IPv6 Operations                                                E. Davies
Internet-Draft                                                Consultant
Expires: September 7, 2006                                   S. Krishnan
                                                               P. Savola
                                                           March 6, 2006

          IPv6 Transition/Co-existence Security Considerations

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The transition from a pure IPv4 network to a network where IPv4 and
   IPv6 co-exist brings a number of extra security considerations that
   need to be taken into account when deploying IPv6 and operating the
   dual-protocol network and the associated transition mechanisms.  This
   document attempts to give an overview of the various issues grouped

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   into three categories:
   o  issues due to the IPv6 protocol itself,
   o  issues due to transition mechanisms, and
   o  issues due to IPv6 deployment.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Issues Due to IPv6 Protocol  . . . . . . . . . . . . . . . . .  4
     2.1.  IPv6 Protocol-specific Issues  . . . . . . . . . . . . . .  4
       2.1.1.  Routing Headers and Hosts  . . . . . . . . . . . . . .  4
       2.1.2.  Routing Headers for Mobile IPv6 and Other Purposes . .  5
       2.1.3.  Site-scope Multicast Addresses . . . . . . . . . . . .  6
       2.1.4.  ICMPv6 and Multicast . . . . . . . . . . . . . . . . .  6
       2.1.5.  Bogus Errored Packets in ICMPv6 Error Messages . . . .  7
       2.1.6.  Anycast Traffic Identification and Security  . . . . .  7
       2.1.7.  Address Privacy Extensions Interact with DDoS
               Defenses . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.8.  Dynamic DNS: Stateless Address Auto-Configuration,
               Privacy Extensions and SEND  . . . . . . . . . . . . .  9
       2.1.9.  Extension Headers  . . . . . . . . . . . . . . . . . .  9
       2.1.10. Fragmentation: Reassembly and Deep Packet
               Inspection . . . . . . . . . . . . . . . . . . . . . . 12
       2.1.11. Fragmentation Related DoS Attacks  . . . . . . . . . . 13
       2.1.12. Link-Local Addresses and Securing Neighbor
               Discovery  . . . . . . . . . . . . . . . . . . . . . . 13
       2.1.13. Securing Router Advertisements . . . . . . . . . . . . 14
       2.1.14. Host to Router Load Sharing  . . . . . . . . . . . . . 15
       2.1.15. Mobile IPv6  . . . . . . . . . . . . . . . . . . . . . 15
     2.2.  IPv4-mapped IPv6 Addresses . . . . . . . . . . . . . . . . 15
     2.3.  Increased End-to-End Transparency  . . . . . . . . . . . . 16
       2.3.1.  IPv6 Networks without NATs . . . . . . . . . . . . . . 16
       2.3.2.  Enterprise Network Security Model for IPv6 . . . . . . 17
     2.4.  IPv6 in IPv6 Tunnels . . . . . . . . . . . . . . . . . . . 18
   3.  Issues Due to Transition Mechanisms  . . . . . . . . . . . . . 18
     3.1.  IPv6 Transition/Co-existence Mechanism-specific Issues . . 19
     3.2.  Automatic Tunneling and Relays . . . . . . . . . . . . . . 19
     3.3.  Tunneling IPv6 Through IPv4 Networks May Break IPv4
           Network Security Assumptions . . . . . . . . . . . . . . . 20
   4.  Issues Due to IPv6 Deployment  . . . . . . . . . . . . . . . . 21
     4.1.  Avoiding the Trap of Insecure IPv6 Service Piloting  . . . 21
     4.2.  DNS Server Problems  . . . . . . . . . . . . . . . . . . . 23
     4.3.  Addressing Schemes and Securing Routers  . . . . . . . . . 23
     4.4.  Consequences of Multiple Addresses in IPv6 . . . . . . . . 24
     4.5.  Deploying ICMPv6 . . . . . . . . . . . . . . . . . . . . . 25
       4.5.1.  Problems Resulting from ICMPv6 Transparency  . . . . . 25
     4.6.  IPsec Transport Mode . . . . . . . . . . . . . . . . . . . 25

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     4.7.  Reduced Functionality Devices  . . . . . . . . . . . . . . 26
     4.8.  Operational Factors when Enabling IPv6 in the Network  . . 26
     4.9.  Ingress Filtering Issues Due to Privacy Addresses  . . . . 27
     4.10. Security Issues Due to ND Proxies  . . . . . . . . . . . . 28
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 30
   Appendix A.  IPv6 Probing/Mapping Considerations . . . . . . . . . 33
   Appendix B.  IPv6 Privacy Considerations . . . . . . . . . . . . . 34
     B.1.  Exposing MAC Addresses . . . . . . . . . . . . . . . . . . 34
     B.2.  Exposing Multiple Devices  . . . . . . . . . . . . . . . . 35
     B.3.  Exposing the Site by a Stable Prefix . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
   Intellectual Property and Copyright Statements . . . . . . . . . . 38

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1.  Introduction

   The transition from a pure IPv4 network to a network where IPv4 and
   IPv6 co-exist brings a number of extra security considerations that
   need to be taken into account when deploying IPv6 and operating the
   dual-protocol network with its associated transition mechanisms.
   This document attempts to give an overview of the various issues
   grouped into three categories:
   o  issues due to the IPv6 protocol itself,
   o  issues due to transition mechanisms, and
   o  issues due to IPv6 deployment.

   It is important to understand that we have to be concerned not about
   replacing IPv4 with IPv6 (in the short term), but with adding IPv6 to
   be operated in parallel with IPv4 [I-D.savola-v6ops-transarch].

   This document also describes two matters that have been wrongly
   identified as potential security concerns for IPv6 in the past and
   explains why they are unlikely to cause problems: considerations
   about probing/mapping IPv6 addresses (Appendix A), and considerations
   with respect to privacy in IPv6 (Appendix B).

2.  Issues Due to IPv6 Protocol

2.1.  IPv6 Protocol-specific Issues

   There are significant differences between the features of IPv6 and
   IPv4: some of these specification changes may result in potential
   security issues.  Several of these issues have been discussed in
   separate drafts but are summarized here to avoid normative references
   that may not become RFCs.  The following specification-related
   problems have been identified, but this is not necessarily a complete

2.1.1.  Routing Headers and Hosts

   All IPv6 nodes must be able to process Routing Headers [RFC2460].
   This RFC can be interpreted, although it is not clearly stated, to
   mean that all nodes (including hosts) must have this processing
   enabled.  This can result in hosts forwarding received traffic if
   there are segments left in the Routing Header when it arrives at the

   A number of potential security issues associated with this behavior
   were documented in [I-D.savola-ipv6-rh-hosts].  Some of these issues
   have been resolved (a separate routing header type is now used for
   Mobile IPv6 [RFC3775] and ICMP Traceback has not been standardized),

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   but two issues remain:

   o  Routing headers can be used to evade access controls based on
      destination addresses.  This could be achieved by sending a packet
      ostensibly to a publicly accessible host address but with a
      routing header containing a 'forbidden' address.  If the publicly
      accessible host is processing routing headers it will forward the
      packet to the destination address in the routing header that would
      have been forbidden by the packet filters if the address had been
      in the destination field when the packet was checked.
   o  If the packet source address in the previous case can be spoofed,
      any host could be used to mediate an anonymous reflection denial-
      of-service attack by having any publicly accessible host redirect
      the attack packets.
   To counteract these threats, if a device is enforcing access controls
   based on destination addresses, it needs to examine both the
   destination address in the base IPv6 header and any way point
   destinations in a routing header that have not yet been reached by
   the packet at the point wher it is being checked.

   Various forms of amplication attack on routers and firewalls using
   the routing header could be envisaged.  A simple form involves
   repeating the address of a way point several times in the routing
   header.  More complex forms could involve alternating way point
   addresses that would result in the packet re-transiting the router or
   firewall.  These attacks can be counteracted by ensuring that routing
   headers do not contain the same way point address more than once, and
   performing ingress/egress filtering to check that the source address
   is appropriate to the destination: packets made to reverse their path
   will fail this test.

