draft-ietf-ntp-bcp-10.txt   draft-ietf-ntp-bcp-11.txt 
Internet Engineering Task Force D. Reilly, Ed. Internet Engineering Task Force D. Reilly, Ed.
Internet-Draft Orolia USA Internet-Draft Orolia USA
Intended status: Best Current Practice H. Stenn Intended status: Best Current Practice H. Stenn
Expires: June 16, 2019 Network Time Foundation Expires: July 21, 2019 Network Time Foundation
D. Sibold D. Sibold
PTB PTB
December 13, 2018 January 17, 2019
Network Time Protocol Best Current Practices Network Time Protocol Best Current Practices
draft-ietf-ntp-bcp-10 draft-ietf-ntp-bcp-11
Abstract Abstract
The Network Time Protocol (NTP) is one of the oldest protocols on the The Network Time Protocol (NTP) is one of the oldest protocols on the
Internet and has been widely used since its initial publication. Internet and has been widely used since its initial publication.
This document is a collection of Best Practices for general operation This document is a collection of Best Practices for general operation
of NTP servers and clients on the Internet. It includes of NTP servers and clients on the Internet. It includes
recommendations for stable, accurate and secure operation of NTP recommendations for stable, accurate and secure operation of NTP
infrastructure. This document is targeted at NTP version 4 as infrastructure. This document is targeted at NTP version 4 as
described in RFC 5905. described in RFC 5905.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 16, 2019. This Internet-Draft will expire on July 21, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. General Network Security Best Practices . . . . . . . . . . . 3 2. General Network Security Best Practices . . . . . . . . . . . 3
2.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. NTP Configuration Best Practices . . . . . . . . . . . . . . 4 3. NTP Configuration Best Practices . . . . . . . . . . . . . . 4
3.1. Keeping NTP up to date . . . . . . . . . . . . . . . . . 4 3.1. Keeping NTP up to date . . . . . . . . . . . . . . . . . 4
3.2. Use enough time sources . . . . . . . . . . . . . . . . . 4 3.2. Use enough time sources . . . . . . . . . . . . . . . . . 4
3.3. Use a diversity of Reference Clocks . . . . . . . . . . . 5 3.3. Use a diversity of Reference Clocks . . . . . . . . . . . 5
3.4. Control Messages . . . . . . . . . . . . . . . . . . . . 6 3.4. Control Messages . . . . . . . . . . . . . . . . . . . . 6
3.5. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 7 3.5. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 6
3.6. Using Pool Servers . . . . . . . . . . . . . . . . . . . 7 3.6. Using Pool Servers . . . . . . . . . . . . . . . . . . . 7
3.7. Leap Second Handling . . . . . . . . . . . . . . . . . . 8 3.7. Leap Second Handling . . . . . . . . . . . . . . . . . . 7
3.7.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 9 3.7.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 8
4. NTP Security Mechanisms . . . . . . . . . . . . . . . . . . . 10 4. NTP Security Mechanisms . . . . . . . . . . . . . . . . . . . 9
4.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . . . 10 4.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . . . 10
4.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Network Time Security . . . . . . . . . . . . . . . . . . 11 4.3. Network Time Security . . . . . . . . . . . . . . . . . . 11
4.4. External Security Protocols . . . . . . . . . . . . . . . 11
5. NTP Security Best Practices . . . . . . . . . . . . . . . . . 11 5. NTP Security Best Practices . . . . . . . . . . . . . . . . . 11
5.1. Minimizing Information Leakage . . . . . . . . . . . . . 11 5.1. Minimizing Information Leakage . . . . . . . . . . . . . 11
5.2. Avoiding Daemon Restart Attacks . . . . . . . . . . . . . 12 5.2. Avoiding Daemon Restart Attacks . . . . . . . . . . . . . 12
5.3. Detection of Attacks Through Monitoring . . . . . . . . . 13 5.3. Detection of Attacks Through Monitoring . . . . . . . . . 13
5.4. Kiss-o'-Death Packets . . . . . . . . . . . . . . . . . . 13 5.4. Kiss-o'-Death Packets . . . . . . . . . . . . . . . . . . 14
5.5. Broadcast Mode Should Only Be Used On Trusted Networks . 14 5.5. Broadcast Mode Should Only Be Used On Trusted Networks . 14
5.6. Symmetric Mode Should Only Be Used With Trusted Peers . . 14 5.6. Symmetric Mode Should Only Be Used With Trusted Peers . . 15
6. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15 6. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15
6.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 15 6.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 15
6.2. Server configuration . . . . . . . . . . . . . . . . . . 15 6.2. Server configuration . . . . . . . . . . . . . . . . . . 16
6.2.1. NTP Pool Project Vendor Subdomains . . . . . . . . . 16
7. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 16 7. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 17 10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18 11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 18 11.2. Informative References . . . . . . . . . . . . . . . . . 19
11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20 11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix A. NTP Implementation by the Network Time Appendix A. Best Practices specific to the Network Time
Foundation . . . . . . . . . . . . . . . . . . . . . 21 Foundation implementation . . . . . . . . . . . . . 21
A.1. Use enough time sources . . . . . . . . . . . . . . . . . 21 A.1. Use enough time sources . . . . . . . . . . . . . . . . . 21
A.2. NTP Control and Facility Messages . . . . . . . . . . . . 21 A.2. NTP Control and Facility Messages . . . . . . . . . . . . 22
A.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22 A.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22
A.4. Leap Second File . . . . . . . . . . . . . . . . . . . . 22 A.4. Leap Second File . . . . . . . . . . . . . . . . . . . . 23
A.5. Leap Smearing . . . . . . . . . . . . . . . . . . . . . . 23 A.5. Leap Smearing . . . . . . . . . . . . . . . . . . . . . . 23
A.6. Configuring ntpd . . . . . . . . . . . . . . . . . . . . 23 A.6. Configuring ntpd . . . . . . . . . . . . . . . . . . . . 23
A.7. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . . . 23 A.7. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction 1. Introduction
NTP version 4 (NTPv4) has been widely used since its publication as NTP version 4 (NTPv4) has been widely used since its publication as
[RFC5905]. This documentation is a collection of best practices for [RFC5905]. This document is a collection of best practices for the
the operation of NTP clients and servers. operation of NTP clients and servers.
The recommendations in this document are intended to help operators The recommendations in this document are intended to help operators
distribute time on their networks more accurately and more securely. distribute time on their networks more accurately and more securely.
It is intended to apply generally to a broad range of networks. Some It is intended to apply generally to a broad range of networks. Some
specific networks may have higher accuracy requirements that require specific networks may have higher accuracy requirements that require
additional techniques beyond what is documented here. additional techniques beyond what is documented here.
Among the best practices covered are recommendations for general Among the best practices covered are recommendations for general
network security, time protocol specific security, and NTP server and network security, time protocol specific security, and NTP server and
client configuration. NTP operation in embedded devices is also client configuration. NTP operation in embedded devices is also
covered. covered.
This document also contains information for protocol implementors who This document also contains information for protocol implementors who
want to develop their own RFC 5905 compliant implementations. want to develop their own implementations that are compliant to RFC
5905.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. General Network Security Best Practices 2. General Network Security Best Practices
2.1. BCP 38 2.1. BCP 38
Many network attacks rely on modifying the IP source address of a Many network attacks rely on modifying the IP source address of a
packet to point to a different IP address than the computer which packet to point to a different IP address than the computer which
originated it. UDP-based protocols such as NTP are generally more originated it. UDP-based protocols such as NTP are generally more
susceptible to spoofing attacks then other connection-oriented susceptible to spoofing attacks than connection-oriented protocols.
protocols. NTP control messages can generate a lot of data in NTP control messages can generate a lot of data in response to a
response to a small query, which makes it more attractive as a vector small query, which makes it attractive as a vector for distributed
for distributed denial-of-service attacks. (NTP Control messages are denial-of-service attacks. (NTP Control messages are discussed
discussed further in Section 3.4). One documented instance of such further in Section 3.4). One documented instance of such an attack
an attack can be found here [1], and further discussion in [IMC14] can be found here [1], and further discussion in [IMC14] and
and [NDSS14]. Mitigating source address spoofing attacks should be a [NDSS14].
priority of anyone administering NTP.
