IPv6 Maintenance (6man) Working Group F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Obsoletes: rfc4941 (if approved) S. Krishnan
Intended status: Standards Track Ericsson Research
Expires: May 6, 2020 T. Narten
IBM Corporation
R. Draves
Microsoft Research
November 3, 2019
Privacy Extensions for Stateless Address Autoconfiguration in IPv6
draft-ietf-6man-rfc4941bis-04
Abstract
Nodes use IPv6 stateless address autoconfiguration to generate
addresses using a combination of locally available information and
information advertised by routers. Addresses are formed by combining
network prefixes with an interface identifier. This document
describes an extension that causes nodes to generate global scope
addresses with randomized interface identifiers that change over
time. Changing global scope addresses 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. This document formally
obsoletes RFC4941.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2020.
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Copyright Notice
Copyright (c) 2019 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
(https://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
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Extended Use of the Same Identifier . . . . . . . . . . . 4
2.2. Possible Approaches . . . . . . . . . . . . . . . . . . . 6
3. Protocol Description . . . . . . . . . . . . . . . . . . . . 6
3.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Generation of Randomized Interface Identifiers . . . . . 8
3.2.1. Simple Randomized Interface Identifiers . . . . . . . 8
3.2.2. Hash-based Generation of Randomized Interface
Identifiers . . . . . . . . . . . . . . . . . . . . . 9
3.3. Generating Temporary Addresses . . . . . . . . . . . . . 10
3.4. Expiration of Temporary Addresses . . . . . . . . . . . . 12
3.5. Regeneration of Temporary Addresses . . . . . . . . . . . 12
3.6. Deployment Considerations . . . . . . . . . . . . . . . . 13
4. Implications of Changing Interface Identifiers . . . . . . . 14
5. Defined Constants . . . . . . . . . . . . . . . . . . . . . . 15
6. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Significant Changes from RFC4941 . . . . . . . . . . . . . . 16
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
Stateless address autoconfiguration [RFC4862] defines how an IPv6
node generates addresses without the need for a Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) server. The security and
privacy implications of such addresses have been discussed in great
detail in [RFC7721],[RFC7217], and RFC7707. This document specifies
an extension for SLAAC to generate temporary addresses, such that the
aforementioned issues are mitigated.
The default address selection for IPv6 has been specified in
[RFC6724]. We note that the determination as to whether to use
stable versus 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. An API such as that specified in
[RFC5014] can enable individual applications to indicate with
sufficient granularity their needs with regards to the use of
temporary addresses.
Section 2 provides background information on the issue. Section 3
describes a procedure for generating temporary interface identifiers
and global scope addresses. Section 4 discusses implications of
changing interface identifiers.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The terms "public address", "stable address", "temporary address",
"constant IID", "stable IID", and "temporary IID" are to be
interpreted as specified in [RFC7721].
The term "global scope addresses" is used in this document to
collectively refer to "Global unicast addresses" as defined in
[RFC4291] and "Unique local addresses" as defined in [RFC4193], and
not to "globally reachable" as defined in [RFC8190].
1.2. Problem Statement
Addresses generated using stateless address autoconfiguration
[RFC4862] contain an embedded interface identifier, which remains
stable over time. Anytime a fixed identifier is used in multiple
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contexts, it becomes possible to correlate seemingly unrelated
activity using this identifier.
The correlation can be performed by
o An attacker who is in the path between the node in question and
the peer(s) to which it is communicating, and who can view the
IPv6 addresses present in the datagrams.
o An attacker who can access the communication logs of the peers
with which the node has communicated.
Since the identifier is embedded within the IPv6 address, which is a
fundamental requirement of communication, it cannot be easily hidden.
This document proposes a solution to this issue by generating
interface identifiers that vary over time.
Note that an attacker, who is on path, may be able to perform
significant correlation based on
o The payload contents of the packets on the wire
o The characteristics of the packets such as packet size and timing
Use of temporary addresses will not prevent such payload-based
correlation, which can only be addressed by widespread deployment of
encryption as advocated in [RFC7624].
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. Any time 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. Although it might appear
that changing an address regularly in such environments would be
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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 could be grouped together for the purposes of
collecting information. If the network contains a very small number
of nodes, say, just one, changing just the interface identifier will
not enhance privacy at all, since the prefix serves as a constant
identifier.
