IPv6 Working Group T. Narten
Internet-Draft IBM Corporation
Expires: March 16, 2005 R. Draves
Microsoft Research
S. Krishnan
Ericsson
September 15, 2004
Privacy Extensions for Stateless Address Autoconfiguration in IPv6
draft-ietf-ipv6-privacy-addrs-v2-00
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 16, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Nodes use IPv6 stateless address autoconfiguration to generate
addresses without the necessity of a Dynamic Host Configuration
Protocol (DHCP) server. Addresses are formed by combining network
prefixes with an interface identifier. On interfaces that contain
embedded IEEE Identifiers, the interface identifier is typically
derived from it. On other interface types, the interface identifier
Narten, et al. Expires March 16, 2005 [Page 1]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
is generated through other means, for example, via random number
generation. This document describes an extension to IPv6 stateless
address autoconfiguration for interfaces whose interface identifier
is derived from an IEEE identifier. Use of the extension causes
nodes to generate global scope addresses from interface identifiers
that change over time, even in cases where the interface contains an
embedded IEEE identifier. Changing the interface identifier (and the
global scope addresses generated from it) over time makes it more
difficult for eavesdroppers and other information collectors to
identify when different addresses used in different transactions
actually correspond to the same node.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Conventions used in this document . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Extended Use of the Same Identifier . . . . . . . . . . . 4
2.2 Address Usage in IPv4 Today . . . . . . . . . . . . . . . 5
2.3 The Concern With IPv6 Addresses . . . . . . . . . . . . . 6
2.4 Possible Approaches . . . . . . . . . . . . . . . . . . . 7
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 9
3.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Generation Of Randomized Interface Identifiers . . . . . . 11
3.2.1 When Stable Storage Is Present . . . . . . . . . . . . 11
3.2.2 In The Absence of Stable Storage . . . . . . . . . . . 12
3.2.3 Alternate approaches . . . . . . . . . . . . . . . . . 13
3.3 Generating Temporary Addresses . . . . . . . . . . . . . . 13
3.4 Expiration of Temporary Addresses . . . . . . . . . . . . 15
3.5 Regeneration of Randomized Interface Identifiers . . . . . 15
3.6 Deployment Considerations . . . . . . . . . . . . . . . . 16
4. Implications of Changing Interface Identifiers . . . . . . . . 18
5. Defined Constants . . . . . . . . . . . . . . . . . . . . . . 19
6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Significant Changes from RFC 3041 . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . 26
Narten, et al. Expires March 16, 2005 [Page 2]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
1. Introduction
Stateless address autoconfiguration [ADDRCONF] defines how an IPv6
node generates addresses without the need for a DHCP server. Some
types of network interfaces come with an embedded IEEE Identifier
(i.e., a link-layer MAC address), and in those cases stateless
address autoconfiguration uses the IEEE identifier to generate a 64-
bit interface identifier [ADDRARCH]. By design, the interface
identifier is likely to be globally unique when generated in this
fashion. The interface identifier is in turn appended to a prefix to
form a 128-bit IPv6 address.
All nodes combine interface identifiers (whether derived from an IEEE
identifier or generated through some other technique) with the
reserved link-local prefix to generate link-local addresses for their
attached interfaces. Additional addresses can then be created by
combining prefixes advertised in Router Advertisements via Neighbor
Discovery [DISCOVERY] with the interface identifier.
Not all nodes and interfaces contain IEEE identifiers. In such
cases, an interface identifier is generated through some other means
(e.g., at random), and the resultant interface identifier is not
globally unique and may also change over time. The focus of this
document is on addresses derived from IEEE identifiers, as the
concern being addressed exists only in those cases where the
interface identifier is globally unique and non-changing. The rest
of this document assumes that IEEE identifiers are being used, but
the techniques described may also apply to interfaces with other
types of globally unique and/or persistent identifiers.
This document discusses concerns associated with the embedding of
non-changing interface identifiers within IPv6 addresses and
describes extensions to stateless address autoconfiguration that can
help mitigate those concerns for individual users and in environments
where such concerns are significant. Section 2 provides background
information on the issue. Section 3 describes a procedure for
generating alternate interface identifiers and global scope
addresses. Section 4 discusses implications of changing interface
identifiers. The term "global scope addresses" is used in this
document to collectively refer to "Global unicast addresses" as
defined in [ADDRARCH] and "Unique local addresses" as defined in
[ULA]
1.1 Conventions used in this document
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].
Narten, et al. Expires March 16, 2005 [Page 3]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
2. Background
This section discusses the problem in more detail, provides context
for evaluating the significance of the concerns in specific
environments and makes comparisons with existing practices.
