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Versions: (draft-gont-opsec-ipv6-implications-on-ipv4-nets) 00 01 02 03 04 05 06 07 RFC 7123

Operational Security Capabilities for                            F. Gont
IP Network Infrastructure (opsec)                 SI6 Networks / UTN-FRH
Internet-Draft                                                    W. Liu
Intended status: Informational                       Huawei Technologies
Expires: July 1, 2013                                  December 28, 2012


             Security Implications of IPv6 on IPv4 Networks
           draft-ietf-opsec-ipv6-implications-on-ipv4-nets-02

Abstract

   This document discusses the security implications of native IPv6
   support and IPv6 transition/co-existence technologies on "IPv4-only"
   networks, and describes possible mitigations for the aforementioned
   issues.

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
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   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 July 1, 2013.

Copyright Notice

   Copyright (c) 2012 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.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Security Implications of Native IPv6 Support . . . . . . . . .  5
     2.1.  Filtering Native IPv6 Traffic  . . . . . . . . . . . . . .  5
   3.  Security Implications of Tunneling Mechanisms  . . . . . . . .  7
     3.1.  Filtering 6in4 . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Filtering 6over4 . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Filtering 6rd  . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Filtering 6to4 . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Filtering ISATAP . . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Filtering Teredo . . . . . . . . . . . . . . . . . . . . . 10
     3.7.  Filtering Tunnel Broker with Tunnel Setup Protocol
           (TSP)  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Additional Considerations when Filtering IPv6 Traffic  . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Summary of filtering rules  . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21




























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1.  Introduction

   Most general-purpose operating systems implement and enable by
   default native IPv6 [RFC2460] support and a number of transition/
   co-existence technologies.  In those cases in which the corresponding
   devices are deployed on networks that are assumed to be IPv4-only,
   native IPv6 support and/or IPv6 transition/co-existence technologies
   could be leveraged by local or remote attackers for a number of
   (illegitimate) purposes.  For example,

   o  A Network Intrusion Detection System (NIDS) might be prepared to
      detect attack patterns for IPv4 traffic, but might be unable to
      detect the same attack patterns when a transition/co-existence
      technology is leveraged for that purpose.

   o  An IPv4 firewall might enforce a specific security policy in IPv4,
      but might be unable to enforce the same policy in IPv6.

   o  Some transition/co-existence mechanisms might cause an internal
      host with otherwise limited IPv4 connectivity to become globally
      reachable over IPv6, therefore resulting in increased (and
      possibly unexpected) host exposure.

         Some transition/co-existence mechanisms (notably Teredo) are
         designed to traverse Network Address Port Translation (NAPT)
         [RFC2663] devices, allowing incoming IPv6 connections from the
         Internet to hosts behind the organizational firewall or NAPT
         (which in many deployments provides a minimum level of
         protection by only allowing those instances of communication
         that have been initiated from the internal network).

   o  IPv6 support might, either inadvertently or as a result of a
      deliberate attack, result in VPN traffic leaks if IPv6-unaware
      Virtual Private Network (VPN) software is employed by dual-stacked
      hosts [I-D.ietf-opsec-vpn-leakages].

   In general, most of the aforementioned security implications can be
   mitigated by enforcing security controls on native IPv6 traffic and
   on IPv4-tunneled traffic.  Among such controls is the enforcement of
   filtering policies, such that undesirable traffic is blocked.  While
   IPv6 widespread/global IPv6 deployment has been slower than expected,
   it is nevertheless happening, and thus filtering IPv6 traffic
   (whether native or transition/co-existence) to mitigate IPv6 security
   implications on IPv4 networks should only be considered as a
   temporary solution to protect a network until IPv6 is deployed.  Only
   in some exceptional cases (such as military operations networks)
   could this approach to mitigating the aforementioned security
   implications be thought of as a longer-term strategy.



