RFC 7217：为 IPv6 无状态地址自动配置（SLAAC）生成语义不透明接口标识符的方法 原文标题：RFC 7217: A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC) | RFC Editor

RFC 7217: A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)

Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 7217 SI6 Networks / UTN-FRH
Category: Standards Track April 2014
ISSN: 2070-1721
 A Method for Generating Semantically Opaque Interface Identifiers
 with IPv6 Stateless Address Autoconfiguration (SLAAC)
Abstract
 This document specifies a method for generating IPv6 Interface
 Identifiers to be used with IPv6 Stateless Address Autoconfiguration
 (SLAAC), such that an IPv6 address configured using this method is
 stable within each subnet, but the corresponding Interface Identifier
 changes when the host moves from one network to another. This method
 is meant to be an alternative to generating Interface Identifiers
 based on hardware addresses (e.g., IEEE LAN Media Access Control
 (MAC) addresses), such that the benefits of stable addresses can be
 achieved without sacrificing the security and privacy of users. The
 method specified in this document applies to all prefixes a host may
 be employing, including link-local, global, and unique-local prefixes
 (and their corresponding addresses).
Status of This Memo
 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in [Section 2 of RFC 5741](https://www.rfc-editor.org/info/rfc5741/#section-2).
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 [http://www.rfc-editor.org/info/rfc7217](https://www.rfc-editor.org/info/rfc7217).
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Copyright Notice
 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 This document is subject to [BCP 78](https://www.rfc-editor.org/bcp/bcp78) and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 ([http://trustee.ietf.org/license-info](http://trustee.ietf.org/license-info)) in effect on the date of
 publication of this document. Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document. Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
 3. Relationship to Other Standards . . . . . . . . . . . . . . . 5
 4. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 6
 5. Algorithm Specification . . . . . . . . . . . . . . . . . . . 7
 6. Resolving DAD Conflicts . . . . . . . . . . . . . . . . . . . 12
 7. Specified Constants . . . . . . . . . . . . . . . . . . . . . 13
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
 10.1. Normative References . . . . . . . . . . . . . . . . . . 15
 10.2. Informative References . . . . . . . . . . . . . . . . . 16
 Appendix A. Possible Sources for the Net_Iface Parameter . . . . 19
 A.1. Interface Index . . . . . . . . . . . . . . . . . . . . . 19
 A.2. Interface Name . . . . . . . . . . . . . . . . . . . . . 19
 A.3. Link-Layer Addresses . . . . . . . . . . . . . . . . . . 19
 A.4. Logical Network Service Identity . . . . . . . . . . . . 20
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1. Introduction
 [[RFC4862](https://www.rfc-editor.org/info/rfc4862/)] specifies Stateless Address Autoconfiguration (SLAAC) for
 IPv6 [[RFC2460](https://www.rfc-editor.org/info/rfc2460/)], which typically results in hosts configuring one or
 more "stable" addresses composed of a network prefix advertised by a
 local router, and an Interface Identifier (IID) that typically embeds
 a hardware address (e.g., an IEEE LAN MAC address) [[RFC4291](https://www.rfc-editor.org/info/rfc4291/)].
 Cryptographically Generated Addresses (CGAs) [[RFC3972](https://www.rfc-editor.org/info/rfc3972/)] are yet
 another method for generating Interface Identifiers; CGAs bind a
 public signature key to an IPv6 address in the SEcure Neighbor
 Discovery (SEND) [[RFC3971](https://www.rfc-editor.org/info/rfc3971/)] protocol.
 Generally, the traditional SLAAC addresses are thought to simplify
 network management, since they simplify Access Control Lists (ACLs)
 and logging. However, they have a number of drawbacks:
 o Since the resulting Interface Identifiers do not vary over time,
 they allow correlation of host activities within the same network,
 thus negatively affecting the privacy of users (see
 [ADDR-GEN-PRIVACY] and [IAB-PRIVACY]).
 o Since the resulting Interface Identifiers are constant across
 networks, the resulting IPv6 addresses can be leveraged to track
 and correlate the activity of a host across multiple networks
 (e.g., track and correlate the activities of a typical client
 connecting to the public Internet from different locations), thus
 negatively affecting the privacy of users.
 o Since embedding the underlying link-layer address in the Interface
 Identifier will result in specific address patterns, such patterns
 may be leveraged by attackers to reduce the search space when
 performing address-scanning attacks [IPV6-RECON]. For example,
 the IPv6 addresses of all hosts manufactured by the same vendor
 (within a given time frame) will likely contain the same IEEE
 Organizationally Unique Identifier (OUI) in the Interface
 Identifier.
 o Embedding the underlying hardware address in the Interface
 Identifier leaks device-specific information that could be
 leveraged to launch device-specific attacks.
 o Embedding the underlying link-layer address in the Interface
 Identifier means that replacement of the underlying interface
 hardware will result in a change of the IPv6 address(es) assigned
 to that interface.
