Port Randomization in the Network Time Protocol Version 4
draft-ietf-ntp-port-randomization-06
Network Time Protocol (ntp) Working Group F. Gont
Internet-Draft G. Gont
Updates: 5905 (if approved) SI6 Networks
Intended status: Standards Track M. Lichvar
Expires: March 19, 2021 Red Hat
September 15, 2020
Port Randomization in the Network Time Protocol Version 4
draft-ietf-ntp-port-randomization-06
Abstract
The Network Time Protocol 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
number. However, in the case of NTP modes where the use of a well-
known port is not required, employing such well-known port
unnecessarily increases the ability of attackers to perform blind/
off-path attacks. This document formally updates RFC5905,
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 19, 2021.
Copyright Notice
Copyright (c) 2020 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Considerations About Port Randomization in NTP . . . . . . . 3
3.1. Mitigation Against Off-path Attacks . . . . . . . . . . . 3
3.2. Effects on Path Selection . . . . . . . . . . . . . . . . 4
3.3. Filtering of NTP traffic . . . . . . . . . . . . . . . . 4
3.4. Effect on NAT devices . . . . . . . . . . . . . . . . . . 5
3.5. Relation to Other Mitigations for Off-Path Attacks . . . 5
4. Update to RFC5905 . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Network Time Protocol (NTP) is one of the oldest Internet
protocols, and currently specified in [RFC5905]. 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 off-path/blind attacks, where an attacker
can send forged packets to one or both NTP peers for achieving 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 [RFC5905]).
Some of these parameters may be easily known or 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 improves the ability of an attacker to perform blind/
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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 a considerable
number of NTP clients employ the NTP well-known port as their local
port, or select predictable ephemeral port numbers, thus improving
the ability of attackers to perform blind/off-path attacks against
NTP.
BCP 156 [RFC6056] already recommends the randomization of transport-
protocol ephemeral ports. This document aligns NTP with the
recommendation in BCP 156 [RFC6056], by formally updating [RFC5905]
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 [RFC2119] [RFC8174] 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 off-path/blind
attacks against transport protocols and upper-layer protocols, such
as [RFC5927] and [RFC4953]. 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, BCP 156 [RFC6056] recommends the
randomization of transport-protocol ephemeral ports. Thus, this
document aims to bring the NTP specification [RFC5905] in line with
the aforementioned recommendation.
We note that the use of port randomization is a transport-layer
mitigation against off-path/blind attacks, and does not preclude (nor
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is it precluded by) other possible mitigations for off-path attacks
that might be implemented by an application protocol (e.g.
[I-D.ietf-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 Multi-Path (ECMP)
algorithm may select the outgoing link by computing a hash over a
number of values, that include 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 may cause different clients in the
same network synchronized to the same NTP server to have a
significant stable offset between their clocks due to their NTP
exchanges consistently taking different paths with different
asymmetry in the network delay.
Section 4 recommends NTP implementations to 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
number, requests/responses cannot be readily differentiated by
inspecting the source and destination port numbers. Implementation
of port randomization for non-symmetrical 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.
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3.4. Effect on NAT devices
Some NAT devices will not translate the source port of a packet when
a privileged port number is employed. In networks where such NAT
devices are employed, use of the NTP well-known port for the client
port will essentially limit the number of hosts that may successfully
employ NTP client implementations.
In the case of NAT devices that will translate the source port even
when a privileged port is employed, packets reaching the external
realm of the NAT will not employ the NTP well-known port as the local
port, since the local port will normally be translated by the NAT
device possibly, but not necessarily, with a random port.
3.5. Relation to Other Mitigations for Off-Path Attacks
Transport-protocol ephemeral port randomization is a best current
practice (BCP 156) that helps mitigate off-path attacks at the
transport-layer. It is orthogonal to other possible mitigations for
off-path attacks that may be implemented at other layers (such as the
use of timestamps in NTP) which may or may not be effective against
some off-path attacks (see e.g. [NTP-FRAG]. This document aligns
NTP with the existing best current practice on ephemeral port
selection, irrespective of other techniques that may (and should) be
implemented for mitigating off-path attacks.