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   This document discusses the security implications of IPv6 and IPv6
   transition/co-existence technologies on (allegedly) IPv4-only
   networks, and provides guidance on how to mitigate the aforementioned
   issues.















































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2.  Security Implications of Native IPv6 Support

   Most popular operating systems include IPv6 support that is enabled
   by default.  This means that even if a network is expected to be
   IPv4-only, much of its infrastructure is nevertheless likely to be
   IPv6 enabled.  For example, hosts are likely to have at least link-
   local IPv6 connectivity which might be exploited by attackers with
   access to the local network.

      [CORE2007] is a security advisory about a buffer overflow which
      could be remotely-exploited by leveraging link-local IPv6
      connectivity that is enabled by default.

   Additionally, unless appropriate measures are taken, an attacker with
   access to an 'IPv4-only' local network could impersonate a local
   router and cause local hosts to enable their 'non-link-local' IPv6
   connectivity (e.g. by sending Router Advertisement messages),
   possibly circumventing security controls that were enforced only on
   IPv4 communications.

      [THC-IPV6] is the first publicly-available toolkit that
      implemented this attack vector (along with many others).

      [IPv6-Toolkit] is a fully-featured trouble-shooting and security
      assessment tool that implements this attack vector (along with
      many others).

      [Waters2011] provides an example of how this could be achieved
      using publicly available tools (besides incorrectly claiming the
      discovery of a "0day vulnerability").

   Native IPv6 support could also possibly lead to VPN traffic leakages
   when hosts employ VPN software that not only does not support IPv6,
   but that does nothing about IPv6 traffic.
   [I-D.ietf-opsec-vpn-leakages] describes this issue, along with
   possible mitigations.

   In general, networks should enforce on native IPv6 traffic the same
   security policies they currently enforce on IPv4 traffic.  However,
   in those networks in which IPv6 has not yet been deployed, and
   enforcing the aforementioned policies is deemed as unfeasible, a
   network administrator might mitigate IPv6-based attack vectors by
   means of appropriate packet filtering.

2.1.  Filtering Native IPv6 Traffic

   Some layer-2 devices might have the ability to selectively filter
   packets based on the type of layer-2 payload.  When such



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   functionality is available, IPv6 traffic could be blocked at those
   layer-2 devices by blocking e.g.  Ethernet frames with the Protocol
   Type field set to 0x86dd [IANA-ETHER].

   SLAAC-based attacks [RFC3756] can be mitigated with technologies such
   as RA-Guard [RFC6105] [I-D.ietf-v6ops-ra-guard-implementation].  In a
   similar way, DHCPv6-based attacks can be mitigated with technologies
   such as DHCPv6-Shield [I-D.ietf-opsec-dhcpv6-shield].  However,
   neither RA-Guard nor DHCPv6-Shield can mitigate attack vectors that
   employ IPv6 link-local addresses, since configuration of such
   addresses does not rely on Router Advertisement messages or DCHPv6-
   server messages.

   Administrators considering the filtering of native IPv6 traffic at
   layer-3 devices are urged to pay attention to the general
   considerations for IPv6 traffic filtering discussed in Section 4.

      If native IPv6 traffic is filtered at layer-2, local IPv6 nodes
      would only get to configure IPv6 link-local addresses.

   In order to mitigate attacks based on native IPv6 traffic, IPv6
   security controls should be enforced on both IPv4 and IPv6 networks.
   The aforementioned controls might include: deploying IPv6-enabled
   NIDS, implementing IPv6 firewalling, etc.

      In some very specific scenarios (e.g., military operations
      networks) in which only IPv4 service might be desired, a network
      administrator might want to disable IPv6 support in all the
      communicating devices.






















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3.  Security Implications of Tunneling Mechanisms

   Unless properly managed, tunneling mechanisms might result in
   negative security implications ([RFC6169] describes the security
   implications of tunneling mechanisms in detail).