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 [ADDR-GEN-PRIVACY] provides additional details regarding how the
 aforementioned vulnerabilities could be exploited and the extent to
 which the method discussed in this document mitigates them.
 The "Privacy Extensions for Stateless Address Autoconfiguration in
 IPv6" [[RFC4941](https://www.rfc-editor.org/info/rfc4941/)] (henceforth referred to as "temporary addresses")
 were introduced to complicate the task of eavesdroppers and other
 information collectors (e.g., IPv6 addresses in web server logs or
 email headers, etc.) to correlate the activities of a host, and
 basically result in temporary (and random) Interface Identifiers.
 These temporary addresses are generated in addition to the
 traditional IPv6 addresses based on IEEE LAN MAC addresses, with the
 temporary addresses being employed for "outgoing communications", and
 the traditional SLAAC addresses being employed for "server" functions
 (i.e., receiving incoming connections).
 It should be noted that temporary addresses can be challenging in a
 number of areas. For example, from a network-management point of
 view, they tend to increase the complexity of event logging,
 troubleshooting, enforcement of access controls, and quality of
 service, etc. As a result, some organizations disable the use of
 temporary addresses even at the expense of reduced privacy
 [BROERSMA]. Temporary addresses may also result in increased
 implementation complexity, which might not be possible or desirable
 in some implementations (e.g., some embedded devices).
 In scenarios in which temporary addresses are deliberately not used
 (possibly for any of the aforementioned reasons), all a host is left
 with is the stable addresses that have typically been generated from
 the underlying hardware addresses. In such scenarios, it may still
 be desirable to have addresses that mitigate address-scanning attacks
 and that, at the very least, do not reveal the host's identity when
 roaming from one network to another -- without complicating the
 operation of the corresponding networks.
 However, even with temporary addresses in place, a number of issues
 remain to be mitigated. Namely,
 o since temporary addresses [RFC4941] do not eliminate the use of
 fixed identifiers for server-like functions, they only partially
 mitigate host-tracking and activity correlation across networks
 (see [ADDR-GEN-PRIVACY] for some example attacks that are still
 possible with temporary addresses).
 o since temporary addresses [RFC4941] do not replace the traditional
 SLAAC addresses, an attacker can still leverage patterns in SLAAC
 addresses to greatly reduce the search space for "alive" nodes
 [GONT-DEEPSEC2011] [CPNI-IPV6] [IPV6-RECON].
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 Hence, there is a motivation to improve the properties of "stable"
 addresses regardless of whether or not temporary addresses are
 employed.
 This document specifies a method to generate Interface Identifiers
 that are stable for each network interface within each subnet, but
 that change as a host moves from one network to another. Thus, this
 method enables keeping the "stability" properties of the Interface
 Identifiers specified in [RFC4291], while still mitigating address-
 scanning attacks and preventing correlation of the activities of a
 host as it moves from one network to another.
2. Terminology
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [[RFC2119](https://www.rfc-editor.org/info/rfc2119/)].
3. Relationship to Other Standards
 The method specified in this document is orthogonal to the use of
 temporary addresses [RFC4941], since it is meant to improve the
 security and privacy properties of the stable addresses that are
 employed along with the aforementioned temporary addresses. In
 scenarios in which temporary addresses are employed, implementation
 of the mechanism described in this document (in replacement of stable
 addresses based on, e.g., IEEE LAN MAC addresses) will mitigate
 address-scanning attacks and also mitigate the remaining vectors for
 correlating host activities based on the host's constant (i.e.,
 stable across networks) Interface Identifiers. On the other hand,
 for hosts that currently disable temporary addresses [RFC4941],
 implementation of this mechanism would mitigate the host-tracking and
 address-scanning issues discussed in Section 1.
 While the method specified in this document is meant to be used with
 SLAAC, this does not preclude this algorithm from being used with
 other address configuration mechanisms, such as DHCPv6 [[RFC3315](https://www.rfc-editor.org/info/rfc3315/)] or
 manual address configuration.
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4. Design Goals
 This document specifies a method for generating Interface Identifiers
 to be used with IPv6 SLAAC, with the following goals:
 o The resulting Interface Identifiers remain stable for each prefix
 used with SLAAC within each subnet for the same network interface.
 That is, the algorithm generates the same Interface Identifier
 when configuring an address (for the same interface) belonging to
 the same prefix within the same subnet.
 o The resulting Interface Identifiers must change when addresses are
 configured for different prefixes. That is, if different
 autoconfiguration prefixes are used to configure addresses for the
 same network interface card, the resulting Interface Identifiers
 must be (statistically) different. This means that, given two
 addresses produced by the method specified in this document, it
 must be difficult for an attacker to tell whether the addresses
 have been generated by the same host.
 o It must be difficult for an outsider to predict the Interface
 Identifiers that will be generated by the algorithm, even with
 knowledge of the Interface Identifiers generated for configuring
 other addresses.
