Operational Considerations for use of DNS in IoT devices
draft-ietf-opsawg-mud-iot-dns-considerations-04

OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Best Current Practice                            W. Pan
Expires: 28 September 2022                           Huawei Technologies
                                                           27 March 2022

        Operational Considerations for use of DNS in IoT devices
            draft-ietf-opsawg-mud-iot-dns-considerations-04

Abstract

   This document details concerns about how Internet of Things devices
   use IP addresses and DNS names.  The issue becomes acute as network
   operators begin deploying RFC8520 Manufacturer Usage Description
   (MUD) definitions to control device access.

   This document makes recommendations on when and how to use DNS names
   in MUD files.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-opsawg-mud-iot-dns-
   considerations/.

   Discussion of this document takes place on the opsawg Working Group
   mailing list (mailto:opsawg@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/opsawg/.

   Source for this draft and an issue tracker can be found at
   https://github.com/mcr/iot-mud-dns-considerations.

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 https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Strategies to map names . . . . . . . . . . . . . . . . . . .   4
   4.  DNS and IP Anti-Patterns for IoT device Manufacturers . . . .   6
     4.1.  Use of IP address literals in-protocol  . . . . . . . . .   7
     4.2.  Use of non-deterministic DNS names in-protocol  . . . . .   8
     4.3.  Use of a too inclusive DNS name . . . . . . . . . . . . .   8
   5.  DNS privacy and outsourcing versus MUD controllers  . . . . .   9
   6.  Recommendations to IoT device manufacturer on MUD and DNS
           usage . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Consistently use DNS  . . . . . . . . . . . . . . . . . .  10
     6.2.  Use primary DNS names controlled by the manufacturer  . .  10
     6.3.  Use Content-Distribution Network with stable names  . . .  10
     6.4.  Do not use geofenced names  . . . . . . . . . . . . . . .  10
     6.5.  Prefer DNS servers learnt from DHCP/Route
           Advertisements  . . . . . . . . . . . . . . . . . . . . .  10
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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1.  Introduction

   [RFC8520] provides a standardized way to describe how a specific
   purpose device makes use of Internet resources.  Access Control Lists
   (ACLs) can be defined in an RFC8520 Manufacturer Usage Description
   (MUD) file that permit a device to access Internet resources by DNS
   name.

   Use of a DNS name rather than IP address in the ACL has many
   advantages: not only does the layer of indirection permit the mapping
   of name to IP address to be changed over time, it also generalizes
   automatically to IPv4 and IPv6 addresses, as well as permitting
   loading balancing of traffic by many different common ways, including
   multi-CDN deployments wherein load balancing can account for
   geography and load.

   At the MUD policy enforcement point -- the firewall -- there is a
   problem.  The firewall has only access to the layer-3 headers of the
   packet.  This includes the source and destination IP address, and if
   not encrypted by IPsec, the destination UDP or TCP port number
   present in the transport header.  The DNS name is not present!

   It has been suggested that one answer to this problem is to provide a
   forced intermediate for the TLS connections.  This could in theory be
   done for TLS 1.2 connections.  The MUD policy enforcement point could
   observe the Server Name Identifier (SNI) [RFC6066].  Some Enterprises
   do this already.  But, as this involves active termination of the TCP
   connection (a forced circuit proxy) in order to see enough of the
   traffic, it requires significant effort.  But, TLS 1.3 provides
   options to encrypt the SNI as the ESNI, which renders the practice
   useless in the end.

   So in order to implement these name based ACLs, there must be a
   mapping between the names in the ACLs and layer-3 IP addresses.  The
   first section of this document details a few strategies that are
   used.

   The second section of this document details how common manufacturer
   anti-patterns get in the way this mapping.

   The third section of this document details how current trends in DNS
   presolution such as public DNS servers, DNS over TLS (DoT), and DNS
   over HTTPS (DoH) cause problems for the strategies employed.  Poor
   interactions with content-distribution networks is a frequent
   pathology that can result.

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   The fourth section of this document makes a series of recommendations
   ("best current practices") for manufacturers on how to use DNS, and
   IP addresses with specific purpose IoT devices.

