draft-ietf-ntp-bcp-08.txt   draft-ietf-ntp-bcp-09.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: May 15, 2019 Network Time Foundation Expires: June 15, 2019 Network Time Foundation
D. Sibold D. Sibold
PTB PTB
November 11, 2018 December 12, 2018
Network Time Protocol Best Current Practices Network Time Protocol Best Current Practices
draft-ietf-ntp-bcp-08 draft-ietf-ntp-bcp-09
Abstract Abstract
The Network Time Protocol (NTP), currently on its fourth version, has The Network Time Protocol (NTP) is one of the oldest protocols on the
been widely used since its initial publication. This documentation Internet and has been widely used since its initial publication.
is a collection of Best Practices for general operation of time This document is a collection of Best Practices for general operation
servers on the Internet from across the NTP community. of NTP servers and clients on the Internet. It includes
recommendations for stable, accurate and secure operation of NTP
infrastructure. This document is targeted at NTP version 4 as
described in RFC 5905.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 May 15, 2019. This Internet-Draft will expire on June 15, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Keeping NTP up to date . . . . . . . . . . . . . . . . . . . 3 2. General Network Security Best Practices . . . . . . . . . . . 3
3. General Network Security Best Practices . . . . . . . . . . . 4 2.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. BCP 38 . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. NTP Configuration Best Practices . . . . . . . . . . . . . . 4
4. NTP Configuration Best Practices . . . . . . . . . . . . . . 4 3.1. Keeping NTP up to date . . . . . . . . . . . . . . . . . 4
4.1. Use enough time sources . . . . . . . . . . . . . . . . . 5 3.2. Use enough time sources . . . . . . . . . . . . . . . . . 4
4.2. Use a diversity of Reference Clocks . . . . . . . . . . . 6 3.3. Use a diversity of Reference Clocks . . . . . . . . . . . 5
4.3. Control Messages . . . . . . . . . . . . . . . . . . . . 6 3.4. Control Messages . . . . . . . . . . . . . . . . . . . . 6
4.4. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 7 3.5. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 7
4.5. Using Pool Servers . . . . . . . . . . . . . . . . . . . 7 3.6. Using Pool Servers . . . . . . . . . . . . . . . . . . . 7
4.6. Leap Second Handling . . . . . . . . . . . . . . . . . . 8 3.7. Leap Second Handling . . . . . . . . . . . . . . . . . . 8
4.6.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 9 3.7.1. Leap Smearing . . . . . . . . . . . . . . . . . . . . 9
5. NTP Security Mechanisms . . . . . . . . . . . . . . . . . . . 10 4. NTP Security Mechanisms . . . . . . . . . . . . . . . . . . . 10
5.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . . . 10 4.1. Pre-Shared Key Approach . . . . . . . . . . . . . . . . . 10
5.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. Autokey . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Network Time Security . . . . . . . . . . . . . . . . . . 11 4.3. Network Time Security . . . . . . . . . . . . . . . . . . 11
6. NTP Security Best Practices . . . . . . . . . . . . . . . . . 11 5. NTP Security Best Practices . . . . . . . . . . . . . . . . . 11
6.1. Minimizing Information Leakage . . . . . . . . . . . . . 11 5.1. Minimizing Information Leakage . . . . . . . . . . . . . 11
6.2. Avoiding Daemon Restart Attacks . . . . . . . . . . . . . 12 5.2. Avoiding Daemon Restart Attacks . . . . . . . . . . . . . 12
6.3. Detection of Attacks Through Monitoring . . . . . . . . . 13 5.3. Detection of Attacks Through Monitoring . . . . . . . . . 13
6.4. Kiss-of-Death Packets . . . . . . . . . . . . . . . . . . 14 5.4. Kiss-o'-Death Packets . . . . . . . . . . . . . . . . . . 13
6.5. Broadcast Mode Should Only Be Used On Trusted Networks . 14 5.5. Broadcast Mode Should Only Be Used On Trusted Networks . 14
6.6. Symmetric Mode Should Only Be Used With Trusted Peers . . 15 5.6. Symmetric Mode Should Only Be Used With Trusted Peers . . 14
7. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15 6. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15
7.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 15 6.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 15
7.2. Server configuration . . . . . . . . . . . . . . . . . . 16 6.2. Server configuration . . . . . . . . . . . . . . . . . . 15
7.2.1. Get a vendor subdomain for pool.ntp.org . . . . . . . 16 6.2.1. Get a vendor subdomain for pool.ntp.org . . . . . . . 16
8. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 16 7. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 18 11.1. Normative References . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . 19 11.2. Informative References . . . . . . . . . . . . . . . . . 19
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20 11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. NTP Implementation by the Network Time Appendix A. NTP Implementation by the Network Time
Foundation . . . . . . . . . . . . . . . . . . . . . 21 Foundation . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . 21
A.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22 A.3. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 22
A.4. Leap Second File . . . . . . . . . . . . . . . . . . . . 22 A.4. Leap Second File . . . . . . . . . . . . . . . . . . . . 22
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 . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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
RFC 5905 [RFC5905]. This documentation is a collection of best [RFC5905]. This documentation is a collection of best practices for
practices from across the NTP community. the 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 recommendatons 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 [RFC5905] compliant want to develop their own RFC 5905 compliant implementations.
implementations.
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. Keeping NTP up to date 2. General Network Security Best Practices
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. Important ways to
detect and protect computers and networks against undefined behavior
and security threats related to time are to keep their NTP
implementations current, use an appropriate number of trustworthy
time sources, and properly monitor their time infrastructure.
There are always new ideas about security on the Internet, and 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. By keeping NTP implementations current,
having "enough" trustworthy time sources (Section 4.1), and properly
monitoring their time infrastructure (Section 4.4), network operators
can make sure that their time infrastructure is operating correctly
and within specification, and is not being attacked or misused.
