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rfc:rfc5991

Internet Engineering Task Force (IETF) D. Thaler Request for Comments: 5991 Microsoft Updates: 4380 S. Krishnan Category: Standards Track Ericsson ISSN: 2070-1721 J. Hoagland

                                                              Symantec
                                                        September 2010
                      Teredo Security Updates

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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5991.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Thaler, et al. Standards Track [Page 1] RFC 5991 Teredo Security Updates September 2010

 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................2
 2. Terminology .....................................................3
 3. Specification ...................................................4
    3.1. Random Address Flags .......................................4
    3.2. Deprecation of Cone Bit ....................................6
 4. Security Considerations .........................................7
 5. Acknowledgments .................................................7
 6. References ......................................................8
    6.1. Normative References .......................................8
    6.2. Informative References .....................................8
 Appendix A.  Implementation Status .................................9
 Appendix B.  Resistance to Address Prediction ......................9

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.  This document updates RFC 4380.
 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

Thaler, et al. Standards Track [Page 2] RFC 5991 Teredo Security Updates September 2010

 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.

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 with which a node tries to communicate.
 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

Thaler, et al. Standards Track [Page 3] RFC 5991 Teredo Security Updates September 2010

    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],
    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].

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 generally 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

Thaler, et al. Standards Track [Page 4] RFC 5991 Teredo Security Updates September 2010

    service port and the NAT port assignment behavior), and it
    therefore 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.
 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.

Thaler, et al. Standards Track [Page 5] RFC 5991 Teredo Security Updates September 2010

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 case 4 of
 the sending details specified in [RFC4380], Section 5.2.4.  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 case 4 is already allowed for reliability reasons
 (as also 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
 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 a 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.

Thaler, et al. Standards Track [Page 6] RFC 5991 Teredo Security Updates September 2010

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
 4,096 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 autoconfiguration [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 [TUNNEL-SEC]: 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.

5. 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 this document and providing comments to make this
 document better.  The authors would also like to thank Alfred Hoenes
 for a careful review of this document.

Thaler, et al. Standards Track [Page 7] RFC 5991 Teredo Security Updates September 2010

6. References

6.1. 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.

6.2. Informative References

 [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 3rd, 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.
 [TUNNEL-SEC]   Hoagland, J., Krishnan, S., and D. Thaler, "Security
                Concerns With IP Tunneling", Work in Progress, March
                2010.

Thaler, et al. Standards Track [Page 8] RFC 5991 Teredo Security Updates September 2010

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.

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
 autoconfiguration (SLAAC) [RFC4862].
 Let's assume that the attacker knows a 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 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.

Thaler, et al. Standards Track [Page 9] RFC 5991 Teredo Security Updates September 2010

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
 USA
 EMail: Jim_Hoagland@symantec.com
 URI:   http://symantec.com/

Thaler, et al. Standards Track [Page 10]

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