Internet Research Task Force (IRTF) F. Gont
Request for Comments: 9414 SI6 Networks
Category: Informational I. Arce
ISSN: 2070-1721 Quarkslab
July 2023
Unfortunate History of Transient Numeric Identifiers
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 Enhancements and Assessments
Research Group (PEARG) in the IRTF.
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 Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the Privacy
Enhancements and Assessments Research Group of the Internet Research
Task Force (IRTF). Documents approved for publication by the IRSG
are not candidates for any level of Internet Standard; see Section 2
of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9414.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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to this document.
Table of Contents
1. Introduction
2. Terminology
3. Threat Model
4. Issues with the Specification of Transient Numeric Identifiers
4.1. IPv4/IPv6 Identification
4.2. TCP Initial Sequence Numbers (ISNs)
4.3. IPv6 Interface Identifiers (IIDs)
4.4. NTP Reference IDs (REFIDs)
4.5. Transport-Protocol Ephemeral Port Numbers
4.6. DNS ID
5. Conclusions
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Networking protocols employ a variety of transient numeric
identifiers for different protocol objects, such as IPv4 and IPv6
Identification values [RFC0791] [RFC8200], IPv6 Interface Identifiers
(IIDs) [RFC4291], transport-protocol ephemeral port numbers
[RFC6056], TCP Initial Sequence Numbers (ISNs) [RFC9293], NTP
Reference IDs (REFIDs) [RFC5905], and DNS IDs [RFC1035]. These
identifiers typically have specific requirements (e.g., uniqueness
during a specified period of time) that must be satisfied such that
they do not result in negative interoperability implications and an
associated failure severity when such requirements are not met
[RFC9415].
| NOTE: Some documents refer to the DNS ID as the DNS "Query ID"
| or "TxID".
For more than 30 years, a large number of implementations of 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, the poor
selection of transient numeric identifiers in such protocols, usually
as a result of insufficient or misleading specifications. While it
is generally trivial to identify an algorithm that can satisfy the
interoperability requirements of a given transient numeric
identifier, empirical evidence exists that doing so without
negatively affecting the security and/or privacy properties of the
aforementioned protocols is prone to error.
For example, implementations have been subject to security and/or
privacy issues resulting from:
* predictable IPv4 or IPv6 Identification values (e.g., see
[Sanfilippo1998a], [RFC6274], and [RFC7739]),
* predictable IPv6 IIDs (e.g., see [RFC7217], [RFC7707], and
[RFC7721]),
* predictable transport-protocol ephemeral port numbers (e.g., see
[RFC6056] and [Silbersack2005]),
* predictable TCP Initial Sequence Numbers (ISNs) (e.g., see
[Morris1985], [Bellovin1989], and [RFC6528]),
* predictable initial timestamps in TCP timestamps options (e.g.,
see [TCPT-uptime] and [RFC7323]), and
* predictable DNS IDs (see, e.g., [Schuba1993] and [Klein2007]).
Recent history indicates that, when new protocols are standardized or
new protocol implementations are produced, the security and privacy
properties of the associated transient numeric identifiers tend to be
overlooked, and inappropriate algorithms to generate such identifiers
are either suggested in the specifications or selected by
implementers. As a result, advice in this area is warranted.
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, and
* 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 and/or privacy
properties of the corresponding protocols is nontrivial. Other
related documents ([RFC9415] and [RFC9416]) provide guidance in this
area, as motivated by the present document.
This document represents the consensus of the Privacy Enhancements
and Assessments Research Group (PEARG).
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 (e.g., see [IANA-PROT]). We note that different
transient numeric 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 the 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
hosts 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
hosts 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 host if, e.g., the address becomes
exposed as a result of the victim host 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 attacker. Alternatively, an attacker may perform
host-tracking if, e.g., the victim host communicates with an
attacker-operated server as it moves from one location to another,
thereby exposing its configured addresses. We note that none of
these scenarios require 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 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 under specified their
transient numeric identifiers, thus leading to implementations that
were vulnerable to numerous off-path attacks. Examples of them are
the specification of TCP local ports in [RFC0793] or the
specification of the DNS ID in [RFC1035].
| NOTE: The TCP local port in an active OPEN request is commonly
| known as the "ephemeral port" of the corresponding TCP
| connection [RFC6056].
