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RFC 7739

Informational
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Security Implications of Predictable Fragment Identification Values

 


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Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 7739                           Huawei Technologies
Category: Informational                                    February 2016
ISSN: 2070-1721


  Security Implications of Predictable Fragment Identification Values

Abstract

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field that, together with the IPv6
   Source Address and the IPv6 Destination Address of a packet,
   identifies fragments that correspond to the same original datagram,
   such that they can be reassembled together by the receiving host.
   The only requirement for setting the Identification field is that the
   corresponding value must be different than that employed for any
   other fragmented datagram sent recently with the same Source Address
   and Destination Address.  Some implementations use a simple global
   counter for setting the Identification field, thus leading to
   predictable Identification values.  This document analyzes the
   security implications of predictable Identification values, and
   provides implementation guidance for setting the Identification field
   of the Fragment Header, such that the aforementioned security
   implications are mitigated.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7739.

Page 2 
Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Security Implications of Predictable Fragment Identification
       Values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Constraints for the Selection of Fragment Identification
       Values  . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Algorithms for Selecting Fragment Identification Values . . .   8
     5.1.  Per-Destination Counter (Initialized to a Random Value) .   8
     5.2.  Randomized Identification Values  . . . . . . . . . . . .   9
     5.3.  Hash-Based Fragment Identification Selection Algorithm  .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Information Leakage Produced by Vulnerable
                Implementations  . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Survey of Fragment Identification Selection
                Algorithms Employed by Popular IPv6 Implementations   18
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   IPv6 specifies the Fragment Header, which is employed for the
   fragmentation and reassembly mechanisms.  The Fragment Header
   contains an "Identification" field that, together with the IPv6
   Source Address and the IPv6 Destination Address of a packet,
   identifies fragments that correspond to the same original datagram,
   such that they can be reassembled together by the receiving host.
   The only requirement for setting the Identification field is that its
   value must be different than that employed for any other fragmented
   datagram sent recently with the same Source Address and Destination
   Address.

   The most trivial algorithm to avoid reusing Identification values too
   quickly is to maintain a global counter that is incremented for each
   fragmented datagram that is transmitted.  However, this trivial
   algorithm leads to predictable Identification values that can be
   leveraged to perform a variety of attacks.

   Section 3 of this document analyzes the security implications of
   predictable Identification values.  Section 4 discusses constraints
   in the possible algorithms for selecting Identification values.
   Section 5 specifies a number of algorithms that could be used for
   generating Identification values that mitigate the issues discussed
   in this document.  Finally, Appendix B contains a survey of the
   algorithms employed by popular IPv6 implementations for generating
   the Identification values.

2.  Terminology

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

3.  Security Implications of Predictable Fragment Identification Values

   Predictable Identification values result in an information leakage
   that can be exploited in a number of ways.  Among others, they may
   potentially be exploited to:

   o  determine the packet rate at which a given system is transmitting
      information

   o  perform stealth port scans to a third party

   o  uncover the rules of a number of firewalls

   o  count the number of systems behind a middle-box

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   o  perform Denial-of-Service (DoS) attacks, or

   o  perform data injection attacks against transport or application
      protocols

   The security implications introduced by predictable Identification
   values in IPv6 are very similar to those of predictable
   Identification values in IPv4.

   NOTE:
      [Sanfilippo1998a] originally pointed out how the IPv4
      Identification field could be examined to determine the packet
      rate at which a given system is transmitting information.  Later,
      [Sanfilippo1998b] described how a system with such an
      implementation could be used to perform a stealth port scan to a
      third (victim) host.  [Sanfilippo1999] explained how to exploit
      this implementation strategy to uncover the rules of a number of
      firewalls.  [Bellovin2002] explained how the IPv4 Identification
      field could be exploited to count the number of systems behind a
      NAT.  [Fyodor2004] is an entire paper on most (if not all) the
      ways to exploit the information provided by the Identification
      field of the IPv4 header (and these results apply in a similar way
      to IPv6).  [Zalewski2003] originally envisioned the exploitation
      of IP fragmentation/reassembly for performing data injection
      attacks against upper-layer protocols.  [Herzberg2013] explores
      the use of IPv4/IPv6 fragmentation and predictable Identification
      values for performing DNS cache poisoning attacks in great detail.
      [RFC6274] covers the security implications of the IPv4 case in
      detail.

