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Network Working Group                                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Informational                                   I. Arce
Expires: September 1, 2018                                     Quarkslab
                                                       February 28, 2018


          Unfortunate History of Transient Numeric Identifiers
                   draft-gont-numeric-ids-history-03

Abstract

   This document performs an analysis of the security and privacy
   implications of different types of "numeric identifiers" used in IETF
   protocols, and tries to categorize them based on their
   interoperability requirements and the associated failure severity
   when such requirements are not met.  It describes a number of
   algorithms that have been employed in real implementations to meet
   such requirements and analyzes their security and privacy properties.
   Additionally, it provides advice on possible algorithms that could be
   employed to satisfy the interoperability requirements of each
   identifier type, while minimizing the security and privacy
   implications, thus providing guidance to protocol designers and
   protocol implementers.  Finally, it provides recommendations for
   future protocol specifications regarding the specification of the
   aforementioned numeric identifiers.

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 September 1, 2018.








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Copyright Notice

   Copyright (c) 2018 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
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   described in the Simplified BSD License.

   This document may not be modified, and derivative works of it may not
   be created, and it may not be published except as an Internet-Draft.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Threat Model  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IPv4/IPv6 Identification  . . . . . . . . . . . . . . . . . .   5
   5.  TCP Initial Sequence Numbers (ISNs) . . . . . . . . . . . . .   8
   6.  IPv6 Interface Identifiers (IIDs) . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Network protocols employ a variety of numeric identifiers for
   different protocol entities, ranging from DNS Transaction IDs (TxIDs)
   to transport protocol numbers (e.g.  TCP ports) or IPv6 Interface
   Identifiers (IIDs).  These identifiers usually have specific
   properties that must be satisfied such that they do not result in
   negative interoperability implications (e.g. uniqueness during a
   specified period of time), and associated failure severities when
   such properties are not met, ranging from soft to hard failures.

   For more than 30 years, a large number of implementations of the TCP/
   IP protocol suite have been subject to a variety of attacks, with
   effects ranging from Denial of Service (DoS) or data injection, to



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   information leakage that could be exploited for pervasive monitoring
   [RFC7528].  The root of these issues has been, in many cases, the
   poor selection of identifiers in such protocols, usually as a result
   of an insufficient or misleading specification.  While it is
   generally trivial to identify an algorithm that can satisfy the
   interoperability requirements for a given identifier, there exists
   practical evidence that doing so without negatively affecting the
   security and/or privacy properties of the aforementioned protocols is
   prone to error.

   For example, implementations have been subject to security and/or
   privacy issues resulting from:

   o  Predictable TCP Initial Sequence Numbers (ISNs) (see e.g.
      [Morris1985])

   o  Predictable ephemeral transport protocol numbers (see e.g.
      [RFC6056] and [Silbersack2005])

   o  Predictable IPv4 or IPv6 Fragment Identifiers (see e.g.
      [RFC5722], [RFC6274], and [RFC7739])

   o  Predictable IPv6 IIDs (see e.g.  [RFC7721] and [RFC7707])

   o  Predictable DNS TxIDs

   Recent history indicate that when new protocols are standardized or
   new protocol implementations are produced, the security and privacy
   properties of the associated identifiers tend to be overlooked and
   inappropriate algorithms to generate identifier values are either
   suggested in the specification or selected by implementers.

   This document contains a non-exhaustive timeline of vulnerability
   disclosures related to some sample transient numeric identifiers and
   other work that has led to advances in this area, with the goal of
   illustrating that:

   o  Vulnerabilities related to how the values for some identifiers are
      generated and assigned have affected implementations for an
      extremely long period of time.

   o  Such vulnerabilities, even when addressed for a given protocol
      version, were later reintroduced in new versions or new
      implementations of the same protocol.

   o  Standardization efforts that discuss and provide advice in this
      area can have a positive effect on protocol specifications and
      protocol implementations.



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   Other related documents ([I-D.gont-numeric-ids-generation] and
   [I-D.gont-numeric-ids-sec-considerations]) provide guidance in this
   area.

