Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation
Intended status: Standards Track S. Goldberg
Expires: April 7, 2019 Boston University
October 4, 2018

Network Time Protocol REFID Updates


RFC 5905, 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.

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Table of Contents

1. Introduction

1.1. The REFID

The interpretation of a REFID is based on the stratum, as documented in RFC 5905, 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 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" value, when it believes that a querying system is not its time source.

The NOT-YOU REFID proposal is backwards-compatible. It can be implemented by one peer in an NTP association without any changes to the other peer.

The NOT-YOU REFID proposal does have a small risk, in that a system that might return NOT-YOU does not have perfect information and it is possible that the remote system peer is contacting "us" via a different network interface. In this case, the remote system might choose us as their system peer, and a degree-one timing loop will occur. In this case, however, the two systems will spiral into worsening stratum positions with increasing root distances, and eventually the loop will break. If any other systems are available as time servers, one of them may 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, section 7.3, "Packet Header Variables", explains how a remote system peer is converted to a REFID. It says:

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 network.

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 fully 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 protocol this situation cannot happen.]

1.4. 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.


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 REFID.

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. However...

Use of the NOT-YOU REFID proposal will hide the current system's system peer from querying systems that the current system believes are not the current system's system peer. Should the current system return the "not you" REFID to a query from its system peer, for example in the case where the system peer sends its query from an unexpected IP address, a one-degree timing loop can occur. Put another way, the responding system has imperfect knowledge about whether or not the sender is its system peer and there are cases where it will offer a NOT-YOU response to its system peer, which can then produce a degree-one timing loop.

Note that this mechanism fully supports degree-one loop detection in the case where the responding NOT-YOU system can accurately detect when it's getting a request from its system peer, and otherwise provides the most basic diagnostic information to third parties.

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 says:

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.

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 proposed "REFID Suggestion" or "I-DO" extension fields are ways to mitigate this potential situation.]

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 reach S16 and 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.

3.3. Questions

Should we reference the REFID Suggestion and I-DO proposals here?

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 field.

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., 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

[CVE-2015-8138] 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.
[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), 2016.

Authors' Addresses

Harlan Stenn Network Time Foundation P.O. Box 918 Talent, OR, 97540 US EMail:
Sharon Goldberg Boston University 111 Cummington St Boston, MA, 02215 US EMail: