Alternative NTP port
draft-ietf-ntp-alternative-port-00

Document Type Active Internet-Draft (ntp WG)
Author Miroslav Lichvar 
Last updated 2020-10-12
Replaces draft-mlichvar-ntp-alternative-port
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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: 5905 (if approved)                                 Oct 12, 2020
Intended status: Standards Track
Expires: April 15, 2021

                          Alternative NTP port
                   draft-ietf-ntp-alternative-port-00

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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on April 15, 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
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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.

   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)

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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
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