


IPv6 Operations Working Group                                  D. Thaler
Internet-Draft                                                 Microsoft
Updates: 4380 (if approved)                                  S. Krishnan
Intended status: Standards Track                                Ericsson
Expires: December 5, 2010                                    J. Hoagland
                                                                Symantec
                                                            June 3, 2010


                        Teredo Security Updates
                 draft-krishnan-v6ops-teredo-update-10

Abstract

   The Teredo protocol defines a set of flags that are embedded in every
   Teredo IPv6 address.  This document specifies a set of security
   updates that modify the use of this flags field, but are backward
   compatible.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 5, 2010.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
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   than English.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Specification  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Random Address Flags . . . . . . . . . . . . . . . . . . .  6
     3.2.  Deprecation of Cone Bit  . . . . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Implementation Status . . . . . . . . . . . . . . . . 13
   Appendix B.  Resistance to address prediction  . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15



















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1.  Introduction

   Teredo [RFC4380] defines a set of flags that are embedded in every
   Teredo IPv6 address.  This document specifies a set of security
   updates that modify the use of this flags field, but are backwards
   compatible.

   The flags field in a Teredo IPv6 address has 13 unused bits out of a
   total of 16 bits.  To guard against address-scanning risks [RFC5157]
   from malicious users, this update randomizes 12 of the 13 unused bits
   when configuring the Teredo IPv6 address.  Even if an attacker were
   able to determine the external (mapped) IPv4 address and port
   assigned by a NAT to the Teredo client, the attacker would still need
   to attack a range of 4,096 IPv6 addresses to determine the actual
   Teredo IPv6 address of the client.

   The cone bit in a Teredo IPv6 address indicates whether a peer needs
   to send Teredo control messages before communicating with a Teredo
   IPv6 address.  Unfortunately, it may also have some value in terms of
   profiling to the extent that it reveals the security posture of the
   network.  If the cone bit is set, an attacker may decide it is
   fruitful to port scan the embedded external IPv4 address and others
   associated with the same organization, looking for open ports.
   Deprecating the cone bit prevents the a priori revelation of the
   security posture of the NAT.


























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2.  Terminology

   This document uses the following terminology, for consistency with
   [RFC4380].

   Cone NAT: A NAT that maps all requests from the same internal IP
   address and port to the same external IP address and port.
   Furthermore, any external host can send a packet to the internal host
   by sending a packet to the mapped external address and port.

   Indirect Bubble: A Teredo control message that is sent to another
   Teredo client via the destination's Teredo server, as specified in
   [RFC4380] section 5.2.4.

   Local Address/Port: The IPv4 address and UDP port from which a Teredo
   client sends Teredo packets.  The local port is referred to as the
   Teredo service port in [RFC4380].  The local address of a node may or
   may not be globally routable because the node can be located behind
   one or more NATs.

   Mapped Address/Port: A global IPv4 address and a UDP port that
   results from the translation of a node's own local address/port by
   one or more NATs.  The node learns these values through the Teredo
   protocol specified in [RFC4380]. the mapped address/port can be
   different for every peer that a node tries to communicate with.

   Network Address Translation (NAT): The process of converting between
   IP addresses used within an intranet or other private network and
   Internet IP addresses.

   Peer: A Teredo client with which another Teredo Client needs to
   communicate.

   Port-Preserving NAT: A NAT that translates a local address/port to a
   mapped address/port such that the mapped port has the same value as
   the local port, as long as that same mapped address/port has not
   already been used for a different local address/port.

   Public Address: An external global address used by a NAT.

   Restricted NAT: A NAT where all requests from the same internal IP
   address and port are mapped to the same external IP address and port.
   Unlike the cone NAT, an external host can send packets to an internal
   host (by sending a packet to the external mapped address and port)
   only if the internal host has first sent a packet to the external
   host.

   Teredo Client: A node that implements the client parts of [RFC4380],



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   has access to the IPv4 Internet and wants to gain access to the IPv6
   Internet.

   Teredo IPv6 Address: An IPv6 address that starts with the prefix
   2001:0000:/32 and is formed as specified in Section 4 of [RFC4380].

   Teredo Server: A node that has a globally routable address on the
   IPv4 Internet, and is used as a helper to provide IPv6 connectivity
   to Teredo clients.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].






































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3.  Specification

3.1.  Random Address Flags

   Teredo addresses are structured and some of the fields contained in
   them are fairly predictable.  This makes the addresses themselves
   easier to predict and opens up a vulnerability.