2.  Terminology

   Identifier:
      A data object in a protocol specification that can be used to
      definitely distinguish a protocol object (a datagram, network
      interface, transport protocol endpoint, session, etc) from all
      other objects of the same type, in a given context.  Identifiers
      are usually defined as a series of bits and represented using
      integer values.  We note that different identifiers may have
      additional requirements or properties depending on their specific
      use in a protocol.  We use the term "identifier" as a generic term
      to refer to any data object in a protocol specification that
      satisfies the identification property stated above.

   Failure Severity:
      The consequences of a failure to comply with the interoperability
      requirements of a given identifier.  Severity considers the worst
      potential consequence of a failure, determined by the system
      damage and/or time lost to repair the failure.  In this document
      we define two types of failure severity: "soft" and "hard".

   Hard Failure:
      A hard failure is a non-recoverable condition in which a protocol
      does not operate in the prescribed manner or it operates with
      excessive degradation of service.  For example, an established TCP
      connection that is aborted due to an error condition constitutes,
      from the point of view of the transport protocol, a hard failure,
      since it enters a state from which normal operation cannot be
      recovered.

   Soft Failure:
      A soft failure is a recoverable condition in which a protocol does
      not operate in the prescribed manner but normal operation can be
      resumed automatically in a short period of time.  For example, a
      simple packet-loss event that is subsequently recovered with a
      retransmission can be considered a soft failure.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].







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3.  Threat Model

   Throughout this document, we assume an attacker does not have
   physical or logical device to the device(s) being attacked.  We
   assume the attacker can simply send any traffic to the target
   devices, to e.g. sample identifiers employed by such devices.

4.  IPv4/IPv6 Identification

   This section presents the timeline of the Identification field both
   for IPv4 and for IPv6.  The reason for presenting both cases in the
   same section is so that it becomes evident that, while the
   Identification value serves the same purpose in both IPv4 and IPv6,
   the work and research done for the IPv4 case did not affect the IPv6
   specifications or implementations.

   The IPv4 Identification value is specified in [RFC0791], which
   specifies the interoperability requirements for the Identification
   field: the sender must choose the Identification field to be unique
   for a given source address, destination address, and protocol for the
   time the datagram (or any fragment of it) could be alive in the
   internet.  It suggests that a node may keep "a table of Identifiers,
   one entry for each destination it has communicated with in the last
   maximum packet lifetime for the internet", and suggests that "since
   the Identifier field allows 65,536 different values, some host may be
   able to simply use unique identifiers independent of destination".
   The above may be read as a suggestion to employ per-destination or
   global counters for the generation of Identification values.  While
   [RFC0791] does not suggest any flawed algorithm for the generation of
   Identification values, it misses a discussion of the security and
   privacy implications of employing predictable.  This has resulted in
   virtually all IP4 implementations generating predictable fragment
   Identification values by means of a global counter, at least at some
   point during the lifetime of such implementations.

   The IPv6 Identification is specified in [RFC2460].  It serves the
   same purpose as its IPv4 counterpart, with the only difference
   residing in the length of the corresponding field, and that while the
   IPv4 Identification field is part of the base IPv4 header, in the
   IPv6 case it is part of the Fragment header (which may or may not be
   present in an IPv6 packet).  [RFC2460] states, in Section 4.5, that
   the Identification must be different than that of any other
   fragmented packet sent recently (within the maximum likely lifetime
   of a packet) with the same Source Address and Destination Address.
   Subsequently, it notes that this requirement can be met by means of a
   wrap-around 32-bit counter that is incremented each time a packet
   must be fragmented, and that it is an implementation choice whether
   to use a global or a per-destination counter.  Thus, the



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   implementation of the IPv6 Identification is similar to that of the
   IPv4 case, with the only difference that in the IPv6 case the
   suggestions to use simple counters is more explicit.

   September 1981:
      [RFC0791] specifies the interoperability requirements for IPv4
      Identification value, but does not specify any requirements in the
      area of security and privacy.

