Network Time Protocol Best Current Practices
draft-ietf-ntp-bcp-01

Internet Engineering Task Force                           D. Reilly, Ed.
Internet-Draft                                    Spectracom Corporation
Intended status: Best Current Practice                          H. Stenn
Expires: April 7, 2017                           Network Time Foundation
                                                               D. Sibold
                                                                     PTB
                                                         October 4, 2016

              Network Time Protocol Best Current Practices
                         draft-ietf-ntp-bcp-01

Abstract

   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
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   This Internet-Draft will expire on April 7, 2017.

Copyright Notice

   Copyright (c) 2016 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
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   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  . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Monitoring  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Using Pool Servers  . . . . . . . . . . . . . . . . . . .   7
     4.6.  Leap Second Handling  . . . . . . . . . . . . . . . . . .   8
       4.6.1.  Leap Smearing . . . . . . . . . . . . . . . . . . . .   9
     4.7.  Configuring ntpd  . . . . . . . . . . . . . . . . . . . .  10
   5.  NTP Security Mechanisms . . . . . . . . . . . . . . . . . . .  10
     5.1.  Pre-Shared Key Approach . . . . . . . . . . . . . . . . .  10
     5.2.  Autokey . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Network Time Security . . . . . . . . . . . . . . . . . .  11
   6.  NTP Security Best Practices . . . . . . . . . . . . . . . . .  12
     6.1.  Minimizing Information Leakage  . . . . . . . . . . . . .  12
     6.2.  Avoiding Daemon Restart Attacks . . . . . . . . . . . . .  12
     6.3.  Detection of Attacks Through Monitoring . . . . . . . . .  13
     6.4.  Broadcast Mode Should Only Be Used On Trusted Networks  .  14
     6.5.  Symmetric Mode Should Only Be Used With Trusted Peers . .  14
   7.  NTP in Embedded Devices . . . . . . . . . . . . . . . . . . .  15
     7.1.  Updating Embedded Devices . . . . . . . . . . . . . . . .  15
     7.2.  KISS Packets  . . . . . . . . . . . . . . . . . . . . . .  15
     7.3.  Server configuration  . . . . . . . . . . . . . . . . . .  16
       7.3.1.  Get a vendor subdomain for pool.ntp.org . . . . . . .  16
   8.  NTP over Anycast  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   protect computers and networks against undefined behavior and
   security threats related to time is to keep their NTP implementations
   current.

   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 can be
   considered insecure after some time.  By keeping NTP implementations
   current, network operators can make sure that older behaviors are not
   exploited.

   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
   practical.

   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.

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   The NTP software can be downloaded from ntp.org [1] and also from
   NTF's github page [2].

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 this.  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 [3] .

4.  NTP Configuration Best Practices

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

4.1.  Use enough time sources

   ntpd takes the available sources of time and submits their timing
   data to intersection and clustering algorithms, looking for the best
   idea of the correct time.

   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 one.

   o  If there are 2 sources of time and they agree well enough, that's
      good.  But if they don't, then ntpd has no way to know which
      source to believe.

   o  If there are 3 sources of time, you can tolerate one of those
      sources becoming unreachable or unusable.  But at that point, we
      are back down to 2 sources.

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   o  4 sources of time is better.  If one of these sources develops a
      problem there are still 3 others.

   But even with 4 or more sources of time, systemic issues can happen.
   During the leap second of June of 2015, several operators implemented
   leap smearing while others did not, and many NTP end nodes became
   very confused.  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.

   Monitor your ntpd instances.  If your times 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.

   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.

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   The ability to use Mode 6 beyond its basic monitoring capabilities
   can be limited to authenticated sessions that provide a 'controlkey',
   and 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 ordinarily 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

   Users can prevent their NTP servers from participating 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.  The
   access to this service is controlled by the restrict statement in
   NTP's configuration file (ntp.conf).  The syntax reads:

   restrict address mask address_mask nomodify

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

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   If a system starts getting unexpected time replies from its time
   servers, that can be an indication that the IP address of the server
   is being forged in requests to that time server, and these abusers
   are trying to convince your time servers to stop serving time to the
   system.

