draft-ietf-opsawg-mud-iot-dns-considerations-13.txt   draft-ietf-opsawg-mud-iot-dns-considerations-16.txt 
OPSAWG Working Group M. Richardson OPSAWG Working Group M. Richardson
Internet-Draft Sandelman Software Works Internet-Draft Sandelman Software Works
Intended status: Best Current Practice W. Pan Intended status: Best Current Practice W. Pan
Expires: 22 September 2024 Huawei Technologies Expires: 4 January 2025 Huawei Technologies
21 March 2024 3 July 2024
Operational Considerations for Use of DNS in IoT Devices Operational Considerations for Use of DNS in IoT Devices
draft-ietf-opsawg-mud-iot-dns-considerations-13 draft-ietf-opsawg-mud-iot-dns-considerations-16
Abstract Abstract
This document details concerns about how Internet of Things (IoT) This document details considerations about how Internet of Things
devices use IP addresses and DNS names. These concerns become acute (IoT) devices use IP addresses and DNS names. These concerns become
as network operators begin deploying RFC 8520 Manufacturer Usage acute as network operators begin deploying RFC 8520 Manufacturer
Description (MUD) definitions to control device access. Usage Description (MUD) definitions to control device access.
Alos, this document makes recommendations on when and how to use DNS Also, this document makes recommendations on when and how to use DNS
names in MUD files. names in MUD files.
About This Document About This Document
This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Status information for this document may be found at Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-opsawg-mud-iot-dns- https://datatracker.ietf.org/doc/draft-ietf-opsawg-mud-iot-dns-
considerations/. considerations/.
Discussion of this document takes place on the opsawg Working Group Discussion of this document takes place on the opsawg Working Group
mailing list (mailto:opsawg@ietf.org), which is archived at mailing list (mailto:opsawg@ietf.org), which is archived at
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https://www.ietf.org/mailman/listinfo/opsawg/.
Source for this draft and an issue tracker can be found at Source for this draft and an issue tracker can be found at
https://github.com/mcr/iot-mud-dns-considerations. https://github.com/mcr/iot-mud-dns-considerations.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 22 September 2024. This Internet-Draft will expire on 4 January 2025.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. A model for MUD controller mapping of names to addresses . . 4 3. A model for MUD controller mapping of DNS names to
3.1. Non-Deterministic Mappings . . . . . . . . . . . . . . . 4 addresses . . . . . . . . . . . . . . . . . . . . . . . . 4
4. DNS and IP Anti-Patterns for IoT Device Manufacturers . . . . 6 3.1. Non-Deterministic Mappings . . . . . . . . . . . . . . . 5
4.1. Use of IP Address Literals in-protocol . . . . . . . . . 6 4. DNS and IP Anti-Patterns for IoT Device Manufacturers . . . . 7
4.1. Use of IP Address Literals . . . . . . . . . . . . . . . 7
4.2. Use of Non-deterministic DNS Names in-protocol . . . . . 8 4.2. Use of Non-deterministic DNS Names in-protocol . . . . . 8
4.3. Use of a Too Generic DNS Name . . . . . . . . . . . . . . 9 4.3. Use of a Too Generic DNS Name . . . . . . . . . . . . . . 9
5. DNS Privacy and Outsourcing versus MUD Controllers . . . . . 9 5. DNS Privacy and Outsourcing versus MUD Controllers . . . . . 10
6. Recommendations To IoT Device Manufacturers on MUD and DNS 6. Recommendations To IoT Device Manufacturers on MUD and DNS
Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Consistently use DNS . . . . . . . . . . . . . . . . . . 10 6.1. Consistently use DNS . . . . . . . . . . . . . . . . . . 10
6.2. Use Primary DNS Names Controlled By The Manufacturer . . 10 6.2. Use Primary DNS Names Controlled By The Manufacturer . . 11
6.3. Use Content-Distribution Network with Stable Names . . . 10 6.3. Use Content-Distribution Network with Stable DNS Names . 11
6.4. Do Not Use Tailored Responses to answer DNS Names . . . . 11 6.4. Do Not Use Tailored Responses to answer DNS Names . . . . 11
6.5. Prefer DNS Servers Learnt From DHCP/Route 6.5. Prefer DNS Servers Learnt From DHCP/Router
Advertisements . . . . . . . . . . . . . . . . . . . . . 11 Advertisements . . . . . . . . . . . . . . . . . . . . . 12
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12 7. Interactions with mDNS and DNSSD . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 13 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 14 10.1. Normative References . . . . . . . . . . . . . . . . . . 15
Appendix A. A Failing Strategy --- Anti-Patterns . . . . . . . . 16 10.2. Informative References . . . . . . . . . . . . . . . . . 15
A.1. Too Slow . . . . . . . . . . . . . . . . . . . . . . . . 16 Appendix A. A Failing Strategy --- Anti-Patterns . . . . . . . . 18
