This document specifies a method for generating IPv6 Interface Identifiers to be used with IPv6 Stateless Address Autoconfiguration (SLAAC), such that addresses configured using this method are stable within each subnet, but the Interface Identifier changes when hosts move from one network to another. This method is meant to be an alternative to generating Interface Identifiers based on hardware address (e.g., using IEEE identifiers), such that the benefits of stable addresses can be achieved without sacrificing the privacy of users. The method specified in this document applies to all prefixes a host may be employing, including link-local, global, and unique- local addresses.
aea1ddd79e402a7e6cae6940341f56386d8efe61f639f9142e54a9dda4b93d71
IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Standards Track June 3, 2013
Expires: December 5, 2013
A method for Generating Stable Privacy-Enhanced Addresses with IPv6
Stateless Address Autoconfiguration (SLAAC)
draft-ietf-6man-stable-privacy-addresses-09
Abstract
This document specifies a method for generating IPv6 Interface
Identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), such that addresses configured using this method are stable
within each subnet, but the Interface Identifier changes when hosts
move from one network to another. This method is meant to be an
alternative to generating Interface Identifiers based on hardware
address (e.g., using IEEE identifiers), such that the benefits of
stable addresses can be achieved without sacrificing the privacy of
users. The method specified in this document applies to all prefixes
a host may be employing, including link-local, global, and unique-
local addresses.
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, 2013.
Copyright Notice
Copyright (c) 2013 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
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(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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Design goals . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Algorithm specification . . . . . . . . . . . . . . . . . . . 8
4. Resolving Duplicate Address Detection (DAD) conflicts . . . . 13
5. Specified Constants . . . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Possible sources for the Net_Iface parameter . . . . 22
A.1. Interface Index . . . . . . . . . . . . . . . . . . . . . 22
A.2. Interface Name . . . . . . . . . . . . . . . . . . . . . . 22
A.3. Link-layer Addresses . . . . . . . . . . . . . . . . . . . 22
A.4. Logical Network Service Identity . . . . . . . . . . . . . 23
Appendix B. Security/privacy issues with traditional SLAAC
addresses . . . . . . . . . . . . . . . . . . . . . . 24
B.1. Correlation of node activities within the same network . . 24
B.2. Correlation of node activities across networks . . . . . . 24
B.3. Host-tracking attacks . . . . . . . . . . . . . . . . . . 24
B.4. Address-scanning attacks . . . . . . . . . . . . . . . . . 25
B.5. Exploitation of device-specific information . . . . . . . 25
Appendix C. Privacy issues still present when temporary
addresses are employed . . . . . . . . . . . . . . . 26
C.1. Host tracking . . . . . . . . . . . . . . . . . . . . . . 26
C.1.1. Tracking hosts across networks #1 . . . . . . . . . . 26
C.1.2. Tracking hosts across networks #2 . . . . . . . . . . 27
C.1.3. Revealing the identity of devices performing
server-like functions . . . . . . . . . . . . . . . . 27
C.2. Address-scanning attacks . . . . . . . . . . . . . . . . . 27
C.3. Information Leakage . . . . . . . . . . . . . . . . . . . 28
C.4. Correlation of node activities within a network . . . . . 28
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
[RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
IPv6 [RFC2460], which typically results in hosts configuring one or
more "stable" addresses composed of a network prefix advertised by a
local router, and an Interface Identifier (IID) that typically embeds
a hardware address (e.g., using IEEE identifiers) [RFC4291].
Cryptographically Generated Addresses (CGAs) [RFC3972] are yet
another method for generating Interface Identifiers, which bind a
public signature key to an IPv6 address in the SEcure Neighbor
Discovery (SEND) [RFC3971] protocol.
Generally, the traditional SLAAC addresses are thought to simplify
network management, since they simplify Access Control Lists (ACLs)
and logging. However, they have a number of drawbacks:
o since the resulting Interface Identifiers do not vary over time,
they allow correlation of node activities within the same network,
thus negatively affecting the privacy of users.
o since the resulting Interface Identifiers are constant across
networks, the resulting IPv6 addresses can be leveraged to track
and correlate the activity of a node across multiple networks
(e.g. track and correlate the activities of a typical client
connecting to the public Internet from different locations), thus
negatively affecting the privacy of users.
o since embedding the underlying link-layer address in the Interface
Identifier will result in specific address patterns, such patterns
may be leveraged by attackers to reduce the search space when
performing address scanning attacks. For example, the IPv6
addresses of all nodes manufactured by the same vendor (at a given
time frame) will likely contain the same IEEE Organizationally
Unique Identifier (OUI) in the Interface Identifier.
o embedding the underlying link-layer address in the Interface
Identifier leaks device-specific information that could be
leveraged to launch device-specific attacks.
o embedding the underlying link-layer address in the Interface
Identifier means that replacement of the underlying interface
hardware will result in a change of the IPv6 address(es) assigned
to that interface.