2.1.2.  Routing Headers for Mobile IPv6 and Other Purposes

   In addition to the basic Routing Header (Type 0), which is intended
   to influence the trajectory of a packet through a network by
   specifying a sequence of router 'waypoints', Routing Header (Type 2)
   has been defined as part of the Mobile IPv6 specifications in
   [RFC3775].  The Type 2 Routing Header is intended for use by hosts to
   handle 'interface local' forwarding needed when packets are sent to
   the care-of address of a mobile node that is away from its home

   It is important that nodes treat the different types of routing
   header appropriately.  It should be possible to apply separate
   filtering rules to the different types of Routing Header.  By design,
   hosts must process Type 2 Routing Headers to support Mobile IPv6 but
   routers should not: to avoid the issues in Section 2.1.1 it may be
   desirable to forbid or limit the processing of Type 0 Routing Headers

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   in hosts and some routers.

   Routing Headers are an extremely powerful and general capability.
   Alternative future uses of Routing Headers need to be carefully
   assessed to ensure that they do not open new avenues of attack that
   can be exploited.

2.1.3.  Site-scope Multicast Addresses

   IPv6 supports multicast addresses with site scope that can
   potentially allow an attacker to identify certain important resources
   on the site if misused.

   Particular examples are the 'all routers' (FF05::2) and 'all Dynamic
   Host Configuration Protocol (DHCP) servers' (FF05::1:3) addresses
   defined in [RFC2375]: an attacker that is able to infiltrate a
   message destined for these addresses on to the site will potentially
   receive in return information identifying key resources on the site.
   This information can then be the target of directed attacks ranging
   from simple flooding to more specific mechanisms designed to subvert
   the device.

   Some of these addresses have current legitimate uses within a site.
   The risk can be minimized by ensuring that all firewalls and site
   boundary routers are configured to drop packets with site scope
   destination addresses.  Also nodes should not join multicast groups
   for which there is no legitimate use on the site and site routers
   should be configured to drop packets directed to these unused

2.1.4.  ICMPv6 and Multicast

   It is possible to launch a denial-of-service (DoS) attack using IPv6
   that could be amplified by the multicast infrastructure.

   Unlike ICMP for IPv4, ICMPv6 [RFC2463] allows error notification
   responses to be sent when certain unprocessable packets are sent to
   multicast addresses.

   The cases in which responses are sent are:
   o  The received packet is longer than the next link MTU: 'Packet Too
      Big' responses are needed to support Path MTU Discovery for
      multicast traffic.
   o  The received packet contains an unrecognized option in a hop-by-
      hop or destination options extension header with the first two
      bits of the option type set to binary '10': 'Parameter Problem'
      responses are intended to inform the source that some or all of
      the recipients cannot handle the option in question.

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   If an attacker can craft a suitable packet sent to a multicast
   destination, it may be possible to elicit multiple responses directed
   at the victim (the spoofed source of the multicast packet).  On the
   other hand, the use of 'reverse path forwarding' checks to eliminate
   loops in multicast forwarding automatically limits the range of
   addresses that can be spoofed.

   In practice an attack using oversize packets is unlikely to cause
   much amplification unless the attacker is able to carefully tune the
   packet size to exploit a network with smaller MTU in the edge than
   the core.  Similarly a packet with an unrecognized hop-by-hop option
   would be dropped by the first router.  However a packet with an
   unrecognized destination option could generate multiple responses.

   In addition to amplification, this kind of attack would potentially
   consume large amounts of forwarding state resources in routers on
   multicast-enabled networks.  These attacks are discussed in more
   detail in [I-D.savola-v6ops-firewalling].

2.1.5.  Bogus Errored Packets in ICMPv6 Error Messages

   Apart from the spurious load on the network, routers and hosts, bogus
   ICMPv6 error messages (types 0 to 127) containing a spoofed errored
   packet can impact higher layer protocols when the alleged errored
   packet is referred to the higher layer at the destination of the
   ICMPv6 packet [RFC2463].  The potentially damaging effects on TCP
   connections and some ways to mitigate the threats are documented in

   Specific countermeasures for particular higher layer protocols are
   beyond the scope of this document but firewalls may be able to help
   counter the threat by inspecting the alleged errored packet embedded
   in the ICMPv6 error message.  The firewall and the receiving host
   should test that the embedded packet contains addresses that would
   have been legitimate (i.e., would have passed ingress/egress
   filtering) for a packet sent from the receiving host.  The
   specification of ICMPv6 and the requirement that networks should have
   a minimum MTU of 1280 octets (as compared with ICMP and IPv4), means
   that the ICMPv6 should normally carry all the header fields of the
   errored packet.  Firewalls and destination hosts should therefore be
   suspicious of ICMPv6 error messages with very truncated errored
   packets (e.g., those that only carry the address fields of the IPv6
   base header.)

2.1.6.  Anycast Traffic Identification and Security

   IPv6 introduces the notion of anycast addresses and services.
   Originally the IPv6 standards disallowed using an anycast address as

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   the source address of a packet.  Responses from an anycast server
   would therefore supply a unicast address for the responding server.
   To avoid exposing knowledge about the internal structure of the
   network, it is recommended that anycast servers now take advantage of
   the ability to return responses with the anycast address as the
   source address if possible.

   If the server needs to use a unicast address for any reason, it may
   be desirable to consider using specialized addresses for anycast
   servers, which are not used for any other part of the network, to
   restrict the information exposed.  Alternatively operators may wish
   to restrict the use of anycast services from outside the domain, thus
   requiring firewalls to filter anycast requests.  For this purpose,
   firewalls need to know which addresses are being used for anycast
   services: these addresses are arbitrary and not distinguishable from
   any other IPv6 unicast address by structure or pattern.

   One particular class of anycast addresses that should be given
   special attention is the set of Subnet-Router anycast addresses
   defined in The IPv6 Addressing Architecture [RFC4291].  All routers
   are required to support these addresses for all subnets for which
   they have interfaces.  For most subnets using global unicast
   addresses, filtering anycast requests to these addresses can be
   achieved by dropping packets with the lower 64 bits (the Interface
   Identifier) set to all zeroes.

2.1.7.  Address Privacy Extensions Interact with DDoS Defenses

   The purpose of the privacy extensions for stateless address auto-
   configuration [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] is to change
   the interface identifier (and hence the global scope addresses
   generated from it) from time to time.  By varying the addresses used,
   eavesdroppers and other information collectors find it more difficult
   to identify which transactions actually relate to a specific node.

   A security issue may result from this if the frequency of node
   address change is sufficiently great to achieve the intended aim of
   the privacy extensions: with a relatively high rate of change, the
   observed behavior of the node could look very like that of a
   compromised node that was the source of a distributed denial of
   service (DDoS).  It would thus be difficult for any future defenses
   against DDoS attacks to distinguish between a high rate of change of
   addresses resulting from genuine use of the privacy extensions and a
   compromised node being used as the source of a DDoS with 'in-prefix'
   spoofed source addresses as described in [I-D.dupont-ipv6-

   Even if a node is well behaved, the change in the address could make

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   it harder for a security administrator to define a policy rule (e.g.
   access control list) that takes into account a specific node.

2.1.8.  Dynamic DNS: Stateless Address Auto-Configuration, Privacy
        Extensions and SEND

   The introduction of Stateless Address Auto-Configuration (SLAAC)
   [RFC2462] with IPv6 provides an additional challenge to the security
   of Dynamic DNS (DDNS).  With manual addressing or the use of DHCP,
   the number of security associations that need to be maintained to
   secure access to the DNS server is limited, assuming any necessary
   updates are carried out by the DHCP server.  This is true equally for
   IPv4 and IPv6.

   Since SLAAC does not make use of a single and potentially trusted
   DHCP server, but depends on the node obtaining the address, securing
   the insertion of updates into DDNS may need a security association
   between each node and the DDNS server.  This is discussed further in

   Using the Privacy Extensions to SLAAC [RFC3041][I-D.ietf-ipv6-
   privacy-addrs-v2] may significantly increase the rate of updates of
   DDNS.  Even if a node using the Privacy Extensions does not publish
   its address for 'forward' lookup (as that would effectively
   compromise the privacy which it is seeking), it may still need to
   update the reverse DNS records so that reverse routability checks can
   be carried out.  If the rate of change needed to achieve real privacy
   has to be increased as is mentioned in Section 2.1.7 the update rate
   for DDNS may be excessive.

   Similarly, the cryptographically generated addresses used by SEND
   [RFC3971] are expected to be periodically regenerated in line with
   recommendations for maximum key lifetimes.  This regeneration could
   also impose a significant extra load on DDNS.