   December 1998:
      [Sanfilippo1998a] finds that predictable IPv4 Identification
      values (generated by most popular implementations) can be
      leveraged to count the number of packets sent by a target node.
      [Sanfilippo1998b] explains how to leverage the same vulnerability
      to implement a port-scanning technique known as dumb/idle scan.  A
      tool that implements this attack is publicly released.

   December 1998:
      [RFC2460] suggests that a global counter be used to generate the
      IPv6 Identification value.

   November 1999:
      [Sanfilippo1999] discusses how to leverage predictable IPv4
      Identification to uncover the rules of a number of firewalls.

   November 1999:
      [Bellovin2002] explains how the IPv4 Identification field can be
      exploited to count the number of systems behind a NAT.

   December 2003:
      [Zalewski2003] explains a technique to perform TCP data injection
      attack based on predictable IPv4 identification values which
      requires less effort than TCP injection attacks performed with
      bare TCP packets.

   November 2005:
      [Silbersack2005] discusses shortcoming in a number of techniques
      to mitigate predictable IPv4 Identification values.

   October 2007:
      [Klein2007] describes a weakness in the pseudo random number
      generator (PRNG) in use for the generation of the IP
      Identification by a number of operating systems.

   June 2011:
      [Gont2011] describes how to perform idle scan attacks in IPv6.

   November 2011:



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      Linux mitigates predictable IPv6 Identification values
      [RedHat2011] [SUSE2011] [Ubuntu2011].

   December 2011:
      [draft-gont-6man-predictable-fragment-id-00] describes the
      security implications of predictable IPv6 Identification values,
      and possible mitigations.  This document is published on the
      Standards Track, meaning to formally update [RFC2460], to
      introduce security and privacy requirements on IPv6 Identification
      values.

   May 2012:
      [Gont2012] notes that some major IPv6 implementations still employ
      predictable IPv6 Identification values.

   March 2013:
      The 6man WG adopts [I-D.gont-6man-predictable-fragment-id], but
      changes the track to "BCP" (while still formally updating
      [RFC2460]), publishing the resulting document as
      [draft-ietf-6man-predictable-fragment-id-00].

   June 2013:
      A patch that implements IPv6-based idle-scan in nmap is submitted
      [Morbitzer2013].

   December 2014:
      The 6man WG changes the status of the aforementioned IETF Internet
      Draft to "Informational" and publishes it as
      [draft-ietf-6man-predictable-fragment-id-02].  As a result, it no
      longer formally updates [RFC2460].

   June 2015:
      [draft-ietf-6man-predictable-fragment-id-08] notes that some
      popular host and router implementations still employ predictable
      IPv6 Identification values.

   February 2016:
      [RFC7739] (based on [I-D.ietf-6man-predictable-fragment-id])
      analyzes the security and privacy implications of predictable IPv6
      Identification values, and provides guidance for selecting an
      algorithm to generate such values.  However, being published on
      the Informational track, it does not formally update [RFC2460].

   June 2016:
      [I-D.ietf-6man-rfc2460bis], revision of [RFC2460], removes the
      suggestion from RFC2460 to employ a global counter for the
      generation of IPv6 Identification values, but does not specify any




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      security and privacy requirements for the IPv6 Identification
      value.

5.  TCP Initial Sequence Numbers (ISNs)

   [RFC0793] suggests that the choice of the ISN of a connection is not
   arbitrary, but aims to reduce the chances of a stale segment from
   being accepted by a new incarnation of a previous connection.
   [RFC0793] suggests the use of a global 32-bit ISN generator that is
   incremented by 1 roughly every 4 microseconds.  However, as a matter
   of fact, protection against stale segments from a previous
   incarnation of the connection is enforced by preventing the creation
   of a new incarnation of a previous connection before 2*MSL have
   passed since a segment corresponding to the old incarnation was last
   seen (where "MSL" is the "Maximum Segment Lifetime" [RFC0793]).  This
   is accomplished by the TIME-WAIT state and TCP's "quiet time" concept
   (see Appendix B of [RFC1323]).  Based on the assumption that ISNs are
   monotonically increasing across connections, many stacks (e.g.,
   4.2BSD-derived) use the ISN of an incoming SYN segment to perform
   "heuristics" that enable the creation of a new incarnation of a
   connection while the previous incarnation is still in the TIME-WAIT
   state (see p. 945 of [Wright1994]).  This avoids an interoperability
   problem that may arise when a node establishes connections to a
   specific TCP end-point at a high rate [Silbersack2005].