Appendix B provides additional details regarding how these
vulnerabilities could be exploited, and the extent to which the
method discussed in this document mitigates them.
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The "Privacy Extensions for Stateless Address Autoconfiguration in
IPv6" [RFC4941] (henceforth referred to as "temporary addresses")
were introduced to complicate the task of eavesdroppers and other
information collectors (e.g. IPv6 addresses in web server logs or
email headers, etc.) to correlate the activities of a node, and
basically result in temporary (and random) Interface Identifiers.
These temporary addresses are generated in addition to the
traditional IPv6 addresses based on IEEE identifiers, with the
"temporary addresses" being employed for "outgoing communications",
and the traditional SLAAC addresses being employed for "server"
functions (i.e., receiving incoming connections).
It should be noted that temporary addresses can be challenging in
a number of areas. For example, from a network-management point
of view, they tend to increase the complexity of event logging,
trouble-shooting, enforcement of access controls and quality of
service, etc. As a result, some organizations disable the use of
temporary addresses even at the expense of reduced privacy
[Broersma]. Temporary addresses may also result in increased
implementation complexity, which might not be possible or
desirable in some implementations (e.g., some embedded devices).
In scenarios in which temporary addresses are deliberately not
used (possibly for any of the aforementioned reasons), all a host
is left with is the stable addresses that have been generated
using e.g. IEEE identifiers. In such scenarios, it may still be
desirable to have addresses that mitigate address scanning
attacks, and that at the very least do not reveal the node's
identity when roaming from one network to another -- without
complicating the operation of the corresponding networks.
However, even with "temporary addresses" in place, a number of issues
remain to be mitigated. Namely,
o since "temporary addresses" [RFC4941] do not eliminate the use of
fixed identifiers for server-like functions, they only partially
mitigate host-tracking and activity correlation across networks
(see Appendix C.1 for some example attacks that are still possible
with temporary addresses).
o since "temporary addresses" [RFC4941] do not replace the
traditional SLAAC addresses, an attacker can still leverage
patterns in those addresses to greatly reduce the search space for
"alive" nodes [Gont-DEEPSEC2011] [CPNI-IPv6]
[I-D.ietf-opsec-ipv6-host-scanning].
Hence, there is a motivation to improve the properties of "stable"
addresses regardless of whether temporary addresses are employed or
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not.
We note that attackers can employ a plethora of probing techniques
[I-D.ietf-opsec-ipv6-host-scanning] to exploit the aforementioned
issues. Some of them (such as the use of ICMPv6 Echo Request and
ICMPv6 Echo Response packets) could mitigated by a personal firewall
at the target host. For other vectors, such listening to ICMPv6
"Destination Unreachable, Address Unreachable" (Type 1, Code 3) error
messages referring to the target addresses
[I-D.ietf-opsec-ipv6-host-scanning], there is nothing a host can do
(e.g., a personal firewall at the target host would not be able to
mitigate this probing technique).
This document specifies a method to generate Interface Identifiers
that are stable/constant for each network interface within each
subnet, but that change as hosts move from one network to another,
thus keeping the "stability" properties of the Interface Identifiers
specified in [RFC4291], while still mitigating address-scanning
attacks and preventing correlation of the activities of a node as it
moves from one network to another.
The method specified in this document is a orthogonal to the use of
"temporary" addresses [RFC4941], since it is meant to improve the
security and privacy properties of the stable addresses that are
employed along with the aforementioned "temporary" addresses. In
scenarios in which "temporary addresses" are employed, implementation
of the mechanism described in this document (in replacement of stable
addresses based on e.g. IEEE identifiers) would mitigate address-
scanning attacks and also mitigate the remaining vectors for
correlating host activities based on the node's IPv6 addresses. On
the other hand, for nodes that currently disable "temporary
addresses" [RFC4941] for some of the reasons described earlier in
this document, implementation of this mechanism will result in stable
privacy-enhanced addresses which address some of the concerns related
to addresses that embed IEEE identifiers [RFC4291], and which
mitigate IPv6 address-scanning attacks.
We note that this method is incrementally deployable, since it does
not pose any interoperability implications when deployed on networks
where other nodes do not implement or employ it. Additionally, we
note that this document does not update or modify IPv6 StateLess
Address Auto-Configuration (SLAAC) [RFC4862] itself, but rather only
specifies an alternative algorithm to generate Interface Identifiers.
Therefore, the usual address lifetime properties (as specified in the
corresponding Prefix Information Options) apply when IPv6 addresses
are generated as a result of employing the algorithm specified in
this document with SLAAC [RFC4862]. Additionally, from the point of
view of renumbering, we note that these addresses behave like the
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traditional IPv6 addresses (that embed a hardware address) resulting
from SLAAC [RFC4862].