There are multiple versions of the NTP protocol in use, and multiple
implementations in use, on many different platforms. The practices
in this document are meant to apply generally to any implementation
of RFC 5905 [RFC5905]. It is recommended that that NTP users select
an implementation that is actively maintained. Users should keep up
to date on any known attacks on their selected implementation, and
deploy updates containing security fixes as soon as practical.
3. General Network Security Best Practices
3.1. 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 then other connection-oriented
protocols. NTP control messages can generate a lot of data in protocols. NTP control messages can generate a lot of data in
response to a small query, which makes it more attractive as a vector response to a small query, which makes it more attractive as a vector
for distributed denial-of-service attacks. (NTP Control messages are for distributed denial-of-service attacks. (NTP Control messages are
discussed further in Section 4.3). One documented instance of such discussed further in Section 3.4). One documented instance of such
an attack can be found here [1], and in [IMC14] and [NDSS14]. an attack can be found here [1], and further discussion in [IMC14]
Mitigating source address spoofing attacks should be a priority of and [NDSS14]. Mitigating source address spoofing attacks should be a
anyone administering NTP. priority of anyone administering NTP.
BCP 38 [RFC2827] was approved in 2000 to address this. BCP 38 BCP 38 [RFC2827] was approved in 2000 to address this. BCP 38 calls
[RFC2827] calls for filtering outgoing and incoming traffic to make for filtering outgoing and incoming traffic to make sure that the
sure that the source and destination IP addresses are consistent with source and destination IP addresses are consistent with the expected
the expected flow of traffic on each network interface. It is flow of traffic on each network interface. It is RECOMMENDED that
recommended that large corporate networks (and ISP's of any size) large corporate networks (and ISP's of any size) implement ingress
implement ingress and egress filtering. More information is and egress filtering. More information is available at the BCP38
available at the BCP38 Info Web page [2] . Info Web page [BCP38INFO] .
4. NTP Configuration Best Practices 3. NTP Configuration Best Practices
This section provides general Best Practices. Best Practices that This section provides Best Practices for NTP configuration and
are implementation specific are compiled in the Appendices. operation. Best Practices that are specific to the NTF
implementation are compiled in Appendix A.
4.1. Use enough time sources 3.1. Keeping NTP up to date
An NTP implementation that is compliant with RFC 5905 [RFC5905] takes Many network security mechanisms rely on time as part of their
the available sources of time and submits this timing data to 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
implementations, on many different platforms. The practices in this
document are meant to apply generally to any implementation of
[RFC5905]. It is RECOMMENDED that that NTP users select an
implementation that is actively maintained. Users should keep up to
date on any known attacks on their selected implementation, and
deploy updates containing security fixes as soon as practical.
3.2. Use enough time sources
An NTP implementation that is compliant with [RFC5905] takes the
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 RFC 5905 [RFC5905] or the detailed description of NTP in should read [RFC5905] or the detailed description of NTP in
MILLS 2006 [MILLS2006]. These available sources must be truly [MILLS2006].
redundant and derive their time from independent sources.
o If there is only 1 source of time, the answer is obvious. It may o If there is only 1 source of time, the answer is obvious. It may
not be a good source of time, but it's the only source of time not be a good source of time, but it's the only source of time
that can be considered. Any issue with the time at the source that can be considered. Any issue with the time at the source
will be passed on to the client. will be passed on to the client.
o If there are 2 sources of time and they agree well enough, then o If there are 2 sources of time and they agree well enough, then
the best "time" can be calculated easily. But if one source the best time can be calculated easily. But if one source fails,
fails, then the solution degrades to the single-source solution then the solution degrades to the single-source solution outlined
outlined above. And if the two sources don't agree, then it's above. And if the two sources don't agree, then it's impossible
impossible to know which one is correct by simply looking at the to know which one is correct by simply looking at the time.
time.
o If there are 3 sources of time, there is more data available to o If there are 3 sources of time, there is more data available to
converge on a "best" time, and this time is more likely to be converge on the best calculated time, and this time is more likely
accurate. And the loss of one of the sources (by becoming to be accurate. And the loss of one of the sources (by becoming
unreachable or unusable) can be tolerated. But at that point, the unreachable or unusable) can be tolerated. But at that point, the
solution degrades to the 2 source solution. solution degrades to the 2 source solution.
o 4 or more sources of time is better, as long as the sources are o 4 or more sources of time is better, as long as the sources are
diverse (Section 4.2). 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. For several hours before and after the June 2015 leap
second, several operators implemented leap smearing while others did second, several operators implemented leap smearing while others did
not, and many NTP end nodes could not determine an accurate time not, and many NTP end nodes could not determine an accurate time
source because 2 of their 4 sources of time gave them consistent UTC/ source because 2 of their 4 sources of time gave them consistent UTC/
POSIX time, while the other 2 gave them consistent leap-smeared time. POSIX time, while the other 2 gave them consistent leap-smeared time.
See Section 4.6.1 for more information. See Section 3.7.1 for more information.
Monitor your NTP instances. If your time sources do not generally Operators SHOULD monitor all of the time sources that are in use. If
agree, find out why and either correct the problems or stop using time sources do not generally agree, find out the cause and either
defective servers. See Section 4.4 for more information. correct the problems or stop using defective servers. See
Section 3.5 for more information.
4.2. 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. RECOMMENDED that several different types of reference clocks be used.
Having a diversity of sources with independent implementations means Having a diversity of sources with independent implementations means
that any one issue is less likely to cause a service interruption. that any 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 must radio-based time source unusable. If the time on your network has to
be correct close to 100% of the time, then even if you are using a be correct close to 100% of the time, then even if you are using a
satellite-based system, you must plan for those rare instances when satellite-based system, operators need to plan for those rare
the system is unavailable (or wrong!). instances when the system is unavailable (or wrong!).