2.1 Extended Use of the Same Identifier
The use of a non-changing interface identifier to form addresses is a
specific instance of the more general case where a constant
identifier is reused over an extended period of time and in multiple
independent activities. Anytime the same identifier is used in
multiple contexts, it becomes possible for that identifier to be used
to correlate seemingly unrelated activity. For example, a network
sniffer placed strategically on a link across which all traffic to/
from a particular host crosses could keep track of which destinations
a node communicated with and at what times. Such information can in
some cases be used to infer things, such as what hours an employee
was active, when someone is at home, etc.
One of the requirements for correlating seemingly unrelated
activities is the use (and reuse) of an identifier that is
recognizable over time within different contexts. IP addresses
provide one obvious example, but there are more. Many nodes also
have DNS names associated with their addresses, in which case the DNS
name serves as a similar identifier. Although the DNS name
associated with an address is more work to obtain (it may require a
DNS query) the information is often readily available. In such
cases, changing the address on a machine over time would do little to
address the concerns raised in this document, unless the DNS name is
changed as well (see Section 4).
Web browsers and servers typically exchange "cookies" with each other
[COOKIES]. Cookies allow web servers to correlate a current activity
with a previous activity. One common usage is to send back targeted
advertising to a user by using the cookie supplied by the browser to
identify what earlier queries had been made (e.g., for what type of
information). Based on the earlier queries, advertisements can be
targeted to match the (assumed) interests of the end-user.
The use of a constant identifier within an address is of special
concern because addresses are a fundamental requirement of
communication and cannot easily be hidden from eavesdroppers and
other parties. Even when higher layers encrypt their payloads,
addresses in packet headers appear in the clear. Consequently, if a
mobile host (e.g., laptop) accessed the network from several
different locations, an eavesdropper might be able to track the
movement of that mobile host from place to place, even if the upper
Narten, et al. Expires March 16, 2005 [Page 4]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
layer payloads were encrypted.
2.2 Address Usage in IPv4 Today
Addresses used in today's Internet are often non-changing in practice
for extended periods of time, especially in non-home environments
(e.g., corporations, campuses, etc.). In such sites, addresses are
assigned statically and typically change infrequently. Over the last
few years, sites have begun moving away from static allocation to
dynamic allocation via DHCP [DHCP]. In theory, the address a client
gets via DHCP can change over time, but in practice servers often
return the same address to the same client (unless addresses are in
such short supply that they are reused immediately by a different
node when they become free). Thus, even within sites using DHCP,
clients frequently end up using the same address for weeks to months
at a time.
For home users accessing the Internet over dialup lines, the
situation is generally different. Such users do not have permanent
connections and are often assigned temporary addresses each time they
connect to their ISP (e.g., AOL). Consequently, the addresses they
use change frequently over time and are shared among a number of
different users. Thus, an address does not reliably identify a
particular device over time spans of more than a few minutes.
A more interesting case concerns always-on connections (e.g., cable
modems, ISDN, DSL, etc.) that result in a home site using the same
address for extended periods of time. This is a scenario that is
just starting to become common in IPv4 and promises to become more of
a concern as always-on internet connectivity becomes widely
available. Although it might appear that changing an address
regularly in such environments would be desirable to lessen privacy
concerns, it should be noted that the network prefix portion of an
address also serves as a constant identifier. All nodes at (say) a
home, would have the same network prefix, which identifies the
topological location of those nodes. This has implications for
privacy, though not at the same granularity as the concern that this
document addresses. Specifically, all nodes within a home would be
grouped together for the purposes of collecting information. This
issue is difficult to address, because the routing prefix part of an
address contains topology information and cannot contain arbitrary
values.
Finally, it should be noted that nodes that need a (non-changing) DNS
name generally have static addresses assigned to them to simplify the
configuration of DNS servers. Although Dynamic DNS [DDNS] can be
used to update the DNS dynamically, it is not yet widely deployed.
In addition, changing an address but keeping the same DNS name does
Narten, et al. Expires March 16, 2005 [Page 5]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
not really address the underlying concern, since the DNS name becomes
a non-changing identifier. Servers generally require a DNS name (so
clients can connect to them), and clients often do as well (e.g.,
some servers refuse to speak to a client whose address cannot be
mapped into a DNS name that also maps back into the same address).
Section 4 describes one approach to this issue.
2.3 The Concern With IPv6 Addresses
The division of IPv6 addresses into distinct topology and interface
identifier portions raises an issue new to IPv6 in that a fixed
portion of an IPv6 address (i.e., the interface identifier) can
contain an identifier that remains constant even when the topology
portion of an address changes (e.g., as the result of connecting to a
different part of the Internet). In IPv4, when an address changes,
the entire address (including the local part of the address) usually
changes. It is this new issue that this document addresses.
