Internet Engineering Task Force H. Stenn
Internet-Draft Network Time Foundation
Intended status: Standards Track March 14, 2016
Expires: September 15, 2016
Network Time Protocol Suggest REFID Extension Field
draft-stenn-ntp-suggest-refid-00
Abstract
NTP has been widely used through several revisions, with the latest
being RFC 5905 [RFC5905]. A core component of the protocol and the
algoritms is the Reference ID, or REFID, which is used to identify
the source of time used for synchronization. Traditionally, when the
source of time was another system the REFID was the IPv4 address of
that other system. The purpose of the REFID is to prevent a one-
degree timing loop, where if A has several timing sources that
include B, if B decides to get its time from A we don't want A then
deciding to get its time from B. The REFID is considered to be
"public data" and is a vital core-component of the base NTP packet.
If a system's REFID is the IPv4 address of its system peer, an
attacker can try to use that information to send spoofed time packets
to either or both the target or the target's server, attempting to
cause a disruption in time service. This proposal is a backward-
compatible way for a time source to tell its peers or clients "If you
use me as your system peer, use this nonce as your REFID." This
nonce SHOULD not be traceable to the original system, and if it is
used as the REFID this type of attack is prevented.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 15, 2016.
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Internet-Draft Network Time Protocol Suggest REFID March 2016
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The REFID . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Suggested REFID Extension Field . . . . . . . . . . . . . 3
4. Generating and Sending the Suggested REFID Extension Field . 4
5. Receiving a Suggested REFID Extension Field . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
NTP has been widely used through several revisions, with the latest
being RFC 5905 [RFC5905]. A core component of the protocol and the
algoritms is the Reference ID, or REFID, which is used to identify
the source of time used for synchronization. Traditionally, when the
source of time was another system, the REFID was the IPv4 address of
that other system. If the remote system was using IPv6 for its
connection, a 4 octet digest value of the IPv6 address was used. The
purpose of the REFID is to prevent a one-degree timing loop, where if
A has several timing sources that include B, if B decides to get its
time from A we don't want A then deciding to get its time from B.
The REFID is considered to be "public data" and is a vital core-
component of the base NTP packet. If a system's REFID is the IPv4
address of its system peer, an attacker can try to use that
information to send spoofed time packets to either or both the target
or the target's server, attempting to cause a disruption in time
service. This proposal is a backward-compatible way for a time
source to tell its peers or clients "If you use me as your system
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peer, use this nonce as your REFID." This nonce, a Suggested REFID,
SHOULD not be traceable to the sending system. If the receiving
system uses this Suggested REFID nonce instead of the IPv4 address as
its REFID, this type of attack and information disclosure is
prevented.
The NTP protocol was designed with a mechanism that allowed for a
depth-1 loop detection to avoid a simple "time loop". Recently, this
mechanism was discovered to be a potential vulnerability exploit.
The best way to mitigate this vulnerability is to decouple the IPv4
address of the server from its REFID. But there is no current way
for a potential time source to tell the other party any other
alternative to use as the REFID. This proposal creates an extension
field to accomplish this.
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].
2. The REFID
The core reason for the REFID in the NTP Protocol is to prevent a
timing loop of degree 1. Put another way, if servers A and B are
exchanging time with each other and server B decides to follow A as
its system peer, the REFID that B will use must be able to identify
server A. The interpretation of a REFID is based on the stratum, as
documented in RFC 5905 [RFC5905], section 7.3, "Packet Header
Variables". At Stratum 2+, which will be the case if servers A and B
are exchanging packets over IPv4, if server B follows A, then B will
have A's IPv4 address as its REFID. When A asks B for its time, A
will see that B is synchronized to A because B will tell A that its
REFID is A's IPv4 address, so when A sees its IP address as B's
REFID, A knows that if it were to follow B for its time then there
would be a timing loop. In this case, A will not select B as a
potential source of time.