A.2. Reveals Patterns of Usage . . . . . . . . . . . . . . . . 16 A.1. Too Slow . . . . . . . . . . . . . . . . . . . . . . . . 18
A.3. Mappings Are Often Incomplete . . . . . . . . . . . . . . 17 A.2. Reveals Patterns of Usage . . . . . . . . . . . . . . . . 18
A.4. Forward Names Can Have Wildcards . . . . . . . . . . . . 17 A.3. Mappings Are Often Incomplete . . . . . . . . . . . . . . 19
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 18 A.4. Forward DNS Names Can Have Wildcards . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
[RFC8520] provides a standardized way to describe how a specific [RFC8520] provides a standardized way to describe how a specific
purpose device makes use of Internet resources. Access Control Lists purpose device makes use of Internet resources. Access Control Lists
(ACLs) can be defined in an RFC 8520 Manufacturer Usage Description (ACLs) can be defined in an RFC 8520 Manufacturer Usage Description
(MUD) file that permit a device to access Internet resources by their (MUD) file that permit a device to access Internet resources by their
DNS names or IP addresses. DNS names or IP addresses.
Use of a DNS name rather than an IP address in an ACL has many 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 advantages: not only does the layer of indirection permit the mapping
of a name to IP address(es) to be changed over time, it also of a name to IP address(es) to be changed over time, it also
generalizes automatically to IPv4 and IPv6 addresses, as well as generalizes automatically to IPv4 and IPv6 addresses, as well as
permitting a variety of load balancing strategies, including multi- permitting a variety of load balancing strategies, including multi-
CDN deployments wherein load balancing can account for geography and CDN deployments wherein load balancing can account for geography and
load. load.
However, the use of DNS names has implications on how ACL are However, the use of DNS names has implications on how ACL are
executed at the MUD policy enforcement point (typically, a firewall). executed at the MUD policy enforcement point (typically, a firewall).
Conceretely, the firewall has access only to the Layer 3 headers of Concretely, the firewall has access only to the Layer 3 headers of
the packet. This includes the source and destination IP address, and the packet. This includes the source and destination IP address, and
if not encrypted by IPsec, the destination UDP or TCP port number if not encrypted by IPsec, the destination UDP or TCP port number
present in the transport header. The DNS name is not present! present in the transport header. The DNS name is not present!
So in order to implement these name based ACLs, there must be a So in order to implement these name based ACLs, there must be a
mapping between the names in the ACLs and IP addresses. mapping between the names in the ACLs and IP addresses.
In order for manufacturers to understand how to configure DNS In order for manufacturers to understand how to configure DNS
associated with name based ACLs, a model of how the DNS resolution associated with name based ACLs, a model of how the DNS resolution
will be done by MUD controllers is necessary. Section 3 models some will be done by MUD controllers is necessary. Section 3 models some
skipping to change at page 4, line 9 skipping to change at page 4, line 14
Section 5 details how current trends in DNS resolution such as public Section 5 details how current trends in DNS resolution such as public
DNS servers, DNS over TLS (DoT) [RFC7858], DNS over HTTPS (DoH) DNS servers, DNS over TLS (DoT) [RFC7858], DNS over HTTPS (DoH)
[RFC8484], or DNS over QUIC (DoQ) [RFC9250] can cause problems with [RFC8484], or DNS over QUIC (DoQ) [RFC9250] can cause problems with
the strategies employed. the strategies employed.
The core of this document, is Section 6, which makes a series of The core of this document, is Section 6, which makes a series of
recommendations ("best current practices") for manufacturers on how recommendations ("best current practices") for manufacturers on how
to use DNS and IP addresses with MUD supporting IoT devices. to use DNS and IP addresses with MUD supporting IoT devices.
Section 7 discusses a set of privacy issues that encrypted DNS (DoT, Section 8 discusses a set of privacy issues that encrypted DNS (DoT,
DoH, for example) are frequently used to deal with. How these DoH, for example) are frequently used to deal with. How these
concerns apply to IoT devices located within a residence or concerns apply to IoT devices located within a residence or
enterprise is a key concern. enterprise is a key concern.
Section 8 also covers some of the negative outcomes should MUD/ Section 9 also covers some of the negative outcomes should MUD/
firewall managers and IoT manufacturers choose not to cooperate. firewall managers and IoT manufacturers choose not to cooperate.