If addresses are generated from an interface identifier, a home
user's address could contain an interface identifier that remains the
same from one dialup session to the next, even if the rest of the
address changes. The way PPP is used today, however, PPP servers
typically unilaterally inform the client what address they are to use
(i.e., the client doesn't generate one on its own). This practice,
if continued in IPv6, would avoid the concerns that are the focus of
this document.
A more troubling case concerns mobile devices (e.g., laptops, PDAs,
etc.) that move topologically within the Internet. Whenever they
move (in the absence of technology such as mobile IP [MOBILEIP]),
they form new addresses for their current topological point of
attachment. This is typified today by the "road warrior" who has
Internet connectivity both at home and at the office. While the
node's address changes as it moves, however, the interface identifier
contained within the address remains the same (when derived from an
IEEE Identifier). In such cases, the interface identifier can be
used to track the movement and usage of a particular machine. For
example, a server that logs usage information together with a source
addresses, is also recording the interface identifier since it is
embedded within an address. Consequently, any data-mining technique
that correlates activity based on addresses could easily be extended
to do the same using the interface identifier. This is of particular
concern with the expected proliferation of next-generation
network-connected devices (e.g., PDAs, cell phones, etc.) in which
large numbers of devices are in practice associated with individual
users (i.e., not shared). Thus, the interface identifier embedded
within an address could be used to track activities of an individual,
even as they move topologically within the internet.
Narten, et al. Expires March 16, 2005 [Page 6]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
In summary, IPv6 addresses on a given interface generated via
Stateless Autoconfiguration contain the same interface identifier,
regardless of where within the Internet the device connects. This
facilitates the tracking of individual devices (and thus potentially
users). The purpose of this document is to define mechanisms that
eliminate this issue, in those situations where it is a concern.
2.4 Possible Approaches
One way to avoid some of the problems discussed above is to use DHCP
for obtaining addresses. With DHCP, the DHCP server could arrange to
hand out addresses that change over time.
Another approach, compatible with the stateless address
autoconfiguration architecture, would be to change the interface id
portion of an address over time and generate new addresses from the
interface identifier for some address scopes. Changing the interface
identifier can make it more difficult to look at the IP addresses in
independent transactions and identify which ones actually correspond
to the same node, both in the case where the routing prefix portion
of an address changes and when it does not.
Many machines function as both clients and servers. In such cases,
the machine would need a DNS name for its use as a server. Whether
the address stays fixed or changes has little privacy implication
since the DNS name remains constant and serves as a constant
identifier. When acting as a client (e.g., initiating
communication), however, such a machine may want to vary the
addresses it uses. In such environments, one may need multiple
addresses: a "public" (i.e., non-secret) server address, registered
in the DNS, that is used to accept incoming connection requests from
other machines, and a "temporary" address used to shield the identity
of the client when it initiates communication. These two cases are
roughly analogous to telephone numbers and caller ID, where a user
may list their telephone number in the public phone book, but disable
the display of its number via caller ID when initiating calls.
To make it difficult to make educated guesses as to whether two
different interface identifiers belong to the same node, the
algorithm for generating alternate identifiers must include input
that has an unpredictable component from the perspective of the
outside entities that are collecting information. Picking
identifiers from a pseudo-random sequence suffices, so long as the
specific sequence cannot be determined by an outsider examining
information that is readily available or easily determinable (e.g.,
by examining packet contents). This document proposes the generation
of a pseudo-random sequence of interface identifiers via an MD5 hash.
Periodically, the next interface identifier in the sequence is
Narten, et al. Expires March 16, 2005 [Page 7]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
generated, a new set of temporary addresses is created, and the
previous temporary addresses are deprecated to discourage their
further use. The precise pseudo-random sequence depends on both a
random component and the globally unique interface identifier (when
available), to increase the likelihood that different nodes generate
different sequences.
Narten, et al. Expires March 16, 2005 [Page 8]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
3. Protocol Description
The goal of this section is to define procedures that:
1. Do not result in any changes to the basic behavior of addresses
generated via stateless address autoconfiguration [ADDRCONF].
2. Create additional global scope addresses based on a random
interface identifier for use with global scope addresses.Such
addresses would be used to initiate outgoing sessions. These
"random" or temporary addresses would be used for a short period
of time (hours to days) and would then be deprecated. Deprecated
address can continue to be used for already established
connections, but are not used to initiate new connections. New
temporary addresses are generated periodically to replace
temporary addresses that expire, with the exact time between
address generation a matter of local policy.
3. Produce a sequence of temporary global scope addresses from a
sequence of interface identifiers that appear to be random in the
sense that it is difficult for an outside observer to predict a
future address (or identifier) based on a current one and it is
difficult to determine previous addresses (or identifiers)
knowing only the present one.