2.1.9.  Extension Headers

   A number of issues relating to the specification of IPv6 Extension
   headers have been identified.  Several of these are discussed in
   [I-D.savola-v6ops-firewalling].  Processing Extension Headers in Middleboxes

   In IPv4 deep packet inspection techniques are used to implement
   policing and filtering both as part of routers and in middleboxes
   such as firewalls.  Fully extending these techniques to IPv6 would
   require inspection of all the extension headers in a packet.  This is
   essential to ensure that policy constraints on the use of certain

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   headers and options are enforced and to remove, at the earliest
   opportunity, packets containing potentially damaging unknown options.

   This requirement appears to conflict with Section 4 of the IPv6
   specification in [RFC2460] which requires that destination options
   are not processed at all until the packet reaches the appropriate
   destination (either the final destination or a routing header

   Also [RFC2460] forbids processing the headers other than in the order
   in which they appear in the packet.

   A further ambiguity relates to whether an intermediate node should
   discard a packet that contains a header or destination option which
   it does not recognize.  If the rules above are followed slavishly, it
   is not (or may not be) legitimate for the intermediate node to
   discard the packet because it should not be processing those headers
   or options.

   [RFC2460] therefore does not appear to take account of the behavior
   of middleboxes and other non-final destinations that may be
   inspecting the packet, and thereby potentially limits the security
   protection of these boxes.  Processing Extension Header Chains

   There is a further problem for middleboxes that want to examine the
   transport headers, which are located at the end of the IPv6 header
   chain.  In order to locate the transport header or other protocol
   data unit, the node has to parse the header chain.

   The IPv6 specification [RFC2460] does not mandate the use of the
   Type-Length-Value format with a fixed layout for the start of each
   header although it is used for the majority of headers currently
   defined.  (Only the Type field is guaranteed in size and offset).

   A middlebox cannot therefore guarantee to be able to process header
   chains that may contain headers defined after the box was
   manufactured.  As noted in Section, middleboxes ought not to
   have to know about all header types in use but still need to be able
   to skip over such headers to find the transport PDU start.  This
   either limits the security that can be applied in firewalls or makes
   it difficult to deploy new extension header types.

   At the time of writing, only the Fragment Header does not fully
   conform to the TLV format used for other extension headers.  In
   practice, many firewalls reconstruct fragmented packets before
   performing deep packet inspection, so this divergence is less

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   problematic than it might have been, and is at least partially
   justified because the full header chain is not present in all

   Destination Options may also contain unknown options.  However, the
   options are encoded in TLV format so that intermediate nodes can skip
   over them during processing, unlike the enclosing extension headers.  Unknown Headers/Destination Options and Security Policy

   A strict security policy might dictate that packets containing either
   unknown headers or destination options are discarded by firewalls or
   other filters.  This requires the firewall to process the whole
   extension header chain, which may be currently in conflict with the
   IPv6 specification as discussed in Section

   Even if the firewall does inspect the whole header chain, it may not
   be sensible to discard packets with items unrecognized by the
   firewall: the intermediate node has no knowledge of which options and
   headers are implemented in the destination node.  Hence it is highly
   desirable to make the discard policy configurable.  This will avoid
   firewalls dropping packets with legitimate items that they do not
   recognize because their hardware or software is not aware of a new
   definition.  Excessive Hop-by-Hop Options

   IPv6 does not limit the number of hop by hop options that can be
   present in a hop-by-hop option header.  The lack of a limit can be
   used to mount denial of service attacks affecting all nodes on a path
   as described in [I-D.krishnan-ipv6-hopbyhop].  Misuse of Pad1 and PadN Options

   IPv6 allows multiple padding options of arbitrary sizes to be placed
   in both Hop-by-Hop and Destination option headers.  There is no
   legitimate reason for having a sequence of padding option fields -
   the required padding can be done with one field and there is
   currently no legitimate reason for padding beyond the next four or,
   at worst, eight octet boundary.  PadN options are required to contain
   zero octets as 'payload': there is, however, no incentive for
   receivers to check this.  It may therefore be possible to use padding
   options as a covert channel.  Firewalls and receiving hosts should
   consider dropping packets that have sequences of Pad0 or PadN options
   or use PadN of more than length 3 or 7, and should actively check
   that PadN does not have other than zero octets in its 'payload'.

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   The IPv6 router alert option specifies a hop-by-hop option that, if
   present, signals the router to take a closer look at the packet.
   This can be used for denial of service attacks.  By sending a large
   number of packets containing a router alert option an attacker can
   deplete the processor cycles on the routers available to legitimate

2.1.10.  Fragmentation: Reassembly and Deep Packet Inspection

   The current specifications of IPv6 in [RFC2460] do not mandate any
   minimum packet size for the fragments of a packet before the last
   one, except for the need to carry the unfragmentable part in all

   The unfragmentable part does not include the transport port numbers
   so that it is possible that the first fragment does not contain
   sufficient information to carry out deep packet inspection involving
   the port numbers.

   Also the reassembly rules for fragmented packets in [RFC2460] do not
   mandate behavior that would minimize the effects of overlapping

   Depending on the implementation of packet reassembly and the
   treatment of packet fragments in firewalls and other nodes that use
   deep packet inspection for traffic filtering, this potentially leaves
   IPv6 open to the sort of attacks described in [RFC1858] and [RFC3128]
   for IPv4.

   There is no reason to allow overlapping packet fragments and overlaps
   could be prohibited in a future revision of the protocol
   specification.  Some implementations already drop all packets with
   overlapped fragments.

   Specifying a minimum size for packet fragments does not help in the
   same way as it does for IPv4 because IPv6 extension headers can be
   made to appear very long: an attacker could insert one or more
   undefined destination options with long lengths and the 'ignore if
   unknown' bit set.  Given the guaranteed minimum MTU of IPv6 it seems
   reasonable that hosts should be able to ensure that the transport
   port numbers are in the first fragment in almost all cases and that
   deep packet inspection should be very suspicious of first fragments
   that do not contain them.

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2.1.11.  Fragmentation Related DoS Attacks

   Packet reassembly in IPv6 hosts also opens up the possibility of
   various fragment-related security attacks.  Some of these are
   analogous to attacks identified for IPv4.  Of particular concern is a
   DoS attack based on sending large numbers of small fragments without
   a terminating last fragment that would potentially overload the
   reconstruction buffers and consume large amounts of CPU resources.

   Mandating the size of packet fragments could reduce the impact of
   this kind of attack by limiting the rate at which fragments could
   arrive and limiting the number of fragments that need to be

2.1.12.  Link-Local Addresses and Securing Neighbor Discovery

   All IPv6 nodes are required to configure a link-local address on each
   interface.  This address is used to communicate with other nodes
   directly connected to the link accessed via the interface, especially
   during the neighbor discovery and auto-configuration processes.
   Link-local addresses are fundamental to the operation of the Neighbor
   Discovery Protocol (NDP) [RFC2461] and SLAAC [RFC2462].  NDP also
   provides the functionality of associating link layer and IP addresses
   provided by the Address Resolution Protocol (ARP) in IPv4 networks.

   The standard version of NDP is subject to a number of security
   threats related to ARP spoofing attacks on IPv4.  These threats have
   been documented in [RFC3756] and mechanisms to combat them specified
   in SEcure Neighbor Discovery (SEND) [RFC3971].  SEND is an optional
   mechanism that is particularly applicable to wireless and other
   environments where it is difficult to physically secure the link.

   Because the link-local address can, by default, be acquired without
   external intervention or control, it allows an attacker to commence
   communication on the link without needing to acquire information
   about the address prefixes in use or communicate with any authorities
   on the link.  This feature gives a malicious node the opportunity to
   mount an attack on any other node that is attached to this link; this
   vulnerability exists in addition to possible direct attacks on NDP.
   Link-local addresses may also facilitate the unauthorized use of the
   link bandwidth ('bandwidth theft') to communicate with another
   unauthorized node on the same link.

   Link-local addresses allocated from the prefix are
   available in IPv4 as well and procedures for using them are described
   in [RFC3927] but the security issues were not as pronounced as for
   IPv6 for the following reasons:

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   o  link-local addresses are not mandatory in IPv4 and are primarily
      intended for isolated or ad hoc networks that cannot acquire a
      routable IPv4 address by other means,
   o  IPv4 link-local addresses are not universally supported across
      operating systems, and
   o  the IPv4 link-local address should be removed when a non-link-
      local address is configured on the interface and will generally
      not be allocated unless other means of acquiring an address are
      not available.

