Internet Engineering Task Force                           D. Reilly, Ed.
Internet-Draft                                                Spectracom
Intended status: Best Current Practice                          H. Stenn
Expires: December 15, 2017                       Network Time Foundation
                                                               D. Sibold
                                                           June 13, 2017

              Network Time Protocol Best Current Practices


   NTP Version 4 (NTPv4) has been widely used since its publication as
   RFC 5905 [RFC5905].  This documentation is a collection of Best
   Practices from across the NTP community.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on December 15, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Keeping NTP up to date  . . . . . . . . . . . . . . . . . . .   3
   3.  General Network Security Best Practices . . . . . . . . . . .   4
     3.1.  BCP 38  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  NTP Configuration Best Practices  . . . . . . . . . . . . . .   4
     4.1.  Use enough time sources . . . . . . . . . . . . . . . . .   4
     4.2.  Use a diversity of Reference Clocks . . . . . . . . . . .   5
     4.3.  Mode 6 and 7  . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Monitoring  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Using Pool Servers  . . . . . . . . . . . . . . . . . . .   8
     4.6.  Leap Second Handling  . . . . . . . . . . . . . . . . . .   8
       4.6.1.  Leap Smearing . . . . . . . . . . . . . . . . . . . .  10
     4.7.  Configuring ntpd  . . . . . . . . . . . . . . . . . . . .  11
   5.  NTP Security Mechanisms . . . . . . . . . . . . . . . . . . .  11
     5.1.  Pre-Shared Key Approach . . . . . . . . . . . . . . . . .  11
     5.2.  Autokey . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Network Time Security . . . . . . . . . . . . . . . . . .  12
   6.  NTP Security Best Practices . . . . . . . . . . . . . . . . .  12
     6.1.  Minimizing Information Leakage  . . . . . . . . . . . . .  12
     6.2.  Avoiding Daemon Restart Attacks . . . . . . . . . . . . .  13
     6.3.  Detection of Attacks Through Monitoring . . . . . . . . .  14
     6.4.  KISS Packets  . . . . . . . . . . . . . . . . . . . . . .  15
     6.5.  Broadcast Mode Should Only Be Used On Trusted Networks  .  15
     6.6.  Symmetric Mode Should Only Be Used With Trusted Peers . .  16
   7.  NTP in Embedded Devices . . . . . . . . . . . . . . . . . . .  17
     7.1.  Updating Embedded Devices . . . . . . . . . . . . . . . .  17
     7.2.  Server configuration  . . . . . . . . . . . . . . . . . .  17
       7.2.1.  Get a vendor subdomain for . . . . . . .  17
   8.  NTP over Anycast  . . . . . . . . . . . . . . . . . . . . . .  18
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   NTP Version 4 (NTPv4) has been widely used since its publication as
   RFC 5905 [RFC5905].  This documentation is a collection of Best
   Practices from across the NTP community.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Keeping NTP up to date

   Many network security mechanisms rely on time as part of their
   operation.  If an attacker 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.  The best way to detect
   and protect computers and networks against undefined behavior and
   security threats related to time is 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, and properly monitoring
   their time infrastructure, network operators can make sure that their
   time infrastructure is operating correctly and within specification,
   and is not being attacked or misused.

   Thousands of individual bugs have been found and fixed in the NTP
   Project's ntpd since the first NTPv4 release in 1997.  Each version
   release contains at least a few bug fixes.  The best way to stay in
   front of these issues is to keep your NTP implementation current.

   There are multiple versions of the NTP protocol in use, and multiple
   implementations in use, on many different platforms.  It is
   recommended that NTP users actively monitor wherever they get their
   software to find out if their versions are vulnerable to any known
   attacks, and deploy updates containing security fixes as soon as

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   The reference implementation of NTP Version 4 from Network Time
   Foundation (NTF) continues to be actively maintained and developed by
   NTF's NTP Project, with help from volunteers and NTF's supporters.
   This NTP software can be downloaded from [1] and also from
   NTF's NTP Project's BitKeeper repository [2] or github page [3].