BCP 38 [RFC2827] was approved in 2000 to address this. BCP 38 calls Mitigating source address spoofing attacks should be a priority of
for filtering outgoing and incoming traffic to make sure that the anyone administering NTP. BCP 38 [RFC2827] was published in 2000 to
source and destination IP addresses are consistent with the expected to provide some level of remediation against address-spoofing
flow of traffic on each network interface. It is RECOMMENDED that attacks. BCP 38 calls for filtering outgoing and incoming traffic to
large corporate networks (and ISP's of any size) implement ingress make sure that the source and destination IP addresses are consistent
with the expected flow of traffic on each network interface. It is
RECOMMENDED that ISP's and large corporate networks implement ingress
and egress filtering. More information is available at the BCP38 and egress filtering. More information is available at the BCP38
Info Web page [BCP38INFO] . Info Web page [2] .
3. NTP Configuration Best Practices 3. NTP Configuration Best Practices
This section provides Best Practices for NTP configuration and This section provides Best Practices for NTP configuration and
operation. Best Practices that are specific to the NTF operation. Best Practices that are specific to the Network Time
implementation are compiled in Appendix A. Foundation implementation are compiled in Appendix A.
3.1. Keeping NTP up to date 3.1. Keeping NTP up to date
Many network security mechanisms rely on time as part of their
operation. If attackers can spoof the time, they may be able to
bypass or neutralize other security elements. For example, incorrect
time can disrupt the ability to reconcile logfile entries on the
affected system with events on other systems. An application which
is secure today could be insecure tomorrow once an unknown bug (or a
known behavior) is exploited in the right way. Even our definition
of what is secure has evolved over the years, so code which was
considered secure when it was written may turn out to be insecure
after some time.
There are multiple versions of the NTP protocol in use, and multiple There are multiple versions of the NTP protocol in use, and multiple
implementations, on many different platforms. The practices in this implementations, on many different platforms. The practices in this
document are meant to apply generally to any implementation of document are meant to apply generally to any implementation of
[RFC5905]. It is RECOMMENDED that that NTP users select an [RFC5905]. NTP users should select an implementation that is
implementation that is actively maintained. Users should keep up to actively maintained. Users should keep up to date on any known
date on any known attacks on their selected implementation, and attacks on their selected implementation, and deploy updates
deploy updates containing security fixes as soon as practical. containing security fixes as soon as practical.
3.2. Use enough time sources 3.2. Use enough time sources
An NTP implementation that is compliant with [RFC5905] takes the An NTP implementation that is compliant with [RFC5905] takes the
available sources of time and submits this timing data to available sources of time and submits this timing data to
sophisticated intersection, clustering, and combining algorithms to sophisticated intersection, clustering, and combining algorithms to
get the best estimate of the correct time. The description of these get the best estimate of the correct time. The description of these
algorithms is beyond the scope of this document. Interested readers algorithms is beyond the scope of this document. Interested readers
should read [RFC5905] or the detailed description of NTP in should read [RFC5905] or the detailed description of NTP in
[MILLS2006]. [MILLS2006].
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diverse (Section 3.3). If one of these sources develops a problem diverse (Section 3.3). If one of these sources develops a problem
there are still at least 3 other time sources. there are still at least 3 other time sources.
Operators who are concerned with maintaining accurate time SHOULD use Operators who are concerned with maintaining accurate time SHOULD use
at least 4 independent, diverse sources of time. Four sources will at least 4 independent, diverse sources of time. Four sources will
provide sufficient backup in case one source goes down. If four provide sufficient backup in case one source goes down. If four
sources are not available, operators MAY use fewer sources, subject sources are not available, operators MAY use fewer sources, subject
to the risks outlined above. to the risks outlined above.
But even with 4 or more sources of time, systemic problems can But even with 4 or more sources of time, systemic problems can
happen. For several hours before and after the June 2015 leap happen. One example involves the leap smearing concept detailed in
second, several operators implemented leap smearing while others did Section 3.7.1. For several hours before and after the June 2015 leap
not, and many NTP end nodes could not determine an accurate time second, several operators configured their NTP servers with leap
source because 2 of their 4 sources of time gave them consistent UTC/ smearing while others did not. Many NTP end nodes could not
POSIX time, while the other 2 gave them consistent leap-smeared time. determine an accurate time source because 2 of their 4 sources of
See Section 3.7.1 for more information. time gave them consistent UTC/POSIX time, while the other 2 gave them
consistent leap-smeared time. This is just one of many potential
causes of disagreement among time sources.
Operators SHOULD monitor all of the time sources that are in use. If Operators are advised to monitor all time sources that are in use.
time sources do not generally agree, find out the cause and either If time sources do not generally agree, operators are encouraged to
correct the problems or stop using defective servers. See investigate the cause of this and either correct the problems or stop
Section 3.5 for more information. using defective servers. See Section 3.5 for more information.
3.3. Use a diversity of Reference Clocks 3.3. Use a diversity of Reference Clocks
When using servers with attached hardware reference clocks, it is When using servers with attached hardware reference clocks, it is
RECOMMENDED that several different types of reference clocks be used. suggested that different types of reference clocks be used. Having a
Having a diversity of sources with independent implementations means diversity of sources with independent implementations means that any
that any one issue is less likely to cause a service interruption. one issue is less likely to cause a service interruption.
Are all clocks on a network from the same vendor? They may have the Are all clocks on a network from the same vendor? They may have the
same bugs. Even devices from different vendors may not be truly same bugs. Even devices from different vendors may not be truly
independent if they share common elements. Are they using the same independent if they share common elements. Are they using the same
base chipset? Are they all running the same version of firmware? base chipset? Are they all running the same version of firmware?
Chipset and firmware bugs can happen, but they can be more difficult Chipset and firmware bugs can happen, but they can be more difficult
to diagnose than application software bugs. When having the correct to diagnose than application software bugs. When having the correct
time is of critical importance, it's ultimately up to operators to time is of critical importance, it's ultimately up to operators to
ensure that their sources are sufficiently independent, even if they ensure that their sources are sufficiently independent, even if they
are not under the operator's control. are not under the operator's control.
A systemic problem with time from any satellite navigation service is A systemic problem with time from any satellite navigation service is
possible and has happened. Sunspot activity can render satellite or possible and has happened. Sunspot activity can render satellite or
radio-based time source unusable. If the time on your network has to radio-based time source unusable. Depending on the application
be correct close to 100% of the time, then even if you are using a requirements, operators may need to consider backup scenarios in the
satellite-based system, operators need to plan for those rare rare circumstance when the satellite system is faulty or unavailable.
instances when the system is unavailable (or wrong!).
3.4. Control Messages 3.4. Control Messages
Some implementations of NTPv4 provide the NTP Control Messages that Some implementations of NTPv4 provide the NTP Control Messages (also
were originally specified in Appendix B of [RFC1305] which defined known as Mode 6 messages) that were originally specified in
NTPv3. These messages were never included the NTPv4 specification, Appendix B of [RFC1305] which defined NTPv3. These messages were
but they are still used. Work is being done to formally document the never included the NTPv4 specification, but they are still used. At
the time of this writing, work is being done to formally document the
structure of these control messages in [I-D.ietf-ntp-mode-6-cmds]. structure of these control messages in [I-D.ietf-ntp-mode-6-cmds].
The NTP Control Messages are designed to permit monitoring and The NTP Control Messages are designed to permit monitoring and
optionally authenticated control of NTP and its configuration. Used optionally authenticated control of NTP and its configuration. Used
properly, these facilities provide vital debugging and performance properly, these facilities provide vital debugging and performance
information and control. Used improperly, these facilities can be an information and control. But these facilities can be a vector for
abuse vector. For this reason, it is RECOMMENDED that publicly- amplification attacks when abused. For this reason, it is
facing NTP servers should block mode 6 queries from outside their RECOMMENDED that publicly-facing NTP servers should block NTP Control
organization. Message queries from outside their organization.