   In the case of TCP, the interoperability requirements for the ISNs
   are probably not clearly spelled out as one would expect.
   Furthermore, the suggestion of employing a global counter in
   [RFC0793] leads to negative security and privacy implications.

   September 1981:
      [RFC0793], suggests the use of a global 32-bit ISN generator,
      whose lower bit is incremented roughly every 4 microseconds.
      However, such an ISN generator makes it trivial to predict the ISN
      that a TCP will use for new connections, thus allowing a variety
      of attacks against TCP.

   February 1985:
      [Morris1985] was the first to describe how to exploit predictable
      TCP ISNs for forging TCP connections that could then be leveraged
      for trust relationship exploitation.

   April 1989:
      [Bellovin1989] discussed the security implications of predictable
      ISNs (along with a range of other protocol-based vulnerabilities).

   February 1995:




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      [Shimomura1995] reported a real-world exploitation of the attack
      described in 1985 (ten years before) in [Morris1985].

   May 1996:
      [RFC1948] was the first IETF effort, authored by Steven Bellovin,
      to address predictable TCP ISNs.  The same concept specified in
      this document for TCP ISNs was later proposed for TCP ephemeral
      ports [RFC6056], TCP Timestamps, and eventually even IPv6
      Interface Identifiers [RFC7217].

   March 2001:
      [Zalewski2001] provides a detailed analysis of statistical
      weaknesses in some ISN generators, and includes a survey of the
      algorithms in use by popular TCP implementations.

   May 2001:
      Vulnerability advisories [CERT2001] [USCERT2001] are released
      regarding statistical weaknesses in some ISN generators, affecting
      popular TCP/IP implementations.

   March 2002:
      [Zalewski2002] updates and complements [Zalewski2001].  It
      concludes that "while some vendors [...] reacted promptly and
      tested their solutions properly, many still either ignored the
      issue and never evaluated their implementations, or implemented a
      flawed solution that apparently was not tested using a known
      approach" [Zalewski2002].

   February 2012:
      [RFC6528], after 27 years of Morris' original work [Morris1985],
      formally updates [RFC0793] to mitigate predictable TCP ISNs.

   August 2014:
      [I-D.eddy-rfc793bis-04], the upcoming revision of the core TCP
      protocol specification, incorporates the algorithm specified in
      [RFC6528] as the recommended algorithm for TCP ISN generation.

6.  IPv6 Interface Identifiers (IIDs)

   IPv6 Interface Identifiers can be generated in multiple ways: SLAAC
   [RFC4862], DHCPv6 [RFC3315], and manual configuration.  This section
   focuses on Interface Identifiers resulting from SLAAC.

   The Interface Identifier of stable (traditional) IPv6 addresses
   resulting from SLAAC have traditionally resulted in the underlying
   link-layer address being embedded in the IID.  IPv6 addresses
   resulting from SLAAC are currently required to employ Modified EUI-64
   format identifiers, which essentially embed the underlying link-layer



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   address of the corresponding network interface.  At the time,
   employing the underlying link-layer address for the IID was seen as a
   convenient way to obtain a unique address.  However, recent awareness
   about the security and privacy implications of this approach, and
   thus ongoing work [I-D.ietf-6man-default-iids] at the IETF is in the
   process of addressing this problem.

   January 1997:
      [RFC2073] specifies the syntax of IPv6 global addresses (referred
      to as "An IPv6 Provider-Based Unicast Address Format" at the
      time), consistent with the IPv6 addressing architecture specified
      in [RFC1884].  Hosts are recommended to "generate addresses using
      link-specific addresses as Interface ID such as 48 bit IEEE-802
      MAC addresses".