3.  General Network Security Best Practices

3.1.  BCP 38

   Many network attacks rely on modifying the IP source address of a
   packet to point to a different IP address than the computer which
   originated it.  This modification/abuse vector has been known for
   quite some time, and BCP 38 [RFC2827] was approved in 2000 to address
   this.  BCP 38 [RFC2827] calls for filtering outgoing and incoming
   traffic to 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 all networks (and ISP's of any
   size) implement ingress and egress filtering.  If a machine on a
   network is sending out packets claiming to be from an address that is
   not on that network, this could be the first indication that there is
   a machine that has been compromised, and is being used abusively.  If
   packets are arriving on an external interface with a source address
   that should only be seen on an internal network, that's a strong
   indication that an attacker is trying to inject spoofed packets into
   the network.  More information is available at the BCP38 Info page
   [4] .

4.  NTP Configuration Best Practices

   These Best Practices, while based on the ntpd reference
   implementation maintained by Network Time Foundation, may be
   applicable to other implementations as well.

4.1.  Use enough time sources

   An NTP implementation (as opposed to an SNTP implementation) takes
   the available sources of time and submits this timing data to
   sophisticated intersection, clustering, and combing algorithms to get
   the best estimate of the correct time.  The description of these
   algorithms is beyond the scope of this document.  Interested readers
   should read RFC 5905 [RFC5905] or the detailed description of NTP in
   MILLS 2006 [MILLS2006]

   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
      that can be considered.  Any issue with the time at the source
      will be passed on to the client.

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   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
      fails, then your solution degrades to the single-source solution
      outlined above.  And if the two sources don't agree, then it's
      impossible to know which one is correct by simply looking at the

   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
      accurate.  And you can tolerate one of the sources becoming
      unreachable or unusable.  But at that point, you are back down to
      2 sources.

   o  4 or more sources of time is better.  If one of these sources
      develops a problem there are still at least 3 other time sources.

   But even with 4 or more sources of time, systemic problems can
   happen.  During the leap second of June of 2015, several operators
   implemented leap smearing while others did 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/POSIX time, while the
   other 2 gave them consistent leap-smeared time.  See Section 4.6.1
   for more information.

   Starting with ntp-4.2.6, the 'pool' directive will spin up "enough"
   associations to provide robust time service, and will disconnect poor
   servers and add in new servers as-needed.  If you have good reason,
   you may use the 'minclock' and 'maxclock' options of the 'tos'
   command to override the default values of how many servers are
   discovered through the 'pool' directive.

   Monitor your ntpd instances.  If your time sources do not generally
   agree, find out why and either correct the problems or stop using
   defective servers.  See Section 4.4 for more information.

4.2.  Use a diversity of Reference Clocks

   When using servers with attached hardware reference clocks, it is
   recommended that several different types of reference clocks be used.
   Having a diversity of sources means 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
   same bugs.  Are they using the same base chipset, regardless of
   whether or not the finished products are from different vendors?  Are
   they all running the same version of firmware?  Chipset and firmware
   bugs can happen, but is often more difficult to diagnose than a
   standard software bug.

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   A systemic problem with time from any satellite navigation service is
   possible and has happened.  Sunspot activity can render satellite or
   radio-based time source unusable.  If the time on your network must
   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
   the system is unavailable (or wrong!).

4.3.  Mode 6 and 7

   NTP Mode 6 (ntpq) and Mode 7 (ntpdc) packets are designed to permit
   monitoring and optional authenticated control of ntpd and its
   configuration.  Used properly, these facilities provide vital
   debugging and performance information and control.  Used improperly,
   these facilities can be an abuse vector.

   Mode 7 queries have been disabled by default in ntpd since 4.2.7p230,
   released on 2011/11/01.  Do not enable Mode 7 unless there is a
   compelling reason to do so.

   The ability to use Mode 6 beyond its basic monitoring capabilities
   can be limited to authenticated sessions that provide a 'controlkey'.
   Similarly, if Mode 7 has been explicitly enabled its use for more
   than basic monitoring can be limited to authenticated sessions that
   provide a 'requestkey'.

   Older versions of the reference implementation of NTP could be abused
   to participate in high-bandwidth DDoS attacks, if the above
   restrictions are not applied.  Starting with ntp-4.2.7p26, released
   in April of 2010, ntpd requires the use of a nonce before replying
   with potentially large response packets.