 o Depending on the specific implementation approach (see Section 5
 and Appendix A), the resulting Interface Identifiers may be
 independent of the underlying hardware (e.g., IEEE LAN MAC
 address). For example, this means that replacing a Network
 Interface Card (NIC) or adding links dynamically to a Link
 Aggregation Group (LAG) will not have the (generally undesirable)
 effect of changing the IPv6 addresses used for that network
 interface.
 o The method specified in this document is meant to be an
 alternative to producing IPv6 addresses based on hardware
 addresses (e.g., IEEE LAN MAC addresses, as specified in
 [[RFC2464](https://www.rfc-editor.org/info/rfc2464/)]). That is, this document does not formally obsolete or
 deprecate any of the existing algorithms to generate Interface
 Identifiers. It is meant to be employed for all of the stable
 (i.e., non-temporary) IPv6 addresses configured with SLAAC for a
 given interface, including global, link-local, and unique-local
 IPv6 addresses.
 We note that this method is incrementally deployable, since it does
 not pose any interoperability implications when deployed on networks
 where other nodes do not implement or employ it. Additionally, we
 note that this document does not update or modify IPv6 Stateless
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 Address Autoconfiguration (SLAAC) [RFC4862] itself, but rather it
 only specifies an alternative algorithm to generate Interface
 Identifiers. Therefore, the usual address lifetime properties (as
 specified in the corresponding Prefix Information Options) apply when
 IPv6 addresses are generated as a result of employing the algorithm
 specified in this document with SLAAC [RFC4862]. Additionally, from
 the point of view of renumbering, we note that these addresses behave
 like the traditional IPv6 addresses (that embed a hardware address)
 resulting from SLAAC [RFC4862].
5. Algorithm Specification
 IPv6 implementations conforming to this specification MUST generate
 Interface Identifiers using the algorithm specified in this section
 as a replacement for any other algorithms for generating "stable"
 addresses with SLAAC (such as those specified in [RFC2464],
 [[RFC2467](https://www.rfc-editor.org/info/rfc2467/)], and [[RFC2470](https://www.rfc-editor.org/info/rfc2470/)]). However, implementations conforming to
 this specification MAY employ the algorithm specified in [RFC4941] to
 generate temporary addresses in addition to the addresses generated
 with the algorithm specified in this document. The method specified
 in this document MUST be employed for generating the Interface
 Identifiers with SLAAC for all the stable addresses, including IPv6
 global, link-local, and unique-local addresses.
 Implementations conforming to this specification SHOULD provide the
 means for a system administrator to enable or disable the use of this
 algorithm for generating Interface Identifiers.
 Unless otherwise noted, all of the parameters included in the
 expression below MUST be included when generating an Interface
 Identifier.
 1. Compute a random (but stable) identifier with the expression:
 RID = F(Prefix, Net_Iface, Network_ID, DAD_Counter, secret_key)
 Where:
 RID:
 Random (but stable) Identifier
 F():
 A pseudorandom function (PRF) that MUST NOT be computable from
 the outside (without knowledge of the secret key). F() MUST
 also be difficult to reverse, such that it resists attempts to
 obtain the secret_key, even when given samples of the output
 of F() and knowledge or control of the other input parameters.
 F() SHOULD produce an output of at least 64 bits. F() could
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 be implemented as a cryptographic hash of the concatenation of
 each of the function parameters. SHA-1 [FIPS-SHS] and SHA-256
 are two possible options for F(). Note: MD5 [[RFC1321](https://www.rfc-editor.org/info/rfc1321/)] is
 considered unacceptable for F() [[RFC6151](https://www.rfc-editor.org/info/rfc6151/)].
 Prefix:
 The prefix to be used for SLAAC, as learned from an ICMPv6
 Router Advertisement message, or the link-local IPv6 unicast
 prefix [RFC4291].
 Net_Iface:
 An implementation-dependent stable identifier associated with
 the network interface for which the RID is being generated.
 An implementation MAY provide a configuration option to select
 the source of the identifier to be used for the Net_Iface
 parameter. A discussion of possible sources for this value
 (along with the corresponding trade-offs) can be found in
 Appendix A.
 Network_ID:
 Some network-specific data that identifies the subnet to which
 this interface is attached -- for example, the IEEE 802.11
 Service Set Identifier (SSID) corresponding to the network to
 which this interface is associated. Additionally, Simple DNA
 [[RFC6059](https://www.rfc-editor.org/info/rfc6059/)] describes ideas that could be leveraged to generate
 a Network_ID parameter. This parameter is OPTIONAL.
 DAD_Counter:
 A counter that is employed to resolve Duplicate Address
 Detection (DAD) conflicts. It MUST be initialized to 0, and
 incremented by 1 for each new tentative address that is
 configured as a result of a DAD conflict. Implementations
 that record DAD_Counter in non-volatile memory for each
 {Prefix, Net_Iface, Network_ID} tuple MUST initialize
 DAD_Counter to the recorded value if such an entry exists in
 non-volatile memory. See Section 6 for additional details.
 secret_key:
 A secret key that is not known by the attacker. The secret
 key SHOULD be of at least 128 bits. It MUST be initialized to
 a pseudo-random number (see [[RFC4086](https://www.rfc-editor.org/info/rfc4086/)] for randomness
 requirements for security) when the operating system is
 installed or when the IPv6 protocol stack is "bootstrapped"
 for the first time. An implementation MAY provide the means
 for the system administrator to display and change the secret
 key.
