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Versions: (draft-gont-ntp-port-randomization) 00 01 06

Network Time Protocol (ntp) Working Group                        F. Gont
Internet-Draft                                                   G. Gont
Updates: 5905 (if approved)                                 SI6 Networks
Intended status: Standards Track                              M. Lichvar
Expires: March 19, 2021                                          Red Hat
                                                      September 15, 2020


       Port Randomization in the Network Time Protocol Version 4
                  draft-ietf-ntp-port-randomization-06

Abstract

   The Network Time Protocol can operate in several modes.  Some of
   these modes are based on the receipt of unsolicited packets, and
   therefore require the use of a well-known port as the local port
   number.  However, in the case of NTP modes where the use of a well-
   known port is not required, employing such well-known port
   unnecessarily increases the ability of attackers to perform blind/
   off-path attacks.  This document formally updates RFC5905,
   recommending the use of transport-protocol ephemeral port
   randomization for those modes where use of the NTP well-known port is
   not required.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 19, 2021.

Copyright Notice

   Copyright (c) 2020 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



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   (https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Considerations About Port Randomization in NTP  . . . . . . .   3
     3.1.  Mitigation Against Off-path Attacks . . . . . . . . . . .   3
     3.2.  Effects on Path Selection . . . . . . . . . . . . . . . .   4
     3.3.  Filtering of NTP traffic  . . . . . . . . . . . . . . . .   4
     3.4.  Effect on NAT devices . . . . . . . . . . . . . . . . . .   5
     3.5.  Relation to Other Mitigations for Off-Path Attacks  . . .   5
   4.  Update to RFC5905 . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Network Time Protocol (NTP) is one of the oldest Internet
   protocols, and currently specified in [RFC5905].  Since its original
   implementation, standardization, and deployment, a number of
   vulnerabilities have been found both in the NTP specification and in
   some of its implementations [NTP-VULN].  Some of these
   vulnerabilities allow for off-path/blind attacks, where an attacker
   can send forged packets to one or both NTP peers for achieving Denial
   of Service (DoS), time-shifts, or other undesirable outcomes.  Many
   of these attacks require the attacker to guess or know at least a
   target NTP association, typically identified by the tuple {srcaddr,
   srcport, dstaddr, dstport, keyid} (see section 9.1 of [RFC5905]).
   Some of these parameters may be easily known or guessed.

   NTP can operate in several modes.  Some of these modes rely on the
   ability of nodes to receive unsolicited packets, and therefore
   require the use of the NTP well-known port (123).  However, for modes
   where the use of a well-known port is not required, employing the NTP
   well-known port improves the ability of an attacker to perform blind/



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   off-path attacks (since knowledge of the port numbers is typically
   required for such attacks).  A recent study [NIST-NTP] that analyzes
   the port numbers employed by NTP clients suggests that a considerable
   number of NTP clients employ the NTP well-known port as their local
   port, or select predictable ephemeral port numbers, thus improving
   the ability of attackers to perform blind/off-path attacks against
   NTP.

   BCP 156 [RFC6056] already recommends the randomization of transport-
   protocol ephemeral ports.  This document aligns NTP with the
   recommendation in BCP 156 [RFC6056], by formally updating [RFC5905]
   such that port randomization is employed for those NTP modes for
   which the use of the NTP well-known port is not needed.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Considerations About Port Randomization in NTP

   The following subsections analyze a number of considerations about
   transport-protocol ephemeral port randomization when applied to NTP.

3.1.  Mitigation Against Off-path Attacks

   There has been a fair share of work in the area of off-path/blind
   attacks against transport protocols and upper-layer protocols, such
   as [RFC5927] and [RFC4953].  Whether the target of the attack is a
   transport protocol instance (e.g., TCP connection) or an upper-layer
   protocol instance (e.g., an application protocol instance), the
   attacker is required to know or guess the five-tuple {Protocol, IP
   Source Address, IP Destination Address, Source Port, Destination
   Port} that identifies the target transport protocol instance or the
   transport protocol instance employed by the target upper-layer
   protocol instance.  Therefore, increasing the difficulty of guessing
   this five-tuple helps mitigate blind/off-path attacks.

   As a result of these considerations, BCP 156 [RFC6056] recommends the
   randomization of transport-protocol ephemeral ports.  Thus, this
   document aims to bring the NTP specification [RFC5905] in line with
   the aforementioned recommendation.

   We note that the use of port randomization is a transport-layer
   mitigation against off-path/blind attacks, and does not preclude (nor



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   is it precluded by) other possible mitigations for off-path attacks
   that might be implemented by an application protocol (e.g.
   [I-D.ietf-ntp-data-minimization]).  For instance, some of the
   aforementioned mitigations may be ineffective against some off-path
   attacks [NTP-FRAG] or may benefit from the additional entropy
   provided by port randomization [NTP-security].