   One key difference between the IPv4 case and the IPv6 case is that,
   in IPv4, the Identification field is part of the fixed IPv4 header
   (and thus usually set for all packets), while in IPv6 the
   Identification field is present only in those packets that carry a
   Fragment Header.  As a result, successful exploitation of the
   Identification field depends on two different factors:

   o  vulnerable Identification generators, and

   o  the ability of an attacker to trigger the use of IPv6
      fragmentation for packets sent from/to the victim node

   The scenarios in which an attacker may successfully perform the
   aforementioned attacks depend on the specific attack type.  For
   example, in order to perform a DoS attack on communications between
   two hosts, an attacker would need to know the IPv6 addresses employed
   by the aforementioned two nodes.  Such knowledge may be readily
   available if the target of the attack is the communication between

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   two specific BGP peers, two specific SMTP servers, or one specific
   primary DNS server and one of its secondary DNS servers, but may not
   be easily available if the goal is a DoS attack on all communications
   between arbitrary IPv6 hosts (e.g., the goal is to perform a DoS
   attack on all communications involving one specific node with
   arbitrary/unknown hosts).  Other attacks, such as performing stealth
   port scans to a third party or determining the packet rate at which a
   given system is transmitting information, only require the attacker
   to know the IPv6 address of a vulnerable implementation.

   As noted in Section 1, some implementations have been known to use
   predictable Identification values.  For instance, Appendix B of this
   document shows that recent versions of a number of popular IPv6
   implementations employ predictable values for the Identification
   field of the Fragment Header.

   Additionally, we note that [RFC2460] states that when an ICMPv6
   Packet Too Big (PTB) error message advertising a Maximum Transfer
   Unit (MTU) smaller than 1280 bytes is received, the receiving host is
   not required to reduce the Path-MTU for the corresponding Destination
   Address, but must simply include a Fragment Header in all subsequent
   packets sent to that destination.  This triggers the use of the so-
   called IPv6 "atomic fragments" [RFC6946]: IPv6 fragments with a
   Fragment Offset equal to 0, and the "M" ("More fragments") bit clear.
   [DEPGEN] documents the motivation of deprecating the generation of
   IPv6 atomic fragments in [RFC2460].

   Thus, an attacker can usually cause a victim host to "fragment" its
   outgoing packets by sending it a forged ICMPv6 Packet Too Big (PTB)
   error message that advertises an MTU smaller than 1280 bytes.

   There are a number of aspects that should be considered, though:

   o  All the implementations the author is aware of record the Path-MTU
      information on a per-destination basis.  Thus, an attacker can
      only cause the victim to enable fragmentation for those packets
      sent to the Source Address of IPv6 packet embedded in the payload
      of the ICMPv6 PTB message.  However, we note that Section 5.2 of
      [RFC1981] notes that an implementation could maintain a single
      system-wide Path MTU (PMTU) value to be used for all packets sent
      to that node.  Clearly, such implementations would exacerbate the
      problem of any attacks based on Path MTU Discovery (PMTUD)
      [RFC5927] or IPv6 fragmentation.

   o  If the victim node implements some of the counter-measures for
      ICMP attacks described in RFC 5927 [RFC5927], it might be
      difficult for an attacker to cause the victim node to employ
      fragmentation for its outgoing packets.  However, many current

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      implementations fail to enforce these validation checks.  For
      example, Linux 2.6.38-8 does not even require received ICMPv6
      error messages to correspond to an ongoing communication instance.

   o  Some implementations (notably Linux) have already been updated
      according to [DEPGEN] such that ICMPv6 PTB messages do not result
      in the generation of IPv6 atomic fragments.

