



Internet Research Task Force (IRTF)                              F. Gont
Internet-Draft                                              SI6 Networks
Intended status: Informational                                   I. Arce
Expires: 14 June 2023                                          Quarkslab
                                                        11 December 2022


          Unfortunate History of Transient Numeric Identifiers
                draft-irtf-pearg-numeric-ids-history-11

Abstract

   This document analyzes the timeline of the specification and
   implementation of different types of "transient numeric identifiers"
   used in IETF protocols, and how the security and privacy properties
   of such protocols have been affected as a result of it.  It provides
   empirical evidence that advice in this area is warranted.  This
   document is a product of the Privacy Enhancement and Assessment
   Research Group (PEARG) in the IRTF.

Status of This Memo

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   This Internet-Draft will expire on 14 June 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Threat Model  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Issues with the Specification of Transient Numeric
           Identifiers . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  IPv4/IPv6 Identification  . . . . . . . . . . . . . . . .   6
     4.2.  TCP Initial Sequence Numbers (ISNs) . . . . . . . . . . .  10
     4.3.  IPv6 Interface Identifiers (IIDs) . . . . . . . . . . . .  12
     4.4.  NTP Reference IDs (REFIDs)  . . . . . . . . . . . . . . .  15
     4.5.  Transport Protocol Ephemeral Port Numbers . . . . . . . .  16
     4.6.  DNS Query ID  . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   Networking protocols employ a variety of transient numeric
   identifiers for different protocol objects, such as IPv4 and IPv6
   Fragment Identifiers [RFC0791] [RFC8200], IPv6 Interface Identifiers
   (IIDs) [RFC4291], transport protocol ephemeral port numbers
   [RFC6056], TCP Initial Sequence Numbers (ISNs) [RFC0793], NTP
   Reference IDs (REFIDs) [RFC5905], and DNS Query IDs [RFC1035].  These
   identifiers typically have specific interoperability requirements
   (e.g. uniqueness during a specified period of time), and associated
   failure severities when such requirements are not met
   [I-D.irtf-pearg-numeric-ids-generation].

   For more than 30 years, a large number of implementations of the IETF
   protocols have been subject to a variety of attacks, with effects
   ranging from Denial of Service (DoS) or data injection, to
   information leakages that could be exploited for pervasive monitoring
   [RFC7258].  The root cause of these issues has been, in many cases,
   poor selection of transient numeric identifiers, usually as a result
   of insufficient or misleading specifications.

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

   *  Predictable IPv4 or IPv6 Fragment Identifiers (see e.g.
      [Sanfilippo1998a], [RFC6274], and [RFC7739])



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   *  Predictable IPv6 IIDs (see e.g.  [RFC7721], [RFC7707], and
      [RFC7217])

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

   *  Predictable TCP Initial Sequence Numbers (ISNs) (see e.g.
      [Morris1985], [Bellovin1989], and [RFC6528])

   *  Predictable DNS Query IDs (see e.g.  [Arce1997] and [Klein2007])

   These examples indicate that when new protocols are standardized or
   implemented, the security and privacy properties of the associated
   transient numeric identifiers tend to be overlooked, and
   inappropriate algorithms to generate such identifiers (i.e. that
   negatively affect the security or privacy properties of the protocol)
   are either suggested in the specification or selected by
   implementers.

   This document contains a non-exhaustive timeline of the specification
   and vulnerability disclosures related to some sample transient
   numeric identifiers, including other work that has led to advances in
   this area.  This analysis indicates that:

   *  Vulnerabilities associated with the inappropriate generation of
      transient numeric identifiers have affected protocol
      implementations for an extremely long period of time.

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

   *  Standardization efforts that discuss and provide advice in this
      area can have a positive effect on IETF specifications and their
      corresponding implementations.

   While it is generally possible to identify an algorithm that can
   satisfy the interoperability requirements for a given transient
   numeric identifier, this document provides empirical evidence that
   doing so without negatively affecting the security or privacy
   properties of the aforementioned protocols is non-trivial.  Other
   related documents ([I-D.irtf-pearg-numeric-ids-generation] and
   [I-D.gont-numeric-ids-sec-considerations]) provide guidance in this
   area, as motivated by the present document.

   This document represents the consensus of the Privacy Enhancement and
   Assessment Research Group (PEARG).




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2.  Terminology

   Transient Numeric 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.  Transient
      numeric identifiers are usually defined as a series of bits, and
      represented using integer values.  These identifiers are typically
      dynamically selected, as opposed to statically-assigned numeric
      identifiers (see e.g.  [IANA-PROT]).  We note that different
      identifiers may have additional requirements or properties
      depending on their specific use in a protocol.  We use the term
      "transient numeric identifier" (or simply "numeric identifier" or
      "identifier" as short forms) as a generic term to refer to any
      data object in a protocol specification that satisfies the
      identification property stated above.

   The terms "constant IID", "stable IID", and "temporary IID" are to be
   interpreted as defined in [RFC7721].

3.  Threat Model

   Throughout this document, we do not consider on-path attacks.  That
   is, we assume the attacker does not have physical or logical access
   to the system(s) being attacked, and that the attacker can only
   observe traffic explicitly directed to the attacker.  Similarly, an
   attacker cannot observe traffic transferred between a sender and the
   receiver(s) of a target protocol, but may be able to interact with
   any of these entities, including by e.g. sending any traffic to them
   to sample transient numeric identifiers employed by the target
   systems when communicating with the attacker.

