Internet Engineering Task Force (IETF) M. Richardson
Request for Comments: 9726 Sandelman Software Works
BCP: 241 W. Pan
Category: Best Current Practice Huawei Technologies
ISSN: 2070-1721 March 2025
Operational Considerations for Use of DNS in Internet of Things (IoT)
Devices
Abstract
This document details considerations about how Internet of Things
(IoT) devices use IP addresses and DNS names. These concerns become
acute as network operators begin deploying Manufacturer Usage
Descriptions (MUD), as specified in RFC 8520, to control device
access.
Also, this document makes recommendations on when and how to use DNS
names in MUD files.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9726.
Copyright Notice
Copyright (c) 2025 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
(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 and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. A Model for MUD Controller Mapping of DNS Names to Addresses
3.1. Non-Deterministic Mappings
4. DNS and IP Anti-Patterns for IoT Device Manufacturers
4.1. Use of IP Address Literals
4.2. Use of Non-Deterministic DNS Names in Protocols
4.3. Use of a Too Generic DNS Name
5. DNS Privacy and Outsourcing versus MUD Controllers
6. Recommendations to IoT Device Manufacturers on MUD and DNS
Usage
6.1. Consistently Use DNS
6.2. Use Primary DNS Names Controlled by the Manufacturer
6.3. Use a Content Distribution Network with Stable DNS Names
6.4. Do Not Use Tailored Responses to Answer DNS Names
6.5. Prefer DNS Servers Learned from DHCP/Router Advertisements
7. Interactions with mDNS and DNS-SD
8. Privacy IANA Considerations
9. Security Privacy Considerations
10. Security Considerations
11. References
10.1.
11.1. Normative References
10.2.
11.2. Informative References
Appendix A. A Failing Strategy: Anti-Patterns
A.1. Too Slow
A.2. Reveals Patterns of Usage
A.3. Mappings Are Often Incomplete
A.4. Forward DNS Names Can Have Wildcards
Contributors
Authors' Addresses
1. Introduction
[RFC8520] provides a standardized way to describe how a device with a
specific purpose device makes use of Internet resources. Access Control
Lists (ACLs) can be defined in a Manufacturer Usage Description (MUD)
file [RFC8520] that permits a device to access Internet resources by
their DNS names or IP addresses.
The use of a DNS name rather than an IP address in an ACL has many
advantages: Not only does the layer of indirection permit the mapping
of a name to IP addresses to be changed over time, but it also
generalizes automatically to IPv4 and IPv6 addresses as well as
permits a variety of load-balancing strategies, including multi-CDN
deployments wherein load-balancing can account for geography and
load.
However, the use of DNS names has implications on how ACLs are
executed at the MUD policy enforcement point (typically, a firewall).
Concretely, the firewall has access only to the Layer 3 headers of
the packet. This includes the source and destination IP addresses
and, if not encrypted by IPsec, the destination UDP or TCP port
number present in the transport header. The DNS name is not present!
So, in order to implement these name-based ACLs, there must be a
mapping between the names in the ACLs and IP addresses.
In order for manufacturers to understand how to configure DNS
associated with name-based ACLs, a model of how the DNS resolution
will be done by MUD controllers is necessary. Section 3 models some
good strategies that could be used.
This model is non-normative but is included so that IoT device
manufacturers can understand how the DNS will be used to resolve the
names they use.
There are some ways of using DNS that will present problems for MUD
controllers, which Section 4 explains.
Section 5 details how current trends in DNS resolution such as public
DNS servers, DNS over TLS (DoT) [RFC7858], DNS over HTTPS (DoH)
[RFC8484], or DNS over QUIC (DoQ) [RFC9250] can cause problems with
the strategies employed.
The core of this document is Section 6, which makes a series of
recommendations ("best current practices") for manufacturers on how
to use DNS and IP addresses with MUD supporting IoT devices. devices described by MUD.
Section 8 9 discusses a set of privacy issues that encrypted DNS (for
example, DoT and DoH) are frequently used to deal with. How these
concerns apply to IoT devices located within a residence or
enterprise is a key concern.
Section 9 10 also covers some of the negative outcomes should MUD/
firewall managers and IoT manufacturers choose not to cooperate.
2. Terminology
This document makes use of terms defined in [RFC8520] and [RFC9499].
The term "anti-pattern" comes from agile software design literature,
as per [antipattern].
