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
draft-ietf-ntp-refid-updates-05
Abstract
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.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source code and issues list for this draft can be found in
https://github.com/hstenn/ietf-ntp-refid-updates
Status of This Memo
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This Internet-Draft will expire on September 26, 2019.
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Copyright Notice
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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.
1.2. NOT-YOU REFID
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
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 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. The NOT-YOU REFID
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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 127.127.127.127 and 127.127.127.128. 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.
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
proposed REFID-SUGGESTION [DRAFT-REFID-SUGGESTION] or I-DO
[DRAFT-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 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
field.
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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,
<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>.
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.
[DRAFT-I-DO]
Stenn, H., "draft-stenn-ntp-i-do", 2018.
[DRAFT-REFID-SUGGESTION]
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),
2016.
[NTP-EXTENSION-FIELD]
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
US
Email: stenn@nwtime.org
Sharon Goldberg
Boston University
111 Cummington St
Boston, MA 02215
US
Email: goldbe@cs.bu.edu
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