   July 1998:
      [RFC2374] specifies "An IPv6 Aggregatable Global Unicast Address
      Format" (obsoleting [RFC2373]) changing the size of the Interface
      ID to 64 bits, and specifies that that IIDs must be constructed in
      IEEE EUI-64 format.  How such identifiers are constructed becomes
      specified in the appropriate "IPv6 over <link>" specification such
      as "IPv6 over Ethernet".

   January 2001:
      [RFC3041] recognizes the problem of network activity correlation,
      and specifies temporary addresses.  Temporary addresses are to be
      used along with stable addresses.

   August 2003:
      [RFC3587] obsoletes [RFC2374], making the TLA/NLA structure
      historic.  The syntax and recommendations for the traditional
      stable IIDs remain unchanged, though.

   February 2006:
      [RFC4291] is published as the latest "IP Version 6 Addressing
      Architecture", requiring the IIDs of the traditional (stable)
      autoconfigured addresses to employ the Modified EUI-64 format.
      The details of constructing such interface identifiers are defined
      in the appropriate "IPv6 over <link>" specifications.

   March 2008:
      [RFC5157] provides hints regarding how patterns in IPv6 addresses
      could be leveraged for the purpose of address scanning.

   December 2011:
      [draft-gont-6man-stable-privacy-addresses-00] notes that the
      traditional scheme for generating stable addresses allows for
      address scanning, and also does not prevent active node tracking.



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      It also specifies an alternative algorithm meant to replace IIDs
      based on Modified EUI-64 format identifiers.

   November 2012:
      The 6man WG adopts [I-D.gont-6man-stable-privacy-addresses] as a
      working group item (as
      [draft-ietf-6man-stable-privacy-addresses-00]).  However, the
      specified algorithm no longer formally replaces the Modified
      EUI-64 format identifiers.

   February 2013:
      An address-scanning tool (scan6 of [IPv6-Toolkit]) that leverages
      IPv6 address patterns is released [Gont2013].

   July 2013:
      [I-D.cooper-6man-ipv6-address-generation-privacy] elaborates on
      the security and privacy implications on all known algorithms for
      generating IPv6 IIDs.

   January 2014:
      The 6man wg publishes [draft-ietf-6man-default-iids-00]
      ("Recommendation on Stable IPv6 Interface Identifiers"),
      recommending [I-D.ietf-6man-stable-privacy-addresses] for the
      generation of stable addresses.

   April 2014:
      [RFC7217] is published, specifying "A Method for Generating
      Semantically Opaque Interface Identifiers with IPv6 Stateless
      Address Autoconfiguration (SLAAC)" as an alternative to (but *not*
      replacement of) Modified EUI-64 format IIDs.

   March 2016:
      [RFC7707] (formerly [I-D.gont-opsec-ipv6-host-scanning] and later
      [I-D.ietf-opsec-ipv6-host-scanning]), about "Network
      Reconnaissance in IPv6 Networks", is published.

   March 2016:
      [RFC7721] (formerly
      [I-D.cooper-6man-ipv6-address-generation-privacy] and later
      [I-D.ietf-6man-ipv6-address-generation-privacy]), about "Security
      and Privacy Considerations for IPv6 Address Generation
      Mechanisms", is published.

   May 2016:
      [draft-gont-6man-non-stable-iids-00] is published, with the goal
      of specifying requirements for non-stable addresses, and updating
      [RFC4941] such that use of only temporary addresses is allowed.




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   May 2016:
      [draft-gont-6man-address-usage-recommendations-00] is published,
      providing an analysis of how different aspects on an address (from
      stability to usage mode) affect their corresponding security and
      privacy implications, and meaning to eventually provide advice in
      this area.

7.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.

8.  Security Considerations

   The entire document is about the security and privacy implications of
   transient numeric identifiers.

9.  Acknowledgements

   The authors would like to thank (in alphabetical order) Dave Crocker,
   Christian Huitema, and Joe Touch, for providing valuable comments on
   earlier versions of this document.

   The authors would like to thank (in alphabetical order) Steven
   Bellovin, Joseph Lorenzo Hall, Gre Norcie, and Martin Thomson, for
   providing valuable comments on [I-D.gont-predictable-numeric-ids], on
   which this document is based.