   Implementations that employ predictable Identification values and
   also fail to enforce validation checks on ICMPv6 error messages
   become vulnerable to the same type of attacks that can be exploited
   with IPv4 fragmentation, discussed earlier in this section.

   One possible way in which predictable Identification values could be
   leveraged for performing a DoS attack is as follows: Let us assume
   that Host A is communicating with Host B, and that an attacker wants
   to perform a DoS attack such communication.  The attacker would learn
   the Identification value currently in use by Host A, possibly by
   sending any packet that would elicit a fragmented response (e.g., an
   ICPMv6 echo request with a large payload).  The attacker would then
   send a forged ICMPv6 PTB error message to Host A (with the IPv6
   Source Address of the embedded IPv6 packet set to the IPv6 address of
   Host A, and the Destination Address of the embedded IPv6 packet set
   to the IPv6 address of a Host B), such that any subsequent packets
   sent by Host A to Host B include a Fragment Header.  Finally, the
   attacker would send forged IPv6 fragments to Host B, with their IPv6
   Source Address set to that of Host A, and Identification values that
   would result in collisions with the Identification values employed
   for the legitimate traffic sent by Host A to Host B.  If Host B
   discards fragments that result in collisions of Identification values
   (e.g., such fragments overlap, and the host implements [RFC5722]),
   the attacker could simply trash the Identification space by sending
   multiple forged fragments with different Identification values, such
   that any subsequent packets from Host A to Host B are discarded at
   Host B as a result of the malicious fragments sent by the attacker.

   NOTE:
      For example, Linux 2.6.38-10 is vulnerable to the aforementioned
      issue.

      [RFC6946] describes an improved processing of these packets that
      would eliminate this specific attack vector, at least in the case
      of TCP connections that employ the Path-MTU Discovery mechanism.

   The aforementioned attack scenario is simply included to illustrate
   the problem of employing predictable Identification values.  We note
   that regardless of the attacker's ability to cause a victim host to

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   employ fragmentation when communicating with third parties, use of
   predictable Identification values makes communication flows that
   employ fragmentation vulnerable to any fragmentation-based attacks.

4.  Constraints for the Selection of Fragment Identification Values

   The Identification field of the Fragment Header is 32-bits long.
   However, when translators (e.g.  [RFC6145]) are employed, the high-
   order 16 bits of the Identification field are effectively ignored.

   NOTE:
      [RFC6145] notes that, when translating in the IPv6-to-IPv4
      direction, "if there is a Fragment Header in the IPv6 packet, the
      last 16 bits of its value MUST be used for the IPv4 identification
      value".

      Additionally, Section 3.3 of [RFC6052] encourages operators to use
      a Network-Specific Prefix (NSP) that maps the IPv4 address space
      into IPv6.  Thus, when an NSP is being used, IPv6 addresses
      representing IPv4 nodes (reached through a stateless translator)
      are indistinguishable from native IPv6 addresses.

   Thus, when translators are employed, the "effective" length of the
   Identification field is 16 bits and, as a result, at least during the
   IPv6/IPv4 transition/co-existence phase, it is probably safer to
   assume that only the low-order 16 bits of the Identification field
   are of use to the destination system.

   Regarding the selection of Identification values, the only
   requirement specified in [RFC2460] is that the Identification value
   must be different than that of any other fragmented packet sent
   recently with the same Source Address and Destination Address.
   Failure to comply with this requirement could lead to the
   interoperability problems discussed in [RFC4963].

   From a security standpoint, unpredictable Identification values are
   desirable.  However, this is somewhat at odds with the "reuse"
   requirements specified in [RFC2460], that specifies that an
   Identification value must be different than that employed for any
   other fragmented packet sent recently with the same Source Address
   and Destination Address.