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] suggests the use of a global counter for the
generation of Identification values when the interoperability
requirement of uniqueness per {IPv6 Source Address, IPv6 Destination
Address} could be achieved with other algorithms that do not result
in negative security and privacy implications [RFC7739].
Finally, there are protocol implementations that simply fail to
comply with existing protocol specifications. For example, some
popular operating systems still fail to implement transport-protocol
ephemeral port randomization, as recommended in [RFC6056], or TCP
Initial Sequence Number randomization, as recommended in [RFC9293].
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 [RFC9415]), 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 protocols, the work and research done for the IPv4
case did not influence IPv6 specifications or implementations.
The IPv4 Identification is specified in [RFC0791], which specifies
the interoperability requirements for the Identification field, i.e.,
the sender must choose the Identification field to be unique for a
given {Source Address, Destination Address, Protocol} for the time
the datagram (or any fragment of it) could be alive in the Internet.
It suggests that a sending protocol module may keep "a table of
Identifiers, one entry for each destination it has communicated with
in the last maximum packet lifetime for the [I]nternet", and it also
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
resulted in many IPv4 implementations generating predictable
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, but rather than
being part of the base header (as in the IPv4 case), it is part of
the Fragment Header (which may or may not be present in an IPv6
packet). Section 4.5 of [RFC1883] states that the Identification
must be different than that of any other fragmented packet sent
recently (within the maximum likely lifetime of a packet) with the
same Source Address and Destination Address. Subsequently, it notes
that this requirement can be met by means of a wrap-around 32-bit
counter that is incremented each time a packet must be fragmented and
that it is an implementation choice whether to use a global or a per-destination per-
destination counter. Thus, the specification 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] is the first revision of the
core IPv6 specification and maintains 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 the IPv4
Identification 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 values 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} set).
December 1998:
[Sanfilippo1998a] finds that predictable IPv4 Identification
values (as 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 "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 values to uncover the rules of a number of
firewalls.
September 2002:
[Fyodor2002] documents the implementation of the "idle scan"
technique in the popular Network Mapper (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.
January 2005:
[Silbersack2005] discusses shortcomings in a number of techniques
to mitigate predictable IPv4 Identification values.
October 2007:
[Klein2007] describes a weakness in the pseudorandom number
generator (PRNG) in use for the generation of IP Identification
values by a number of operating systems.
June 2011:
[Gont2011] describes how to perform 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.
March 2013:
The 6man WG adopts [draft-gont-6man-predictable-fragment-id-03]
but changes the track to "BCP" (while still formally updating
[RFC2460]), posting the resulting document as
[draft-ietf-6man-predictable-fragment-id-00].
June 2013:
A patch to incorporate support for IPv6-based idle 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 posts 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 [draft-ietf-6man-predictable-fragment-id-10])
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 as an
"Informational" RFC, it does not formally update [RFC2460] and
does not introduce security and privacy requirements on the
generation of IPv6 Identification values.
June 2016:
[draft-ietf-6man-rfc2460bis-05], a draft 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:
[draft-ietf-6man-rfc2460bis-13] is finally published as [RFC8200],
obsoleting [RFC2460] and pointing to [RFC7739] for sample
algorithms for the generation of IPv6 Identification values.
However, it does not introduce security and privacy requirements
on the generation of IPv6 Identification values.
October 2019:
[IPID-DEV] notes that the IPv6 Identification 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 has 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 implementation will use for new connections, thus allowing a
variety of attacks against TCP.
February 1985:
[Morris1985] is 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] discusses the security considerations for
predictable ISNs (along with a range of other protocol-based
vulnerabilities).
January 1995:
[Shimomura1995] reports a real-world exploitation of the
vulnerability described in [Morris1985] ten years before (in
1985).
May 1996:
[RFC1948] is the first IETF effort, authored by Steven Bellovin,
to address predictable TCP ISNs. However, [RFC1948] does not
formally update [RFC0793]. Note: 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 [RFC9415])
[OpenBSD-TCP-ISN-I].
December 2000:
OpenBSD implements TCP ISN randomization using simple
randomization (please see Section 7.1 of [RFC9415])
[OpenBSD-TCP-ISN-R].
March 2001:
[Zalewski2001] provides a detailed analysis of statistical
weaknesses in some TCP ISN generators and includes a survey of the
algorithms in use by popular TCP implementations. Vulnerability
advisories [USCERT2001] were released regarding statistical
weaknesses in some TCP ISN generators, affecting popular TCP
implementations. Other vulnerability advisories on the same
vulnerability, such as [CERT2001], were published later on.