   The Privacy Considerations section concerns itself with issues that
   DNS-over-TLS and DNS-over-HTTPS are frequently used to deal with.
   How these concerns apply to IoT devices located within a residence or
   enterprise is a key concern.

   The Security Considerations section covers some of the negative
   outcomes should MUD/firewall managers and IoT manufacturers choose
   not to cooperate.

2.  Terminology

   Although this document is not an IETF Standards Track publication, it
   adopts the conventions for normative language to provide clarity of
   instructions to the implementer.  The key words "MUST", "MUST NOT",
   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

3.  Strategies to map names

   The most naive method is to try to map IP addresses to names using
   the in-addr.arpa (IPv4), and ipv6.arpa (IPv6) mappings.  This fails
   for a number of reasons:

   1.  it can not be done fast enough,

   2.  it reveals usage patterns of the devices,

   3.  the mapping are often incomplete,

   4.  even if the mapping is present, due to virtual hosting, it may
       not map back to the name used in the ACL.

   This is not a successful strategy, its use is NOT RECOMMENDED.

   XXX --- explain in detail how this can fail.

   XXX --- explain N:1 vs 1:1 for virtual hosting.

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   The simplest successful strategy for translating names is for a MUD
   controller to take is to do a DNS lookup on the name (a forward
   lookup), and then use the resulting IP addresses to populate the
   physical ACLs.

   There are still a number of failures possible.

   The most important one is in the mapping of the names to IP addresses
   may be non-deterministic.  [RFC1794] describes the very common
   mechanism that returns DNS A (or reasonably AAAA) records in a
   permutted order.  This is known as Round Robin DNS, and it has been
   used for many decades.  The device is intended to use the first IP
   address that is returned, and each query returns addresses in a
   different ordering, splitting the load among many servers.

   This situation does not result in failures as long as all possible A/
   AAAA records are returned.  The MUD controller and the device get a
   matching set, and the ACLs that are setup cover all possibilities.

   There are a number of circumstances in which the list is not
   exhaustive.  The simplest is when the round robin does not return all
   addresses.  This is routinely done by geographical DNS load balancing
   system.  It can also happen if there are more addresses than will
   conveniently fit into a DNS reply.  The reply will be marked as
   truncated.  (If DNSSEC resolution will be done, then the entire RR
   must be retrieved over TCP (or using a larger EDNS(0) size) before
   being validated)

   However, in a geographical DNS load balancing system, different
   answers are given based upon the locality of the system asking.
   There may also be further layers of round-robin indirection.

   Aside from the list of records being incomplete, the list may have
   changed between the time that the MUD controller did the lookup and
   the time that the IoT device does the lookup, and this change can
   result in a failure for the ACL to match.

   In order to compensate for this, the MUD controller SHOULD regularly
   do DNS lookups in order to get never have stale data.  These lookups
   need to be rate limited in order to avoid load.  It may be necessary
   to avoid local recursive DNS servers.  The MUD controller SHOULD
   incorporate its own recursive caching DNS server.  Properly designed
   recursive servers should cache data for many minutes to days, while
   the underlying DNS data can change at a higher frequency, providing
   different answers to different queries!

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   A MUD controller that is aware of which recursive DNS server that the
   IoT device will use can instead query that server on a periodic
   basis.  Doing so provides three advantages:

   1.  any geographic load balancing will base the decision on the
       geolocation of the recursive DNS server, and the recursive name
       server will provide the same answer to the MUD controller as to
       the IoT device.

   2.  the resulting name to IP address mapping in the recursive name
       server will be cached, and will remain the same for the entire
       advertised Time-To-Live reported in the DNS query return.  This
       also allows the MUD controller to avoid doing unnecessary
       queries.

   3.  if any addresses have been omitted in a round-robin DNS process,
       the cache will have the set of addresses that were returned.

   The solution of using the same caching recursive resolver as the
   target device is very simple when the MUD controllers is located in a
   residential CPE device.  The device is usually also the policy
   enforcement point for the ACLs, and a caching resolver is typically
   located on the same device.  In addition the convenience, there is a
   shared fate advantage: as all three components are running on the
   same device, if the device is rebooted, clearing the cache, then all
   three components will get restarted when the device is restarted.