   As mentioned above, there are two general ways to use Mode 6 and Mode
   7 requests.  One way is to query ntpd for information, and this mode
   can be disabled with:

   restrict ... noquery

   The second way to use Mode 6 and Mode 7 requests is to modify ntpd's
   behavior.  Modification of ntpd's configuration requires an
   authenticated session BY default.  If no authentication keys have
   been specified no modifications can be made.  For additional
   protection, the ability to perform these modifications can be
   controlled with:

   restrict ... nomodify

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   Users can prevent their NTP servers from considering query/
   configuration traffic by default by adding the following to their
   ntp.conf file:

   restrict default -4 nomodify notrap nopeer noquery

   restrict default -6 nomodify notrap nopeer noquery

   restrict source nomodify notrap noquery
   # nopeer is OK if you don't use the 'pool' directive

4.4.  Monitoring

   The reference implementation of NTP allows remote monitoring.  Access
   to this service is generally controlled by the "noquery" directive in
   NTP's configuration file (ntp.conf) via a "restrict" statement.  The
   syntax reads:

   restrict address mask address_mask noquery

   Monitor ntpd instances so machines that are "out of sync" can be
   quickly identified.  Monitor system logs for messages from ntpd so
   problems and abuse attempts can be quickly identified.

   If a system starts getting unexpected time replies from its time
   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
   trying to convince that time server to stop serving time to that

   If a system is a broadcast client and its syslog shows that it is
   receiving "early" time messages from its server, that is an
   indication that somebody may be forging packets from a broadcast

   If a server's syslog shows messages that indicates it is receiving
   timestamps that are earlier than the current system time, then either
   the system clock is unusually fast or somebody is trying to launch a
   replay attack against that server.

   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
   machines that are allowed to serve time to the group.

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4.5.  Using Pool Servers

   It only takes a small amount of bandwidth and system resources to
   synchronize one NTP client, but NTP servers that can service tens of
   thousands of clients take more resources to run.  Users who want to
   synchronize their computers should only synchronize to servers that
   they have permission to use.

   The NTP pool project is a group of volunteers who have donated their
   computing and bandwidth resources to freely distribute time from
   primary time sources to others on the Internet.  The time is
   generally of good quality, but comes with no guarantee whatsoever.
   If you are interested in using the pool, please review their
   instructions at

   If you are a vendor who wishes to provide time service to your
   customers or clients, consider joining the pool and providing a
   "vendor zone" thru the pool project.

   If you want to synchronize many computers, consider running your own
   NTP servers that are synchronized by the pool, and synchronizing your
   clients to your in-house NTP servers.  This reduces the load on the

   If you would like to contribute a server with a static IP address and
   a permanent Internet conenction to the pool, please consult the
   instructions at .

4.6.  Leap Second Handling

   UTC is kept in agreement with the astronomical time UT1 [7] to within
   +/- 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
   rotational rate of the earth.  Leap seconds are not introduced at a
   fixed rate.  They are announced by the IERS (International Earth
   rotation and Reference systems Service) in its Bulletin C [8] when
   necessary to keep UTC and UT1 aligned.

   NTP time is based on the UTC timescale, and the protocol has the
   capability to broadcast leap second information.  Some GNSS systems
   (like GPS) or radio transmitters (like DCF77) broadcast leap second
   information, so if you are synced to an ntp server that is ultimately
   synced to a source that provides leap second notification you will
   get advance notification of impending leap seconds automatically.

   Since the length of the UT1 day is generally slowly increasing [9],
   all leap seconds that have been introduced since the practice started
   in 1972 have been "positive" leap seconds, where a second is added to

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   UTC.  NTP also supports a "negative" leap second, where a second is
   removed from UTC, should that ever become necessary (or useful).

   While earlier versions of NTP contained some ambiguity regarding when
   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
   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
   wait until the last day of the month before broadcasting leap
   seconds.  Doing this will prevent older clients from applying a leap
   second at the wrong time.  Note well that NTPv4 allows a maximum poll
   interval of 17, or 131,072 seconds, which is longer than a day.

   The IETF maintains a leap second list [10] for NTP users who are not
   receiving leap second information through an automatic source.  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
   ntpd.conf to apply and use the file:

   leapfile "/path/to your/leap-file"

   You may need to restart ntpd to apply this change.