"CDNs" refers to Content Distribution Networks, such as those
described in [RFC6707], Section 1.1.
3. A Model for MUD Controller Mapping of DNS Names to Addresses
This section details a strategy that a MUD controller could take.
Within the limits of the DNS use detailed in Section 6, this process
could work. The methods detailed in Appendix A just will not work.
The simplest successful strategy for translating DNS names for a MUD controller to take translate
DNS names is to do a DNS lookup on the name (a forward lookup) and
then use the resulting IP addresses to populate the actual ACLs.
There are a number of possible failures, and the goal of this section
is to explain how some common DNS usages may fail.
3.1. Non-Deterministic Mappings
Most importantly, the mapping of the DNS names to IP addresses should could
be non-deterministic.
[RFC1794] describes the very common mechanism that returns DNS A (or
reasonably AAAA) records in a permuted order. This is known as
"round-robin DNS" and has been used for many decades. The historical
intent is that the requestor will tend to use the first IP address
that is returned. As each query results in addresses being in a
different order, the effect is to split 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 set up cover all possibilities.
There are a number of circumstances in which the list is not
exhaustive. The simplest is when the round-robin DNS does not return
all addresses. This is routinely done by geographical DNS load-
balancing systems: Only the addresses that the balancing system
wishes to be used are returned.
Failure can also occur if there are more addresses than what 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 did the lookup, and this change can
result in a failure for the ACL to match. If the IoT device did not
use the same recursive servers as the MUD controller, then tailored
DNS replies and/or truncated round-robin results could return a
different and non-overlapping set of addresses.
In order to compensate for this, the MUD controller performs regular
DNS lookups in order to never have stale data. These lookups must be
rate-limited to avoid excessive load on the DNS servers, and it may
be necessary to avoid local recursive resolvers. A MUD controller
that incorporates its own recursive caching DNS client will be able
to observe the TTL on the entries and cause them to expire
appropriately. This cache will last for at least some number of
minutes and up to some number of days (respecting the TTL), 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 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 mapping (of name to IP address) in the recursive
name server will be cached and will remain the same for the
entire advertised TTL 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 same 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 controller is located in a
residential Customer Premises Equipment (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 to
convenience, there is a shared fate advantage: As all three
components are running on the same device, if the device is rebooted
(which clears the cache), then all three components will get
restarted when the device is restarted.
The solution is more complex and sometimes more fragile when the MUD
controller is located elsewhere in an enterprise or remotely in a
cloud, such as when a Software-Defined Network (SDN) is used to
manage the ACLs. The DNS servers for a particular device may not be
known to the MUD controller, and the MUD controller may not even be
permitted to make recursive queries to that server if it is known.
In this case, additional installation-specific mechanisms are
probably needed to get the right view of the DNS.
A critical failure can occur when the device makes a new DNS request
and receives a new set of IP addresses, but the MUD controller's copy
of the addresses has not yet reached their TTL. In that case, the
MUD controller still has the old addresses implemented in the ACLs,
but the IoT device has a new address not previously returned to the
MUD controller. This can result in a connectivity failure.
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 [antipattern].
This section describes a number of things that 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
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 an update is required.
The current firmware model of the device is sometimes provided, and
then the vendor's authoritative server provides a determination if a
new version is required and, if so, what version. In simpler cases,
an HTTPS endpoint 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 either firmware is signed internally [RFC9019] so
that it can be verified, or a hash of the blob is provided.
An authoritative server might be tempted to provide an IP address
literal inside the protocol. An argument for doing this is that it
eliminates problems with firmware updates that might be caused by a
lack of DNS or by incompatibilities with DNS. For instance, a bug
that causes interoperability issues with some recursive servers would
become unpatchable for devices that were forced to use that recursive
resolver type.
But, there are several problems with the 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, assuming that only one URL can be
provided. A DNS name can contain both kinds of addresses and can
also contain many different IP addresses of each kind. An update
server might believe that if the connection were on IPv4, then an
IPv4 literal would be acceptable. However, due to NAT64 [RFC6146], a
device with only IPv6 connectivity will often be able to reach an
IPv4 firmware update server by name (through DNS64 [RFC6147]) but not
be able to reach an arbitrary IPv4 address.
A MUD file definition for this access would need to resolve to the set of IP
addresses that might be returned by the update server. This can be
done with IP address literals in the MUD file, but this may require
continuing updates to the MUD file if the addresses change
frequently. A DNS name in the MUD could resolve to the set of all
possible IPv4 and IPv6 addresses that would be used, with DNS
providing a level of indirection that obviates the need to update the
MUD file itself.
