Internet Engineering Task Force (IETF) F. Gont

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 7217 SI6 Networks / UTN-FRH
Category: Standards Track April 2014
ISSN: 2070-1721

A Method for Generating Semantically Opaque Interface Identifiers
with IPv6 Stateless Address Autoconfiguration (SLAAC)

Abstract

This document specifies a method for generating IPv6 Interface
Identifiers to be used with IPv6 Stateless Address Autoconfiguration
(SLAAC), such that an IPv6 address configured using this method is
stable within each subnet, but the corresponding Interface Identifier
changes when the host moves from one network to another. This method
is meant to be an alternative to generating Interface Identifiers
based on hardware addresses (e.g., IEEE LAN Media Access Control
(MAC) addresses), such that the benefits of stable addresses can be
achieved without sacrificing the security and 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 prefixes
(and their corresponding addresses).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7217.

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Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Relationship to Other Standards . . . . . . . . . . . . . . . 5
4. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Algorithm Specification . . . . . . . . . . . . . . . . . . . 7
6. Resolving DAD Conflicts . . . . . . . . . . . . . . . . . . . 12
7. Specified Constants . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Possible Sources for the Net_Iface Parameter . . . . 19
A.1. Interface Index . . . . . . . . . . . . . . . . . . . . . 19
A.2. Interface Name . . . . . . . . . . . . . . . . . . . . . 19
A.3. Link-Layer Addresses . . . . . . . . . . . . . . . . . . 19
A.4. Logical Network Service Identity . . . . . . . . . . . . 20

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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., an IEEE LAN MAC address) [RFC4291].
Cryptographically Generated Addresses (CGAs) [RFC3972] are yet
another method for generating Interface Identifiers; CGAs 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 host activities within the same network,
thus negatively affecting the privacy of users (see
[ADDR-GEN-PRIVACY] and [IAB-PRIVACY]).

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 host 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 [IPV6-RECON]. For example,
the IPv6 addresses of all hosts manufactured by the same vendor
(within a given time frame) will likely contain the same IEEE
Organizationally Unique Identifier (OUI) in the Interface
Identifier.

o Embedding the underlying hardware 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.

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[ADDR-GEN-PRIVACY] provides additional details regarding how the
aforementioned vulnerabilities could be exploited and the extent to
which the method discussed in this document mitigates them.

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 host, and
basically result in temporary (and random) Interface Identifiers.
These temporary addresses are generated in addition to the
traditional IPv6 addresses based on IEEE LAN MAC addresses, 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,
troubleshooting, 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 typically been generated from
the underlying hardware addresses. 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 host'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 [ADDR-GEN-PRIVACY] 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 SLAAC
addresses to greatly reduce the search space for "alive" nodes
[GONT-DEEPSEC2011] [CPNI-IPV6] [IPV6-RECON].

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Hence, there is a motivation to improve the properties of "stable"
addresses regardless of whether or not temporary addresses are
employed.

This document specifies a method to generate Interface Identifiers
that are stable for each network interface within each subnet, but
that change as a host moves from one network to another. Thus, this
method enables 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
host as it moves from one network to another.

2. Terminology

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 [RFC2119].

3. Relationship to Other Standards

The method specified in this document is 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 LAN MAC addresses) will mitigate
address-scanning attacks and also mitigate the remaining vectors for
correlating host activities based on the host's constant (i.e.,
stable across networks) Interface Identifiers. On the other hand,
for hosts that currently disable temporary addresses [RFC4941],
implementation of this mechanism would mitigate the host-tracking and
address-scanning issues discussed in Section 1.

While the method specified in this document is meant to be used with
SLAAC, this does not preclude this algorithm from being used with
other address configuration mechanisms, such as DHCPv6 [RFC3315] or
manual address configuration.

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4. Design Goals

This document specifies a method for generating Interface Identifiers
to be used with IPv6 SLAAC, with the following goals:

o The resulting Interface Identifiers remain stable for each prefix
used with SLAAC within each subnet for the same network interface.
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 must 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 to tell whether the addresses
have been generated by the same host.