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 2. The Interface Identifier is finally obtained by taking as many
 bits from the RID value (computed in the previous step) as
 necessary, starting from the least significant bit.
 We note that [RFC4291] requires that the Interface IDs of all
 unicast addresses (except those that start with the binary
 value 000) be 64 bits long. However, the method discussed in
 this document could be employed for generating Interface IDs
 of any arbitrary length, albeit at the expense of reduced
 entropy (when employing Interface IDs smaller than 64 bits).
 The resulting Interface Identifier SHOULD be compared against the
 reserved IPv6 Interface Identifiers [[RFC5453](https://www.rfc-editor.org/info/rfc5453/)] [IANA-RESERVED-IID]
 and against those Interface Identifiers already employed in an
 address of the same network interface and the same network
 prefix. In the event that an unacceptable identifier has been
 generated, this situation SHOULD be handled in the same way as
 the case of duplicate addresses (see Section 6).
 This document does not require the use of any specific PRF for the
 function F() above, since the choice of such PRF is usually a trade-
 off between a number of properties (processing requirements, ease of
 implementation, possible intellectual property rights, etc.), and
 since the best possible choice for F() might be different for
 different types of devices (e.g., embedded systems vs. regular
 servers) and might possibly change over time.
 Including the SLAAC prefix in the PRF computation causes the
 Interface Identifier to vary across each prefix (link-local, global,
 etc.) employed by the host and, consequently, also across networks.
 This mitigates the correlation of activities of multihomed hosts
 (since each of the corresponding addresses will typically employ a
 different prefix), host-tracking (since the network prefix will
 change as the host moves from one network to another), and any other
 attacks that benefit from predictable Interface Identifiers (such as
 IPv6 address-scanning attacks).
 The Net_Iface is a value that identifies the network interface for
 which an IPv6 address is being generated. The following properties
 are required for the Net_Iface parameter:
 o It MUST be constant across system bootstrap sequences and other
 network events (e.g., bringing another interface up or down).
 o It MUST be different for each network interface simultaneously in
 use.
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 Since the stability of the addresses generated with this method
 relies on the stability of all arguments of F(), it is key that the
 Net_Iface parameter be constant across system bootstrap sequences and
 other network events. Additionally, the Net_Iface parameter must
 uniquely identify an interface within the host, such that two
 interfaces connecting to the same network do not result in duplicate
 addresses. Different types of operating systems might benefit from
 different stability properties of the Net_Iface parameter. For
 example, a client-oriented operating system might want to employ
 Net_Iface identifiers that are attached to the NIC, such that a
 removable NIC always gets the same IPv6 address, irrespective of the
 system communications port to which it is attached. On the other
 hand, a server-oriented operating system might prefer Net_Iface
 identifiers that are attached to system slots/ports, such that
 replacement of a NIC does not result in an IPv6 address change.
 Appendix A discusses possible sources for the Net_Iface along with
 their pros and cons.
 Including the optional Network_ID parameter when computing the RID
 value above causes the algorithm to produce a different Interface
 Identifier when connecting to different networks, even when
 configuring addresses belonging to the same prefix. This means that
 a host would employ a different Interface Identifier as it moves from
 one network to another even for IPv6 link-local addresses or Unique
 Local Addresses (ULAs) [[RFC4193](https://www.rfc-editor.org/info/rfc4193/)]. In those scenarios where the
 Network_ID is unknown to the attacker, including this parameter might
 help mitigate attacks where a victim host connects to the same subnet
 as the attacker and the attacker tries to learn the Interface
 Identifier used by the victim host for a remote network (see
 Section 8 for further details).
 The DAD_Counter parameter provides the means to intentionally cause
 this algorithm to produce different IPv6 addresses (all other
 parameters being the same). This could be necessary to resolve DAD
 conflicts, as discussed in detail in Section 6.
 Note that the result of F() in the algorithm above is no more secure
 than the secret key. If an attacker is aware of the PRF that is
 being used by the victim (which we should expect), and the attacker
 can obtain enough material (i.e., addresses configured by the
 victim), the attacker may simply search the entire secret-key space
 to find matches. To protect against this, key lengths of at least
 128 bits should be adequate. The secret key is initialized at system
 installation time to a pseudorandom number, thus allowing this
 mechanism to be enabled and used automatically, without user
 intervention. Providing a mechanism to display and change the
 secret_key would allow an administrator to cause a new/replacement
 system (with the same implementation of this specification) to
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 generate the same IPv6 addresses as the system being replaced. We
 note that since the privacy of the scheme specified in this document
 relies on the secrecy of the secret_key parameter, implementations
 should constrain access to the secret_key parameter to the extent
 practicable (e.g., require superuser privileges to access it).