The ability to use Mode 6 beyond its basic monitoring capabilities The ability to use NTP Control Messages beyond their basic monitoring
SHOULD be limited to authenticated sessions that provide a capabilities SHOULD be limited to authenticated sessions that provide
'controlkey'. It MAY also be limited through mechanisms outside of a 'controlkey'. It can also be limited through mechanisms outside of
the NTP specification, such as Access Control Lists, that only allow the NTP specification, such as Access Control Lists, that only allow
access from approved IP addresses. access from approved IP addresses.
The NTP Control Messages responses are much larger than the The NTP Control Messages responses are much larger than the
corresponding queries. Thus, they can be abused in high-bandwidth corresponding queries. Thus, they can be abused in high-bandwidth
DDoS attacks. To provide protection for such abuse NTP server DDoS attacks. Section 2.1 gives more information on how to provide
operators on large networks SHOULD deploy ingress filtering in protection for this abuse by implementing BCP 38.
accordance with BCP 38 [RFC2827].
3.5. Monitoring 3.5. Monitoring
Operators SHOULD use their NTP implementation's remote monitoring Operators SHOULD use their NTP implementation's remote monitoring
capabilities to quickly identify servers which are out of sync, and capabilities to quickly identify servers which are out of sync, and
ensure correctness of the service. Operators SHOULD also monitor ensure correctness of the service. Operators SHOULD also monitor
system logs for messages so problems and abuse attempts can be system logs for messages so problems and abuse attempts can be
quickly identified. quickly identified.
If a system starts getting unexpected time replies from its time If a system starts to receive NTP Reply packets from a time server
servers, that can be an indication that the IP address of the system that do not correspond to any requests sent by the system, that can
is being forged in requests to its time server. The goal of this be an indication that an attacker is forging that system's IP address
attack is to convince the time server to stop serving time to the in requests to the remote time server. The goal of this attack would
system whose address is being forged. be to convince the time server to stop serving time to the system
whose address is being forged.
If a system is a broadcast client and its system log shows that it is If a system is a broadcast client and its system log shows that it is
receiving early time messages from its server, that is an indication receiving early time messages from its server, that is an indication
that somebody may be forging packets from a broadcast server. that somebody may be forging packets from a broadcast server.
(Broadcast client and server modes are defined in Section 3 of
[RFC5905])
If a server's system log shows messages that indicates it is If a server's system log shows messages that indicates it is
receiving timestamps that are earlier than the current system time, receiving NTP timestamps that are much earlier than the current
then either the system clock is unusually fast or somebody is trying system time, then either the system clock is unusually fast or
to launch a replay attack against that server. somebody is trying to launch a replay attack against that server.
3.6. Using Pool Servers 3.6. Using Pool Servers
It only takes a small amount of bandwidth and system resources to It only takes a small amount of bandwidth and system resources to
synchronize one NTP client, but NTP servers that can service tens of synchronize one NTP client, but NTP servers that can service tens of
thousands of clients take more resources to run. Users who want to thousands of clients take more resources to run. Network operators
synchronize their computers SHOULD only synchronize to servers that and advanced users who want to synchronize their computers MUST only
they have permission to use. synchronize to servers that they have permission to use.
The NTP pool project is a group of volunteers who have donated their The NTP Pool Project is a group of volunteers who have donated their
computing and bandwidth resources to freely distribute time from computing and bandwidth resources to freely distribute time from
primary time sources to others on the Internet. The time is primary time sources to others on the Internet. The time is
generally of good quality, but comes with no guarantee whatsoever. generally of good quality but comes with no guarantee whatsoever. If
If you are interested in using the pool, please review their you are interested in using this pool, please review their
instructions at http://www.pool.ntp.org/en/use.html [2]. instructions at http://www.pool.ntp.org/en/use.html [3].
If you are a vendor who wishes to provide time service to your Vendors can obtain their own subdomain that is part of the NTP Pool
customers or clients, consider joining the pool and providing a Project. This offers vendors the ability to safely make use of the
"vendor zone" through the pool project. time distributed by the pool for their devices. Details are
available at http://www.pool.ntp.org/en/vendors.html [4] .
If you want to synchronize many computers, consider running your own If there is a need to synchronize many computers, an operator may
NTP servers that are synchronized by the pool, and synchronizing your want to run local NTP servers that are synchronized to the NTP Pool
clients to your in-house NTP servers. This reduces the load on the Project. NTP users on that operator's networks can then be
pool. synchronized to local NTP servers.
3.7. Leap Second Handling 3.7. Leap Second Handling
UTC is kept in agreement with the astronomical time UT1 [3] to within UTC is kept in agreement with the astronomical time UT1 [5] to within
+/- 0.9 seconds by the insertion (or possibly a deletion) of a leap +/- 0.9 seconds by the insertion (or possibly a deletion) of a leap
second. UTC is an atomic time scale whereas UT1 is based on the second. UTC is an atomic time scale whereas UT1 is based on the
rotational rate of the earth. Leap seconds are not introduced at a rotational rate of the earth. Leap seconds are not introduced at a
fixed rate. They are announced by the International Earth Rotation fixed rate. They are announced by the International Earth Rotation
and Reference Systems Service (IERS) in its Bulletin C [4] when and Reference Systems Service (IERS) in its Bulletin C [6] when
necessary to keep UTC and UT1 aligned. necessary to keep UTC and UT1 aligned.
NTP time is based on the UTC timescale, and the protocol has the NTP time is based on the UTC timescale, and the protocol has the
capability to broadcast leap second information. Some Global capability to broadcast leap second information. Some Global
Navigation Satellite Systems (like GPS) or radio transmitters (like Navigation Satellite Systems (like GPS) or radio transmitters (like
DCF77) broadcast leap second information, so if you are synced to an DCF77) broadcast leap second information. If an NTP client is synced
NTP server that is ultimately synced to a source that provides leap to an NTP server that provides leap second notification, the client
second notification you will get advance notification of impending will get advance notification of impending leap seconds
leap seconds automatically. automatically.
Since the length of the UT1 day is generally slowly increasing [5], Since the length of the UT1 day is generally slowly increasing [7],
all leap seconds that have been introduced since the practice started all leap seconds that have been introduced since the practice started
in 1972 have been positive leap seconds, where a second is added to in 1972 have been positive leap seconds, where a second is added to
UTC. NTP also supports a negative leap second, where a second is UTC. NTP also supports a negative leap second, where a second is
removed from UTC, if that ever becomes necessary. removed from UTC, if that ever becomes necessary.
While earlier versions of NTP contained some ambiguity regarding when While earlier versions of NTP contained some ambiguity regarding when
a leap second that is broadcast by a server should be applied by a a leap second that is broadcast by a server should be applied by a
client, RFC 5905 is clear that leap seconds are only applied on the client, RFC 5905 is clear that leap seconds are only applied on the
last day of a month. However, because some older clients may apply last day of a month. However, because some older clients may apply
it at the end of the current day, it is RECOMMENDED that NTP servers it at the end of the current day, it is RECOMMENDED that NTP servers
wait until the last day of the month before broadcasting leap wait until the last day of the month before broadcasting leap
seconds. Doing this will prevent older clients from applying a leap seconds. Doing this will prevent older clients from applying a leap
second at the wrong time. Note well that NTPv4's longest polling second at the wrong time. When implementing this recommendation,
interval exceeds one day and thus a leap second announcement may be operators should ensure that clients are not configured to use
missed. polling intervals greater than 24 hours, so the leap second
notification is not missed.
In circumstances where an NTP server is not receiving leap second In circumstances where an NTP server is not receiving leap second
information from an automated source, certain organizations maintain information from an automated source, certain organizations maintain
files which are updated every time a new leap second is announced: files which are updated every time a new leap second is announced:
NIST: ftp://time.nist.gov/pub/leap-seconds.list NIST: ftp://time.nist.gov/pub/leap-seconds.list
US Navy (maintains GPS Time): ftp://tycho.usno.navy.mil/pub/ntp/leap- US Navy (maintains GPS Time): ftp://tycho.usno.navy.mil/pub/ntp/leap-
seconds.list seconds.list
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standard leap second event, that minute will have 61 (or possibly 59) standard leap second event, that minute will have 61 (or possibly 59)
seconds in it, and some applications (and even some OS's) are known seconds in it, and some applications (and even some OS's) are known
to have problems with that. to have problems with that.
