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Versions: 00 01 02 03 draft-ietf-v6ops-security-overview

Internet Engineering Task Force                                P. Savola
Internet Draft                                                 CSC/FUNET
Expiration Date: August 2004
                                                           February 2004

          IPv6 Transition/Co-existence Security Considerations


Status of this Memo

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

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   The transition/co-existance from IPv4 to IPv4/IPv6 causes one to
   consider the security considerations of such a process.  In this
   memo, I try to give an overview of different aspects relating to IPv6
   grouped in three categories: issues due to IPv6 protocol itself,
   issues due to transition mechanisms, and issues due to IPv6

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Table of Contents

   1.  Introduction  ...............................................   2
   2.  Issues Due to IPv6 Protocol  ................................   3
     2.1.  IPv6 Protocol-specific Issues  ..........................   3
     2.2.  IPv4-mapped IPv6 Addresses  .............................   4
     2.3.  Increased End-to-End Transparency  ......................   4
   3.  Issues Due to Transition Mechanisms  ........................   5
     3.1.  IPv6 Transition/Co-existence Mechanism-specific Issues  .   5
     3.2.  Automatic Tunneling and Relays  .........................   5
     3.3.  Tunneling May Break Security Assumptions  ...............   6
   4.  Issues Due to IPv6 Deployment  ..............................   7
     4.1.  IPv6 Service Piloting Done Insecurely  ..................   7
     4.2.  Enabling IPv6 by Default Brings the Usability Down  .....   8
     4.3.  Operational Factors when Enabling IPv6 in the Network  ..   8
   5.  Acknowledgements  ...........................................   9
   6.  Security Considerations  ....................................   9
   7.  References  .................................................   9
     7.1.  Normative  ..............................................   9
     7.2.  Informative  ............................................   9
   Author's Address  ...............................................  11
   A.  IPv6 Probing/Mapping Considerations  ........................  11
   B.  IPv6 Privacy Considerations  ................................  12
     B.1.  Exposing MAC Addresses  .................................  12
     B.2.  Exposing Multiple Devices  ..............................  12
   Intellectual Property Statement  ................................  13
   Full Copyright Statement  .......................................  13

1. Introduction

   The transition/co-existance from IPv4 to IPv4/IPv6 causes one to
   consider the security considerations of such a process.  In this
   memo, I try to give an overview of different aspects relating to IPv6
   grouped in three categories: issues due to IPv6 protocol itself,
   issues due to transition mechanisms, and issues due to IPv6

   A view of IPv6 transition has been presented in a separate document
   [TRANS]; it is important to read it at least cursorily to understand
   that the point is not about replacing IPv4 with IPv6 (in the short
   term), but adding IPv6 alongside IPv4.

   This document also (at the moment, may be removed in future versions)
   describes two "non-issues", in Appedix A and B: considerations about
   probing/mapping IPv6 addresses, and considerations wrt. privacy in

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2. Issues Due to IPv6 Protocol

2.1. IPv6 Protocol-specific Issues

   Some features of IPv6 are a bit different from IPv4, and may include
   some potential problems specification-wise. Some examples include at

     o how hosts should interact with routing headers (they must act as
       forwarders) [RHHOSTS]

     o how routing headers may be too generic contructs to be useful for
       e.g. MIPv6 purposes [RHHAOSEC]

     o how home address option was previously specified (fixed now)

     o how ICMPv6 messages, in some cases, may be generated in response
       to multicast packets (where in IPv4 they can't) [FW]

     o how the privacy IPv6 addresses may not actually provide all that
       much privacy (ie. the applicability is unclear) [3041HARM]

     o how IPv6 has been specified wrt. middleboxes such as firewalls
       (e.g. when new extension headers etc. are used) [FW]

   On the other hand, there are several aspects where IPv4 security is
   weak have been made stronger (at least by additional specifications),
   at least:

     o threats related to local links, comparable to different ARP
       spoofing techniques; the ND threats have been documented
       [SENDREQ] and fixes specified [SEND]

     o Mobile IPv6 depends on the return routability checks for its
       security; this seems relatively robust form of security; the
       design has been described in [RRSEC]

   Appendix A lists (typically bogus) considerations related to IPv6
   network mapping or probing.  Appendix B lists mainly unfound claims
   about the lack of privacy in IPv6.

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2.2. IPv4-mapped IPv6 Addresses

   Overloaded functionality is always a double-edged sword: it may yield
   some deployment benefits, but often also incurs the price which comes
   with ambiguity.