   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
   server.

   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 identify the IPs of
   machines that are allowed to serve time to the group.

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 collection of volunteers who have donated
   their computing and bandwidth resources to provide time on the
   Internet for free.  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 http://www.pool.ntp.org/en/
   use.html .

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

   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 pool.ntp.org [5] .

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4.6.  Leap Second Handling

   UTC is kept in agreement with the astronomical time UT1 [6] to witin
   +0.9 second (in absolute value) by the insertion of leap seconds.
   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 when necessary to keep UTC and UT1 aligned by the
   IERS (International Earth rotation and Reference systems Service) in
   its Bulletin C [7].

   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 have a Stratum-1 server synced to GNSS (or you
   are synced to a lower stratum server that is ultimately synced to
   GNSS), you will get advance notification of impending leap seconds
   automatically.

   Since the length of the UT1 day is slowly increasing [8], all leap
   seconds that have been introduced since the practice started 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 removed
   from UTC, should that ever become necessary.

   While earlier versions of NTP contained some ambiguity regarding when
   leap seconds could occur, RFC 5905 is clear that leap seconds are
   processed at the end of a month.  If an upstream server is
   broadcasting that a leap second is pending, RFC5905-compliant servers
   should apply it at the end of the last minute of the last day of the
   month.

   The IETF maintains a leap second list [9] 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 the file:

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

   You will need to restart to apply the changes.

   Files are also available from other sources:

   NIST: ftp://time.nist.gov/pub/leap-seconds.list

   US Navy (maintains GPS Time): ftp://tycho.usno.navy.mil/pub/ntp/leap-
   seconds.list

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   IERS (announces leap seconds):
   https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list

   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 ntpd 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 only 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.

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.  (86400s,
   which is 1 day, has 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 agree on what time they think it is!

   The purpose of Leap Smearing is to enable software that doesn't deal
   with the leap second event properly to function correctly, 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.
   Support 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
   http://bk1.ntp.org/ntp-stable/README.leapsmear?PAGE=anno .

   Clients that are connected to leap smearing servers must never apply
   the "standard" NTP leap second handling.  So if they are using ntpd,

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   these clients must never have a leap second file loaded, and the
   smearing servers must never advertise 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
   configured to use a mixture of smeared and non-smeared servers.

4.7.  Configuring ntpd

   See https://support.ntp.org/bin/view/Support/ConfiguringNTP 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 it
   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 is 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.  If the NTP daemon is
   built against an OpenSSL library, NTP can also base the calculation
   of the MAC upon the SHA-1 or any other digest algorithm supported by
   each side's OpenSSL library.

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   To use this approach the communication partners have to exchange the
   key, which consists of a keyid with a value between 1 and 65534,
   inclusive, and a label which indicates the chosen digest algorithm.
   Each communication partner adds this information to 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
   file:

   server address key keyid

   Note that the NTP process has to trust the applied key.  An NTP
   process explicitly has to add each key it want to trust to a 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.  By 2011, security researchers had identified
   computational areas in the Autokey protocol that, while secure at the
   time of its original design, were no longer secure.

   We recommend that Autokey NOT BE USED.  Know that as of the fall of
   2011, a common(?) laptop computer could crack the security cookie
   used in the Autokey protocol in 30 minutes' time.  If you must use
   Autokey, know that your session keys should be set to expire in under
   30 minutes' time.

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 2016.

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6.  NTP Security Best Practices

6.1.  Minimizing Information Leakage

   The base NTP packet leaks important information (including reference
   ID and reference time) that can 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
   inspecting the fields in the mode 4 response packet.  NTP control
   queries also leak important information (including reference ID,
   expected origin timestamp, etc) that can 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
   response.

   As such, access control should be used to limit the exposure of this
   information to third parties.

   All hosts should only respond to NTP control queries from authorized
   parties.  One way to do this is to only allow control queries from
   authorized IP addresses.

   A host that is not supposed to act as an NTP server that provides
   timing information to other hosts should additionally drop incoming
   mode 3 timing queries.