   These vulnerabilities can be mitigated in several ways.  A general
   solution will require
   o  authenticating the link layer connectivity, for example by using
      IEEE 802.1x functionality, port-based MAC address security
      (locking), or physical security, and
   o  using SEcure Neighbor Discovery (SEND) to create a
      cryptographically generated link-local address as described in
      [RFC3971] that is tied to the authenticated link layer address.
   This solution would be particularly appropriate in wireless LAN
   deployments where it is difficult to physically secure the

   In wired environments, where the physical infrastructure is
   reasonably secure, it may be sufficient to ignore communication
   requests originating from a link-local address for other than local
   network management purposes.  This requires that nodes should only
   accept packets with link-local addresses for a limited set of
   protocols including NDP, MLD and other functions of ICMPv6.

2.1.13.  Securing Router Advertisements

   As part of the Neighbor Discovery process, routers on a link
   advertise their capabilities in Router Advertisement messages.  The
   version of NDP defined in [RFC2461] does not protect the integrity of
   these messages or validate the assertions made in the messages with
   the result that any node that connects to the link can maliciously
   claim to offer routing services that it will not fulfil, and
   advertise inappropriate prefixes and parameters.  These threats have
   been documented in [RFC3756].

   A malicious node may also be able to carry out a DoS attack by
   deprecating an established valid prefix (by advertising it with a
   zero lifetime).  Similar DoS attacks are possible if the optional
   Router Selection mechanism is implemented as described in the
   security considerations of [RFC4191].

   SEND [RFC3971] can be used to provide verification that routers are
   authorized to provide the services they advertise through a

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   certificate-based mechanism.  This capability of SEND is also
   particularly appropriate for wireless environments where clients are
   reliant on the assertions of the routers rather than a physically
   secured connection.

2.1.14.  Host to Router Load Sharing

   If a host deploys the optional Host to Router Load Sharing mechanism
   [RFC4311] a malicious application could carry out a DoS attack on one
   or more of the load sharing routers if the application is able to use
   knowledge of the load sharing algorithm to synthesize traffic that
   subverts the load sharing algorithm and directs a large volume of
   bogus traffic towards a subset of the routers.  The likelihood of
   such an attack can be reduced if the implementation uses a
   sufficiently sophisticated load sharing algorithm as described in the
   security considerations of [RFC4311].

2.1.15.  Mobile IPv6

   Mobile IPv6 offers significantly enhanced security compared with
   Mobile IPv4 especially when using optimized routing and care-of
   addresses.  Return routability checks are used to provide relatively
   robust assurance that the different addresses that a mobile node uses
   as it moves through the network do indeed all refer to the same node.
   The threats and solutions are described in [RFC3775] and a more
   extensive discussion of the security aspects of the design can be
   found in [RFC4225].  Obsolete Home Address Option in Mobile IPv6

   The Home Address option specified in early drafts of Mobile IPv6
   would have allowed a trivial source spoofing attack: hosts were
   required to substitute the source address of incoming packets with
   the address in the option, thereby potentially evading checks on the
   packet source address.  This is discussed at greater length in
   [I-D.savola-ipv6-rh-ha-security].  The version of Mobile IPv6 as
   standardized in [RFC3775] has removed this issue by ensuring that the
   Home Address destination option is only processed if there is a
   corresponding binding cache entry and securing Binding Update

   A number of pre-standard implementations of Mobile IPv6 were
   available that implemented this obsolete and insecure option: care
   should be taken to avoid running such obsolete systems.

2.2.  IPv4-mapped IPv6 Addresses

   Overloaded functionality is always a double-edged sword: it may yield

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   some deployment benefits, but often also incurs the price that comes
   with ambiguity.

   One example of such is IPv4-mapped IPv6 addresses: a representation
   of an IPv4 address as an IPv6 address inside an operating system.
   Since the original specification, the use of IPv4-mapped addresses
   has been extended to a transition mechanism, Stateless IP/ICMP
   Translation algorithm (SIIT) [RFC2765], where they are potentially
   used in the addresses of packets on the wire.

   Therefore, it becomes difficult to unambiguously discern whether an
   IPv4 mapped address is really an IPv4 address represented in the IPv6
   address format *or* an IPv6 address received from the wire (which may
   be subject to address forgery, etc.).

   In addition, special cases like these, while giving deployment
   benefits in some areas, require a considerable amount of code
   complexity (e.g. in the implementations of bind() system calls and
   reverse DNS lookups) that is probably undesirable.  Some of these
   issues are discussed in [I-D.cmetz-v6ops-v4mapped-api-harmful] and

   In practice, although the packet translation mechanisms of SIIT are
   specified for use in the Network Address Translator - Protocol
   Translator (NAT-PT) [RFC2765], NAT-PT uses a mechanism different from
   IPv4-mapped IPv6 addresses for communicating embedded IPv4 addresses
   in IPv6 addresses.  Also SIIT is not recommended for use as a
   standalone transition mechanism.  Given the issues that have been
   identified, it seems appropriate that mapped addresses should not be
   used on the wire.  However, changing application behavior by
   deprecating the use of mapped addresses in the operating system
   interface would have significant impact on application porting
   methods [RFC4038] and needs further study.

2.3.  Increased End-to-End Transparency

   One of the major design aims of IPv6 has been to maintain the
   original IP architectural concept of end-to-end transparency.
   Transparency can help foster technological innovation in areas such
   as peer-to-peer communication but maintaining the security of the
   network at the same time requires some modifications in the network
   architecture.  Ultimately, it is also likely to need changes in the
   security model as compared with the norms for IPv4 networks.

2.3.1.  IPv6 Networks without NATs

   The necessity of introducing Network Address Translators (NATs) into
   IPv4 networks, resulting from a shortage of IPv4 addresses, has

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   removed the end-to-end transparency of most IPv4 connections: the use
   of IPv6 would restore this transparency.  However, the use of NATs,
   and the associated private addressing schemes, has become
   inappropriately linked to the provision of security in enterprise
   networks.  The restored end-to-end transparency of IPv6 networks can
   therefore be seen as a threat by poorly informed enterprise network
   managers.  Some seem to want to limit the end-to-end capabilities of
   IPv6, for example by deploying private, local addressing and
   translators, even when it is not necessary because of the abundance
   of IPv6 addresses.

   Recommendations for designing an IPv6 network to meet the perceived
   security and connectivity requirements implicit in the current usage
   of IPv4 NATs whilst maintaining the advantages of IPv6 end-to-end
   transparency are described in IPv6 Network Architecture Protection

2.3.2.  Enterprise Network Security Model for IPv6

   The favored model for enterprise network security in IPv4 stresses
   the use of a security perimeter policed by autonomous firewalls and
   incorporating the NATs.  Both perimeter firewalls and NATs introduce
   asymmetry and reduce the transparency of communications through these
   perimeters.  The symmetric bidirectionality and transparency that are
   extolled as virtues of IPv6 may seem to be at odds with this model.
   Consequently network managers may even see them as undesirable
   attributes, in conflict with their need to control threats to and
   attacks on the networks they administer.

   It is worth noting that IPv6 does not *require* end-to-end
   connectivity.  It merely provides end-to-end addressability; the
   connectivity can still be controlled using firewalls (or other
   mechanisms), and it is indeed wise to do so.

   A number of matters indicate that IPv6 networks should migrate
   towards an improved security model, which will increase the overall
   security of the network while at the same time facilitating end-to-
   end communication:
   o  Increased usage of end-to-end security especially at the network
      layer.  IPv6 mandates the provision of IPsec capability in all
      nodes and increasing usage of end-to-end security is a challenge
      to current autonomous firewalls that are unable to perform deep
      packet inspection on encrypted packets.  It is also incompatible
      with NATs because they modify the packets, even when packets are
      only authenticated rather than encrypted.
   o  Acknowledgement that over-reliance on the perimeter model is
      potentially dangerous.  An attacker who can penetrate today's
      perimeters will have free rein within the perimeter, in many

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      cases.  Also a successful attack will generally allow the attacker
      to capture information or resources and make use of them.
   o  Development of mechanisms such as 'Trusted Computing' that will
      increase the level of trust that network managers are able to
      place on hosts.
   o  Development of centralized security policy repositories and secure
      distribution mechanisms that, in conjunction with trusted hosts,
      will allow network managers to place more reliance on security
      mechanisms at the end points.  The mechanisms are likely to
      include end-node firewalling and intrusion detection systems as
      well as secure protocols that allow end points to influence the
      behavior of perimeter security devices.
   o  Review of the role of perimeter devices with increased emphasis on
      intrusion detection, network resource protection and coordination
      to thwart distributed denial of service attacks.