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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2. Design goals
This document specifies a method for selecting Interface Identifiers
to be used with IPv6 SLAAC, with the following goals:
o The resulting Interface Identifiers remain constant/stable for
each prefix used with SLAAC within each subnet. That is, the
algorithm generates the same Interface Identifier when configuring
an address (for the same interface) belonging to the same prefix
within the same subnet.
o The resulting Interface Identifiers do change when addresses are
configured for different prefixes. That is, if different
autoconfiguration prefixes are used to configure addresses for the
same network interface card, the resulting Interface Identifiers
must be (statistically) different. This means that, given two
addresses produced by the method specified in this document, it
must be difficult for an attacker tell whether the addresses have
been generated/used by the same node.
o It must be difficult for an outsider to predict the Interface
Identifiers that will be generated by the algorithm, even with
knowledge of the Interface Identifiers generated for configuring
other addresses.
o Depending on the specific implementation approach (see Section 3
and Appendix A), the resulting Interface Identifiers may be
independent of the underlying hardware (e.g. link-layer address).
This means that e.g. replacing a Network Interface Card (NIC) will
not have the (generally undesirable) effect of changing the IPv6
addresses used for that network interface.
o The method specified in this document is meant to be an
alternative to producing IPv6 addresses based on e.g. IEEE
identifiers (as specified in [RFC2464]). It is meant to be
employed for all of the stable (i.e. non-temporary) IPv6 addresses
configured with SLAAC for a given interface, including global,
link-local, and unique-local IPv6 addresses.
We note that of use of the algorithm specified in this document is
(to a large extent) orthogonal to the use of "temporary addresses"
[RFC4941]. When employed along with "temporary addresses", the
method specified in this document will mitigate address-scanning
attacks and correlation of node activities across networks (see
Appendix C and [IAB-PRIVACY]). On the other hand, hosts that do not
implement/use "temporary addresses" but employ the method specified
in this document would, at the very least, mitigate the host-tracking
and address scanning issues discussed in the previous section.
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3. Algorithm specification
IPv6 implementations conforming to this specification MUST generate
Interface Identifiers using the algorithm specified in this section
in replacement of any other algorithms used for generating "stable"
addresses (such as those specified in [RFC2464]). However,
implementations conforming to this specification MAY employ the
algorithm specified in [RFC4941] to generate temporary addresses in
addition to the addresses generated with the algorithm specified in
this document. The method specified in this document MUST be
employed for generating the Interface Identifiers for all the stable
addresses of a given interface, including IPv6 global, link-local,
and unique-local addresses.
This means that this document does not formally obsolete or
deprecate any of the existing algorithms to generate Interface
Identifiers (e.g. such as that specified in [RFC2464]). However,
those IPv6 implementations that employ this specification MUST
generate all of their "stable" addresses as specified in this
document.
Implementations conforming to this specification SHOULD provide the
means for a system administrator to enable or disable the use of this
algorithm for generating Interface Identifiers.
Unless otherwise noted, all of the parameters included in the
expression below MUST be included when generating an Interface
Identifier.
1. Compute a random (but stable) identifier with the expression:
RID = F(Prefix, Net_Iface, Network_ID, DAD_Counter, secret_key)
Where:
RID:
Random (but stable) Interface Identifier
F():
A pseudorandom function (PRF) that is not computable from the
outside (without knowledge of the secret key), which should
produce an output of at least 64 bits.The PRF could be
implemented as a cryptographic hash of the concatenation of
each of the function parameters.
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Prefix:
The prefix to be used for SLAAC, as learned from an ICMPv6
Router Advertisement message, or the link-local IPv6 unicast
prefix.
Net_Iface:
An implementation-dependent stable identifier associated with
the network interface for which the RID is being generated.
An implementation MAY provide a configuration option to select
the source of the identifier to be used for the Net_Iface
parameter. A discussion of possible sources for this value
(along with the corresponding trade-offs) can be found in
Appendix A.
Network_ID:
Some network specific data that identifies the subnet to which
this interface is attached. For example the IEEE 802.11
Service Set Identifier (SSID) corresponding to the network to
which this interface is associated. This parameter is
OPTIONAL.
DAD_Counter:
A counter that is employed to resolve Duplicate Address
Detection (DAD) conflicts. It MUST be initialized to 0, and
incremented by 1 for each new tentative address that is
configured as a result of a DAD conflict. Implementations
that record DAD_Counter in non-volatile memory for each
{Prefix, Net_Iface, Network_ID} tuple MUST initialize
DAD_Counter to the recorded value if such an entry exists in
non-volatile memory). See Section 4 for additional details.
secret_key:
A secret key that is not known by the attacker. The secret
key MUST be initialized at operating system installation time
to a pseudo-random number (see [RFC4086] for randomness
requirements for security). An implementation MAY provide the
means for the the system administrator to change or display
the secret key.