3.2.  Effects on Path Selection

   Intermediate systems implementing the Equal-Cost Multi-Path (ECMP)
   algorithm may select the outgoing link by computing a hash over a
   number of values, that include the transport-protocol source port.
   Thus, as discussed in [NTP-CHLNG], the selected client port may have
   an influence on the measured offset and delay.

   If the source port is changed with each request, packets in different
   exchanges will be more likely to take different paths, which could
   cause the measurements to be less stable and have a negative impact
   on the stability of the clock.

   Network paths to/from a given server are less likely to change
   between requests if port randomization is applied on a per-
   association basis.  This approach minimizes the impact on the
   stability of NTP measurements, but may cause different clients in the
   same network synchronized to the same NTP server to have a
   significant stable offset between their clocks due to their NTP
   exchanges consistently taking different paths with different
   asymmetry in the network delay.

   Section 4 recommends NTP implementations to randomize the ephemeral
   port number of client/server associations.  The choice of whether to
   randomize the port number on a per-association or a per-request basis
   is left to the implementation.

3.3.  Filtering of NTP traffic

   In a number of scenarios (such as when mitigating DDoS attacks), a
   network operator may want to differentiate between NTP requests sent
   by clients, and NTP responses sent by NTP servers.  If an
   implementation employs the NTP well-known port for the client port
   number, requests/responses cannot be readily differentiated by
   inspecting the source and destination port numbers.  Implementation
   of port randomization for non-symmetrical modes allows for simple
   differentiation of NTP requests and responses, and for the
   enforcement of security policies that may be valuable for the
   mitigation of DDoS attacks, when all NTP clients in a given network
   employ port randomization.




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3.4.  Effect on NAT devices

   Some NAT devices will not translate the source port of a packet when
   a privileged port number is employed.  In networks where such NAT
   devices are employed, use of the NTP well-known port for the client
   port will essentially limit the number of hosts that may successfully
   employ NTP client implementations.

   In the case of NAT devices that will translate the source port even
   when a privileged port is employed, packets reaching the external
   realm of the NAT will not employ the NTP well-known port as the local
   port, since the local port will normally be translated by the NAT
   device possibly, but not necessarily, with a random port.

3.5.  Relation to Other Mitigations for Off-Path Attacks

   Transport-protocol ephemeral port randomization is a best current
   practice (BCP 156) that helps mitigate off-path attacks at the
   transport-layer.  It is orthogonal to other possible mitigations for
   off-path attacks that may be implemented at other layers (such as the
   use of timestamps in NTP) which may or may not be effective against
   some off-path attacks (see e.g.  [NTP-FRAG].  This document aligns
   NTP with the existing best current practice on ephemeral port
   selection, irrespective of other techniques that may (and should) be
   implemented for mitigating off-path attacks.

4.  Update to RFC5905

   The following text from Section 9.1 ("Peer Process Variables") of
   [RFC5905]:

      dstport: UDP port number of the client, ordinarily the NTP port
      number PORT (123) assigned by the IANA.  This becomes the source
      port number in packets sent from this association.

   is replaced with:

      dstport: UDP port number of the client.  In the case of broadcast
      server mode (5) and symmetric modes (1 and 2), it SHOULD contain
      the NTP port number PORT (123) assigned by the IANA.  In the
      client mode (3), it SHOULD contain a randomized port number, as
      specified in [RFC6056].  The value in this variable becomes the
      source port number of packets sent from this association.  The
      randomized port number SHOULD NOT be shared with other
      associations.

      If a client implementation performs ephemeral port randomization
      on a per-request basis, it SHOULD close the corresponding socket/



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      port after each request/response exchange.  In order to prevent
      duplicate or delayed server packets from eliciting ICMP port
      unreachable error messages at the client, the client MAY wait for
      more responses from the server for a specific period of time (e.g.
      3 seconds) before closing the UDP socket/port.



      NOTES:
         The choice of whether to randomize the ephemeral port number on
         a per-request or a per-association basis is left to the
         implementation, and should consider the possible effects on
         path selection along with its possible impact on time
         measurement.

         On most current operating systems, which implement ephemeral
         port randomization [RFC6056], an NTP client may normally rely
         on the operating system to perform ephemeral port
         randomization.  For example, NTP implementations using POSIX
         sockets may achieve ephemeral port randomization by *not*
         binding the socket with the bind() function, or binding it to
         port 0, which has a special meaning of "any port". connect()ing
         the socket will make the port inaccessible by other systems
         (that is, only packets from the specified remote socket will be
         received by the application).