4. By default, generate a set of addresses from the same
(randomized) interface identifier, one address for each prefix
for which a global address has been generated via stateless
address autoconfiguration. Using the same interface identifier
to generate a set of temporary addresses reduces the number of IP
multicast groups a host must join. Nodes join the solicited-node
multicast address for each unicast address they support, and
solicited-node addresses are dependent only on the low-order bits
of the corresponding address. This default behaviour was made to
address the concern that a node that joins a large number of
multicast groups may be required to put its interface into
promiscuous mode, resulting in possible reduced performance.
A node highly concerned about privacy MAY use different interface
identifiers on different prefixes, resulting in a set of global
addresses that cannot be easily tied to each other. This may be
useful, for example, to a mobile node using multiple wireless
interfaces to connect to multiple independent networks.
3.1 Assumptions
The following algorithm assumes that each interface maintains an
associated randomized interface identifier. When temporary addresses
are generated, the current value of the associated randomized
interface identifier is used. The actual value of the identifier
changes over time as described below, but the same identifier can be
Narten, et al. Expires March 16, 2005 [Page 9]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
used to generate more than one temporary address.
The algorithm also assumes that for a given temporary address, an
implementation can determine the corresponding public address from
which it was generated. When a temporary address is deprecated, a
new temporary address is generated. The specific valid and preferred
lifetimes for the new address are dependent on the corresponding
lifetime values in the public address.
Finally, this document assumes that when a node initiates outgoing
communication, temporary addresses can be given preference over
public addresses, when the device is configured to do so. This is
consistent with on-going work that addresses the topic of
source-address selection in the more general case [ADDR_SELECT] and
also means that all connections initiated by the node can use
temporary addresses without requiring application-specific
enablement. This document also assumes that an API will exist that
allows individual applications to indicate whether they prefer to use
temporary or public addresses and override the system defaults.
Narten, et al. Expires March 16, 2005 [Page 10]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
3.2 Generation Of Randomized Interface Identifiers
We describe two approaches for the generation and maintenance of the
randomized interface identifier. The first assumes the presence of
stable storage that can be used to record state history for use as
input into the next iteration of the algorithm across system
restarts. A second approach addresses the case where stable storage
is unavailable and there is a need to generate randomized interface
identifiers without previous state.
3.2.1 When Stable Storage Is Present
The following algorithm assumes the presence of a 64-bit "history
value" that is used as input in generating a randomized interface
identifier. The very first time the system boots (i.e., out-of-the-
box), a random value SHOULD be generated using techniques that help
ensure the initial value is hard to guess [RANDOM]. Whenever a new
interface identifier is generated, a value generated by the
computation is saved in the history value for the next iteration of
the algorithm.
A randomized interface identifier is created as follows:
1. Take the history value from the previous iteration of this
algorithm (or a random value if there is no previous value) and
append to it the interface identifier generated as described in
[ADDRARCH].
2. Compute the MD5 message digest [MD5] over the quantity created in
the previous step.
3. Take the left-most 64-bits of the MD5 digest and set bit 6 (the
left-most bit is numbered 0) to zero. This creates an interface
identifier with the universal/local bit indicating local
significance only.
4. Compare the generated identifier against a list of known values
that should not be used. Inappropriate values include those used
in reserved anycast addresses [RFC2526], those used by ISATAP
[ISATAP], the value 0, and those already assigned to an address
on the local device. In the event that an unacceptable
identifier has been generated, the node MUST restart the process
at step 1 above, using the right-most 64 bits of the MD5 digest
obtained in step 2 in place of the history value in step 1.
5. Save the generated identifier as the associated randomized
interface identifier.
6. Take the rightmost 64-bits of the MD5 digest computed in step 2)
and save them in stable storage as the history value to be used
in the next iteration of the algorithm.
MD5 was chosen for convenience, and because its particular properties
Narten, et al. Expires March 16, 2005 [Page 11]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
were adequate to produce the desired level of randomization. IPv6
nodes are already required to implement MD5 as part of IPsec [IPSEC],
thus the code will already be present on IPv6 machines.
In theory, generating successive randomized interface identifiers
using a history scheme as above has no advantages over generating
them at random. In practice, however, generating truly random
numbers can be tricky. Use of a history value is intended to avoid
the particular scenario where two nodes generate the same randomized
interface identifier, both detect the situation via DAD, but then
proceed to generate identical randomized interface identifiers via
the same (flawed) random number generation algorithm. The above
algorithm avoids this problem by having the interface identifier
(which will often be globally unique) used in the calculation that
generates subsequent randomized interface identifiers. Thus, if two
nodes happen to generate the same randomized interface identifier,
they should generate different ones on the followup attempt.