4.3. Control Messages 3.4. Control Messages
Some implementations of NTPv4 provide the NTP Control Messages which Some implementations of NTPv4 provide the NTP Control Messages that
originally have been specified in Appendix B of [RFC1305] which were originally specified in Appendix B of [RFC1305] which defined
defined NTPv3, but never have been part of the NTPv4 specification. NTPv3. These messages were never included the NTPv4 specification,
(Work is being done to formally document the structure of these but they are still used. Work is being done to formally document the
control messages in draft-ietf-ntp-mode-6-cmds [CTRLMSG] .) 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. Used improperly, these facilities can be an
abuse vector. For this reason, it is recommended that publicly- abuse vector. For this reason, it is RECOMMENDED that publicly-
facing NTP servers should block mode 6 queries from outside their facing NTP servers should block mode 6 queries from outside their
organization. organization.
The ability to use Mode 6 beyond its basic monitoring capabilities The ability to use Mode 6 beyond its basic monitoring capabilities
can be limited to authenticated sessions that provide a 'controlkey'. SHOULD be limited to authenticated sessions that provide a
It can also be limited through mechanisms outside of the NTP 'controlkey'. It MAY also be limited through mechanisms outside of
specification, such as Access Control Lists, that only allow access the NTP specification, such as Access Control Lists, that only allow
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. To provide protection for such abuse NTP server
operators should deploy ingress filtering BCP 38 [RFC2827]. operators on large networks SHOULD deploy ingress filtering in
accordance with BCP 38 [RFC2827].
4.4. Monitoring 3.5. Monitoring
Use your NTP implementation's remote monitoring capabilities to Operators SHOULD use their NTP implementation's remote monitoring
quickly identify servers which are out of sync, and ensure capabilities to quickly identify servers which are out of sync, and
correctness of the service. Monitor system logs for messages so ensure correctness of the service. Operators SHOULD also monitor
problems and abuse attempts can be quickly identified. system logs for messages so problems and abuse attempts can be
quickly identified.
If a system starts getting unexpected time replies from its time If a system starts getting unexpected time replies from its time
servers, that can be an indication that the IP address of the system servers, that can be an indication that the IP address of the system
is being forged in requests to its time server, and these abusers are is being forged in requests to its time server. The goal of this
trying to convince that time server to stop serving time to that attack is to convince the time server to stop serving time to the
system. 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 receiving early time messages from its server, that is an indication
indication that somebody may be forging packets from a broadcast that somebody may be forging packets from a broadcast server.
server.
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 timestamps that are earlier than the current system time,
then either the system clock is unusually fast or somebody is trying then either the system clock is unusually fast or somebody is trying
to launch a replay attack against that server. to launch a replay attack against that server.
4.5. 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. Users who want to
synchronize their computers should only synchronize to servers that synchronize their computers SHOULD only synchronize to servers that
they have permission to use. 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 you are interested in using the pool, please review their If you are interested in using the pool, please review their
instructions at http://www.pool.ntp.org/en/use.html [3]. instructions at http://www.pool.ntp.org/en/use.html [2].
If you are a vendor who wishes to provide time service to your If you are a vendor who wishes to provide time service to your
customers or clients, consider joining the pool and providing a customers or clients, consider joining the pool and providing a
"vendor zone" through the pool project. "vendor zone" through the pool project.
If you want to synchronize many computers, consider running your own If you want to synchronize many computers, consider running your own
NTP servers that are synchronized by the pool, and synchronizing your NTP servers that are synchronized by the pool, and synchronizing your
clients to your in-house NTP servers. This reduces the load on the clients to your in-house NTP servers. This reduces the load on the
pool. pool.
4.6. Leap Second Handling 3.7. Leap Second Handling
UTC is kept in agreement with the astronomical time UT1 [4] to within UTC is kept in agreement with the astronomical time UT1 [3] 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 [5] when and Reference Systems Service (IERS) in its Bulletin C [4] 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, so if you are synced to an
NTP server that is ultimately synced to a source that provides leap NTP server that is ultimately synced to a source that provides leap
second notification you will get advance notification of impending second notification you will get advance notification of impending
leap seconds automatically. leap seconds automatically.
Since the length of the UT1 day is generally slowly increasing [6], Since the length of the UT1 day is generally slowly increasing [5],
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, should that ever become 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. Note well that NTPv4's longest polling
interval exceeds one day and thus a leap second announcement may be interval exceeds one day and thus a leap second announcement may be
missed. 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
IERS (announces leap seconds): IERS (announces leap seconds):
https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list
4.6.1. Leap Smearing 3.7.1. Leap Smearing
Some NTP installations may instead make use of a technique called
"Leap Smearing". With this method, instead of introducing an extra
second (or eliminating a second), NTP time will be slewed in small
increments over a comparably large window of time (called the smear
interval) around the leap second event. The smear interval should be
large enough to make the rate that the time is slewed small, so that
clients will follow the smeared time without objecting. Periods
ranging from 2 to 24 hours have been used successfully. During the
adjustment window, all the NTP clients' times may be offset from UTC
by as much as a full second, depending on the implementation. But at
least all clients will generally agree on what time they think it is!
Operators should NOTE WELL that using a leap-smear can cause your Some NTP installations make use of a technique called Leap Smearing.
reported time to be "legally indefensible" and/or be a breach of With this method, instead of introducing an extra second (or
compliance regulations. eliminating a second) on a leap second event, NTP time will be slewed
in small increments over a comparably large window of time (called
the smear interval) around the leap second event. The smear interval
should be large enough to make the rate that the time is slewed
small, so that clients will follow the smeared time without
objecting. Periods ranging from 2 to 24 hours have been used
successfully. During the adjustment window, all the NTP clients'
times may be offset from UTC by as much as a full second, depending
on the implementation. But at least all clients will generally agree
on what time they think it is.
