opsec                                                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice BCP                                              W. Liu
Expires: August 8, December 7, 2014                            Huawei Technologies
                                                         G. Van de Velde
                                                           Cisco Systems
                                                        February 4,
                                                            June 5, 2014

         DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers


   This document specifies a mechanism for protecting hosts connected to
   a broadcast switched 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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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 8, December 7, 2014.

Copyright Notice

   Copyright (c) 2014 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 .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .   3  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .   3  5
   4.  DHCPv6-Shield Configuration  . . . . . . . . . . . . . . . . .   4  7
   5.  DHCPv6-Shield Implementation Advice  . . . . . . . . . . . . .   4  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .   6 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .   6 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .   7 . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . .   7 . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . .   7 . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . .   8 . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .   8 . 16

1.  Introduction

   This document specifies a mechanism for protecting hosts connected to
   a broadcast switched network against rogue DHCPv6 servers [RFC3315].  This
   mechanism is analogous to the RA-Guard mechanism [RFC6104] [RFC6105]
   [RFC7113] 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 (henceforth
   "DHCPv6-server "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 a specified port
   specific ports of the layer-2 device.

   Before the DCHPv6-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 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.

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

   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 follows

   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
      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 (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 a
   DHCPv6-Shield 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 (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-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. specification.

   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: [RFC7112] 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

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

          RATIONALE: [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.  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-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) 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
   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. 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., DoS).

   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.

   We note that if an attacker sends a fragmented DHCPv6 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 Crsten Schmoll, Robert Sleigh, Mark
   Smith, and Eric Vyncke, 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
   [I-D.ietf-v6ops-ra-guard-implementation] [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
   [I-D.ietf-v6ops-ra-guard-implementation], [RFC7113], 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.

   [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

   [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.


   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", draft-ietf-v6ops-ra-
              guard-implementation-07 (work in progress), November 2012. RFC 7113, February 2014.

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

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