   Files are also available from other sources:


   US Navy (maintains GPS Time):

   IERS (announces leap seconds):

   ntpd servers with a manually configured leap second file will ignore
   leap second information broadcast from upstream NTP servers until the
   leap second file expires.  If no valid leap second file is available
   then a leap second notification from an attached reference clock is
   always accepted by ntpd.

   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
   majority of servers must provide the notification before it is
   accepted.  Before 4.2.6, a leap second notification would be accepted
   if a single upstream server of a group of configured servers provided
   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
   GPS receiver in one server, but this behavior was once chosen because

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   in the "early days" there was a greater chance that leap second
   information would be available from a very limited number of sources.

4.6.1.  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 sucessfully.  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!

   NOTE WELL that using a leap-smear can cause your reported time to be
   "legally indefensible" and/or be a breach of compliance regulations.

   The purpose of Leap Smearing is to enable systems that don't deal
   with the leap second event properly to function consistently, at the
   expense of fidelity to UTC during the smear window.  During a
   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
   to have problems with that.

   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
   configured by default and must be added at compile time.  In
   addition, no leap smearing will occur unless a leap smear interval is
   specified in ntpd.conf .  For more information, refer to [11].

   Clients that are connected to leap smearing servers MUST NOT apply
   the "standard" NTP leap second handling.  So if they are using ntpd,
   these clients must never have a leap second file loaded, and the
   smearing servers must never advertise to clients that a leap second
   is pending.

   Leap Smearing MUST NOT be used for public-facing NTP servers, as they
   will disagree with non-smearing servers (as well as UTC) during the
   leap smear interval.  However, be aware that some public-facing
   servers may be configured this way anyway in spite of this guidance.

   System Administrators are advised to be aware of impending leap
   seconds and how the servers (inside and outside their organization)
   they are using deal with them.  Individual clients must never be

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   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
   same leap smear configuration.

4.7.  Configuring ntpd

   Configuration access to ntpd is controlled by the "modify" status in
   NTP's configuration file (ntp.conf), which is controlled by a
   "restrict" statement.  The syntax is:

   restrict address mask address_mask nomodify

   See for
   additional information on configuring ntpd.

5.  NTP Security Mechanisms

   In the standard configuration NTP packets are exchanged unprotected
   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
   content of the NTP packet, which leads to degradation of the time
   synchronization or the transmission of false time information.  A
   profound threat analysis for time synchronization protocols are given
   in RFC 7384 [RFC7384].  NTP provides two internal security mechanisms
   to protect authenticity and integrity of the NTP packets.  Both
   measures protect the NTP packet by means of a Message Authentication
   Code (MAC).  Neither of them encrypts the NTP's payload, because this
   payload information is not considered to be confidential.

5.1.  Pre-Shared Key Approach

   This approach applies a symmetric key for the calculation of the MAC,
   which protects authenticity and integrity of the exchanged packets
   for a association.  NTP does not provide a mechanism for the exchange
   of the keys between the associated nodes.  Therefore, for each
   association, keys have to be exchanged securely by external means.
   It is recommended that each association be protected by its own
   unique key.  NTP does not provide a mechanism to automatically
   refresh the applied keys.  It is therefore recommended that the
   participants periodically agree on a fresh key.  The calculation of
   the MAC may always be based on an MD5 hash, and an AES-128-CMAC hash
   is expected to soon be allowed as well.  If the NTP daemon is built
   against an OpenSSL library, NTP can also base the calculation of the
   MAC upon any other digest algorithm supported by each side's OpenSSL

   To use this approach the communication partners have to exchange the
   key, which consists of a keyid with a value between 1 and 65534,

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   inclusive, and a label which indicates the chosen digest algorithm.
   Each communication partner adds this information to their key file in
   the form:

   keyid label key

   The key file contains the key in clear text.  Therefore it should
   only be readable by the NTP process.  Different keys are added line
   by line to the key file.

