Alternative NTP port

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Last updated 2020-04-06
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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: RFC5905 (if approved)                              Apr 06, 2020
Intended status: Standards Track
Expires: October 8, 2020

                          Alternative NTP port


   This document specifies an alternative port for the Network Time
   Protocol (NTP) which is restricted in modes in order to avoid
   amplification attacks.

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Lichvar                  Expires October 8, 2020                [Page 1]
Internet-Draft            Alternative NTP port                  Apr 2020

1.  Introduction

   There are several modes specified for NTP.  NTP packets have a 3-bit
   field for the mode.  Modes 1, 2, 3, 4, and 5 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 does not allow traffic amplification.
   (Explain why?)

   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.

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

   Longer NTP packets (using extension fields) are needed by NTS

   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Lichvar                  Expires October 8, 2020                [Page 2]
Internet-Draft            Alternative NTP port                  Apr 2020

2.  Alternative port

   The port TBD is an alternative port for NTP.  The protocol and the
   format of NTP packets sent from or to this port is unchanged.  The
   port MAY be used in modes 1 (active), 2 (passive), 3 (client), 4
   (server), and 5 (broadcast).  The port MUST NOT be used in modes 0
   (unspecified), 6 (control), and 7 (private).

   An NTP server SHOULD receive requests on both the original NTP port
   (123) and the alternative port (TBD).  It MUST respond from the same
   port which received the request.  If a request is received on the
   alternative port in a mode which is not allowed on this port, the
   request MUST be discarded with no response.

   When an NTP client is started, it SHOULD send the first request to
   the alternative port.  The client SHOULD be switching between the two
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