   Several of the technologies required to support an enhanced security
   model are still under development, including secure protocols to
   allow end points to control firewalls: the complete security model
   utilizing these technologies is now emerging but still requires some

   In the meantime, initial deployments will need to make use of similar
   firewalling and intrusion detection techniques to IPv4 that may limit
   end-to-end transparency temporarily, but should be prepared to use
   the new security model as it develops and avoid the use of NATs by
   the use of the architectural techniques described in [I-D.ietf-v6ops-
   nap].  In particular, using NAT-PT [RFC2766] as a general purpose
   transition mechanism should be avoided as it is likely to limit the
   exploitation of end-to-end security and other IPv6 capabilities in
   future as explained in [I-D.ietf-v6ops-natpt-to-exprmntl].

2.4.  IPv6 in IPv6 Tunnels

   IPv6 in IPv6 tunnels can be used to circumvent security checks, so it
   is essential to filter packets both at tunnel ingress and egress
   points (the encapsulator and decapsulator) to ensure that both the
   inner and outer addresses are accpetable, and the tunnel is not being
   used to carry inappropriate traffic.  The security discussions in
   [RFC3964], which is primarily about the 6to4 transition tunneling
   mecahnism (see Section 3.1) contains useful discussion of possible
   attacks and ways to counteract these threats.

3.  Issues Due to Transition Mechanisms

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3.1.  IPv6 Transition/Co-existence Mechanism-specific Issues

   The more complicated the IPv6 transition/co-existence becomes, the
   greater the danger that security issues will be introduced either
   o  in the mechanisms themselves,
   o  in the interaction between mechanisms, or
   o  by introducing unsecured paths through multiple mechanisms.
   These issues may or may not be readily apparent.  Hence it would be
   desirable to keep the mechanisms simple, as few in number as possible
   and built from as small pieces as possible to simplify analysis.

   One case where such security issues have been analyzed in detail is
   the 6to4 tunneling mechanism [RFC3964].

   As tunneling has been proposed as a model for several more cases than
   are currently being used, its security properties should be analyzed
   in more detail.  There are some generic dangers to tunneling:

   o  it may be easier to avoid ingress filtering checks
   o  it is possible to attack the tunnel interface: several IPv6
      security mechanisms depend on checking that Hop Limit equals 255
      on receipt and that link-local addresses are used.  Sending such
      packets to the tunnel interface is much easier than gaining access
      to a physical segment and sending them there.
   o  automatic tunneling mechanisms are typically particularly
      dangerous as there is no pre-configured association between end
      points.  Accordingly, at the receiving end of the tunnel packets
      have to be accepted and decapsulated from any source.
      Consequently, special care should be taken when specifying
      automatic tunneling techniques.

3.2.  Automatic Tunneling and Relays

   Two mechanisms have been specified that use automatic tunneling and
   are intended for use outside a single domain.  These mechanisms
   encapsulate the IPv6 packet directly in an IPv4 packet in the case of
   6to4 [RFC3056] or in an IPv4 UDP packet in the case of Teredo
   [RFC4380].  In each case packets can be sent and received by any
   similarly equipped nodes in the IPv4 Internet.

   As mentioned in Section 3.1, a major vulnerability in such approaches
   is that receiving nodes must allow decapsulation of traffic sourced
   from anywhere in the Internet.  This kind of decapsulation function
   must be extremely well secured because of the wide range of potential

   An even more difficult problem is how these mechanisms are able to
   establish communication with native IPv6 nodes or between the

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   automatic tunneling mechanisms: such connectivity requires the use of
   some kind of "relay".  These relays could be deployed in various
   locations such as:
   o  all native IPv6 nodes,
   o  native IPv6 sites,
   o  in IPv6-enabled ISPs, or
   o  just somewhere in the Internet.

   Given that a relay needs to trust all the sources (e.g., in the 6to4
   case, all 6to4 routers) that are sending it traffic, there are issues
   in achieving this trust and at the same time scaling the relay system
   to avoid overloading a small number of relays.

   As authentication of such a relay service is very difficult to
   achieve, and particularly so in some of the possible deployment
   models, relays provide a potential vehicle for address spoofing,
   (reflected) Denial-of-Service attacks, and other threats.

   Threats related to 6to4 and measures to combat them are discussed in
   [RFC3964].  [RFC4380] incorporates extensive discussion of the
   threats to Teredo and measures to combat them.

3.3.  Tunneling IPv6 Through IPv4 Networks May Break IPv4 Network
      Security Assumptions

   NATs and firewalls have been deployed extensively in the IPv4
   Internet, as discussed in Section 2.3.  Operators who deploy them
   typically have some security/operational requirements in mind (e.g. a
   desire to block inbound connection attempts), which may or may not be

   The addition of tunneling can change the security model that such
   deployments are seeking to enforce.  IPv6-over-IPv4 tunneling using
   protocol 41 is typically either explicitly allowed, or disallowed
   implicitly.  Tunneling IPv6 over IPv4 encapsulated in UDP constitutes
   a more difficult problem as UDP must usually be allowed to pass
   through NATs and firewalls.  Consequently, using UDP implies the
   ability to punch holes in NAT's and firewalls although, depending on
   the implementation, this ability may be limited or only achieved in a
   stateful manner.  In practice, the mechanisms have been explicitly
   designed to traverse both NATs and firewalls in a similar fashion.

   One possible view is that use of tunneling is especially questionable
   in home/SOHO environments where the level of expertise in network
   administration is typically not very high; in these environments the
   hosts may not be as tightly managed as in others (e.g., network
   services might be enabled unnecessarily), leading to possible
   security break-ins or other vulnerabilities.

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   Holes can be punched both intentionally and unintentionally.  In
   cases where the administrator or user makes an explicit decision to
   create the hole, this is less of a problem, although (for example)
   some enterprises might want to block IPv6 tunneling explicitly if
   employees were able to create such holes without reference to
   administrators.  On the other hand, if a hole is punched
   transparently, it is likely that a proportion of users will not
   understand the consequences: this will very probably result in a
   serious threat sooner or later.

   When deploying tunneling solutions, especially tunneling solutions
   that are automatic and/or can be enabled easily by users who do not
   understand the consequences, care should be taken not to compromise
   the security assumptions held by the users.

   For example, NAT traversal should not be performed by default unless
   there is a firewall producing a similar by-default security policy to
   that provided by IPv4 NAT.  IPv6-in-IPv4 (protocol 41) tunneling is
   less of a problem, as it is easier to block if necessary; however, if
   the host is protected in IPv4, the IPv6 side should be protected as

   As has been shown in Appendix A, it is relatively easy to determine
   the IPv6 address corresponding to an IPv4 address in tunneling
   deployments.  It is therefore vital NOT to rely on "security by
   obscurity" i.e., assuming that nobody is able to guess or determine
   the IPv6 address of the host especially when using automatic
   tunneling transition mechanisms.

   The network architecture must provide separate IPv4 and IPv6
   firewalls with tunnelled IPv6 traffic arriving encapsulated in IPv4
   packets routed through the IPv4 firewall before being decapsulated,
   and then through the IPv6 firewall as shown in Figure 1.

                +--------+      +--------+      +--------+
      Site      | Native | IPv6 |v6 in v4| IPv4 | Native |      Public
   Network <--->|  IPv6  |<---->| Tunnel |<---->|  IPv4  |<---> Internet
                |Firewall|      |Endpoint|      |Firewall|
                +--------+      +--------+      +--------+

   Figure 1: Tunnelled Traffic and Firewalls

4.  Issues Due to IPv6 Deployment

4.1.  Avoiding the Trap of Insecure IPv6 Service Piloting

   Because IPv6 is a new service for many networks, network managers

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   will often opt to make a pilot deployment in a part of the network to
   gain experience and understand the problems as well as the benefits
   that may result from a full production quality IPv6 service.

   Unless IPv6 service piloting is done in a manner that is as secure as
   possible there is a risk that security in the pilot that does not
   match up to what is achievable with current IPv4 production service
   can adversely impact the overall assessment of the IPv6 pilot
   deployment.  This may result in a decision to delay or even avoid
   deploying an IPv6 production service.  For example, hosts and routers
   might not be protected by IPv6 firewalls, even if the corresponding
   IPv4 service is fully protected by firewalls as described in
   [I-D.ietf-v6ops-v6onbydefault].  This is particularly critical where
   IPv6 capabilities are turned on by default in new equipment or new
   releases of operating systems: network managers may not be fully
   aware of the security exposure that this creates.

