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IETF RFC 9109
Last modified on Tuesday, August 24th, 2021
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Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 9109 G. Gont
Updates: 5905 SI6 Networks
Category: Standards Track M. Lichvar
ISSN: 2070-1721 Red Hat
August 2021
Network Time Protocol Version 4: Port Randomization
 Abstract
The Network Time Protocol (NTP) can operate in several modes. Some
of these modes are based on the receipt of unsolicited packets and
therefore require the use of a well-known port as the local port.
However, in the case of NTP modes where the use of a well-known port
is not required, employing such a well-known port unnecessarily
facilitates the ability of attackers to perform blind/off-path
attacks. This document formally updates RFC 5905, recommending the
use of transport-protocol ephemeral port randomization for those
modes where use of the NTP well-known port is not required.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9109.
Copyright Notice
Copyright (c) 2021 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Considerations about Port Randomization in NTP
3.1. Mitigation against Off-Path Attacks
3.2. Effects on Path Selection
3.3. Filtering of NTP Traffic
3.4. Effect on NAPT Devices
4. Update to RFC 5905
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The Network Time Protocol (NTP) is one of the oldest Internet
protocols and is currently specified in [RFC 5905]. Since its
original implementation, standardization, and deployment, a number of
vulnerabilities have been found both in the NTP specification and in
some of its implementations [NTP-VULN]. Some of these
vulnerabilities allow for blind/off-path attacks, where an attacker
can send forged packets to one or both NTP peers to achieve Denial of
Service (DoS), time shifts, or other undesirable outcomes. Many of
these attacks require the attacker to guess or know at least a target
NTP association, typically identified by the tuple {srcaddr, srcport,
dstaddr, dstport, keyid} (see Section 9.1 of [RFC 5905]). Some of
these parameters may be known or easily guessed.
NTP can operate in several modes. Some of these modes rely on the
ability of nodes to receive unsolicited packets and therefore require
the use of the NTP well-known port (123). However, for modes where
the use of a well-known port is not required, employing the NTP well-
known port unnecessarily facilitates the ability of attackers to
perform blind/off-path attacks (since knowledge of the port numbers
is typically required for such attacks). A recent study [NIST-NTP]
that analyzes the port numbers employed by NTP clients suggests that
numerous NTP clients employ the NTP well-known port as their local
port, or select predictable ephemeral port numbers, thus
unnecessarily facilitating the ability of attackers to perform blind/
off-path attacks against NTP.
BCP 156 [RFC 6056] already recommends the randomization of transport-
protocol ephemeral ports. This document aligns NTP with the
recommendation in BCP 156 [RFC 6056] by formally updating [RFC 5905]
such that port randomization is employed for those NTP modes for
which the use of the NTP well-known port is not needed.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. Considerations about Port Randomization in NTP
The following subsections analyze a number of considerations about
transport-protocol ephemeral port randomization when applied to NTP.
3.1. Mitigation against Off-Path Attacks
There has been a fair share of work in the area of blind/off-path
attacks against transport protocols and upper-layer protocols, such
as [RFC 4953] and [RFC 5927]. Whether the target of the attack is a
transport-protocol instance (e.g., TCP connection) or an upper-layer
protocol instance (e.g., an application-protocol instance), the
attacker is required to know or guess the five-tuple {Protocol, IP
Source Address, IP Destination Address, Source Port, Destination
Port} that identifies the target transport-protocol instance or the
transport-protocol instance employed by the target upper-layer
protocol instance. Therefore, increasing the difficulty of guessing
this five-tuple helps mitigate blind/off-path attacks.
As a result of these considerations, transport-protocol ephemeral
port randomization is a best current practice (BCP 156) that helps
mitigate off-path attacks at the transport layer. This document
aligns the NTP specification [RFC 5905] with the existing best current
practice on transport-protocol ephemeral port selection, irrespective
of other techniques that may (and should) be implemented for
mitigating off-path attacks.
We note that transport-protocol ephemeral port randomization is a
transport-layer mitigation against blind/off-path attacks and does
not preclude (nor is it precluded by) other possible mitigations for
off-path attacks that might be implemented at other layers (e.g.,
[NTP-DATA-MINIMIZATION]). For instance, some of the aforementioned
mitigations may be ineffective against some off-path attacks
[NTP-FRAG] or may benefit from the additional entropy provided by
port randomization [NTP-security].
3.2. Effects on Path Selection
Intermediate systems implementing the Equal-Cost Multipath (ECMP)
algorithm may select the outgoing link by computing a hash over a
number of values, including the transport-protocol source port.
Thus, as discussed in [NTP-CHLNG], the selected client port may have
an influence on the measured offset and delay.
If the source port is changed with each request, packets in different
exchanges will be more likely to take different paths, which could
cause the measurements to be less stable and have a negative impact
on the stability of the clock.
