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Network Time Protocol (ntp) Working Group                        F. Gont
Internet-Draft                                                   G. Gont
Obsoletes: rfc5905 (if approved)                            SI6 Networks
Intended status: Standards Track                          April 16, 2019
Expires: October 18, 2019


       Port Randomization in the Network Time Protocol Version 4
                  draft-gont-ntp-port-randomization-00

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 service/well-known port as the local
   port number.  However, in the case of NTP modes where the use of a
   service/well-known port is not required, employing such well-known/
   service port unnecessarily increases the ability of attackers to
   perform blind/off-path attacks, since knowledge of such port number
   is typically required for such attacks.  This document formally
   updates RFC5905, recommending the use of port randomization for those
   modes where use of the NTP service 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
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   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 October 18, 2019.

Copyright Notice

   Copyright (c) 2019 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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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   This document may not be modified, and derivative works of it may not
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Update to RFC5905 . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The Network Time Protocol (NTP) is one of the oldest Internet
   protocols, and currently specified in [RFC5905].  Since its 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, and other undesirable outcomes.  Many
   of these attacks require the attacker to guess or know at least a
   target association, typically identified by the tuple {srcaddr,
   srcport, dstaddr, dstport, keyid}. Some of these parameters may be
   easily known or guessed.

   NTP can operate in several modes.  Some of these modes rely on the
   ability to receive unsolicited packets, and therefore require the use
   of a service/well-known port number.  However, for modes where the
   use of a service/well-known port is not required, employing such
   well-known/service port improves the ability of an attacker to
   perform blind/off-path attacks (since knowledge of such port number
   is typically required for such attacks).  This document formally
   updates RFC5905, recommending the use of port randomization for those
   modes where use of the NTP service port is not required.




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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 [RFC2119].

3.  Update to RFC5905

   The specification of the "srcport" and "dstport" peer process
   variables from Section 9.1 ("Peer Process Variables") of [RFC5905] is
   updated as follows:

   srcport: UDP port number of the server or reference clock.  This
   becomes the destination port number in packets sent from this
   association.  When operating in symmetric modes (1 and 2), this field
   must contain the NTP port number PORT (123) assigned by the IANA.  In
   other modes, it SHOULD contain a randomized transport-protocol, as
   specified in [RFC6056].

   dstport: UDP port number of the client.  In the case of broadcast
   server mode (5) and symmetric modes (1 and 2), it must contain the
   NTP port number PORT (123) assigned by the IANA.  In other cases, it
   SHOULD contain a randomized transport-protocol, as specified in
   [RFC6056].  The value in this variable becomes the source port number
   in packets sent from this association.

4.  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.

5.  Security Considerations

   The security implications of predictable numeric identifiers
   [I-D.gont-predictable-numeric-ids] (and of predictable transport-
   protocol port numbers [RFC6056] in particular) have been known for a
   long time now.  However, the NTP specification have 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 service port
   (123) for the srcport and dstport variables is not required for all
   operating modes, and 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-




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   protocol port randomization when use of the NTP service port is not
   required.

6.  Acknowledgments

   The authors would like to thank (in alphabetical order) [TBD] for
   providing valuable comments on earlier versions of this document.

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

7.  References

7.1.  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>.

7.2.  Informative References

   [I-D.gont-predictable-numeric-ids]
              Gont, F. and I. Arce, "Security and Privacy Implications
              of Numeric Identifiers Employed in Network Protocols",
              draft-gont-predictable-numeric-ids-03 (work in progress),
              March 2019.

   [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.

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




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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:   http://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





























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