 Furthermore, in order to prevent leakages of the secret_key
 parameter, it should not be used for any purposes other than being a
 parameter to the scheme specified in this document.
 We note that all of the bits in the resulting Interface IDs are
 treated as "opaque" bits [[RFC7136](https://www.rfc-editor.org/info/rfc7136/)]. For example, the universal/local
 bit of Modified EUI-64 format identifiers is treated as any other bit
 of such an identifier. In theory, this might result in IPv6 address
 collisions and DAD failures that would otherwise not be encountered.
 However, this is not deemed as a likely issue because of the
 following considerations:
 o The interface IDs of all addresses (except those of addresses that
 start with the binary value 000) are 64 bits long. Since the
 method specified in this document results in random Interface IDs,
 the probability of DAD failures is very small.
 o Real-world data indicates that MAC address reuse is far more
 common than assumed [HD-MOORE]. This means that even IPv6
 addresses that employ (allegedly) unique identifiers (such as IEEE
 LAN MAC addresses) might result in DAD failures and, hence,
 implementations should be prepared to gracefully handle such
 occurrences. Additionally, some virtualization technologies
 already employ hardware addresses that are randomly selected, and,
 hence, cannot be guaranteed to be unique [IPV6-RECON].
 o Since some popular and widely deployed operating systems (such as
 Microsoft Windows) do not embed hardware addresses in the
 Interface IDs of their stable addresses, reliance on such unique
 identifiers is reduced in the deployed world (fewer deployed
 systems rely on them for the avoidance of address collisions).
 Finally, we note that since different implementations are likely to
 use different values for the secret_key parameter, and may also
 employ different PRFs for F() and different sources for the Net_Iface
 parameter, the addresses generated by this scheme should not expected
 to be stable across different operating-system installations. For
 example, a host that is dual-boot or that is reinstalled may result
 in different IPv6 addresses for each operating system and/or
 installation.
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6. Resolving DAD Conflicts
 If, as a result of performing DAD [RFC4862], a host finds that the
 tentative address generated with the algorithm specified in Section 5
 is a duplicate address, it SHOULD resolve the address conflict by
 trying a new tentative address as follows:
 o DAD_Counter is incremented by 1.
 o A new Interface Identifier is generated with the algorithm
 specified in Section 5, using the incremented DAD_Counter value.
 Hosts SHOULD introduce a random delay between 0 and IDGEN_DELAY
 seconds (see Section 7) before trying a new tentative address, to
 avoid lockstep behavior of multiple hosts.
 This procedure may be repeated a number of times until the address
 conflict is resolved. Hosts SHOULD try at least IDGEN_RETRIES (see
 Section 7) tentative addresses if DAD fails for successive generated
 addresses, in the hopes of resolving the address conflict. We also
 note that hosts MUST limit the number of tentative addresses that are
 tried (rather than indefinitely try a new tentative address until the
 conflict is resolved).
 In those unlikely scenarios in which duplicate addresses are detected
 and the order in which the conflicting hosts configure their
 addresses varies (e.g., because they may be bootstrapped in different
 orders), the algorithm specified in this section for resolving DAD
 conflicts could lead to addresses that are not stable within the same
 subnet. In order to mitigate this potential problem, hosts MAY
 record the DAD_Counter value employed for a specific {Prefix,
 Net_Iface, Network_ID} tuple in non-volatile memory, such that the
 same DAD_Counter value is employed when configuring an address for
 the same Prefix and subnet at any other point in time. We note that
 the use of non-volatile memory is OPTIONAL, and hosts that do not
 implement this feature are still compliant to this protocol
 specification.
 In the event that a DAD conflict cannot be solved (possibly after
 trying a number of different addresses), address configuration would
 fail. In those scenarios, hosts MUST NOT automatically fall back to
 employing other algorithms for generating Interface Identifiers.
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7. Specified Constants
 This document specifies the following constant:
 IDGEN_RETRIES:
 defaults to 3.
 IDGEN_DELAY:
 defaults to 1 second.
8. Security Considerations
 This document specifies an algorithm for generating Interface
 Identifiers to be used with IPv6 Stateless Address Autoconfiguration
 (SLAAC), as an alternative to e.g., Interface Identifiers that embed
 hardware addresses (such as those specified in [RFC2464], [RFC2467],
 and [RFC2470]). When compared to such identifiers, the identifiers
 specified in this document have a number of advantages:
 o They prevent trivial host-tracking based on the IPv6 address,
 since when a host moves from one network to another the network
 prefix used for autoconfiguration and/or the Network ID (e.g.,
 IEEE 802.11 SSID) will typically change; hence, the resulting
 Interface Identifier will also change (see [ADDR-GEN-PRIVACY]).
 o They mitigate address-scanning techniques that leverage
 predictable Interface Identifiers (e.g., known Organizationally
 Unique Identifiers) [IPV6-RECON].
 o They may result in IPv6 addresses that are independent of the
 underlying hardware (i.e., the resulting IPv6 addresses do not
 change if a network interface card is replaced) if an appropriate
 source for Net_Iface (see Section 5) is employed.
 o They prevent the information leakage produced by embedding
 hardware addresses in the Interface Identifier (which could be
 exploited to launch device-specific attacks).
 o Since the method specified in this document will result in
 different Interface Identifiers for each configured address,
 knowledge or leakage of the Interface Identifier employed for one
 stable address will not negatively affect the security/privacy of
 other stable addresses configured for other prefixes (whether at
 the same time or at some other point in time).