A third problem involves the use of HTTPS. It is often more
difficult to get TLS certificates for an IP address, and so it is
less likely that the firmware download will be protected by TLS. An
IP address literal in the TLS ServerNameIndicator [RFC6066] might not
provide enough context for a web server to distinguish which of the
(potentially many) tenants the client wishes to reach. This drives
the use of an IP address per tenant, and for IPv4 (at least), this is
no longer a sustainable use of IP addresses.
Finally, it is common in some CDNs to use multiple layers of DNS
CNAMEs in order to isolate the content owner's naming system from
changes in how the distribution network is organized.
When a name or address is returned within an update protocol for
which a MUD rule cannot be written, then the MUD controller is unable
to authorize the connection. In order for the connection to be
authorized, the set of names returned within the update protocol
needs to be known ahead of time and must be from a finite set of
possibilities. Such a set of names or addresses can be placed into
the MUD file as an ACL in advance, and the connections can be
authorized.
4.2. Use of Non-Deterministic DNS Names in Protocols
A second pattern is for a control protocol to connect to a known HTTP
endpoint. This is easily described in MUD. References within that
control protocol are made to additional content at other URLs. The
values of those URLs do not fit any easily described pattern and may
point to arbitrary DNS names.
Those DNS names are often within some third-party CDN system or may
be arbitrary DNS names in a cloud-provider storage system (e.g.,
[AmazonS3] or [Akamai]). Some of the name components may be
specified by the third-party CDN provider.
Such DNS names may be unpredictably chosen by the CDN and not the
device manufacturer and therefore impossible to insert into a MUD
file. Implementation of the CDN system may also involve HTTP
redirections to downstream CDN systems.
Even if the CDN provider's chosen DNS names are deterministic, they
may change at a rate much faster than MUD files can be updated.
This situation applies to firmware updates but also applies to many
other kinds of content: video content, in-game content, etc.
A solution may be to use a deterministic DNS name within the control
of the device manufacturer. The device manufacturer is asked to
point a CNAME to the CDN, to a name that might look like
"g7.a.example", with the expectation that the CDN vendor's provider's DNS will
do all the appropriate work to geolocate the transfer. This can be
fine for a MUD file, as the MUD controller, if located in the same
geography as the IoT device, can follow the CNAME and collect the set
of resulting IP addresses along with the TTL for each. Then, the MUD
controller can take charge of refreshing that mapping at intervals
driven by the TTL.
In some cases, a complete set of geographically distributed servers
may be known ahead of time (or that it changes very slowly), and the
device manufacturer can list all those IP addresses in the DNS for
the name that it lists in the MUD file. As long as the active set of
addresses used by the CDN is a strict subset of that list, then the
geolocated name can be used for the content download itself.
4.3. Use of a Too Generic DNS Name
Some CDNs make all customer content available at a single URL (such
as "s3.example.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.
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 in 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 endpoints (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 DoT and DoH. [RFC9499] details
the terms. But, even with the unencrypted DNS (a.k.a. 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 devices, revealing the DNS queries to
those outside entities may constitute a privacy concern. For other
users, the use of an insecure local resolver may constitute a privacy
concern.
As described in Section 3, the MUD controller needs to have access to
the same resolver or resolvers as the IoT device. If the IoT device
does not use the DNS servers provided to it via DHCP or Router
Advertisements, then the MUD controller will need to be told which
servers will in fact be used. As yet, there is no protocol to do
this, but future work could provide this as an extension to MUD.
Until such time as such a protocol exists, the best practice is for
the IoT device to always use the DNS servers provided by DHCP or
Router Advertisements.
6. Recommendations to IoT Device Manufacturers 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 [RFC9238]).
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 actual list of expected connections to put in the MUD file. An
IoT manufacturer must now spend some time reviewing the network
communications by their device.
This document discusses a number of challenges that occur relating to
how DNS requests are made and resolved, and the goal of this section
is to make recommendations on how to modify IoT systems to work well
with MUD.
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.
DNS names should always be used.
6.2. Use Primary DNS Names Controlled by the Manufacturer
The second recommendation is to allocate and use DNS names within
zones controlled by the manufacturer. These DNS names can be
populated with an alias (see [RFC9499], Section 2) that points to the
production system. Ideally, a different name is used for each
logical function, allowing 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; they allow for an infinite
number of possible DNS names.
