OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Best Current Practice                 22 September 2020
Expires: 26 March 2021

        Operational Considerations for use of DNS in IoT devices


   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 explains the problem through a series of examples of
   what can go wrong, and then provides some advice on how a device
   manufacturer can best make deal with these issues.  The
   recommendations have an impact upon device and network protocol

   {RFC-EDITOR, please remove.  Markdown and issue tracker for this
   document is at https://github.com/mcr/iot-mud-dns-considerations.git

Status of This Memo

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   This Internet-Draft will expire on 26 March 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   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  . . . . . . . . .   6
     4.2.  Use of non-deterministic DNS names in-protocol  . . . . .   7
     4.3.  Use of a too inclusive DNS name . . . . . . . . . . . . .   7
   5.  DNS privacy and outsourcing vs MUD controllers  . . . . . . .   7
   6.  Recommendations to IoT device manufacturer on MUD and DNS
           usage . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Consistently use DNS  . . . . . . . . . . . . . . . . . .   8
     6.2.  Use primary DNS names controlled by the manufacturer  . .   8
     6.3.  Use Content-Distribution Network with stable names  . . .   9
     6.4.  Prefer DNS servers learnt from DHCP/Route
           Advertisements  . . . . . . . . . . . . . . . . . . . . .   9
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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

   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

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   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!

   In theory, on TLS 1.2 connections the MUD policy enforcement point
   might observe the Server Name Identifier (SNI), in practice it
   involves active termination of the TCP connection (a forced circuit
   proxy) in order to see enough of the traffic.  And to what end?  TLS
   1.3 provides options to encrypt it (ESNI).

   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

   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.

   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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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   This document is a Best Current Practices (BCP) document.  It uses
   the above language where it needs to make a normative requirement on

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, and it is not used.

   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.

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   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.  These lookups need to be rate limited in order to
   avoid load.  It may be necessary to avoid recursive DNS servers in
   order to avoid receiving cached data.  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!

   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

   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.

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

   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.

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 an
   authoritatve server.  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 more complex case supports situations in which the device needs
   to be running the latest patch release before it can apply the next
   major release.  For instance, a device running 1.4 must upgrade to at
   least version 1.9 before it is able to download version 2.0 of the

   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.

   The challenge for a MUD controller is in the details of the URL that
   is provided.  An authoritative server might be tempted to provided an
   IP address literal inside the protocol: there are two arguments 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.  This
   model is typical of on-demand video systems including Netflix (see
   in quite a number of other situations.  Third-party content-
   distribution networks (CDN) tend to use DNS names in order to isolate
   the content-owner from changes to the distribution network.

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   [BEHAVE-BCP-REF] gives other good reasons why IP address literals are
   bad ideas; in particular they work very poorly when devices have IPv6
   capabilities, and are on IPv6-only networks with NAT64 (see

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]).

   *INSERT* examples of non-deterministic CDN content.

   Since it is not possible to predict a name for where the content will
   be, it is not possible to include that into the MUD file.

   This applies to the firmware update situation as well.

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 any S3 bucket, potentially attacking
   other buckets.

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

5.  DNS privacy and outsourcing vs MUD controllers

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

   There are significant privacy issues with having IoT devices sending
   their DNS queries to an outside entity.  Doing it over a secure
   transport (DoT/DoH) is clearly better than doing so on port 53.  The
   providers of the secure resolver service will, however, still see the
   IoT device queries.

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   A described above in Section 3 the MUD controller needs to have
   access to the same resolver(s) as the IoT device.  Use of the QuadX
   resolvers at first seems to present less of a problem than use of
   some other less well known resolver.  While any system may use QuadX,
   in most cases those services are massively replicated via anycast:
   there is no guarantee that a MUD controller will speak to the same
   instance, or get the same geographic anycast result.

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

   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.

6.1.  Consistently use DNS

   The first 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.

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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.  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].

   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.
   The recommendation here is to do this only when the 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.

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   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 DoT 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.  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 embarassment.

   IoT device manufacturers are encouraged to anonymizing ways to do
   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 provide the update
   notification.  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 {{inprotocol} 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)

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8.  Security Considerations

   This document deals with conflicting Security requirements: devices
   which an operator wants to manage using [RFC8520] vs 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

9.1.  Normative References

   [Akamai]   "Akamai", 2019,

   [AmazonS3] "Amazon S3", 2019,

              Hoffman, P., "Terminology for DNS Transports and
              Location", Work in Progress, Internet-Draft, draft-ietf-
              dnsop-terminology-ter-02, 3 August 2020,

              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-
              12, 17 September 2020, <http://www.ietf.org/internet-

              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, <http://www.ietf.org/internet-drafts/draft-peterson-

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              Reddy.K, T., Wing, D., Richardson, M., 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, <http://www.ietf.org/

   [RFC1794]  Brisco, T., "DNS Support for Load Balancing", RFC 1794,
              DOI 10.17487/RFC1794, April 1995,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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,

9.2.  Informative References

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              Richardson, M., Latour, J., and H. Gharakheili, "On
              loading MUD URLs from QR codes", Work in Progress,
              Internet-Draft, draft-richardson-opsawg-securehomegateway-
              mud-05, 8 September 2020, <http://www.ietf.org/internet-

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

Appendix A.  Appendices

Author's Address

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

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