Network Time Protocol REFID Updates

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Last updated 2019-03-25
Replaces draft-stenn-ntp-ipv6-refid-hash, draft-stenn-ntp-not-you-refid
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Internet Engineering Task Force                                 H. Stenn
Internet-Draft                                   Network Time Foundation
Intended status: Standards Track                             S. Goldberg
Expires: September 26, 2019                            Boston University
                                                          March 25, 2019

                  Network Time Protocol REFID Updates


   RFC 5905 [RFC5905], section 7.3, "Packet Header Variables", defines
   the value of the REFID, the system peer for the responding host.  In
   the past, for IPv4 associations the IPv4 address is used, and for
   IPv6 associations the first four octets of the MD5 hash of the IPv6
   are used.  There are two recognized shortcomings to this approach,
   and this proposal addresses them.  One is that knowledge of the
   system peer is "abusable" information and should not be generally
   available.  The second is that the four octet hash of the IPv6
   address looks very much like an IPv4 address, and this is confusing.


   The source code and issues list for this draft can be found in

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 26, 2019.

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Copyright Notice

   Copyright (c) 2019 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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   ( in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  The REFID . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  IPv6 REFID  . . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Proposal  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Augmenting the IPv6 REFID Hash  . . . . . . . . . . . . . . .   5
     3.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Potential Problems  . . . . . . . . . . . . . . . . . . .   6
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

1.1.  The REFID

   The interpretation of a REFID is based on the stratum, as documented
   in RFC 5905 [RFC5905], section 7.3, "Packet Header Variables".  The
   core reason for the REFID in the NTP Protocol is to prevent a degree-
   one timing loop, where server B decides to follow A as its time
   source, and A then decides to follow B as its time source.

   At Stratum 2+, which will be the case if two servers A and B are
   exchanging timing information, then if server B follows A as its time
   source, A's address will be B's REFID.  When A uses IPv4, the default

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   REFID is A's IPv4 address.  When A uses IPv6, the default REFID is a
   four-octet digest of A's IPv6 address.  Now, if A queries B for its
   time, then A will learn that B is using A as its time source by
   observing A's address in the REFID field of the response packet sent
   by B.  Thus, A will not select B as a potential time source, as this
   would cause a timing loop.


   The traditional REFID mechanism, however, also allows a third-party C
   to learn that A is the time source that is being used by B.  When A
   is using IPv4, C can learn this by querying B for its time, and
   observing that the REFID in B's response is the IPv4 address of A.
   Meanwhile, when A is using IPv6, then C can again query B for its
   time, and then can use an offline dictionary attack to attempt to
   determine the IPv6 address that corresponds to the digest value in
   the response sent by B.  C could construct the necessary dictionary
   by compiling a list of publicly accessible IPv6 servers.  Remote
   attackers can use this technique to attempt to identify the time
   sources used by a target, and then send spoofed packets to the target
   or its time source in an attempt to disrupt time service, as was done
   e.g., in [NDSS16] or [CVE-2015-8138].

   The REFID thus unnecessarily leaks information about a target's time
   server to remote attackers.  The best way to mitigate this
   vulnerability is to decouple the IP address of the time source from
   the REFID.  To do this, a system can use an otherwise-impossible
   value for its REFID, called the NOT-YOU REFID value, when it believes
   that a querying system is not its time source.

   The NOT-YOU REFID proposal is backwards-compatible and provides the
   bare minimum diagnostic information to third parties.  It can be
   implemented by one peer in an NTP association without any changes to
   the other peer.  This holds as long as responding NOT-YOU system can
   accurately detect when it's getting a request from its system peer.

   The NOT-YOU REFID proposal does have a small risk.  Consider system A
   that returns the NOT-YOU REFID and system B that has two network
   interfaces B1 and B2.  Suppose that system A is using system B as his
   time source, via network interface B1.  Now suppose that system B
   queries system A for time via network interface B2.  In this case,
   system A returns the NOT-YOU REFID value to system B, since system A
   does not realize that network interface B1 and B2 belong to the same
   system.  In this case, system B might choose system A as its time
   source, and a degree-one timing loop will occur.  In this case,
   however, the two systems will spiral into degrading stratum positions
   with increasing root distances, and eventually the loop will break.
   If any other systems are available as time servers, one of them will

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   become the new system peer.  However, unless or until this happens
   the two spiraling systems will have degraded time quality.

1.3.  IPv6 REFID

   In an environment where all time queries made to a server can be
   trusted, an operator might well choose to expose the real REFID.  RFC
   5905 [RFC5905], section 7.3, "Packet Header Variables", explains how
   a remote system peer is converted to a REFID.  It says:

      If using the IPv4 address family, the identifier is the four-octet
      IPv4 address.  If using the IPv6 family, it is the first four
      octets of the MD5 hash of the IPv6 address. ...

   However, the MD5 hash of an IPv6 address often looks like a valid
   IPv4 address.  When this happens, an operator cannot tell if the
   REFID refers to an IPv6 address or and IPv4.  Specifically, the NTP
   Project has received a report where the generated IPv6 hash decoded
   to the IPv4 address of a different machine on the system peer's

   This proposal offers a way for a system to generate a REFID for a
   IPv6 system peer that does not conflict with an IPv4-based REFID.