6.3. Use a Content Distribution Network with Stable DNS Names
When aliases point to a CDN, give preference to stable DNS names that
point to appropriately load-balanced targets. CDNs that employ very
low 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 Tailored Responses to Answer DNS Names
[RFC7871] defines the edns-client-subnet (ECS) EDNS0 option and
explains how authoritative servers sometimes answer queries
differently based upon the IP address of the end system making the
request. Ultimately, the decision is based upon some topological
notion of closeness. This is often used to provide tailored
responses to clients, providing them with a geographically
advantageous answer.
When the MUD controller makes its DNS query, it is critical that it
receives an answer that is based upon the same topological decision
as when the IoT device makes its query.
There are probably ways in which the MUD controller could use the
edns-client-subnet option to make a query that would get the same
treatment as when the IoT device makes its query. If this worked,
then it would receive the same answer as the IoT device.
In practice it could be quite difficult if the IoT device uses a
different Internet connection, a different firewall, or a different
recursive DNS server. The edns-client-server edns-client-subnet option might be ignored
or overridden by any of the DNS infrastructure.
Some tailored responses might only reorder the replies so that the
most preferred address is first. Such a system would be acceptable
if the MUD controller had a way to know that the list was complete.
But, due to the above problems, a strong recommendation is to avoid
using tailored responses as part of the DNS names in the MUD file.
6.5. Prefer DNS Servers Learned from DHCP/Router Advertisements
The best practice is for IoT devices to do DNS with the DHCP-provided
DNS servers or with DNS servers learned from Router Advertisements
[RFC8106].
The Adaptive DNS Discovery (ADD) Working Group has written [RFC9462]
and [RFC9463] to provide information to end devices on how to find
locally provisioned secure/private DNS servers.
Use of public resolvers instead of the locally provided DNS resolver,
whether Do53, DoQ, DoT, or DoH, is discouraged.
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, detailed in Section 6.4. The public
resolver might not return the same tailored names that the MUD
controller would get.
It is recommended that non-local resolvers are only used when the
locally provided resolvers provide no answers to any queries at all
and do so repeatedly. The status of the operator-provided resolvers
needs to be re-evaluated on a periodic basis.
Finally, if a device will ever attempt to use a non-local resolver, resolvers,
then the address addresses of that resolver needs those resolvers need to be listed in the MUD
file as a destination destinations that is are to be permitted. This needs to include
the port numbers (i.e., 53, 853 for DoT, 443 for DoH) that will be
used as well.
7. Interactions with mDNS and DNS-SD
Unicast DNS requests are not the only way to map names to IP
addresses. IoT devices might also use Multicast DNS (mDNS)
[RFC6762], both to be discovered by other devices and also to
discover other devices.
mDNS replies include A and AAAA records, and it is conceivable that
these replies contain addresses that are not local to the link on
which they are made. This could be the result of another device that
contains malware. An unsuspecting IoT device could be led to contact
some external host as a result. Protecting against such things is
one of the benefits of MUD.
In the unlikely case that the external host has been listed as a
legitimate destination in a MUD file, communication will continue as
expected. As an example, an IoT device might look for a name like
"update.local" in order to find a source of firmware updates. It
could be led to connect to some external host that was listed as
"update.example" in the MUD file. This should work fine if the name
"update.example" does not require any kind of tailored reply.
In residential networks, there has typically not been more than one
network (although this is changing through work like
[AUTO-STUB-NETWORKS]), but on campus or enterprise networks, having
more than one network is not unusual. In such networks, mDNS is
being replaced with DNS-based Service Discovery (DNS-SD) [RFC8882],
and in such a situation, connections could be initiated to other
parts of the network. Such connections might traverse the MUD policy
enforcement point (an intra-department firewall) and could very well
be rejected because the MUD controller did not know about that
interaction.
[RFC8250] includes a number of provisions for controlling internal
communications, including complex communications like same
manufacturer ACLs. To date, this aspect of MUD has been difficult to
describe. This document does not consider internal communications to
be in scope.
8. IANA Considerations
This document has no IANA actions.
9. Privacy Considerations
The use of non-local DNS servers exposes the list of DNS names
resolved to a third party, including passive eavesdroppers.
The use of DoT and DoH eliminates the threat from passive
eavesdropping but still exposes the list to the operator of the DoT
or DoH server. There are additional methods to help preserve
privacy, such as that described by [RFC9230].
