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Internet Engineering Task Force                                P. Savola
Internet Draft                                                 CSC/FUNET
Expiration Date: April 2004
                                                            October 2003

                  Firewalling Considerations for IPv6


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   There are quite a few potential problems regarding firewalling or
   packet filtering in IPv6 environment.  These include slight ambiguity
   in the IPv6 specification, problems parsing packets beyond unknown
   Extension Headers and Destination Options, and introduction of end-
   to-end encrypted traffic and peer-to-peer applications.  There may
   also be need to extend packet matching to include some Extension
   Header or Destination Option fields.  A number of often-raised, but
   not necessary relevant, issues are also summarized.  This memo
   discusses these issues to raise awareness and proposes some tentative
   solutions or workarounds.

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Table of Contents

   1.  Introduction  ...............................................   2
     1.1.  Terminology  ............................................   3
   2.  Ambiguous Text in the IPv6 Specification  ...................   4
     2.1.  The Problem  ............................................   4
     2.2.  Possible Solutions  .....................................   5
   3.  Parsing Extension Header Chains  ............................   6
     3.1.  The Problem  ............................................   6
     3.2.  Possible Solutions  .....................................   6
   4.  Parsing Unknown Destination Options and Security Policy  ....   7
     4.1.  The Problem  ............................................   7
     4.2.  Possible Solutions  .....................................   7
   5.  Firewalls and End-to-End IPsec-encrypted ESP-traffic  .......   8
     5.1.  The Problem  ............................................   8
     5.2.  Possible Solutions  .....................................   8
   6.  Firewalls and Interactions with Peer-to-Peer Applications  ..   9
     6.1.  The Problem  ............................................   9
     6.2.  Possible Solutions  .....................................   9
   7.  Other Issues Associated with IPv6 Firewalls  ................  10
     7.1.  IPv4 ARP vs IPv6 Neighbor Discovery  ....................  10
     7.2.  Filtering Specific Neighbor Discovery Messages  .........  10
     7.3.  Firewall Policies and Multiple Addresses per Node  ......  11
     7.4.  Firewall Transparency in the Network  ...................  11
   8.  Security Considerations  ....................................  12
   9.  Acknowledgements  ...........................................  12
   10.  References  ................................................  12
     10.1.  Normative References  ..................................  12
     10.2.  Informative References  ................................  12
   Author's Address  ...............................................  13
   A.  Possible Desirable Header Field Matching Extensions  ........  13
   B.  Amplification DoS Attack Using IPv6 Multicast  ..............  14
   Intellectual Property Statement  ................................  15
   Full Copyright Statement  .......................................  15

1. Introduction

   There are quite a few potential problems regarding firewalling or
   packet filtering in IPv6 environment.  These include slight ambiguity
   in the IPv6 specification, problems parsing packets beyond unknown
   Extension Headers and Destination Options, and introduction of end-
   to-end encrypted traffic and peer-to-peer applications.  There may
   also be need to extend packet matching to include some Extension
   Header or Destination Option fields.  A number of often-raised, but
   not necessary relevant, issues are also summarized.

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   This memo discusses these issues to raise awareness and proposes some
   tentative solutions or workarounds.  This document is not meant as a
   guide on how to set up IPv6 firewall policies (for example), but
   rather as a list of problematic issues to be considered by firewall
   developers, subject matter experts and those partipating in the IPv6
   standardization effort.

   On-link attacks using Neighbor Discovery are similar to ones
   available through IPv4 ARP, and not typically applicable to
   firewalls, and are therefore out of scope.  A good summary of the
   issues is available [SENDREQ].  However, one should note that IPv4
   ARP traffic is done using link-layer protocols, and is not generally
   seen (or blocked, even with a "deny all" policy) by firewalls; with
   IPv6, all such traffic is part of ICMPv6 Neighbor Discovery protocol
   suite, and thus more visible at the IP layer.

