Operational Security Capabilities for
opsec                                                            F. Gont
IP Network Infrastructure (opsec)
Internet-Draft                                    SI6 Networks / UTN-FRH
Internet-Draft                                                    W. Liu
Intended status: BCP Best Current Practice                            W. Liu
Expires: April 25, 2014                              Huawei Technologies
Expires: June 15, 2013
                                                         G. Van de Velde
                                                           Cisco Systems
                                                       December 12, 2012
                                                        October 22, 2013

         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
   at 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 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
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   This Internet-Draft will expire on June 15, 2013. April 25, 2014.

Copyright Notice

   Copyright (c) 2012 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  DHCPv6-Shield Configuration . . . . . . . . . . . . . . . . .   4
   5.  DHCPv6-Shield Implementation Advice . . . . . . . . . . . . .  5
   4.   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  8
   5.   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  9
   6.   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 10
   7.   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 11
     7.2.   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 11   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 13   8

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 [RFC4861] messages.

   The basic concept behind DHCPv6-Shield is that a layer-2 device
   filters DHCPv6 messages meant to DHCPv6 clients, clients (henceforth
   "DHCPv6-server messages"), according to a number of different
   criteria.  The most basic filtering criterion
   being is that the aforementioned DHCPv6 DHCPv6-server
   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 DHCPv6-server messages are to
   be 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 DHCPv6-server messages meant for DHCPv6 clients only if they are received on layer-2
   ports that have been explicitly configured for such purpose.

2.  Requirements Language
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].


3.  Terminology

   For the purposes of this document, the terms Extension Header, Header
   Chain, First Fragment, and Upper-layer Header are used as follows

   Extension Header:

      Extension Headers are defined in Section 4 of [RFC2460].  As a
      result of [I-D.ietf-6man-ext-transmit], [IANA-PROTO] provides a
      list of assigned Internet Protocol Numbers and designates which of
      those protocol numbers also represent extension headers.

   First Fragment:

      An IPv6 fragment with fragment offset equal to 0.

   IPv6 Header Chain:

      The header chain contains an initial IPv6 header, zero or more
      IPv6 extension headers, and optionally, a single upper-layer
      header.  If an upper-layer header is present, it terminates the
      header chain; otherwise the "No Next Header" value (Next Header =
      59) terminates it.

      The first member of the header chain is always an IPv6 header.
      For a subsequent header to qualify as a member of the header
      chain, it must be referenced by the "Next Header" field of the
      previous member of the header chain.  However, if a second IPv6
      header appears in the header chain, as is the case when IPv6 is
      tunneled over IPv6, the second IPv6 header is considered to be an
      upper-layer header and terminates the header chain.  Likewise, if
      an Encapsulating Security Payload (ESP) header appears in the
      header chain it is considered to be an upper-layer header and it
      terminates the header chain.

   Upper-layer Header:

      In the general case, the upper-layer header is the first member of
      the header chain that is neither an IPv6 header nor an IPv6
      extension header.  However, if either an ESP header, or a second
      IPv6 header occur in the header chain, they are considered to be
      upper layer headers and they terminate the header chain.

      Neither the upper-layer payload, nor any protocol data following
      the upper-layer payload, is considered to be part of the header
      chain.  In a simple example, if the upper-layer header is a TCP
      header, the TCP payload is not part of the header chain.  In a
      more complex example, if the upper-layer header is an ESP header,
      neither the payload data, nor any of the fields that follow the
      payload data in the ESP header are part of the header chain.

4.  DHCPv6-Shield Configuration

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


5.  DHCPv6-Shield Implementation Advice

   The following filtering rules MUST be enforced as part of an DHCPv6-
   Shield a
   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. client (i.e., a DHCPv6-server message).

             RATIONALE: [RFC6564] specifies a uniform format for IPv6
             Extension Header, Headers, 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-
             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

             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).

   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 DHCPv6-server 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

   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
   IPv6 implementations [SI6-FRAG] with respect to their fragment
   reassembly policy seems to indicate that most current implementations
   comply with [RFC5722].


6.  IANA Considerations

   This document has no actions for IANA.


7.  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

   As noted in Section 3, 5, 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 packet 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.


8.  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.

   Section 3 of this document was borrowed from
   [I-D.ietf-6man-oversized-header-chain], authored by Fernando Gont,
   Vishwas Manral, and Ron Bonica.

   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.


9.  References


9.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [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.


              Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", draft-ietf-6man-ext-
              transmit-05 (work in progress), October 2013.

9.2.  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. F., Manral, "Security V., and Interoperability
              Implications R. Bonica, "Implications of
              Oversized IPv6 Header Chains",
              draft-ietf-6man-oversized-header-chain-02 draft-ietf-6man-oversized-
              header-chain-08 (work in progress), November 2012. October 2013.

              Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)",
              draft-ietf-v6ops-ra-guard-implementation-07 draft-ietf-v6ops-ra-
              guard-implementation-07 (work in progress), November 2012.

              Internet Assigned Numbers Authority, "Protocol Numbers",
              February 2013, <http://www.iana.org/assignments/protocol-

              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).

Authors' Addresses
   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

   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

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