   One example of such is IPv4-mapped IPv6 addresses: a representation
   of IPv4 address as an IPv6 address inside an operating system.  Since
   then, IPv4-mapped addresses have been extended to be used with a
   transition mechanism [SIIT], 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 arenas, require some amount of code complexity (e.g.
   in the implementations of bind() system calls) which we might be
   better off without [V4MAPPEDA] [V4MAPPEDW].

   At least, the mapped addresses should be disallowed on the wire.
   Changing the application behavior would have significant impact on
   application porting methods, though.

2.3. Increased End-to-End Transparency

   With IPv6, increased end-to-end transparency in general can sometimes
   be seen as a threat.  Some seem to want limited end-to-end
   capabilities, e.g.  in the form of private, local addressing, even
   when it is not necessary.

   People have gotten used to the perceived, dubious security benefits
   of NATs and perimeter firewalls, and the bidirectionality and
   transparency that IPv6 can provide may seem undesirable at times.

   This is a really important issue especially for most enterprise
   network managers.

   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.

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3. Issues Due to Transition Mechanisms

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

   The more complicated the IPv6 transition/co-existence becomes, the
   more danger there is to introduce security issues in the mechanisms
   (which may or may not be readily apparent).  Therefore it would be
   desirable to keep the mechanisms simple, in as small pieces as

   One case where such security issues have been analyzed is [6TO4SEC].

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

     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 the other end-point is unspecified, and packets have
       to be accepted and decapsulated from everywhere.  Therefore,
       special care should be observed when specifying automatic
       tunneling techniques.

3.2. Automatic Tunneling and Relays

   Two mechanisms have been (or are being) specified which use automatic
   tunneling over IPv4 or UDP/IPv4 between the nodes enabling the same
   mechanism for connectivity: 6to4 and Teredo (respectively).

   The first obvious issue (as mentioned above) in such approaches is
   that such nodes must allow decapsulation of traffic from anywhere in
   the Internet.  That kind of decapsulation function must be extremely
   well secured as it's so wide open.

   Even more difficult problem is how these mechanisms are able to
   communicate with native IPv6 nodes or between the automatic tunneling
   mechanisms: such connectivity requires the use of some kind of
   "relays".  These relays could be deployed e.g., in all native IPv6
   nodes, native IPv6 sites, IPv6 ISPs, or just somewhere in the
   Internet.  This has some obvious trust and scaling issues.  As

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   authentication of such a relay service is very difficult, and more so
   in some of those deployment models, relays provide a means to for
   address spoofing, (reflected) Denial-of-Service attacks, and other

   Threats related to 6to4 are discussed in [6TO4SEC].

3.3. Tunneling May Break Security Assumptions

   NATs and firewalls have been deployed extensively in the IPv4
   Internet, for the good or the bad.  People who deploy them typically
   have some security/operational requirements in mind (e.g. a desire to
   block inbound connection attempts), whether misguided or not.

   Tunneling can change that model.  IPv6-over-IPv4 tunneling is
   typically explicitly allowed or disallowed implicitly.  Tunneling
   IPv6 over IPv4 with UDP, however, is often an entirely different
   thing: as UDP must usually be allowed through, at least in part and
   in a possibly stateful manner, one can "punch holes" in NAT's and
   firewalls using UDP.  Actually, the mechanisms have been explicitly
   designed to traverse both NATs and firewalls in a similar fashion.

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

   Holes can be punched both intentionally and unintentionally. In case
   it is a willing choice from the administrator/user, this is less of a
   problem (but e.g., enterprises might want to block IPv6 tunneling
   explicitly if some employees would do something like this willingly
   on their own).  On the other hand, if a hole is punched
   transparently, without people understanding the consequences, it will
   very probably result in a serious threat sooner or later.

   When deploying tunneling solutions, especially tunneling solutions
   which are automatic and/or can be enabled easily by users not
   understanding 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 as
   IPv4 NAT provides.  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 well.

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   As has been shown in Appendix A, it is relatively easy to find out
   IPv6 address of an IPv4, so one should never rely on "security by
   obscurity" i.e., relying that nobody is able to guess or know the
   IPv6 address of the host.

4. Issues Due to IPv6 Deployment

4.1. IPv6 Service Piloting Done Insecurely

   In many cases, IPv6 service piloting is done in a manner which is
   considered to be less secure than as one would do with IPv4.  For
   example, hosts and routers might not be protected by IPv6 firewalls,
   even if in IPv4 firewalls are being used.