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].
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's original work
[Morris1985], formally updates [RFC0793] to mitigate predictable
TCP ISNs.
August 2014:
The algorithm specified in [RFC6528] becomes the recommended
("SHOULD") algorithm for TCP ISN generation in
[draft-eddy-rfc793bis-04], an early revision of the core TCP
specification [RFC9293].
August 2022:
[RFC9293], a revision of the core TCP specification, is published,
adopting 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 Stateless Address Autoconfiguration (SLAAC)
[RFC4862], DHCPv6 [RFC8415], and manual configuration. This section
focuses on Interface Identifiers resulting from SLAAC.
The Interface Identifier of stable IPv6 addresses resulting from
SLAAC originally 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), which is consistent with the IPv6 addressing architecture
specified in [RFC1884]. Hosts are recommended to "generate
addresses using link-specific addresses as Interface ID such as 48
bit IEEE-802 MAC addresses".
July 1998:
[RFC2374] specifies "An IPv6 Aggregatable Global Unicast Address
Format" (obsoleting [RFC2073]), changing the size of the IID to 64
bits, and specifies that IIDs must be constructed in IEEE 64-bit
Extended Unique Identifier (EUI-64) format. How such identifiers
are constructed is specified in the corresponding "IPv6 over
<link>" specifications, such as "IPv6 over Ethernet".
January 2001:
[RFC3041] recognizes the problem of IPv6 network activity
correlation and specifies IPv6 temporary addresses. Temporary
addresses are to be used along with stable addresses.
August 2003:
[RFC3587] obsoletes [RFC2374], making the Top-Level Aggregator
(TLA) / Next-Level Aggregator (NLA) structure historic, though the
syntax and recommendations for the stable IIDs remain unchanged.
February 2006:
[RFC4291] is published as the latest "IP Version 6 Addressing
Architecture", requiring the IIDs of "all unicast addresses,
except those that start with the binary value 000" 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
original scheme for generating stable addresses allows for IPv6
address scanning and for active host tracking (even when IPv6
temporary addresses are employed). It also specifies an
alternative algorithm meant to replace IIDs based on Modified
EUI-64 format identifiers.
November 2012:
The 6man WG adopts [draft-gont-6man-stable-privacy-addresses-01]
as a working group item (as
[draft-ietf-6man-stable-privacy-addresses-00]). However, the
document no longer formally updates [RFC4291]; 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].
July 2013:
[draft-cooper-6man-ipv6-address-generation-privacy-00] elaborates
on the security and privacy properties of all known algorithms for
generating IPv6 IIDs.
January 2014:
The 6man WG posts [draft-ietf-6man-default-iids-00]
("Recommendation on Stable IPv6 Interface Identifiers"),
recommending [draft-ietf-6man-stable-privacy-addresses-17] for the
generation of stable addresses.
April 2014:
[RFC7217] (formerly [draft-ietf-6man-stable-privacy-addresses-17])
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 [draft-gont-opsec-ipv6-host-scanning-02] and
later [draft-ietf-opsec-ipv6-host-scanning-08]), about "Network
Reconnaissance in IPv6 Networks", is published.
March 2016:
[RFC7721] (formerly
[draft-cooper-6man-ipv6-address-generation-privacy-00] and later
[draft-ietf-6man-ipv6-address-generation-privacy-08]), about
"Security and Privacy Considerations for IPv6 Address Generation
Mechanisms", is published.
May 2016:
[draft-gont-6man-non-stable-iids-00] is posted, 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 posted,
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:
[draft-ietf-6man-default-iids-16], produced by the 6man WG, is
published as [RFC8064] ("Recommendation on Stable IPv6 Interface
Identifiers"), 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.
March 2018:
[draft-fgont-6man-rfc4941bis-00] is posted (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,
posted 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:
[draft-ietf-6man-rfc4941bis-12] is approved by the IESG for
publication as an RFC.
February 2021:
[draft-ietf-6man-rfc4941bis-12] 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 the first four octets of the MD5
hash of the IPv6 address of the time source.
July 2016:
[draft-stenn-ntp-not-you-refid-00] is posted, 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 common REFID. It is
subsequently adopted by the NTP WG as
[draft-ietf-ntp-refid-updates-00].
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 a lack of WG support, the
[draft-ietf-ntp-refid-updates-00] 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. "Ephemeral ports" refer to the local ports employed in
active OPEN requests, that is, typically the local port numbers
employed on the side initiating the communication.
