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

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
                 draft-mlichvar-ntp-alternative-port-00

Abstract

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

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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
   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.

3.  IANA Considerations

   IANA is requested to allocate a (system?)  UDP/TCP port.

4.  References

4.1.  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>.

4.2.  Informative References

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   [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-07 (work in progress), November 2019.

   [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.

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com

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