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


   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

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,
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   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
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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   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",
   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

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

   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.

              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.

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

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


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              SI6 Networks, "IPv6 NIDS evasion and improvements in IPv6
              fragmentation/reassembly", 2012, <http://

              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

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

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