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 5
and Appendix A), the resulting Interface Identifiers may be
independent of the underlying hardware (e.g., IEEE LAN MAC
address). For example, this means that replacing a Network
Interface Card (NIC) or adding links dynamically to a Link
Aggregation Group (LAG) 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 hardware
addresses (e.g., IEEE LAN MAC addresses, as specified in
[RFC2464]). That is, this document does not formally obsolete or
deprecate any of the existing algorithms to generate Interface
Identifiers. 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 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

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Address Autoconfiguration (SLAAC) [RFC4862] itself, but rather it
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 traditional IPv6 addresses (that embed a hardware address)
resulting from SLAAC [RFC4862].

5. Algorithm Specification

IPv6 implementations conforming to this specification MUST generate
Interface Identifiers using the algorithm specified in this section
as a replacement for any other algorithms for generating "stable"
addresses with SLAAC (such as those specified in [RFC2464],
[RFC2467], and [RFC2470]). 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 with SLAAC for all the stable addresses, including IPv6
global, link-local, and unique-local addresses.

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) Identifier

F():
A pseudorandom function (PRF) that MUST NOT be computable from
the outside (without knowledge of the secret key). F() MUST
also be difficult to reverse, such that it resists attempts to
obtain the secret_key, even when given samples of the output
of F() and knowledge or control of the other input parameters.
F() SHOULD produce an output of at least 64 bits. F() could

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be implemented as a cryptographic hash of the concatenation of
each of the function parameters. SHA-1 [FIPS-SHS] and SHA-256
are two possible options for F(). Note: MD5 [RFC1321] is
considered unacceptable for F() [RFC6151].

Prefix:
The prefix to be used for SLAAC, as learned from an ICMPv6
Router Advertisement message, or the link-local IPv6 unicast
prefix [RFC4291].

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. Additionally, Simple DNA
[RFC6059] describes ideas that could be leveraged to generate
a Network_ID parameter. 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 6 for additional details.

secret_key:
A secret key that is not known by the attacker. The secret
key SHOULD be of at least 128 bits. It MUST be initialized to
a pseudo-random number (see [RFC4086] for randomness
requirements for security) when the operating system is
installed or when the IPv6 protocol stack is "bootstrapped"
for the first time. An implementation MAY provide the means
for the system administrator to display and change the secret
key.

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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 bits 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
entropy (when employing Interface IDs smaller than 64 bits).

The resulting Interface Identifier SHOULD be compared against the
reserved IPv6 Interface Identifiers [RFC5453] [IANA-RESERVED-IID]
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 6).

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.

Including the SLAAC prefix in the PRF computation causes the
Interface Identifier to vary across each prefix (link-local, global,
etc.) employed by the host and, consequently, also across networks.
This mitigates the correlation of activities of multihomed hosts
(since each of the corresponding addresses will typically employ a
different prefix), host-tracking (since the network prefix will
change as the host moves from one network to another), and any other
attacks that benefit from predictable Interface Identifiers (such as
IPv6 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).

o It MUST be different for each network interface simultaneously in
use.

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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 parameter be constant across system bootstrap sequences and
other network events. Additionally, the Net_Iface parameter must
uniquely identify an interface within the host, 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 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 NIC 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 causes 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) [RFC4193]. In those scenarios where the
Network_ID is unknown to the attacker, including this parameter might
help mitigate attacks where a victim host connects to the same subnet
as the attacker and the attacker tries to learn the Interface
Identifier used by the victim host for a remote network (see
Section 8 for further details).

The DAD_Counter parameter provides the means to intentionally cause
this algorithm to produce different IPv6 addresses (all other
parameters being the same). This could be necessary to resolve DAD
conflicts, as discussed in detail in Section 6.