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 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 Center publishes details of what became
known as the "Kaminsky Attack" [VU-800113] [Kaminsky2008] on the
DNS. The attack exploits the lack of ephemeral port randomization
and DNS ID randmization 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 of available ports (53 or 1024 and above) that is are as large
as possible and practicable. 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 resolvers themselves, rather than in the underlying transport
protocols.
January 2011:
[RFC6056] notes that many TCP and UDP implementations result in
predictable ephemeral 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:
[draft-gont-ntp-port-randomization-00] notes that some NTP
implementations employ the NTP service port (123) as the local
port for nonsymmetric modes and aims to update the NTP
specification to recommend port randomization in such cases, which
is in line with [RFC6056]. The proposal experiences some pushback
in the relevant working group (NTP WG) [NTP-PORTR] but is finally
adopted as a working group item as
[draft-ietf-ntp-port-randomization-00].
August 2021:
[draft-ietf-ntp-port-randomization-08] is finally published as
[RFC9109].
4.6. DNS ID
The DNS ID [RFC1035] can be employed to match DNS replies to
outstanding DNS queries.
| NOTE: Some documents refer to the DNS ID as the DNS "Query ID"
| or "TxID".
November 1987:
[RFC1035] specifies that the DNS 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 this
numeric identifier, nor does it suggest an algorithm for
generating it.
August 1993:
[Schuba1993] describes DNS cache poisoning attacks that require
the attacker to guess the DNS ID.
June 1995:
[Vixie1995] suggests that both the UDP source port and the DNS 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 DNS 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 DNS ID employed for a victim with a high probability of
success, thus allowing for DNS cache poisoning attacks.
March 2007:
[Klein2007c] finds that the Microsoft Windows DNS server generates
predictable DNS ID values.
July 2007:
[Klein2007b] finds that a popular DNS server software (BIND 9)
that randomizes the DNS ID is still subject to DNS cache poisoning
attacks by forging a large number of queries and leveraging the
birthday paradox.
October 2007:
[Klein2007] finds that OpenBSD's DNS software (based on the BIND
DNS server of the Internet Systems Consortium (ISC)) generates
predictable DNS ID values.
January 2009:
[RFC5452] is published, requiring resolvers to randomize the DNS
ID of queries and to verify that the DNS ID of a reply matches
that of the DNS query as part of the DNS reply validation process.
May 2010:
[Economou2010] finds that the Windows SMTP Service implements its
own DNS resolver that results in predictable DNS ID values.
Additionally, it fails to validate that the DNS ID of a reply
matches that of the DNS query that supposedly elicited it.
5. Conclusions
For more than 30 years, a large number of implementations of 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, the poor
selection of transient numeric identifiers in such protocols, 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 and/or privacy
properties of the aforementioned protocols is nontrivial. It is thus
evident that advice in this area is warranted.
[RFC9416] 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. [RFC9415] specifies a number of sample algorithms for
generating transient numeric identifiers with specific
interoperability requirements and failure severities.
6. IANA Considerations
This document has no IANA actions.
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.
[RFC9415] analyzes and categorizes transient numeric identifiers
based on their interoperability requirements and their associated
failure severities and recommends possible algorithms that can be
employed to comply with those requirements without negatively
affecting the security and privacy properties of the corresponding
protocols.
[RFC9416] 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 or privacy properties of
these transient numeric identifiers are mitigated.
8. References
8.1. 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>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC0793] Postel, J., "Transmission Control Protocol", RFC 793,
DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, DOI 10.17487/RFC1323, May
1992, <https://www.rfc-editor.org/info/rfc1323>.
[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>.
[RFC1884] Hinden, R., Ed. and S. Deering, Ed., "IP Version 6
Addressing Architecture", RFC 1884, DOI 10.17487/RFC1884,
December 1995, <https://www.rfc-editor.org/info/rfc1884>.
[RFC2073] Rekhter, Y., Lothberg, P., Hinden, R., Deering, S., and J.
Postel, "An IPv6 Provider-Based Unicast Address Format",
RFC 2073, DOI 10.17487/RFC2073, January 1997,
<https://www.rfc-editor.org/info/rfc2073>.