   Section 5 of this document borrows text from [RFC7528], authored by
   Fernando Gont and Steven Bellovin.

10.  References

10.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1323]  Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
              1992, <https://www.rfc-editor.org/info/rfc1323>.




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   [RFC1884]  Hinden, R., Ed. and S. Deering, Ed., "IP Version 6
              Addressing Architecture", RFC 1884, DOI 10.17487/RFC1884,
              December 1995, <https://www.rfc-editor.org/info/rfc1884>.

   [RFC2073]  Rekhter, Y., Lothberg, P., Hinden, R., Deering, S., and J.
              Postel, "An IPv6 Provider-Based Unicast Address Format",
              RFC 2073, DOI 10.17487/RFC2073, January 1997,
              <https://www.rfc-editor.org/info/rfc2073>.

   [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>.

   [RFC2373]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, DOI 10.17487/RFC2373, July 1998,
              <https://www.rfc-editor.org/info/rfc2373>.

   [RFC2374]  Hinden, R., O'Dell, M., and S. Deering, "An IPv6
              Aggregatable Global Unicast Address Format", RFC 2374,
              DOI 10.17487/RFC2374, July 1998,
              <https://www.rfc-editor.org/info/rfc2374>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              DOI 10.17487/RFC3041, January 2001,
              <https://www.rfc-editor.org/info/rfc3041>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3587]  Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
              Unicast Address Format", RFC 3587, DOI 10.17487/RFC3587,
              August 2003, <https://www.rfc-editor.org/info/rfc3587>.

   [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>.






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   [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>.

   [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>.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, DOI 10.17487/RFC5722, December 2009,
              <https://www.rfc-editor.org/info/rfc5722>.

   [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>.

   [RFC6528]  Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <https://www.rfc-editor.org/info/rfc6528>.

   [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>.

10.2.  Informative References

   [Bellovin1989]
              Bellovin, S., "Security Problems in the TCP/IP Protocol
              Suite", Computer Communications Review, vol. 19, no. 2,
              pp. 32-48, 1989,
              <https://www.cs.columbia.edu/~smb/papers/ipext.pdf>.

   [Bellovin2002]
              Bellovin, S., "A Technique for Counting NATted Hosts",
              IMW'02 Nov. 6-8, 2002, Marseille, France, 2002.







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   [CERT2001]
              CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in
              TCP/IP Initial Sequence Numbers", 2001,
              <http://www.cert.org/advisories/CA-2001-09.html>.

   [CPNI-TCP]
              Gont, F., "Security Assessment of the Transmission Control
              Protocol (TCP)",  United Kingdom's Centre for the
              Protection of National Infrastructure (CPNI) Technical
              Report, 2009, <http://www.gont.com.ar/papers/
              tn-03-09-security-assessment-TCP.pdf>.

   [draft-gont-6man-address-usage-recommendations-00]
              Gont, F. and W. Liu, "IPv6 Address Usage Recommendations",
              draft-gont-6man-address-usage-recommendations-00 (work in
              progress), May 2016.

   [draft-gont-6man-non-stable-iids-00]
              Gont, F. and W. Liu, "Recommendation on Non-Stable IPv6
              Interface Identifiers", draft-gont-6man-non-stable-iids-00
              (work in progress), May 2016.

   [draft-gont-6man-predictable-fragment-id-00]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-gont-6man-predictable-
              fragment-id-00 (work in progress), December 2011.

   [draft-gont-6man-stable-privacy-addresses-00]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-gont-6man-stable-privacy-addresses-00
              (work in progress), December 2011.

   [draft-ietf-6man-default-iids-00]
              Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-00 (work in progress), July
              2014.

   [draft-ietf-6man-predictable-fragment-id-00]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-00 (work in progress), March 2013.

   [draft-ietf-6man-predictable-fragment-id-02]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-02 (work in progress), December 2014.



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   [draft-ietf-6man-predictable-fragment-id-08]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-08 (work in progress), June 2015.

   [draft-ietf-6man-stable-privacy-addresses-00]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-ietf-6man-stable-privacy-addresses-00
              (work in progress), May 2012.