 We note that while some probing techniques (such as the use of ICMPv6
 Echo Request and ICMPv6 Echo Response packets) could be mitigated by
 a personal firewall at the target host, for other probing vectors,
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 such as listening to ICMPv6 "Destination Unreachable, Address
 Unreachable" (Type 1, Code 3) error messages that refer to the target
 addresses [IPV6-RECON], there is nothing a host can do (e.g., a
 personal firewall at the target host would not be able to mitigate
 this probing technique). Hence, the method specified in this
 document is still of value for hosts that employ personal firewalls.
 In scenarios in which an attacker can connect to the same subnet as a
 victim host, the attacker might be able to learn the Interface
 Identifier employed by the victim host for an arbitrary prefix by
 simply sending a forged Router Advertisement [[RFC4861](https://www.rfc-editor.org/info/rfc4861/)] for that
 prefix, and subsequently learning the corresponding address
 configured by the victim host (either listening to the Duplicate
 Address Detection packets or to any other traffic that employs the
 newly configured address). We note that a number of factors might
 limit the ability of an attacker to successfully perform such an
 attack:
 o First-Hop security mechanisms such as Router Advertisement Guard
 (RA-Guard) [[RFC6105](https://www.rfc-editor.org/info/rfc6105/)] [[RFC7113](https://www.rfc-editor.org/info/rfc7113/)] could prevent the forged Router
 Advertisement from reaching the victim host.
 o If the victim implementation includes the (optional) Network_ID
 parameter for computing F() (see Section 5), and the Network_ID
 employed by the victim for a remote network is unknown to the
 attacker, the Interface Identifier learned by the attacker would
 differ from the one used by the victim when connecting to the
 legitimate network.
 In any case, we note that at the point in which this kind of attack
 becomes a concern, a host should consider employing SEND [RFC3971] to
 prevent an attacker from illegitimately claiming authority for a
 network prefix.
 We note that this algorithm is meant to be an alternative to
 Interface Identifiers such as those specified in [RFC2464], but it is
 not meant as an alternative to temporary Interface Identifiers (such
 as those specified in [RFC4941]). Clearly, temporary addresses may
 help to mitigate the correlation of activities of a host within the
 same network, and they may also reduce the attack exposure window
 (since temporary addresses are short-lived when compared to the
 addresses generated with the method specified in this document). We
 note that the implementation of this specification would still
 benefit those hosts employing temporary addresses, since it would
 mitigate host-tracking vectors still present when such addresses are
 used (see [ADDR-GEN-PRIVACY]) and would also mitigate address-
 scanning techniques that leverage patterns in IPv6 addresses that
 embed IEEE LAN MAC addresses. Finally, we note that the method
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
 described in this document addresses some of the privacy concerns
 arising from the use of IPv6 addresses that embed IEEE LAN MAC
 addresses, without the use of temporary addresses, thus possibly
 offering an interesting trade-off for those scenarios in which the
 use of temporary addresses is not feasible.
9. Acknowledgements
 The algorithm specified in this document has been inspired by Steven
 Bellovin's work ([[RFC1948](https://www.rfc-editor.org/info/rfc1948/)]) in the area of TCP sequence numbers.
 The author would like to thank (in alphabetical order) Mikael
 Abrahamsson, Ran Atkinson, Karl Auer, Steven Bellovin, Matthias
 Bethke, Ben Campbell, Brian Carpenter, Tassos Chatzithomaoglou, Tim
 Chown, Alissa Cooper, Dominik Elsbroek, Stephen Farrell, Eric Gray,
 Brian Haberman, Bob Hinden, Christian Huitema, Ray Hunter, Jouni
 Korhonen, Suresh Krishnan, Eliot Lear, Jong-Hyouk Lee, Andrew
 McGregor, Thomas Narten, Simon Perreault, Tom Petch, Michael
 Richardson, Vincent Roca, Mark Smith, Hannes Frederic Sowa, Martin
 Stiemerling, Dave Thaler, Ole Troan, Lloyd Wood, James Woodyatt, and
 He Xuan, for providing valuable comments on earlier versions of this
 document.
 Hannes Frederic Sowa produced a reference implementation of this
 specification for the Linux kernel.
 Finally, the author wishes to thank Nelida Garcia and Guillermo Gont
 for their love and support.