   A NTP client establishes a protected association by appending the
   option "key keyid" to the server statement in the NTP configuration

   server address key keyid

   Note that the NTP process has to trust the applied key.  A key is
   deemed trusted when its keyid is added to the list of trusted keys by
   the "trustedkey" statement in the NTP configuration file.

   trustedkey keyid_1 keyid_2 ... keyid_n

5.2.  Autokey

   Autokey was designed in 2003 to provide a means for clients to
   authenticate servers.  However, security researchers have identified
   vulnerabilities in the Autokey protocol, which make the protocol
   "useless". [13]


5.3.  Network Time Security

   Work has begun on an enhanced replacement for Autokey, which is
   called Network Time Security (NTS) [NTS].  NTS was published in the
   summer of 2013.  As of October 2016, this effort was at draft #15,
   and about to begin 'final call'.  The first unicast implementation of
   NTS was started in the summer of 2015 and is expected to be released
   in early 2017.

6.  NTP Security Best Practices

6.1.  Minimizing Information Leakage

   The base NTP packet leaks important information (including reference
   ID and reference time) that may be used in attacks [NDSS16],
   [CVE-2015-8138], [CVE-2016-1548].  A remote attacker can learn this
   information by sending mode 3 queries to a target system and

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   inspecting the fields in the mode 4 response packet.  NTP control
   queries also leak important information (including reference ID,
   expected origin timestamp, etc.) that may be used in attacks
   [CVE-2015-8139].  A remote attacker can learn this information by
   sending control queries to a target system and inspecting the

   As such, access control should be used to limit the exposure of this
   information to inappropriate third 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
   timing information to other hosts may additionally log and drop
   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
   query.  A much better appropach might be to filter mode 3 queries at
   the edge, or make sure mode 3 queries are allowed from trusted
   systems or networks.

   An "leaf-node host" is host that is using NTP solely for the purpose
   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
   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
   these servers containing timing information.)  To minimize
   information leakage, leaf-node hosts should drop all incoming NTP
   packets except mode 4 response packets that come from unknown
   sources.  Note well that proper monitoring of an ntpd instance
   includes checking the time of that ntpd instance.

6.2.  Avoiding Daemon Restart Attacks

   RFC 5905 [RFC5905] says NTP clients should not accept time shifts
   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
   the following two conditions hold:

   1.  The operating system automatically restarts the NTP daemon when
       it quits.  (Modern *NIX operating systems are replacing
       traditional init systems with process supervisors, such as
       systemd, which can be configured to automatically restart any

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       daemons that quit.  This behavior is the default in CoreOS and
       Arch Linux.  It is likely to become the default behavior in other
       systems as they migrate legacy init scripts to process
       supervisors such as systemd.)

   2.  If, against long-standing recommendation, ntpd is always started
       with the -g option, it will ignore the panic threshold when it is
       restarted.  The -g option SHOULD only be provided in cold-start

   In such cases, if the attacker can send the target an offset that
   exceeds the panic threshold, the client will quit.  Then, when the
   client restarts, it ignores the panic threshold and accepts the
   attacker's large offset.

   Hosts running with the above two conditions should be aware that the
   panic threshold does not protect them from attacks.  The recommended
   and natural solution is not to run hosts with these conditions.
   Specifically, only use the -g flag in cold-start situations, or when
   sufficient oversight and checking is in place to make sure that the
   use of -g is appropriate.

   As an alternative, the following steps could be taken to mitigate the
   risk of attack.

   o  Monitor 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.

   o  Request manual intervention when a timestep larger than the panic
      threshold is detected.

   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  Add "minsane" and "minclock" parameters to the ntp.conf file so
      ntpd waits until "enough" trusted sources of time agree on the
      correct time.

6.3.  Detection of Attacks Through Monitoring

   Users should monitor their NTP instances to detect attacks.  Many
   known attacks on NTP have particular signatures.  Common attack
   signatures include:

   1.  "Bogus packets" - A packet whose origin timestamp does not match
       the value that expected by the client.

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   2.  "Zero origin packet" - A packet with a origin timestamp set to
       zero [CVE-2015-8138].

   3.  A packet with an invalid cryptographic MAC [CCR16].

   The observation of many such packets could indicate that the client
   is under attack.

   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.  See
   Section 6.4 for more information.

6.4.  KISS Packets

   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.
   It is important for all NTP devices to respect these packets and back
   off when asked to do so by a server.  It is even more important for
   an embedded device, which may not have exposed a control interface
   for NTP.