   [Fyodor2004]
              Fyodor, "Idle scanning and related IP ID games", 2004,
              <http://www.insecure.org/nmap/idlescan.html>.

   [Gont2011]
              Gont, F., "Hacking IPv6 Networks (training course)", Hack
              In Paris 2011 Conference Paris, France, June 2011.

   [Gont2012]
              Gont, F., "Recent Advances in IPv6 Security", BSDCan 2012
              Conference Ottawa, Canada. May 11-12, 2012, May 2012.

   [Gont2013]
              Gont, F., "Beta release of the SI6 Network's IPv6 Toolkit
              (help wanted!)", Message posted to the IPv6 Hackers
              mailing-list  Message-ID:
              <51184548.3030105@si6networks.com>, 1995,
              <https://lists.si6networks.com/pipermail/
              ipv6hackers/2013-February/000947.html>.

   [I-D.cooper-6man-ipv6-address-generation-privacy]
              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-cooper-6man-ipv6-address-generation-privacy-00 (work
              in progress), July 2013.

   [I-D.eddy-rfc793bis-04]
              Eddy, W., "Transmission Control Protocol Specification",
              draft-eddy-rfc793bis-04 (work in progress), August 2014.

   [I-D.gont-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-gont-6man-predictable-
              fragment-id-03 (work in progress), January 2013.






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   [I-D.gont-6man-stable-privacy-addresses]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", draft-gont-6man-stable-privacy-addresses-01
              (work in progress), March 2012.

   [I-D.gont-numeric-ids-generation]
              Gont, F. and I. Arce, "On the Generation of Transient
              Numeric Identifiers", draft-gont-numeric-ids-generation-01
              (work in progress), July 2017.

   [I-D.gont-numeric-ids-sec-considerations]
              Gont, F. and I. Arce, "Security Considerations for
              Transient Numeric Identifiers Employed in Network
              Protocols", draft-gont-numeric-ids-sec-considerations-01
              (work in progress), July 2017.

   [I-D.gont-opsec-ipv6-host-scanning]
              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-gont-opsec-ipv6-host-scanning-02 (work in
              progress), October 2012.

   [I-D.gont-predictable-numeric-ids]
              Gont, F. and I. Arce, "Security and Privacy Implications
              of Numeric Identifiers Employed in Network Protocols",
              draft-gont-predictable-numeric-ids-01 (work in progress),
              July 2017.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-16 (work in progress),
              September 2016.

   [I-D.ietf-6man-ipv6-address-generation-privacy]
              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-ietf-6man-ipv6-address-generation-privacy-08 (work
              in progress), September 2015.

   [I-D.ietf-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-10 (work in progress), October 2015.







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   [I-D.ietf-6man-rfc2460bis]
              Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", draft-ietf-6man-rfc2460bis-13 (work
              in progress), May 2017.

   [I-D.ietf-6man-stable-privacy-addresses]
              Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", draft-ietf-6man-stable-
              privacy-addresses-17 (work in progress), January 2014.

   [I-D.ietf-opsec-ipv6-host-scanning]
              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-ietf-opsec-ipv6-host-scanning-08 (work in
              progress), August 2015.

   [IPv6-Toolkit]
              SI6 Networks, "SI6 Networks' IPv6 Toolkit",
              <https://www.si6networks.com/tools/ipv6toolkit>.

   [Joncheray1995]
              Joncheray, L., "A Simple Active Attack Against TCP", Proc.
              Fifth Usenix UNIX Security Symposium, 1995.

   [Klein2007]
              Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
              Predictable IP ID Vulnerability", 2007,
              <http://www.trusteer.com/files/OpenBSD_DNS_Cache_Poisoning
              _and_Multiple_OS_Predictable_IP_ID_Vulnerability.pdf>.

   [Morbitzer2013]
              Morbitzer, M., "[PATCH] TCP Idle Scan in IPv6",  Message
              posted to the nmap-dev mailing-list, 2013,
              <http://seclists.org/nmap-dev/2013/q2/394>.