   Teredo prefix:  This field is 32 bits and has a single IANA assigned
      value

   Server:  This field is 32 bits and is set to the server in use.  The
      server to use is usually statically configured on the client.
      This means that overall entropy of the server field will be low,
      i.e., that the server will not be hard to predict.  Attackers
      could confine their guessing to the most popular server IP
      addresses.

   Flags:  The flags field is 16 bits in length, but [RFC4380] provides
      for only one of these bits (the cone bit) to vary.

   Client port:  This 16 bit field corresponds to the external port
      number assigned to the client's Teredo service port.  Thus the
      value of this field depends on two factors (the chosen Teredo
      service port and the NAT port assignment behavior) and therefore
      it is harder to predict the entropy this field will have.  If
      clients tend to use a predictable port number and NATs are often
      port-preserving, then the port number can be rather predictable.

   Client IPv4 address:  This 32 bit field corresponds to the external
      IPv4 address the NAT has assigned for the client port.  In
      principle, this can be any address in the assigned part of the
      IPv4 unicast address space.  However, if an attacker is looking
      for the address of a specific Teredo client, they will have to
      have the external IPv4 address pretty well narrowed down.  Certain
      IPv4 address ranges could also become well known for having a
      higher concentration of Teredo clients, making it easier to find
      an arbitrary Teredo client.  These addresses could correspond to
      large organizations that allow Teredo such as a university or
      enterprise or to Internet Service Providers that only provide
      their customers with RFC 1918 addresses.

   Optimizations in scanning can also reduce the number of addresses
   that need to be checked.  For example, for addresses behind a cone
   NAT, it would likely be easy to probe if a specific port number is
   open on an IPv4 address, prior to trying to form a Teredo address for
   that address and port.




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   Hence the Flags field specified in [RFC4380] section 4 is updated as
   follows:

                        1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|z|Random1|U|G|    Random2    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   C: This flag is specified in [RFC4380] and its use is modified in
   Section 3.2 below.

   z: This flag is reserved.  It MUST be set to zero when the address is
   constructed, as specified in [RFC4380].

   Random1: MUST be set to a random value.

   U: This flag is specified in [RFC4380].

   G: This flag is specified in [RFC4380].

   Random2: MUST be set to a random value.

3.2.  Deprecation of Cone Bit

   The qualification procedure is specified in [RFC4380] section 5.2.1,
   and is modified as follows.  Teredo clients SHOULD completely skip
   the first phase of the qualification procedure and implement only the
   second phase where it uses the Teredo link local address with the
   cone bit set to zero.  Consequently, a distinction between cone and
   restricted NATs can no longer be made.  Teredo communication will
   still succeed, but at the expense of forcing peers to skip step 4 of
   the sending details specified in [RFC4380].  This will result in the
   same number of indirect bubbles being sent as if the other end were a
   peer behind a restricted NAT.  Even though the peer behind the cone
   NAT does not need these indirect bubbles, it replies to these
   indirect bubbles just like it would to any other indirect bubbles.
   Skipping step 4 is already allowed for reliability reasons (as
   specified in [RFC4380] section 5.2.4), and hence this does not break
   interoperability, but the result of skipping the first phase of
   qualification is to force that behavior (which is less efficient, but
   potentially more reliable) to be taken by peers.

   In addition, clients and relays SHOULD ignore the cone bit in the
   address of a Teredo peer and treat it as if it were always clear, as
   specified in [RFC4380] section 5.2.4 (last paragraph).

   Teredo servers MUST NOT ignore the cone bit for the following



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   reasons.

   o  The cone bit in the IPv6 source address of a Router Solicitation
      (RS) from a client controls what IPv4 source address the server
      should use when sending a Router Advertisement (RA).  If this
      behavior is not preserved, legacy clients will conclude that they
      are behind a cone NAT even when they are not (because the client
      WILL receive the RA where previously it would not, since cone bit
      set to 1 requires the server to respond from another IP address).
      They will then set their cone bit and lose connectivity.

   o  When the Teredo server sends RAs (or bubbles if it's also a relay)
      the cone bit in its own teredo address is set, indicating that it
      doesn't require bubbles to reach it.





































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4.  Security Considerations

   The basic threat model for Teredo is described in detail in [RFC4380]
   section 7, but briefly the goal is that a Teredo client should be as
   secure as if a host were directly attached to an untrusted Internet
   link.  This document specifies updates to [RFC4380] that improve the
   security of the base Teredo mechanism regarding specific threats.