3.2.2 In The Absence of Stable Storage
In the absence of stable storage, no history value will be available
across system restarts to generate a pseudo-random sequence of
interface identifiers. Consequently, the initial history value used
above SHOULD be generated at random. A number of techniques might be
appropriate. Consult [RANDOM] for suggestions on good sources for
obtaining random numbers. Note that even though machines may not
have stable storage for storing a history value, they will in many
cases have configuration information that differs from one machine to
another (e.g., user identity, security keys, serial numbers, etc.).
One approach to generating a random initial history value in such
cases is to use the configuration information to generate some data
bits (which may remain constant for the life of the machine, but will
vary from one machine to another), append some random data and
compute the MD5 digest as before.
Narten, et al. Expires March 16, 2005 [Page 12]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
3.2.3 Alternate approaches
Please note that there are other approaches to generate random
interface identifiers, albeit with different goals and applicability.
One such approach is CGA [CGA], which generates a random interface
identifier based on the public key of the node. The goal of CGAs is
to prove ownership of an address and to prevent spoofing and stealing
of existing IPv6 addresses. The CGA random interface identifier
generation algorithm may not be suitable for privacy addresses
because of the following properties
o It requires the node to have a public key. This means that the
node can still be identified by its public key
o The random interface identifier process is computationally
intensive and hence discourages frequent regeneration
3.3 Generating Temporary Addresses
[ADDRCONF] describes the steps for generating a link-local address
when an interface becomes enabled as well as the steps for generating
addresses for other scopes. This document extends [ADDRCONF] as
follows. When processing a Router Advertisement with a Prefix
Information option carrying a global scope prefix for the purposes of
address autoconfiguration (i.e., the A bit is set), the node MUST
perform the following steps:
1. Process the Prefix Information Option as defined in [ADDRCONF],
either creating a new public address or adjusting the lifetimes
of existing addresses, both public and temporary. When adjusting
the lifetime of an existing temporary address, there are two
cases to consider:
A. In some cases, the lifetimes in a received option will be
shorter than the lifetimes of an existing temporary addresses
derived from the prefix given in the option. This
corresponds to the case where the lifetimes have been
reconfigured by a system administrator to have a shorter
lifetime. Perform the following procedure to determine how
to update the lifetime of the temporary address:
B.
+ If the received Valid Lifetime is greater than 2 hours
update the lifetime of the temporary address to the
received lifetime.
+ If the RemainingLifetime of the temporary address is less
than or equal to 2 hours ignore the received option.
+ Otherwise set the valid lifetime to 2 hours.
C. These steps are necessary to prevent a denial of service
attack where a bogus advertisement contains prefixes with
very small Valid Lifetimes
Narten, et al. Expires March 16, 2005 [Page 13]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
D. In many cases, the lifetime of a received option will extend
the lifetime of a public address. For example, a site might
advertise short lifetimes (on the order of hours or minutes)
that are effectively extended with each new RA. In such
cases, the lifetimes of temporary addresses should be
extended, subject to the overall constraint that no temporary
addresses should ever remain "valid" or "preferred" for a
time longer than (TEMP_VALID_LIFETIME-DESYNC_FACTOR) or
(TEMP_PREFERRED_LIFETIME-DESYNC_FACTOR) respectively. The
configuration variables TEMP_VALID_LIFETIME and
TEMP_PREFERRED_LIFETIME correspond to approximate target
lifetimes for temporary addresses.
One way an implementation can satisfy the above constraints is to
associate with each temporary address a creation time (called
CREATION_TIME) that indicates the time at which the address was
created. When updating the preferred lifetime of an existing
temporary address, it would be set to expire at whichever time is
earlier: the time indicated by the received lifetime or
(CREATION_TIME + TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR). A
similar approach can be used with the valid lifetime.
2. When a new public address is created as described in [ADDRCONF]
(because the prefix advertised does not match the prefix of any
address already assigned to the interface, and the Valid Lifetime
in the option is not zero), the node SHOULD also create a new
temporary address.
3. When creating a temporary address, the lifetime values MUST be
derived from the corresponding public address as follows:
* Its Valid Lifetime is the lower of the Valid Lifetime of the
public address or TEMP_VALID_LIFETIME
* Its Preferred Lifetime is the lower of the Preferred Lifetime
of the public address or
TEMP_PREFERRED_LIFETIME-DESYNC_FACTOR.
4. A temporary address is created only if this calculated Preferred
Lifetime is greater than REGEN_ADVANCE time units. In
particular, an implementation MUST NOT create a temporary address
with a zero Preferred Lifetime.