2. The Interface Identifier is finally obtained by taking as many
bits from the RID value (computed in the previous step) as
necessary, starting from the least significant bit.
We note that [RFC4291] requires that, the Interface IDs of all
unicast addresses (except those that start with the binary
value 000) be 64-bit long. However, the method discussed in
this document could be employed for generating Interface IDs
of any arbitrary length, albeit at the expense of reduced
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entropy (when employing Interface IDs smaller than 64 bits).
The resulting Interface Identifier should be compared against the
Subnet-Router Anycast [RFC4291] and the Reserved Subnet Anycast
Addresses [RFC2526], and against those Interface Identifiers
already employed in an address of the same network interface and
the same network prefix. In the event that an unacceptable
identifier has been generated, this situation should be handled
in the same way as the case of duplicate addresses (see
Section 4).
This document does not require the use of any specific PRF for the
function F() above, since the choice of such PRF is usually a trade-
off between a number of properties (processing requirements, ease of
implementation, possible intellectual property rights, etc.), and
since the best possible choice for F() might be different for
different types of devices (e.g. embedded systems vs. regular
servers) and might possibly change over time.
Note that the result of F() in the algorithm above is no more secure
than the secret key. If an attacker is aware of the PRF that is
being used by the victim (which we should expect), and the attacker
can obtain enough material (i.e. addresses configured by the victim),
the attacker may simply search the entire secret-key space to find
matches. To protect against this, the secret key should be of a
reasonable length. Key lengths of at least 128 bits should be
adequate. The secret key is initialized at system installation time
to a pseudo-random number, thus allowing this mechanism to be
enabled/used automatically, without user intervention.
Including the SLAAC prefix in the PRF computation causes the
Interface Identifier to vary across each prefix (link-local, global,
etc.) employed by the node and, as consequently, also across
networks. This mitigates the correlation of activities of multi-
homed nodes (since each of the corresponding addresses will employ a
different Interface ID), host-tracking (since the network prefix will
change as the node moves from one network to another), and any other
attacks that benefit from predictable Interface Identifiers (such as
address scanning attacks).
The Net_Iface is a value that identifies the network interface for
which an IPv6 address is being generated. The following properties
are required for the Net_Iface parameter:
o it MUST be constant across system bootstrap sequences and other
network events (e.g., bringing another interface up or down)
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o it MUST be different for each network interface simultaneously in
use
Since the stability of the addresses generated with this method
relies on the stability of all arguments of F(), it is key that the
Net_Iface be constant across system bootstrap sequences and other
network events. Additionally, the Net_Iface must uniquely identify
an interface within the node, such that two interfaces connecting to
the same network do not result in duplicate addresses. Different
types of operating systems might benefit from different stability
properties of the Net_Iface parameter. For example, a client-
oriented operating system might want to employ Net_Iface identifiers
that are attached to the underlying network interface card (NIC),
such that a removable NIC always gets the same IPv6 address,
irrespective of the system communications port to which it is
attached. On the other hand, a server-oriented operating system
might prefer Net_Iface identifiers that are attached to system slots/
ports, such that replacement of a network interface card does not
result in an IPv6 address change. Appendix A discusses possible
sources for the Net_Iface, along with their pros and cons.
Including the optional Network_ID parameter when computing the RID
value above would cause the algorithm to produce a different
Interface Identifier when connecting to different networks, even when
configuring addresses belonging to the same prefix. This means that
a host would employ a different Interface Identifier as it moves from
one network to another even for IPv6 link-local addresses or Unique
Local Addresses (ULAs). In those scenarios where the Network_ID is
unknown to the attacker, including this parameter might help mitigate
attacks where a victim node connects to the same subnet as the
attacker, and the attacker tries to learn the Interface Identifier
used by the victim node for a remote network (see Section 7 for
further details).
The DAD_Counter parameter provides the means to intentionally cause
this algorithm produce a different IPv6 addresses (all other
parameters being the same). This could be necessary to resolve
Duplicate Address Detection (DAD) conflicts, as discussed in detail
in Section 4.
Finally, we note that all of the bits in the resulting Interface IDs
are treated as "opaque" bits. For example, the universal/local bit
of Modified EUI-64 format identifiers is treated as any other bit of
such identifier. In theory, this might result in Duplicate Address
Detection (DAD) failures that would otherwise not be encountered.