   For example, when analyzing vulnerabilities associated with TCP
   Initial Sequence Numbers (ISNs), we consider the attacker is unable
   to capture network traffic corresponding to a TCP connection between
   two other hosts.  However, we consider the attacker is able to
   communicate with any of these hosts (e.g., establish a TCP connection
   with any of them), to e.g. sample the TCP ISNs employed by these
   systems when communicating with the attacker.

   Similarly, when considering host-tracking attacks based on IPv6
   interface identifiers, we consider an attacker may learn the IPv6
   address employed by a victim node if e.g. the address becomes exposed
   as a result of the victim node communicating with an attacker-
   operated server.  Subsequently, an attacker may perform host-tracking
   by probing a set of target addresses composed by a set of target
   prefixes and the IPv6 interface identifier originally learned by the



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   attacker.  Alternatively, an attacker may perform host tracking if
   e.g. the victim node communicates with an attacker-operated server as
   it moves from one location to another, those exposing its configured
   addresses.  We note that none of these scenarios requires the
   attacker observe traffic not explicitly directed to the attacker.

4.  Issues with the Specification of Transient Numeric Identifiers

   While assessing IETF protocol specifications regarding the use of
   transient numeric identifiers, we have found that most of the issues
   discussed in this document arise as a result of one of the following
   conditions:

   *  Protocol specifications that under-specify the requirements for
      their transient numeric identifiers

   *  Protocol specifications that over-specify their transient numeric
      identifiers

   *  Protocol implementations that simply fail to comply with the
      specified requirements

   A number of IETF protocol specifications have simply overlooked the
   security and privacy implications of transient numeric identifiers.
   Examples of them are the specification of TCP ephemeral ports in
   [RFC0793], the specification of TCP sequence numbers in [RFC0793], or
   the specification of the DNS TxID in [RFC1035].

   On the other hand, there are a number of IETF protocol specifications
   that over-specify some of their associated transient numeric
   identifiers.  For example, [RFC4291] essentially overloads the
   semantics of IPv6 Interface Identifiers (IIDs) by embedding link-
   layer addresses in the IPv6 IIDs, when the interoperability
   requirement of uniqueness could be achieved in other ways that do not
   result in negative security and privacy implications [RFC7721].
   Similarly, [RFC2460] suggested the use of a global counter for the
   generation of Fragment Identification values, when the
   interoperability properties of uniqueness per {Src IP, Dst IP} could
   be achieved with other algorithms that do not result in negative
   security and privacy implications [RFC7739].

   Finally, there are implementations that simply fail to comply with
   the corresponding IETF protocol specifications or recommendations.
   For example, some popular operating systems (notably Microsoft
   Windows) still fail to implement transport protocol ephemeral port
   randomization, as recommended in [RFC6056].





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   The following subsections document the timelines for a number of
   sample transient numeric identifiers, that illustrate how the problem
   discussed in this document has affected protocols from different
   layers over time.  These sample transient numeric identifiers have
   different interoperability requirements and failure severities (see
   Section 6 of [I-D.irtf-pearg-numeric-ids-generation]), and thus are
   considered to be representative of the problem being analyzed in this
   document.

4.1.  IPv4/IPv6 Identification

   This section presents the timeline of the Identification field
   employed by IPv4 (in the base header) and IPv6 (in Fragment Headers).
   The reason for presenting both cases in the same section is to make
   it 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 IPv6 specifications or implementations.

   The IPv4 Identification 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, hosts may be able to simply use
   unique identifiers independent of destination".  The above has been
   interpreted numerous times 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, the specification omits a
   discussion of the security and privacy implications of predictable
   Identification values.  This has resulted in many IPv4
   implementations generating predictable fragment Identification values
   by means of a global counter, at least at some point in time.

   The IPv6 Identification was originally specified in [RFC1883].  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).  [RFC1883] 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



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   packet must be fragmented, and that it is an implementation choice
   whether to use a global or a per-destination counter.  Thus, the
   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.  [RFC2460] was
   the first revision of the core IPv6 specification, and maintained the
   same text for the specification of the IPv6 Identification field.
   [RFC8200], the second revision of the core IPv6 specification,
   removes the suggestion from [RFC2460] to use a counter for the
   generation of IPv6 Identification values, and points to [RFC7739] for
   sample algorithms for their generation.

   September 1981:
      [RFC0791] specifies the interoperability requirements for IPv4
      Identification value, but does not perform a vulnerability
      assessment of this transient numeric identifier.

   December 1995:
      [RFC1883], the first specification of the IPv6 protocol, is
      published.  It suggests that a counter be used to generate the
      IPv6 Identification value, and notes that it is an implementation
      choice whether to maintain a single counter for the node or
      multiple counters, e.g., one for each of the node's possible
      source addresses, or one for each active (source address,
      destination address) combination.

   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], a revision of the IPv6 specification, is published,
      obsoleting [RFC1883].  It maintains the same specification of the
      IPv6 Identification field as its predecessor ([RFC1883]).

   December 1998:
      OpenBSD implements randomization of the IPv4 Identification field
      [OpenBSD-IPv4-ID].

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





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   September 2002:
      [Fyodor2002] documents the implementation of the "idle/dumb scan"
      technique in the popular nmap tool.

   November 2002:
      [Bellovin2002] explains how the IPv4 Identification field can be
      exploited to count the number of systems behind a NAT.

   October 2003:
      OpenBSD implements randomization of the IPv6 Identification field
      [OpenBSD-IPv6-ID].

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

   November 2005:
      [Silbersack2005] discusses shortcomings 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 dumb/idle scan attacks in
      IPv6.

   November 2011:
      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 has the Intended Status
      of "Standards Track", with the intention to formally update
      [RFC2460], to introduce security and privacy requirements on the
      generation of IPv6 Identification values.

