Alternative NTP port
draft-ietf-ntp-alternative-port-01

Document Type Active Internet-Draft (ntp WG)
Author Miroslav Lichvar 
Last updated 2021-06-11 (latest revision 2021-02-15)
Replaces draft-mlichvar-ntp-alternative-port
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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: 5905 (if approved)                                 Feb 15, 2021
Intended status: Standards Track
Expires: August 19, 2021

                          Alternative NTP port
                   draft-ietf-ntp-alternative-port-01

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.

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

Copyright Notice

   Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . .   5
   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 [RFC5906], which allows an NTP response to be
   longer than the request, e.g. packets containing the Certificate
   Message or Cookie Message extension field.  Autokey is rarely used.
   If it is enabled on a publicly accessible server, the access needs to
   be tightly controlled to limit denial-of-service (DoS) attacks
   exploiting the amplification.

   The modes 6 and 7 of NTP allow significant traffic amplification,
   which has been exploited in large-scale DoS attacks on the Internet.
   Publicly accessible servers that support these modes need to be
   configured to not respond to requests using the modes, as recommended
   in BCP 233 [RFC8633], but the number of servers that still do that is
   significant enough to require specific mitigations.

   Network operators have implemented different mitigations.  They are
   not documented and may change over time.  Some of the mitigations
   that have been observed are:

   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)

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   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.  However, this mitigation can be
   implemented only on devices which can inspect the UDP payload.

   The number of public servers in the pool.ntp.org project has dropped
   since 2013, when the large-scale attacks started.

   The length-specific filtering and rate limiting has an impact on the
   Network Time Security [RFC8915] authentication, which uses extension
   fields in NTPv4 packets.

   This document specifies an alternative port for NTP which is
   restricted to a subset of the NTP protocol which does not allow
   amplification in order to enable safe 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.

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   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:

      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 that supports the alternative port MUST 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 an NTP
      extension field, it MUST verify for each response that it is not
      longer than the request.

      When an NTP client is started, it SHOULD send the first request to
      the alternative port.  The client SHOULD alternate 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, two peers using the alternative port MUST send packets
      with an equal length in order to synchronize with each other.  The
      peers MAY still use different polling intervals as packets sent at
      subsequent polls are considered to be separate requests and
      responses.

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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>

      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, Ragnar
   Sundblad, and Steven Sommars 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>.

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   [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>.

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-10 (work in progress), September 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>.

   [RFC8633]  Reilly, D., Stenn, H., and D. Sibold, "Network Time
              Protocol Best Current Practices", BCP 223, RFC 8633,
              DOI 10.17487/RFC8633, July 2019,
              <https://www.rfc-editor.org/info/rfc8633>.

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/info/rfc8915>.

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com

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