      Of the plethora of tunneling mechanism that have so far been
      standardized and widely implemented, the so-called "automatic
      tunneling" mechanisms (such as Teredo, ISATAP, and 6to4) are of
      particular interest from a security standpoint, since they might
      be employed without prior consent or action of the user or network
      administrator.

   Therefore, tunneling mechanisms should be a concern not only to
   network administrators that have consciously deployed them, but also
   to network and security administrators whose security policies might
   be bypassed by exploiting these mechanisms.

      [CERT2009] contains some examples of how tunnels can be leveraged
      to bypass firewall rules.

   The aforementioned issues could be mitigated by applying the common
   security practice of only allowing traffic deemed as "necessary"
   (i.e., the so-called "default deny" policy).  Thus, when such policy
   is enforced IPv6 transition/co-existence traffic would be blocked by
   default, and would only be allowed as a result of an explicit
   decision (rather than as a result of lack of awareness about such
   traffic).

      It should be noted that this type of policy is usually enforced at
      a network that is the target of such traffic (such as an
      enterprise network).  IPv6 transition traffic should generally
      never be filtered e.g. by an ISP when it is transit traffic.

   In those scenarios in which transition/co-existence traffic is meant
   to be blocked, it is highly recommended that, in addition to the
   enforcement of filtering policies at the organizational perimeter,
   the corresponding transition/co-existence mechanisms be disabled on
   each node connected to the organizational network.  This would not
   only prevent security breaches resulting from accidental use of these
   mechanisms, but would also disable this functionality altogether,
   possibly mitigating vulnerabilities that might be present in the host
   implementation of these transition/co-existence mechanisms.

   IPv6-in-IPv4 tunnelling mechanisms (such as 6to4 or configured
   tunnels) can generally be blocked by dropping IPv4 packets that
   contain a Protocol field set to 41.  Security devices such as NIDS
   might also include signatures that detect such transition/



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   co-existence traffic.

   Administrators considering the filtering of transition/co-existence
   traffic are urged to pay attention to the general considerations for
   IPv6 traffic filtering discussed in Section 4

3.1.  Filtering 6in4

   Probably the most basic type of tunnel employed for connecting IPv6
   "islands" is the so-called "6in4", in which IPv6 packets are
   encapsulated within IPv4 packets.  These tunnels are typically result
   from manual configuration at the two tunnel endpoints.

   6in4 tunnels can be blocked by blocking IPv4 packets with a Protocol
   field of 41.

3.2.  Filtering 6over4

   [RFC2529] specifies a mechanism known as 6over4 or 'IPv6 over IPv4'
   (or colloquially as 'virtual Ethernet'), which comprises a set of
   mechanisms and policies to allow isolated IPv6 hosts located on
   physical links with no directly-connected IPv6 router, to become
   fully functional IPv6 hosts by using an IPv4 domain that supports
   IPv4 multicast as their virtual local link.

      This transition technology has never been widely deployed, because
      of the low level of deployment of multicast in most networks.

   6over4 encapsulates IPv6 packets in IPv4 packets with their Protocol
   field set to 41.  As a result, simply filtering all IPv4 packets that
   have a Protocol field equal to 41 will filter 6over4 (along with many
   other transition technologies).

   A more selective filtering could be enforced such that 6over4 traffic
   is filtered while other transition traffic is still allowed.  Such a
   filtering policy would block all IPv4 packets that have their
   Protocol field set to 41, and that have a Destination Address that
   belongs to the prefix 239.0.0.0/8.

   This filtering policy basically blocks 6over4 Neighbor Discovery
   traffic directed to multicast addresses, thus preventing Stateless
   Address Auto-configuration (SLAAC), address resolution, etc.
   Additionally, it would prevent the 6over multicast addresses from
   being leveraged for the purpose of network reconnaissance.







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3.3.  Filtering 6rd

   6rd builds upon the mechanisms of 6to4 to enable the rapid deployment
   of IPv6 on IPv4 infrastructures, while avoiding some downsides of
   6to4.  Usage of 6rd was originally documented in [RFC5569], and the
   mechanism was generalized to other access technologies and formally
   standardized in [RFC5969].