The purpose of Leap Smearing is to enable systems that don't deal The purpose of Leap Smearing is to enable systems that don't deal
with the leap second event properly to function consistently, at the with the leap second event properly to function consistently, at the
expense of fidelity to UTC during the smear window. During a expense of fidelity to UTC during the smear window. During a
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
be synchronized with UTC or civil time SHOULD NOT use leap smearing,
because the distributed time cannot be guaranteed to be traceable to
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. So these clients must never
have a leap second file loaded, and the smearing servers must never have 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
f or 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.
System Administrators are advised to be aware of impending leap System Administrators are advised to be aware of impending leap
seconds and how the servers (inside and outside their organization) seconds and how the servers (inside and outside their organization)
they are using deal with them. Individual clients must never be they are using deal with them. Individual clients MUST NOT be
configured to use a mixture of smeared and non-smeared servers. If a configured to use a mixture of smeared and non-smeared servers. If a
client uses smeared servers, the servers it uses must all have the client uses smeared servers, the servers it uses must all have the
same leap smear configuration. same leap smear configuration.
5. NTP Security Mechanisms 4. NTP Security Mechanisms
In the standard configuration NTP packets are exchanged unprotected In the standard configuration NTP packets are exchanged unprotected
between client and server. An adversary that is able to become a between client and server. An adversary that is able to become a
Man-In-The-Middle is therefore able to drop, replay or modify the Man-In-The-Middle is therefore able to drop, replay or modify the
content of the NTP packet, which leads to degradation of the time content of the NTP packet, which leads to degradation of the time
synchronization or the transmission of false time information. A synchronization or the transmission of false time information. A
profound threat analysis for time synchronization protocols is given threat analysis for time synchronization protocols is given in
in RFC 7384 [RFC7384]. NTP provides two internal security mechanisms [RFC7384]. NTP provides two internal security mechanisms to protect
to protect authenticity and integrity of the NTP packets. Both authenticity and integrity of the NTP packets. Both measures protect
measures protect the NTP packet by means of a Message Authentication the NTP packet by means of a Message Authentication Code (MAC).
Code (MAC). Neither of them encrypts the NTP's payload, because this Neither of them encrypts the NTP's payload, because this payload
payload information is not considered to be confidential. information is not considered to be confidential.
5.1. 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 have to be exchanged securely by external each association, keys SHOULD be exchanged securely by external
means, and they have to be protected from disclosure. It is means, and they SHOULD be protected from disclosure. It is
recommended that each association be protected by its own unique key. RECOMMENDED that each association be protected by its own unique key.
It is recommended that participants agree to refresh keys It is RECOMMENDED that participants agree to refresh keys
periodically. However, NTP does not provide a mechanism to assist in periodically. However, NTP does not provide a mechanism to assist in
doing so. doing so.
RFC 5905 [RFC5905] specifies a hash which must be supported for [RFC5905] specifies a hash which must be supported for calculation of
calculation of the MAC, but other algorithms may be supported as the MAC, but other algorithms may be supported as well. The MD5 hash
well. The MD5 hash is now considered to be too weak. is now considered to be too weak. Implementations will soon be
Implementations will soon be available based on AES-128-CMAC available based on AES-128-CMAC [I-D.ietf-ntp-mac], and users are
[NTPMAC], and users are encouraged to use that when it is available. encouraged to use that when it is available.
To use this approach the communication partners have to exchange the To use this approach the communication partners have to exchange the
key, which consists of a keyid with a value between 1 and 65534, key, which consists of a keyid with a value between 1 and 65534,
inclusive, and a label which indicates the chosen digest algorithm. inclusive, and a label which indicates the chosen digest algorithm.
Each communication partner adds this information to its own key file. 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
should only be readable by the NTP process. Different keys are added SHOULD only be readable by the NTP process. Different keys are added
line by line to the key file. line by line to the key file.
An NTP client establishes a protected association by appending the An NTP client establishes a protected association by appending the
key to the server statement in its configuration file. Note that the key to the server statement in its configuration file. Note that the
NTP process has to trust the applied key. NTP process has to trust the applied key.
5.2. Autokey 4.2. Autokey
Autokey was specified in 2010 to provide automated key management and Autokey was specified in 2010 to provide automated key management and
authentication of NTP servers. However, security researchers have authentication of NTP servers. However, security researchers have
identified vulnerabilities in the Autokey protocol, which make the identified vulnerabilities [6] in the Autokey protocol.
protocol "useless". [7]
Autokey SHOULD NOT BE USED. Autokey SHOULD NOT be used.
5.3. Network Time Security 4.3. Network Time Security
Work is in progress on an enhanced replacement for Autokey, which is Work is in progress on an enhanced replacement for Autokey. Refer to
called Network Time Security (NTS) [NTSFORNTP]. As of July 2018, [I-D.ietf-ntp-using-nts-for-ntp] for more information.
this effort was at draft #12, and in the 'Working Group Last Call'
process. Readers are encouraged to adopt its mechanisms.
6. 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 your
implementation for more information. implementation for more information.
6.1. 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
response. 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. attackers not on the list. Hosts SHOULD only respond to NTP control
queries from authorized parties.
Hosts should only respond to NTP control queries from authorized
parties. One way to do this is to only allow control queries from
authenticated sources via authorized IP addresses.
