Teredo Security Updates
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 5991.
|Authors||James Hoagland , Suresh Krishnan , Dave Thaler|
|Last updated||2015-10-14 (Latest revision 2010-06-03)|
|RFC stream||Internet Engineering Task Force (IETF)|
|IESG||IESG state||RFC 5991 (Proposed Standard)|
|Responsible AD||Jari Arkko|
|Send notices to||(None)|
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 5, 2010. 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 Thaler, et al. Expires December 5, 2010 [Page 1] Internet-Draft Teredo Security Updates June 2010 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 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 . . . . . . . . . . . . . . . . . . . . . . . . . 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 Thaler, et al. Expires December 5, 2010 [Page 2] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 3] Internet-Draft Teredo Security Updates June 2010 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], Thaler, et al. Expires December 5, 2010 [Page 4] Internet-Draft Teredo Security Updates June 2010 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]. Thaler, et al. Expires December 5, 2010 [Page 5] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 6] Internet-Draft Teredo Security Updates June 2010 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 Thaler, et al. Expires December 5, 2010 [Page 7] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 8] Internet-Draft Teredo Security Updates June 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 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. Thaler, et al. Expires December 5, 2010 [Page 9] Internet-Draft Teredo Security Updates June 2010 5. IANA Considerations [RFC Editor: please remove this section prior to publication.] This document has no IANA Actions. Thaler, et al. Expires December 5, 2010 [Page 10] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 11] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 12] Internet-Draft Teredo Security Updates June 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. Thaler, et al. Expires December 5, 2010 [Page 13] Internet-Draft Teredo Security Updates June 2010 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. Thaler, et al. Expires December 5, 2010 [Page 14] Internet-Draft Teredo Security Updates June 2010 Authors' Addresses Dave Thaler Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: +1 425 703 8835 Email: email@example.com Suresh Krishnan Ericsson 8400 Decarie Blvd. Town of Mount Royal, QC Canada Phone: +1 514 345 7900 x42871 Email: firstname.lastname@example.org James Hoagland Symantec Corporation 350 Ellis St. Mountain View, CA 94043 US Email: Jim_Hoagland@symantec.com URI: http://symantec.com/ Thaler, et al. Expires December 5, 2010 [Page 15]