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





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   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 to incorporate support for IPv6-based idle/dumb scans in
      nmap is submitted [Morbitzer2013].

   December 2014:
      The 6man WG changes the Intended Status of
      [draft-ietf-6man-predictable-fragment-id-01] to "Informational"
      and publishes it as [draft-ietf-6man-predictable-fragment-id-02].
      As a result, it no longer formally updates [RFC2460], and security
      and privacy requirements on the generation of IPv6 Identification
      values are eliminated.

   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 with
      the Intended Status of "Informational", it does not formally
      update [RFC2460], and does not introduce security and privacy
      requirements on the generation of IPv6 Identification values.

   June 2016:
      [I-D.ietf-6man-rfc2460bis], revision of [RFC2460], removes the
      suggestion from RFC2460 to use a counter for the generation of
      IPv6 Identification values, but does not perform a vulnerability
      assessment of the generation of IPv6 Identification values, and
      does not introduce security and privacy requirements on the
      generation of IPv6 Identification values.

   July 2017:
      [I-D.ietf-6man-rfc2460bis] is finally published as [RFC8200],
      obsoleting [RFC2460], and pointing to [RFC7739] for sample
      algorithms for the generation of IPv6 Fragment Identification
      values.  However, it does not introduce security and privacy
      requirements on the generation of IPv6 Identification values.





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   June 2019:
      [IPID-DEV] notes that the IPv6 ID generators of two popular
      operating systems are flawed.

4.2.  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].

   The interoperability requirements for TCP ISNs are probably not as
   clearly spelled out as one would expect.  Furthermore, the suggestion
   of employing a global counter in [RFC0793] negatively affects the
   security and privacy properties of the protocol.

   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 instance 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 considerations for
      predictable ISNs (along with a range of other protocol-based
      vulnerabilities).




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   February 1995:
      [Shimomura1995] reported a real-world exploitation of the attack
      described in [Morris1985] ten years before (in 1985).

   May 1996:
      [RFC1948] was the first IETF effort, authored by Steven Bellovin,
      to address predictable TCP ISNs.  However, [RFC1948] does not
      formally update [RFC0793].  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].

   July 1996:
      OpenBSD implements TCP ISN randomization based on random
      increments (please see Appendix A.2 of
      [I-D.irtf-pearg-numeric-ids-generation]) [OpenBSD-TCP-ISN-I].

   December 2000:
      OpenBSD implements TCP ISN randomization using simple
      randomization (please see Section 7.1 of
      [I-D.irtf-pearg-numeric-ids-generation]) [OpenBSD-TCP-ISN-R].

   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] were released
      regarding statistical weaknesses in some ISN generators, affecting
      popular TCP 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].












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   June 2007:
      OpenBSD implements TCP ISN randomization based on the algorithm
      specified in [RFC1948] (currently obsoleted and replaced by
      [RFC6528]) for the TCP endpoint that performs the active open,
      while keeping the simple randomization scheme for the endpoint
      performing the passive open [OpenBSD-TCP-ISN-H].  This provides
      monotonically-increasing ISNs for the client side (allowing the
      BSD heuristics to work as expected), while avoiding any patterns
      in the ISN generation for the server side.

   February 2012:
      [RFC6528], published 27 years after 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 ("SHOULD") algorithm for TCP ISN
      generation.

4.3.  IPv6 Interface Identifiers (IIDs)

   IPv6 Interface Identifiers can be generated as a result of different
   mechanisms, including SLAAC [RFC4862], DHCPv6 [RFC8415], 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.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 properties of this approach [RFC7707]
   [RFC7721] has led to the replacement of this flawed scheme with an
   alternative one [RFC7217] [RFC8064] that does not negatively affect
   the security and privacy properties of the protocol.

   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".






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   July 1998:
      [RFC2374] specifies "An IPv6 Aggregatable Global Unicast Address
      Format" (obsoleting [RFC2373]) changing the size of the IID to 64
      bits, and specifies that IIDs must be constructed in IEEE EUI-64
      format.  How such identifiers are constructed becomes specified in
      the corresponding "IPv6 over <link>" specifications, 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) IPv6
      addresses resulting from SLAAC to employ the Modified EUI-64
      format.  The details of constructing such interface identifiers
      are defined in the corresponding "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.
      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
      document no longer formally updates [RFC4291], and therefore 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].



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   July 2013:
      [I-D.cooper-6man-ipv6-address-generation-privacy] elaborates on
      the security and privacy properties of 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] (formerly [I-D.ietf-6man-stable-privacy-addresses]) 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.

   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 properties, and meaning to eventually provide advice in
      this area.

   February 2017:
      The 6man WG publishes [RFC8064] ("Recommendation on Stable IPv6
      Interface Identifiers") (formerly [I-D.ietf-6man-default-iids]),
      with requirements for stable addresses and a recommendation to
      employ [RFC7217] for the generation of stable addresses.  It
      formally updates a large number of RFCs.



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   March 2018:
      [draft-fgont-6man-rfc4941bis-00] is published (as suggested by the
      6man WG), to address flaws in [RFC4941] by revising it (as an
      alternative to the [draft-gont-6man-non-stable-iids-00] effort,
      published in March 2016).

   July 2018:
      [draft-fgont-6man-rfc4941bis-00] is adopted (as
      [draft-ietf-6man-rfc4941bis-00]) as a WG item of the 6man WG.

   December 2020:
      [I-D.ietf-6man-rfc4941bis] is approved by the IESG for publication
      as an RFC.

   February 2021:
      [I-D.ietf-6man-rfc4941bis] is finally published as [RFC8981].