A host that is not supposed to act as an NTP server that provides A host that is not supposed to act as an NTP server that provides
timing information to other hosts may additionally log and drop timing information to other hosts MAY additionally log and drop
incoming mode 3 timing queries from unexpected sources. Note well incoming mode 3 timing queries from unexpected sources. Note well
that the easiest way to monitor ntpd's status is to send it a mode 3 that the easiest way to monitor ntpd's status is to send it a mode 3
query. It is recommended that operators should filter mode 3 queries query. It is recommended that operators SHOULD filter mode 3 queries
at the edge, or make sure mode 3 queries are allowed only from at the edge, or make sure mode 3 queries are allowed only from
trusted systems or networks. 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 ntpd instance includes Note well that proper monitoring of an NTP server instance includes
checking the time of that ntpd instance. checking the time of that NTP server instance.
6.2. Avoiding Daemon Restart Attacks Please refer to [I-D.ietf-ntp-data-minimization] for more
information.
RFC 5905 [RFC5905] says NTP clients should not accept time shifts 5.2. Avoiding Daemon Restart Attacks
greater than the panic threshold. Specifically, RFC 5905 says "PANIC
means the offset is greater than the panic threshold PANICT (1000 s)
and SHOULD cause the program to exit with a diagnostic message to the
system log."
However, this behavior can be exploited by attackers [NDSS16], when [RFC5905] says NTP clients should not accept time shifts greater than
the following two conditions hold: the panic threshold. Specifically, RFC 5905 says "PANIC means the
offset is greater than the panic threshold PANICT (1000 s) and SHOULD
cause the program to exit with a diagnostic message to the system
log."
1. The operating system automatically restarts the NTP daemon when However, this behavior can be exploited by attackers as described in
[NDSS16], when the following two conditions hold:
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. It is likely to become the default behavior in other
systems as they migrate legacy init scripts to process systems as they migrate legacy init scripts to process
supervisors such as systemd.) 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 the exceeds the panic threshold, the client will quit. Then, when it
client restarts, it ignores the panic threshold and accepts the restarts, it ignores the panic threshold and accepts the attacker's
attacker's large offset. large offset.
Hosts running with the above two conditions should be aware that the Operators SHOULD be aware that when operating with the above two
panic threshold does not protect them from attacks. The recommended conditions, the panic threshold offers no protection from attacks.
and natural solution is not to run hosts with these conditions. The natural solution is not to run hosts with these conditions.
Specifically, only ignore the panic threshold in cold-start Specifically, operators SHOULD NOT ignore the panic threshold in all
situations if sufficient oversight and checking is in place to make cold-start situations unless sufficient oversight and checking is in
sure that this is appropriate. place to make sure that this type of attack cannot happen.
As an alternative, the following steps could be taken to mitigate the As an alternative, the following steps MAY be taken by operators to
risk of attack. mitigate the risk of attack:
o Monitor NTP system log to detect when the NTP daemon has quit due o Monitor the NTP system log to detect when the NTP daemon has quit
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 Implementations should prevent the NTP daemon from taking time o Add 'minsane' and 'minclock' parameters to the ntp.conf file so
steps that set the clock to a time earlier than the compile date ntpd waits until enough trusted sources of time agree on the
of the NTP daemon.
o Add "minsane" and "minclock" parameters to the ntp.conf file so
ntpd waits until "enough" trusted sources of time agree on the
correct time. correct time.
6.3. Detection of Attacks Through Monitoring In addition, implementations SHOULD prevent the NTP daemon from
taking time steps that set the clock to a time earlier than the
compile date of the NTP daemon.
Users should monitor their NTP instances to detect attacks. Many 5.3. Detection of Attacks Through Monitoring
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.
Also, Kiss-o'-Death (KoD) packets can be used in denial of service 5.4. Kiss-o'-Death Packets
attacks. Thus, the observation of even just one KoD packet with a
high poll value could be sign that the client is under attack. See
Section 6.4 for more information.
6.4. Kiss-of-Death Packets
The "Kiss-o'-Death" (KoD) packet is a rate limiting mechanism where a The "Kiss-o'-Death" (KoD) packet is a rate limiting mechanism where a
server can tell a misbehaving client to "back off" its query rate. server can tell a misbehaving client to reduce its query rate. It is
It is important for all NTP devices to respect these packets and back RECOMMENDED that all NTP devices respect these packets and back off
off when asked to do so by a server. It is even more important for when asked to do so by a server. It is even more important for an
an embedded device, which may not have exposed a control interface embedded device, which may not have an exposed control interface for
for NTP. 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 RFC 5905 [RFC5905]. If the client uses the poll is undefined in [RFC5905]. If the client uses the poll interval
interval value sent by the server in the KoD packet, it must not value sent by the server in the KoD packet, it MUST NOT simply accept
simply accept any value. Using large interval values may open a any value. Using large interval values may open a vector for a
vector for a denial-of-service attack that causes the client to stop denial-of-service attack that causes the client to stop querying its
querying its server [NDSS16]. server [NDSS16].
The KoD mechanism relies on clients behaving properly in order to be The KoD mechanism relies on clients behaving properly in order to be
effective. Some clients ignore the KoD packet entirely, and other effective. Some clients ignore the KoD packet entirely, and other
poorly-implemented clients might unintentionally increase their poll poorly-implemented clients might unintentionally increase their poll
rate and simulate a denial of service attack. Server administrators rate and simulate a denial of service attack. Server administrators
should be prepared for this and take measures outside of the NTP SHOULD be prepared for this and take measures outside of the NTP
protocol to drop packets from misbehaving clients when these clients protocol to drop packets from misbehaving clients when these clients
are detected. are detected.
6.5. Broadcast Mode Should Only Be Used On Trusted Networks Also, Kiss-o'-Death (KoD) packets can be used in denial of service
attacks. Thus, the observation of even just one KoD packet with a
high poll value could be sign that the client is under attack.
