Shield is that a layer-2 device
filters DHCPv6 messages meant to DHCPv6 clients, according to a
number of different criteria. The most basic filtering criterion
being that the aforementioned DHCPv6 messages are discarded by the
layer-2 device unless they are received on a specified port of the
layer-2 device.

Before the DCHPv6-Shield device is deployed, the administrator
specifies the layer-2 port(s) on which DHCPv6 packets meant for
DHCPv6 clients are allowed. Only those ports to which a DHCPv6
server is to be connected should be specified as such. Once
deployed, the DHCPv6-Shield device inspects received packets, and
allows (i.e. passes) DHCPv6 messages meant for DHCPv6 clients only if
they are received on layer-2 ports that have been explicitly
configured for such purpose.

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].
























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2. DHCPv6-Shield Configuration

Before being deployed for production, the DHCPv6-Shield device MUST
me configured with respect to which layer-2 ports are allowed to send
DHCPv6 packets to DHCPv6 clients. Only those layer-2 ports
explicitly configured for such purpose will be allowed to send DHCPv6
packets to DHCPv6 clients.












































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3. DHCPv6-Shield Implementation Advice

The following filtering rules MUST be enforced as part of an DHCPv6-
Shield implementation on those ports that are not allowed to send
DHCPv6 packets to DHCPv6 clients:

1. Try to identify whether the packet is a DHCPv6 packet meant for a
DHCPv6 client, by parsing the IPv6 header chain. When doing so,
enforce a limit on the maximum number of Extension Headers that
is allowed for each packet, and if such limit is hit before the
upper-layer protocol is identified, silently drop the packet.

[RFC6564] specifies a uniform format for IPv6 Extension
Header, thus meaning that an IPv6 node should be able to parse
an IPv6 header chain even if it contains Extension Headers
that are not currently supported by that node.

2. If the layer-2 device is unable to identify whether the packet is
a DHCPv6 packet meant for a DHCPv6 client or not (i.e., the
packet is a first-fragment, and the necessary information is
missing), silently drop the packet.

Note: This rule should only be applied to non-fragmented IPv6
datagrams and IPv6 fragments with a Fragment Offset of 0 (non-
first fragments can be safely passed, since they will never
reassemble into a complete datagram if the first fragment is
successfully dropped by DHCPv6-Shield).

3. If the packet is identified to be a DHCPv6 packet meant for a
DHCPv6 client, silently drop the packet.

A packet is said to be "a DCHPv6 packet meant for a DHCPv6
client if the encapsulated transport protocol is UDP, and the
UDP Destination Port is 546.

4. In all other cases, pass the packet as usual.

Note: For the purpose of enforcing the DHCPv6-Shield filtering
policy, an ESP header [RFC4303] should be considered to be an
"upper-layer protocol" (that is, it should be considered the last
header in the IPv6 header chain). This means that packets
employing ESP would be passed by the DHCPv6-Shield device to the
intended destination. If the destination host does not have a
security association with the sender of the aforementioned IPv6
packet, the packet would be dropped. Otherwise, if the packet is
considered valid by the IPsec implementation at the receiving host
and it encapsulates a DHCPv6 message, it is up to the receiving
host what to do with such packet.



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Rule #2 has been defined as a default rule to drop packets that
cannot be positively identified as not being DHCPv6 packets meant for
DHCPv6 clients (possibly because the packet contains fragments that
do not contain the entire IPv6 header chain). This means that, at
least in theory, DHCPv6-Shield could result in false-positive
blocking of some legitimate non-DHCPv6 packets that could not be
positively identified as being non-DHCPv6. However, as noted in
[I-D.gont-6man-oversized-header-chain], IPv6 packets that fail to
include the entire IPv6 header chain are anyway unlikely to survive
in real networks. Whilst currently legitimate from a specifications
standpoint, they are virtually impossible to police with state-less
filters and firewalls, and are hence likely to be blocked by such
filters and firewalls.

The aforementioned filtering rules implicitly handle the case of
fragmented packets: if the DHCPv6-Shield device fails to identify the
upper-layer protocol as a result of the use of fragmentation, the
corresponding packets would be silently dropped.

Finally, we note that IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of DHCPv6-based attacks. However, a recent assessment of
IPv6 implementations [SI6-FRAG] with respect to their fragment
reassembly policy seems to indicate that most current implementations
comply with [RFC5722].


























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4. IANA Considerations

This document has no actions for IANA.
















































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5. Security Considerations

The mechanism specified in this document can be used to mitigate
DHCPv6-based attacks. Attack vectors based on other messages (such
as ICMPv6 Router Advertisements) are out of the scope of this
document.

Mitigation of such attack vectors is discussed in other documents,
such as [RFC6105], [I-D.ietf-v6ops-ra-guard-implementation] and
[draft-gont-opsec-ipv6-ndp-shield].

As noted in Section 3, IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of DHCPv6-based attacks. However, most current
implementations seem to comply with [RFC5722], and hence forbid IPv6
overlapping fragments.

We note that if an attacker sends a fragmented DHCPv6 packets on a
port not allowed to send such packets, the first-fragment would be
dropped, and the rest of the fragments would be passed. This means
that the victim node would tie memory buffers for the aforementioned
fragments, which would never reassemble into a complete datagram. If
a large number of such packets were sent by an attacker, and the
victim node failed to implement proper resource management for the
fragment reassembly buffer, this could lead to a Denial of Service
(DoS). However, this does not really introduce a new attack vector,
since an attacker could always perform the same attack by sending
forged fragmented datagram in which at least one of the fragments is
missing. [CPNI-IPv6] discusses some resource management strategies
that could be implemented for the fragment reassembly buffer.





















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6. Acknowledgements

This document is heavily based on the document
[I-D.ietf-v6ops-ra-guard-implementation] authored by Fernando Gont.
Thus, the author would like to thank Ran Atkinson, Karl Auer, Robert
Downie, Washam Fan, David Farmer, Marc Heuse, Nick Hilliard, Ray
Hunter, Joel Jaeggli, Simon Perreault, Arturo Servin, Gunter van de
Velde, James Woodyatt, and Bjoern A. Zeeb, for providing valuable
comments on [I-D.ietf-v6ops-ra-guard-implementation], on which this
document is based.









































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7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.

[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.

[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.

7.2. Informative References

[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.

[I-D.gont-6man-oversized-header-chain]
Gont, F. and V. Manral, "Security and Interoperability
Implications of Oversized IPv6 Header Chains",
draft-gont-6man-oversized-header-chain-01 (work in
progress), April 2012.

[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)",
draft-ietf-v6ops-ra-guard-implementation-03 (work in
progress), May 2012.

[draft-gont-opsec-ipv6-ndp-shield]
Gont, F., "Neighbor Discovery Shield (ND-Shield)", IETF
Internet Draft, draft-gont-opsec-ipv6-ndp-shield, work in
progress, May 2012.

[SI6-FRAG]



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SI6 Networks, "IPv6 NIDS evasion and improvements in IPv6
fragmentation/reassembly", 2012, <http://
blog.si6networks.com/2012/02/
ipv6-nids-evasion-and-improvements-in.html>.

[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).










































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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








































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