4. Update to RFC5905
The following text from Section 9.1 ("Peer Process Variables") of
[RFC5905]:
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 the IANA. In the
client mode (3), it SHOULD contain a randomized port number, as
specified in [RFC6056]. 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.
If a client implementation performs ephemeral port randomization
on a per-request basis, it SHOULD close the corresponding socket/
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port after each request/response exchange. In order to prevent
duplicate or delayed server packets from eliciting ICMP port
unreachable error messages 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:
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 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 [RFC6056], 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". connect()ing
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. Implementation Status
[RFC Editor: Please remove this section before publication of this
document as an RFC.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
OpenNTPD:
[OpenNTPD] has never explicitly set the local port of NTP clients,
and thus employs the ephemeral port selection algorithm
implemented by the operating system. Thus, on all operating
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systems that implement port randomization (such as current
versions of OpenBSD, Linux, and FreeBSD), OpenNTPD will employ
port randomization for client ports.
chrony:
[chrony] by default does not set the local client port, and thus
employs the ephemeral port selection algorithm implemented by the
operating system. Thus, on all operating systems that implement
port randomization (such as current versions of OpenBSD, Linux,
and FreeBSD), chrony will employ port randomization for client
ports.
nwtime.org's sntp client:
sntp does not explicitly set the local port, and thus employs the
ephemeral port selection algorithm implemented by the operating
system. Thus, on all operating systems that implement port
randomization (such as current versions of OpenBSD, Linux, and
FreeBSD), it will employ port randomization for client ports.
6. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
7. Security Considerations
The security implications of predictable numeric identifiers
[I-D.irtf-pearg-numeric-ids-generation] (and of predictable
transport-protocol port numbers [RFC6056] in particular) have been
known for a long time now. However, the NTP specification has
traditionally followed a pattern of employing common settings and
code even when not strictly necessary, which at times has resulted in
negative security and privacy implications (see e.g.
[I-D.ietf-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 off-path/blind attacks against NTP. Therefore, this document
formally updates [RFC5905], recommending the use of transport-
protocol port randomization when use of the NTP well-known port is
not required.
This issue has been tracked by US-CERT with VU#597821, and has been
assigned CVE-2019-11331.
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8. Acknowledgments
The authors would like to thank (in alphabetical order) Ivan Arce,
Dhruv Dhody, Todd Glassey, Watson Ladd, Aanchal Malhotra, Danny
Mayer, Gary E. Miller, Tomoyuki Sahara, Dieter Sibold, Steven
Sommars, Kristof Teichel, and Ulrich Windl, for providing valuable
comments on earlier 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 nwtime.org's NTP implementation.
Fernando would like to thank Nelida Garcia and Jorge Oscar Gont, for
their love and support.
9. References
9.1. 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>.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[chrony] "chrony", <https://chrony.tuxfamily.org/>.
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[I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-04 (work
in progress), March 2019.
[I-D.irtf-pearg-numeric-ids-generation]
Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", draft-irtf-pearg-numeric-ids-
generation-03 (work in progress), September 2020.
[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,
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, Monterey, California pp. 271-290,
January 2017, <http://leapsecond.com/ntp/
NTP_Paper_Sommars_PTTI2017.pdf>.
[NTP-FRAG]
Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
"Attacking the Network Time Protocol", NDSS'17, San Diego,
CA. Feb 2017, 2017,
<http://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf>.
[NTP-security]
Malhotra, A., Van Gundy, M., Varia, V., Kennedy, H.,
Gardner, J., and S. Goldberg, "The Security of NTP's
Datagram Protocol", Cryptology ePrint Archive Report
2016/1006, 2016, <https://eprint.iacr.org/2016/1006>.
[NTP-VULN]
Network Time Foundation, "Security Notice", Network Time
Foundation's NTP Support Wiki ,
<https://support.ntp.org/bin/view/Main/SecurityNotice>.
[OpenNTPD]
"OpenNTPD Project", <https://www.openntpd.org>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.
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[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Authors' Addresses
Fernando Gont
SI6 Networks
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: https://www.si6networks.com
Guillermo Gont
SI6 Networks
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: ggont@si6networks.com
URI: https://www.si6networks.com
Miroslav Lichvar
Red Hat
Purkynova 115
Brno 612 00
Czech Republic
Email: mlichvar@redhat.com
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