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

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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
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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
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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.