Operational Security Capabilities for                            F. Gont
IP Network Infrastructure (opsec)                 SI6 Networks / UTN-FRH
Internet-Draft                                                    W. Liu
Intended status: BCP                                 Huawei Technologies
Expires: June 15, 2013                                   G. Van de Velde
                                                           Cisco Systems
                                                       December 12, 2012


         DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
                   draft-ietf-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.

   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 June 15, 2013.

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  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
































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1.  Introduction

   This document specifies a mechanism for protecting hosts connected to
   a broadcast network against rogue DHCPv6 servers [RFC3315].  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.  DHCPv6-Shield MUST parse the IPv6 entire header chain present in
       the packet, to identify whether it is a DHCPv6 packet meant for a
       DHCPv6 client.

          RATIONALE: [RFC6564] specifies a uniform format for IPv6
          Extension Header, thus meaning that an IPv6 node can parse an
          IPv6 header chain even if it contains Extension Headers that
          are not currently supported by that node.  Additionally,
          [I-D.ietf-6man-oversized-header-chain] requires that if a
          packet is fragmented, the first fragment contains the entire
          IPv6 header chain.

          DHCPv6-Shield implementations MUST NOT enforce a limit on the
          number of bytes they can inspect (starting from the beginning
          of the IPv6 packet), since this could introduce false-
          positives: legitimate packets could be dropped simply because
          the DHCPv6-Shield device does not parse the entire IPv6 header
          chain present in the packet.  An implementation that has such
          an implementation-specific limit MUST NOT claim compliance
          with this specification, and MUST pass the packet when such
          implementation-specific limit is reached.

   2.  When parsing the IPv6 header chain, if the packet is a first-
       fragment (i.e., a packet containing a Fragment Header with the
       Fragment Offset set to 0) and it fails to contain the entire IPv6
       header chain (i.e., all the headers starting from the IPv6 header
       up to, and including, the upper-layer header), DHCPv6-Shield MUST
       drop the packet, and SHOULD log the packet drop event in an
       implementation-specific manner as a security fault.

          RATIONALE: [I-D.ietf-6man-oversized-header-chain] specifies
          that the first-fragment (i.e., the fragment with the Fragment
          Offset set to 0) MUST contain the entire IPv6 header chain,
          and allows intermediate systems such as routers to drop those
          packets that fail to comply with this requirement.

          NOTE: This rule should only be applied to 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 they are part of a DHCPv6 packet meant for a
          DHCPv6 client received on a port where such packets are not
          allowed).



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   3.  When parsing the IPv6 header chain, if the packet is identified
       to be a DHCPv6 packet meant for a DHCPv6 client, DHCPv6-Shield
       MUST drop the packet, and SHOULD log the packet drop event in an
       implementation-specific manner as a security fault.

   4.  In all other cases, DHCPv6-Shield MUST 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 encapsulates a DHCPv6 message, it is up to the receiving host
      what to do with such packet.

   If a packet is dropped due to this filtering policy, then the packet
   drop event SHOULD be logged in an implementation-specific manner as a
   security fault.  The logging mechanism SHOULD include a drop counter
   dedicated to DHCPv6-Shield packet drops.

   In order to protect current end-node IPv6 implementations, 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 (because the packet is a fragment that fails to include 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-server) packets.  However, as noted in
   [I-D.ietf-6man-oversized-header-chain], IPv6 packets that fail to
   include the entire IPv6 header chain are virtually impossible to
   police with state-less filters and firewalls, and hence are unlikely
   to survive in real networks.  [I-D.ietf-6man-oversized-header-chain]
   requires that hosts employing fragmentation include the entire IPv6
   header chain in the first fragment (the fragment with the Fragment
   Offset set to 0), thus eliminating the aforementioned false
   positives.

   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 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



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   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.

   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

   The authors would like to thank (in alphabetical order) Jean-Michel
   Combes, Juergen Schoenwaelder, and Mark Smith, for providing valuable
   comments on earlier versions of this document.

   This document is heavily based on the document
   [I-D.ietf-v6ops-ra-guard-implementation] authored by Fernando Gont.
   Thus, the authors 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.ietf-6man-oversized-header-chain]
              Gont, F. and V. Manral, "Security and Interoperability
              Implications of Oversized IPv6 Header Chains",
              draft-ietf-6man-oversized-header-chain-02 (work in
              progress), November 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-07 (work in
              progress), November 2012.

   [SI6-FRAG]
              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>.




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   [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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Authors' Addresses

   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


   Will Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com


   Gunter Van de Velde
   Cisco Systems
   De Kleetlaan 6a
   Diegem  1831
   Belgium

   Phone: +32 2704 5473
   Email: gunter@cisco.com





















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