The use of unencrypted (Do53) requests to a local DNS server exposes
the list to any internal passive eavesdroppers. For some situations,
that may be significant, particularly if unencrypted WiFi is used.
Use of an encrypted DNS connection to a local DNS recursive resolver
is the preferred choice.
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., the attacker can take
advantage of any known vulnerability on the device).
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 4.1 postulates that the version
number needs to be provided to an intelligent agent that can decide
the correct route to do upgrades. [RFC9019] provides a wide variety
of ways to accomplish the same thing without having to divulge the
current version number.
9.
10. Security Considerations
This document deals with conflicting security requirements:
* devices that an operator wants to manage using [RFC8520]
* 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 careful planning
on how the DNS can be safely and effectively be used by MUD
controllers and IoT devices.
When an IoT device with an inaccurate MUD file is deployed into a
network that uses MUD, there is a significant possibility that the
device will cause a spurious security exception to be raised. There
is significant evidence that such spurious exceptions can cause
significant overhead to personnel. In particular, repeated spurious
exceptions are likely to cause the entire exception process to be
turned off. When MUD alerts are turned off, then even legitimate
exceptions are ignored. This is very much a Boy Who Calls Wolf
[boywhocriedwolf] situation.
In order to avoid this situation, and for MUD alerts to be given
appropriate attention, it is key that IoT device manufacturers create
accurate MUD files. This may require some significant thought and
even rework of key systems so that all network access required by the
IoT device can be described by a MUD file. This level of informed
cooperation within the IoT device vendor and with MUD controller
manufacturers is key to getting significant return on investment from
MUD.
Manufacturers are encouraged to write MUD files that are good enough
rather than perfect. If in doubt, they should write MUD files that
are somewhat more permissive if the files result in no spurious
alerts.
10.
11. References
10.1.
11.1. Normative References
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
DOI 10.17487/RFC1794, April 1995,
<https://www.rfc-editor.org/info/rfc1794>.
[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>.
[RFC8250] Elkins, N., Hamilton, R., and M. Ackermann, "IPv6
Performance and Diagnostic Metrics (PDM) Destination
Option", RFC 8250, DOI 10.17487/RFC8250, September 2017,
<https://www.rfc-editor.org/info/rfc8250>.
[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>.
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/info/rfc9019>.
[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.
10.2.
11.2. Informative References
[Akamai] Wikipedia, "Akamai Technologies", 26 February 2025,
<https://en.wikipedia.org/w/
index.php?title=Akamai_Technologies&oldid=1277665363>.
[AmazonS3] Wikipedia, "Amazon S3", 14 March 2025,
<https://en.wikipedia.org/w/
index.php?title=Amazon_S3&oldid=1280379498>.
[antipattern]
Agile Alliance, "AntiPattern",
<https://www.agilealliance.org/glossary/antipattern>.
[AUTO-STUB-NETWORKS]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-06, 4 November
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
snac-simple-06>.
[awss3virtualhosting]
Tech Monitor, "Down to the Wire: AWS Delays 'Path-Style'
S3 Deprecation at Last Minute", 24 September 2020,
<https://techmonitor.ai/techonology/cloud/aws-s3-path-
deprecation>.
[boywhocriedwolf]
Wikipedia, "The Boy Who Cried Wolf", 6 February 2025,
<https://en.wikipedia.org/w/
index.php?title=The_Boy_Who_Cried_Wolf&oldid=1274257821>.
[mudmaker] "MUD Maker", <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>.
[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>.
[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.
[RFC6707] Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
Distribution Network Interconnection (CDNI) Problem
Statement", RFC 6707, DOI 10.17487/RFC6707, September
2012, <https://www.rfc-editor.org/info/rfc6707>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>.
[RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 8106, DOI 10.17487/RFC8106, March 2017,
<https://www.rfc-editor.org/info/rfc8106>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8882] Huitema, C. and D. Kaiser, "DNS-Based Service Discovery
(DNS-SD) Privacy and Security Requirements", RFC 8882,
DOI 10.17487/RFC8882, September 2020,
<https://www.rfc-editor.org/info/rfc8882>.
[RFC9230] Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.A.
Wood, "Oblivious DNS over HTTPS", RFC 9230,
DOI 10.17487/RFC9230, June 2022,
<https://www.rfc-editor.org/info/rfc9230>.