   6rd can be blocked by blocking IPv4 packets with the Protocol field
   set to 41.

3.4.  Filtering 6to4

   6to4 [RFC3056] is an address assignment and router-to-router, host-
   to-router, and router-to-host automatic tunnelling mechanism that is
   meant to provide IPv6 connectivity between IPv6 sites and hosts
   across the IPv4 Internet.

      The security considerations for 6to4 are discussed in detail in
      [RFC3964].  [RFC6343] provides advice to network operators about
      6to4 (some of which relates to security mitigations).

   As discussed in Section 3, all IPv6-in-IPv4 traffic, including 6to4,
   could be easily blocked by filtering IPv4 that contain their Protocol
   field set to 41.  This is the most effective way of filtering such
   traffic.

   If 6to4 traffic is meant to be filtered while other IPv6-in-IPv4
   traffic is allowed, then more finer-grained filtering rules could be
   applied.  For example, 6to4 traffic could be filtered by applying
   filtering rules such as:

   o  Filter outgoing IPv4 packets that have the Destination Address set
      to an address that belongs to the prefix 192.88.99.0/24.

   o  Filter incoming IPv4 packets that have the Source Address set to
      an address that belongs to the prefix 192.88.99.0/24.

         These rules assume that the corresponding nodes employ the
         "Anycast Prefix for 6to4 Relay Routers" [RFC3068].

         It has been suggested that 6to4 relays send their packets with
         their IPv4 Source Address set to 192.88.99.1.

   o  Filter outgoing IPv4 packets that have the Destination Address set
      to the IPv4 address of well-known 6to4 relays.





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   o  Filter incoming IPv4 packets that have the Source Address set to
      the IPv4 address of well-known 6to4 relays.

      These last two filtering policies will generally be unnecessary,
      and possibly unfeasible to enforce (given the number of potential
      6to4 relays, and the fact that many relays might remain unknown to
      the network administrator).  If anything, they should be applied
      with the additional requirement that such IPv4 packets have their
      Protocol field set to 41, to avoid the case where other services
      available at the same IPv4 address as a 6to4 relay are mistakenly
      made inaccessible.

   If the filtering device has capabilities to inspect the payload of
   IPv4 packets, then the following filtering rules could be enforced:

   o  Filter outgoing IPv4 packets that have their Protocol field set to
      41, and that have an IPv6 Source Address (embedded in the IPv4
      payload) that belongs to the prefix 2002::/16.

   o  Filter incoming IPv4 packets that have their Protocol field set to
      41, and that have an IPv6 Destination address (embedded in the
      IPv4 payload) that belongs to the prefix 2002::/16.

3.5.  Filtering ISATAP

   ISATAP [RFC5214] is an Intra-site tunnelling protocol, and thus it is
   generally expected that such traffic will not traverse the
   organizational firewall of an IPv4-only.  Nevertheless, ISATAP can be
   easily blocked by blocking IPv4 packets with a Protocol field of 41.

   The most popular operating system that includes an implementation of
   ISATAP in the default installation is Microsoft Windows.  Microsoft
   Windows obtains the ISATAP router address by resolving the domain
   name isatap.<localdomain> DNS A resource records.  Additionally, they
   try to learn the ISATAP router address by employing Link-local
   Multicast Name Resolution (LLMNR) [RFC4795] to resolve the name
   "isatap".  As a result, blocking ISATAP by preventing hosts from
   successfully performing name resolution for the aforementioned names
   and/or by filtering packets with specific IPv4 destination addresses
   is both difficult and undesirable.

3.6.  Filtering Teredo

   Teredo [RFC4380] is an address assignment and automatic tunnelling
   technology that provides IPv6 connectivity to dual-stack nodes that
   are behind one or more Network Address Port Translation (NAPT)
   [RFC2663] devices, by encapsulating IPv6 packets in IPv4-based UDP
   datagrams.  Teredo is meant to be a 'last resort' IPv6 connectivity



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   technology, to be used only when other technologies such as 6to4
   cannot be deployed (e.g., because the edge device has not been
   assigned a public IPv4 address).