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, key lengths of at least
128 bits should be adequate. The secret key is initialized at system
installation time to a pseudorandom number, thus allowing this
mechanism to be enabled and used automatically, without user
intervention. Providing a mechanism to display and change the
secret_key would allow an administrator to cause a new/replacement
system (with the same implementation of this specification) to

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generate the same IPv6 addresses as the system being replaced. We
note that since the privacy of the scheme specified in this document
relies on the secrecy of the secret_key parameter, implementations
should constrain access to the secret_key parameter to the extent
practicable (e.g., require superuser privileges to access it).
Furthermore, in order to prevent leakages of the secret_key
parameter, it should not be used for any purposes other than being a
parameter to the scheme specified in this document.

We note that all of the bits in the resulting Interface IDs are
treated as "opaque" bits [RFC7136]. For example, the universal/local
bit of Modified EUI-64 format identifiers is treated as any other bit
of such an identifier. In theory, this might result in IPv6 address
collisions and DAD failures that would otherwise not be encountered.
However, this is not deemed as a likely issue because of the
following considerations:

o The interface IDs of all addresses (except those of addresses that
start with the binary value 000) are 64 bits 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 [HD-MOORE]. This means that even IPv6
addresses that employ (allegedly) unique identifiers (such as IEEE
LAN MAC addresses) might result in DAD failures and, hence,
implementations should be prepared to gracefully handle such
occurrences. Additionally, some virtualization technologies
already employ hardware addresses that are randomly selected, and,
hence, cannot be guaranteed to be unique [IPV6-RECON].

o Since some popular and widely deployed operating systems (such as
Microsoft Windows) do not embed hardware addresses in the
Interface IDs of their stable addresses, reliance on such unique
identifiers is reduced in the deployed world (fewer deployed
systems rely on them for the avoidance of address collisions).

Finally, we note that since different implementations are likely to
use different values for the secret_key parameter, and may also
employ different PRFs for F() and different sources for the Net_Iface
parameter, the addresses generated by this scheme should not expected
to be stable across different operating-system installations. For
example, a host that is dual-boot or that is reinstalled may result
in different IPv6 addresses for each operating system and/or
installation.

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6. Resolving DAD Conflicts

If, as a result of performing DAD [RFC4862], a host finds that the
tentative address generated with the algorithm specified in Section 5
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 5, using the incremented DAD_Counter value.

Hosts SHOULD introduce a random delay between 0 and IDGEN_DELAY
seconds (see Section 7) before trying a new tentative address, to
avoid lockstep behavior of multiple hosts.

This procedure may be repeated a number of times until the address
conflict is resolved. Hosts SHOULD try at least IDGEN_RETRIES (see
Section 7) 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 the order in which the conflicting hosts configure their
addresses varies (e.g., because they may be bootstrapped in different
orders), 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, hosts 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. We note that
the use of non-volatile memory is OPTIONAL, and hosts that do not
implement this feature are still compliant to this protocol
specification.

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, hosts MUST NOT automatically fall back to
employing other algorithms for generating Interface Identifiers.

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7. Specified Constants

This document specifies the following constant:

IDGEN_RETRIES:
defaults to 3.

IDGEN_DELAY:
defaults to 1 second.

8. 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
hardware addresses (such as those specified in [RFC2464], [RFC2467],
and [RFC2470]). When compared to such identifiers, the identifiers
specified in this document have a number of advantages:

o They prevent trivial host-tracking based on the IPv6 address,
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; hence, the resulting
Interface Identifier will also change (see [ADDR-GEN-PRIVACY]).

o They mitigate address-scanning techniques that leverage
predictable Interface Identifiers (e.g., known Organizationally
Unique Identifiers) [IPV6-RECON].

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 (see Section 5) 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 or leakage of the Interface Identifier employed for one
stable address 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).

We note that while some probing techniques (such as the use of ICMPv6
Echo Request and ICMPv6 Echo Response packets) could be mitigated by
a personal firewall at the target host, for other probing vectors,

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such as listening to ICMPv6 "Destination Unreachable, Address
Unreachable" (Type 1, Code 3) error messages that refer to the target
addresses [IPV6-RECON], 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). Hence, the method specified in this
document is still of value for hosts that employ personal firewalls.