   Implementation or policy-specific issues are mainly out of scope but
   partially touched on in section 7 about "non-problems"; these include
   e.g. issues of node-specific state creation (could be problematic if
   networks were brute-force scanned) and applicability of existing
   policies (e.g. blocking ICMPv6 would have very bad effects,
   particularly if certain link-local messages receive no special

   In section 2, slightly ambiguous text in the IPv6 specification is
   discussed.  In section 3, a syntactical problem with parsing unknown
   Extension Headers is pointed out.  In section 4, a similar problem
   with Destination Options is discussed in the context of security
   policy.  In section 5, implications of end-to-end encrypted traffic
   are considerated.  In section 6, similar implications of peer-to-peer
   applications are mentioned.  In section 7, a number of often-raised,
   but not necessary relevant, issues are summarized.  In appendix A,
   some possibly useful packet matching extensions for IPv6 are brought

   A possible generic denial-of-service attack using multicast and
   including amplification has also been noticed; as it is not firewall-
   specific, it is described (for the lack of a better place) with in
   Appendix B.

1.1. Terminology

   In this document, the term "firewall" is used to mean any kind of
   packet filter; no special features (like statefullness or
   application-specific packet inspection) is assumed.

   When considering firewalls, one should note that there are several
   ways to place and implement a firewall; in principle:

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      1. network firewall

      2. network, user-controlled firewall

      3. end-node, admin-controlled firewall

      4. end-node firewall

   Note that the second seems very rare, and the third is not really
   common yet.  The reason why the entity which controls the firewall is
   explicitly mentioned is because it has significant implications on
   trust relations and which kind of solutions to the problems are

   The first is configured solely by an administrator, and placed in the
   network to block or pass the traffic of multiple nodes or network in
   an aggregated fashion.

   The second is also configured by an administrator and placed in the
   network, but the user may be able to affect some policy decisions
   made in the firewall e.g. by some signalling protocol; ie. the policy
   of the firewall can, to some extent, be influenced by the end-nodes.

   The third, also sometimes called a distributed firewall, is a
   firewall placed in the end-nodes, but controlled in some co-ordinated
   fashion by an administrator [DISFW].

   The last is the typical end-node firewall, policy set by the end-
   user, or sometimes even the applications run on the end-node.

2. Ambiguous Text in the IPv6 Specification

2.1. The Problem

   The [IPV6] specification forbids skipping over any of the headers
   before processing them or processing them at all before reaching the
   destination (section 4):

   "With one exception, Extension Headers are not examined or processed
   by any node along a packet's delivery path, until the packet reaches
   the node (or each of the set of nodes, in the case of multicast)
   identified in the Destination Address field of the IPv6 header.
   There, normal demultiplexing on the Next Header field of the IPv6
   header invokes the module to process the first Extension Header, or
   the upper-layer header if no Extension Header is present.  The
   contents and semantics of each Extension Header determine whether or
   not to proceed to the next header.  Therefore, Extension Headers must
   be processed strictly in the order they appear in the packet; a

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   receiver must not, for example, scan through a packet looking for a
   particular kind of Extension Header and process that header prior to
   processing all preceding ones."


   "If, as a result of processing a header, a node is required to
   proceed to the next header but the Next Header value in the current
   header is unrecognized by the node, it should discard the packet and
   send an ICMP Parameter Problem message to the source of the packet,
   with an ICMP Code value of 1 ("unrecognized Next Header type
   encountered") and the ICMP Pointer field containing the offset of the
   unrecognized value within the original packet.  The same action
   should be taken if a node encounters a Next Header value of zero in
   any header other than an IPv6 header."

   Similar applies to the specified Destination Options processing
   behaviour: if the Option Type has been specified so that the packet
   should not be processed further in the case of unrecognized options
   (ie. the highest-order two bits are not "00"), should the firewall
   also discard the packet and/or send ICMP Parameter Problem message
   back to the source?

   Are these also to be done by intermediate nodes (which, by some
   definition, should not be processing Extension Headers or Destination
   Options Header with Hop-by-Hop options as an exception); this seems

   This wording clearly does not take into the account that there might
   be middleboxes, or non-final destinations, that could be processing
   the packet.

2.2. Possible Solutions

   The correct behaviour must be made clear; the wording should be
   clarified.  Clarifications might be needed at least on:

      1. whether intermediate nodes should be taken into account in the
         text describing the header processing

      2. intermediate nodes' behaviour when detecting unrecognized

   It seems to be obvious that the firewalls will always inspect the
   headers, and in whichever order they want; see the next sections for
   descriptions of the specific problems.