   As noted in [RFC4380], in order for a Teredo client to configure its
   Teredo IPv6 address, it must contact a Teredo server, through the
   Teredo service port (UDP port number 3544).

   To prevent the Teredo initialization process from succeeding, and
   hence prevent the use of Teredo, an organizational firewall could
   filter outgoing UDP packets with a Destination Port of 3544.

      It is clear that such a filtering policy does not prevent an
      attacker from running its own Teredo server in the public
      Internet, using a non-standard UDP port for the Teredo service
      port (i.e., a port number other than 3544).

   If the filtering device has capabilities to inspect the payload of
   IPv4 packets, the following (additional) filtering policy could be
   enforced:

   o  Filter outgoing IPv4/UDP packets that have that embed an IPv6
      packet with the "Version" field set to 6, and an IPv6 Source
      Address that belongs to the prefix 2001::/32.

   o  Filter incoming IPv4/UDP packets that have that embed an IPv6
      packet with the "Version" field set to 6, and an IPv6 Destination
      Address that belongs to the prefix 2001::/32.

      These two filtering rules could, at least in theory, result in
      false positives.  Additionally, they would generally require the
      filtering device to reassemble fragments prior to enforcing
      filtering rules, since the information required to enforce them
      might be missing in the received fragments (which should be
      expected if Teredo is being employed for malicious purposes).

   The most popular operating system that includes an implementation of
   Teredo in the default installation is Microsoft Windows.  Microsoft
   Windows obtains the Teredo server addresses (primary and secondary)
   by resolving the domain name teredo.ipv6.microsoft.com into DNS A
   records.  A network administrator might want to prevent Microsoft
   Windows hosts from obtaining Teredo service by filtering at the
   organizational firewall outgoing UDP datagrams (i.e.  IPv4 packets
   with the Protocol field set to 17) that contain in the IPv4
   Destination Address any of the IPv4 addresses that the domain name
   teredo.ipv6.microsoft.com maps to.  Additionally, the firewall would
   filter incoming UDP datagrams from any of the IPv4 addresses to which
   the domain names of well-known Teredo servers (such as



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   teredo.ipv6.microsoft.com) resolve.

      As these IPv4 addresses might change over time, an administrator
      should obtain these addresses when implementing the filtering
      policy, and should also be prepared to keep this list up to date.

      The corresponding addresses can be easily obtained from a UNIX
      host by issuing the command 'dig teredo.ipv6.microsoft.com a'
      (without quotes).

   It should be noted that even with all these filtering policies in
   place, a node in the internal network might still be able to
   communicate with some Teredo clients.  That is, it could configure an
   IPv6 address itself (without even contacting a Teredo server), and
   might send Teredo traffic to those peers for which intervention of
   the host's Teredo server is not required (e.g., Teredo clients behind
   a cone NAT).

3.7.  Filtering Tunnel Broker with Tunnel Setup Protocol (TSP)

   The tunnel broker model enables dynamic configuration of tunnels
   between a tunnel client and a tunnel server.  The tunnel broker
   provides a control channel for creating, deleting or updating a
   tunnel between the tunnel client and the tunnel server.
   Additionally, the tunnel broker may register the user IPv6 address
   and name in the DNS.  Once the tunnel is configured, data can flow
   between the tunnel client and the tunnel server.  [RFC3053] describes
   the Tunnel Broker model, while [RFC5572] specifies the Tunnel Setup
   Protocol (TSP), which can be used by clients to communicate with the
   Tunnel Broker.

