IPv6 Operations Working Group D. Thaler
Internet-Draft Microsoft
Expires: April 17, 2009 S. Krishnan
Ericsson
J. Hoagland
Symantec
October 14, 2008
Teredo Security Updates
draft-krishnan-v6ops-teredo-update-04
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Random Address Flags . . . . . . . . . . . . . . . . . . . 4
3.2. Deprecation of Cone Bit . . . . . . . . . . . . . . . . . 6
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Appendix A: Implementation Status . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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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 attacher 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, can 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.
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
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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 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 [RFC4380] section 2.14.
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 can be used to better predict the
address.
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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 a 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.
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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 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).
4. Acknowledgments
The authors would like to thank Remi Denis-Courmont, Fred Templin,
Jordi Palet Martinez, James Woodyatt, Christian Huitema and Tom Yu
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.
5. Security Considerations
The basic threat model for Teredo is described in detail in [RFC4380]
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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.
Opening a hole in an enterprise firewall
[I-D.ietf-v6ops-tunnel-security-concerns]: Teredo is NOT RECOMMENDED
as a solution for managed networks. Administrators of such networks
may wish to filter all Teredo traffic at the boundaries of their
networks.
6. IANA Considerations
[RFC Editor: please remove this section prior to publication.]
This document has no IANA Actions.
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 Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-00 (work in
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progress), July 2008.
[RFC5157] Chown, T., "IPv6 Implications for Network Scanning",
RFC 5157, March 2008.
Appendix A. 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.
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
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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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