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3. Parsing Extension Header Chains

3.1. The Problem

   IPv4 [RFC1122] [RFC1812] silently ignores options it does not
   recognize; options have a specific, pre-defined format.  IPv6
   Extension Headers are structured differently: the header format can
   change, and generally it is not possible to parse the header, or
   proceed to the following Extension Headers unless the processing of
   the previous header has been implemented.

   The above is problematic as it is often the case that a packet filter
   will want to examine the terminal headers, e.g. TCP or UDP.  That is
   not possible if there is a problem processing any one of the
   preceding headers.

   Skipping over unknown headers and letting the packet through might be
   dangerous, if the unknown header would significantly change how the
   packet would be interpreted by the end-node.

   One should note that all the currently defined Extension Headers,
   except Fragmentation, are encoded in the Type, Length, Value (TLV)

3.2. Possible Solutions

   In the generic case, even ignoring the IPv6 specification, unknown
   headers cannot be skipped over except by making some very wild
   guesses of the content.  Thus the solutions (or work-arounds) are:

      1. always keep the packet filter up-to-date, so that it can parse
         all types of Extension Headers,

      2. never introduce new Extension Headers, except possibly in a
         very controlled manner; use Destination Options instead, or

      3. standardize the format (for at least the first N bytes
         including at least the length and the next header value) of
         possibly later specified new Extension Headers (for example,
         that all the new ones must be in TLV format), so that skipping
         over headers could be possible.

   The first is not a workable solution in a generic case at least if
   it's expected that new Extension Headers should be introducable: the
   lifetime of firewall devices and software seems to be much longer
   than one would expect.  For example, it is good to consider the case
   of buggy firewalls and ECN support [ECN]: even though software fixes
   may have been available for a long time, upgrades have not taken

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   place, hindering the deployment of new technology.  It seems that
   keeping up with Extension Headers is not possible: software firewalls
   and their patch cycle are a problem big enough already, without
   considering hardware firewalls which could need new hardware
   implementations as well.

   The second seems quite a bleak work-around, but as currently
   specified, there is little choice; most (if not all) new features can
   probably be implemented using Destination Options.  However, it's
   still good to document and understand this deployment and
   specification deadlock.

   The third might be doable but it would require some standardization

4. Parsing Unknown Destination Options and Security Policy

4.1. The Problem

   Similar to the above, Destination Options may also include unknown
   options.  However, the options are encoded in the TLV-format.  So,
   skipping over unknown options is technically possible.

   However, especially in a very controlled environments, where a
   firewall may implement a strict security policy, it may be desirable
   to reject any packets whose options the firewall does not recognize
   (which may cause the end-nodes to do something that has not been
   anticipated in the security policy controlled by the firewall).

   Skipping over unknown destination options and letting the packet
   through might be dangerous, if the unknown option would significantly
   change how the packet would be interpreted by the end-node.

4.2. Possible Solutions

   No protocol action seems to be necessary provided that the
   implementation would not, in this case, send ICMPv6 messages or
   discard packets upon receiving an unknown header.

   However, it may be desirable for firewall implementations to have a
   feature controlling the handling behaviour of unrecognized
   Destination Options.

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5. Firewalls and End-to-End IPsec-encrypted ESP-traffic

5.1. The Problem

   With the promise of the restoration of end-to-end transparency, and
   if at least some of the challenges for implementing Public Key
   Infrastructures are worked around, it may be possible that the amount
   of end-to-end encrypted traffic will increase enermously.  The
   traffic is likely to be encrypted using IPsec.

   In this case, on-the-path observers (such as a firewall) do not have
   the possibility to examine the usually critical headers (such as
   TCP/UDP).  This may result in an administrative decision to disable
   IPsec-encrypted traffic by filtering it out completely, as the end-
   nodes' adherence to the security policies cannot be verified.

5.2. Possible Solutions

   It would be desirable, if the users wish to do so, be able to have
   the firewall or some node the firewall is configured to trust as an
   intermediary in IPsec Security Parameter Index (SPI)
   negotiation/configuration, as that is the only visible way to
   demultiplex encrypted content between two the source and destination.
   However, even though this may mitigate the risks somewhat, but it
   appears that SPI's could be reused (without the intermediary) in such
   a way that entirely different kind of traffic could be sent.  There
   is no fix for this, by the definition of end-to-end encryption.