   The other possible alternative, in some places, is that no service
   piloting is done at all because IPv6 firewalls may not be widely used
   -- and IPv6 deployment suffers (of course, this is also one of the
   nice excuses for not doing IPv6).

   This problem may be partially due to a slow speed of IPv6-capable
   firewall development and deployment.  However, it is also a problem
   with a lack of information: actually, there are quite a few IPv6
   packet filters and firewalls already, which could be used for
   sufficient access controls, but network administrators may not be
   aware of them yet.

   However, there appears to be a real lack in two areas: "personal
   firewalls" and enterprise firewalls; the same devices that support
   and are used for IPv4 today are often expected to also become
   IPv6-capable -- even though this is not really required.  That is,
   IPv4 access could be filtered by one firewall, and when IPv6 access
   is added, it could be protected by another firewall; they don't have
   to be the same, and even their models don't have to be the same.

   Another, smaller factor may be that due to a few decisions on how
   IPv6 was built, it's more difficult for firewalls to be implemented
   and work under all the cases (e.g. when new extension headers etc.
   are used) [FW]: it's a bit more difficult for intermediate nodes to
   process the IPv6 header chains than IPv4 packets.

   A similar argument, stated to hinder IPv6 deployment, has been the
   lack of Intrusion Detection Systems (IDS).  It's not clear whether
   this is more of an excuse than a real reason.

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4.2. Enabling IPv6 by Default Brings the Usability Down

   A practical disadvantage of enabling IPv6 as of this writing is that
   it typically brings the observed service level down a bit; that is,
   the usability suffers.

   This is due to at least three reasons:

     o global IPv6 routing is still rather unstable, leading to packet
       loss, lower throughput, and higher delay [6BMESS]

     o some applications can not properly handle both IPv4 and IPv6 or
       may have problems handling all the fallbacks and failure modes
       (and in some cases, e.g. if the TCP timeout kicks in, this may be
       very difficult)

     o some DNS server implementations have flaws that severely affect
       DNS queries for IPv6 addresses [DNSA4]

   Actually, some would be 100% ready to release IPv6 services (e.g.
   web) today, but that would mean trouble for many of their dual-
   stacked customers or users all over the world so they don't: these
   are often published under a separate domain or subdomain, and are
   practically not used that often.

   These issues are also described at some length in [ONBYDEF].

4.3. Operational Factors when Enabling IPv6 in the Network

   You have to be careful when enabling IPv6 in the network gear for
   multiple reasons:

   IPv6-enabled router software may be unstable(r) yet; either IPv6 is
   unstable, or the software you have to run to be able to run IPv6 is
   different (from non-IPv6 parts) from the one you would run otherwise,
   making the software in practice more unstable -- and raising the bar
   for IPv6 adoption.

   IPv6 processing may not happen at (near) line speed (or in the same
   level as IPv4).  A high amount of IPv6 traffic (even legitimate, e.g.
   NNTP) could easily overload the software-based IPv6 processing and
   cause harm also to IPv4 processing, affecting availability. That is,
   if people don't feel confident enough in the IPv6 support, they will
   be hesitant to enable it in their "production" networks.

   Sometimes required features may be missing from the vendors' software
   releases; an example is a software enabling IPv6 telnet/SSH access,
   but having no ability to turn it off or limit access to it!

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   Sometimes the default IPv6 configuration is insecure.  For example,
   in one vendor, if you've 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's possible to run the both in one routing
   protocol, or have two separate routing protocols; either approach has
   its tradeoffs.  If multiple routing protocols are used, one should
   note that this causes double the number of processing when links flap
   or recalculation is otherwise needed -- which might more easily
   overload the routers' CPU, causing slightly slower convergence time.

5. Acknowledgements

   Alain Durand, Alain Baudot, Luc Beloeil, and Andras Kis-Szabo
   provided feedback to improve this memo.  Michael Wittsend and Michael
   Cole discussed issues relating to probing/mapping and privacy.

6. Security Considerations

   This memo tries to give an overview of security considerations of the
   different aspects of IPv6.

7. References

7.1. Normative

   [TRANS]     Savola, P., "A View on IPv6 Transition Architecture",
               draft-savola-v6ops-transarch-03.txt, Jan 2004.

7.2. Informative

   [3041HARM]  Dupont, F., Savola, P., "RFC 3041 Considered Harmful",
               draft-dupont-ipv6-rfc3041harmful-04.txt, Feb 2004.

