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rfc:rfc7610

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.

Gont, et al. Best Current Practice [Page 1] RFC 7610 DHCPv6-Shield August 2015

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
 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. 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

Gont, et al. Best Current Practice [Page 2] RFC 7610 DHCPv6-Shield August 2015

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.

Gont, et al. Best Current Practice [Page 3] RFC 7610 DHCPv6-Shield August 2015

 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

Gont, et al. Best Current Practice [Page 4] RFC 7610 DHCPv6-Shield August 2015

    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.

Gont, et al. Best Current Practice [Page 5] RFC 7610 DHCPv6-Shield August 2015

        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.

Gont, et al. Best Current Practice [Page 6] RFC 7610 DHCPv6-Shield August 2015

 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

Gont, et al. Best Current Practice [Page 7] RFC 7610 DHCPv6-Shield August 2015

 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.

Gont, et al. Best Current Practice [Page 8] RFC 7610 DHCPv6-Shield August 2015

 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>.

Gont, et al. Best Current Practice [Page 9] RFC 7610 DHCPv6-Shield August 2015

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>.

Gont, et al. Best Current Practice [Page 10] RFC 7610 DHCPv6-Shield August 2015

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.

Gont, et al. Best Current Practice [Page 11] RFC 7610 DHCPv6-Shield August 2015

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

Gont, et al. Best Current Practice [Page 12]

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