Per RFC 5905 [RFC5905], NTP's broadcast mode is authenticated using 5.5. Broadcast Mode Should Only Be Used On Trusted Networks
symmetric key cryptography. The broadcast server and all of its
broadcast clients share a symmetric cryptographic key, and the
broadcast server uses this key to append a message authentication
code (MAC) to the broadcast packets it sends.
Importantly, all broadcast clients that listen to this server must Per [RFC5905], NTP's broadcast mode is authenticated using symmetric
key cryptography. The broadcast server and all of its broadcast
clients share a symmetric cryptographic key, and the broadcast server
uses this key to append a message authentication code (MAC) to the
broadcast packets it sends.
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 client must 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
trusted network. trusted network.
6.6. Symmetric Mode Should Only Be Used With Trusted Peers 5.6. Symmetric Mode Should Only Be Used With Trusted Peers
In symmetric mode, two peers Alice and Bob can both push and pull In symmetric mode, two peers Alice and Bob can both push and pull
synchronization to and from each other using either ephemeral synchronization to and from each other using either ephemeral
symmetric passive (mode 2) or persistent symmetric active (NTP mode symmetric passive (mode 2) or persistent symmetric active (NTP mode
1) packets. The persistent association is preconfigured and 1) packets. The persistent association is preconfigured and
initiated at the active peer but not preconfigured at the passive initiated at the active peer but not preconfigured at the passive
peer (Bob). Upon receipt of a mode 1 NTP packet from Alice, Bob peer (Bob). Upon receipt of a mode 1 NTP packet from Alice, Bob
mobilizes a new ephemeral association if he does not have one mobilizes a new ephemeral association if he does not have one
already. This is a security risk for Bob because an arbitrary already. This is a security risk for Bob because an arbitrary
attacker can attempt to change Bob's time by asking Bob to become its attacker can attempt to change Bob's time by asking Bob to become its
symmetric passive peer. symmetric passive peer.
For this reason, a host (Bob) should only allow symmetric passive For this reason, a host SHOULD only allow symmetric passive
associations to be established with trusted peers. Specifically, Bob associations to be established with trusted peers. Specifically, a
should require each of its symmetric passive association to be host SHOULD require each of its symmetric passive association to be
cryptographically authenticated. Each symmetric passive association cryptographically authenticated. Each symmetric passive association
should be authenticated under a different cryptographic key. SHOULD be authenticated under a different cryptographic key.
The use of a different cryptographic key per peer prevents a Sybil
attack, where a single malicious peer uses the same cryptographic key
to set up multiple symmetric associations a target, and thus bias the
results of the target's Byzantine fault tolerant peer selection
algorithms.
7. NTP in Embedded Devices 6. NTP in Embedded Devices
Readers of this BCP already understand how important accurate time is As computing becomes more ubiquitous, there will be many small
for network computing. And as computing becomes more ubiquitous, embedded devices that require accurate time. These devices may not
there will be many small "Internet of Things" devices that require have a persistent battery-backed clock, so using NTP to set the
accurate time. These devices may not have a persistent battery- correct time on power-up may be critical for proper operation. These
backed clock, so using NTP to set the correct time on power-up may be devices may not have a traditional user interface, but if they
critical for proper operation. These devices may not have a connect to the Internet they will be subject to the same security
traditional user interface, but if they connect to the Internet they threats as traditional deployments.
will be subject to the same security threats as traditional
deployments.
7.1. Updating Embedded Devices 6.1. Updating Embedded Devices
Vendors of embedded devices have a special responsibility to pay Vendors of embedded devices MUST pay attention to the current state
attention to the current state of NTP bugs and security issues, of protocol security issues and bugs in their chosen implementation,
because their customers don't have the ability to update their NTP because their customers don't have the ability to update their NTP
implementation on their own. Those devices may have a single implementation on their own. Those devices may have a single
firmware upgrade, provided by the manufacturer, that updates all firmware upgrade, provided by the manufacturer, that updates all
capabilities at once. This means that the vendor assumes the 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 the latest NTP
updates applied. updates applied.
This should also include the ability to update information regarding Vendors of embedded devices SHOULD also include the ability to update
which NTP server to connect to on these devices. information regarding which NTP server to connect to on these
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 [8]. and Abuse [7].
7.2. Server configuration 6.2. Server configuration
Vendors of embedded devices that need time synchronization should Vendors of embedded devices that need time synchronization SHOULD
also carefully consider where they get their time from. There are also carefully consider where they get their time from. There are
several public-facing NTP servers available, but they may not be several public-facing NTP servers available, but they may not be
prepared to service requests from thousands of new devices on the prepared to service requests from thousands of new devices on the
Internet. Internet. Vendors SHOULD only synchronize to servers that they have
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.
Vendors should read RFC 4085 [RFC4085], which advises against Vendors should read [RFC4085], which advises against embedding
embedding globally-routable IP addresses in products, and offers globally-routable IP addresses in products, and offers several better
several better alternatives. alternatives.
7.2.1. Get a vendor subdomain for pool.ntp.org 6.2.1. Get a vendor subdomain for pool.ntp.org
The NTP Pool Project offers a program where vendors can obtain their 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 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 ability to safely make use of the time distributed by the Pool for
their devices. Vendors are encouraged to support the pool if they their devices. Vendors are encouraged to support the pool if they
participate. For more information, visit http://www.pool.ntp.org/en/ participate. For more information, visit http://www.pool.ntp.org/en/
vendors.html [9] . vendors.html [8] .
8. NTP over Anycast 7. NTP over Anycast
Anycast is described in BCP 126 [RFC4786]. (Also see RFC 7094 Anycast is described in BCP 126 [RFC4786]. (Also see [RFC7094]).