10. References
10.1. Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", [BCP 14](https://www.rfc-editor.org/bcp/bcp14), [RFC 2119](https://www.rfc-editor.org/info/rfc2119/), March 1997.
 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
 (IPv6) Specification", [RFC 2460](https://www.rfc-editor.org/info/rfc2460/), December 1998.
 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
 and M. Carney, "Dynamic Host Configuration Protocol for
 IPv6 (DHCPv6)", [RFC 3315](https://www.rfc-editor.org/info/rfc3315/), July 2003.
 [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
 Neighbor Discovery (SEND)", [RFC 3971](https://www.rfc-editor.org/info/rfc3971/), March 2005.
 [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
 [RFC 3972](https://www.rfc-editor.org/info/rfc3972/), March 2005.
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
 Requirements for Security", [BCP 106](https://www.rfc-editor.org/bcp/bcp106), [RFC 4086](https://www.rfc-editor.org/info/rfc4086/), June 2005.
 [[RFC4122](https://www.rfc-editor.org/info/rfc4122/)] Leach, P., Mealling, M., and R. Salz, "A Universally
 Unique IDentifier (UUID) URN Namespace", [RFC 4122](https://www.rfc-editor.org/info/rfc4122/), July
 2005.
 [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
 Addresses", [RFC 4193](https://www.rfc-editor.org/info/rfc4193/), October 2005.
 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
 Architecture", [RFC 4291](https://www.rfc-editor.org/info/rfc4291/), February 2006.
 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
 "Neighbor Discovery for IP version 6 (IPv6)", [RFC 4861](https://www.rfc-editor.org/info/rfc4861/),
 September 2007.
 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
 Address Autoconfiguration", [RFC 4862](https://www.rfc-editor.org/info/rfc4862/), September 2007.
 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
 Extensions for Stateless Address Autoconfiguration in
 IPv6", [RFC 4941](https://www.rfc-editor.org/info/rfc4941/), September 2007.
 [RFC5453] Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC
 5453, February 2009.
 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
 Interface Identifiers", [RFC 7136](https://www.rfc-editor.org/info/rfc7136/), February 2014.
10.2. Informative References
 [ADDR-GEN-PRIVACY]
 Cooper, A., Gont, F., and D. Thaler, "Privacy
 Considerations for IPv6 Address Generation Mechanisms",
 Work in Progress, February 2014.
 [BROERSMA] Broersma, R., "IPv6 Everywhere: Living with a Fully
 IPv6-enabled environment", Australian IPv6 Summit 2010,
 Melbourne, VIC Australia, October 2010,
 <[http://www.ipv6.org.au/10ipv6summit/talks/](http://www.ipv6.org.au/10ipv6summit/talks/Ron_Broersma.pdf)
 [Ron_Broersma.pdf](http://www.ipv6.org.au/10ipv6summit/talks/Ron_Broersma.pdf)>.
 [CPNI-IPV6]
 Gont, F., "Security Assessment of the Internet Protocol
 version 6 (IPv6)", UK Centre for the Protection of
 National Infrastructure, (available on request).
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
 [FIPS-SHS] NIST, "Secure Hash Standard (SHS)", FIPS Publication
 180-4, March 2012, <[http://csrc.nist.gov/publications/](http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf)
 [fips/fips180-4/fips-180-4.pdf](http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf)>.
 [GONT-DEEPSEC2011]
 Gont, F., "Results of a Security Assessment of the
 Internet Protocol version 6 (IPv6)", DEEPSEC 2011
 Conference, Vienna, Austria, November 2011,
 <[http://www.si6networks.com/presentations/deepsec2011/](http://www.si6networks.com/presentations/deepsec2011/fgont-deepsec2011-ipv6-security.pdf)
 [fgont-deepsec2011-ipv6-security.pdf](http://www.si6networks.com/presentations/deepsec2011/fgont-deepsec2011-ipv6-security.pdf)>.
 [HD-MOORE] Moore, HD., "The Wild West", Louisville, Kentucky, U.S.A,
 DerbyCon 2012, September 2012, <[https://speakerdeck.com/](https://speakerdeck.com/hdm/derbycon-2012-the-wild-west)
 [hdm/derbycon-2012-the-wild-west](https://speakerdeck.com/hdm/derbycon-2012-the-wild-west)>.
 [IAB-PRIVACY]
 IAB, "Privacy and IPv6 Addresses", July 2011,
 <[http://www.iab.org/wp-content/IAB-uploads/2011/07/](http://www.iab.org/wp-content/IAB-uploads/2011/07/IPv6-addresses-privacy-review.txt)
 [IPv6-addresses-privacy-review.txt](http://www.iab.org/wp-content/IAB-uploads/2011/07/IPv6-addresses-privacy-review.txt)>.
 [IANA-RESERVED-IID]
 IANA, "Reserved IPv6 Interface Identifiers",
 <[http://www.iana.org/assignments/ipv6-interface-ids](http://www.iana.org/assignments/ipv6-interface-ids)>.
