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RFC 7610

DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers

Pages: 12
Best Current Practice: 199

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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 7610                        SI6 Networks / UTN-FRH
BCP: 199                                                          W. Liu
Category: Best Current Practice                      Huawei Technologies
ISSN: 2070-1721                                          G. Van de Velde
                                                          Alcatel-Lucent
                                                             August 2015


         DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers

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'); 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 memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7610.
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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
   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. Requirements Language ...........................................3 3. Terminology .....................................................3 4. DHCPv6-Shield Configuration .....................................5 5. DHCPv6-Shield Implementation Requirements .......................5 6. Security Considerations .........................................7 7. References ......................................................9 7.1. Normative References .......................................9 7.2. Informative References ....................................10 Acknowledgements ..................................................11 Authors' Addresses ................................................12
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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 Router Advertisement Guard (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, DHCPv6-Shield does 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.
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   For the purposes of this document, the terms "IPv6 Extension Header",
   "First Fragment", "IPv6 Header Chain", and "Upper-Layer Header" are
   used as specified in [RFC7112]:

   IPv6 Extension Header:

      IPv6 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 IPv6 Extension Headers.

   First Fragment:

      An IPv6 fragment with a Fragment Offset equal to 0.

   IPv6 Header Chain:

      The IPv6 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
      IPv6 Header Chain; otherwise, the "No Next Header" value (Next
      Header = 59) terminates it.

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

      Neither the upper-layer payload nor any protocol data following
      the upper-layer payload is considered to be part of the IPv6
      Header Chain.  In a simple example, if the Upper-Layer Header is a
      TCP header, the TCP payload is not part of the IPv6 Header Chain.
      In a more complex example, if the Upper-Layer Header is an ESP
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      header, neither the payload data nor any of the fields that follow
      the payload data in the ESP header are part of the IPv6 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 are allowed to receive DHCPv6 packets to pass to DHCPv6 clients.

5. DHCPv6-Shield Implementation Requirements

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 or not 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 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) contain the entire IPv6 Header Chain. [RFC7112] also allows intermediate systems such as routers to drop packets that fail to comply with this requirement.
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          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 or not 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
          [IPV6-UEH]).  [RFC7045] requires that nodes be configurable
          with respect to whether or not 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.

          RATIONALE: Ultimately, the goal of DHCPv6-Shield is to 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, what to do with such a packet
      is up to the receiving host.
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   The rules above indicate that if a packet is dropped due to this
   filtering policy, the packet drop event should 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 such that the default is for 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) to be dropped.  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 stateless filters and
   firewalls; hence, they 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. 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
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   an implementation must stop searching for recognizable header types
   in a packet due to such limitations, the device SHOULD be
   configurable to either pass or drop that packet.

   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 a layer 2 domain with several cascading switches,
   there will be an ingress port on the host's local switch that will
   need to be enabled for receiving DHCPv6-server messages.  However,
   this local switch will be reliant on the upstream devices filtering
   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 to 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 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 a
   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 [RFC7513] to
   mitigate IPv6 address spoofing attacks.
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   Finally, we note that other mechanisms for mitigating attacks based
   on DHCPv6-server messages are available that have different
   deployment considerations.  For example, [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.

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>. [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, <http://www.rfc-editor.org/info/rfc3315>. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005, <http://www.rfc-editor.org/info/rfc4303>. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <http://www.rfc-editor.org/info/rfc4861>. [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments", RFC 5722, DOI 10.17487/RFC5722, December 2009, <http://www.rfc-editor.org/info/rfc5722>. [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing of IPv6 Extension Headers", RFC 7045, DOI 10.17487/RFC7045, December 2013, <http://www.rfc-editor.org/info/rfc7045>. [RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of Oversized IPv6 Header Chains", RFC 7112, DOI 10.17487/RFC7112, January 2014, <http://www.rfc-editor.org/info/rfc7112>.
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7.2. Informative References

[CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). [IANA-PROTO] IANA, "Protocol Numbers", <http://www.iana.org/assignments/protocol-numbers>. [IPV6-UEH] Gont, F., Liu, W., Krishnan, S., and H. Pfeifer, "IPv6 Universal Extension Header", Work in Progress, draft-gont-6man-rfc6564bis-00, April 2014. [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement Problem Statement", RFC 6104, DOI 10.17487/RFC6104, February 2011, <http://www.rfc-editor.org/info/rfc6104>. [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, DOI 10.17487/RFC6105, February 2011, <http://www.rfc-editor.org/info/rfc6105>. [RFC7113] Gont, F., "Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)", RFC 7113, DOI 10.17487/RFC7113, February 2014, <http://www.rfc-editor.org/info/rfc7113>. [RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address Validation Improvement (SAVI) Solution for DHCP", RFC 7513, DOI 10.17487/RFC7513, May 2015, <http://www.rfc-editor.org/info/rfc7513>. [SECURE-DHCPV6] Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs", Work in Progress, draft-ietf-dhc-secure-dhcpv6-07, September 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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Acknowledgements

The authors would like to thank Mike Heard, who provided detailed feedback on earlier draft 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, Carsten Schmoll, Juergen Schoenwaelder, Robert Sleigh, Donald Smith, Mark Smith, Hannes Tschofenig, Eric Vyncke, and Qin Wu for providing valuable comments on earlier draft 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 [RFC7113], authored by Fernando Gont. Thus, the authors would like to thank the following individuals for providing valuable comments on [RFC7113]: 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. The authors would like to thank Joel Jaeggli for his advice and guidance throughout the IETF process. Fernando Gont would like to thank Diego Armando Maradona for his magic and inspiration.
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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 (Shucheng) Liu Huawei Technologies Bantian, Longgang District Shenzhen 518129 China Email: liushucheng@huawei.com 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