   Finally, since Identification values need to be selected for each
   outgoing datagram that requires fragmentation, the performance impact
   should be considered when choosing an algorithm for the selection of
   Identification values.

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5.  Algorithms for Selecting Fragment Identification Values

   There are a number of algorithms that may be used for setting the
   Identification field such that the security issues discussed in this
   document are avoided.  This section presents three of those.

   The algorithm in Section 5.1 typically leads to a low Identification
   reuse frequency at the expense of keeping per-destination state; this
   algorithm only uses a Pseudorandom Number Generator (PNRG) when the
   host communicates with a new destination.  The algorithm in
   Section 5.2 may result in a higher Identification reuse frequency.
   It also uses a PRNG for each datagram that needs to be fragmented.
   Hence, the algorithm in Section 5.1 will likely result in better
   performance properties.  Finally, the algorithm in Section 5.3
   achieves a similar Identification reuse frequency to that of the
   algorithm in Section 5.1 without the need of keeping state, but
   possibly at the expense of lower per-packet performance.

   NOTE:
      Since the specific algorithm to be employed for the PRNGs in
      Section 5.1 and Section 5.2, and the specific algorithms to be
      employed for the hash functions in Section 5.3 have not been
      specified, it is impossible to provide a quantitative performance
      comparison of the algorithms described in this section.

5.1.  Per-Destination Counter (Initialized to a Random Value)

   This algorithm consists of the following steps:

   1.  Whenever a packet must be sent with a Fragment Header, the
       sending host should look up in the Destination Cache an entry
       corresponding to the Destination Address of the packet.

   2.  If such an entry exists, it contains the last Identification
       value used for that Destination Address.  Therefore, such a value
       should be incremented by 1 and used for setting the
       Identification field of the outgoing packet.  Additionally, the
       updated value should be recorded in the corresponding entry of
       the Destination Cache [RFC4861].

   3.  If such an entry does not exist, it should be created, and the
       Identification value for that destination should be initialized
       with a random value (e.g., with a Pseudorandom Number Generator),
       and used for setting the Identification field of the Fragment
       Header of the outgoing fragmented datagram.

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   The advantages of this algorithm are:

   o  It is simple to implement, with the only complexity residing in
      the PRNG used to initialize the Identification value contained in
      each entry of the Destination Cache.

   o  The Identification reuse frequency will typically be lower than
      that achieved by a global counter (when sending traffic to
      multiple destinations), since this algorithm uses per-destination
      counters (rather than a single system-wide counter).

   o  It has good performance properties (once the corresponding entry
      in the Destination Cache has been created and initialized, each
      subsequent Identification value simply involves the increment of a
      counter).

   The possible drawbacks of this algorithm are:

   o  If, as a result of resource management, an entry of the
      Destination Cache must be removed, the last Identification value
      used for that Destination will be lost.  Thus, subsequent traffic
      to that destination would cause that entry to be recreated and
      reinitialized to random value, thus possibly leading to
      Identification "collisions".

   o  Since the Identification values are predictable by the destination
      host, a vulnerable host might possibly leak to third parties the
      Identification values used by other hosts to send traffic to it
      (i.e., Host B could leak to Host C the Identification values that
      Host A is using to send packets to Host B).  Appendix A describes
      one possible scenario for such leakage in detail.

5.2.  Randomized Identification Values

   Clearly, use of a Pseudorandom Number Generator for selecting the
   Identification would be desirable from a security standpoint.  With
   such a scheme, the Identification of each fragmented datagram would
   be selected as:

                  Identification = random()

   where "random()" is the PRNG.

   The specific properties of such scheme would clearly depend on the
   specific PRNG employed.  For example, some PRNGs may result in higher
   Identification reuse frequencies than others, in the same way that
   some PRNGs may be more expensive (in terms of processing requirements
   and/or implementation complexity) than others.