Operators who have legal obligations or other strong requirements to Operators who have legal obligations or other strong requirements to
be synchronized with UTC or civil time SHOULD NOT use leap smearing, be synchronized with UTC or civil time SHOULD NOT use leap smearing,
because the distributed time cannot be guaranteed to be traceable to because the distributed time cannot be guaranteed to be traceable to
UTC during the smear interval. UTC during the smear interval.
Clients that are connected to leap smearing servers MUST NOT apply Clients that are connected to leap smearing servers MUST NOT apply
the standard NTP leap second handling. So these clients must never the standard NTP leap second handling. These clients must never have
have a leap second file loaded, and the smearing servers must never a leap second file loaded, and the smearing servers must never
advertise to clients that a leap second is pending. advertise to clients that a leap second is pending.
Any use of leap smearing servers should be limited to within a Any use of leap smearing servers should be limited to within a
single, well-controlled environment. Leap Smearing MUST NOT be used single, well-controlled environment. Leap Smearing MUST NOT be used
for public-facing NTP servers, as they will disagree with non- for public-facing NTP servers, as they will disagree with non-
smearing servers (as well as UTC) during the leap smear interval, and smearing servers (as well as UTC) during the leap smear interval, and
there is no standardized way for a client to detect that a server is there is no standardized way for a client to detect that a server is
using leap smearing. However, be aware that some public-facing using leap smearing. However, be aware that some public-facing
servers may be configured this way anyway in spite of this guidance. servers may be configured this way anyway in spite of this guidance.
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the NTP packet by means of a Message Authentication Code (MAC). the NTP packet by means of a Message Authentication Code (MAC).
Neither of them encrypts the NTP's payload, because this payload Neither of them encrypts the NTP's payload, because this payload
information is not considered to be confidential. information is not considered to be confidential.
4.1. Pre-Shared Key Approach 4.1. Pre-Shared Key Approach
This approach applies a symmetric key for the calculation of the MAC, This approach applies a symmetric key for the calculation of the MAC,
which protects authenticity and integrity of the exchanged packets which protects authenticity and integrity of the exchanged packets
for an association. NTP does not provide a mechanism for the for an association. NTP does not provide a mechanism for the
exchange of the keys between the associated nodes. Therefore, for exchange of the keys between the associated nodes. Therefore, for
each association, keys SHOULD be exchanged securely by external each association, keys MUST be exchanged securely by external means,
means, and they SHOULD be protected from disclosure. It is and they MUST be protected from disclosure. It is RECOMMENDED that
RECOMMENDED that each association be protected by its own unique key. each association be protected by its own unique key. It is
It is RECOMMENDED that participants agree to refresh keys RECOMMENDED that participants agree to refresh keys periodically.
periodically. However, NTP does not provide a mechanism to assist in However, NTP does not provide a mechanism to assist in doing so.
doing so. Each communication partner will need to keep track of its keys in its
own local key storage.
[RFC5905] specifies a hash which must be supported for calculation of [RFC5905] specifies using the MD5 hash algorithm for calculation of
the MAC, but other algorithms may be supported as well. The MD5 hash the MAC, but other algorithms may be supported as well. The MD5 hash
is now considered to be too weak. Implementations will soon be is now considered to be too weak and unsuitable for cryptographic
available based on AES-128-CMAC [I-D.ietf-ntp-mac], and users are usage. [RFC6151] has more information on the algorithm's weaknesses.
encouraged to use that when it is available. Implementations will soon be available based on AES-128-CMAC
[I-D.ietf-ntp-mac], and users SHOULD use that when it is available.
To use this approach the communication partners have to exchange the
key, which consists of a keyid with a value between 1 and 65534,
inclusive, and a label which indicates the chosen digest algorithm.
Each communication partner adds this information to its own key file.
Some implementations store the key in clear text. Therefore it Some implementations store the key in clear text. Therefore it MUST
SHOULD only be readable by the NTP process. Different keys are added only be readable by the NTP process.
line by line to the key file.
An NTP client establishes a protected association by appending the An NTP client has to be able to link a key to a particular server in
key to the server statement in its configuration file. Note that the order to establish a protected association. This linkage is
NTP process has to trust the applied key. implementation specific. Once applied, a key will be trusted until
the link is removed.
4.2. Autokey 4.2. Autokey
Autokey was specified in 2010 to provide automated key management and [RFC5906] specifies the Autokey protocol. It was published in 2010
authentication of NTP servers. However, security researchers have to provide automated key management and authentication of NTP
identified vulnerabilities [6] in the Autokey protocol. servers. However, security researchers have identified
vulnerabilities [8] in the Autokey protocol.
Autokey SHOULD NOT be used. Autokey SHOULD NOT be used.
4.3. Network Time Security 4.3. Network Time Security
Work is in progress on an enhanced replacement for Autokey. Refer to Work is in progress on an enhanced replacement for Autokey. Refer to
[I-D.ietf-ntp-using-nts-for-ntp] for more information. [I-D.ietf-ntp-using-nts-for-ntp] for more information.
4.4. External Security Protocols
If applicable, external security Protocols such as IPsec and MACsec
can be applied to enhance integrity and authenticity protection of
NTP time synchronization packets. Usage of such external security
protocols can decrease time synchronization performance [RFC7384].
Therefore, operators are advised to carefully evaluate if the
decreased time synchronization performance meets their specific
timing requirements.
5. NTP Security Best Practices 5. NTP Security Best Practices
This section lists some general NTP security practices, but these This section lists some general NTP security practices, but these
issues may (or may not) have been mitigated in particular versions of issues may (or may not) have been mitigated in particular versions of
particular implementations. Contact the maintainers of your particular implementations. Contact the maintainers of the relevant
implementation for more information. implementation for more information.
5.1. Minimizing Information Leakage 5.1. Minimizing Information Leakage
The base NTP packet leaks important information (including reference The base NTP packet leaks important information (including reference
ID and reference time) that may be used in attacks [NDSS16], ID and reference time) that may be used in attacks [NDSS16],
[CVE-2015-8138], [CVE-2016-1548]. A remote attacker can learn this [CVE-2015-8138], [CVE-2016-1548]. A remote attacker can learn this
information by sending mode 3 queries to a target system and information by sending mode 3 queries to a target system and
inspecting the fields in the mode 4 response packet. NTP control inspecting the fields in the mode 4 response packet. NTP control
queries also leak important information (including reference ID, queries also leak important information (including reference ID,
expected origin timestamp, etc.) that may be used in attacks expected origin timestamp, etc.) that may be used in attacks
[CVE-2015-8139]. A remote attacker can learn this information by [CVE-2015-8139]. A remote attacker can learn this information by
sending control queries to a target system and inspecting the sending control queries to a target system and inspecting the leaked
response. information in the response.
As such, mechanisms outside of the NTP protocol, such as Access As such, mechanisms outside of the NTP protocol, such as Access
Control Lists, SHOULD be used to limit the exposure of this Control Lists, SHOULD be used to limit the exposure of this
information to allowed IP addresses, and keep it from remote information to allowed IP addresses, and keep it from remote
attackers not on the list. Hosts SHOULD only respond to NTP control attackers not on the list. Hosts SHOULD only respond to NTP control
queries from authorized parties. queries from authorized parties.
A host that is not supposed to act as an NTP server that provides An NTP client that does not provide time on the network can
timing information to other hosts MAY additionally log and drop additionally log and drop incoming mode 3 timing queries from
incoming mode 3 timing queries from unexpected sources. Note well unexpected sources. Note well that the easiest way to monitor the
that the easiest way to monitor ntpd's status is to send it a mode 3 status of an NTP instance is to send it a mode 3 query, so it may not
query. It is recommended that operators SHOULD filter mode 3 queries be desirable to drop all mode 3 queries. As an alternative,
at the edge, or make sure mode 3 queries are allowed only from operators SHOULD either filter mode 3 queries from outside their
trusted systems or networks. networks, or make sure mode 3 queries are allowed only from trusted
systems or networks.