However, this is not deemed as a real issue, because of the following
considerations:
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o The interface IDs of all addresses (except those of addresses that
that start with the binary value 000) are 64-bit long. Since the
method specified in this document results in random Interface IDs,
the probability of DAD failures is very small.
o Real world data indicates that MAC address reuse is far more
common than assumed [HDMoore]. This means that even IPv6
addresses that employ (allegedly) unique identifiers (such as IEEE
identifiers) might result in DAD failures, and hence
implementations should be prepared to gracefully handle such
occurrences.
o Since some popular and widely-deployed operating systems (such as
Microsoft Windows) do not employ unique hardware identifiers for
the Interface IDs of their stable addresses, reliance on such
unique identifiers is more reduced in the deployed world (fewer
deployed systems rely on them for the avoidance of address
collisions).
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4. Resolving Duplicate Address Detection (DAD) conflicts
If as a result of performing Duplicate Address Detection (DAD)
[RFC4862] a host finds that the tentative address generated with the
algorithm specified in Section 3 is a duplicate address, it SHOULD
resolve the address conflict by trying a new tentative address as
follows:
o DAD_Counter is incremented by 1.
o A new Interface Identifier is generated with the algorithm
specified in Section 3, using the incremented DAD_Counter value.
This procedure may be repeated a number of times until the address
conflict is resolved. Hosts SHOULD try at least IDGEN_RETRIES (see
Section 5) tentative addresses if DAD fails for successive generated
addresses, in the hopes of resolving the address conflict. We also
note that hosts MUST limit the number of tentative addresses that are
tried (rather than indefinitely try a new tentative address until the
conflict is resolved).
In those (unlikely) scenarios in which duplicate addresses are
detected and in which the order in which the conflicting nodes
configure their addresses may vary (e.g., because they may be
bootstrapped in different order), the algorithm specified in this
section for resolving DAD conflicts could lead to addresses that are
not stable within the same subnet. In order to mitigate this
potential problem, nodes MAY record the DAD_Counter value employed
for a specific {Prefix, Net_Iface, Network_ID} tuple in non-volatile
memory, such that the same DAD_Counter value is employed when
configuring an address for the same Prefix and subnet at any other
point in time.
In the event that a DAD conflict cannot be solved (possibly after
trying a number of different addresses), address configuration would
fail. In those scenarios, nodes MUST NOT automatically fall back to
employing other algorithms for generating Interface Identifiers.
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5. Specified Constants
This document specifies the following constant:
IDGEN_RETRIES:
defaults to 3.
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6. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
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7. Security Considerations
This document specifies an algorithm for generating Interface
Identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), as an alternative to e.g. Interface Identifiers that embed
IEEE identifiers (such as those specified in [RFC2464]). When
compared to such identifiers, the identifiers specified in this
document have a number of advantages:
o They prevent trivial host-tracking, since when a host moves from
one network to another the network prefix used for
autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID)
will typically change, and hence the resulting Interface
Identifier will also change (see Appendix C.1).
o They mitigate address-scanning techniques which leverage
predictable Interface Identifiers (e.g., known Organizationally
Unique Identifiers) [I-D.ietf-opsec-ipv6-host-scanning].
o They may result in IPv6 addresses that are independent of the
underlying hardware (i.e. the resulting IPv6 addresses do not
change if a network interface card is replaced) if an appropriate
source for Net_Iface (Section 3) is employed.
o They prevent the information leakage produced by embedding
hardware addresses in the Interface Identifier (which could be
exploited to launch device-specific attacks).
o Since the method specified in this document will result in
different Interface Identifiers for each configured address,
knowledge/leakage of the Interface Identifier employed for one
stable address of will not negatively affect the security/privacy
of other stable addresses configured for other prefixes (whether
at the same time or at some other point in time).
In scenarios in which an attacker can connect to the same subnet as a
victim node, the attacker might be able to learn the Interface
Identifier employed by the victim node for an arbitrary prefix, by
simply sending a forged Router Advertisement [RFC4861] for that
prefix, and subsequently learning the corresponding address
configured by the victim node (either listening to the Duplicate
Address Detection packets, or to any other traffic that employs the
newly configured address). We note that a number of factors might
limit the ability of an attacker to successfully perform such an
attack:
o First-Hop security mechanisms such as RA-Guard [RFC6105]
[I-D.ietf-v6ops-ra-guard-implementation] could prevent the forged
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Router Advertisement from reaching the victim node
o If the victim implementation includes the (optional) Network_ID
parameter for computing F() (see Section 3), and the Network_ID
employed by the victim for a remote network is unknown to the
attacker, the Interface Identifier learned by the attacker would
differ from the one used by the victim when connecting to the
legitimate network.
In any case, we note that at the point in which this kind of attack
becomes a concern, a host should consider employing Secure Neighbor
Discovery (SEND) [RFC3971] to prevent an attacker from illegitimately
claiming authority for a network prefix.