   This proposal is not backwards-compatible.  It SHOULD be implemented
   by both peers in an NTP association.  In the scenario where A and B
   are peering using IPv6, where A is the system peer and does not
   understand IPv6 REFID, and B is subordinate and is using IPv6 REFID,
   A will not be able to determine that B is using A as its system peer
   and a degree-one timing loop can form.

   If both peers implement the IPv6 REFID this situation cannot happen.

   If at least one of the peers implements the proposed I-DO
   [DRAFT-I-DO] protocol this situation cannot happen.

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].


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2.1.  Proposal

   When enabled, this proposal allows the one-degree loop detection to
   work and useful diagnostic information to be provided to trusted
   partners while keeping potentially abusable information from being
   disclosed to ostensibly uninterested parties.  It does this by
   returning the normal REFID to queries that come from trusted
   addresses or from an address that the current system believes is its
   time source (aka its "system peer"), and otherwise returning one of
   two special IP addresses that is interpreted to mean "not you".  The
   "not you" IP addresses are and  If
   an IPv6 query is received from an address whose four-octet hash
   equals one of these two addresses and we believe the querying host is
   not our system peer, the other NOT-YOU address is returned as the

   This mechanism is correct and transparent when the system responding
   with a NOT-YOU can accurately detect when it's getting a timing query
   from its system peer.  A querying system that uses IPv4 continues to
   check that its IPv4 address does not appear in the REFID before
   deciding whether to take time from the current system.  A querying
   system that uses IPv6 continues to check that the four-octet hash of
   its IPv6 address does not appear in the REFID before deciding whether
   to take time from the current system.

3.  Augmenting the IPv6 REFID Hash

3.1.  Background

   In a trusted network, the S2+ REFID is generated based on the network
   system peer.  RFC 5905 [RFC5905] says:

      If using the IPv4 address family, the identifier is the four-octet
      IPv4 address.  If using the IPv6 family, it is the first four
      octets of the MD5 hash of the IPv6 address.

   This means that the IPv4 representation of the IPv6 hash would be:
   b1.b2.b3.b4 .  This proposal is that the system MAY also use
   255.b2.b3.b4 as its REFID.  This reduces the risk of ambiguity, since
   addresses beginning with 255 are "reserved", and thus will not
   collide with valid IPv4 on the network.

   When using the REFID to check for a timing loop for an IPv6
   association, if the code that checks the first four-octets of the
   hash fails to match then the code must check again, using 0xFF as the
   first octet of the hash.

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3.2.  Potential Problems

   There is a 1 in 16,777,216 chance that the REFID hashes of two IPv6
   addresses will be identical, producing a false-positive loop
   detection.  With a sufficient number of servers, the risk of this
   problem becomes a non-issue.  The use of the NOT-YOU REFID and/or the
   [DRAFT-I-DO] extension fields are ways to mitigate this potential

   Unrealistically, if only two instances of NTP are communicating via
   IPv6 and system A implements this new IPv6 REFID hash and system B
   does not, system B will not be able to detect this loop condition.
   In this case, the two machines will slowly increase their stratum
   until they become unsynchronized.  This situation is considered to be
   unrealistic because, for this to happen, each system would have to
   have only the other system available as a time source, for example,
   in a misconfigured "orphan mode" setup.  There is no risk of this
   happening in an NTP network with 3 or more time sources, or in a
   properly-configured "time island" setup.

4.  Acknowledgements

   For the "not-you" REFID, we acknowledge useful discussions with
   Aanchal Malhotra and Matthew Van Gundy.

   For the IPv6 REFID, we acknowledge Dan Mahoney (and perhaps others)
   for suggesting the idea of using an "impossible" first-octet value to
   indicate an IPv6 refid hash.

5.  IANA Considerations

   This memo requests IANA to allocate a pseudo Extension Field Type of
   0xFFFF so the proposed "I-Do" exchange can report whether or not the
   "IPv6 REFID Hash" is supported.

6.  Security Considerations

   Many systems running NTP are configured to return responses to timing
   queries by default.  These responses contain a REFID field, which
   generally reveals the address of the system's time source if that
   source is an IPv4 address.  This behavior can be exploited by remote
   attackers who wish to first learn the address of a target's time
   source, and then attack the target and/or its time source.  As such,
   the NOT-YOU REFID proposal is designed to harden NTP against these
   attacks by limiting the amount of information leaked in the REFID

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   Systems running NTP should reveal the identity of their system in
   peer in their REFID only when they are on a trusted network.  The
   IPv6 REFID proposal provides one way to do this, when the system peer
   uses addresses in the IPv6 family.

7.  References

7.1.  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,

   [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,

7.2.  Informative References

              Van Gundy, M. and J. Gardner, "Network Time Protocol
              Origin Timestamp Check Impersonation Vulnerability (CVE-
              2015-8138)", in TALOS VULNERABILITY REPORT (TALOS-
              2016-0077), 2016.

              Stenn, H., "draft-stenn-ntp-i-do", 2018.

              Stenn, H., "draft-stenn-ntp-suggest-refid", 2018.

   [NDSS16]   Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
              "Attacking the Network Time Protocol", in ISOC Network and
              Distributed System Security Symposium 2016 (NDSS'16),

              Stenn, H., "draft-stenn-ntp-extension-fields", 2018.

Authors' Addresses

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   Harlan Stenn
   Network Time Foundation
   P.O. Box 918
   Talent, OR  97540


   Sharon Goldberg
   Boston University
   111 Cummington St
   Boston, MA  02215


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