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
[RFC6265]. 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
layer payloads were encrypted.
Using temporary address alone may not be sufficient to prevent all
forms of tracking. It is however quite clear that some usage of
temporary addresses is necessary to improve user privacy.
The security and privacy implications of IPv6 addresses are discussed
in detail in [RFC7721], [RFC7707], and [RFC7217].
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2.2. Possible Approaches
One way to avoid having a stable non-changing address is to use
DHCPv6 [RFC8415] for obtaining addresses. Section 12 of [RFC8415]
discusses the use of DHCPv6 for the assignment and management of
"temporary addresses", which are never renewed and provide the same
property of temporary addresses described in this document with
regards to the privacy concern.
Another approach, compatible with the stateless address
autoconfiguration architecture, would be to change the interface
identifier portion of an address over time. 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 stable 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.
On the other hand, a machine that functions only as a client may want
to employ only temporary addresses for public communication.
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.
3. Protocol Description
The goal of this section is to define procedures that can generate
IPv6 addresses with the following properties:
1. Temporary addresses can be employed for initiating outgoing
sessions.
2. Temporary addresses are used for a short period of time
(typically hours to days) and are subsequently deprecated.
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Deprecated addresses can continue to be used for already
established connections, but are not used to initiate new
connections.
3. New temporary addresses are generated periodically to replace
temporary addresses that expire.
4. Temporary addresses must have a limited lifetime (limited "valid
lifetime" and "preferred lifetime" from [RFC4862]), that should
be statistically different for different addresses. The lifetime
of an address should be further reduced when privacy-meaningful
events (such as a node attaching to a different network, or the
regeneration of a new randomized MAC address) takes place.
5. By default, one address is generated for each prefix advertised
for stateless address autoconfiguration. The resulting Interface
Identifiers must be statistically different when addresses are
configured for different prefixes. That is, when temporary
addresses are generated for different autoconfiguration prefixes
for the same network interface, the resulting Interface
Identifiers must be statistically different. This means that,
given two addresses that employ different prefixes, it must be
difficult for an outside entity to tell whether the addresses
correspond to the same network interface or even whether they
have been generated by the same host.
6. It must be difficult for an outside entity to predict the
Interface Identifiers that will be employed for temporary
addresses, even with knowledge of the algorithm/method employed
to generate them and/or knowledge of the Interface Identifiers
previously employed for other temporary addresses. These
Interface Identifiers must be semantically opaque [RFC7136] and
must not follow any specific patterns.
3.1. Assumptions
The following algorithm assumes that for a given temporary address,
an implementation can determine the prefix 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
set for the prefix from which it was generated.
Finally, this document assumes that when a node initiates outgoing
communication, temporary addresses can be given preference over
stable addresses (if available), when the device is configured to do
so. [RFC6724] mandates implementations to provide a mechanism, which
allows an application to configure its preference for temporary
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addresses over stable addresses. It also allows for an
implementation to prefer temporary addresses by default, so that the
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 stable addresses and
override the system defaults.
3.2. Generation of Randomized Interface Identifiers
The following subsections specify some possible algorithms for
generating temporary interface identifiers that follow the guidelines
in Section 3 of this document. The algorithm specified in
Section 3.2.1 benefits from a Pseudo-Random Number Generator (PRNG)
available on the system. On the other hand, the algorithm specified
in Section 3.2.2 allows for code reuse by nodes that implement
[RFC7217].
3.2.1. Simple Randomized Interface Identifiers
One possible approach would be to select a pseudorandom number of the
appropriate length. A node employing this algorithm should generate
IIDs as follows:
1. Obtain a random number (see [RFC4086] for randomness requirements
for security)
2. The Interface Identifier is obtained by taking as many bits from
the aforementioned random number (obtained in the previous step)
as necessary. Note: there are no special bits in an Interface
Identifier [RFC7136].
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).
3. 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, a new interface identifier should be generated, by
repeating the algorithm from the first step.
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3.2.2. Hash-based Generation of Randomized Interface Identifiers
The algorithm in [RFC7217] can be augmented for the generation of
temporary addresses. The benefit of this would be that a node could
employ a single algorithm for generating stable and temporary
addresses, by employing appropriate parameters.