5. New temporary addresses MUST be created by appending the
interface's current randomized interface identifier to the prefix
that was used to generate the corresponding public address.
6. The node MUST Perform duplicate address detection (DAD) on the
generated temporary address. If DAD indicates the address is
already in use, the node MUST generate a new randomized interface
identifier as described in Section 3.2 above, and repeat the
previous steps as appropriate up to 5 times. If after 5
consecutive attempts no non-unique address was generated, the
node MUST log a system error and MUST NOT attempt to generate
temporary addresses for that interface. Note that DAD MUST be
performed on every unicast address generated from this randomized
Narten, et al. Expires March 16, 2005 [Page 14]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
interface identifier.
3.4 Expiration of Temporary Addresses
When a temporary address becomes deprecated, a new one MUST be
generated. This is done by repeating the actions described in
Section 3.3, starting at step 3). Note that, except for the
transient period when a temporary address is being regenerated, in
normal operation at most one temporary address corresponding to a
public address should be in a non-deprecated state at any given time.
Note that if a temporary address becomes deprecated as result of
processing a Prefix Information Option with a zero Preferred
Lifetime, then a new temporary address MUST NOT be generated. The
Prefix Information Option will also deprecate the corresponding
public address. To insure that a preferred temporary address is
always available, a new temporary address SHOULD be regenerated
slightly before its predecessor is deprecated. This is to allow
sufficient time to avoid race conditions in the case where generating
a new temporary address is not instantaneous, such as when duplicate
address detection must be run. The node SHOULD start the address
regeneration process REGEN_ADVANCE time units before a temporary
address would actually be deprecated.
As an optional optimization, an implementation MAY remove a
deprecated temporary address that is not in use by applications or
upper-layers. For TCP connections, such information is available in
control blocks. For UDP-based applications, it may be the case that
only the applications have knowledge about what addresses are
actually in use. Consequently, one may need to use heuristics in
deciding when an address is no longer in use (e.g., the default
TEMP_VALID_LIFETIME suggested above).
3.5 Regeneration of Randomized Interface Identifiers
The frequency at which temporary addresses changes depends on how a
device is being used (e.g., how frequently it initiates new
communication) and the concerns of the end user. The most egregious
privacy concerns appear to involve addresses used for long periods of
time (weeks to months to years). The more frequently an address
changes, the less feasible collecting or coordinating information
keyed on interface identifiers becomes. Moreover, the cost of
collecting information and attempting to correlate it based on
interface identifiers will only be justified if enough addresses
contain non-changing identifiers to make it worthwhile. Thus, having
large numbers of clients change their address on a daily or weekly
basis is likely to be sufficient to alleviate most privacy concerns.
There are also client costs associated with having a large number of
Narten, et al. Expires March 16, 2005 [Page 15]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
addresses associated with a node (e.g., in doing address lookups, the
need to join many multicast groups, etc.). Thus, changing addresses
frequently (e.g., every few minutes) may have performance
implications.
Nodes following this specification SHOULD generate new temporary
addresses on a periodic basis. This can be achieved automatically by
generating a new randomized interface identifier at least once every
(TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR) time units.
As described above, generating a new temporary address REGEN_ADVANCE
time units before a temporary address becomes deprecated produces
addresses with a preferred lifetime no larger than
TEMP_PREFERRED_LIFETIME. The value DESYNC_FACTOR is a random value
(different for each client) that ensures that clients don't
synchronize with each other and generate new addresses at exactly the
same time. When the preferred lifetime expires, a new temporary
address MUST be generated using the new randomized interface
identifier.
Because the precise frequency at which it is appropriate to generate
new addresses varies from one environment to another, implementations
SHOULD provide end users with the ability to change the frequency at
which addresses are regenerated. The default value is given in
TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time
at which to invalidate a temporary address depends on how
applications are used by end users. Thus, the default value given of
one week (TEMP_VALID_LIFETIME) may not be appropriate in all
environments. Implementations SHOULD provide end users with the
ability to override both of these default values.
Finally, when an interface connects to a new link, a new randomized
interface identifier SHOULD be generated immediately together with a
new set of temporary addresses. If a device moves from one ethernet
to another, generating a new set of temporary addresses from a
different randomized interface identifier ensures that the device
uses different randomized interface identifiers for the temporary
addresses associated with the two links, making it more difficult to
correlate addresses from the two different links as being from the
same node. The process by which a node can determine whether a link
change has occurred is described by Detecting Network Attachment
[DNA]
3.6 Deployment Considerations
Devices implementing this specification MUST provide a way for the
end user to explicitly enable or disable the use of temporary
addresses. In addition, a site might wish to disable the use of
temporary addresses in order to simply network debugging and
Narten, et al. Expires March 16, 2005 [Page 16]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
operations. Consequently, implementations SHOULD provide a way for
trusted system administrators to enable or disable the use of
temporary addresses.