[RFC2374] Hinden, R., O'Dell, M., and S. Deering, "An IPv6
Aggregatable Global Unicast Address Format", RFC 2374,
DOI 10.17487/RFC2374, July 1998,
<https://www.rfc-editor.org/info/rfc2374>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
DOI 10.17487/RFC3041, January 2001,
<https://www.rfc-editor.org/info/rfc3041>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<https://www.rfc-editor.org/info/rfc4941>.
[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>.
[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>.
[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[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>.
[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>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
8.2. Informative References
[Arce1997] Arce, I. and E. Kargieman, "BIND Vulnerabilities and
Solutions", April 1997,
<http://www.openbsd.org/advisories/sni_12_resolverid.txt>.
[Bellovin1989]
Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", Computer Communications Review, vol. 19, no. 2,
pp. 32-48, April 1989,
<https://www.cs.columbia.edu/~smb/papers/ipext.pdf>.
[Bellovin2002]
Bellovin, S., "A Technique for Counting NATted Hosts",
IMW'02, Marseille, France, DOI 10.1145/637201.637243,
November 2002,
<https://www.cs.columbia.edu/~smb/papers/fnat.pdf>.
[CERT2001] CERT/CC, "CERT Advisory CA-2001-09: Statistical Weaknesses
in TCP/IP Initial Sequence Numbers", May 2001,
<https://resources.sei.cmu.edu/asset_files/
WhitePaper/2001_019_001_496192.pdf>.
[draft-cooper-6man-ipv6-address-generation-privacy-00]
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>.
[draft-eddy-rfc793bis-04]
Eddy, W., Ed., "Transmission Control Protocol
Specification", Work in Progress, Internet-Draft, draft-
eddy-rfc793bis-04, 25 August 2014,
<https://www.ietf.org/archive/id/draft-eddy-rfc793bis-
04.txt>.
[draft-fgont-6man-rfc4941bis-00]
Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Privacy Extensions for Stateless Address
Autoconfiguration in IPv6", Work in Progress, Internet-
Draft, draft-fgont-6man-rfc4941bis-00, 25 March 2018,
<https://www.ietf.org/archive/id/draft-fgont-6man-
rfc4941bis-00.txt>.
[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://www.ietf.org/archive/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://www.ietf.org/archive/id/draft-gont-6man-non-
stable-iids-00.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://www.ietf.org/archive/id/draft-gont-6man-
predictable-fragment-id-00.txt>.
[draft-gont-6man-predictable-fragment-id-03]
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>.
[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://www.ietf.org/archive/id/draft-gont-6man-stable-
privacy-addresses-00.txt>.
[draft-gont-6man-stable-privacy-addresses-01]
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>.
[draft-gont-ntp-port-randomization-00]
Gont, F. and G. Gont, "Port Randomization in the Network
Time Protocol Version 4", Work in Progress, Internet-
Draft, draft-gont-ntp-port-randomization-00, 16 April
2019, <https://www.ietf.org/archive/id/draft-gont-ntp-
port-randomization-00.txt>.
[draft-gont-opsec-ipv6-host-scanning-02]
Gont, F. and T. Chown, "Network Reconnaissance in IPv6
Networks", Work in Progress, Internet-Draft, draft-gont-
opsec-ipv6-host-scanning-02, 23 October 2012,
<https://www.ietf.org/archive/id/draft-gont-opsec-ipv6-
host-scanning-02.txt>.
[draft-gont-predictable-numeric-ids-03]
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://datatracker.ietf.org/doc/html/draft-gont-
predictable-numeric-ids-03>.
[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, 24 January 2014,
<https://www.ietf.org/archive/id/draft-ietf-6man-default-
iids-00.txt>.
[draft-ietf-6man-default-iids-16]
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-16, 28 September 2016,
<https://www.ietf.org/archive/id/draft-ietf-6man-default-
iids-16.txt>.
[draft-ietf-6man-ipv6-address-generation-privacy-08]
Cooper, A., Gont, F., and D. Thaler, "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>.
[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://www.ietf.org/archive/id/draft-ietf-6man-
predictable-fragment-id-00.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, 29 April 2014,
<https://www.ietf.org/archive/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://datatracker.ietf.org/doc/html/draft-ietf-
6man-predictable-fragment-id-02>.
[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://www.ietf.org/archive/id/draft-ietf-6man-
predictable-fragment-id-08.txt>.
