Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation
Intended status: Standards Track S. Goldberg
Expires: May 17, 2017 Boston University
November 13, 2016
Network Time Protocol REFID Updates
draft-ietf-ntp-refid-updates-00
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 at least three shortcomings to this approach,
and this proposal will address the three so noted. 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 third is that a growing number of low-stratum servers
want to offer leap-smeared time to their clients, and there is no
obvious way to know if a server is offering accurate time or leap-
smeared time.
Status of This Memo
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This Internet-Draft will expire on May 17, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . 3
1.4. Leap-Smear REFID . . . . . . . . . . . . . . . . . . . . 4
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. The NOT-YOU REFID . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Augmenting the IPv6 REFID Hash . . . . . . . . . . . . . . . 5
3.1. Background . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Potential Problems . . . . . . . . . . . . . . . . . . . 6
3.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 6
4. The REFID sent to clients during a Leap-Smear . . . . . . . . 7
4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Leap Smear REFID . . . . . . . . . . . . . . . . . . . . 7
4.3. Questions . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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
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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, since
this would cause a timing loop.
1.2. NOT-YOU REFID
This 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
publically 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.
1.3. IPv6 REFID
In a trusted situation, 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
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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. Having said this,
however, in a properly-designed NTP network there is negligible risk
of a degree-one timing loop if only one system implements and uses
the IPv6 REFID. This backward incompatability can be avoided by
using the proposed I-DO protocol.
1.4. Leap-Smear REFID
RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming
method of distributing time on networks. Leap Seconds will continue
to exist for a good number of years' time, and since the timescale
mandated by POSIX effectively ignores any instances where there are
not 86,400 seconds' time in a day something must be done to reliably
synchronize clocks during the application of leap second corrections.
One mechanism for dealing with the application that has recently
become visible is to apply the leap second using a "smear", where the
time reported by leap-second aware servers is gradually adjusted so
there is no major disruption to time synchronization when processing
a leap second.
While the proper handling of leap seconds can be expected from up-to-
date software and time servers, there are large numbers of out-of-
date software installations and systems that are just not able to
properly handle a leap second correction.
This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is.
This proposal is backwards-compatible in all but poorly-designed NTP
networks. The entire point of providing NTP servers that offer leap-
smeared time in response to CLIENT requests is to provide smooth time
to clients that are unable to properly handle leap seconds. If an
operator is skilled enough to provide leap-smeared time to a subset
of clients that cannot properly handle leap seconds, they can be
expected to know enough to avoid using leap-smeared time between time
servers that are expected to be able to properly handle leap seconds.
Leap smears are expected to be implemented on a limited number of
time servers where there is a base of client systems that cannot
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handle a leap second correction. Furthermore, even in a poorly-
designed NTP network the "window of risk" lasts only as long as it
takes for the leap second to be smeared.
1.5. 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
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 a
special IP address that is interpreted to mean "not you". The "not
you" IP address is 127.127.127.127 when the query is made from an
IPv4 address, or when the query is made from an IPv6 address whose
four-octect hash does not equal 127.127.127.127. The "not you" IP
address is 127.127.127.128 when the query is made from an address
whose four-octect hash equals 127.127.127.127.
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.
This 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. Note well, however, that the current system
will return the "not you" value to a query from its system peer if
the system peer sends its query from an unexpected IP address. 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 will
then produce a degree-one timing loop.
3. Augmenting the IPv6 REFID Hash
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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 . The 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 one side implements this new IPv4 REFID hash and the other
side does not, the "other side" 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 the only current
way this could happen would be for there to only be these two
instances of NTP available as time sources 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 ask IANA to allocate a pseudo Extension Field Type of
0xFFFF (for example) so the proposed "I-Do" exchange can report
whether or not the "IPv6 REFID Hash" is supported?