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   Discussion of the properties of possible PRNGs is considered out of
   the scope of this document.  However, we do note that some PRNGs
   employed in the past by some implementations have been found to be
   predictable [Klein2007].  Please see [RFC4086] for randomness
   requirements for security.

5.3.  Hash-Based Fragment Identification Selection Algorithm

   Another alternative is to implement a hash-based algorithm similar to
   that specified in [RFC6056] for the selection of transport port
   numbers.  With such a scheme, the Identification value of each
   fragmented datagram would be selected with the expression:

   Identification = F(Src IP, Dst IP, secret1)  +
                    counter[G(Src IP, Dst Pref, secret2)]

   where:

   Identification:
      Identification value to be used for the fragmented datagram.

   F():
      Hash function.

   Src IP:
      IPv6 Source Address of the datagram to be fragmented.

   Dst IP:
      IPv6 Destination Address of the datagram to be fragmented.

   secret1:
      Secret data unknown to the attacker.  This value can be
      initialized to a pseudo-random value during the system
      bootstrapping sequence.  It should remain constant at least while
      there could be previously sent fragments still in the network or
      at the fragment reassembly buffer of the corresponding destination
      system(s).

   counter[]:
      System-wide array of 32-bit counters (e.g. with 8K elements or
      more).  Each counter should be initialized to a pseudo-random
      value during the system bootstrapping sequence.

   G():
      Hash function.  It may or may not be the same hash function as
      that used for F().

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   Dst Pref:
      IPv6 "Destination Prefix" of the datagram to be fragmented (can be
      assumed to be the first eight bytes of the Destination Address of
      such packet).  Note: the "Destination Prefix" (rather than
      Destination Address) is used, such that the ability of an attacker
      of searching the "increments" space by using multiple addresses of
      the same subnet is reduced.

   secret2:
      Secret data unknown to the attacker.  This value can be
      initialized to a pseudo-random value during the system
      bootstrapping sequence.  It should remain constant at least while
      there could be previously sent fragments still in the network or
      at the fragment reassembly buffer of the corresponding destination
      system(s).

   NOTE:
      counter[G(src IP, Dst Pref, secret2)] should be incremented by one
      each time an Identification value is selected.

   The output of F() will be constant for each (Src IP, Dst IP) pair.
   Similarly, the output of G() will be constant for each (Src IP, Dst
   Pref) pair.  Thus, the resulting Identification value will be the
   result of a random offset plus a linear function (provided by
   counter[]), therefore resulting in a monotonically increasing
   sequence of Identification values for each (src IP, Dst IP) pair.

   NOTE:
      F() essentially provides the unpredictability (by off-path
      attackers) of the resulting Identification values, while counter[]
      provides a linear function such that the Identification values are
      different for each fragmented packet while the Identification
      reuse frequency is minimized.

   The advantages of this algorithm are:

   o  The Identification reuse frequency will typically be lower than
      that achieved by a global counter (when sending traffic to
      multiple destinations), since this algorithm uses multiple system-
      wide counters (rather than a single system-wide counter).  The
      extent to which the reuse frequency will be lower depends on the
      number of elements in counter[], and the number of other active
      flows that result in the same value of G() (and hence cause the
      same counter to be incremented for each datagram that is
      fragmented).

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   o  It is possible to implement the algorithm such that good
      performance is achieved.  For example, the result of F() could be
      stored in the Destination Cache (such that it need not be
      recomputed for each packet that must be sent) along with the
      computed index/argument for counter[].

      NOTE:
         If this implementation approach is followed, and an entry of
         the Destination Cache must be removed as a result of resource
         management, the last Identification value used for that
         Destination will *not* be lost.  This is an improvement over
         the algorithm specified in Section 5.1.