4.4.  NTP Reference IDs (REFIDs)

   The NTP [RFC5905] Reference ID is a 32-bit code identifying the
   particular server or reference clock.  Above stratum 1 (secondary
   servers and clients), this value can be employed to avoid degree-one
   timing loops; that is, scenarios where two NTP peers are (mutually)
   the time source of each other.  If using the IPv4 address family, the
   identifier is the four-octet IPv4 address.  If using the IPv6 address
   family, it is the first four octets of the MD5 hash of the IPv6
   address.

   June 2010:
      [RFC5905], "Network Time Protocol Version 4: Protocol and
      Algorithms Specification" is published.  It specifies that for NTP
      peers with stratum higher than 1 the REFID embeds the IPv4 Address
      of the time source or an MD5 hash of the IPv6 address of the time
      source.

   July 2016:
      [draft-stenn-ntp-not-you-refid-00] is published, describing the
      information leakage produced via the NTP REFID.  It proposes that
      NTP returns a special REFID when a packet employs an IP Source
      Address that is not believed to be a current NTP peer, but
      otherwise generates and returns the traditional REFID.  It is
      subsequently adopted by the NTP WG as
      [I-D.ietf-ntp-refid-updates].








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   April 2019:
      [Gont-NTP] notes that the proposed fix specified in
      [draft-ietf-ntp-refid-updates-00] is, at the very least, sub-
      optimal.  As a result of lack of WG support, the effort is
      eventually abandoned.

4.5.  Transport Protocol Ephemeral Port Numbers

   Most (if not all) transport protocols employ "port numbers" to
   demultiplex packets to the corresponding transport protocol
   instances.

   August 1980:
      [RFC0768] notes that the UDP source port is optional and
      identifies the port of the sending process.  It does not specify
      interoperability requirements for source port selection, nor does
      it suggest possible ways to select port numbers.  Most popular
      implementations end up selecting source ports from a system-wide
      global counter.

   September 1981:
      [RFC0793] (the TCP specification) essentially describes the use of
      port numbers, and specifies that port numbers should result in a
      unique socket pair (local address, local port, remote address,
      remote port).  How ephemeral ports (i.e. port numbers for "active
      opens") are selected, and the port range from which they are
      selected, are left unspecified.

   July 1996:
      OpenBSD implements ephemeral port randomization [OpenBSD-PR].

   July 2008:
      The CERT Coordination Centre published details of what became
      known as the "Kaminsky Attack" [VU-800113] [Kaminsky2008] on the
      DNS.  The attack exploited the lack of source port randomization
      in many major DNS implementations to perform cache poisoning in an
      effective and practical manner.

   January 2009:
      [RFC5452] mandates the use of port randomization for DNS
      resolvers, and mandates that implementations must randomize ports
      from the range (53 or 1024, and above) or the largest possible
      port range.  It does not recommend possible algorithms for port
      randomization, although the document specifically targets DNS
      resolvers, for which a simple port randomization suffices (e.g.
      Algorithm 1 of [RFC6056]).  This document led to the
      implementation of port randomization in the DNS resolver
      themselves, rather than in the underlying transport-protocols.



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   January 2011:
      [RFC6056] notes that many TCP and UDP implementations result in
      predictable port numbers, and also notes that many implementations
      select port numbers from a small portion of the whole port number
      space.  It recommends the implementation and use of ephemeral port
      randomization, proposes a number of possible algorithms for port
      randomization, and also recommends to randomize port numbers over
      the range 1024-65535.

   March 2016:
      [NIST-NTP] reports a non-normal distribution of the ephemeral port
      numbers employed by the NTP clients of an Internet Time Service.

   April 2019:
      [I-D.gont-ntp-port-randomization] notes that some NTP
      implementations employ the NTP service port (123) as the local
      port for non-symmetric modes, and aims to update the NTP
      specification to recommend port randomization in such cases, in
      line with [RFC6056].  The proposal experiences some push-back in
      the relevant working group (NTP WG) [NTP-PORTR], but is finally
      adopted as a working group item as
      [I-D.ietf-ntp-port-randomization].

   August 2021:
      [I-D.ietf-ntp-port-randomization] is finally published as
      [RFC9109].

4.6.  DNS Query ID

   The DNS Query ID [RFC1035] can be employed to match DNS replies to
   outstanding DNS queries.

   November 1987:
      [RFC1035] specifies that the ID is a 16 bit identifier assigned by
      the program that generates any kind of query, and that this
      identifier is copied in the corresponding reply and can be used by
      the requester to match up replies to outstanding queries.  It does
      not specify the interoperability requirements for these numeric
      identifiers, nor does it suggest an algorithm for generating them.

   1993:
      [Schuba1993] describes DNS cache poisoning attacks that require
      the attacker to guess the Query ID.








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   June 1995:
      [Vixie1995] suggests that both the UDP source port and the ID of
      query packets should be randomized, although that might not
      provide enough entropy to prevent an attacker from guessing these
      values.

   April 1997:
      [Arce1997] finds that implementations employ predictable UDP
      source ports and predictable Query IDs, and argues that both
      should be randomized.

   November 2002:
      [Sacramento2002] finds that by spoofing multiple requests for the
      same domain name from different IP addresses, an attacker may
      guess the Query ID employed for a victim with a high probability
      of success, thus performing DNS cache poisoning attacks.

   July 2007:
      [Klein2007b] finds that a popular DNS server software (BIND 9)
      that randomizes the Query ID is still subject to DNS cache
      poisoning attacks by forging a large number of queries and
      leveraging the birthday paradox.

   March 2007:
      [Klein2007c] finds that Microsoft Windows DNS Server generates
      predictable Query ID values.