   In some cases a perceived lack of availability of IPv6 firewalls and
   other security capabilities, such as intrusion detection systems may
   have lead network managers to resist any kind of IPv6 service
   deployment.  These problems may be partly due to the relatively slow
   development and deployment of IPv6-capable security equipment, but
   the major problems appear to have been a lack of information, and
   more importantly a lack of documented operational experience on which
   managers can draw.  In actual fact, as of the time of writing (2006)
   there are a significant number of alternative IPv6 packet filters and
   firewalls already in existence, which could be used for provide
   sufficient access controls.

   However, there are a small number of areas that where the available
   equipment and capabilities may still be a barrier to secure
   o  'Personal firewalls' intended for use on hosts are not yet widely
   o  Enterprise firewalls are at an early stage of development and may
      not provide the full range of capabilities needed to implement the
      necessary IPv6 filtering rules. network managers often expect the
      same devices that support and are used for IPv4 today to also
      become IPv6-capable -- even though this is not really required and
      the equipment may not have the requisite hardware capabilities to
      support fast packet filtering for IPv6.  Suggestions for the
      appropriate deployment of firewalls are given in Section 3.3 -- as
      will be seen from this section it is usually desirable that the
      firewalls are in separate boxes and there is no necessity for them
      to be same model of equipment.
   o  A lesser factor may be that some design decisions in the IPv6
      protocol make it more difficult for firewalls to be implemented
      and work in all cases, and to be fully future proof (e.g. when new

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      extension headers are used) as discussed in Section 2.1.9: it is
      significantly more difficult for intermediate nodes to process the
      IPv6 header chains than IPv4 packets.
   o  Adequate Intrusion Detection Systems (IDS) are more difficult to
      construct for IPv6.  IDSs are now beginning to become available
      but the pattern-based mechanisms used for IPv4 may not be the most
      appropriate for long-term development of these systems as end-to-
      end encryption becomes more prevalent.  Future systems may be more
      reliant on traffic flow pattern recognition.
   o  Implementations of high availability capabilities supporting IPv6
      are also in short supply.  In particular, development of the IPv6
      version of the Virtual Router Redundancy Protocol (VRRP)
      [I-D.ietf-vrrp-ipv6-spec] has lagged the development of the main
      IPv6 protocol although alternatives may be available for some

   In all these areas developments are ongoing and they should not be
   considered a long-term bar to the deployment of IPv6 either as a
   pilot or production service.  The necessary tools are now available
   to make a secure IPv6 deployment and with careful selection of
   components and design of the network architecture a successful pilot
   or production IPv6 service can be deployed.  Recommendations for
   secure deployment and appropriate management of IPv6 networks can be
   found in the documentation archives of the European Union 6net
   project [SIXNET] and in the Deployment Guide published by the IPv6
   Promotion Council of Japan [JpIPv6DC].

4.2.  DNS Server Problems

   Some DNS server implementations have flaws that severely affect DNS
   queries for IPv6 addresses as discussed in [RFC4074].  These flaws
   can be used for DoS attacks affecting both IPv4 and IPv6 by inducing
   caching DNS servers to believe that a domain is broken and causing
   the server to block access to all requests for the domain for a
   precautionary period.

4.3.  Addressing Schemes and Securing Routers

   Whilst in general terms brute force scanning of IPv6 subnets is
   essentially impossible due to the enormously larger address space of
   IPv6 and the 64 bit interface identifiers (see Appendix A), this will
   be obviated if administrators do not take advantage of the large
   space to use unguessable interface identifiers.

   Because the unmemorability of complete IPv6 addresses there is a
   temptation for administrators to use small integers as interface
   identifiers when manually configuring them, as might happen on point-
   to-point links or when provisioning complete addresses from a DHCPv6

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   server.  Such allocations make it easy for an attacker to find active
   nodes that they can then port scan.

   To make use of the larger address space properly, administrators
   should be very careful when entering IPv6 addresses in their
   configurations (e.g.  Access Control List), since numerical IPv6
   addresses are more prone to human error than IPv4 due to their length
   and unmemorability.

   It is also essential to ensure that the management interfaces of
   routers are well secured as the router will usually contain a
   significant cache of neighbor addresses in its neighbor cache.

4.4.  Consequences of Multiple Addresses in IPv6

   One positive consequence of IPv6 is that nodes that do not require
   global access can communicate locally just by the use of a link-local
   address (if very local access is sufficient) or across the site by
   using a Unique Local Address (ULA).  In either case it is easy to
   ensure that access outside the assigned domain of activity can be
   controlled by simple filters (which may be the default for link-
   locals).  However, the security hazards of using link-local addresses
   for non-management purposes as documented in Section 2.1.12 should be
   borne in mind.

   On the other hand, the possibility that a node or interface can have
   multiple global scope addresses makes access control filtering both
   on ingress and egress more complex and requires higher maintenance
   levels.  Vendors and network administrators need to be aware that
   multiple addresses are the norm rather than the exception in IPv6:
   when building and selecting tools for security and management a
   highly desirable feature is the ability to be able to 'tokenize'
   access control lists and configurations in general to cater for
   multiple addresses and/or address prefixes.

   The addresses could be from the same network prefix (for example,
   privacy mechanisms [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] will
   periodically create new addresses taken from the same prefix and two
   or more of these may be active at the same time), or from different
   prefixes (for example, when a network is multihomed or is
   implementing anycast services).  In either case, it is possible that
   a single host could be using several different addresses with
   different prefixes.  It would be desirable that the security
   administrator should be able to identify that the same host is behind
   all these addresses.

   Some network administrators may find the mutability of addresses when
   privacy mechanisms are used in their network to be undesirable

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   because of the current difficulties in maintaining access control
   lists and knowing the origin of traffic.  In general, disabling the
   use of privacy addresses is only possible if the full stateful DHCPv6
   mechanism [RFC3315] is used to allocate IPv6 addresses and DHCPv6
   requests for privacy addresses are not honored.

4.5.  Deploying ICMPv6

   In IPv4 it is commonly accepted that some filtering of ICMP packets
   by firewalls is essential to maintain security.  Because of the
   extended use that is made of ICMPv6 [RFC2461] with a multitude of
   functions, the simple set of dropping rules that are usually applied
   in IPv4 need to be significantly developed for IPv6.  The blanket
   dropping of all ICMP messages that is used in some very strict
   environments is simply not possible for IPv6.

   In an IPv6 firewall, policy needs to allow some messages through the
   firewall but also has to permit certain messages to and from the
   firewall, especially those with link-local sources on links to which
   the firewall is attached.  These messages must be permitted to ensure
   that Neighbor Discovery [RFC2462], Multicast Listener Discovery
   [RFC2710], [RFC3810] and Stateless Address Configuration [RFC2463]
   work as expected.

   Recommendations for filtering ICMPv6 messages can be found in

4.5.1.  Problems Resulting from ICMPv6 Transparency

   As described in Section 4.5, certain ICMPv6 error packets need to be
   passed through a firewall in both directions.  This means that some
   ICMPv6 error packets can be exchanged between inside and outside
   without any filtering.

   Using this feature, malicious users can communicate between the
   inside and outside of a firewall bypassing the administrator's
   inspection (proxy, firewall etc.).  For example it might be possible
   to carry out a covert conversation through the payload of ICMPv6
   error messages or tunnel inappropriate encapsulated IP packets in
   ICMPv6 error messages.  This problem can be alleviated by filtering
   ICMPv6 errors using a stateful packet inspection mechanism to ensure
   that the packet carried as a payload is associated with legitimate
   traffic to or from the protected network.

4.6.  IPsec Transport Mode

   IPsec provides security to end-to-end communications at the network
   layer (layer 3).  The security features available include access

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   control, connectionless integrity, data origin authentication,
   protection against replay attacks, confidentiality, and limited
   traffic flow confidentiality (see [RFC4301] section 2.1).  IPv6
   mandates the implementation of IPsec in all conforming nodes, making
   the usage of IPsec to secure end-to-end communication possible in a
   way that is generally not available to IPv4.