   TSP can use either TCP or UDP as the transport protocol.  In both
   cases TSP uses port number 3653, which has been assigned by the IANA
   for this purpose.  As a result, TSP (the Tunnel Broker control
   channel) can be blocked by blocking TCP and UDP packets originating
   from the local network and destined to UDP port 3653 or TCP port
   3653.  Additionally, the data channel can be blocked by blocking UDP
   packets originated from the local network and destined to UDP port
   3653, and IPv4 packets with a Protocol field set to 41.












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4.  Additional Considerations when Filtering IPv6 Traffic

   IPv6 deployments in the Internet are continually increasing, and some
   hosts default to preferring IPv6 connectivity whenever it is
   available.  This is likely to cause IPv6-capable hosts to attempt to
   reach an ever-increasing number of popular destinations via IPv6,
   even if this IPv6 connectivity is provided relies on a transition
   technology over an IPv4-only network.

   A large source of IPv6 brokenness today comes from nodes that believe
   that they have functional IPv6 connectivity, but the path to their
   destination fails somewhere upstream [Anderson2010] [Anderson2011]
   [Huston2010b] [Huston2012].  Upstream filtering of transition
   technologies or situations where a misconfigured node attempts to
   "provide" native IPv6 service on a given network without proper
   upstream IPv6 connectivity may result in hosts attempting to reach
   remote nodes via IPv6, and depending on the absence or presence and
   specific implementation details of "Happy Eyeballs" [RFC6555], there
   might be a non-trivial timeout period before the host falls back to
   IPv4 [Huston2010a] [Huston2012].

   For this reason, networks attempting to prevent IPv6 traffic from
   traversing their devices should consider configuring their local
   recursive DNS servers to ignore queries for AAAA DNS records, or even
   consider filtering AAAA records at the network ingress point to
   prevent end hosts from attempting their own DNS resolution.  This
   will ensure that end hosts which are on an IPv4-only network will
   only receive DNS A records, and they will be unlikely to attempt to
   use (likely broken) IPv6 connectivity to reach their desired
   destinations.  Additionally, it is generally deemed as good practice
   to signal the packet drop to the source node, such that the source
   node is able to react to such packet drop in a more appropriate and
   timely way.

      A firewall could signal the packet drop by means of an ICMPv6
      error message (or TCP [RFC0793] RST segment if appropriate), such
      that the source node can e.g. quickly react as described in
      [RFC5461].

      For obvious reasons, if the traffic being filtered is IPv6
      transition/co-existence traffic, the signalling packet should be
      sent by means of the corresponding IPv6 transition/co-existence
      technology.








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5.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.














































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6.  Security Considerations

   This document discusses the security implications of IPv6 on IPv4
   networks, and describes a number of techniques to mitigate the
   aforementioned issues.  In general, the possible mitigations boil
   down to enforcing on native IPv6 and IPv6 transition/co-existence
   traffic the same security policies currently enforced for IPv4
   traffic, and/or blocking the aforementioned traffic when it is deemed
   as undesirable.










































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7.  Acknowledgements

   The authors would like to thank Wes George, who contributed most of
   the text that comprises Section 4 of this document.

   The authors would like to thank (in alphabetical order) Ran Atkinson,
   Brian Carpenter, Panos Kampanakis, David Malone, Arturo Servin,
   Donald Smith, Tina Tsou, and Eric Vyncke, for providing valuable
   comments on earlier versions of this document.

   This document is based on the results of the the project "Security
   Assessment of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6],
   carried out by Fernando Gont on behalf of the UK Centre for the
   Protection of National Infrastructure (CPNI).  Fernando Gont would
   like to thank the UK CPNI for their continued support.




































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8.  References

8.1.  Normative References

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

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              January 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, January 2010.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification",
              RFC 5969, August 2010.

   [RFC5572]  Blanchet, M. and F. Parent, "IPv6 Tunnel Broker with the
              Tunnel Setup Protocol (TSP)", RFC 5572, February 2010.

8.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",



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              RFC 2663, August 1999.

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
              6to4", RFC 3964, December 2004.

   [RFC5461]  Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
              February 2009.