   IPv6 address scanning [RFC5157] by off-path attackers: The Teredo
   IPv6 Address format defined in [RFC4380] section 4 makes it
   relatively easy for a malicious user to conduct an address-scan to
   determine IPv6 addresses by guessing the external (mapped) IPv4
   address and port assigned to the Teredo client.  The random address
   bits guard against address-scanning risks by providing a range of
   4096 IPv6 addresses per external IPv4 address/port.  As a result,
   even if a malicious user were able to determine the external (mapped)
   IPv4 address and port assigned to the Teredo client, the malicious
   user would still need to attack a range of 4,096 IPv6 addresses to
   determine the actual Teredo IPv6 address of the client.  Appendix B
   compares the address prediction resistance of a Teredo address
   following this specification to that of an address formed using
   standard IPv6 Stateless Address Auto-configuration [RFC4862].

   In order to prevent adversaries from easily guessing the values of
   the random bits and hence the address, the Random1 and Random2 bits
   in the Teredo Flags field MUST be constructed following the
   recommendations for Random number generation as specified in
   [NIST-RANDOM] and [RFC4086].

   Opening a hole in an enterprise firewall
   [I-D.ietf-v6ops-tunnel-security-concerns]: Teredo is NOT RECOMMENDED
   as a solution for networks that wish to implement strict controls for
   what traffic passes to and from the Internet.  Administrators of such
   networks may wish to filter all Teredo traffic at the boundaries of
   their networks.
















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5.  IANA Considerations

   [RFC Editor: please remove this section prior to publication.]

   This document has no IANA Actions.














































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6.  Acknowledgments

   The authors would like to thank Remi Denis-Courmont, Fred Templin,
   Jordi Palet Martinez, James Woodyatt, Christian Huitema, Tom Yu, Jari
   Arkko, David Black, Tim Polk and Sean Turner for reviewing earlier
   versions of the document and providing comments to make this document
   better.  The authors would also like to thank Alfred Hoenes for a
   careful review of the document.











































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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

7.2.  Informative References

   [I-D.ietf-v6ops-tunnel-security-concerns]
              Hoagland, J., Krishnan, S., and D. Thaler, "Security
              Concerns With IP Tunneling",
              draft-ietf-v6ops-tunnel-security-concerns-02 (work in
              progress), March 2010.

   [NIST-RANDOM]
              "NIST SP 800-90, Recommendation for Random Number
              Generation Using Deterministic Random Bit Generators",
              March 2007, <http://csrc.nist.gov/publications/nistpubs/
              800-90/SP800-90revised_March2007.pdf>.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",
              RFC 5157, March 2008.


















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Appendix A.  Implementation Status

   Deprecation of the cone bit as specified in this document is
   implemented in Windows Vista and Windows Server 2008.

   The random flags specified in this document are implemented in
   Windows Vista SP1 and Windows Server 2008.

   All Windows implementations automatically disable Teredo if they
   detect that they are on a managed network with a domain controller.









































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Appendix B.  Resistance to address prediction

   This section compares the address prediction resistance of a Teredo
   address as compared to an address formed using IPv6 stateless address
   auto-configuration (SLAAC) [RFC4862].

   Let's assume that the attacker knows Teredo Client's external IPv4
   address and Ethernet card's vendor.  Since the attacker knows the
   Client's external IPv4 address he does not have to search this space.
   The attacker does not know the external port (16 bits) and the value
   of the random bits (12 bits) and he has to search this space.  This
   gives the attacker a total search space of 28 bits (16+12).  This
   compares very favorably with the 24 bits of search space required to
   find an address configured using SLAAC (when the Ethernet card's
   vendor is known) as described in Section 2.3 of [RFC5157].  Without
   the 12 random bits the search space is limited to only 16 bits and
   this is significantly worse than the 24 bits of search space provided
   by SLAAC.

   As the knowledge of the attacker decreases the number of bits of
   search space in both the cases is likely to increase in a relatively
   similar fashion.  The predictability of Teredo addresses will stay
   comparable to that of SLAAC addresses with the added 12 bits of
   search space, but will be significantly worse without the random
   bits.


























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Authors' Addresses

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com


   Suresh Krishnan
   Ericsson
   8400 Decarie Blvd.
   Town of Mount Royal, QC
   Canada

   Phone: +1 514 345 7900 x42871
   Email: suresh.krishnan@ericsson.com


   James Hoagland
   Symantec Corporation
   350 Ellis St.
   Mountain View, CA  94043
   US

   Email: Jim_Hoagland@symantec.com
   URI:   http://symantec.com/





















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