We note that this algorithm is meant to be an alternative to
Interface Identifiers such as those specified in [RFC2464], but is
not meant as an alternative to temporary Interface Identifiers (such
as those specified in [RFC4941]). Clearly, temporary addresses may
help to mitigate the correlation of activities of a node within the
same network, and may also reduce the attack exposure window (since
temporary addresses are short-lived when compared to the addresses
generated with the method specified in this document). We note that
implementation of this algorithm would still benefit those hosts
employing "temporary addresses", since it would mitigate host-
tracking vectors still present when such addresses are used (see
Appendix C.1), and would also mitigate address-scanning techniques
that leverage patterns in IPv6 addresses that embed IEEE identifiers.
Finally, we note that the method described in this document addresses
some of the privacy concerns arising from the use of IPv6 addresses
that embed IEEE identifiers, without the use of temporary addresses,
thus possibly offering an interesting trade-off for those scenarios
in which the use of temporary addresses is not feasible.
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8. Acknowledgements
The algorithm specified in this document has been inspired by Steven
Bellovin's work ([RFC1948]) in the area of TCP sequence numbers.
The author would like to thank (in alphabetical order) Ran Atkinson,
Karl Auer, Steven Bellovin, Matthias Bethke, Ben Campbell, Brian
Carpenter, Tassos Chatzithomaoglou, Tim Chown, Alissa Cooper, Dominik
Elsbroek, Brian Haberman, Bob Hinden, Christian Huitema, Ray Hunter,
Jouni Korhonen, Eliot Lear, Jong-Hyouk Lee, Andrew McGregor, Tom
Petch, Michael Richardson, Mark Smith, Dave Thaler, Ole Troan, James
Woodyatt, and He Xuan, for providing valuable comments on earlier
versions of this document.
This document is based on the technical report "Security Assessment
of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI).
Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
their continued support.
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9. References
9.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
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[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
9.2. Informative References
[RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
RFC 1948, May 1996.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[I-D.ietf-opsec-ipv6-host-scanning]
Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", draft-ietf-opsec-ipv6-host-scanning-01 (work in
progress), April 2013.
[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)",
draft-ietf-v6ops-ra-guard-implementation-07 (work in
progress), November 2012.
[HDMoore] HD Moore, "The Wild West", Louisville, Kentucky, U.S.A.
September 25-29, 2012,
<https://speakerdeck.com/hdm/derbycon-2012-the-wild-west>.
[Gont-DEEPSEC2011]
Gont, "Results of a Security Assessment of the Internet
Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
Vienna, Austria, November 2011, <http://
www.si6networks.com/presentations/deepsec2011/
fgont-deepsec2011-ipv6-security.pdf>.
[Gont-BRUCON2012]
Gont, "Recent Advances in IPv6 Security", BRUCON 2012
Conference, Ghent, Belgium, September 2012, <http://
www.si6networks.com/presentations/brucon2012/
fgont-brucon2012-recent-advances-in-ipv6-security.pdf>.
[Broersma]
Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
enabled environment", Australian IPv6 Summit 2010,
Melbourne, VIC Australia, October 2010, <http://
www.ipv6.org.au/10ipv6summit/talks/Ron_Broersma.pdf>.
[IAB-PRIVACY]
IAB, "Privacy and IPv6 Addresses", July 2011, <http://
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www.iab.org/wp-content/IAB-uploads/2011/07/
IPv6-addresses-privacy-review.txt>.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
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Appendix A. Possible sources for the Net_Iface parameter
The following subsections describe a number of possible sources for
the Net_Iface parameter employed by the F() function in Section 3.
The choice of a specific source for this value represents a number of
trade-offs, which may vary from one implementation to another.
A.1. Interface Index
The Interface Index [RFC3493] [RFC3542] of an interface uniquely
identifies an interface within a node. However, these identifiers
might or might not have the stability properties required for the
Net_Iface value employed by this method. For example, the Interface
Index might change upon removal or installation of a network
interface (typically one with a smaller value for the Interface
Index, when such a naming scheme is used), or when network interfaces
happen to be initialized in a different order. We note that some
implementations are known to provide configuration knobs to set the
Interface Index for a given interface. Such configuration knobs
could be employed to prevent the Interface Index from changing (e.g.
as a result of the removal of a network interface).
A.2. Interface Name
The Interface Name (e.g., "eth0", "em0", etc) tends to be more stable
than the underlying Interface Index, since such stability is
required/desired when interface names are employed in network
configuration (firewall rules, etc.). The stability properties of
Interface Names depend on implementation details, such as what is the
namespace used for Interface Names. For example, "generic" interface
names such as "eth0" or "wlan0" will generally be invariant with
respect to network interface card replacements. On the other hand,
vendor-dependent interface names such as "rtk0" or the like will
generally change when a network interface card is replaced with one
from a different vendor.