   A related approach could be have an intermediate firewall or security
   gateway act as some kind of IPsec proxy, either by formally specified
   means or by performing a "man-in-the-middle" -type "attack" on all
   the IPsec traffic.  Whether this would work or be useful is not
   clear.  A similar proposal is to require the private key storage in
   the security gateway; however, such an architecture would be a very
   attracting target and if compromised, would severely compromise the
   value of IPsec encryption.

   One could try to encode some interesting values, e.g. protocol
   numbers and ports, in the Flow Label field; one problem here is the
   relatively limited length of 20 bits.  But this would have to be done
   in the source node, which is not usually (at least completely)
   trusted in this context.  Also, according to [FLOWLAB], nodes must
   not assume any properties in the Flow Label values.

   An approach which has been proposed in the past in many forms has
   been to specify an IPsec-like ESP-protocol which would allow
   revealing only some portions of the packets, for example transport-
   layer headers [ESVP].  To some extent, this would be helpful, but not

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   necessarily enough; for example, application-specific checking might
   require access to the whole packet, especially if a different
   solution to the problem noted in the next section is not found.

   A possible approach would be to try to shift the focus, at least
   partially, to end-node firewalls; if end-nodes are not particularly
   trusted, an end-node, admin-controlled firewall might be provide a
   reasonable tradeoff between security policy and cryptography.

   There appears to be no network-based solution for this, which is
   indeed a feature of end-to-end cryptography.

6. Firewalls and Interactions with Peer-to-Peer Applications

6.1. The Problem

   As above, the restoration of end-to-end transparency provides a
   possibility for a more wide-spread use of peer-to-peer applications.
   Such applications are often a bit problematic from the firewall
   perspective: it is often the practise to allow outbound (from the
   protected site) traffic while allowing in only the related traffic
   (and naturally some other administratively permitted traffic).  Being
   able to run (some) peer-to-peer applications easily in a controlled
   environment would be valuable.

6.2. Possible Solutions

   One workaround would be to try to standardize some default port
   ranges (in an application-specific manner) for such applications as
   these, for example in the above 32768 range.  In this way, a site
   could enable/disable (default) port ranges for (some) peer-to-peer
   applications at will.  A major disadvantage here would be that this
   could violate the trust model: some applications could intentionally
   try to use some other's port range to gain entry through the firewall
   even if the default range for that specific application was blocked.
   This would imply a requirement for at least some form of trust.

   Another, but possibly quite a complex solution would be to implement
   some form of peer-to-peer "pinholing" [MIDCOM].  This hasn't yet been
   standardized even for IPv4 (though the concept is quite protocol-
   independent).  A problem with model is that generally there is no
   trust relation between the firewall and the host (or an application
   at the host): how would it help if a host (or misbehaving application
   at the host) would be able to request opening a hole in the firewall?
   So, there certainly seem to be very significant tradeoffs and threat
   models to consider here.

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   A possible approach, as above, would be to try to shift, at least
   partially, the focus to end-node firewalls; if end-nodes are not
   particularly trusted, an end-node, admin-controlled firewall might be
   provide a reasonable tradeoff between security policy and

7. Other Issues Associated with IPv6 Firewalls

   This section tries to summarize some issues which have been brough up
   in conjunction with IPv6 firewalling, but which are not seen as
   problems as such.

7.1. IPv4 ARP vs IPv6 Neighbor Discovery

   A number of people have been confused about the fact that IPv4 ARP
   runs at the link-layer, while IPv6 Neighbor Discovery is part of
   ICMPv6.  When an IPv4 "deny all [IP] traffic" -rule blocks
   "everything except ARP", the same IPv6 rule would also deny the
   similar functions provided by Neighbor Discovery.

   This just seems to be an issue people have to be educated on.

7.2. Filtering Specific Neighbor Discovery Messages

   A typically similar set of people who have been confused of the role
   of Neighbor Discovery (see above) also seem to be confused on what a
   firewall should do with certain Neighbor Discovery packets.