A "leaf-node host" is a host that is using NTP solely for the purpose A "leaf-node host" is a host that is using NTP solely for the purpose
of adjusting its own system time. Such a host is not expected to of adjusting its own system time. Such a host is not expected to
provide time to other hosts, and relies exclusively on NTP's basic provide time to other hosts, and relies exclusively on NTP's basic
mode to take time from a set of servers. (That is, the host sends mode to take time from a set of servers. (That is, the host sends
mode 3 queries to its servers and receives mode 4 responses from mode 3 queries to its servers and receives mode 4 responses from
these servers containing timing information.) To minimize these servers containing timing information.) To minimize
information leakage, leaf-node hosts SHOULD drop all incoming NTP information leakage, leaf-node hosts SHOULD drop all incoming NTP
packets except mode 4 response packets that come from known sources. packets except mode 4 response packets that come from known sources.
Note well that proper monitoring of an NTP server instance includes An exception to this can be made if a leaf-node host is being
checking the time of that NTP server instance. actively monitored, in which case incoming packets from the
monitoring server can be allowed.
Please refer to [I-D.ietf-ntp-data-minimization] for more Please refer to [I-D.ietf-ntp-data-minimization] for more
information. information.
5.2. Avoiding Daemon Restart Attacks 5.2. Avoiding Daemon Restart Attacks
[RFC5905] says NTP clients should not accept time shifts greater than [RFC5905] says NTP clients should not accept time shifts greater than
the panic threshold. Specifically, RFC 5905 says "PANIC means the the panic threshold. Specifically, RFC 5905 says "PANIC means the
offset is greater than the panic threshold PANICT (1000 s) and SHOULD offset is greater than the panic threshold PANICT (1000 s) and SHOULD
cause the program to exit with a diagnostic message to the system cause the program to exit with a diagnostic message to the system
log." log."
However, this behavior can be exploited by attackers as described in However, this behavior can be exploited by attackers as described in
[NDSS16], when the following two conditions hold: [NDSS16], when the following two conditions hold:
1. The operating system automatically restarts the NTP client when 1. The operating system automatically restarts the NTP client when
it quits. (Modern *NIX operating systems are replacing it quits. (Modern *NIX operating systems are replacing
traditional init systems with process supervisors, such as traditional init systems with process supervisors, such as
systemd, which can be configured to automatically restart any systemd, which can be configured to automatically restart any
daemons that quit. This behavior is the default in CoreOS and daemons that quit. This behavior is the default in CoreOS and
Arch Linux. It is likely to become the default behavior in other Arch Linux. As of the time of this writing, it appears likely to
systems as they migrate legacy init scripts to process become the default behavior in other systems as they migrate
supervisors such as systemd.) legacy init scripts to process supervisors such as systemd.)
2. The NTP client is configured to ignore the panic threshold on all 2. The NTP client is configured to ignore the panic threshold on all
restarts. restarts.
In such cases, if the attacker can send the target an offset that In such cases, if the attacker can send the target an offset that
exceeds the panic threshold, the client will quit. Then, when it exceeds the panic threshold, the client will quit. Then, when it
restarts, it ignores the panic threshold and accepts the attacker's restarts, it ignores the panic threshold and accepts the attacker's
large offset. large offset.
Operators SHOULD be aware that when operating with the above two Operators need to be aware that when operating with the above two
conditions, the panic threshold offers no protection from attacks. conditions, the panic threshold offers no protection from attacks.
The natural solution is not to run hosts with these conditions. The natural solution is not to run hosts with these conditions.
Specifically, operators SHOULD NOT ignore the panic threshold in all Specifically, operators SHOULD NOT ignore the panic threshold in all
cold-start situations unless sufficient oversight and checking is in cold-start situations unless sufficient oversight and checking is in
place to make sure that this type of attack cannot happen. place to make sure that this type of attack cannot happen.
As an alternative, the following steps MAY be taken by operators to As an alternative, the following steps MAY be taken by operators to
mitigate the risk of attack: mitigate the risk of attack:
o Monitor the NTP system log to detect when the NTP daemon has quit o Monitor the NTP system log to detect when the NTP daemon has quit
due to a panic event, as this could be a sign of an attack. due to a panic event, as this could be a sign of an attack.
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o Monitor the NTP system log to detect when the NTP daemon has quit o Monitor the NTP system log to detect when the NTP daemon has quit
due to a panic event, as this could be a sign of an attack. due to a panic event, as this could be a sign of an attack.
o Request manual intervention when a timestep larger than the panic o Request manual intervention when a timestep larger than the panic
threshold is detected. threshold is detected.
o Configure the ntp client to only ignore the panic threshold in a o Configure the ntp client to only ignore the panic threshold in a
cold start situation. cold start situation.
o Add 'minsane' and 'minclock' parameters to the ntp.conf file so o Increase the minimum number of servers required before the NTP
ntpd waits until enough trusted sources of time agree on the client adjusts the system clock. This will make the NTP client
correct time. wait until enough trusted sources of time agree before declaring
the time to be correct.
In addition, implementations SHOULD prevent the NTP daemon from In addition, the following steps SHOULD be taken by those who
taking time steps that set the clock to a time earlier than the implement the NTP protocol:
compile date of the NTP daemon.
o Prevent the NTP daemon from taking time steps that set the clock
to a time earlier than the compile date of the NTP daemon.
o Prevent the NTP daemon from putting 'INIT' in the reference ID of
its NTP packets upon initializing. This will make it more
difficult for attackers to know when the daemon reboots.
5.3. Detection of Attacks Through Monitoring 5.3. Detection of Attacks Through Monitoring
Operators SHOULD monitor their NTP instances to detect attacks. Many Operators SHOULD monitor their NTP instances to detect attacks. Many
known attacks on NTP have particular signatures. Common attack known attacks on NTP have particular signatures. Common attack
signatures include: signatures include:
1. Bogus packets - A packet whose origin timestamp does not match 1. Bogus packets - A packet whose origin timestamp does not match
the value that expected by the client. the value that expected by the client.
2. Zero origin packet - A packet with an origin timestamp set to 2. Zero origin packet - A packet with an origin timestamp set to
zero [CVE-2015-8138]. zero [CVE-2015-8138].
3. A packet with an invalid cryptographic MAC [CCR16]. 3. A packet with an invalid cryptographic MAC [CCR16].
The observation of many such packets could indicate that the client The observation of many such packets could indicate that the client
is under attack. is under attack.
5.4. Kiss-o'-Death Packets 5.4. Kiss-o'-Death Packets
The "Kiss-o'-Death" (KoD) packet is a rate limiting mechanism where a The "Kiss-o'-Death" (KoD) packet includes a rate management mechanism
server can tell a misbehaving client to reduce its query rate. It is where a server can tell a misbehaving client to reduce its query
RECOMMENDED that all NTP devices respect these packets and back off rate. KoD packets in general (and the RATE packet in particular) are
when asked to do so by a server. It is even more important for an defined in Section 7.4 of [RFC5905]. It is RECOMMENDED that all NTP
embedded device, which may not have an exposed control interface for devices respect these packets and back off when asked to do so by a
NTP. server. It is even more important for an embedded device, which may
not have an exposed control interface for NTP.
That said, a client MUST only accept a KoD packet if it has a valid That said, a client MUST only accept a KoD packet if it has a valid
origin timestamp. Once a RATE packet is accepted, the client should origin timestamp. Once a RATE packet is accepted, the client should
increase its poll interval value (thus decreasing its polling rate) increase its poll interval value (thus decreasing its polling rate)
up to a reasonable maximum. This maximum can vary by implementation up to a reasonable maximum. This maximum can vary by implementation
but should not exceed a poll interval value of 13 (2 hours). The but should not exceed a poll interval value of 13 (2 hours). The
mechanism to determine how much to increase the poll interval value mechanism to determine how much to increase the poll interval value
is undefined in [RFC5905]. If the client uses the poll interval is undefined in [RFC5905]. If the client uses the poll interval
value sent by the server in the KoD packet, it MUST NOT simply accept value sent by the server in the RATE packet, it MUST NOT simply
any value. Using large interval values may open a vector for a accept any value. Using large interval values may open a vector for
denial-of-service attack that causes the client to stop querying its a denial-of-service attack that causes the client to stop querying
server [NDSS16]. its server [NDSS16].