   The possible drawbacks of this algorithm are:

   o  Since the Identification values are predictable by the destination
      host, a vulnerable host could possibly leak to third parties the
      Identification values used by other hosts to send traffic to it
      (i.e., Host B could leak to Host C the Identification values that
      Host A is using to send packets to Host B).  Appendix A describes
      a possible scenario in which that information leakage could take
      place.  We note, however, that this algorithm makes the
      aforementioned attack less reliable for the attacker, since each
      counter could be possibly shared by multiple traffic flows (i.e.,
      packets destined to other destinations might cause the same
      counter to be incremented).

   This algorithm might be preferable (over the one specified in
   Section 5.1) in those scenarios in which a node is expected to
   communicate with a large number of destinations, and thus it is
   desirable to limit the amount of information to be maintained in
   memory.

   NOTE:
      In such scenarios, if the algorithm specified in Section 5.1 were
      implemented, entries from the Destination Cache might need to be
      pruned frequently, thus increasing the risk of Identification
      "collisions".

6.  Security Considerations

   This document discusses the security implications of predictable
   Identification values, and provides implementation guidance such that
   the aforementioned security implications can be mitigated.

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   A number of possible algorithms are described, to provide some
   implementation alternatives to implementers.  We note that the
   selection of such an algorithm usually implies a number of trade-offs
   (security, performance, implementation complexity, interoperability
   properties, etc.).

7.  References

7.1.  Normative References

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

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

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <http://www.rfc-editor.org/info/rfc6052>.

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

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   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <http://www.rfc-editor.org/info/rfc6145>.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,
              <http://www.rfc-editor.org/info/rfc6946>.

7.2.  Informative References

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

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010,
              <http://www.rfc-editor.org/info/rfc5927>.

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

   [DEPGEN]   Gont, F., Liu, S., and T. Anderson, "Generation of IPv6
              Atomic Fragments Considered Harmful", Work in Progress,
              draft-ietf-6man-deprecate-atomfrag-generation-05, January
              2016.

   [Bellovin2002]
              Bellovin, S., "A Technique for Counting NATted Hosts",
              IMW'02 Nov. 6-8, 2002, Marseille, France,
              DOI 10.1145/637201.637243, 2002.

   [Fyodor2004]
              Lyon, G., "TCP Idle Scan", from Chapter 5 of "Nmap Network
              Scanning", 2004,
              <http://www.insecure.org/nmap/idlescan.html>.

   [Herzberg2013]
              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", Technical Report 13-03, March 2013,
              <http://u.cs.biu.ac.il/~herzbea/security/13-03-frag.pdf>.

   [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>.

Top      ToC       Page 15 
   [Sanfilippo1998a]
              Sanfilippo, S., "Subject: about the ip header id", message
              to Bugtraq mailing list, 14 December 1998,
              <http://diswww.mit.edu/menelaus.mit.edu/bt/8704>.

   [Sanfilippo1998b]
              Sanfilippo, S., "Subject: new tcp scan method", message
              to Bugtraq mailing list, 18 December 1998,
              <http://diswww.mit.edu/menelaus.mit.edu/bt/8736>.

   [Sanfilippo1999]
              Sanfilippo, S., "Subject: more about IP ID", message
              to Bugtraq mailing list, 20 November 1999,
              <http://diswww.mit.edu/menelaus.mit.edu/bt/12686>.

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

   [Zalewski2003]
              Zalewski, M., "Subject: A new TCP/IP blind data injection
              technique?", message to Bugtraq mailing list, 11 December
              2003, <http://lcamtuf.coredump.cx/ipfrag.txt>.

Top      ToC       Page 16 
Appendix A.  Information Leakage Produced by Vulnerable Implementations

   Section 3 provides a number of references describing a number of ways
   in which a vulnerable implementation may reveal the Identification
   values to be used in subsequent packets, thus opening the door to a
   number of attacks.  In all of those scenarios, a vulnerable
   implementation leaks/reveals its own Identification number.