   Where the solution is more complex is when the MUD controller is
   located elsewhere in an Enteprise, or remotely in a cloud such as
   when a Software Defines Network (SDN) is used to manage the ACLs.
   The DNS servers for a particular device may not be known to the MUD
   controller, nor the MUD controller be even permitted to make recusive
   queries that server if it is known.  In this case, additional
   installation specific mechanisms are probably needed to get the right
   view of DNS.

4.  DNS and IP Anti-Patterns for IoT device Manufacturers

   In many design fields, there are good patterns that should be
   emulated, and often there are patterns that should not be emulated.
   The latter are called anti-patterns, as per [antipatterns].

   This section describes a number of things with IoT manufacturers have
   been observed to do in the field, each of which presents difficulties
   for MUD enforcement points.

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4.1.  Use of IP address literals in-protocol

   A common pattern for a number of devices is to look for firmware
   updates in a two step process.  An initial query is made (often over
   HTTPS, sometimes with a POST, but the method is immaterial) to a
   vendor system that knows whether or not an update is required.

   The current firmware model of the device is sometimes provided and
   then the authoritative server provides a determination if a new
   version is required, and if so, what version.  In simpler cases, an
   HTTPS end point is queried which provides the name and URL of the
   most recent firmware.

   The authoritative upgrade server then responds with a URL of a
   firmware blob that the device should download and install.  Best
   practice is that firmware is either signed internally
   ([I-D.ietf-suit-architecture]) so that it can be verified, or a hash
   of the blob is provided.

   An authoritative server might be tempted to provided an IP address
   literal inside the protocol: there are two arguments (anti-patterns)
   for doing this.

   One is that it eliminates problems to firmware updates that might be
   caused by lack of DNS, or incompatibilities with DNS.  For instance
   the bug that causes interoperability issues with some recursive
   servers would become unpatchable for devices that were forced to use
   that recursive resolver type.

   A second reason to avoid a DNS in the URL is when an inhouse content-
   distribution system is involved that involves on-demand instances
   being added (or removed) from a cloud computing architecture.

   But, there are many problems with use of IP address literals for the
   location of the firmware.

   The first is that the update service server must decide whether to
   provide an IPv4 or an IPv6 literal.  A DNS name can contain both
   kinds of addresses, and can also contain many different IP addresses
   of each kind.

   The second problem is that it forces the MUD file definition to
   contain the exact same IP address literals.  It must also contain an
   ACL for each address literal.  DNS provides a useful indirection
   method that naturally aggregates the addresses.

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   A third problem involves the use of HTTPS.  IP address literals do
   not provide enough context for TLS ServerNameIndicator to be useful
   [RFC6066].  This limits the firmware repository to be a single tenant
   on that IP address, and for IPv4 (at least), this is no longer a
   sustainable use of IP addresses.

   And with any non-determistic name or address that is returned, the
   MUD controller is not challenged to validate the transaction, as it
   can not see into the communication.

   Third-party content-distribution networks (CDN) tend to use DNS names
   in order to isolate the content-owner from changes to the
   distribution network.

4.2.  Use of non-deterministic DNS names in-protocol

   A second pattern is for a control protocol to connect to a known HTTP
   end point.  This is easily described in MUD.  Within that control
   protocol references are made to additional content at other URLs.
   The values of those URLs do not fit any easily described pattern and
   may point at arbitrary names.

   Those names are often within some third-party Content-Distribution-
   Network (CDN) system, or may be arbitrary names in a cloud-provider
   storage system such as Amazon S3 (such [AmazonS3], or [Akamai]).

   Such names may be unpredictably chosen by the content provider, and
   not the content owner, and so impossible to insert into a MUD file.

   Even if the content provider chosen names are deterministic they may
   change at a rate much faster than MUD files can be updated.

   This in particular may apply to the location where firmware updates
   may be retrieved.