   To secure IPv6 end-to-end communications, IPsec transport mode would
   generally be the solution of choice.  However, use of these IPsec
   security features can result in novel problems for network
   administrators and decrease the effectiveness of perimeter firewalls
   because of the increased prevalence of encrypted packets on which the
   firewalls cannot perform deep packet inspection and filtering.

   One example of such problems is the lack of security solutions in the
   middlebox, including effective content-filtering, ability to provide
   DoS prevention based on the expected TCP protocol behavior, and
   intrusion detection.  Future solutions to this problem are discussed
   in Section 2.3.2.  Another example is an IPsec-based DoS (e.g.,
   sending malformed ESP/AH packets) that can be especially detrimental
   to software-based IPsec implementations.

4.7.  Reduced Functionality Devices

   With the deployment of IPv6 we can expect the attachment of a very
   large number of new IPv6-enabled devices with scarce resources and
   low computing capacity.  The resource limitations are generally
   because of a market requirement for cost reduction.  Although the
   IPv6 Node Requirements [I-D.ietf-ipv6-node-requirements] specifies
   some mandatory security capabilities for every conformant node, these
   do not include functions required for a node to be able to protect
   itself.  Accordingly, some such devices may not be able even to
   perform the minimum set of functions required to protect themselves
   (e.g. 'personal' firewall, automatic firmware update, enough CPU
   power to endure DoS attacks).  This means a different security scheme
   may be necessary for such reduced functionality devices.

4.8.  Operational Factors when Enabling IPv6 in the Network

   There are a number of reasons that make it essential to take
   particular care when enabling IPv6 in the network equipment:

   Initially, IPv6-enabled router software may be less mature than
   current IPv4-only implementations and there is less experience with
   configuring IPv6 routing, which can result in disruptions to the IPv6
   routing environment and (IPv6) network outages.

   IPv6 processing may not happen at (near) line speed (or at a

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   comparable performance level to IPv4 in the same equipment).  A high
   level of IPv6 traffic (even legitimate, e.g.  Network News Transport
   Protocol, NNTP) could easily overload IPv6 processing especially when
   it is software-based without the hardware support typical in high-end
   routers.  This may potentially have deleterious knock-on effects on
   IPv4 processing, affecting availability of both services.
   Accordingly, if people don't feel confident enough in the IPv6
   capabilities of their equipment, they will be reluctant to enable it
   in their "production" networks.

   Sometimes essential features may be missing from early releases of
   vendors' software; an example is provision of software enabling IPv6
   telnet/SSH access (e.g., to the configuration application of a
   router), but without the ability to turn it off or limit access to

   Sometimes the default IPv6 configuration is insecure.  For example,
   in one vendor's implementation, if you have restricted IPv4 telnet to
   only a few hosts in the configuration, you need to be aware that IPv6
   telnet will be automatically enabled, that the configuration commands
   used previously do not block IPv6 telnet, IPv6 telnet is open to the
   world by default, and that you have to use a separate command to also
   lock down the IPv6 telnet access.

   Many operator networks have to run interior routing protocols for
   both IPv4 and IPv6.  It is possible to run the both in one routing
   protocol, or have two separate routing protocols; either approach has
   its tradeoffs [RFC4029].  If multiple routing protocols are used, one
   should note that this causes double the amount of processing when
   links flap or recalculation is otherwise needed -- which might more
   easily overload the router's CPU, causing slightly slower convergence

4.9.  Ingress Filtering Issues Due to Privacy Addresses

   [RFC3041][I-D.ietf-ipv6-privacy-addrs-v2] describes a method for
   creating temporary addresses on IPv6 nodes to address privacy issues
   created by the use of a constant identifier.  In a large network
   implementing such a mechanism new temporary addresses may be created
   at a fairly high rate.  This might make it hard for ingress filtering
   mechanisms to distinguish between legitimately changing temporary
   addresses and spoofed source addresses, which are "in-prefix" (i.e.,
   they use a topologically correct prefix and non-existent interface
   ID).  This can be addressed by using finer grained access control
   mechanisms on the network egress point.

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4.10.  Security Issues Due to ND Proxies

   In order to span a single subnet over multiple physical links, a new
   capability is being introduced in IPv6 to proxy Neighbor Discovery
   messages.  This node will be called an NDProxy (see [I-D.ietf-ipv6-
   ndproxy].  NDProxies are susceptible to the same security issues as
   the ones faced by hosts using unsecured Neighbor Discovery or ARP.
   These proxies may process unsecured messages, and update the neighbor
   cache as a result of such processing, thus allowing a malicious node
   to divert or hijack traffic.  This may undermine the advantages of
   using SEND [RFC3971].

   To resolve the security issues introduced by NDProxies, SEND needs to
   be extended to be NDProxy aware.

5.  IANA Considerations

   This memo does not contain any actions for IANA.

6.  Security Considerations

   This memo attempts to give an overview of security considerations of
   the different aspects of IPv6, particularly as they relate to the
   transition to a network in which IPv4- and IPv6-based communications
   need to coexist.

7.  Acknowledgements

   Alain Durand, Alain Baudot, Luc Beloeil, Tim Chown, Andras Kis-Szabo,
   Vishwas Manral, Janos Mohacsi, Alvaro Vives and Mark Smith provided
   feedback to improve this memo.  Satoshi Kondo, Shinsuke Suzuki and
   Alvaro Vives provided additional inputs in cooperation with the
   Deployment Working Group of the Japanese IPv6 Promotion Council and
   the Euro6IX IST co-funded project, together with inputs from Jordi
   Palet, Brian Carpenter, and Peter Bieringer.  Michael Wittsend and
   Michael Cole discussed issues relating to probing/mapping and

8.  References

8.1.  Normative References

              Narten, T., "Privacy Extensions for Stateless Address

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              Autoconfiguration in IPv6",
              draft-ietf-ipv6-privacy-addrs-v2-04 (work in progress),
              December 2005.

              Aoun, C. and E. Davies, "Reasons to Move NAT-PT to
              Experimental", draft-ietf-v6ops-natpt-to-exprmntl-03 (work
              in progress), October 2005.

              Hinden, R., "Virtual Router Redundancy Protocol for IPv6",
              draft-ietf-vrrp-ipv6-spec-07 (work in progress),
              October 2004.

   [RFC2375]  Hinden, R. and S. Deering, "IPv6 Multicast Address
              Assignments", RFC 2375, July 1998.

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

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [RFC2463]  Conta, A. and S. Deering, "Internet Control Message
              Protocol (ICMPv6) for the Internet Protocol Version 6
              (IPv6) Specification", RFC 2463, December 1998.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

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

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

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

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

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

8.2.  Informative References

   [FNAT]     Bellovin, S., "Technique for Counting NATted Hosts", Proc.
              Second Internet Measurement Workshop , November 2002,

              Chown, T., "IPv6 Implications for TCP/UDP Port Scanning",
              draft-chown-v6ops-port-scanning-implications-02 (work in
              progress), October 2005.

              Metz, C. and J. Hagino, "IPv4-Mapped Address API
              Considered Harmful",
              draft-cmetz-v6ops-v4mapped-api-harmful-01 (work in
              progress), October 2003.

              Davies, E. and J. Mohacsi, "Best Current Practice for
              Filtering ICMPv6 Messages in Firewalls",
              draft-davies-v6ops-icmpv6-filtering-bcp-00 (work in
              progress), July 2005.

              Dupont, F. and P. Savola, "RFC 3041 Considered Harmful",
              draft-dupont-ipv6-rfc3041harmful-05 (work in progress),
              June 2004.

              Gont, F., "ICMP attacks against TCP",
              draft-gont-tcpm-icmp-attacks-05 (work in progress),
              October 2005.

              Durand, A., "Operational Considerations and Issues with
              IPv6 DNS", draft-ietf-dnsop-ipv6-dns-issues-12 (work in
              progress), October 2005.

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              Thaler, D., "Neighbor Discovery Proxies (ND Proxy)",
              draft-ietf-ipv6-ndproxy-04 (work in progress),
              October 2005.

              Loughney, J., "IPv6 Node Requirements",
              draft-ietf-ipv6-node-requirements-11 (work in progress),
              August 2004.

              Velde, G., "IPv6 Network Architecture Protection",
              draft-ietf-v6ops-nap-02 (work in progress), October 2005.

              Roy, S., Durand, A., and J. Paugh, "Issues with Dual Stack
              IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03
              (work in progress), July 2004.