The use of temporary addresses may cause unexpected difficulties with
some applications. As described below, some servers refuse to accept
communications from clients for which they cannot map the IP address
into an DNS name. In addition, some applications may not behave
robustly if temporary addresses are used and an address expires
before the application has terminated, or if it opens multiple
sessions, but expects them to all use the same addresses.
Consequently, the use of temporary addresses SHOULD be disabled by
default in order to minimize potential disruptions. Individual
applications, which have specific knowledge about the normal duration
of connections, MAY override this as appropriate.
If a very small number of nodes(say only one) use a given prefix for
extended periods of time, just changing the interface identifier
part of the prefix may not be sufficient to ensure privacy, since
the prefix acts as a constant identifier. The procedures described
in this document are most effective when the prefix is reasonably non
static or is used by a fairly large number of nodes
Narten, et al. Expires March 16, 2005 [Page 17]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
4. Implications of Changing Interface Identifiers
The IPv6 addressing architecture goes to some lengths to ensure that
interface identifiers are likely to be globally unique where easy to
do so. The widespread use of temporary addresses may result in a
significant fraction of Internet traffic not using addresses in which
the interface id portion is globally unique. Consequently, usage of
the algorithms in this document may complicate providing such a
future flexibility, if global uniqueness is necessary.
The desires of protecting individual privacy vs. the desire to
effectively maintain and debug a network can conflict with each
other. Having clients use addresses that change over time will make
it more difficult to track down and isolate operational problems.
For example, when looking at packet traces, it could become more
difficult to determine whether one is seeing behavior caused by a
single errant machine, or by a number of them.
Some servers refuse to grant access to clients for which no DNS name
exists. That is, they perform a DNS PTR query to determine the DNS
name, and may then also perform an AAAA query on the returned name to
verify that the returned DNS name maps back into the address being
used. Consequently, clients not properly registered in the DNS may
be unable to access some services. As noted earlier, however, a
node's DNS name (if non-changing) serves as a constant identifier.
The wide deployment of the extension described in this document could
challenge the practice of inverse-DNS-based "authentication," which
has little validity, though it is widely implemented. In order to
meet server challenges, nodes could register temporary addresses in
the DNS using random names (for example a string version of the
random address itself).
Use of the extensions defined in this document may complicate
debugging and other operational troubleshooting activities.
Consequently, it may be site policy that temporary addresses should
not be used. Consequently, implementations must provide a method for
the end user or trusted administrator to override the use of
temporary addresses.
Narten, et al. Expires March 16, 2005 [Page 18]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
5. Defined Constants
Constants defined in this document include:
TEMP_VALID_LIFETIME -- Default value: 1 week. Users should be able
to override the default value.
TEMP_PREFERRED_LIFETIME -- Default value: 1 day. Users should be
able to override the default value.
REGEN_ADVANCE -- 5 seconds
MAX_DESYNC_FACTOR -- 10 minutes. Upper bound on DESYNC_FACTOR.
DESYNC_FACTOR -- A random value within the range 0 -
MAX_DESYNC_FACTOR. It is computed once at system start (rather than
each time it is used) and must never be greater than
(TEMP_VALID_LIFETIME - REGEN_ADVANCE).
Narten, et al. Expires March 16, 2005 [Page 19]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
6. Future Work
An implementation might want to keep track of which addresses are
being used by upper layers so as to be able to remove a deprecated
temporary address from internal data structures once no upper layer
protocols are using it (but not before). This is in contrast to
current approaches where addresses are removed from an interface when
they become invalid [ADDRCONF], independent of whether or not upper
layer protocols are still using them. For TCP connections, such
information is available in control blocks. For UDP-based
applications, it may be the case that only the applications have
knowledge about what addresses are actually in use. Consequently, an
implementation generally will need to use heuristics in deciding when
an address is no longer in use (e.g., as is suggested in Section
3.4).
The determination as to whether to use public vs. temporary
addresses can in some cases only be made by an application. For
example, some applications may always want to use temporary
addresses, while others may want to use them only in some
circumstances or not at all. Suitable API extensions will likely
need to be developed to enable individual applications to indicate
with sufficient granularity their needs with regards to the use of
temporary addresses. Recommendations on DNS practices to avoid the
problem described in Section 4 when reverse DNS lookups fail may be
needed.
While this document discusses ways of obscuring a user's permanent IP
address, the method described is believed to be ineffective against
sophisticated forms of traffic analysis. To increase effectiveness,
one may need to consider use of more advanced techniques, such as
Onion Routing [ONION].