   [Morris1985]
              Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP
              Software", CSTR 117, AT&T Bell Laboratories, Murray Hill,
              NJ, 1985,
              <https://pdos.csail.mit.edu/~rtm/papers/117.pdf>.

   [RedHat2011]
              RedHat, "RedHat Security Advisory RHSA-2011:1465-1:
              Important: kernel security and bug fix update", 2011,
              <https://rhn.redhat.com/errata/RHSA-2011-1465.html>.






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   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, DOI 10.17487/RFC1948, May 1996,
              <https://www.rfc-editor.org/info/rfc1948>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC5157]  Chown, T., "IPv6 Implications for Network Scanning",
              RFC 5157, DOI 10.17487/RFC5157, March 2008,
              <https://www.rfc-editor.org/info/rfc5157>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <https://www.rfc-editor.org/info/rfc6056>.

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
              <https://www.rfc-editor.org/info/rfc6274>.

   [RFC7528]  Higgs, P. and J. Piesing, "A Uniform Resource Name (URN)
              Namespace for the Hybrid Broadcast Broadband TV (HbbTV)
              Association", RFC 7528, DOI 10.17487/RFC7528, April 2015,
              <https://www.rfc-editor.org/info/rfc7528>.

   [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>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [Sanfilippo1998a]
              Sanfilippo, S., "about the ip header id", Post to Bugtraq
              mailing-list, Mon Dec 14 1998,
              <http://seclists.org/bugtraq/1998/Dec/48>.



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   [Sanfilippo1998b]
              Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list,
              1998, <http://www.kyuzz.org/antirez/papers/dumbscan.html>.

   [Sanfilippo1999]
              Sanfilippo, S., "more ip id", Post to Bugtraq mailing-
              list, 1999,
              <http://www.kyuzz.org/antirez/papers/moreipid.html>.

   [Shimomura1995]
              Shimomura, T., "Technical details of the attack described
              by Markoff in NYT", Message posted in USENET's
              comp.security.misc newsgroup  Message-ID:
              <3g5gkl$5j1@ariel.sdsc.edu>, 1995,
              <http://www.gont.com.ar/docs/post-shimomura-usenet.txt>.

   [Silbersack2005]
              Silbersack, M., "Improving TCP/IP security through
              randomization without sacrificing interoperability",
              EuroBSDCon 2005 Conference, 2005,
              <http://citeseerx.ist.psu.edu/viewdoc/
              download?doi=10.1.1.91.4542&rep=rep1&type=pdf>.

   [SUSE2011]
              SUSE, "SUSE Security Announcement: Linux kernel security
              update (SUSE-SA:2011:046)", 2011,
              <http://lists.opensuse.org/
              opensuse-security-announce/2011-12/msg00011.html>.

   [Ubuntu2011]
              Ubuntu, "Ubuntu: USN-1253-1: Linux kernel
              vulnerabilities", 2011,
              <http://www.ubuntu.com/usn/usn-1253-1/>.

   [USCERT2001]
              US-CERT, "US-CERT Vulnerability Note VU#498440: Multiple
              TCP/IP implementations may use statistically predictable
              initial sequence numbers", 2001,
              <http://www.kb.cert.org/vuls/id/498440>.

   [Wright1994]
              Wright, G. and W. Stevens, "TCP/IP Illustrated, Volume 2:
              The Implementation", Addison-Wesley, 1994.

   [Zalewski2001]
              Zalewski, M., "Strange Attractors and TCP/IP Sequence
              Number Analysis", 2001,
              <http://lcamtuf.coredump.cx/oldtcp/tcpseq.html>.



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   [Zalewski2002]
              Zalewski, M., "Strange Attractors and TCP/IP Sequence
              Number Analysis - One Year Later", 2001,
              <http://lcamtuf.coredump.cx/newtcp/>.

   [Zalewski2003]
              Zalewski, M., "A new TCP/IP blind data injection
              technique?", 2003,
              <http://lcamtuf.coredump.cx/ipfrag.txt>.

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


   Ivan Arce
   Quarkslab

   Email: iarce@quarkslab.com
   URI:   https://www.quarkslab.com























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