opsec                                                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice                            W. Liu
Expires: January 7, 2016                             Huawei Technologies
                                                         G. Van de Velde
                                                          Alcatel-Lucent
                                                            July 6, 2015


         DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
                   draft-ietf-opsec-dhcpv6-shield-08

Abstract

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

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 January 7, 2016.

Copyright Notice

   Copyright (c) 2015 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, et al.             Expires January 7, 2016                [Page 1]


Internet-Draft                DHCPv6-Shield                    July 2015


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

1.  Introduction

   This document specifies DHCPv6-Shield: a mechanism for protecting
   hosts connected to a switched network against rogue DHCPv6 servers
   [RFC3315].  The basic concept behind DHCPv6-Shield is that a layer-2
   device filters DHCPv6 messages intended for DHCPv6 clients
   (henceforth "DHCPv6-server messages"), according to a number of
   different criteria.  The most basic filtering criterion is that
   DHCPv6-server messages are discarded by the layer-2 device unless
   they are received on specific ports of the layer-2 device.

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

   DHCPv6-Shield is analogous to the RA-Guard mechanism [RFC6104]
   [RFC6105] [RFC7113], intended for protection against rogue Router
   Advertisement [RFC4861] messages.

   We note that DHCPv6-Shield mitigates only DHCPv6-based attacks
   against hosts.  Attack vectors based on other messages meant for
   network configuration (such as ICMPv6 Router Advertisements) are not
   addressed by DHCPv6-Shield itself.  In a similar vein,



Gont, et al.             Expires January 7, 2016                [Page 2]


Internet-Draft                DHCPv6-Shield                    July 2015


   DHCPv6-Shielddoes not mitigate attacks against DHCPv6 servers (e.g.,
   Denial of Service).

2.  Requirements Language

   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].

3.  Terminology

   DHCPv6-Shield:

      the set of filtering rules specified in this document, meant to
      mitigate attacks that employ DHCPv6-server packets.

   DHCPv6-Shield device:

      A layer-2 device (typically a layer-2 switch) that enforces the
      filtering policy specified in this document.

   For the purposes of this document, the terms Extension Header, Header
   Chain, First Fragment, and Upper-layer Header are used as specified
   in [RFC7112]:

   IPv6 Extension Header:

      Extension Headers are defined in Section 4 of [RFC2460].  As a
      result of [RFC7045], [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



Gont, et al.             Expires January 7, 2016                [Page 3]


Internet-Draft                DHCPv6-Shield                    July 2015


      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 is
   explicitly configured with respect to which layer-2 ports are allowed
   to receive DHCPv6 packets destined to DHCPv6 clients (i.e.
   DHCPv6-server messages).  Only those layer-2 ports explicitly
   configured for such purpose will be allowed to receive DHCPv6 packets
   to DHCPv6 clients.

5.  DHCPv6-Shield Implementation Requirements

   The following are the filtering rules that are enforced as part of a
   DHCPv6-Shield implementation on those ports that are not allowed to
   receive DHCPv6 packets to DHCPv6 clients:

   1.  DHCPv6-Shield implementations MUST parse the entire IPv6 header
       chain present in the packet, to identify whether it is a DHCPv6
       packet meant for a DHCPv6 client (i.e., a DHCPv6-server message).

          RATIONALE: 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-negatives: DHCP6-server packets received on ports not
          allowed to receive such packets could be allowed simply




Gont, et al.             Expires January 7, 2016                [Page 4]


Internet-Draft                DHCPv6-Shield                    July 2015


          because the DHCPv6-Shield device does not parse the entire
          IPv6 header chain present in the packet.

   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 ought to log the packet drop event in an
       implementation-specific manner as a security fault.

          RATIONALE: Packets that fail to contain the entire IPv6 header
          chain could otherwise be leveraged for circumventing
          DHCPv6-Shield.  [RFC7112] requires that the first-fragment
          (i.e., the fragment with the Fragment Offset set to 0)
          contains 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).

   3.  DHCPv6-Shield MUST provide a configuration knob that controls
       whether packets with unrecognized Next Header values are dropped;
       this configuration knob MUST default to "drop".  When parsing the
       IPv6 header chain, if the packet contains an unrecognized Next
       Header value and the configuration knob is configured to "drop",
       DHCPv6-Shield MUST drop the packet, and ought to log the packet
       drop event in an implementation-specific manner as a security
       fault.

          RATIONALE: An unrecognized Next Header value could possibly
          identify an IPv6 Extension Header, and thus be leveraged to
          conceal a DHCPv6-server packet (since there is no way for
          DHCPv6-Shield to parse past unrecognized Next Header values
          [I-D.gont-6man-rfc6564bis]).  [RFC7045] requires that nodes be
          configurable with respect to whether packets with unrecognized
          headers are forwarded, and allows the default behavior to be
          that such packets be dropped.

   4.  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 alert.



Gont, et al.             Expires January 7, 2016                [Page 5]


Internet-Draft                DHCPv6-Shield                    July 2015


          RATIONALE: Ultimately, the goal of DHCPv6-Shield is drop
          DHCPv6 packets destined to DHCPv6 clients (i.e.  DHCPv6-server
          messages) that are received on ports that have not been
          explicitly configured to allow the receipt of such packets.

   5.  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.