[RFC9238] Richardson, M., Latour, J., and H. Habibi Gharakheili,
"Loading Manufacturer Usage Description (MUD) URLs from QR
Codes", RFC 9238, DOI 10.17487/RFC9238, May 2022,
<https://www.rfc-editor.org/info/rfc9238>.
[RFC9250] Huitema, C., Dickinson, S., and A. Mankin, "DNS over
Dedicated QUIC Connections", RFC 9250,
DOI 10.17487/RFC9250, May 2022,
<https://www.rfc-editor.org/info/rfc9250>.
[RFC9462] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", RFC 9462,
DOI 10.17487/RFC9462, November 2023,
<https://www.rfc-editor.org/info/rfc9462>.
[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, November 2023,
<https://www.rfc-editor.org/info/rfc9463>.
Appendix A. A Failing Strategy: Anti-Patterns
Attempts to map IP addresses to DNS names in real time often fail for
a number of reasons:
1. It can not be done fast enough.
2. It reveals usage patterns of the devices.
3. The mappings 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 for the reasons explained below.
A.1. Too Slow
Mappings of IP addresses to DNS names require a DNS lookup in the in-
addr.arpa or ip6.arpa space. For a cold DNS cache, this will
typically require 2 to 3 NS record lookups to locate the DNS server
that holds the information required. At 20 to 100 ms per round trip,
this easily adds up to a significant amount of time before the packet
that caused the lookup can be released.
While subsequent connections to the same site (and subsequent packets
in the same flow) will not be affected if the results are cached, the
effects will be felt. The ACL results can be cached for a period of
time given by the TTL of the DNS results, but the DNS lookup must be
repeated, e.g., in a few hours or days, when the cached binding (of
IP address to name) expires.
A.2. Reveals Patterns of Usage
By doing the DNS lookups when the traffic occurs, then a passive
attacker can see when the device is active and may be able to derive
usage patterns. They could determine when a home was occupied or
not. This does not require access to all on-path data, just to the
DNS requests to the bottom level of the DNS tree.
A.3. Mappings Are Often Incomplete
An IoT manufacturer with a cloud service provider that fails to
include an A or AAAA record as part of their forward name publication
will find that the new server is simply not used. The operational
feedback for that mistake is immediate. The same is not true for
reverse DNS mappings: They can often be incomplete or incorrect for
months or even years without a visible effect on operations.
IoT manufacturer cloud service providers often find it difficult to
update reverse DNS maps in a timely fashion, assuming that they can
do it at all. Many cloud-based solutions dynamically assign IP
addresses to services, often as the service grows and shrinks,
reassigning those IP addresses to other services quickly. The use of
HTTP 1.1 Virtual Hosting may allow addresses and entire front-end
systems to be reused dynamically without even reassigning the IP
addresses.
In some cases, there are multiple layers of CNAME between the
original name and the target service name. This is often due to a
load-balancing layer in the DNS followed by a load-balancing layer at
the HTTP level.
The reverse DNS mapping for the IP address of the load balancer
usually does not change. If hundreds of web services are funneled
through the load balancer, it would require hundreds of PTR records
to be deployed. This would easily exceed the UDP/DNS and EDNS0
limits and require all queries to use TCP, which would further slow
down loading of the records.
The enumeration of all services/sites that have been at that load
balancer might also constitute a security concern. To limit the
churn of DNS PTR records and reduce failures of the MUD ACLs,
operators would want to add all possible DNS names for each reverse
DNS mapping, whether or not the DNS load-balancing in the forward DNS
space lists that endpoint at that moment.
A.4. Forward DNS Names Can Have Wildcards
In some large hosting providers, content is hosted through a domain
name that is published as a DNS wildcard (and uses a wildcard
certificate). For instance, github.io, which is used for hosting
content, including the Editors' copy of Internet-Drafts stored on
GitHub, does not actually publish any DNS names. Instead, a wildcard
exists to answer all potential DNS names: Requests are routed
appropriately once they are received.
This kind of system works well for self-managed hosted content.
However, while it is possible to insert up to a few dozen PTR
records, many thousands of entries are not possible, nor is it
possible to deal with the unlimited (infinite) number of
possibilities that a wildcard supports.
Therefore, it would be impossible for the PTR reverse lookup to ever
work with these wildcard DNS names.
Contributors
Tirumaleswar Reddy.K
Nokia
Authors' Addresses
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com