The KoD mechanism relies on clients behaving properly in order to be The KoD rate management mechanism relies on clients behaving properly
effective. Some clients ignore the KoD packet entirely, and other in order to be effective. Some clients ignore the RATE packet
poorly-implemented clients might unintentionally increase their poll entirely, and other poorly-implemented clients might unintentionally
rate and simulate a denial of service attack. Server administrators increase their poll rate and simulate a denial of service attack.
SHOULD be prepared for this and take measures outside of the NTP Server administrators are advised to be prepared for this and take
protocol to drop packets from misbehaving clients when these clients measures outside of the NTP protocol to drop packets from misbehaving
are detected. clients when these clients are detected.
Also, Kiss-o'-Death (KoD) packets can be used in denial of service Kiss-o'-Death (KoD) packets can be used in denial of service attacks.
attacks. Thus, the observation of even just one KoD packet with a Thus, the observation of even just one RATE packet with a high poll
high poll value could be sign that the client is under attack. value could be sign that the client is under attack. And KoD packets
are commonly accepted even when not cryptographically authenticated,
which increases the risk of denial of service attacks.
5.5. Broadcast Mode Should Only Be Used On Trusted Networks 5.5. Broadcast Mode Should Only Be Used On Trusted Networks
Per [RFC5905], NTP's broadcast mode is authenticated using symmetric Per [RFC5905], NTP's broadcast mode is authenticated using symmetric
key cryptography. The broadcast server and all of its broadcast key cryptography. The broadcast server and all its broadcast clients
clients share a symmetric cryptographic key, and the broadcast server share a symmetric cryptographic key, and the broadcast server uses
uses this key to append a message authentication code (MAC) to the this key to append a message authentication code (MAC) to the
broadcast packets it sends. broadcast packets it sends.
Importantly, all broadcast clients that listen to this server have to Importantly, all broadcast clients that listen to this server have to
know the cryptographic key. This mean that any client can use this know the cryptographic key. This mean that any client can use this
key to send valid broadcast messages that look like they come from key to send valid broadcast messages that look like they come from
the broadcast server. Thus, a rogue broadcast client can use its the broadcast server. Thus, a rogue broadcast client can use its
knowledge of this key to attack the other broadcast clients. knowledge of this key to attack the other broadcast clients.
For this reason, an NTP broadcast server and all its clients have to For this reason, an NTP broadcast server and all its clients have to
trust each other. Broadcast mode SHOULD only be run from within a trust each other. Broadcast mode SHOULD only be run from within a
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As computing becomes more ubiquitous, there will be many small As computing becomes more ubiquitous, there will be many small
embedded devices that require accurate time. These devices may not embedded devices that require accurate time. These devices may not
have a persistent battery-backed clock, so using NTP to set the have a persistent battery-backed clock, so using NTP to set the
correct time on power-up may be critical for proper operation. These correct time on power-up may be critical for proper operation. These
devices may not have a traditional user interface, but if they devices may not have a traditional user interface, but if they
connect to the Internet they will be subject to the same security connect to the Internet they will be subject to the same security
threats as traditional deployments. threats as traditional deployments.
6.1. Updating Embedded Devices 6.1. Updating Embedded Devices
Vendors of embedded devices MUST pay attention to the current state Vendors of embedded devices are advised to pay attention to the
of protocol security issues and bugs in their chosen implementation, current state of protocol security issues and bugs in their chosen
because their customers don't have the ability to update their NTP implementation, because their customers don't have the ability to
implementation on their own. Those devices may have a single update their NTP implementation on their own. Those devices may have
firmware upgrade, provided by the manufacturer, that updates all a single firmware upgrade, provided by the manufacturer, that updates
capabilities at once. This means that the vendor assumes the all capabilities at once. This means that the vendor assumes the
responsibility of making sure their devices have the latest NTP responsibility of making sure their devices have an up-to-date and
updates applied. secure NTP implementation.
Vendors of embedded devices SHOULD also include the ability to update Vendors of embedded devices SHOULD include the ability to update the
information regarding which NTP server to connect to on these list of NTP servers used by the device.
devices.
There is a catalog of NTP server abuse incidents, some of which There is a catalog of NTP server abuse incidents, some of which
involve embedded devices, on the Wikipedia page for NTP Server Misuse involve embedded devices, on the Wikipedia page for NTP Server Misuse
and Abuse [7]. and Abuse [9].
6.2. Server configuration 6.2. Server configuration
Vendors of embedded devices that need time synchronization SHOULD Vendors of embedded devices with preconfigured NTP servers need to
also carefully consider where they get their time from. There are carefully consider which servers to use. There are several public-
several public-facing NTP servers available, but they may not be facing NTP servers available, but they may not be prepared to service
prepared to service requests from thousands of new devices on the requests from thousands of new devices on the Internet. Vendors MUST
Internet. Vendors SHOULD only synchronize to servers that they have only preconfigure NTP servers that they have permission to use.
permission to use.
Vendors are encouraged to invest resources into providing their own Vendors are encouraged to invest resources into providing their own
time servers for their devices to connect to. time servers for their devices to connect to. This may be done
through the NTP Pool Project, as documented in Section 3.6.
Vendors should read [RFC4085], which advises against embedding Vendors should read [RFC4085], which advises against embedding
globally-routable IP addresses in products, and offers several better globally-routable IP addresses in products, and offers several better
alternatives. alternatives.
6.2.1. NTP Pool Project Vendor Subdomains
The NTP Pool Project offers a program where vendors can obtain their
own subdomain that is part of the NTP Pool. This offers vendors the
ability to safely make use of the time distributed by the Pool for
their devices. Vendors are encouraged to support the pool if they
participate. For more information, visit http://www.pool.ntp.org/en/
vendors.html [8] .
7. NTP over Anycast 7. NTP over Anycast
Anycast is described in BCP 126 [RFC4786]. (Also see [RFC7094]). Anycast is described in BCP 126 [RFC4786]. (Also see [RFC7094]).
With anycast, a single IP address is assigned to multiple interfaces, With anycast, a single IP address is assigned to multiple servers,
and routers direct packets to the closest active interface. and routers direct packets to the closest active server.
Anycast is often used for Internet services at known IP addresses, Anycast is often used for Internet services at known IP addresses,
such as DNS. Anycast can also be used in large organizations to such as DNS. Anycast can also be used in large organizations to
simplify configuration of a large number of NTP clients. Each client simplify configuration of many NTP clients. Each client can be
can be configured with the same NTP server IP address, and a pool of configured with the same NTP server IP address, and a pool of anycast
anycast servers can be deployed to service those requests. New servers can be deployed to service those requests. New servers can
servers can be added to or taken from the pool, and other than a be added to or taken from the pool, and other than a temporary loss
temporary loss of service while a server is taken down, these of service while a server is taken down, these additions can be
additions can be transparent to the clients. transparent to the clients.
Note well that using a single anycast address for NTP presents its Note well that using a single anycast address for NTP presents its
own potential issues. It means each client will likely use a single own potential issues. It means each client will likely use a single
time server source. A key element of a robust NTP deployment is each time server source. A key element of a robust NTP deployment is each
client using multiple sources of time. With multiple time sources, a client using multiple sources of time. With multiple time sources, a
client will analyze the various time sources, selecting good ones, client will analyze the various time sources, selecting good ones,
and disregarding poor ones. If a single Anycast address is used, and disregarding poor ones. If a single Anycast address is used,
this analysis will not happen. this analysis will not happen. This can be mitigated by creating
multiple, separate anycast pools so clients can have multiple sources
of time while still gaining the configuration benefits of the anycast
pools.