   October 2007:
      [Klein2007] finds that OpenBSD's DNS software (based on ISC's BIND
      DNS Server) generates predictable Query ID values.

   January 2009:
      [RFC5452] is published, requiring resolvers to randomize the Query
      ID of DNS queries, and to verify that the Query ID of a DNS reply
      matches that of the DNS query as part of the DNS reply validation
      process.

   May 2010:
      [Economou2010] finds that Windows SMTP Service implements its own
      DNS resolver that results in predictable Query ID values.
      Additionally, it fails to validate that the Query ID of DNS reply
      matches the one from the DNS query that supposedly elicited the
      reply.








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5.  Conclusions

   For more than 30 years, a large number of implementations of the IETF
   protocols have been subject to a variety of attacks, with effects
   ranging from Denial of Service (DoS) or data injection, to
   information leakages that could be exploited for pervasive monitoring
   [RFC7258].  The root cause of these issues has been, in many cases,
   poor selection of transient numeric identifiers, usually as a result
   of insufficient or misleading specifications.

   While it is generally possible to identify an algorithm that can
   satisfy the interoperability requirements for a given transient
   numeric identifier, this document provides empirical evidence that
   doing so without negatively affecting the security or privacy
   properties of the aforementioned protocols is non-trivial.  It is
   thus evident that advice in this area is warranted.

   [I-D.gont-numeric-ids-sec-considerations] aims at requiring future
   IETF protocol specifications to contain analysis of the security and
   privacy properties of any transient numeric identifiers specified by
   the protocol, and to recommend an algorithm for the generation of
   such transient numeric identifiers.
   [I-D.irtf-pearg-numeric-ids-generation] specifies a number of sample
   algorithms for generating transient numeric identifiers with specific
   interorability requirements and failure severities.

6.  IANA Considerations

   There are no IANA registries within this document.

7.  Security Considerations

   This document analyzes the timeline of the specification and
   implementation of the transient numeric identifiers of some sample
   IETF protocols, and how the security and privacy properties of such
   protocols have been affected as a result of it.  It provides concrete
   evidence that advice in this area is warranted.

   [I-D.gont-numeric-ids-sec-considerations] formally requires IETF
   protocol specifications to specify the interoperability requirements
   for their transient numeric identifiers, to do a warranted
   vulnerability assessment of such transient numeric identifiers, and
   to recommend possible algorithms for their generation, such that the
   interoperability requirements are complied with, while any negative
   security and privacy properties of these transient numeric
   identifiers are mitigated.





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   [I-D.irtf-pearg-numeric-ids-generation] analyzes and categorizes
   transient numeric identifiers based on their interoperability
   requirements and their associated failure severities, and recommends
   possible algorithms that can comply with those requirements without
   negatively affecting the security and privacy properties of the
   corresponding protocols.

8.  Acknowledgements

   The authors would like to thank (in alphabetical order) Bernard
   Aboba, Dave Crocker, Spencer Dawkins, Theo de Raadt, Sara Dickinson,
   Guillermo Gont, Christian Huitema, Colin Perkins, Vincent Roca, Kris
   Shrishak, Joe Touch, Brian Trammell, and Christopher Wood, 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 4.2 of this document borrows text from [RFC6528], authored by
   Fernando Gont and Steven Bellovin.

   The authors would like to thank Sara Dickinson and Christopher Wood,
   for their guidance during the publication process of this document.

   The authors would like to thank Diego Armando Maradona for his magic
   and inspiration.

9.  References

9.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

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

   [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>.

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



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   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, DOI 10.17487/RFC1883,
              December 1995, <https://www.rfc-editor.org/info/rfc1883>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,
              <https://www.rfc-editor.org/info/rfc5452>.

9.2.  Informative References

   [OpenBSD-PR]
              OpenBSD, "Implementation of port randomization", 29 July
              1996, <https://cvsweb.openbsd.org/src/sys/netinet/
              in_pcb.c?rev=1.6>.

   [VU-800113]
              CERT/CC, "Multiple DNS implementations vulnerable to cache
              poisoning (Vulnerability Note VU#800113)", 8 July 2008,
              <https://www.kb.cert.org/vuls/id/800113>.






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   [IANA-PROT]
              IANA, "Protocol Registries",
              <https://www.iana.org/protocols>.

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

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [I-D.ietf-6man-rfc4941bis]
              Gont, F., Krishnan, S., Narten, T., and R. P. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", Work in Progress, Internet-
              Draft, draft-ietf-6man-rfc4941bis-12, 2 November 2020,
              <https://www.ietf.org/archive/id/draft-ietf-6man-
              rfc4941bis-12.txt>.

   [I-D.gont-opsec-ipv6-host-scanning]
              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", Work in Progress, Internet-Draft, draft-gont-
              opsec-ipv6-host-scanning-02, 22 October 2012,
              <https://www.ietf.org/archive/id/draft-gont-opsec-ipv6-
              host-scanning-02.txt>.

   [I-D.ietf-opsec-ipv6-host-scanning]
              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", Work in Progress, Internet-Draft, draft-ietf-
              opsec-ipv6-host-scanning-08, 28 August 2015,
              <https://www.ietf.org/archive/id/draft-ietf-opsec-ipv6-
              host-scanning-08.txt>.

   [I-D.gont-6man-stable-privacy-addresses]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", Work in Progress, Internet-Draft, draft-gont-
              6man-stable-privacy-addresses-01, 31 March 2012,
              <https://www.ietf.org/archive/id/draft-gont-6man-stable-
              privacy-addresses-01.txt>.