   An "end 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, end hosts should drop all incoming NTP packets
   except mode 4 response packets that come from its configured servers.

6.2.  Avoiding Daemon Restart Attacks

   [RFC5905] says NTP clients should not accept time shifts greater than
   the panic threshold.  Specifically, RFC5905 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

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       traditional init systems with process supervisors, such as
       systemd, which can be configured to automatically restart any
       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 systemd.)

   2.  The NTP daemon ignores the panic threshold when it is restarted.
       (This is sometimes called the -g option.)

   In such cases, the attacker can send the target an offset that
   exceeds the panic threshold, causing the client to 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.  A natural
   solution is not to run hosts with these conditions.

   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  Modify the NTP daemon so that it "hangs" (ie does not quit, but
      just waits for a better timing samples but does not modify the
      local clock) when it receives a large offset.

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.

   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].

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   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 (e.g. poll>10) could be sign that the client is under
   attack.

6.4.  Broadcast Mode Should Only Be Used On Trusted Networks

   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 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.

6.5.  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 arrival 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

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   results of the target's Byzantine fault tolerant peer selection
   algorithms.

7.  NTP in Embedded Devices

   Readers of this BCP 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 deployments.

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 the NTP server
   address.

   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 [12].

7.2.  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.

   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.

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7.3.  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
   Internet.

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

7.3.1.  Get a vendor subdomain for pool.ntp.org

   The NTP Pool Project offers a program where vendors can obtain their
   own subdomain that is part of the NTP Pool.  This offers vendors the
   ability to safely make use of the time distributed by the Pool for
   their devices.  Vendors are encouraged to support the pool if they
   participate.  For more information, visit http://www.pool.ntp.org/en/
   vendors.html .

8.  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
   interface.

   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.

   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 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

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   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
   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, and Martin Burnicki.

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

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   check the status of these drafts, and make use of the methods they
   describe.

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <http://www.rfc-editor.org/info/rfc4786>.

   [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,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <http://www.rfc-editor.org/info/rfc7094>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <http://www.rfc-editor.org/info/rfc7384>.

12.2.  Informative References

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

   [CVE-2015-8138]
              Van Gundy, M. and J. Gardner, "NETWORK TIME PROTOCOL
              ORIGIN TIMESTAMP CHECK IMPERSONATION VULNERABILITY", 2016,
              <http://www.talosintel.com/reports/TALOS-2016-0077>.

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   [CVE-2015-8139]
              Van Gundy, M., "NETWORK TIME PROTOCOL NTPQ AND NTPDC
              ORIGIN TIMESTAMP DISCLOSURE VULNERABILITY", 2016,
              <http://www.talosintel.com/reports/TALOS-2016-0078>.

   [CVE-2016-1548]
              Gardner, J. and M. Lichvar, "Xleave Pivot: NTP Basic Mode
              to Interleaved", 2016,
              <http://blog.talosintel.com/2016/04/
              vulnerability-spotlight-further-ntpd_27.html>.

   [NDSS16]   Malhotra, A., Cohen, I., Brakke, E., and S. Goldberg,
              "Attacking the Network Time Protocol", NDSS'16, San Diego,
              CA. , 2016, <https://eprint.iacr.org/2015/1020.pdf>.

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

   [NTSFORNTP]
              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

   [1] http://www.ntp.org/downloads.html

   [2] https://github.com/ntp-project/ntp

   [3] http://www.bcp38.info

   [5] http://www.pool.ntp.org/en/join.html

   [6] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time

   [7] https://www.iers.org/IERS/EN/Publications/Bulletins/
       bulletins.html

   [8] https://en.wikipedia.org/wiki/Solar_time#Mean_solar_time

   [9] https://www.ietf.org/timezones/data/leap-seconds.list

   [12] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse

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Authors' Addresses

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

   Email: denis.reilly@spectracom.orolia.com

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

   Email: stenn@nwtime.org

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

   Phone: +49-(0)531-592-8420
   Fax:   +49-531-592-698420
   Email: dieter.sibold@ptb.de

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