   It has been argued that a firewall should be able to filter out
   specific proxy-ND behaviour, unauthorized ND Redirects, wrong Router
   Advertisements, verify that packets coming from a node advertising to
   be reachable at some link-layer address to really come from that
   link-layer address, etc.

   However, there are a number of strong arguments why this should not
   be done in a firewall.  First, all of these messages are strictly on-
   link -- they are not routed.  Thus, firewalling such messages would
   only be of questionable use in end-node firewalls (to protect against
   on-link abuse).  On the other hand, as [SENDREQ] points out, the
   physical link is actually rather difficult to secure: in addition to
   enabling IP-level protections, one also has to secure link-layer
   -level security.  Such very fine-grained, ND-specific features would
   seem to be clearly belong to the (Secure) Neighbor Discovery (or its
   implementation) itself - not the firewalls.

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7.3. Firewall Policies and Multiple Addresses per Node

   Often, firewall policies try to specify "a node" or "a port in a
   node".  This naturally gets more complicated if the nodes have
   multiple addresses: typically at least a link-local address and a
   global address.

   This is not problematic in itself, as the use of link-local addresses
   is restricted on the link (and could even be considered out of scope
   for most firewalls) and because one should not use link-local
   addresses except for specific purpose protocols.  Multiple global
   addresses (e.g. from multihoming) can be worked out by
   implementation-specific methods, e.g., by making it easier to
   identify a node by the Interface ID part of the address when
   desirable, and creating the rules for all the prefixes by one pseudo-
   rule.  However, the policies must be tuned manually if different
   security properties have been assigned to different prefixes.

   All in all, it seems desirable to make setting policies easier also
   with multiple addresses, but this doesn't seem to be a problem as

7.4. Firewall Transparency in the Network

   Many want to deploy firewalls which do not participate in the network
   at all, e.g. by not sending ICMPv6 unreachable packets for denied
   targets, but rather silently discarding any traffic they do not
   allow. In this kind of scenario, it may be desirable to even deploy
   the firewall to function as a bridge, not as a router.

   Some others want to deploy firewalls to be visible: either so that
   the address of the firewall can be seen from the messages it sends,
   or so that the firewall tries to be "transparent", i.e., forge the
   replies as if they were coming from the destination nodes the
   connecting node were trying to reach (e.g., by setting the source
   address of an ICMPv6 message to be that of the destination address,
   or by forging TCP RST, etc.).

   There are trade-offs both ways.  A visible firewall is  extremely
   useful when the firewall is used to set "friendly" restrictions (e.g.
   internally to in an Enterprise), because there will be no TCP
   timeouts (and similar delays) when accidentally trying something that
   is not allowed: an immediate ICMPv6 message or a TCP RST allows an
   immediate abort.  The first may be useful in very hostile scenarios,
   where sending ICMPv6 (or other) messages might just exacerbate the
   issue (e.g. in the form of ICMPv6 reflection or ICMPv6 storms);
   however, note that ICMPv6 specification specifies rate-limiting for
   this specific purpose.

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   In any case, the firewall transparency considerations do not seem to
   be specific to IPv6 and are not problems as such.

8. Security Considerations

   This draft discusses security considerations related to IPv6
   firewalling.  When discussing potential solutions for problems, the
   weaknesses are also pointed out.

   In general, the firewall often does not, and cannot, trust the
   node(s) it protects.  These may even belong to different
   administrative entit(y/ies).  In that case, making compromises will
   usually open some holes in the firewall.

9. Acknowledgements

   Brian Carpenter suggested an IPv6 firewall could support P2P
   pinholing. Soo Guan Eng provided commentary.  Andras Kis-Szabo and
   Changming Liu provided a number of comments and useful commentary.

10. References

10.1. Normative References

   [ADDRARCH]      Hinden, R., Deering, S., "IP Version 6 Addressing
               Architecture", RFC3513, April 2003.

   [IPV6]      Deering, S., Hinden, R., "Internet Protocol, Version 6
               (IPv6) Specification", RFC 2460, December 1998.

10.2. Informative References

   [DISFW]     Bellovin, S., "Distributed Firewalls", ;login: in Nov
               1999, http://www.research.att.com/~smb/papers/distfw.html

   [ECN]       Garzik, J., "ECN-under-Linux Unofficial Vendor Support
               Page", http://gtf.org/garzik/ecn/, March 2002.