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

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169, April 2011.

   [RFC6343]  Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
              RFC 6343, August 2011.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

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

   [I-D.ietf-opsec-vpn-leakages]
              Gont, F., "Virtual Private Network (VPN) traffic leakages
              in dual-stack hosts/ networks",
              draft-ietf-opsec-vpn-leakages-00 (work in progress),
              December 2012.

   [I-D.ietf-opsec-dhcpv6-shield]
              Gont, F., Liu, W., and G. Velde, "DHCPv6-Shield:
              Protecting Against Rogue DHCPv6 Servers",
              draft-ietf-opsec-dhcpv6-shield-00 (work in progress),
              December 2012.

   [IANA-ETHER]
              IANA, "Ether Types", 2012,
              <http://www.iana.org/assignments/ethernet-numbers>.

   [CERT2009]



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              CERT, "Bypassing firewalls with IPv6 tunnels", 2009, <http
              ://www.cert.org/blogs/vuls/2009/04/
              bypassing_firewalls_with_ipv6.html>.

   [CORE2007]
              CORE, "OpenBSD's IPv6 mbufs remote kernel buffer
              overflow", 2007,
              <http://www.coresecurity.com/content/open-bsd-advisorie>.

   [Huston2010a]
              Huston, G., "IPv6 Measurements", 2010,
              <http://www.potaroo.net/stats/1x1/>.

   [Huston2010b]
              Huston, G., "Flailing IPv6", 2010,
              <http://www.potaroo.net/ispcol/2010-12/6to4fail.pdf>.

   [Huston2012]
              Huston, G., "Bemused Eyeballs: Tailoring Dual Stack
              Applications for a CGN Environment", 2012,
              <http://www.potaroo.net/ispcol/2012-05/notquite.pdf>.

   [Anderson2010]
              Anderson, T., "Measuring and combating IPv6 brokenness",
               RIPE 61, Roma, November 2010,
              <http://ripe61.ripe.net/presentations/162-ripe61.pdf>.

   [Anderson2011]
              Anderson, T., "IPv6 dual-stack client loss in Norway",
              2011, <http://www.fud.no/ipv6/>.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

   [IPv6-Toolkit]
              "IPv6 Toolkit",
              <http://www.si6networks.com/tools/ipv6toolkit>.

   [THC-IPV6]
              "The Hacker's Choice IPv6 Attack Toolkit",
              <http://www.thc.org/thc-ipv6/>.

   [Waters2011]
              Waters, A., "SLAAC Attack - 0day Windows Network
              Interception Configuration Vulnerability", 2011,
              <http://resources.infosecinstitute.com/slaac-attack/>.



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Appendix A.  Summary of filtering rules

   +------------+------------------------------------------------------+
   | Technology |                    Filtering rules                   |
   +------------+------------------------------------------------------+
   |   Native   |                   EtherType 0x86DD                   |
   |    IPv6    |                                                      |
   +------------+------------------------------------------------------+
   |    6in4    |                      IP proto 41                     |
   +------------+------------------------------------------------------+
   |   6over4   |                      IP proto 41                     |
   +------------+------------------------------------------------------+
   |     6rd    |                      IP proto 41                     |
   +------------+------------------------------------------------------+
   |    6to4    |                      IP proto 41                     |
   +------------+------------------------------------------------------+
   |   ISATAP   |                      IP proto 41                     |
   +------------+------------------------------------------------------+
   |   Teredo   |                  UDP Dest Port 3544                  |
   +------------+------------------------------------------------------+
   |   TB with  |   (IP proto 41) || (UDP Dest Port 3653 || TCP Dest   |
   |     TSP    |                      Port 3653)                      |
   +------------+------------------------------------------------------+

                    Table 1: Summary of filtering rules

      NOTE: the table above describes general and simple filtering rules
      for blocking the corresponding traffic.  More finer-grained rules
      might be available in each of the corresponding sections of this
      document.





















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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 Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com































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