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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: 5905 (if approved)                                 Sep 16, 2020
Intended status: Standards Track
Expires: March 20, 2021


                          Alternative NTP port
                 draft-mlichvar-ntp-alternative-port-02

Abstract

   This document updates RFC 5905 to specify an alternative port for the
   Network Time Protocol (NTP) which is restricted to NTP messages that
   do not allow traffic amplification in order to make NTP safe for the
   Internet.

Status of This Memo

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

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   This Internet-Draft will expire on March 20, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Alternative port - update to RFC 5905 . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   There are several modes specified for NTP.  NTP packets in versions
   2, 3, and 4 have a 3-bit field for the mode.  Modes 1 (active), 2
   (passive), 3 (client), 4 (server), and 5 (broadcast) are used for
   synchronization of clocks.  They are specified in RFC 5905 [RFC5905].
   Modes 6 and 7 are used for other purposes, like monitoring and remote
   management of NTP servers and clients.  The mode 6 is specified in
   Control Messages Protocol for Use with Network Time Protocol Version
   4 [I-D.ietf-ntp-mode-6-cmds].

   The first group of modes typically does not allow any traffic
   amplification, i.e. the response is not larger than the request.  An
   exception is Autokey specified in RFC 5906 [RFC5906].  Autokey is
   rarely supported on public NTP servers.

   However, the modes 6 and 7 allow significant traffic amplification,
   which has been exploited in large-scale denial-of-service (DoS)
   attacks over the Internet.

   Over time, network operators have been observed to implement the
   following mitigations:

   1.  Blocked UDP packets with destination or source port 123

   2.  Blocked UDP packets with destination or source port 123 and
       specific length (e.g. longer than 48 octets)

   3.  Blocked UDP packets with destination or source port 123 and NTP
       mode 6 or 7

   4.  Limited rate of UDP packets with destination or source port 123

   From those, only the 3rd approach does not have an impact on
   synchronization of clocks with NTP.



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   The number of public servers in the pool.ntp.org project has dropped
   in large part due to the mitigations (citation?).

   Longer NTP packets (using extension fields) are needed by NTS
   [I-D.ietf-ntp-using-nts-for-ntp].

   This document specifies an alternative port for NTP which is
   restricted to the safe modes in order to enable synchronization of
   clocks in networks where the port 123 is blocked or rate limited.

1.1.  Requirements Language

   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.

2.  Alternative port - update to RFC 5905

   The table in "Figure 6: Global Parameters" in Section 7.2 of
   [RFC5905] is extended with:

                +---------+-------+----------------------+
                | Name    | Value | Description          |
                +---------+-------+----------------------+
                | ALTPORT | TBD   | Alternative NTP port |
                +---------+-------+----------------------+

   The following text from Section 9.1 of [RFC5905]:

      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 can contain any number consistent with
      local policy.

   is replaced with:

      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) or the
      alternative NTP port ALTPORT (TBD) assigned by the IANA.  In other
      modes, it can contain any number consistent with local policy.

   The following text is added to the Section 9.1:



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      The port ALTPORT (TBD) is an alternative port to the port PORT
      (123).  The protocol and the format of NTP packets sent from and
      to this port is unchanged.  Both NTP requests and responses MAY be
      sent from the alternative port.  An NTP packet MUST NOT be sent
      from the alternative port if it is a response which has a longer
      UDP payload than the request, or the number of NTP packets in a
      single response is larger than one.

      Only modes 1 (active), 2 (passive), 3 (client), 4 (server), and 5
      (broadcast) are generally usable on this port.

      An NTP server SHOULD receive requests in the client mode on both
      the PORT (123) and ALTPORT (TBD) ports.  If it responds, it MUST
      send the response from the port which received the request.  If
      the server supports any extension fields in NTP packets, it MUST
      verify that each response is not larger than the request, even if
      the number of extension fields is constant and they have a
      constant length.

      When an NTP client is started, it SHOULD send the first request to
      the alternative port.  The client SHOULD be switching between the
      two ports until a valid response is received.  The client MAY send
      a limited number of requests to both ports at the same time in
      order to speed up the discovery of the responding port.  When both
      ports are responding, the client SHOULD prefer the alternative
      port.

      An NTP server which supports NTS SHOULD include the NTPv4 Port
      Negotiation record in NTS-KE responses to specify the alternative
      port as the port to which the client should send NTP requests.

      In the symmetric modes (active and passive) NTP packets are
      considered to be requests and responses at the same time.
      Therefore, the peers MUST send packets with an equal length in
      order to synchronize with each other.  The peers MAY use different
      polling intervals (packets sent at subsequent polls are considered
      to be separate requests and responses).

3.  IANA Considerations

   IANA is requested to allocate the following port in the Service Name
   and Transport Protocol Port Number Registry [RFC6335]:

      Service Name: ntp-alt

      Transport Protocol: udp

      Assignee: IESG <iesg@ietf.org>



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      Contact: IETF Chair <chair@ietf.org>

      Description: Network Time Protocol

      Reference: [[this memo]]

      Port Number: [[TBD]], selected by IANA from the System Port range

4.  Security Considerations

   A Man-in-the-middle (MITM) attacker can selectively block requests
   sent to the alternative port to force a client to select the original
   port and get a degraded NTP service with a significant packet loss.
   The client needs to periodically try the alternative port to recover
   from the degraded service when the attack stops.

5.  Acknowledgements

   The author would like to thank Daniel Franke, Dhruv Dhody, and Ragnar
   Sundblad for their useful comments.

6.  References

6.1.  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>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [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>.






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6.2.  Informative References

   [I-D.ietf-ntp-mode-6-cmds]
              Haberman, B., "Control Messages Protocol for Use with
              Network Time Protocol Version 4", draft-ietf-ntp-mode-
              6-cmds-09 (work in progress), June 2020.

   [I-D.ietf-ntp-using-nts-for-ntp]
              Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", draft-ietf-ntp-using-nts-for-ntp-28 (work in
              progress), March 2020.

   [RFC5906]  Haberman, B., Ed. and D. Mills, "Network Time Protocol
              Version 4: Autokey Specification", RFC 5906,
              DOI 10.17487/RFC5906, June 2010,
              <https://www.rfc-editor.org/info/rfc5906>.

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com
























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