   [ESVP]      Kasera, S. (ed.), "IP Encapsulating Security Variable
               Payload (ESVP)", work-in-progress,
               draft-kasera-esvp-00.txt, October 2002.

   [FLOWLAB]   Rajahalme, J., et al., "IPv6 Flow Label Specification",
               work-in-progress, draft-ietf-ipv6-flow-label-07.txt,
               April 2002.

   [ICMPV6]    Conta, A., Deering, S., "Internet Control Message
               Protocol (ICMPv6)", RFC2463, December 1998.

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   [MIDCOM]    Srisuresh, P. et al, "Middlebox communication
               architecture and framework", RFC3303, August 2002.

   [MIPV6]     Johnson, D., et al, "Mobility Support in IPv6",
               draft-ietf-mobileip-ipv6-24.txt, work-in-progress,
               July 2003.

   [RFC1122]   Braden, R. (Editor), "Requirements for Internet Hosts
               -- Communication Layers", RFC1122, October 1989.

   [RFC1812]   Baker, F. (Editor), "Requirements for IP Version 4
               Routers", RFC1812, June 1995.

   [RHHAOSEC]  Savola, P. "Security of IPv6 Routing Header and
               Home Address Options", work-in-progress,
               draft-savola-ipv6-rh-ha-security-03.txt, December 2002.

   [SENDREQ]   Nikander, P., el al., "IPv6 Neighbor Discovery trust
               models and threats", work-in-progress,
               draft-ietf-send-psreq-03.txt, April 2003.

Author's Address

   Pekka Savola
   Espoo, Finland
   EMail: psavola@funet.fi

A. Possible Desirable Header Field Matching Extensions

   As Destination options and Extension Header types are taken into use,
   it may be desirable for a firewall to support some matching against
   certain header fields.  These include, for example:

     - whether or not a specific Extension Header or a Destination
       Option is detected
     - behaviour when an unknown (or specified) Extension Header or
       Destination Option is  detected
     - (Routing Header -specific) being able to match segments left
       (mainly, whether it is zero or not), type and the next-to-be-
       swapped destination(s) [RHHAOSEC]
     - (Home Address Option [MIPV6] -specific) being able to match
       against the home address
     - (ESP/AH -specific) being able to match against SPI
     - (Tunneled-traffic specific) being able to match against the
       embedded IPv4 address in e.g. 6to4, ISATAP, etc.

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   Some of these are much more useful than others; the list is only
   meant to give ideas about possibly useful (in some scenarios, at
   least) functionalities.

B. Amplification DoS Attack Using IPv6 Multicast

   It is possible to launch a denial-of-service attack using IPv6,
   including a form of amplification based on multicast infrastructure.

   Multicast address must not be used as a source address ([ADDRARCH],
   section 2.7), but explicit checks to drop those packets or respond to
   them have not been specified (that is, [ADDRARCH] specifies that
   nodes MUST discard certain kinds of packets if received, but these
   are not listed as such).  However, such attacks are not considered

   By crafting packets, sent to multicast destinations, which are
   certain to contain a response to the source address (the victim's
   address) would seem to be potentially useful for an attacker:

        src = <victim> (unicast-address)
        dst = <multicast address> (w/ as many receivers as possible)

        next-header=200 (something undefined)
        next-header=destination-options (or hop-by-hop options)
        options-header=<an unknown option type starting with binary "10">

   Now, both of these amount to the same thing: every node which
   processes the packet and adheres to [IPV6] will generate an ICMPv6
   parameter problem back to the source.

   [ICMPV6] does not permit the first approach with an unknown extension
   header, but additionally permits ICMPv6 message generation with Path
   MTU Discovery.

   This is different with IPv4, where no ICMP errors are generated in
   response to packets sent to multicast addresses [RFC1122].  Clearly,
   when specifying, allowing the flexibility to define whether a
   response to multicast packets would be sent was considered a feature,
   but dangerous features have a tendency to being turned to bad uses.

   In addition to amplification, this kind of attack would have an
   effect on multicast-enabled router network as a large amount of
   multicast forwarding state would be created.

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