   [6BMESS]    Savola, P., "Moving from 6bone to IPv6 Internet",
               draft-savola-v6ops-6bone-mess-01.txt, Nov 2002.

   [6TO4SEC]   Savola, P., Patel, C., "Security Considerations for
               6to4",  draft-ietf-v6ops-6to4-security-01.txt, Feb 2004.

   [DNSA4]     Morishita., Y., Jinmei, T., "Common Misbehavior against
               DNS Queries for IPv6 Addresses", draft-morishita-dnsop-
               misbehavior-against-aaaa-00.txt, Jun 2003.

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   [FNAT]      Bellovin, S., "A Technique for Counting NATted Hosts",
               Second Internet Measurement Workshop,
               Nov 2003.

   [FW]        Savola, P. "Firewalling Considerations for IPv6",
               draft-savola-v6ops-firewalling-02.txt, Oct 2003.

   [MAPPING]   Schild, C., Strauf, C., "Guide to Mapping IPv4 to IPv6
               Subnets", draft-schild-v6ops-guide-v4mapping-00.txt,
               Dec 2003.

   [ONBYDEF]   Roy, S., et al., "Dual Stack IPv6 on by Default",
               draft-ietf-v6ops-v6onbydefault-00.txt, Jun 2003.

   [PORTSCAN]  Chown, T., "IPv6 Implications for TCP/UDP Port Scanning",
               Oct 2003.

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

   [RHHOSTS]   Savola, P. "Note about Routing Header Processing on IPv6
               Hosts", draft-savola-ipv6-rh-hosts-00.txt, Feb 2002.

   [RRSEC]     Nikander, P, et al., "Mobile IP version 6 Route
               Optimization Security Design Background",
               draft-nikander-mobileip-v6-ro-sec-02.txt, Dec 2003.

   [SEND]      Arkko, J., et al., "SEcure Neighbor Discovery (SEND)",
               draft-ietf-send-ndopt-03.txt, Jan 2004.

   [SENDREQ]   Nikander, P., et al., "IPv6 Neighbor Discovery trust
               models and threats", draft-ietf-send-psreq-04.txt,
               Oct 2003.

   [SIIT]      Nordmark, E., "Stateless IP/ICMP Translation Algorithm",
               RFC276, February 2000.

   [V4MAPPEDA] Metz, C., Hagino, J., "IPv4-Mapped address API considered
               harmful", draft-cmetz-v6ops-v4mapped-api-harmful-01.txt,
               Oct 2003.

   [V4MAPPEDW] Metz, C., Hagino, J., "IPv4-Mapped Addresses on the Wire
               Considered Harmful",
               draft-itojun-v6ops-v4mapped-harmful-02.txt, Oct 2003.

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Author's Address

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

A. IPv6 Probing/Mapping Considerations

   Some want the IPv6 numbering topology (either at network or node
   level) [MAPPING] match IPv4 as exactly as possible, the others see
   this as a security threat because IPv6 could have different security
   properties than IPv4.

   That is, if an attacker knows the IPv4 address of the node, 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
   the similar security policies are in place.

   On the other hand, brute-force scanning or probing is unfeasible

   For example, automatic tunneling mechanisms use rather 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 address.  Further than that, it's 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 Linux and BSD implementations default to
   2002:V4ADDR::1.  This could also be used as one way to identify an

   One proposal has been to randomize the addresses or Subnet identifier
   in the address of the 6to4 router.  This doesn't 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, though.

   To conclude, it seems that with an IPv4 address, the respective IPv6
   address, when automatic tunneling mechanism is being used, could
   possibly be guessed with relative ease.  This has significant
   implications if the IPv6 security policy isn't the same as IPv4.

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B. IPv6 Privacy Considerations

   It has 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.

  B.1. Exposing MAC Addresses

   The MAC address, which with stateless address autoconfiguration,
   results in an EUI64, 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., in the fear of a resulting burglary (e.g.,
   if a crook identifies expensive equipment from the MAC addresses).

   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; this
   is typically only available in the private customer database of the

  B.2. Exposing Multiple Devices

   Another presented concern is whether the user wants to show off as
   having a lot of computers or other devices at a network; NAT "hides"
   everything behind an address, but is not perfect either [FNAT].

   One practical reason why some may find this desirable is being able
   to thwart certain ISPs' business models, where one should pay extra
   for additional computers (and not the connectivity as a whole).

   Similar feasibility issues as described above apply.  To a degree,
   the counting avoidance could be performed 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.

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Intellectual Property Statement

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