In scenarios in which an attacker can connect to the same subnet as a
victim host, the attacker might be able to learn the Interface
Identifier employed by the victim host 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 host (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 Router Advertisement Guard
(RA-Guard) [RFC6105] [RFC7113] could prevent the forged Router
Advertisement from reaching the victim host.

o If the victim implementation includes the (optional) Network_ID
parameter for computing F() (see Section 5), 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 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 it 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 host within the
same network, and they 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 the implementation of this specification would still
benefit those hosts employing temporary addresses, since it would
mitigate host-tracking vectors still present when such addresses are
used (see [ADDR-GEN-PRIVACY]) and would also mitigate address-
scanning techniques that leverage patterns in IPv6 addresses that
embed IEEE LAN MAC addresses. Finally, we note that the method

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described in this document addresses some of the privacy concerns
arising from the use of IPv6 addresses that embed IEEE LAN MAC
addresses, 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.

9. 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) Mikael
Abrahamsson, Ran Atkinson, Karl Auer, Steven Bellovin, Matthias
Bethke, Ben Campbell, Brian Carpenter, Tassos Chatzithomaoglou, Tim
Chown, Alissa Cooper, Dominik Elsbroek, Stephen Farrell, Eric Gray,
Brian Haberman, Bob Hinden, Christian Huitema, Ray Hunter, Jouni
Korhonen, Suresh Krishnan, Eliot Lear, Jong-Hyouk Lee, Andrew
McGregor, Thomas Narten, Simon Perreault, Tom Petch, Michael
Richardson, Vincent Roca, Mark Smith, Hannes Frederic Sowa, Martin
Stiemerling, Dave Thaler, Ole Troan, Lloyd Wood, James Woodyatt, and
He Xuan, for providing valuable comments on earlier versions of this
document.

Hannes Frederic Sowa produced a reference implementation of this
specification for the Linux kernel.

Finally, the author wishes to thank Nelida Garcia and Guillermo Gont
for their love and support.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 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.

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[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.

[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 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.

[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC
5453, February 2009.

[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, February 2014.

10.2. Informative References

[ADDR-GEN-PRIVACY]
Cooper, A., Gont, F., and D. Thaler, "Privacy
Considerations for IPv6 Address Generation Mechanisms",
Work in Progress, February 2014.

[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>.

[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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[FIPS-SHS] NIST, "Secure Hash Standard (SHS)", FIPS Publication
180-4, March 2012, <http://csrc.nist.gov/publications/
fips/fips180-4/fips-180-4.pdf>.

[GONT-DEEPSEC2011]
Gont, F., "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>.

[HD-MOORE] Moore, HD., "The Wild West", Louisville, Kentucky, U.S.A,
DerbyCon 2012, September 2012, <https://speakerdeck.com/
hdm/derbycon-2012-the-wild-west>.

[IAB-PRIVACY]
IAB, "Privacy and IPv6 Addresses", July 2011,
<http://www.iab.org/wp-content/IAB-uploads/2011/07/
IPv6-addresses-privacy-review.txt>.

[IANA-RESERVED-IID]
IANA, "Reserved IPv6 Interface Identifiers",
<http://www.iana.org/assignments/ipv6-interface-ids>.

[IPV6-RECON]
Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", Work in Progress, January 2014.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[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.

[RFC2467] Crawford, M., "Transmission of IPv6 Packets over FDDI
Networks", RFC 2467, December 1998.

[RFC2470] Crawford, M., Narten, T., and S. Thomas, "Transmission of
IPv6 Packets over Token Ring Networks", RFC 2470, December
1998.

[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6", RFC
3493, February 2003.

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[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.

[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059, November
2010.

[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.

[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.

[RFC7113] Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113, February 2014.

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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 5.
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 that interface within the 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 or 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 USB-based network interface cards that 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 in
which neither Interface Indexes nor Interface Names have the
stability properties specified in Section 5 for Net_Iface, an

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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 addresses 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.

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: [email protected]
URI: http://www.si6networks.com

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