Network paths to/from a given server are less likely to change
between requests if port randomization is applied on a per-
association basis. This approach minimizes the impact on the
stability of NTP measurements, but it may cause different clients in
the same network synchronized to the same NTP server to have a
significant stable offset between their clocks. This is due to their
NTP exchanges consistently taking different paths with different
asymmetry in the network delay.
Section 4 recommends that NTP implementations randomize the ephemeral
port number of client/server associations. The choice of whether to
randomize the port number on a per-association or a per-request basis
is left to the implementation.
3.3. Filtering of NTP Traffic
In a number of scenarios (such as when mitigating DDoS attacks), a
network operator may want to differentiate between NTP requests sent
by clients and NTP responses sent by NTP servers. If an
implementation employs the NTP well-known port for the client port,
requests/responses cannot be readily differentiated by inspecting the
source and destination port numbers. Implementation of port
randomization for nonsymmetrical modes allows for simple
differentiation of NTP requests and responses and for the enforcement
of security policies that may be valuable for the mitigation of DDoS
attacks, when all NTP clients in a given network employ port
randomization.
3.4. Effect on NAPT Devices
Some NAPT devices will reportedly not translate the source port of a
packet when a system port number (i.e., a port number in the range
0-1023) [RFC 6335] is employed. In networks where such NAPT devices
are employed, use of the NTP well-known port for the client port may
limit the number of hosts that may successfully employ NTP client
implementations at any given time.
| NOTES:
|
| NAPT devices are defined in Section 4.1.2 of [RFC 2663].
|
| The reported behavior is similar to the special treatment of
| UDP port 500, which has been documented in Section 2.3 of
| [RFC 3715].
In the case of NAPT devices that will translate the source port even
when a system port is employed, packets reaching the external realm
of the NAPT will not employ the NTP well-known port as the source
port, as a result of the port translation function being performed by
the NAPT device.
4. Update to RFC 5905
The following text from Section 9.1 (Peer Process Variables) of
[RFC 5905]:
| dstport: UDP port number of the client, ordinarily the NTP port
| number PORT (123) assigned by the IANA. This becomes the
| source port number in packets sent from this association.
is replaced with:
| dstport: UDP port number of the client. In the case of broadcast
| server mode (5) and symmetric modes (1 and 2), it SHOULD
| contain the NTP port number PORT (123) assigned by IANA. In
| the client mode (3), it SHOULD contain a randomized port
| number, as specified in [RFC 6056]. The value in this variable
| becomes the source port number of packets sent from this
| association. The randomized port number SHOULD NOT be shared
| with other associations, to avoid revealing the randomized port
| to other associations.
|
| If a client implementation performs transport-protocol
| ephemeral port randomization on a per-request basis, it SHOULD
| close the corresponding socket/port after each request/response
| exchange. In order to prevent duplicate or delayed server
| packets from eliciting ICMP port unreachable error messages
| [RFC 792] [RFC 4443] at the client, the client MAY wait for more
| responses from the server for a specific period of time (e.g.,
| 3 seconds) before closing the UDP socket/port.
|
|
| NOTES:
|
| Randomizing the ephemeral port number on a per-request basis
| will better mitigate blind/off-path attacks, particularly if
| the socket/port is closed after each request/response
| exchange, as recommended above. The choice of whether to
| randomize the ephemeral port number on a per-request or a
| per-association basis is left to the implementation, and it
| should consider the possible effects on path selection along
| with its possible impact on time measurement.
|
| On most current operating systems, which implement ephemeral
| port randomization [RFC 6056], an NTP client may normally
| rely on the operating system to perform ephemeral port
| randomization. For example, NTP implementations using POSIX
| sockets may achieve ephemeral port randomization by _not_
| binding the socket with the bind() function or binding it to
| port 0, which has a special meaning of "any port". Using
| the connect() function for the socket will make the port
| inaccessible by other systems (that is, only packets from
| the specified remote socket will be received by the
| application).
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
The security implications of predictable numeric identifiers
[PEARG-NUMERIC-IDS] (and of predictable transport-protocol port
numbers [RFC 6056] in particular) have been known for a long time now.
However, the NTP specification has traditionally followed a pattern
of employing common settings even when not strictly necessary, which
at times has resulted in negative security and privacy implications
(see, e.g., [NTP-DATA-MINIMIZATION]). The use of the NTP well-known
port (123) for the srcport and dstport variables is not required for
all operating modes. Such unnecessary usage comes at the expense of
reducing the amount of work required for an attacker to successfully
perform blind/off-path attacks against NTP. Therefore, this document
formally updates [RFC 5905], recommending the use of transport-
protocol port randomization when use of the NTP well-known port is
not required.
This issue has been assigned CVE-2019-11331 [VULN-REPORT] in the U.S.
National Vulnerability Database (NVD).