We note that Interface Names might still change when network
interfaces are added or removed once the system has been bootstrapped
(for example, consider Universal Serial Bus-based network interface
cards which might be added or removed once the system has been
bootstrapped).
A.3. Link-layer Addresses
Link-layer addresses typically provide for unique identifiers for
network interfaces; although, for obvious reasons, they generally
change when a network interface card is replaced. In scenarios where
neither Interface Indexes nor Interface Names have the stability
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properties specified in Section 3 for Net_Iface, an implementation
might want to employ the link-layer address of the interface for the
Net_Iface parameter, albeit at the expense of making the
corresponding IPv6 addresses dependent on the underlying network
interface card (i.e., the corresponding IPv6 address would typically
change upon replacement of the underlying network interface card).
A.4. Logical Network Service Identity
Host operating systems with a conception of logical network service
identity, distinct from network interface identity or index, may keep
a Universally Unique Identifier (UUID) [RFC4122] or similar
identifier with the stability properties appropriate for use as the
Net_Iface parameter.
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Appendix B. Security/privacy issues with traditional SLAAC addresses
This section provides additional details regarding security/privacy
issues arising from traditional SLAAC addresses- Namely, it provides
additional details regarding how those issues could be exploited, and
the extent to which the method specified in this document mitigates
such issues.
B.1. Correlation of node activities within the same network
Since traditional SLAAC addresses employ Interface Identifiers that
are constant within the same network, such identifiers can be
leveraged to correlate the activities of a node within the same
network. One sample scenario is that in which a client repeatedly
connects to a server over a period of time, and hence, based on the
stable Interface Identifier, the server can correlate all
communication instances as being initiated by the same node.
The method specified in this document does not mitigate this attack
vector, since it produces Interface Identifiers that are constant
within a given network.
This attack vector could only be mitigated by employing "temporary
addresses" [RFC4941]. However, as noted earlier in this document,
in scenarios in which there is a reduced number of nodes in a
given network, mitigation of this vector might be difficult (if at
all possible) -- even with "temporary addresses" [RFC4941] in
place.
B.2. Correlation of node activities across networks
Since traditional SLAAC addresses employ Interface Identifiers that
are constant across networks, such identifiers can be leveraged to
correlate the activities of a node across networks. One sample
scenario is that in which a client repeatedly connects to a server
over a period of time, and hence, based on the stable Interface
Identifier, the server can correlate all communication instances as
being initiated by the same node. Since the method specified in this
document results in Interface Identifiers that are not constant
across networks, this attack vector is mitigated.
B.3. Host-tracking attacks
Since traditional SLAAC addresses employ Interface Identifiers that
are constant across networks, such identifiers can be leveraged to
track a node across networks.
For example, let us assume that the attacker knows the Interface
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Identifier employed by the target node. If the target node contacts
an attacker-operated node each time it moves from one network to
another, the attacker will be able to track the node as it moves from
one network to another.
An active version of this attack would imply that, once the Interface
Identifier is known to the attacker the attacker probes whether there
is an address with that Interface Identifier in each target network
(i.e., in each network the client might connect to). If such address
is found to be "alive", then the attacker could infer that the target
node has connected to the corresponding network.
This vector is discussed in detail in Appendix C.1.2.
Since the method specified in this document results in Interface
Identifiers that are not constant across networks, this attack vector
is mitigated.
B.4. Address-scanning attacks
Since traditional SLAAC addresses typically embed the underlying
link-layer address, the aforementioned addresses follow specific
patterns that can be leveraged to reduce the search space when
performing IPv6 address-scanning attacks (this is discussed in detail
in [I-D.ietf-opsec-ipv6-host-scanning]). The method specified in
this document produces random (but table within each subnet)
Interface Identifiers, thus mitigating this attack vector.
B.5. Exploitation of device-specific information
Since traditional SLAAC addresses typically embed the underlying
link-layer address, the aforementioned addresses leaks device-
specific information that might be leveraged to launch device-
specific attacks. For example, an attacker with knowledge about a
specific vulnerability in devices manufactured by some vendor might
easily identify potential targets by looking at the Interface
Identifier of a list of IPv6 addresses. The method specified in this
document produces random (but table within each subnet) Interface
Identifiers, thus mitigating this attack vector.
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Appendix C. Privacy issues still present when temporary addresses are
employed
It is not unusual for people to assume or expect that all the
security/privacy implications of traditional SLAAC addresses are
mitigated when "temporary addresses" [RFC4941] are employed.
However, as noted in Section 1 of this document and [IAB-PRIVACY],
since temporary addresses are employed in addition to (rather than in
replacement of) traditional SLAAC addresses, many of the security/
privacy implications of traditional SLAAC addresses are not mitigated
by the use of temporary addresses.