   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
   increase its poll interval value (thus decreasing its polling rate)
   up to a reasonable maximum.  This maximum can vary by implementation
   but should not exceed a poll interval value of 13 (2 hours).  The
   mechanism to determine how much to increase the poll interval value
   is undefined in RFC 5905 [RFC5905].  If the client uses the poll
   interval value sent by the server in the KoD packet, it must not
   simply accept any value.  Using large interval values may open a
   vector for a denial-of-service attack that causes the client to stop
   querying its server [NDSS16].

   The KoD mechanism relies on clients behaving properly in order to be
   effective.  Some clients ignore the KoD packet entirely, and other
   poorly-implemented clients might unintentionally increase their poll
   rate and simulate a denial of service attack.  Server administrators
   should be prepared for this and take measures outside of the NTP
   protocol to drop packets from misbehaving clients.

6.5.  Broadcast Mode Should Only Be Used On Trusted Networks

   Per RFC 5905 [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.

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   Importantly, all broadcast clients that listen to this server must
   know the cryptographic key.  This mean that any client can use this
   key to send valid broadcast messages that look like they come from
   the broadcast server.  Thus, a rogue broadcast client can use its
   knowledge of this key to attack the other broadcast clients.

   For this reason, an NTP broadcast server and all its client must
   trust each other.  Broadcast mode should only be run from within a
   trusted network.

   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.
   Use this feature.  Note, however, that an adversarial client that
   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
   because the origin timestamp on broadcast mode packets is not
   validated by the client.  By contrast, client/server and symmetric
   modes do require origin timestamp validation, making it more
   difficult to spoof packets [CCR16].

6.6.  Symmetric Mode Should Only Be Used With Trusted Peers

   In symmetric mode, two peers Alice and Bob can both push and pull
   synchronization to and from each other using either ephemeral
   symmetric passive (mode 2) or persistent symmetric active (NTP mode
   1) packets.  The persistent association is preconfigured and
   initiated at the active peer but not preconfigured at the passive
   peer (Bob).  Upon receipt of a mode 1 NTP packet from Alice, Bob
   mobilizes a new ephemeral association if he does not have one
   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
   symmetric passive peer.

   For this reason, a host (Bob) should only allow symmetric passive
   associations to be established with trusted peers.  Specifically, Bob
   should require each of its symmetric passive association to be
   cryptographically authenticated.  Each symmetric passive association
   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

   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.
   Use this feature.

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7.  NTP in Embedded Devices

   Readers of this BCP likely already understand how important accurate
   time is for network computing.  And as computing becomes more
   ubiquitous, there will be many small "Internet of Things" devices
   that require accurate time.  These embedded devices may not have a
   traditional user interface, but if they connect to the Internet they
   will be subject to the same security threats as traditional

7.1.  Updating Embedded Devices

   Vendors of embedded devices have a special responsibility to pay
   attention to the current state of NTP bugs and security issues,
   because their customers don't have the ability to update their NTP
   implementation on their own.  Those devices may have a single
   firmware upgrade, provided by the manufacturer, that updates all
   capabilities at once.  This means that the vendor assumes the
   responsibility of making sure their devices have the latest NTP
   updates applied.

   This should also include the ability to update any NTP server
   addresses on these devices.

   There is a catalog of NTP server abuse incidents, some of which
   involve embedded devices, on the Wikipedia page for NTP Server Misuse
   and Abuse [14].

7.2.  Server configuration

   Vendors of embedded devices that need time synchronization should
   also carefully consider where they get their time from.  There are
   several public-facing NTP servers available, but they may not be
   prepared to service requests from thousands of new devices on the

   Vendors are encouraged to invest resources into providing their own
   time servers for their devices to connect to.

7.2.1.  Get a vendor subdomain for

   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
   vendors.html .

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8.  NTP over Anycast

   Anycast is described in BCP 126 [RFC4786].  (Also see RFC 7094
   [RFC7094]).  With anycast, a single IP address is assigned to
   multiple interfaces, and routers direct packets to the closest active

   Anycast is often used for Internet services at known IP addresses,
   such as DNS.  Anycast can also be used in large organizations to
   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
   anycast servers can be deployed to service those requests.  New
   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
   additions can be transparent to the clients.

   NOTE WELL: Using a single anycast address for NTP should be done with
   care.  It means each client will likely use a single 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
   will analyze the various time sources, selecting good ones, and
   disregarding poor ones.  If a single Anycast address is used, this
   analysis will not happen.