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC 5905, June 2010,
<https://www.rfc-editor.org/info/RFC 5905>.
[RFC 6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC 6056, January 2011,
<https://www.rfc-editor.org/info/RFC 6056>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
7.2. Informative References
[NIST-NTP] Sherman, J. and J. Levine, "Usage Analysis of the NIST
Internet Time Service", Journal of Research of the
National Institute of Standards and Technology, Volume
121, DOI 10.6028/jres.121.003, March 2016,
<https://tf.nist.gov/general/pdf/2818.pdf>.
[NTP-CHLNG]
Sommars, S., "Challenges in Time Transfer using the
Network Time Protocol (NTP)", Proceedings of the 48th
Annual Precise Time and Time Interval Systems and
Applications Meeting, pp. 271-290,
DOI 10.33012/2017.14978, January 2017,
<http://leapsecond.com/ntp/
NTP_Paper_Sommars_PTTI2017.pdf>.
[NTP-DATA-MINIMIZATION]
Franke, D. and A. Malhotra, "NTP Client Data
Minimization", Work in Progress, Internet-Draft, draft-
ietf-ntp-data-minimization-04, 25 March 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-ntp-
data-minimization-04>.
[NTP-FRAG] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
"Attacking the Network Time Protocol", NDSS '16,
DOI 10.14722/ndss.2016.23090, February 2016,
<https://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf>.
[NTP-security]
Malhotra, A., Van Gundy, M., Varia, M., Kennedy, H.,
Gardner, J., and S. Goldberg, "The Security of NTP's
Datagram Protocol", Cryptology ePrint Archive Report
2016/1006, DOI 10.1007/978-3-319-70972-7_23, February
2017, <https://eprint.iacr.org/2016/1006.pdf>.
[NTP-VULN] "Network Time Foundation",
<http://support.ntp.org/bin/view/Main/SecurityNotice>.
[PEARG-NUMERIC-IDS]
Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", Work in Progress, Internet-Draft,
draft-irtf-pearg-numeric-ids-generation-07, 2 February
2021, <https://datatracker.ietf.org/doc/html/draft-irtf-
pearg-numeric-ids-generation-07>.
[RFC 792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC 792, September 1981,
<https://www.rfc-editor.org/info/RFC 792>.
[RFC 2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations",
RFC 2663, DOI 10.17487/RFC 2663, August 1999,
<https://www.rfc-editor.org/info/RFC 2663>.
[RFC 3715] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715,
DOI 10.17487/RFC 3715, March 2004,
<https://www.rfc-editor.org/info/RFC 3715>.
[RFC 4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC 4443, March 2006,
<https://www.rfc-editor.org/info/RFC 4443>.
[RFC 4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC 4953, July 2007,
<https://www.rfc-editor.org/info/RFC 4953>.
[RFC 5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC 5927, July 2010,
<https://www.rfc-editor.org/info/RFC 5927>.
[RFC 6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC 6335, August 2011,
<https://www.rfc-editor.org/info/RFC 6335>.
[VULN-REPORT]
The MITRE Corporation, "CVE-2019-1133", National
Vulnerability Database, August 2020,
<https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-
2019-11331>.
Acknowledgments
The authors would like to thank (in alphabetical order) Ivan Arce,
Roman Danyliw, Dhruv Dhody, Lars Eggert, Todd Glassey, Blake Hudson,
Benjamin Kaduk, Erik Kline, Watson Ladd, Aanchal Malhotra, Danny
Mayer, Gary E. Miller, Bjorn Mork, Hal Murray, Francesca Palombini,
Tomoyuki Sahara, Zaheduzzaman Sarker, Dieter Sibold, Steven Sommars,
Jean St-Laurent, Kristof Teichel, Brian Trammell, Éric Vyncke, Ulrich
Windl, and Dan Wing for providing valuable comments on earlier draft
versions of this document.
Watson Ladd raised the problem of DDoS mitigation when the NTP well-
known port is employed as the client port (discussed in Section 3.3
of this document).
The authors would like to thank Harlan Stenn for answering questions
about a popular NTP implementation (see <https://www.nwtime.org>).
Fernando Gont would like to thank Nelida Garcia and Jorge Oscar Gont
for their love and support.
Authors' Addresses
Fernando Gont
SI6 Networks
Evaristo Carriego 2644
1706 Haedo, Provincia de Buenos Aires
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Guillermo Gont
SI6 Networks
Evaristo Carriego 2644
1706 Haedo, Provincia de Buenos Aires
Argentina
Phone: +54 11 4650 8472
Email: ggont@si6networks.com
URI: https://www.si6networks.com
Miroslav Lichvar
Red Hat
Purkynova 115
612 00 Brno
Czech Republic
Email: mlichvar@redhat.com
RFC TOTAL SIZE: 20815 bytes
PUBLICATION DATE: Tuesday, August 24th, 2021
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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