   This section presents a different attack scenario, in which a
   vulnerable implementation leaks/reveals the Identification number of
   a non-vulnerable implementation.  That is, a vulnerable
   implementation (Host A) leaks the current Identification value in use
   by a third-party host (Host B) to send fragmented datagrams from Host
   B to Host A.

   NOTE:
      For the most part, this section is included to illustrate how a
      vulnerable implementation might be leveraged to leak out the
      Identification value of an otherwise non-vulnerable
      implementation.

   The following scenarios assume:

   Host A:
      An IPv6 host that implements the algorithm specified in
      Section 5.1, implements [RFC5722], but does not implement
      [RFC6946].

   Host B:
      Victim node.  Selects the Identification values from a global
      counter.

   Host C:
      Attacker.  Can forge the IPv6 Source Address of his packets at
      will.

   In the following scenarios, large ICMPv6 Echo Request packets are
   employed to "sample" the Identification value of a host.  We note
   that while the figures show only one packet for the ICMPv6 Echo
   Request and the ICMPv6 Echo Reply packets, each of those packets will
   typically comprise two fragments, such that the corresponding
   unfragmented datagram is larger than the MTU of the networks to which
   Host B and Host C are attached.  Additionally, the following
   scenarios assume that Host A employs a Fragment Header when sending
   traffic to Host B (typically the so-called "IPv6 atomic fragments"
   [RFC6946]): this behavior may be triggered by forged ICMPv6 PTB
   messages that advertise an MTU smaller than 1280 bytes (assuming the
   victim still generates atomic fragments [DEPGEN]).

Top      ToC       Page 17 
   In lines #1-#2 (and lines #7-#8), the attacker samples the current
   Identification value at Host B.  In line #3, the attacker sends a
   forged TCP SYN segment to Host A.  In line 4, the attacker sends a
   forged TCP segment to Host B as an incomplete IPv6 fragmented
   datagram (e.g., a single fragment with Fragment Offset=0, More
   fragments=1).  If corresponding TCP port is closed, and the attacker
   fails when trying to produce a collision of Identification values
   (see line #4), the following packet exchange might take place:

       A                          B                              C

   #1                              <------ Echo Req #1 -----------
   #2                              --- Echo Repl #1, FID=5000 --->
   #3  <------------------- SYN #1, src= B -----------------------
   #4                              <--- SYN/ACK, FID=42 src=A ----
   #5  ---- SYN/ACK, FID=9000 --->
   #6  <----- RST, FID= 5001 -----
   #7                              <-------- Echo Req #2 ---------
   #8                              --- Echo Repl #2, FID=5002 --->

   The RST segment in line #6 is elicited by the SYN/ACK segment from
   line #5 (illegitimately elicited by the SYN segment from line #3).
   The packet from line #4, sent as an incomplete IPv6 datagram,
   eventually times out.

   On the other hand, if the attacker succeeds to produce a collision of
   Identification values, the following packet exchange could take
   place:

       A                          B                              C

   #1                              <------- Echo Req #1 ----------
   #2                              --- Echo Repl #1, FID=5000 --->
   #3  <------------------- SYN #1, src= B -----------------------
   #4                              <-- SYN/ACK, FID=9000 src=A ---
   #5  ---- SYN/ACK, FID=9000 --->
                           ... (RFC5722) ...
   #6                              <------- Echo Req #2 ----------
   #7                              ---- Echo Repl #2, FID=5001 -->

   Clearly, the Identification value sampled from the second ICMPv6 Echo
   Reply packet ("Echo Repl #2") implicitly indicates whether the
   Identification value in the forged SYN/ACK (see line #4 in both
   figures) was the current Identification value in use by Host A.

   As a result, the attacker could employ this technique to learn the
   current Identification value used by host A to send packets to host
   B, even when Host A itself has a non-vulnerable implementation.

Top      ToC       Page 18 
Appendix B.  Survey of Fragment Identification Selection Algorithms
             Employed by Popular IPv6 Implementations

   This section includes a survey of the Identification selection
   algorithms employed by some popular operating systems.