Nodes would employ the following algorithm for generating the
temporary IID:
1. Compute a random identifier with the expression:
RID = F(Prefix, MAC_Address, Network_ID, Time, DAD_Counter,
secret_key)
Where:
RID:
Random 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
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.
MAC_Address:
The MAC address corresponding to the underlying network
interface card. Employing the MAC address in this expression
(in replacement of the Net_Iface parameter of the expression
in RFC7217) means that the re-generation of a randomized MAC
address will result in a different temporary address.
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
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[RFC6059] describes ideas that could be leveraged to generate
a Network_ID parameter. This parameter is SHOULD be employed
if some form of "Network_ID" is available.
Time:
An implementation-dependent representation of time. One
possible example is the representation in UNIX-like systems
[OPEN-GROUP], that measure time in terms of the number of
seconds elapsed since the Epoch (00:00:00 Coordinated
Universal Time (UTC), 1 January 1970).
DAD_Counter:
A counter that is employed to resolve Duplicate Address
Detection (DAD) conflicts.
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
"bootstrapped".
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. 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, the value DAD_Counter should be incremented by 1, and
the algorithm should be restarted from the first step.
3.3. Generating Temporary Addresses
[RFC4862] 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 [RFC4862] 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 [RFC4862],
adjusting the lifetimes of existing temporary addresses. If a
received option may extend the lifetimes of temporary addresses,
with the overall constraint that no temporary addresses should
ever remain "valid" or "preferred" for a time longer than
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(TEMP_VALID_LIFETIME) 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.
2. 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.
3. If the node has not configured any temporary address for the
corresponding prefix, the node SHOULD create a new temporary
address for such prefix.
4. When creating a temporary address, the lifetime values MUST be
derived from the corresponding prefix as follows:
* Its Valid Lifetime is the lower of the Valid Lifetime of the
prefix and TEMP_VALID_LIFETIME
* Its Preferred Lifetime is the lower of the Preferred Lifetime
of the prefix and TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR.
5. 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.
6. New temporary addresses MUST be created by appending a randomized
interface identifier (generates as described in Section 3.2 of
this document) to the prefix that was received.
7. 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, and repeat the previous steps as appropriate up to
TEMP_IDGEN_RETRIES times. If after TEMP_IDGEN_RETRIES
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
interface identifier.
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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 4). Note that, except for the
transient period when a temporary address is being regenerated, in
normal operation at most one temporary address per prefix should be
in a non-deprecated state at any given time on a given interface.
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. To
ensure 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 as detailed in Section 6.
3.5. Regeneration of Temporary Addresses
The frequency at which temporary addresses change 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
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 by generating a
new temporary address at least once every (TEMP_PREFERRED_LIFETIME -
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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 suggested default
value 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 (different) link, a new
set of temporary addresses MUST be generated immediately. If a
device moves from one ethernet to another, generating a new set of
temporary addresses 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 node MAY follow
any process available to it, to determine that the link change has
occurred. One such process is described by "Simple Procedures for
Detecting Network Attachment in IPv6" [RFC6059]. Detecting link
changes would prevent link down/up events from causing temporary
addresses to be (unnecessarily) regenerated.
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 simplify network debugging and
operations. Consequently, implementations SHOULD provide a way for
trusted system administrators to enable or disable the use of
temporary addresses.
Additionally, sites might wish to selectively enable or disable the
use of temporary addresses for some prefixes. For example, a site
might wish to disable temporary address generation for "Unique local"
[RFC4193] prefixes while still generating temporary addresses for all
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other global prefixes. Another site might wish to enable temporary
address generation only for the prefixes 2001::/16 and 2002::/16
while disabling it for all other prefixes. To support this behavior,
implementations SHOULD provide a way to enable and disable generation
of temporary addresses for specific prefix subranges. This per-
prefix setting SHOULD override the global settings on the node with
respect to the specified prefix subranges. Note that the per-prefix
setting can be applied at any granularity, and not necessarily on a
per subnet basis.
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 a 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.
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 address 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.
4. Implications of Changing Interface Identifiers
The desires of protecting individual privacy versus 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).
