Internet Engineering Task Force D. Reilly, Ed.
Internet-Draft Spectracom
Intended status: Best Current Practice H. Stenn
Expires: October 15, 2017 Network Time Foundation
D. Sibold
PTB
April 13, 2017
Network Time Protocol Best Current Practices
draft-ietf-ntp-bcp-03
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 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
(http://trustee.ietf.org/license-info) 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 . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . 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. Broadcast Mode Should Only Be Used On Trusted Networks . 14
6.5. Symmetric Mode Should Only Be Used With Trusted Peers . . 15
7. NTP in Embedded Devices . . . . . . . . . . . . . . . . . . . 15
7.1. Updating Embedded Devices . . . . . . . . . . . . . . . . 16
7.2. KISS Packets . . . . . . . . . . . . . . . . . . . . . . 16
7.3. Server configuration . . . . . . . . . . . . . . . . . . 16
7.3.1. Get a vendor subdomain for pool.ntp.org . . . . . . . 17
8. NTP over Anycast . . . . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . 19
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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",
"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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This 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 is
normally only 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 developed and maintained by 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
might 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 some NTP clients follwed 2
servers that offered UTC time (with leap second announcements) and 2
servers that offered leap smeared time (with no leap second
announcements). 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. The default values built
in to ntpd should be correct. If you have good reasons, you may use
the 'minclock' and 'maxclock' options of the 'tos' command to
override the default values of how many servers are discovered and
used through the 'pool' directive.
Properly monitor your NTP 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 might 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 are 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 needs
to be correct close to 100% of the time, then even if you are using a
satellite-based time source you must plan for those rare instances
when the time source 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
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debugging and performance information, and control facilities. 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 is
limited by default to authenticated sessions that provide and use a
'controlkey'. Similarly, if Mode 7 has been explicitly enabled its
use for more than basic monitoring is limited by default to
authenticated sessions that provide and use a 'requestkey'.
Older versions of the reference implementation of NTP likely do not
have the protections listed above and 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
Adminitstrators can prevent their NTP servers from responding to
these directive in the general case 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
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4.4. Monitoring
The reference implementation of NTP allows remote monitoring. Access
to this service is controlled by the restrict statement in ntpd's
configuration file (ntp.conf). The syntax is:
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.
If a system starts getting unexpected time replies from its time
servers, that is a likely indication that an abuser is forging your
server's IP in time requests to your time server in an attempt to
convince your time servers to stop serving time to your 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 might be forging packets from a broadcast
server. Broadcast time should only be used in trusted networks.
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.
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
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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 http://www.pool.ntp.org/en/join.html .
4.6. Leap Second Handling
UTC is kept in agreement with the astronomical time UT1 [6] to within
+/- 0.9 seconds by the insertion or 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 [7] 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 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 generally 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, in the event that the IERS announces one.
While earlier versions of NTP contained some ambiguity regarding when
a leap second that is broadcast by a server is 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 might 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 in NTPv4 the maximum allowed poll
interval is 17, or about 1.5 days' time. In this situation, it's
possible that a client will miss the leap second announcement.
The IETF maintains a leap second list [9]. The use of a leap second
list 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"
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You may need to restart ntpd to apply this change.
The leap second list file is 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
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. While this was useful behavior
in the past, when information about pending leap seconds was less
available. Now, we have the combination of 1) easy availablilty of
the leap second file and software to use it, and 2) a greater
awareness of the potential for hostile behavior. In this new light,
we have shifted from "believe a single warning" to "follow the best
authoritative source". The best authoritative source is the leap
seconds list file, The next best source would be an attached refclock
that can provide notification of leap second adjustments, and the
next best source would be the majority consensus from upstream
servers. There is still a risk of misbehavior if a "master" NTP
server gets an invalid leap second warning, e.g. due to an incorrect
leap second list file, or a faulty GPS receiver.