This section discusses a (non-exhaustive) number of scenarios in
which host security/privacy is still negatively affected as a result
of employing Interface Identifiers that are constant across networks
(e.g., those resulting from embedding IEEE identifiers), even when
temporary addresses [RFC4941] are employed. It aims to clarify the
motivation of employing the method specified in this document in
replacement of the traditional SLAAC addresses even when privacy/
temporary addresses [RFC4941] are employed.
C.1. Host tracking
This section describes two attack scenarios which illustrate that
host-tracking may still be possible when privacy/temporary addresses
[RFC4941] are employed. These examples should remind us that one
should not disclose more than it is really needed for achieving a
specific goal (and an Interface Identifier that is constant across
different networks does exactly that: it discloses more information
than is needed for providing a stable address).
C.1.1. Tracking hosts across networks #1
A host configures its stable addresses with the constant Interface
Identifier, and runs any application that needs to perform a server-
like function (e.g. a peer-to-peer application). As a result of
that, an attacker/user participating in the same application (e.g.,
P2P) would learn the constant Interface Identifier used by the host
for that network interface.
Some time later, the same host moves to a completely different
network, and employs the same P2P application. The attacker now
interacts with the same host again, and hence can learn its newly-
configured stable address. Since the Interface Identifier is the
same as the one used before, the attacker can infer that it is
communicating with the same device as before.
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C.1.2. Tracking hosts across networks #2
Once an attacker learns the constant Interface Identifier employed by
the victim host for its stable address, the attacker is able to
"probe" a network for the presence of such host at any given network.
See Appendix C.1.1 for just one example of how an attacker could
learn such value. Other examples include being able to share the
same network segment at some point in time (e.g. a conference
network or any public network), etc.
For example, if an attacker learns that in one network the victim
used the Interface Identifier 1111:2222:3333:4444 for its stable
addresses, then he could subsequently probe for the presence of such
device in the network 2011:db8::/64 by sending a probe packet (ICMPv6
Echo Request, or any other probe packet) to the address 2001:db8::
1111:2222:3333:4444.
C.1.3. Revealing the identity of devices performing server-like
functions
Some devices, such as storage devices, may typically perform server-
like functions and may be usually moved from one network to another.
Such devices are likely to simply disable (or not even implement)
privacy/temporary addresses [RFC4941]. If the aforementioned devices
employ Interface Identifiers that are constant across networks, it
would be trivial for an attacker to tell whether the same device is
being used across networks by simply looking at the Interface
Identifier. Clearly, performing server-like functions should not
imply that a device discloses its identity (i.e., that the attacker
can tell whether it is the same device providing some function in two
different networks, at two different points in time).
The scheme proposed in this document prevents such information
leakage by causing nodes to generate different Interface Identifiers
when moving from one network to another, thus mitigating this kind of
privacy attack.
C.2. Address-scanning attacks
While it is usually assumed that IPv6 address-scanning attacks are
unfeasible, an attacker can leverage address patterns in IPv6
addresses to greatly reduce the search space
[I-D.ietf-opsec-ipv6-host-scanning] [Gont-BRUCON2012]. Addresses
that embed IEEE identifiers result in one of such patterns that could
be leveraged to reduce the search space when other nodes employ the
same IEEE OUI (Organizationally Unique Identifier).
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As noted earlier in this document, temporary addresses [RFC4941] do
not replace/eliminate the use of IPv6 addresses that embed IEEE
identifiers (they are employed in addition to those), and hence hosts
implementing [RFC4941] would still be vulnerable to address-scanning
attacks. The method specified in this document is meant as an
alternative to addresses that embed IEEE identifiers, and hence
eliminates such patterns (thus mitigating the aforementioned address-
scanning attacks).
C.3. Information Leakage
IPv6 addresses embedding IEEE identifiers leak information about the
device (Network Interface Card vendor, or even Operating System
and/or software type), which could be leveraged by an attacker with
device/software-specific vulnerabilities knowledge to quickly find
possible targets. Since temporary addresses do not replace the
traditional SLAAC addresses that typically embed IEEE identifiers,
employing temporary addresses does not eliminate this possible
information leakage.
C.4. Correlation of node activities within a network
In scenarios in which the number of nodes connected to a subnetwork
is small, preventing the correlation of the activities of those nodes
within such network might be difficult (if at all possible) to
achieve, even with temporary addresses [RFC4941] in place. As a
trivial example, consider a scenario where there is a single node (or
a reduced number of nodes) connected to a specific network. An
attacker could detect new addresses in use at that network along with
addresses that are no longer in use, and infer which addresses are
being employed by which hosts. This task is made particularly easier
by the fact that use of "temporary addresses" can be easily inferred
(since they follow different patterns from that of traditional SLAAC
addresses), and since they are re-generated periodically (i.e., after
a specific amount of time has elapsed).
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Author's Address
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
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