[RFC7094]). With anycast, a single IP address is assigned to With anycast, a single IP address is assigned to multiple interfaces,
multiple interfaces, and routers direct packets to the closest active and routers direct packets to the closest active interface.
interface.
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 a large number of NTP clients. Each client
can be configured with the same NTP server IP address, and a pool of can be configured with the same NTP server IP address, and a pool of
anycast servers can be deployed to service those requests. New anycast servers can be deployed to service those requests. New
servers can be added to or taken from the pool, and other than a servers can be added to or taken from the pool, and other than a
temporary loss of service while a server is taken down, these temporary loss of service while a server is taken down, these
additions can be transparent to the clients. additions can be transparent to the clients.
NOTE WELL: Using a single anycast address for NTP should be done with Note well that using a single anycast address for NTP presents its
care. It means each client will likely use a single time server own potential issues. It means each client will likely use a single
source. A key element of a robust NTP deployment is each client time server source. A key element of a robust NTP deployment is each
using multiple sources of time. With multiple time sources, a client client using multiple sources of time. With multiple time sources, a
will analyze the various time sources, selecting good ones, and client will analyze the various time sources, selecting good ones,
disregarding poor ones. If a single Anycast address is used, this and disregarding poor ones. If a single Anycast address is used,
analysis will not happen. this analysis will not happen.
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 may arbitrarily enter and leave the network, the
server a particular client is connected to may change. This may server a particular client is connected to may change. This may
cause a small shift in time from the perspective of the client when cause a small shift in time from the perspective of the client when
the server it is connected to changes. It is recommended that the server it is connected to changes. It is RECOMMENDED that
anycast only be deployed in environments where these small shifts can anycast only be deployed in environments where these small shifts can
be tolerated. 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 at least one is also RECOMMENDED that each Anycast NTP server have an alternative
Unicast interface, so its performance can be checked independently of method of access, such as an alternate Unicast IP address, so its
the anycast routing scheme. performance can be checked independently of the anycast routing
scheme.
One useful application in large networks is to use a hybrid unicast/ One useful application in large networks is to use a hybrid unicast/
anycast approach. Stratum 1 NTP servers can be deployed with unicast anycast approach. Stratum 1 NTP servers can be deployed with unicast
interfaces at several sites. Each site may have several Stratum 2 interfaces at several sites. Each site may have several Stratum 2
servers with two ethernet interfaces, or a single interface which can servers with two ethernet interfaces, or a single interface which can
support multiple addresses. One interface has a unique unicast IP support multiple addresses. One interface has a unique unicast IP
address. The second has an anycast IP interface (with a shared IP address. The second has an anycast IP interface (with a shared IP
address per location). The unicast interfaces can be used to obtain address per location). The unicast interfaces can be used to obtain
time from the Stratum 1 servers globally (and perhaps peer with the time from the Stratum 1 servers globally (and perhaps peer with the
other Stratum 2 servers at their site). Clients at each site can be other Stratum 2 servers at their site). Clients at each site can be
configured to use the shared anycast address for their site, configured to use the shared anycast address for their site,
simplifying their configuration. Keeping the anycast routing simplifying their configuration. Keeping the anycast routing
restricted on a per-site basis will minimize the disruption at the restricted on a per-site basis will minimize the disruption at the
client if its closest anycast server changes. Each Stratum 2 server client if its closest anycast server changes. Each Stratum 2 server
can be uniquely identified on their unicast interface, to make can be uniquely identified on their unicast interface, to make
monitoring easier. monitoring easier.
9. Acknowledgments 8. Acknowledgments
The authors wish to acknowledge the contributions of Sue Graves, The authors wish to acknowledge the contributions of Sue Graves,
Samuel Weiler, Lisa Perdue, Karen O'Donoghue, David Malone, Sharon Samuel Weiler, Lisa Perdue, Karen O'Donoghue, David Malone, Sharon
Goldberg, Martin Burnicki, Miroslav Lichvar, Daniel Fox Franke, and Goldberg, Martin Burnicki, Miroslav Lichvar, Daniel Fox Franke,
Robert Nagy. Robert Nagy, and Brian Haberman.
10. IANA Considerations 9. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
11. Security Considerations 10. Security Considerations
Time is a fundamental component of security on the internet. The Time is a fundamental component of security on the internet. The
absence of a reliable source of current time subverts many common web absence of a reliable source of current time subverts many common web
authentication schemes, e.g., by allowing the use of expired authentication schemes, e.g., by allowing the use of expired
credentials or by allowing for replay of messages only intended to be credentials or by allowing for replay of messages only intended to be
processed once. processed once.
Much of this document directly addresses how to secure NTP servers. Much of this document directly addresses how to secure NTP servers.
In particular, see Section 3, Section 5, and Section 6. In particular, see Section 2, Section 4, and Section 5.
There are several general threats to time synchronization protocols There are several general threats to time synchronization protocols
which are discussed in RFC 7384 [RFC7384]. which are discussed in [RFC7384].
[NTSFORNTP] is an Internet-Draft that specifies the Network Time [I-D.ietf-ntp-using-nts-for-ntp] specifies the Network Time Security
Security (NTS) mechanism and applies it specifically to NTP. Readers (NTS) mechanism and applies it to NTP. Readers are encouraged to
are encouraged to check the status of the draft, and make use of the check the status of the draft, and make use of the methods it
methods it describes. describes.
12. References 11. References
12.1. Normative References 11.1. Normative References
[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
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>.
skipping to change at page 19, line 28 skipping to change at page 19, line 5
[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 11.2. Informative References
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informative References [BCP38INFO]
"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.