Open Issues
1) Implementations should allow system administrators to configure
the use of temporary addresses. We've considered the possibility of
using Router Advertisements to configure a host's use of temporary
addresses, but that has a major drawback: in some situations (for
example a home user receiving RAs from an ISP's router), the
administrator of the host and the administrator of the router may
have different opinions about the use of temporary addresses. Any
configuration mechanism that disables the use of temporary addresses
without input from the user MUST ensure that the host's administrator
has authorized the disabling.
Narten, et al. Expires March 16, 2005 [Page 20]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
7. Significant Changes from RFC 3041
This section summarizes the changes in this document relative to RFC
3041 that an implementer of RFC 3041 should be aware of.
1. Added wording to exclude certain interface identifiers from the
range of acceptable interface identifiers. Interface IDs such
as 0, those for reserved anycast addresses [RFC2526], etc.
2. Added a configuration knob that provides the end user with a way
to enable or disable the use of temporary addresses.
3. Under RFC 3041, RAs with short lifetimes (e.g., 1 hour) that
always send the same lifetime for long periods of time (e.g.,
days to weeks) resulted in temporary addresses being created
with lifetimes of only 1 hour. Additional rules were added to
increase the Lifetime of temporary addresses when the advertised
lifetimes were short.
4. DAD is now run on all temporary addresses, not just the first one
generated from an interface identifier.
5. Changed the default setting for usage of temporary addresses to
be disabled.
6. Added a security considerations section to highlight the ingress
filtering issues which can be caused by the use of temporary
addresses as described in this document
7. Removed references to site-local addresses
8. Added a check for denial of service attacks using low valid
lifetimes in router advertisements
9. Changed the document to use RFC2119 language
10. The node is now allowed to generate different interface
identifiers for different prefixes, if it so desires.
11. DAD is now performed on all unicast addresses created from the
same interface identifier instead of just the first one
Narten, et al. Expires March 16, 2005 [Page 21]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
8. Security Considerations
Ingress filtering is being deployed as a means of preventing the use
of spoofed source addresses in Distributed Denial of Service(DDoS)
attacks. In a network with a large number of nodes, new temporary
addresses are created at a fairly high rate. This might make it
difficult for ingress filtering mechanisms to distinguish between
legitimately changing temporary addresses and spoofed source
addresses which "in-prefix"(They use a topologically correct prefix
and non-existent interface ID). This can be addressed by using
access control mechanisms on a per address basis on the network
egress point.
Narten, et al. Expires March 16, 2005 [Page 22]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
9. Acknowledgements
The authors would like to acknowledge the contributions of the ipv6
working group and, in particular, Ran Atkinson, Matt Crawford, Steve
Deering, Allison Mankin, Peter Bieringer, Jari Arkko, Pekka Nikander,
Pekka Savola, and Francis Dupont for their detailed comments.
10 References
[ADDRARCH]
Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ADDRCONF]
Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[ADDR_SELECT]
Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[CGA] Aura, T., "Cryptographically Generated Addresses (CGA)",
draft-ietf-send-cga-06 (work in progress), April 2004.
[COOKIES] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, October 2000.
[DDNS] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[DISCOVERY]
Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[DNA] Choi, J. and G. Daley, "Detecting Network Attachment in
IPv6 Goals", draft-ietf-dna-goals-01 (work in progress),
September 2004.
[IPSEC] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[ISATAP] Templin, F., Gleeson, T., Talwar, M. and D. Thaler,
"Intra-Site Automatic Tunnel Addressing Protocol
Narten, et al. Expires March 16, 2005 [Page 23]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
(ISATAP)", draft-ietf-ngtrans-isatap-22 (work in
progress), May 2004.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[MOBILEIP]
Perkins, C., "IP Mobility Support", RFC 2002, October
1996.
[ONION] Reed, MGR., Syverson, PFS. and DMG. Goldschlag, "Proxies
for Anonymous Routing", Proceedings of the 12th Annual
Computer Security Applications Conference, San Diego, CA,
December 1996.
[RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[ULA] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", draft-ietf-ipv6-unique-local-addr-05 (work in
progress), June 2004.
Authors' Addresses
Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC
USA
EMail: narten@raleigh.ibm.com
Richard Draves
Microsoft Research
One Microsoft Way
Redmond, WA
USA
EMail: richdr@microsoft.com
Narten, et al. Expires March 16, 2005 [Page 24]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
EMail: suresh.krishnan@ericsson.com
Narten, et al. Expires March 16, 2005 [Page 25]
Internet-Draft Privacy Extensions to Address Autoconf September 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Narten, et al. Expires March 16, 2005 [Page 26]