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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.
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_PREFERRED_LIFETIME - REGEN_ADVANCE).
TEMP_IDGEN_RETRIES -- Default value: 3
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 [RFC4862], 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.
Recommendations on DNS practices to avoid the problem described in
Section 4 when reverse DNS lookups fail may be needed. [RFC4472]
contains a more detailed discussion of the DNS-related issues.
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While this document discusses ways of obscuring a user's 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].
7. Security Considerations
Ingress filtering has been and 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 are "in-prefix" (using 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.
8. Significant Changes from RFC4941
This section summarizes the changes in this document relative to RFC
4941 that an implementer of RFC 4941 should be aware of.
1. Discussion of IEEE-based IIDs has been removed, since the current
recommendation ([RFC8064]) is to employ [RFC7217]).
2. The document employs the terminology from [RFC7721].
3. Sections 2.2 and 2.3 of [RFC4941] have been removed since the
topic has been discussed in more detail in e.g. [RFC7721].
4. The algorithm specified in Section 3.2.1 and 3.2.2 of [RFC4941]
was replaced by two possible alternative algorithms.
5. Generation of stable addresses is not implied or required by this
document.
6. Temporary addresses are *not* disabled by default.
7. Section 3.2.3 from [RFC4941] was removed, based on the
explanation of that very section of RFC4941.
8. All the verified errata for [RFC4941] has been incorporated.
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9. Acknowledgments
The authors would like to thank (in alphabetical order) Brian
Carpenter, Tim Chown, Lorenzo Colitti, David Farmer, Tom Herbert, Bob
Hinden, Christian Huitema, Dave Plonka, Michael Richardson, Mark
Smith, and Johanna Ullrich for providing valuable comments on earlier
versions of this document.
This document incoporates errata submitted for [RFC4941] by (in
alphabetical order) Jiri Bohac and Alfred Hoenes.
This document is based on [RFC4941] (a revision of RFC3041). Suresh
Krishnan was the sole author of RFC4941. He would like to
acknowledge the contributions of the ipv6 working group and, in
particular, Jari Arkko, Pekka Nikander, Pekka Savola, Francis Dupont,
Brian Haberman, Tatuya Jinmei, and Margaret Wasserman for their
detailed comments.
Rich Draves and Thomas Narten were the authors of RFC 3041. They
would like to acknowledge the contributions of the ipv6 working group
and, in particular, Ran Atkinson, Matt Crawford, Steve Deering,
Allison Mankin, and Peter Bieringer.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers",
RFC 5453, DOI 10.17487/RFC5453, February 2009,
<https://www.rfc-editor.org/info/rfc5453>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
RFC 8064, DOI 10.17487/RFC8064, February 2017,
<https://www.rfc-editor.org/info/rfc8064>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8190] Bonica, R., Cotton, M., Haberman, B., and L. Vegoda,
"Updates to the Special-Purpose IP Address Registries",
BCP 153, RFC 8190, DOI 10.17487/RFC8190, June 2017,
<https://www.rfc-editor.org/info/rfc8190>.
10.2. Informative References
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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>.
[IANA-RESERVED-IID]
IANA, "Reserved IPv6 Interface Identifiers",
<http://www.iana.org/assignments/ipv6-interface-ids>.
[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.
[OPEN-GROUP]
The Open Group, "The Open Group Base Specifications Issue
7 / IEEE Std 1003.1-2008, 2016 Edition",
Section 4.16 Seconds Since the Epoch, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
contents.html>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472,
DOI 10.17487/RFC4472, April 2006,
<https://www.rfc-editor.org/info/rfc4472>.
[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
DOI 10.17487/RFC5014, September 2007,
<https://www.rfc-editor.org/info/rfc5014>.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
DOI 10.17487/RFC6059, November 2010,
<https://www.rfc-editor.org/info/rfc6059>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
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[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
<https://www.rfc-editor.org/info/rfc7707>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
Authors' Addresses
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
Suresh Krishnan
Ericsson Research
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Email: suresh.krishnan@ericsson.com
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Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC
USA
Email: narten@us.ibm.com
Richard Draves
Microsoft Research
One Microsoft Way
Redmond, WA
USA
Email: richdr@microsoft.com
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