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4. The REFID sent to clients during a Leap-Smear
4.1. Background
RFC 5905 [RFC5905] and earlier versions of NTP are the overwhelming
method of distributing time on networks. Leap Seconds will continue
to exist for a good number of years' time, and since the timescale
mandated by POSIX effectively ignores any instances where there are
not 86,400 seconds' time in a day, something must be done to reliably
synchronize clocks during the application of leap second corrections.
One mechanism for dealing with the application that has recently
become visible is to apply the leap second using a "smear", where the
time reported by leap-second aware servers is gradually adjusted so
there is no major disruption to time synchronization when processing
a leap second.
While the proper handling of leap seconds can be expected from up-to-
date software and time servers, there are large numbers of out-of-
date software installations and systems that are just not able to
properly handle a leap second correction.
This proposal offers a way for a system to generate a REFID that
indicates that the time being supplied in the NTP packet already
contains an amount of leap smear correction, and what that amount is.
4.2. Leap Smear REFID
RFC 5905 [RFC5905] defines the data type of NTP time values in
Section 6, "Data Types":
All NTP time values are represented in twos-complement format,
with bits numbered in big-endian (as described in Appendix A of
[RFC0791]) fashion from zero starting at the left, or high-order,
position. ...
The 32 bit signed integer seconds portion and the 32 bit unsigned
fractional seconds portion, or 32:32 format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fraction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NTP Timestamp Format (32:32)
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This format provides coverage for 136 years' time to a precision of
232 picoseconds. If a leap-second addition is being completely
smeared just before before the stroke of the next POSIX second then
the smear correction will be (0,1). If this was the only way to
apply a leap smear correction then we could simply use an unsigned
value to represent the correction. But while the first popular leap
smear implementation applied the correction over an appropriate
number of hours' time before the actual leap second so the system
time was corrected at the stroke of 00:00, that meant that the
difference between system time and UTC spent half of the duration of
the smear application at [.5,1) "off" of correct time. The second
popular implementation of the leap smear applied the first half-
second correction before the stroke of 00:00 for a correction range
of (0,.5] and the last half-second correction starting at the stroke
of 00:00 for a [-.5,0) correction range. This also means we need a
signed value to represent the amount of correction.
The REFID of a system that is supplying smeared time to client
requests while leap-smear correction is active would be 254.b1.b2.b3,
where the three octets (b1, b2, and b3) are a 2:22 formatted value,
yielding precision to 238 nanoseconds, or about a quarter of a
microsecond.
Note that if an NTP server decides to offer smeared time corrections
to clients, it SHOULD only offer this time in response to CLIENT time
requests. An NTP server that is offering smeared time SHOULD NOT
send smeared time in any peer exchanges. Also, CLIENT machines
SHOULD NOT be distributing time (smeared or otherwise) to other
systems.
We also note that during the application of a leap smear, the REFID
from a system offering smeared time cannot provide detection of a
timing loop. This is not expected to be a problem because time
server systems are not expected to make CLIENT connections with each
other, so they should not be receiving smeared time. Moreso, if a
time server is configured to make CLIENT connections to a server that
offers smeared time, with the mechanism described here it can detect
when it is getting smeared time, and either ignore time from that
source, or "undo" the leap smear correction and use the corrected
time for that sample.
This proposal is not an attempt to justify servers offering leap
smeared time. It is only an attempt to make it easy and visible to
identify when a server is offering or client is receiving smeared
time, and provide the client a means to know the amount of smear
correction as of the latest successful poll.
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4.3. Questions
Should we ask IANA to allocate a pseudo Extension Field Type of
0xFFFE (for example) so the proposed "I-Do" exchange can report
whether or not this server will offer leap smeared time in response
to CLIENT time requests, identifying the amount of correction using
the above REFID?
5. 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.
For the Leap Smear REFID, we acknowledge useful discussions with
Juergen Perlinger.
6. IANA Considerations
This memo makes no requests of IANA.
7. 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.
8. References
8.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc5905>.
8.2. 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: stenn@nwtime.org
Sharon Goldberg
Boston University
111 Cummington St
Boston, MA 02215
US
Email: goldbe@cs.bu.edu
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