4.6.1. Leap Smearing
Some NTP installations may make use of a technique called "Leap
Smearing" to propagate a leap second correction. 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
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smeared time without objecting. A smear interval of 86400 seconds,
which is 1 day, has been used sucessfully. During the adjustment
window, all the NTP clients' times could 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 systems that don't deal
with the leap second event properly to function smoothly, 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 OSes) 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 availabled by default and has to be specifically 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 not apply
the "standard" NTP leap second handling. So if they are using ntpd,
these clients must not have a leap second file loaded, and the
smearing servers must not advertise 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 might 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 not be
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
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
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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 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. 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.
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.
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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, by 2011, security researchers had
identified grave vulnerabilities leading to a complete breach of the
protocol in real time on commodity hardware, which renders it
"completely useless". [12])
It is strongly recommended that Autokey not be used.
5.3. Network Time Security
Work has begun on an enhanced replacement for Autokey, which is
called Network Time Security (NTS) [NTS]. NTS was first published as
an Internet-Draft 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 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 prevent 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
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.
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A "leaf client" is a host that is using NTP solely for the purpose of
adjusting its own time. A leaf client should not be a time server to
other hosts. That is, a leaf client sends mode 3 queries to its
servers and receives mode 4 responses from these servers containing
timing information. To minimize information leakage, leaf clients
should drop all incoming NTP packets except for packets coming from
trusted monitoring systems and mode 4 response packets that come from
its configured time sources.
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 is designed to be used only in cold-start
situations. If it is used in more general situations it can be
exploited by attackers [NDSS16] when ntpd is restarted with a
disabled panic gate check.
If your operating system has init scripts, these scripts should not
disable panic gate checking on restarts.
A growing number of operating systems use process supervisors such as
systemd 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 Linux-based systems as they
migrate legacy init scripts to systemd. These scripts should not
disable panic gate checking on restarts.
If, against long-standing recommendations, a system disables panic
gate checking on all restarts, an 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.
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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].
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 this key is used by
the broadcast server to build and by the broadcast clients to
authenticate the Message Authentication Code (MAC) that protects NTP
broadcast packets.
Put another way, all broadcast clients that listen to broadcast
servers know and share the same 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 with
knowledge of this key cab attack broadcast clients.
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For this reason, all NTP broadcast servers and clients need to 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.
Updated NTP broadcast clients are protected against and detect these
attacks by reporting unexpected or inconsistent broadcast packets,
and by ignoring broadcast packets that arrive "too early". Monitor
your NTP log files.
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 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 for each peer association
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
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 "Internet of Things" devices that
require accurate time. These embedded devices might 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.
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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 and
fix them quickly, because their customers don't have the ability to
update their NTP implementation on their own. Those devices might
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 [13].
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 erroneously and destructively increase
their poll rate and create a low-level 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.
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 might 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.
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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], with additional
information at RFC 7094 [RFC7094]. With anycast, 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 could be used in large organizations to
simplify configuration of a large number of NTP clients, but note
well this simplification comes at the cost of degraded NTP behavior
and performance. 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 possible brief loss of service immediately
after server is taken down (and before packets are directed to a new
server), these additions are 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 are allowed to arbitrarily enter and leave the
network or as the routing behavior changes, the server a particular
client is connected to could change. This can cause a shift in the
delay and symmetry between the client and the server.
NOTE WELL: Using a single anycast address for NTP should be done with
care. It means there is likely to be an apparent single time server
source for the client population. A key element of a robust NTP
deployment is multiple sources of time. With multiple time servers a
client can analyze the various time sources, selecting good ones, and
disregarding poor ones. Users that want this benefit should not rely
on a single Anycast address for NTP.
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 are allowed to arbitrarily enter and leave the
network, the server any given client is connected to could change.
It is recommended that anycast be deployed in environments where
these small shifts can be tolerated.
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Configuration of an anycast interface is independent of NTP. Clients
will always connect to the closest anycast 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 all servers and clients. If a 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 could 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, Martin Burnicki, Miroslav Lichvar, and Daniel Fox Franke.
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 might assume that system time cannot be changed
and is always monotonic, and vulnerabilites could 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
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applies the methods in [NTS] specifically to NTP. At the time of
this writing, these are still drafts. Readers are encouraged to
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
[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://lists.ntp.org/pipermail/ntpwg/2011-August/001714.html
[13] https://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse
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Authors' Addresses
Denis Reilly (editor)
Spectracom
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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