[CTRLMSG] Mills, D. and B. Haberman, "Control Messages Protocol for
Use with Network Time Protocol Version 4", draft-ietf-ntp-
mode-6-cmds-05 (work in progress), March 2018.
[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>.
[CVE-2015-8139] [CVE-2015-8139]
Van Gundy, M., "NETWORK TIME PROTOCOL NTPQ AND NTPDC Van Gundy, M., "NETWORK TIME PROTOCOL NTPQ AND NTPDC
ORIGIN TIMESTAMP DISCLOSURE VULNERABILITY", 2016, ORIGIN TIMESTAMP DISCLOSURE VULNERABILITY", 2016,
<http://www.talosintel.com/reports/TALOS-2016-0078>. <http://www.talosintel.com/reports/TALOS-2016-0078>.
[CVE-2016-1548] [CVE-2016-1548]
Gardner, J. and M. Lichvar, "Xleave Pivot: NTP Basic Mode Gardner, J. and M. Lichvar, "Xleave Pivot: NTP Basic Mode
to Interleaved", 2016, to Interleaved", 2016,
<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]
Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-03 (work
in progress), September 2018.
[I-D.ietf-ntp-mac]
Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", draft-ietf-ntp-mac-05
(work in progress), October 2018.
[I-D.ietf-ntp-mode-6-cmds]
Haberman, B., "Control Messages Protocol for Use with
Network Time Protocol Version 4", draft-ietf-ntp-mode-
6-cmds-06 (work in progress), September 2018.
[I-D.ietf-ntp-using-nts-for-ntp]
Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
Sundblad, "Network Time Security for the Network Time
Protocol", draft-ietf-ntp-using-nts-for-ntp-14 (work in
progress), October 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.
[NDSS14] Rossow, C., "Amplification Hell: Revisiting Network [NDSS14] Rossow, C., "Amplification Hell: Revisiting Network
Protocols for DDoS Abuse", NDSS'14, San Diego, CA. , 2014. Protocols for DDoS Abuse", NDSS'14, San Diego, CA. , 2014.
[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>.
[NTPMAC] Malhotra, A. and S. Goldberg, "Message Authentication Code [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
for the Network Time Protocol", draft-ietf-ntp-mac-04 Requirement Levels", BCP 14, RFC 2119,
(work in progress), March 2018. DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[NTSFORNTP] [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
Sibold, D., Roettger, S., and K. Teichel, "Using the 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
Network Time Security Specification to Secure the Network May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Time Protocol", draft-ietf-ntp-using-nts-for-ntp-12 (work
in progress), July 2018.
12.3. 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.bcp38.info [2] http://www.pool.ntp.org/en/use.html
[3] http://www.pool.ntp.org/en/use.html
[4] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time [3] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time
[5] https://www.iers.org/IERS/EN/Publications/Bulletins/ [4] https://www.iers.org/IERS/EN/Publications/Bulletins/
bulletins.html bulletins.html
[6] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time [5] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time
[7] https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html [6] https://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html
[8] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse [7] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse
[9] http://www.pool.ntp.org/en/vendors.html [8] http://www.pool.ntp.org/en/vendors.html
[10] http://www.ntp.org/downloads.html [9] http://www.ntp.org/downloads.html
[11] http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [10] http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno
[12] https://support.ntp.org/bin/view/Support/ConfiguringNTP [11] https://support.ntp.org/bin/view/Support/ConfiguringNTP
Appendix A. NTP Implementation by the Network Time Foundation Appendix A. NTP Implementation by the Network Time Foundation
The Network Time Foundation (NTF) provides the reference The Network Time Foundation (NTF) provides the reference
implementation of NTP, well-known under the name "ntpd". It is implementation of NTP, well-known under the name "ntpd". It is
actively maintained and developed by NTF's NTP Project, with help actively maintained and developed by NTF's NTP Project, with help
from volunteers and NTF's supporters. This NTP software can be from volunteers and NTF's supporters. This NTP software can be
downloaded from ntp.org [10]. downloaded from ntp.org [9].
A.1. Use enough time sources A.1. Use enough time sources
In addition to the recommendation given in Section Section 4.1 the In addition to the recommendation given in Section Section 3.2 the
ntpd implementation provides the 'pool' directive. Starting with ntpd implementation provides the 'pool' directive. Starting with
ntp-4.2.6, this directive will spin up "enough" associations to ntp-4.2.6, this directive will spin up enough associations to provide
provide robust time service, and will disconnect poor servers and add robust time service, and will disconnect poor servers and add in new
in new servers as-needed. If you have good reason, you may use the servers as-needed. If you have good reason, you may use the
'minclock' and 'maxclock' options of the 'tos' command to override 'minclock' and 'maxclock' options of the 'tos' command to override
the default values of how many servers are discovered through the the default values of how many servers are discovered through the
'pool' directive. 'pool' 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
skipping to change at page 23, line 19 skipping to change at page 23, line 8
if a single upstream server of a group of configured servers provided if a single upstream server of a group of configured servers provided
a leap second notification. This would lead to misbehavior if single a leap second notification. This would lead to misbehavior if single
NTP servers sent an invalid leap second warning, e.g. due to a faulty NTP servers sent an invalid leap second warning, e.g. due to a faulty
GPS receiver in one server, but this behavior was once chosen because GPS receiver in one server, but this behavior was once chosen because
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 [11]. http://bk.ntp.org/ntp-stable/README.leapsmear?PAGE=anno [10].
A.6. Configuring ntpd A.6. Configuring ntpd
See https://support.ntp.org/bin/view/Support/ConfiguringNTP [12] for See https://support.ntp.org/bin/view/Support/ConfiguringNTP [11] 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 keyid information to their
key file in the form: key file in the form:
keyid label key keyid label key
An ntpd client establishes a protected association by appending the An ntpd client establishes a protected association by appending the
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