   The above indicates that if a packet is dropped due to this filtering
   policy, the packet drop event be logged in an implementation-specific
   manner as a security fault.  It is useful for the logging mechanism
   to include a per-port 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-server packets (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 [RFC7112], 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.  [RFC7112] 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
   IPv6 implementations [SI6-FRAG] with respect to their fragment



Gont, et al.             Expires January 7, 2016                [Page 6]


Internet-Draft                DHCPv6-Shield                    July 2015


   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 recommendations in this document represent the ideal behavior of
   a DHCPv6 shield device.  However, in order to implement DHCPv6 shield
   on the fast path, it may be necessary to limit the depth into the
   packet that can be scanned before giving up.  In circumstances where
   there is such a limitation, it is recommended that implementations
   drop packets after attempting to find a protocol header up to that
   limit, whatever it is.  Ideally, such devices should be configurable
   with a list of protocol header identifiers so that if new transport
   protocols are standardized after the device is released, they can be
   added to the list of protocol header types that the device
   recognizes.  Since any protocol header that is not a UDP header would
   be passed by the DHCPv6 shield algorithm, this would allow such
   devices to avoid blocking the use of new transport protocols.  When
   an implementation must stop searching for recognizable header types
   in a packet due to such limitations, whether the device passes or
   drop that packet SHOULD be configurable.

   The mechanism specified in this document can be used to mitigate
   DHCPv6-based attacks against hosts.  Attack vectors based on other
   messages meant for network configuration (such as ICMPv6 Router
   Advertisements) are out of the scope of this document.  Additionally,
   the mechanism specified in this document does not mitigate attacks
   against DHCPv6 servers (e.g., Denial of Service).

   If deployed in layer-2 domain with several cascading switches, there
   will be an ingress port on the host's local switch which will need to
   be enabled for receiving DHCPv6-server messages.  However, this local
   switch will be reliant on the upstream devices to have filtered out
   rogue DHCPv6-server messages, as the local switch has no way of
   determining which upstream DHCP-server messages are valid.
   Therefore, in order to be effective DHCPv6 Shield should be deployed
   and enabled on all layer-2 switches of a given layer-2 domain.

   As noted in Section 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.




Gont, et al.             Expires January 7, 2016                [Page 7]


Internet-Draft                DHCPv6-Shield                    July 2015


   We note that if an attacker sends a fragmented DHCPv6 packet on a
   port not allowed to receive 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.

   Additionally, we note that the security of a site employing DHCPv6
   Shield could be further improved by deploying [I-D.ietf-savi-dhcp],
   to mitigate IPv6 address spoofing attacks.

   Finally, we note that other mechanisms for mitigating attacks based
   on DHCPv6-server messages are available that have different
   deployment considerations.  For example, [I-D.ietf-dhc-secure-dhcpv6]
   allows for authentication of DHCPv6-server packets if the IPv6
   addresses of the DHCPv6 servers can be pre-configured at the client
   nodes.

8.  Acknowledgements

   The authors would like to thank Mike Heard, who provided detailed
   feedback on earlier versions of this document and helped a lot in
   producing a technically-sound document throughout the whole
   publication process.

   The authors would like to thank (in alphabetical order) Ben Campbell,
   Jean-Michel Combes, Sheng Jiang, Ted Lemon, Pete Resnick, Juergen
   Schoenwaelder, Carsten Schmoll, Robert Sleigh, Donald Smith, Mark
   Smith, Hannes Tschofenig, Eric Vyncke, and Qin Wu, for providing
   valuable comments on earlier versions of this document.

   Part of Section 3 of this document was borrowed from [RFC7112],
   authored by Fernando Gont, Vishwas Manral, and Ron Bonica.

   This document is heavily based on the document [RFC7113] authored by
   Fernando Gont.  Thus, the authors would like to thank Ran Atkinson,
   Karl Auer, Robert Downie, Washam Fan, David Farmer, Mike Heard, 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 [RFC7113], on which this
   document is based.



Gont, et al.             Expires January 7, 2016                [Page 8]


Internet-Draft                DHCPv6-Shield                    July 2015


   The authors would like to thank Joel Jaeggli for his advice and
   guidance throughout the IETF process.

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.

   [RFC7112]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", RFC 7112, January 2014.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045, December 2013.

9.2.  Informative References

   [I-D.ietf-dhc-secure-dhcpv6]
              Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs", draft-
              ietf-dhc-secure-dhcpv6-07 (work in progress), September
              2012.

   [I-D.gont-6man-rfc6564bis]
              Gont, F., Will, W., Krishnan, S., and H. Pfeifer, "IPv6
              Universal Extension Header", draft-gont-6man-rfc6564bis-00
              (work in progress), April 2014.

   [RFC6104]  Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
              Problem Statement", RFC 6104, February 2011.




Gont, et al.             Expires January 7, 2016                [Page 9]


Internet-Draft                DHCPv6-Shield                    July 2015


   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113, February 2014.

   [IANA-PROTO]
              Internet Assigned Numbers Authority, "Protocol Numbers",
              February 2013, <http://www.iana.org/assignments/protocol-
              numbers/protocol-numbers.txt>.

   [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>.

   [I-D.ietf-savi-dhcp]
              Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-34 (work in progress),
              February 2015.

   [CPNI-IPv6]
              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
   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



Gont, et al.             Expires January 7, 2016               [Page 10]


Internet-Draft                DHCPv6-Shield                    July 2015


   Gunter Van de Velde
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp, Antwerp  2018
   Belgium

   Phone: +32 476 476 022
   Email: gunter.van_de_velde@alcatel-lucent.com











































Gont, et al.             Expires January 7, 2016               [Page 11]