If clients are connected to an NTP server via anycast, the client If clients are connected to an NTP server via anycast, the client
does not know which particular server they are connected to. As does not know which particular server they are connected to. As
anycast servers may arbitrarily enter and leave the network, the anycast servers enter and leave the network, or the network topology
server a particular client is connected to may change. This may changes, the server a particular client is connected to may change.
cause a small shift in time from the perspective of the client when This may cause a small shift in time from the perspective of the
the server it is connected to changes. It is RECOMMENDED that client when the server it is connected to changes. In extreme cases
anycast only be deployed in environments where these small shifts can where the network topology is changing rapidly, this could cause the
be tolerated. server seen by a client to rapidly change as well, which can lead to
larger time inaccuracies. It is RECOMMENDED that anycast only be
deployed in environments where this behavior can be tolerated
Configuration of an anycast interface is independent of NTP. Clients Configuration of an anycast interface is independent of NTP. Clients
will always connect to the closest server, even if that server is will always connect to the closest server, even if that server is
having NTP issues. It is RECOMMENDED that anycast NTP having NTP issues. It is RECOMMENDED that anycast NTP
implementations have an independent method of monitoring the implementations have an independent method of monitoring the
performance of NTP on a server. If the server is not performing to performance of NTP on a server. If the server is not performing to
specification, it should remove itself from the Anycast network. It specification, it should remove itself from the Anycast network. It
is also RECOMMENDED that each Anycast NTP server have an alternative is also RECOMMENDED that each Anycast NTP server have an alternative
method of access, such as an alternate Unicast IP address, so its method of access, such as an alternate Unicast IP address, so its
performance can be checked independently of the anycast routing performance can be checked independently of the anycast routing
skipping to change at page 18, line 17 skipping to change at page 18, line 32
[I-D.ietf-ntp-using-nts-for-ntp] specifies the Network Time Security [I-D.ietf-ntp-using-nts-for-ntp] specifies the Network Time Security
(NTS) mechanism and applies it to NTP. Readers are encouraged to (NTS) mechanism and applies it to NTP. Readers are encouraged to
check the status of the draft, and make use of the methods it check the status of the draft, and make use of the methods it
describes. describes.
11. References 11. References
11.1. Normative References 11.1. 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>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <https://www.rfc-editor.org/info/rfc2827>. May 2000, <https://www.rfc-editor.org/info/rfc2827>.
[RFC4085] Plonka, D., "Embedding Globally-Routable Internet [RFC4085] Plonka, D., "Embedding Globally-Routable Internet
Addresses Considered Harmful", BCP 105, RFC 4085, Addresses Considered Harmful", BCP 105, RFC 4085,
DOI 10.17487/RFC4085, June 2005, DOI 10.17487/RFC4085, June 2005,
<https://www.rfc-editor.org/info/rfc4085>. <https://www.rfc-editor.org/info/rfc4085>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <https://www.rfc-editor.org/info/rfc4786>. December 2006, <https://www.rfc-editor.org/info/rfc4786>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
11.2. Informative References [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>.
[BCP38INFO] 11.2. Informative References
"BCP38 Info Page", <http://www.bcp38.info>.
[CCR16] Malhotra, A. and S. Goldberg, "Attacking NTP's [CCR16] Malhotra, A. and S. Goldberg, "Attacking NTP's
Authenticated Broadcast Mode", SIGCOMM Computer Authenticated Broadcast Mode", SIGCOMM Computer
Communications Review (CCR) , 2016. Communications Review (CCR) , 2016.
[CVE-2015-8138] [CVE-2015-8138]
Van Gundy, M. and J. Gardner, "NETWORK TIME PROTOCOL Van Gundy, M. and J. Gardner, "NETWORK TIME PROTOCOL
ORIGIN TIMESTAMP CHECK IMPERSONATION VULNERABILITY", 2016, ORIGIN TIMESTAMP CHECK IMPERSONATION VULNERABILITY", 2016,
<http://www.talosintel.com/reports/TALOS-2016-0077>. <http://www.talosintel.com/reports/TALOS-2016-0077>.
skipping to change at page 19, line 23 skipping to change at page 19, line 43
<http://blog.talosintel.com/2016/04/ <http://blog.talosintel.com/2016/04/
vulnerability-spotlight-further-ntpd_27.html>. vulnerability-spotlight-further-ntpd_27.html>.
[I-D.ietf-ntp-data-minimization] [I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-03 (work Minimization", draft-ietf-ntp-data-minimization-03 (work
in progress), September 2018. in progress), September 2018.
[I-D.ietf-ntp-mac] [I-D.ietf-ntp-mac]
Malhotra, A. and S. Goldberg, "Message Authentication Code Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", draft-ietf-ntp-mac-05 for the Network Time Protocol", draft-ietf-ntp-mac-06
(work in progress), October 2018. (work in progress), January 2019.
[I-D.ietf-ntp-mode-6-cmds] [I-D.ietf-ntp-mode-6-cmds]
Haberman, B., "Control Messages Protocol for Use with Haberman, B., "Control Messages Protocol for Use with
Network Time Protocol Version 4", draft-ietf-ntp-mode- Network Time Protocol Version 4", draft-ietf-ntp-mode-
6-cmds-06 (work in progress), September 2018. 6-cmds-06 (work in progress), September 2018.
[I-D.ietf-ntp-using-nts-for-ntp] [I-D.ietf-ntp-using-nts-for-ntp]
Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time Sundblad, "Network Time Security for the Network Time
Protocol", draft-ietf-ntp-using-nts-for-ntp-14 (work in Protocol", draft-ietf-ntp-using-nts-for-ntp-15 (work in
progress), October 2018. progress), December 2018.
[IMC14] Czyz, J., Kallitsis, M., Gharaibeh, M., Papadopoulos, C., [IMC14] Czyz, J., Kallitsis, M., Gharaibeh, M., Papadopoulos, C.,
Bailey, M., and M. Karir, "Taming the 800 Pound Gorilla: Bailey, M., and M. Karir, "Taming the 800 Pound Gorilla:
The Rise and Decline of NTP DDoS Attacks", Internet The Rise and Decline of NTP DDoS Attacks", Internet
Measurement Conference , 2014. Measurement Conference , 2014.
[MILLS2006] [MILLS2006]
Mills, D., "Computer network time synchronization: the Mills, D., "Computer network time synchronization: the
Network Time Protocol", CRC Press , 2006. Network Time Protocol", CRC Press , 2006.
skipping to change at page 20, line 10 skipping to change at page 20, line 32
[NDSS16] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg, [NDSS16] Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
"Attacking the Network Time Protocol", NDSS'16, San Diego, "Attacking the Network Time Protocol", NDSS'16, San Diego,
CA. , 2016, <https://eprint.iacr.org/2015/1020.pdf>. CA. , 2016, <https://eprint.iacr.org/2015/1020.pdf>.
[RFC1305] Mills, D., "Network Time Protocol (Version 3) [RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305, Specification, Implementation and Analysis", RFC 1305,
DOI 10.17487/RFC1305, March 1992, DOI 10.17487/RFC1305, March 1992,
<https://www.rfc-editor.org/info/rfc1305>. <https://www.rfc-editor.org/info/rfc1305>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC5906] Haberman, B., Ed. and D. Mills, "Network Time Protocol
Requirement Levels", BCP 14, RFC 2119, Version 4: Autokey Specification", RFC 5906,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC5906, June 2010,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc5906>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil, [RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094, "Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014, DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>. <https://www.rfc-editor.org/info/rfc7094>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>. October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[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>.
11.3. URIs 11.3. URIs
[1] https://blog.cloudflare.com/technical-details-behind-a-400gbps- [1] https://blog.cloudflare.com/technical-details-behind-a-400gbps-
ntp-amplification-ddos-attack/ ntp-amplification-ddos-attack/
[2] http://www.pool.ntp.org/en/use.html [2] http://www.bcp38.info
[3] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time [3] http://www.pool.ntp.org/en/use.html
[4] https://www.iers.org/IERS/EN/Publications/Bulletins/ [4] http://www.pool.ntp.org/en/vendors.html
bulletins.html
[5] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time [5] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time
[6] https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html [6] https://www.iers.org/IERS/EN/Publications/Bulletins/
bulletins.html
[7] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse [7] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time
[8] http://www.pool.ntp.org/en/vendors.html [8] https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html
[9] http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [9] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse
[10] https://support.ntp.org/bin/view/Support/ConfiguringNTP [10] http://www.ntp.org/downloads.html
Appendix A. NTP Implementation by the Network Time Foundation [11] http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno
The Network Time Foundation (NTF) provides the reference [12] https://support.ntp.org/bin/view/Support/ConfiguringNTP
implementation of NTP, well-known under the name "ntpd". It is
actively maintained and developed by NTF's NTP Project, with help Appendix A. Best Practices specific to the Network Time Foundation
from volunteers and NTF's supporters. This NTP software can be implementation
downloaded from <http://www.ntp.org/downloads.html>
The Network Time Foundation (NTF) provides a widely used
implementation of NTP, known as ntpd [10]. It is an evolution of the
first NTP implementations developed by David Mills at the University
of Delaware. This appendix contains additional recommendations
specific to this implementation.