3. The Suggested REFID Extension Field
Since there is no way in the base NTP packet for "this" instance of
an NTP server to tell the "other" instance what REFID it should use
if the "other" instance decides to use "this" instance as its system
peer, the best available way to convey this information is via an
extension field.
If an incoming packet contains an unrecognized extension field, one
of two things will happen. Either that extension field will be
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ignored, or the entire packet will be dropped. If an extension field
is present there ordinarily SHOULD be a MAC following the extension
field. Some extension fields are unable to be "signed" by a MAC,
regardless of whether or not that MAC is a traditional MAC or an
extension field MAC.
The following paragraph is included for informational purposes.
Another proposal is on its way that addresses and clarifies extension
fields, perhaps in the form of an errata to
https://datatracker.ietf.org/doc/draft-ietf-ntp-extension-field .
To assist in the validation of an NTP packet that contains extension
fields, if the Field Type has the 0x2000 bit set in it, there MAY be
a MAC after this extension field. If the Field Type does not have
the 0x2000 bit set in it, there SHOULD be a MAC after this extension
field.
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
+---------------+---------------+-------------------------------+
| Field Type | Field Length |
+-------------------------------+-------------------------------+
| Suggested REFID |
+---------------------------------------------------------------+
NTP Extension Field: REFID Suggestion
Field Type: TBD (Recommendation for IANA: 0x0006 (Suggested REFID,
MAC required), 0x2006 (Suggested REFID, MAC OPTIONAL))
Field Length: 0x0008
Suggested REFID: The 4 octets of the suggested REFID. This value
SHOULD be 0xFDxxxxxx, where the bottom 3 octets SHOULD be random
values.
Examples: When decoded as an IPv4 address, suggested REFIDs would
decode as 253.0.0.0 thru 253.255.255.255.
4. Generating and Sending the Suggested REFID Extension Field
A system that decides to send a Suggested REFID extension field
SHOULD generate a new Suggested REFID for each new association. It
MAY generate a new Suggested REFID for any association in any
response. In addition to remembering the IP-based REFID, the sender
should also remember the Suggested REFID.
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Since the core NTPv4 and earlier protocols do not contain any way to
tell the recipient what to use as a REFID and RFC 5905 [RFC5905] uses
the IPv4 address of the sender as the REFID if the association is
effected over an IPv4 connection, this means that an attacker can
simply send an NTP client request to a server knowing that server's
system peer will be returned as the REFID in the response packet. At
this point, an attacker can, if that REFID is an IPv4 address, begin
to launch attacks at the target forging the putative IP of the
target's time source, or the attacker can start forging packets to
the putative time server claiming to be from the target, in an
attempt to cause the time server to limit or deny time service to the
target.
Using a nonce for the REFID that is only recognized by the sending
machine effectively prevents this type of attack.
If servers S1, S2, and S3 are all exchanging time with each other,
there is a 1 in 16,777,216 (2^24) chance that two different servers
in the same group will happen to choose the same nonce, and that
would produce a false-positive timing loop detection. If the
Suggested REFID is never changed, this false-positive condition will
occur for potentially a long time. This small risk can be reduced by
periodically generating a new Suggested REFID.
5. Receiving a Suggested REFID Extension Field
An NTP server keeps track of the IP address it uses to talk to a
client. If an NTP server chooses to send a Suggested REFID to an
association, it MUST remember this value. When checking for a timing
loop, the Suggested REFID must also be included in the list of tested
values.
6. Acknowledgements
The author wishes to acknowledge the contributions of Martin Burnicki
and Sam Weiler.
7. IANA Considerations
This memo requests IANA to allocate NTP Extension Field Types 0x0006
(Suggested REFID, MAC required), 0x2006 (Suggested REFID, MAC
OPTIONAL) for this purpose.
8. Security Considerations
Additional information TBD
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9. 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,
<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>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <http://www.rfc-editor.org/info/rfc7384>.
Author's Address
Harlan Stenn
Network Time Foundation
P.O. Box 918
Talent, OR 97540
US
Email: stenn@nwtime.org
Stenn Expires September 15, 2016 [Page 6]