              Metz, C. and J. Hagino, "IPv4-Mapped Addresses on the Wire
              Considered Harmful",
              draft-itojun-v6ops-v4mapped-harmful-02 (work in progress),
              October 2003.

              Krishnan, S., "Arrangement of Hop-by-Hop options",
              draft-krishnan-ipv6-hopbyhop-00 (work in progress),
              June 2004.

              Savola, P., "Security of IPv6 Routing Header and Home
              Address Options", draft-savola-ipv6-rh-ha-security-02
              (work in progress), March 2002.

              Savola, P., "Note about Routing Header Processing on IPv6
              Hosts", draft-savola-ipv6-rh-hosts-00 (work in progress),
              February 2002.

              Savola, P., "Firewalling Considerations for IPv6",
              draft-savola-v6ops-firewalling-02 (work in progress),
              October 2003.

              Savola, P., "A View on IPv6 Transition Architecture",
              draft-savola-v6ops-transarch-03 (work in progress),

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              January 2004.

              Schild, C., "Guide to Mapping IPv4 to IPv6 Subnets",
              draft-schild-v6ops-guide-v4mapping-00 (work in progress),
              January 2004.

              Deployment WG, "IPv6 Deployment Guideline (2005 Edition)",
              IPv6 Promotion Council (Japan) Deployment Working Group,
              2005, <http://www.v6pc.jp/>.

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              February 2000.

   [RFC3128]  Miller, I., "Protection Against a Variant of the Tiny
              Fragment Attack (RFC 1858)", RFC 3128, June 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

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

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4029]  Lind, M., Ksinant, V., Park, S., Baudot, A., and P.
              Savola, "Scenarios and Analysis for Introducing IPv6 into
              ISP Networks", RFC 4029, March 2005.

   [RFC4038]  Shin, M-K., Hong, Y-G., Hagino, J., Savola, P., and E.
              Castro, "Application Aspects of IPv6 Transition",
              RFC 4038, March 2005.

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   [RFC4074]  Morishita, Y. and T. Jinmei, "Common Misbehavior Against
              DNS Queries for IPv6 Addresses", RFC 4074, May 2005.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4225]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
              Nordmark, "Mobile IP Version 6 Route Optimization Security
              Design Background", RFC 4225, December 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4311]  Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load
              Sharing", RFC 4311, November 2005.

   [SIXNET]   6Net, "Large Scale International IPv6 Pilot Network", EU
              Information Society Technologies Project , 2005,

Appendix A.  IPv6 Probing/Mapping Considerations

   One school of thought wants the IPv6 numbering topology (either at
   network or node level) [I-D.schild-v6ops-guide-v4mapping] to match
   IPv4 as exactly as possible, whereas others see IPv6 as giving more
   flexibility to the address plans, not wanting to constrain the design
   of IPv6 addressing.  Mirroring the address plans may also be seen as
   a security threat because an IPv6 deployment may have different
   security properties from IPv4.

   Given the relatively immature state of IPv6 network security, if an
   attacker knows the IPv4 address of the node and believes it to be
   dual-stacked with IPv4 and IPv6, he might want to try to probe the
   corresponding IPv6 address, based on the assumption that the security
   defenses might be lower.  This might be the case particularly for
   nodes which are behind a NAT in IPv4, but globally addressable in
   IPv6.  Naturally, this is not a concern if similar and adequate
   security policies are in place.

   On the other hand, brute-force scanning or probing of addresses is
   computationally infeasible due to the large search space of interface
   identifiers on most IPv6 subnets (somewhat less than 64 bits wide,
   depending on how identifiers are chosen), always provided that
   identifiers are chosen at random out of the available space, as
   discussed in [I-D.chown-v6ops-port-scanning-implications].

   For example, automatic tunneling mechanisms typically use

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   deterministic methods for generating IPv6 addresses, so probing/
   port-scanning an IPv6 node is simplified.  The IPv4 address is
   embedded at least in 6to4, Teredo and ISATAP addresses.
   Additionally, it is possible (in the case of 6to4 in particular) to
   learn the address behind the prefix; for example, Microsoft 6to4
   implementation uses the address 2002:V4ADDR::V4ADDR while older Linux
   and FreeBSD implementations default to 2002:V4ADDR::1.  This could
   also be used as one way to identify an implementation and hence
   target any specific weaknesses.

   One proposal has been to randomize the addresses or subnet identifier
   in the address of the 6to4 router.  This does not really help, as the
   6to4 router (whether a host or a router) will return an ICMPv6 Hop
   Limit Exceeded message, revealing the IP address.  Hosts behind the
   6to4 router can use methods such as RFC 3041 addresses to conceal
   themselves, provided that they are not meant to be reachable by
   sessions started from elsewhere: they would still require a globally
   accessible static address if they wish to receive communications
   initiated elsewhere.

   To conclude, it seems that when an automatic tunneling mechanism is
   being used, given an IPv4 address, the corresponding IPv6 address
   could possibly be guessed with relative ease.  This has significant
   implications if the IPv6 security policy is less adequate than that
   for IPv4.

Appendix B.  IPv6 Privacy Considerations

   The generation of IPv6 addresses of IPv6 addresses from MAC addresses
   potentially allows the behavior of users to be tracked in a way which
   may infringe their privacy.  [RFC3041] specifies mechanisms which can
   be used to reduce the risk of infringement.  It has also been claimed
   that IPv6 harms the privacy of the user, either by exposing the MAC
   address, or by exposing the number of nodes connected to a site.

   Additional discussion of privacy issues can be found in the IPv6
   Network Architecture Protection document [I-D.ietf-v6ops-nap].

B.1.  Exposing MAC Addresses

   Using stateless address autoconfiguration results in the MAC address
   being incorporated in an EUI64 that exposes the model of network
   card.  The concern has been that a user might not want to expose the
   details of the system to outsiders, e.g., fearing a resulting
   burglary if a thief identifies expensive equipment from the vendor
   identifier embedded in MAC addresses.

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   In most cases, this seems completely unfounded.  First, such an
   address must be learned somehow -- this is a non-trivial process; the
   addresses are visible e.g., in web site access logs, but the chances
   that a random web site owner is collecting this kind of information
   (or whether it would be of any use) are quite slim.  Being able to
   eavesdrop the traffic to learn such addresses (e.g., by the
   compromise of DSL or Cable modem physical media) seems also quite
   far-fetched.  Further, using RFC 3041 addresses for such purposes is
   straightforward if worried about the risk.  Second, the burglar would
   have to be able to map the IP address to the physical location;
   typically this would only be possible with information from the
   private customer database of the ISP and, for large sites, the
   administrative records of the site.

B.2.  Exposing Multiple Devices

   Another concern that has been aired involves the user wanting to
   conceal the presence of a large number of computers or other devices
   connected to a network; NAT can "hide" all this equipment behind a
   single address, but is not perfect either [FNAT].

   One practical reason why some administrators may find this desirable
   is being able to thwart certain ISPs' business models.  These models
   require payment based on the number of connected computers, rather
   than the connectivity as a whole.

   Similar feasibility issues as described above apply.  To a degree,
   the number of machines present could be obscured by the sufficiently
   frequent re-use of RFC 3041 addresses -- that is, if during a short
   period, dozens of generated addresses seem to be in use, it's
   difficult to estimate whether they are generated by just one host or
   multiple hosts.

B.3.  Exposing the Site by a Stable Prefix

   When an ISP provides IPv6 connectivity to its customers, it delegates
   a fixed global routing prefix (usually a /48) to them.

   Due to this fixed allocation, it is easier to correlate the global
   routing prefix to a network site.  In case of consumer users, this
   correlation leads to a privacy issue, since a site is often
   equivalent to an individual or a family in such a case.  That is,
   some users might be concerned about being able to be tracked based on
   their /48 allocation if it is static [I-D.dupont-ipv6-
   rfc3041harmful].  On the other hand many users may find having a
   static allocation desirable as it allows them to offer services
   hosted in their network more easily.

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   This problem remains unsolved even when a user changes his/her
   interface ID or subnet ID, because malicious users can still discover
   this binding.  This problem can be solved by untraceable IPv6
   addresses as described in [I-D.ietf-v6ops-nap].

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Authors' Addresses

   Elwyn B. Davies
   Soham, Cambs

   Phone: +44 7889 488 335
   Email: elwynd@dial.pipex.com

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC  H4P 2N2

   Phone: +1 514-345-7900
   Email: suresh.krishnan@ericsson.com

   Pekka Savola

   Email: psavola@funet.fi

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