A.1. Use enough time sources A.1. Use enough time sources
In addition to the recommendation given in Section Section 3.2 the In addition to the recommendation given in Section 3.2 the ntpd
ntpd implementation provides the 'pool' directive. Starting with implementation provides the 'pool' directive. Starting with ntp-
ntp-4.2.6, this directive will spin up enough associations to provide 4.2.6, using this directive in the ntp.conf file will spin up enough
robust time service, and will disconnect poor servers and add in new associations to provide robust time service, and will disconnect poor
servers as-needed. If you have good reason, you may use the servers and add in new servers as-needed. The 'minclock' and
'minclock' and 'maxclock' options of the 'tos' command to override 'maxclock' options of the 'tos' command may be used to override the
the default values of how many servers are discovered through the default values of how many servers are discovered through the 'pool'
'pool' directive. directive.
A.2. NTP Control and Facility Messages A.2. NTP Control and Facility Messages
In addition to NTP Control Messages the ntpd implementation also In addition to NTP Control Messages the ntpd implementation also
offers the Mode 7 commands for monitoring and configuration. offers the Mode 7 commands for monitoring and configuration.
If Mode 7 has been explicitly enabled to be used for more than basic If Mode 7 has been explicitly enabled to be used for more than basic
monitoring it should be limited to authenticated sessions that monitoring it should be limited to authenticated sessions that
provide a 'requestkey'. provide a 'requestkey'.
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protection, the ability to perform these modifications can be protection, the ability to perform these modifications can be
controlled with: controlled with:
restrict ... nomodify restrict ... nomodify
Users can prevent their NTP servers from considering query/ Users can prevent their NTP servers from considering query/
configuration traffic by default by adding the following to their configuration traffic by default by adding the following to their
ntp.conf file: ntp.conf file:
restrict default -4 nomodify notrap nopeer noquery restrict default -4 nomodify notrap nopeer noquery
restrict default -6 nomodify notrap nopeer noquery restrict default -6 nomodify notrap nopeer noquery
restrict source nomodify notrap noquery restrict source nomodify notrap noquery
# nopeer is OK if you don't use the 'pool' directive
A.3. Monitoring A.3. Monitoring
The reference implementation of NTP allows remote monitoring. Access The ntpd implementation allows remote monitoring. Access to this
to this service is generally controlled by the "noquery" directive in service is generally controlled by the "noquery" directive in NTP's
NTP's configuration file (ntp.conf) via a "restrict" statement. The configuration file (ntp.conf) via a "restrict" statement. The syntax
syntax reads: reads:
restrict address mask address_mask noquery restrict address mask address_mask noquery
If a system is using broadcast mode and is running ntp-4.2.8p6 or If a system is using broadcast mode and is running ntp-4.2.8p6 or
later, use the 4th field of the ntp.keys file to specify the IPs of later, use the 4th field of the ntp.keys file to specify the IPs of
machines that are allowed to serve time to the group. machines that are allowed to serve time to the group.
A.4. Leap Second File A.4. Leap Second File
The use of leap second files requires ntpd 4.2.6 or later. After The use of leap second files requires ntpd 4.2.6 or later. After
fetching the leap seconds file onto the server, add this line to fetching the leap seconds file onto the server, add this line to
ntpd.conf to apply and use the file: ntpd.conf to apply and use the file, substituting the proper path:
leapfile "/path/to your/leap-file" leapfile "/path/to/leap-file"
You may need to restart ntpd to apply this change. There may need to restart ntpd to apply this change.
ntpd servers with a manually configured leap second file will ignore ntpd servers with a manually configured leap second file will ignore
leap second information broadcast from upstream NTP servers until the leap second information broadcast from upstream NTP servers until the
leap second file expires. If no valid leap second file is available leap second file expires. If no valid leap second file is available
then a leap second notification from an attached reference clock is then a leap second notification from an attached reference clock is
always accepted by ntpd. always accepted by ntpd.
If no valid leap second file is available, a leap second notification If no valid leap second file is available, a leap second notification
may be accepted from upstream NTP servers. As of ntp-4.2.6, a may be accepted from upstream NTP servers. As of ntp-4.2.6, a
majority of servers must provide the notification before it is majority of servers must provide the notification before it is
skipping to change at page 23, line 12 skipping to change at page 23, line 39
in the "early days" there was a greater chance that leap second in the "early days" there was a greater chance that leap second
information would be available from a very limited number of sources. information would be available from a very limited number of sources.
A.5. Leap Smearing A.5. Leap Smearing
Leap Smearing was introduced in ntpd versions 4.2.8.p3 and 4.3.47, in Leap Smearing was introduced in ntpd versions 4.2.8.p3 and 4.3.47, in
response to client requests. Support for leap smearing is not response to client requests. Support for leap smearing is not
configured by default and must be added at compile time. In configured by default and must be added at compile time. In
addition, no leap smearing will occur unless a leap smear interval is addition, no leap smearing will occur unless a leap smear interval is
specified in ntpd.conf . For more information, refer to specified in ntpd.conf . For more information, refer to
http://bk.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [9]. http://bk.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [11].
A.6. Configuring ntpd A.6. Configuring ntpd
See https://support.ntp.org/bin/view/Support/ConfiguringNTP [10] for See https://support.ntp.org/bin/view/Support/ConfiguringNTP [12] for
additional information on configuring ntpd. additional information on configuring ntpd.
A.7. Pre-Shared Keys A.7. Pre-Shared Keys
Each communication partner must add the keyid information to their Each communication partner must add the key information to their key
key file in the form: file in the form:
keyid label key keyid type key
where "keyid" is a number between 1 and 65534, inclusive, "type" is
an ASCII character which defines the key format, and "key" is the key
itself.
An ntpd client establishes a protected association by appending the An ntpd client establishes a protected association by appending the
option "key keyid" to the server statement in ntp.conf: option "key keyid" to the server statement in ntp.conf:
server address key keyid server address key keyid
substituting the server address in the "address" field and the
numerical keyid to use with that server in the "keyid" field.
A key is deemed trusted when its keyid is added to the list of A key is deemed trusted when its keyid is added to the list of
trusted keys by the "trustedkey" statement in ntp.conf. trusted keys by the "trustedkey" statement in ntp.conf.
trustedkey keyid_1 keyid_2 ... keyid_n trustedkey keyid_1 keyid_2 ... keyid_n
Starting with ntp-4.2.8p7 the ntp.keys file accepts an optional 4th Starting with ntp-4.2.8p7 the ntp.keys file accepts an optional 4th
column, a comma-separated list of IPs that are allowed to serve time. column, a comma-separated list of IPs that are allowed to serve time.
Use this feature. Note, however, that an adversarial client that Use this feature. Note, however, that an adversarial client that
knows the symmetric broadcast key could still easily spoof its source knows the symmetric broadcast key could still easily spoof its source
IP to an IP that is allowed to serve time. (This is easy to do IP to an IP that is allowed to serve time. (This is easy to do
because the origin timestamp on broadcast mode packets is not because the origin timestamp on broadcast mode packets is not
validated by the client. By contrast, client/server and symmetric validated by the client. By contrast, client/server and symmetric
modes do require origin timestamp validation, making it more modes do require origin timestamp validation, making it more
difficult to spoof packets [CCR16]. difficult to spoof packets [CCR16]).
Authors' Addresses Authors' Addresses
Denis Reilly (editor) Denis Reilly (editor)
Orolia USA Orolia USA
1565 Jefferson Road, Suite 460 1565 Jefferson Road, Suite 460
Rochester, NY 14623 Rochester, NY 14623
US US
Email: denis.reilly@orolia.com Email: denis.reilly@orolia.com
Harlan Stenn Harlan Stenn
Network Time Foundation Network Time Foundation
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