4.3.  Use of a too inclusive DNS name

   Some CDNs make all customer content at a single URL (such as
   s3.amazonaws.com).  This seems to be ideal from a MUD point of view:
   a completely predictable URL.

   The problem is that a compromised device could then connect to the
   contents of any bucket, potentially attacking the data from other
   customers.

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   Exactly what the risk is depends upon what the other customers are
   doing: it could be limited to simply causing a distributed denial of
   service attack resulting to high costs to those customers, or such an
   attack could potentially include writing content.

   Amazon has recognized the problems associated with this practice, and
   aims to change it to a virtual hosting model, as per
   [awss3virtualhosting].

   The MUD ACLs provide only for permitting end points (hostnames and
   ports), but do not filter URLs (nor could filtering be enforced
   within HTTPS).

5.  DNS privacy and outsourcing versus MUD controllers

   [RFC7858] and [RFC8094] provide for DNS over TLS (DoT) and DNS over
   HTTPS (DoH).  [I-D.ietf-dnsop-terminology-ter] details the terms.
   But, even with traditional DNS over Port-53 (Do53), it is possible to
   outsource DNS queries to other public services, such as those
   operated by Google, CloudFlare, Verisign, etc.

   For some users and classes of device, revealing the DNS queries to
   those outside entities may consititute a privacy concern.  For other
   users the use of an insecure local resolver may constitute a privacy
   concern.

   A described above in Section 3 the MUD controller needs to have
   access to the same resolver(s) as the IoT device.

6.  Recommendations to IoT device manufacturer on MUD and DNS usage

   Inclusion of a MUD file with IoT devices is operationally quite
   simple.  It requires only a few small changes to the DHCP client code
   to express the MUD URL.  It can even be done without code changes via
   the use of a QR code affixed to the packaging (see
   [I-D.richardson-mud-qrcode].

   The difficult part is determining what to put into the MUD file
   itself.  There are currently tools that help with the definition and
   analysis of MUD files, see [mudmaker].  The remaining difficulty is
   now the semantic contents of what is in the MUD file.  An IoT
   manufacturer must now spend some time reviewing what the network
   communications that their device does.

   This document has discussed a number of challenges that occur
   relating to how DNS requests are made and resolved, and it is the
   goal of this section to make recommendations on how to modify IoT
   systems to work well with MUD.

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6.1.  Consistently use DNS

   For the reasons explained in Section 4.1, the most important
   recommendation is to avoid using IP address literals in any protocol.
   Names should always be used.

6.2.  Use primary DNS names controlled by the manufacturer

   The second recommendation is to allocate and use names within zones
   controlled by the manufacturer.  These names can be populated with an
   alias (see [RFC8499] section 2) that points to the production system.
   Ideally, a different name is used for each logical function, allowing
   for different rules in the MUD file to be enabled and disabled.

   While it used to be costly to have a large number of aliases in a web
   server certificate, this is no longer the case.  Wildcard
   certificates are also commonly available which allowed for an
   infinite number of possible names.

6.3.  Use Content-Distribution Network with stable names

   When aliases point to a Content-Distribution Network (CDN), prefer to
   use stable names that point to appropriately load balanced targets.
   CDNs that employ very low time-to-live (TTL) values for DNS make it
   harder for the MUD controller to get the same answer as the IoT
   Device.  A CDN that always returns the same set of A and AAAA
   records, but permutes them to provide the best one first provides a
   more reliable answer.

6.4.  Do not use geofenced names

   Due the problems with different answers from different DNS servers,
   described above, a strong recommendation is to avoid using such
   things.

6.5.  Prefer DNS servers learnt from DHCP/Route Advertisements

   IoT Devices should prefer doing DNS to the network provided DNS
   servers.  Whether this is restricted to Classic DNS (Do53) or also
   includes using DoT/DoH is a local decision, but a locally provided
   DoT server SHOULD be used, as recommended by
   [I-D.reddy-dprive-bootstrap-dns-server] and [I-D.peterson-doh-dhcp].

   The ADD WG is currently only focusing on insecure discovery
   mechanisms like DHCP/RA [I-D.btw-add-home] and DNS based discovery
   mechanisms ({?{I-D.pauly-add-deer}}).  Secure discovery of network
   provided DoH/DoT resolver is possible using the mechanisms discussed
   in [I-D.reddy-add-enterprise] section-4.

