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
September 2010 Teredo Security Updates
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
Status of This Memo
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Table of Contents
1. Introduction ....................................................22. Terminology .....................................................33. Specification ...................................................43.1. Random Address Flags .......................................43.2. Deprecation of Cone Bit ....................................64. Security Considerations .........................................75. Acknowledgments .................................................76. References ......................................................86.1. Normative References .......................................86.2. Informative References .....................................8Appendix A. Implementation Status .................................9Appendix B. Resistance to Address Prediction ......................91. 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
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.
This document uses the following terminology, for consistency with
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
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],
has access to the IPv4 Internet, and wants to gain access to the
Teredo IPv6 Address: An IPv6 address that starts with the prefix
2001:0000:/32 and is formed as specified in Section 4 of
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].
3.1. 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
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
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 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
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
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 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
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
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.
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.
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.
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
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