[draft-ietf-6man-predictable-fragment-id-10]
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-rfc2460bis-05]
Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", Internt-Draft draft-ietf-6man-
rfc2460bis-05, Work in Progress, Internet-Draft,
draft-ietf-6man-rfc2460bis-05, 28 June 2016,
<https://www.ietf.org/archive/id/draft-ietf-6man-
rfc2460bis-05.txt>.
[draft-ietf-6man-rfc2460bis-13]
Deering, S. and R. Hinden, "draft-ietf-6man-rfc2460bis-
13", 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>.
[draft-ietf-6man-rfc4941bis-00]
Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Privacy Extensions for Stateless Address
Autoconfiguration in IPv6", Work in Progress, Internet-
Draft, draft-ietf-6man-rfc4941bis-00, 2 July 2018,
<https://www.ietf.org/archive/id/draft-ietf-6man-
rfc4941bis-00.txt>.
[draft-ietf-6man-rfc4941bis-12]
Gont, F., Krishnan, S., Narten, T., and R. 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>.
[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://www.ietf.org/archive/id/draft-ietf-6man-stable-
privacy-addresses-00.txt>.
[draft-ietf-6man-stable-privacy-addresses-17]
Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", Work in Progress, Internet-
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>.
[draft-ietf-ntp-port-randomization-00]
Gont, F., Gont, G., and M. Lichvar, "Port Randomization in
the Network Time Protocol Version 4", Work in Progress,
October 2019, <https://www.ietf.org/archive/id/draft-ietf-
ntp-port-randomization-00.txt>.
[draft-ietf-ntp-port-randomization-08]
Gont, F., Gont, G., and M. Lichvar, "Port Randomization in
the Network Time Protocol Version 4", Work in Progress,
June 2021, <https://www.ietf.org/archive/id/draft-ietf-
ntp-port-randomization-08.txt>.
[draft-ietf-ntp-refid-updates-00]
Stenn, H. and S. Goldberg, "Network Time Protocol REFID
Updates", Work in Progress, Internet-Draft, draft-ietf-
ntp-refid-updates-00, 13 November 2016,
<https://www.ietf.org/archive/id/draft-ietf-ntp-refid-
updates-00.txt>.
[draft-ietf-opsec-ipv6-host-scanning-08]
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>.
[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://www.ietf.org/archive/id/draft-stenn-ntp-not-you-
refid-00.txt>.
[Economou2010]
Economou, N., "Windows SMTP Service DNS query Id
vulnerabilities", Advisory ID Internal CORE-2010-0427, May
2010, <https://www.coresecurity.com/core-labs/advisories/
core-2010-0424-windows-smtp-dns-query-id-bugs>.
[Fyodor2002]
Fyodor, "Idle scanning and related IP ID games", September
2002,
<https://nmap.org/presentations/CanSecWest03/CD_Content/
idlescan_paper/idlescan.html>.
[Gont-NTP] Gont, F., "[Ntp] Comments on draft-ietf-ntp-refid-updates-
05", message to the IETF NTP mailing list, 16 April 2019,
<https://mailarchive.ietf.org/arch/msg/ntp/
NkfTHxUUOdp14Agh3h1IPqfcRRg>.
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specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
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[RFC5157] Chown, T., "IPv6 Implications for Network Scanning",
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[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,
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[RFC6274] Gont, F., "Security Assessment of the Internet Protocol
Version 4", RFC 6274, DOI 10.17487/RFC6274, July 2011,
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[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>.
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Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
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[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
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[RFC7739] Gont, F., "Security Implications of Predictable Fragment
Identification Values", RFC 7739, DOI 10.17487/RFC7739,
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[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
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Autoconfiguration in IPv6", RFC 8981,
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[RFC9109] Gont, F., Gont, G., and M. Lichvar, "Network Time Protocol
Version 4: Port Randomization", RFC 9109,
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[RFC9415] Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", RFC 9415, DOI 10.17487/RFC9415, June July
2023, <https://www.rfc-editor.org/info/rfc9415>.
[RFC9416] Gont, F. and I. Arce, "Security Considerations for
Transient Numeric Identifiers Employed in Network
Protocols", BCP 72, RFC 9416, DOI 10.17487/RFC9416, June July
2023, <https://www.rfc-editor.org/info/rfc9416>.
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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
[draft-gont-predictable-numeric-ids-03], 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.
Authors' Addresses
Fernando Gont
SI6 Networks
Segurola y Habana
4310 7mo piso
Ciudad Autonoma de 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
Argentina
Email: iarce@quarkslab.com
URI: https://www.quarkslab.com