5.  Implementation Status

   [RFC Editor: Please remove this section before publication of this
   document as an RFC.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   OpenNTPD:
      [OpenNTPD] has never explicitly set the local port of NTP clients,
      and thus employs the ephemeral port selection algorithm
      implemented by the operating system.  Thus, on all operating



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      systems that implement port randomization (such as current
      versions of OpenBSD, Linux, and FreeBSD), OpenNTPD will employ
      port randomization for client ports.

   chrony:
      [chrony] by default does not set the local client port, and thus
      employs the ephemeral port selection algorithm implemented by the
      operating system.  Thus, on all operating systems that implement
      port randomization (such as current versions of OpenBSD, Linux,
      and FreeBSD), chrony will employ port randomization for client
      ports.

   nwtime.org's sntp client:
      sntp does not explicitly set the local port, and thus employs the
      ephemeral port selection algorithm implemented by the operating
      system.  Thus, on all operating systems that implement port
      randomization (such as current versions of OpenBSD, Linux, and
      FreeBSD), it will employ port randomization for client ports.

6.  IANA Considerations

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

7.  Security Considerations

   The security implications of predictable numeric identifiers
   [I-D.irtf-pearg-numeric-ids-generation] (and of predictable
   transport-protocol port numbers [RFC6056] in particular) have been
   known for a long time now.  However, the NTP specification has
   traditionally followed a pattern of employing common settings and
   code even when not strictly necessary, which at times has resulted in
   negative security and privacy implications (see e.g.
   [I-D.ietf-ntp-data-minimization]).  The use of the NTP well-known
   port (123) for the srcport and dstport variables is not required for
   all operating modes.  Such unnecessary usage comes at the expense of
   reducing the amount of work required for an attacker to successfully
   perform off-path/blind attacks against NTP.  Therefore, this document
   formally updates [RFC5905], recommending the use of transport-
   protocol port randomization when use of the NTP well-known port is
   not required.

   This issue has been tracked by US-CERT with VU#597821, and has been
   assigned CVE-2019-11331.






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8.  Acknowledgments

   The authors would like to thank (in alphabetical order) Ivan Arce,
   Dhruv Dhody, Todd Glassey, Watson Ladd, Aanchal Malhotra, Danny
   Mayer, Gary E.  Miller, Tomoyuki Sahara, Dieter Sibold, Steven
   Sommars, Kristof Teichel, and Ulrich Windl, for providing valuable
   comments on earlier versions of this document.

   Watson Ladd raised the problem of DDoS mitigation when the NTP well-
   known port is employed as the client port (discussed in Section 3.3
   of this document).

   The authors would like to thank Harlan Stenn for answering questions
   about nwtime.org's NTP implementation.

   Fernando would like to thank Nelida Garcia and Jorge Oscar Gont, for
   their love and support.

9.  References

9.1.  Normative References

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

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

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [chrony]   "chrony", <https://chrony.tuxfamily.org/>.







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   [I-D.ietf-ntp-data-minimization]
              Franke, D. and A. Malhotra, "NTP Client Data
              Minimization", draft-ietf-ntp-data-minimization-04 (work
              in progress), March 2019.

   [I-D.irtf-pearg-numeric-ids-generation]
              Gont, F. and I. Arce, "On the Generation of Transient
              Numeric Identifiers", draft-irtf-pearg-numeric-ids-
              generation-03 (work in progress), September 2020.

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

   [NTP-CHLNG]
              Sommars, S., "Challenges in Time Transfer Using the
              Network Time Protocol (NTP)", Proceedings of the 48th
              Annual Precise Time and Time Interval Systems and
              Applications Meeting, Monterey, California  pp. 271-290,
              January 2017, <http://leapsecond.com/ntp/
              NTP_Paper_Sommars_PTTI2017.pdf>.

   [NTP-FRAG]
              Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
              "Attacking the Network Time Protocol", NDSS'17, San Diego,
              CA.  Feb 2017, 2017,
              <http://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf>.

   [NTP-security]
              Malhotra, A., Van Gundy, M., Varia, V., Kennedy, H.,
              Gardner, J., and S. Goldberg, "The Security of NTP's
              Datagram Protocol", Cryptology ePrint Archive Report
              2016/1006, 2016, <https://eprint.iacr.org/2016/1006>.

   [NTP-VULN]
              Network Time Foundation, "Security Notice", Network Time
              Foundation's NTP Support Wiki ,
              <https://support.ntp.org/bin/view/Main/SecurityNotice>.

   [OpenNTPD]
              "OpenNTPD Project", <https://www.openntpd.org>.

   [RFC4953]  Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, DOI 10.17487/RFC4953, July 2007,
              <https://www.rfc-editor.org/info/rfc4953>.




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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

Authors' Addresses

   Fernando Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

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


   Guillermo Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: ggont@si6networks.com
   URI:   https://www.si6networks.com


   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com











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