   If clients are connected to an NTP server via anycast, the client
   does not know which particular server they are connected to.  As
   anycast servers may arbitrarily enter and leave the network, the
   server a particular client is connected to may change.  This may
   cause a small shift in time from the perspective of the client when
   the server it is connected to changes.  It is recommended that
   anycast only be deployed in environments where these small shifts can
   be tolerated.

   Configuration of an anycast interface is independent of NTP.  Clients
   will always connect to the closest server, even if that server is
   having NTP issues.  It is recommended that anycast NTP
   implementations have an independent method of monitoring the
   performance of NTP on a server.  If the server is not performing to
   specification, it should remove itself from the Anycast network.  It
   is also recommended that each Anycast NTP server have at least one
   Unicast interface, so its performance can be checked independently of
   the anycast routing scheme.

   One useful application in large networks is to use a hybrid unicast/
   anycast approach.  Stratum 1 NTP servers can be deployed with unicast
   interfaces at several sites.  Each site may have several Stratum 2
   servers with two ethernet interfaces.  One interface has a unique
   unicast IP address.  The second has an anycast IP interface (with a

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   shared IP address per location).  The unicast interfaces can be used
   to obtain 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 configured to use the shared anycast address for their
   site, simplifying their configuration.  Keeping the anycast routing
   restricted on a per-site basis will minimize the disruption at the
   client if its closest anycast server changes.  Each Stratum 2 server
   can be uniquely identified on their unicast interface, to make
   monitoring easier.

9.  Acknowledgements

   The authors wish to acknowledge the contributions of Sue Graves,
   Samuel Weiler, Lisa Perdue, Karen O'Donoghue, David Malone, Sharon
   Goldberg, Martin Burnicki, Miroslav Lichvar, Daniel Fox Franke, and
   Robert Nagy.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Security Considerations

   Time is a fundamental component of security on the internet.
   Credentials and certificates can expire.  Logins and other forms of
   access can be revoked after a period of time, or at a scheduled time.
   And some applications may assume that system time cannot be changed
   and is always monotonic, and vulnerabilites may be exposed if a time
   in the past is forced into a system.  Therefore, any system
   adminstrator concerned with security should be concerned with how the
   current time gets into their system.

   [NTS] is an Internet-Draft of a collection of methods to secure time
   transfer over networks.  [NTSFORNTP] is an Internet-Draft that
   applies the methods in [NTS] specifically to NTP.  At the time of
   this writing, these are still drafts.  Readers are encourages to
   check the status of these drafts, and make use of the methods they

12.  References

12.1.  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,

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   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <>.

12.2.  Informative References

   [CCR16]    Malhotra, A. and S. Goldberg, "Attacking NTP's
              Authenticated Broadcast Mode", SIGCOMM Computer
              Communications Review (CCR) , 2016.

              Van Gundy, M. and J. Gardner, "NETWORK TIME PROTOCOL


              Gardner, J. and M. Lichvar, "Xleave Pivot: NTP Basic Mode
              to Interleaved", 2016,

              Mills, D., "Computer network time synchronization: the
              Network Time Protocol", CRC Press , 2006.

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   [NDSS16]   Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
              "Attacking the Network Time Protocol", NDSS'16, San Diego,
              CA. , 2016, <>.

   [NTS]      Sibold, D., Roettger, S., and K. Teichel, "Network Time
              Security", draft-ietf-ntp-network-time-security-15 (work
              in progress), September 2016.

              Sibold, D., Roettger, S., and K. Teichel, "Using the
              Network Time Security Specification to Secure the Network
              Time Protocol", draft-ietf-ntp-using-nts-for-ntp-06 (work
              in progress), September 2016.

12.3.  URIs












Authors' Addresses

   Denis Reilly (editor)
   1565 Jefferson Road, Suite 460
   Rochester, NY  14623


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   Harlan Stenn
   Network Time Foundation
   P.O. Box 918
   Talent, OR  97540


   Dieter Sibold
   Physikalisch-Technische Bundesanstalt
   Bundesallee 100
   Braunschweig  D-38116

   Phone: +49-(0)531-592-8420
   Fax:   +49-531-592-698420

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