   NOTE:
      The survey was produced with the SI6 Networks' IPv6 toolkit
      [SI6-IPv6].

   +------------------------------+------------------------------------+
   |       Operating System       |             Algorithm              |
   +------------------------------+------------------------------------+
   |        Cisco IOS 15.3        |    Predictable (Global Counter,    |
   |                              |          Init=0, Incr=1)           |
   +------------------------------+------------------------------------+
   |         FreeBSD 9.0          |       Unpredictable (Random)       |
   +------------------------------+------------------------------------+
   |        Linux 3.0.0-15        |    Predictable (Global Counter,    |
   |                              |          Init=0, Incr=1)           |
   +------------------------------+------------------------------------+
   |        Linux-current         |  Unpredictable (Per-dest Counter,  |
   |                              |        Init=random, Incr=1)        |
   +------------------------------+------------------------------------+
   |          NetBSD 5.1          |       Unpredictable (Random)       |
   +------------------------------+------------------------------------+
   |       OpenBSD-current        |   Unpredictable (Random, SKIP32)   |
   +------------------------------+------------------------------------+
   |          Solaris 10          |   Predictable (Per-dst Counter,    |
   |                              |          Init=0, Incr=1)           |
   +------------------------------+------------------------------------+
   |        Windows XP SP2        |    Predictable (Global Counter,    |
   |                              |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |   Windows XP Professional    |    Predictable (Global Counter,    |
   |          32bit, SP3          |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |  Windows Vista (Build 6000)  |    Predictable (Global Counter,    |
   |                              |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |    Windows Vista Business    |    Predictable (Global Counter,    |
   |          64bit, SP1          |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |    Windows 7 Home Premium    |    Predictable (Global Counter,    |
   |                              |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |    Windows Server 2003 R2    |    Predictable (Global Counter,    |
   |     Standard 64bit, SP2      |          Init=0, Incr=2)           |

Top      ToC       Page 19 
   +------------------------------+------------------------------------+
   | Windows Server 2008 Standard |    Predictable (Global Counter,    |
   |          32bit, SP1          |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |    Windows Server 2008 R2    |    Predictable (Global Counter,    |
   |     Standard 64bit, SP1      |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   | Windows Server 2012 Standard |    Predictable (Global Counter,    |
   |            64bit             |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |    Windows 7 Home Premium    |    Predictable (Global Counter,    |
   |          32bit, SP1          |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   |  Windows 7 Ultimate 32bit,   |    Predictable (Global Counter,    |
   |             SP1              |          Init=0, Incr=2)           |
   +------------------------------+------------------------------------+
   | Windows 8 Enterprise 32 bit  |  Unpredictable (Alg. from Section  |
   |                              |                5.3)                |
   +------------------------------+------------------------------------+

   Table 1: Fragment Identification algorithms employed by different OSs

   NOTE:
      In the text above, "predictable" should be taken as "easily
      guessable by an off-path attacker, by sending a few probe
      packets".

Top      ToC       Page 20 
Acknowledgements

   The author would like to thank Ivan Arce for proposing the attack
   scenario described in Appendix A.

   The author would like to thank Ivan Arce, Stephen Bensley, Ron
   Bonica, Tassos Chatzithomaoglou, Guillermo Gont, Brian Haberman, Bob
   Hinden, Sheng Jiang, Tatuya Jinmei, Merike Kaeo, Will Liu, Juan
   Antonio Matos, Simon Perreault, Hosnieh Rafiee, Meral Shirazipour,
   Mark Smith, Dave Thaler, and Klaas Wierenga, for providing valuable
   comments on earlier draft versions of this document.

   This document is based on work performed by Fernando Gont on behalf
   of the UK Centre for the Protection of National Infrastructure
   (CPNI).

   The author would like to thank Buffy for her love and support.

Author's Address

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com