 [IPV6-RECON]
 Gont, F. and T. Chown, "Network Reconnaissance in IPv6
 Networks", Work in Progress, January 2014.
 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", [RFC 1321](https://www.rfc-editor.org/info/rfc1321/),
 April 1992.
 [RFC1948] Bellovin, S., "Defending Against Sequence Number Attacks",
 [RFC 1948](https://www.rfc-editor.org/info/rfc1948/), May 1996.
 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
 Networks", [RFC 2464](https://www.rfc-editor.org/info/rfc2464/), December 1998.
 [RFC2467] Crawford, M., "Transmission of IPv6 Packets over FDDI
 Networks", [RFC 2467](https://www.rfc-editor.org/info/rfc2467/), December 1998.
 [RFC2470] Crawford, M., Narten, T., and S. Thomas, "Transmission of
 IPv6 Packets over Token Ring Networks", [RFC 2470](https://www.rfc-editor.org/info/rfc2470/), December
 1998.
 [[RFC3493](https://www.rfc-editor.org/info/rfc3493/)] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
 Stevens, "Basic Socket Interface Extensions for IPv6", RFC
 3493, February 2003.
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
 [[RFC3542](https://www.rfc-editor.org/info/rfc3542/)] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
 "Advanced Sockets Application Program Interface (API) for
 IPv6", [RFC 3542](https://www.rfc-editor.org/info/rfc3542/), May 2003.
 [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
 Detecting Network Attachment in IPv6", [RFC 6059](https://www.rfc-editor.org/info/rfc6059/), November
 2010.
 [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
 Mohacsi, "IPv6 Router Advertisement Guard", [RFC 6105](https://www.rfc-editor.org/info/rfc6105/),
 February 2011.
 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
 for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
 [RFC 6151](https://www.rfc-editor.org/info/rfc6151/), March 2011.
 [RFC7113] Gont, F., "Implementation Advice for IPv6 Router
 Advertisement Guard (RA-Guard)", [RFC 7113](https://www.rfc-editor.org/info/rfc7113/), February 2014.
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
Appendix A. Possible Sources for the Net_Iface Parameter
 The following subsections describe a number of possible sources for
 the Net_Iface parameter employed by the F() function in Section 5.
 The choice of a specific source for this value represents a number of
 trade-offs, which may vary from one implementation to another.
A.1. Interface Index
 The Interface Index [RFC3493] [RFC3542] of an interface uniquely
 identifies that interface within the node. However, these
 identifiers might or might not have the stability properties required
 for the Net_Iface value employed by this method. For example, the
 Interface Index might change upon removal or installation of a
 network interface (typically one with a smaller value for the
 Interface Index, when such a naming scheme is used) or when network
 interfaces happen to be initialized in a different order. We note
 that some implementations are known to provide configuration knobs to
 set the Interface Index for a given interface. Such configuration
 knobs could be employed to prevent the Interface Index from changing
 (e.g., as a result of the removal of a network interface).
A.2. Interface Name
 The Interface Name (e.g., "eth0", "em0", etc.) tends to be more
 stable than the underlying Interface Index, since such stability is
 required or desired when interface names are employed in network
 configuration (firewall rules, etc.). The stability properties of
 Interface Names depend on implementation details, such as what is the
 namespace used for Interface Names. For example, "generic" interface
 names such as "eth0" or "wlan0" will generally be invariant with
 respect to network interface card replacements. On the other hand,
 vendor-dependent interface names such as "rtk0" or the like will
 generally change when a network interface card is replaced with one
 from a different vendor.
 We note that Interface Names might still change when network
 interfaces are added or removed once the system has been bootstrapped
 (for example, consider USB-based network interface cards that might
 be added or removed once the system has been bootstrapped).
A.3. Link-Layer Addresses
 Link-layer addresses typically provide for unique identifiers for
 network interfaces; although, for obvious reasons, they generally
 change when a network interface card is replaced. In scenarios in
 which neither Interface Indexes nor Interface Names have the
 stability properties specified in Section 5 for Net_Iface, an
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RFC 7217 Stable and Opaque IIDs with SLAAC April 2014
 implementation might want to employ the link-layer address of the
 interface for the Net_Iface parameter, albeit at the expense of
 making the corresponding IPv6 addresses dependent on the underlying
 network interface card (i.e., the corresponding IPv6 addresses would
 typically change upon replacement of the underlying network interface
 card).
A.4. Logical Network Service Identity
 Host operating systems with a conception of logical network service
 identity, distinct from network interface identity or index, may keep
 a Universally Unique Identifier (UUID) [RFC4122] or similar
 identifier with the stability properties appropriate for use as the
 Net_Iface parameter.
Author's Address
 Fernando Gont
 SI6 Networks / UTN-FRH
 Evaristo Carriego 2644
 Haedo, Provincia de Buenos Aires 1706
 Argentina
 Phone: +54 11 4650 8472
 EMail: fgont@si6networks.com
 URI: http://www.si6networks.com
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