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   Use of public QuadX resolver instead of the provided DNS resolver,
   whether Do53, DoT or DoH is discouraged.  Should the network provide
   such a resolver for use, then there is no reason not to use it, as
   the network operator has clearly thought about this.

   Some manufacturers would like to have a fallback to using a public
   resolver to mitigate against local misconfiguration.  There are a
   number of reasons to avoid this, or at least do this very carefully.

   It is recommended that use of non-local resolvers is only done when
   the locally provided resolvers provide no answers to any queries at
   all, and do so repeatedly.  The use of the operator provided
   resolvers SHOULD be retried on a periodic basis, and once they
   answer, there should be no further attempts to contact public
   resolvers.

   Finally, the list of public resolvers that might be contacted MUST be
   listed in the MUD file as destinations that are to be permitted!
   This should include the port numbers (53, 853 for DoT, 443 for DoH)
   that will be used as well.

7.  Privacy Considerations

   The use of non-local DNS servers exposes the list of names resolved
   to a third parties, including passive eavesdroppers.

   The use of DoT and DoH eliminates the minimizes threat from passive
   eavesdropped, but still exposes the list to the operator of the DoT
   or DoH server.  There are additional methods, such as described by
   [I-D.pauly-dprive-oblivious-doh].

   The use of unencrypted (Do53) requests to a local DNS server exposes
   the list to any internal passive eavesdroppers, and for some
   situations that may be significant, particularly if unencrypted WiFi
   is used.  Use of Encrypted DNS connection to a local DNS recursive
   resolver is a preferred choice, assuming that the trust anchor for
   the local DNS server can be obtained, such as via
   [I-D.reddy-dprive-bootstrap-dns-server].

   IoT devices that reach out to the manufacturer at regular intervals
   to check for firmware updates are informing passive eavesdroppers of
   the existence of a specific manufacturer's device being present at
   the origin location.

   Identifying the IoT device type empowers the attacker to launch
   targeted attacks to the IoT device (e.g., Attacker can advantage of
   the device vulnerability).

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   While possession of a Large (Kitchen) Appliance at a residence may be
   uninteresting to most, possession of intimate personal devices (e.g.,
   "sex toys") may be a cause for embarrassment.

   IoT device manufacturers are encouraged to find ways to anonymize
   their update queries.  For instance, contracting out the update
   notification service to a third party that deals with a large variety
   of devices would provide a level of defense against passive
   eavesdropping.  Other update mechanisms should be investigated,
   including use of DNSSEC signed TXT records with current version
   information.  This would permit DoT or DoH to convey the update
   notification in a private fashion.  This is particularly powerful if
   a local recursive DoT server is used, which then communicates using
   DoT over the Internet.

   The more complex case of section Section 4.1 postulates that the
   version number needs to be provided to an intelligent agent that can
   decided the correct route to do upgrades.  The current
   [I-D.ietf-suit-architecture] specification provides a wide variety of
   ways to accomplish the same thing without having to divulge the
   current version number.

   The use of a publically specified firmware update protocol would also
   enhance privacy of IoT devices.  In such a system the IoT device
   would never contact the manufacturer for version information or for
   firmware itself.  Instead, details of how to query and where to get
   the firmware would be provided as a MUD extension, and a Enterprise-
   wide mechanism would retrieve firmware, and then distribute it
   internally.  Aside from the bandwidth savings of downloading the
   firmware only once, this also makes the number of devices active
   confidential, and provides some evidence about which devices have
   been upgraded and which ones might still be vulnerable.  (The
   unpatched devices might be lurking, powered off, lost in a closet)

8.  Security Considerations

   This document deals with conflicting Security requirements:

   1.  devices which an operator wants to manage using [RFC8520]

   2.  requirements for the devices to get access to network resources
       that may be critical to their continued safe operation.

   This document takes the view that the two requirements do not need to
   be in conflict, but resolving the conflict requires some advance
   planning by all parties.