   [I-D.ietf-6man-stable-privacy-addresses]
              Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", Work in Progress, Internet-



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              Draft, draft-ietf-6man-stable-privacy-addresses-17, 27
              January 2014, <https://www.ietf.org/archive/id/draft-ietf-
              6man-stable-privacy-addresses-17.txt>.

   [I-D.cooper-6man-ipv6-address-generation-privacy]
              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              Work in Progress, Internet-Draft, draft-cooper-6man-ipv6-
              address-generation-privacy-00, 15 July 2013,
              <https://www.ietf.org/archive/id/draft-cooper-6man-ipv6-
              address-generation-privacy-00.txt>.

   [I-D.ietf-6man-ipv6-address-generation-privacy]
              Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              Work in Progress, Internet-Draft, draft-ietf-6man-ipv6-
              address-generation-privacy-08, 23 September 2015,
              <https://www.ietf.org/archive/id/draft-ietf-6man-ipv6-
              address-generation-privacy-08.txt>.

   [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>, 2013,
              <https://lists.si6networks.com/pipermail/
              ipv6hackers/2013-February/000947.html>.

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

   [draft-gont-6man-stable-privacy-addresses-00]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", Work in Progress, Internet-Draft, draft-gont-
              6man-stable-privacy-addresses-00, 15 December 2011,
              <https://tools.ietf.org/id/draft-gont-6man-stable-privacy-
              addresses-00.txt>.

   [draft-ietf-6man-stable-privacy-addresses-00]
              Gont, F., "A method for Generating Stable Privacy-Enhanced
              Addresses with IPv6 Stateless Address Autoconfiguration
              (SLAAC)", Work in Progress, Internet-Draft, draft-ietf-
              6man-stable-privacy-addresses-00, 18 May 2012,
              <https://tools.ietf.org/id/draft-ietf-6man-stable-privacy-
              addresses-00.txt>.





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   [draft-gont-6man-address-usage-recommendations-00]
              Gont, F. and W. Liu, "IPv6 Address Usage Recommendations",
              Work in Progress, Internet-Draft, draft-gont-6man-address-
              usage-recommendations-00, 27 May 2016,
              <https://tools.ietf.org/id/draft-gont-6man-address-usage-
              recommendations-00.txt>.

   [draft-gont-6man-non-stable-iids-00]
              Gont, F. and W. Liu, "Recommendation on Non-Stable IPv6
              Interface Identifiers", Work in Progress, Internet-Draft,
              draft-gont-6man-non-stable-iids-00, 23 May 2016,
              <https://tools.ietf.org/id/draft-gont-6man-non-stable-
              iids-00.txt>.

   [draft-ietf-6man-default-iids-00]
              Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              Work in Progress, Internet-Draft, draft-ietf-6man-default-
              iids-00, 28 July 2014, <https://tools.ietf.org/id/draft-
              ietf-6man-default-iids-00.txt>.

   [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>.

   [draft-ietf-6man-rfc4941bis-00]
              Gont, F., Krishnan, S.K., Narten, T.N., and R.D. Draves,
              "Privacy Extensions for Stateless Address
              Autoconfiguration in IPv6", Work in Progress, Internet-
              Draft, draft-ietf-6man-rfc4941bis-00, 2 July 2018,
              <https://tools.ietf.org/id/draft-ietf-6man-rfc4941bis-
              00.txt>.

   [draft-fgont-6man-rfc4941bis-00]
              Gont, F., Krishnan, S.K., Narten, T.N., and R.D. Draves,
              "Privacy Extensions for Stateless Address
              Autoconfiguration in IPv6", Work in Progress, Internet-
              Draft, draft-fgont-6man-rfc4941bis-00, 25 March 2018,
              <https://tools.ietf.org/id/draft-fgont-6man-rfc4941bis-
              00.txt>.










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   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and W. S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              Work in Progress, Internet-Draft, draft-ietf-6man-default-
              iids-16, 28 September 2016,
              <https://www.ietf.org/archive/id/draft-ietf-6man-default-
              iids-16.txt>.

   [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>.

   [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>.

   [I-D.gont-predictable-numeric-ids]
              Gont, F. and I. Arce, "Security and Privacy Implications
              of Numeric Identifiers Employed in Network Protocols",
              Work in Progress, Internet-Draft, draft-gont-predictable-
              numeric-ids-03, 11 March 2019,
              <https://www.ietf.org/archive/id/draft-gont-predictable-
              numeric-ids-03.txt>.

   [I-D.gont-numeric-ids-sec-considerations]
              Gont, F. and I. Arce, "Security Considerations for
              Transient Numeric Identifiers Employed in Network
              Protocols", Work in Progress, Internet-Draft, draft-gont-
              numeric-ids-sec-considerations-08, 10 December 2022,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              gont-numeric-ids-sec-considerations/>.

   [I-D.irtf-pearg-numeric-ids-generation]
              Gont, F. and I. Arce, "On the Generation of Transient
              Numeric Identifiers", Work in Progress, Internet-Draft,
              draft-irtf-pearg-numeric-ids-generation-11, 11 July 2022,
              <https://www.ietf.org/archive/id/draft-irtf-pearg-numeric-
              ids-generation-11.txt>.

   [I-D.ietf-6man-rfc2460bis]
              Deering, S. E. and R. M. Hinden, "Internet Protocol,
              Version 6 (IPv6) Specification", Work in Progress,
              Internet-Draft, draft-ietf-6man-rfc2460bis-13, 19 May
              2017, <https://www.ietf.org/archive/id/draft-ietf-6man-
              rfc2460bis-13.txt>.





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   [draft-stenn-ntp-not-you-refid-00]
              Goldberg, S. and H. Stenn, "Network Time Protocol Not You
              REFID", Work in Progress, Internet-Draft, draft-stenn-ntp-
              not-you-refid-00, 8 July 2016, <https://tools.ietf.org/id/
              draft-stenn-ntp-not-you-refid-00.txt>.

   [draft-ietf-ntp-refid-updates-00]
              Goldberg, S. and H. Stenn, "Network Time Protocol Not You
              REFID", Work in Progress, Internet-Draft, draft-ietf-ntp-
              refid-updates-00, 13 November 2016,
              <https://tools.ietf.org/id/draft-ietf-ntp-refid-updates-
              00.txt>.

   [Gont-NTP] Gont, F., "[Ntp] Comments on draft-ietf-ntp-refid-updates-
              05", Post to the NTP WG mailing list Message-ID:
              <d871d66d-4043-d8d0-f924-2191ebb2e2ce@si6networks.com>, 16
              April 2019, <https://mailarchive.ietf.org/arch/msg/ntp/
              NkfTHxUUOdp14Agh3h1IPqfcRRg>.

   [I-D.ietf-ntp-refid-updates]
              Stenn, H. and S. Goldberg, "Network Time Protocol REFID
              Updates", Work in Progress, Internet-Draft, draft-ietf-
              ntp-refid-updates-05, 25 March 2019,
              <https://www.ietf.org/archive/id/draft-ietf-ntp-refid-
              updates-05.txt>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

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

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

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

   [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>.

   [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>.

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

   [CERT2001] CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in
              TCP/IP Initial Sequence Numbers", 2001,
              <https://resources.sei.cmu.edu/asset_files/
              WhitePaper/2001_019_001_496192.pdf>.

   [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,
              <https://www.gont.com.ar/docs/post-shimomura-usenet.txt>.

   [I-D.eddy-rfc793bis-04]
              Eddy, W., "Transmission Control Protocol Specification",
              Work in Progress, Internet-Draft, draft-eddy-rfc793bis-04,
              25 August 2014,
              <https://tools.ietf.org/id/draft-eddy-rfc793bis-04.txt>.

   [OpenBSD-TCP-ISN-I]
              OpenBSD, "Implementation of TCP ISN randomization based on
              random increments", 29 July 1996,
              <https://cvsweb.openbsd.org/src/sys/netinet/
              tcp_subr.c?rev=1.6>.





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   [OpenBSD-TCP-ISN-R]
              OpenBSD, "Implementation of TCP ISN randomization based on
              simple randomization", 13 December 2000,
              <https://cvsweb.openbsd.org/src/sys/netinet/
              tcp_subr.c?rev=1.37>.

   [OpenBSD-TCP-ISN-H]
              OpenBSD, "Implementation of RFC1948 for TCP ISN
              randomization", 13 December 2000,
              <https://cvsweb.openbsd.org/src/sys/netinet/
              tcp_subr.c?rev=1.97>.

   [I-D.gont-ntp-port-randomization]
              Gont, F. and G. Gont, "Port Randomization in the Network
              Time Protocol Version 4", Work in Progress, Internet-
              Draft, draft-gont-ntp-port-randomization-04, 6 August
              2019, <https://www.ietf.org/archive/id/draft-gont-ntp-
              port-randomization-04.txt>.

   [I-D.ietf-ntp-port-randomization]
              Gont, F., Gont, G., and M. Lichvar, "Network Time Protocol
              Version 4: Port Randomization", Work in Progress,
              Internet-Draft, draft-ietf-ntp-port-randomization-08, 10
              June 2021, <https://www.ietf.org/archive/id/draft-ietf-
              ntp-port-randomization-08.txt>.

   [RFC9109]  Gont, F., Gont, G., and M. Lichvar, "Network Time Protocol
              Version 4: Port Randomization", RFC 9109,
              DOI 10.17487/RFC9109, August 2021,
              <https://www.rfc-editor.org/info/rfc9109>.

   [NTP-PORTR]
              Gont, F., "[Ntp] New rev of the NTP port randomization I-D
              (Fwd: New Version Notification for draft-gont-ntp-port-
              randomization-01.txt)", 2019,
              <https://mailarchive.ietf.org/arch/browse/
              ntp/?gbt=1&index=n09Sb61WkH03lSRtamkELXwEQN4>.

   [NIST-NTP] Sherman, J.A. and J. Levine, "Usage Analysis of the NIST
              Internet Time Service", Journal of Research of the
              National Institute of Standards and Technology Volume 121,
              8 March 2016, <https://tf.nist.gov/general/pdf/2818.pdf>.

   [IPID-DEV] Klein, A. and B. Pinkas, "From IP ID to Device ID and
              KASLR Bypass (Extended Version)", June 2019,
              <https://arxiv.org/pdf/1906.10478.pdf>.





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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>.

   [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>.

   [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>.

   [Bellovin2002]
              Bellovin, S. M., "A Technique for Counting NATted Hosts",
              IMW'02 Nov. 6-8, 2002, Marseille, France, 2002,
              <https://www.cs.columbia.edu/~smb/papers/fnat.pdf>.

   [Fyodor2002]
              Fyodor, "Idle scanning and related IP ID games", 2002,
              <http://www.insecure.org/nmap/idlescan.html>.

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

   [Sanfilippo1998b]
              Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list,
              1998, <https://github.com/antirez/hping/raw/master/docs/
              SPOOFED_SCAN.txt>.

   [Sanfilippo1999]
              Sanfilippo, S., "more ip id", Post to Bugtraq mailing-
              list, 1999,
              <https://github.com/antirez/hping/raw/master/docs/MORE-
              FUN-WITH-IPID>.