9.  References

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9.1.  Normative References

   [Akamai]   "Akamai", 2019,
              <https://en.wikipedia.org/wiki/Akamai_Technologies>.

   [AmazonS3] "Amazon S3", 2019,
              <https://en.wikipedia.org/wiki/Amazon_S3>.

   [I-D.ietf-dnsop-terminology-ter]
              Hoffman, P., "Terminology for DNS Transports and
              Location", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-terminology-ter-02, 3 August 2020,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-
              terminology-ter-02.txt>.

   [I-D.ietf-suit-architecture]
              Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
              Firmware Update Architecture for Internet of Things", Work
              in Progress, Internet-Draft, draft-ietf-suit-architecture-
              16, 27 January 2021, <https://www.ietf.org/archive/id/
              draft-ietf-suit-architecture-16.txt>.

   [I-D.peterson-doh-dhcp]
              Peterson, T., "DNS over HTTP resolver announcement Using
              DHCP or Router Advertisements", Work in Progress,
              Internet-Draft, draft-peterson-doh-dhcp-01, 21 October
              2019, <https://www.ietf.org/archive/id/draft-peterson-doh-
              dhcp-01.txt>.

   [I-D.reddy-dprive-bootstrap-dns-server]
              Reddy, T., Wing, D., Richardson, M. C., and M. Boucadair,
              "A Bootstrapping Procedure to Discover and Authenticate
              DNS-over-TLS and DNS-over-HTTPS Servers", Work in
              Progress, Internet-Draft, draft-reddy-dprive-bootstrap-
              dns-server-08, 6 March 2020,
              <https://www.ietf.org/archive/id/draft-reddy-dprive-
              bootstrap-dns-server-08.txt>.

   [RFC1794]  Brisco, T., "DNS Support for Load Balancing", RFC 1794,
              DOI 10.17487/RFC1794, April 1995,
              <https://www.rfc-editor.org/info/rfc1794>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/info/rfc8094>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

9.2.  Informative References

   [antipatterns]
              "AntiPattern", 12 July 2021,
              <https://www.agilealliance.org/glossary/antipattern>.

   [awss3virtualhosting]
              "Down to the Wire: AWS Delays 'Path-Style' S3 Deprecation
              at Last Minute", 12 July 2021,
              <https://techmonitor.ai/techonology/cloud/aws-s3-path-
              deprecation>.

   [I-D.btw-add-home]
              Boucadair, M., Reddy, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for
              Encrypted DNS Discovery", Work in Progress, Internet-
              Draft, draft-btw-add-home-12, 22 January 2021,
              <https://www.ietf.org/archive/id/draft-btw-add-home-
              12.txt>.

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   [I-D.pauly-dprive-oblivious-doh]
              Kinnear, E., McManus, P., Pauly, T., Verma, T., and C. A.
              Wood, "Oblivious DNS Over HTTPS", Work in Progress,
              Internet-Draft, draft-pauly-dprive-oblivious-doh-11, 17
              February 2022, <https://www.ietf.org/archive/id/draft-
              pauly-dprive-oblivious-doh-11.txt>.

   [I-D.reddy-add-enterprise]
              Reddy, T. and D. Wing, "DNS-over-HTTPS and DNS-over-TLS
              Server Deployment Considerations for Enterprise Networks",
              Work in Progress, Internet-Draft, draft-reddy-add-
              enterprise-00, 23 June 2020,
              <https://www.ietf.org/archive/id/draft-reddy-add-
              enterprise-00.txt>.

   [I-D.richardson-mud-qrcode]
              Richardson, M., Latour, J., and H. H. Gharakheili, "On
              loading MUD URLs from QR codes", Work in Progress,
              Internet-Draft, draft-richardson-mud-qrcode-07, 21 March
              2022, <https://www.ietf.org/archive/id/draft-richardson-
              mud-qrcode-07.txt>.

   [mudmaker] "Mud Maker", 2019, <https://mudmaker.org>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

Appendix A.  Appendices

Authors' Addresses

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

   Wei Pan
   Huawei Technologies
   Email: william.panwei@huawei.com

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