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

   [OpenBSD-IPv4-ID]
              OpenBSD, "Randomization of the IPv4 Identification field",
              26 December 1998,
              <https://cvsweb.openbsd.org/src/sys/netinet/
              ip_id.c?rev=1.1>.




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   [OpenBSD-IPv6-ID]
              OpenBSD, "Randomization of the IPv6 Identification field",
              1 October 2003,
              <https://cvsweb.openbsd.org/src/sys/netinet6/
              ip6_id.c?rev=1.1>.

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

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

   [Arce1997] Arce, I. and E. Kargieman, "BIND Vulnerabilities and
              Solutions", 1997,
              <http://www.openbsd.org/advisories/sni_12_resolverid.txt>.

   [Klein2007]
              Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S
              Predictable IP ID Vulnerability", 2007,
              <https://dl.packetstormsecurity.net/papers/attack/OpenBSD_
              DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP_ID_Vuln
              erability.pdf>.

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

   [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>.

   [Ubuntu2011]
              Ubuntu, "Ubuntu: USN-1253-1: Linux kernel
              vulnerabilities", 2011,
              <https://ubuntu.com/security/notices/USN-1253-1>.

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





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   [Gont2012] Gont, F., "Recent Advances in IPv6 Security", BSDCan 2012
              Conference Ottawa, Canada. May 11-12, 2012, May 2012,
              <https://www.si6networks.com/files/presentations/
              bsdcan2012/fgont-bsdcan2012-recent-advances-in-
              ipv6-security.pdf>.

   [I-D.gont-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-gont-6man-predictable-fragment-id-03, 9 January
              2013, <https://www.ietf.org/archive/id/draft-gont-6man-
              predictable-fragment-id-03.txt>.

   [I-D.ietf-6man-predictable-fragment-id]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-ietf-6man-predictable-fragment-id-10, 9 October
              2015, <https://www.ietf.org/archive/id/draft-ietf-6man-
              predictable-fragment-id-10.txt>.

   [draft-ietf-6man-predictable-fragment-id-01]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-ietf-6man-predictable-fragment-id-01, 30 April 2014,
              <https://tools.ietf.org/id/draft-ietf-6man-predictable-
              fragment-id-01.txt>.

   [draft-ietf-6man-predictable-fragment-id-02]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-ietf-6man-predictable-fragment-id-02, 19 December
              2014, <https://tools.ietf.org/id/draft-ietf-6man-
              predictable-fragment-id-02.txt>.

   [draft-gont-6man-predictable-fragment-id-00]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-gont-6man-predictable-fragment-id-00, 15 December
              2011, <https://tools.ietf.org/id/draft-gont-6man-
              predictable-fragment-id-00.txt>.

   [draft-ietf-6man-predictable-fragment-id-00]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-ietf-6man-predictable-fragment-id-00, 22 March 2013,
              <https://tools.ietf.org/id/draft-ietf-6man-predictable-
              fragment-id-00.txt>.




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   [draft-ietf-6man-predictable-fragment-id-08]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, Internet-Draft,
              draft-ietf-6man-predictable-fragment-id-08, 9 June 2015,
              <https://tools.ietf.org/id/draft-ietf-6man-predictable-
              fragment-id-08.txt>.

   [Schuba1993]
              Schuba, C., "ADDRESSING WEAKNESSES IN THE DOMAIN NAME
              SYSTEM PROTOCOL", 1993,
              <http://ftp.cerias.purdue.edu/pub/papers/christoph-schuba/
              schuba-DNS-msthesis.pdf>.

   [Vixie1995]
              Vixie, P., "DNS and BIND Security Issues", 5th Usenix
              Security Symposium May 2, 1995, 2 May 1995, <https://www.u
              senix.org/legacy/publications/library/proceedings/
              security95/full_papers/vixie.pdf>.

   [Klein2007b]
              Klein, A., "BIND 9 DNS Cache Poisoning", March 2007,
              <https://citeseerx.ist.psu.edu/viewdoc/
              summary?doi=10.1.1.86.4474>.

   [Klein2007c]
              Klein, A., "Windows DNS Server Cache Poisoning", March
              2007, <https://dl.packetstormsecurity.net/papers/attack/
              Windows_DNS_Cache_Poisoning.pdf>.

   [Sacramento2002]
              Sacramento, V., "CAIS-ALERT: Vulnerability in the sending
              requests control of BIND", 19 November 2002,
              <https://seclists.org/bugtraq/2002/Nov/331>.

   [Kaminsky2008]
              Kaminsky, D., "Black Ops 2008: It's The End Of The Cache
              As We Know It", August 2008,
              <https://www.blackhat.com/presentations/bh-jp-08/bh-jp-08-
              Kaminsky/BlackHat-Japan-08-Kaminsky-DNS08-BlackOps.pdf>.

   [Economou2010]
              Economou, N., "Windows SMTP Service DNS query Id
              vulnerabilities", Advisory ID Internal CORE-2010-0427 May
              4, 2010, 4 May 2010, <https://www.coresecurity.com/core-
              labs/advisories/core-2010-0424-windows-smtp-dns-query-id-
              bugs>.





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Authors' Addresses

   Fernando Gont
   SI6 Networks
   Segurola y Habana 4310 7mo piso
   Ciudad Autonoma de Buenos Aires
   Buenos Aires
   Argentina
   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com


   Ivan Arce
   Quarkslab
   Segurola y Habana 4310 7mo piso
   Ciudad Autonoma de Buenos Aires
   Buenos Aires
   Argentina
   Email: iarce@quarkslab.com
   URI:   https://www.quarkslab.com































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