2. Terminology 2. Terminology
Although this document is not an IETF Standards Track publication, it Although this document is not an IETF Standards Track publication, it
adopts the conventions for normative language to provide clarity of adopts the conventions for normative language to provide clarity of
instructions to the implementer. The key words "MUST", "MUST NOT", instructions to the implementer. The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119] document are to be interpreted as described in BCP 14 [RFC2119]
[RFC8174] when, and only when, they appear in all capitals, as shown [RFC8174] when, and only when, they appear in all capitals, as shown
here. here.
This document makes use of terms defined in [RFC8520] and This document makes use of terms defined in [RFC8520] and
[I-D.ietf-dnsop-rfc8499bis]. [I-D.ietf-dnsop-rfc8499bis].
The term "anti-pattern" comes from agile software design literature, The term "anti-pattern" comes from agile software design literature,
as per [antipatterns]. as per [antipatterns].
3. A model for MUD controller mapping of names to addresses CDN refers to Content Distribution Networks, such as described by
[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. This section details a strategy that a MUD controller could take.
Within the limits of DNS use detailed in Section 6, this process can Within the limits of DNS use detailed in Section 6, this process can
work. The methods detailed in Appendix A just will not work. work. The methods detailed in Appendix A just will not work.
The simplest successful strategy for translating names for a MUD The simplest successful strategy for translating DNS names for a MUD
controller to take is to do a DNS lookup on the name (a forward 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 lookup), and then use the resulting IP addresses to populate the
actual ACLs. actual ACLs.
There a number of possible failures, and the goal of this section is There a number of possible failures, and the goal of this section is
to explain how some common DNS usages may fail. to explain how some common DNS usages may fail.
3.1. Non-Deterministic Mappings 3.1. Non-Deterministic Mappings
The most important one is that the mapping of the names to IP The most important one is that the mapping of the DNS names to IP
addresses may be non-deterministic. addresses may be non-deterministic.
[RFC1794] describes the very common mechanism that returns DNS A (or [RFC1794] describes the very common mechanism that returns DNS A (or
reasonably AAAA) records in a permuted order. This is known as Round reasonably AAAA) records in a permuted order. This is known as Round
Robin DNS, and it has been used for many decades. The historical Robin DNS, and it has been used for many decades. The historical
intent is that the requestor will tend to use the first IP address intent is that the requestor will tend to use the first IP address
that is returned. As each query results in addresses in a different that is returned. As each query results in addresses in a different
ordering, the effect is to split the load among many servers. ordering, the effect is to split the load among many servers.
This situation does not result in failures as long as all possible A/ This situation does not result in failures as long as all possible A/
skipping to change at page 6, line 28 skipping to change at page 6, line 35
The solution of using the same caching recursive resolver as the The solution of using the same caching recursive resolver as the
target device is very simple when the MUD controller is located in a target device is very simple when the MUD controller is located in a
residential CPE device. The device is usually also the policy residential CPE device. The device is usually also the policy
enforcement point for the ACLs, and a caching resolver is typically enforcement point for the ACLs, and a caching resolver is typically
located on the same device. In addition to convenience, there is a located on the same device. In addition to convenience, there is a
shared fate advantage: as all three components are running on the shared fate advantage: as all three components are running on the
same device, if the device is rebooted, clearing the cache, then all same device, if the device is rebooted, clearing the cache, then all
three components will get restarted when the device is restarted. three components will get restarted when the device is restarted.
Where the solution is more complex is when the MUD controller is Where the solution is more complex and sometimes more fragile is when
located elsewhere in an Enterprise, or remotely in a cloud such as the MUD controller is located elsewhere in an Enterprise, or remotely
when a Software Defined Network (SDN) is used to manage the ACLs. in a cloud such as when a Software Defined Network (SDN) is used to
The DNS servers for a particular device may not be known to the MUD manage the ACLs. The DNS servers for a particular device may not be
controller, nor the MUD controller be even permitted to make known to the MUD controller, nor the MUD controller be even permitted
recursive queries to that server if it is known. In this case, to make recursive queries to that server if it is known. In this
additional installation specific mechanisms are probably needed to case, additional installation specific mechanisms are probably needed
get the right view of the DNS. 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 time to live. In that
case, the MUD controller still has the old address(es) 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 4. DNS and IP Anti-Patterns for IoT Device Manufacturers
In many design fields, there are good patterns that should be In many design fields, there are good patterns that should be
emulated, and often there are patterns that should not be emulated. emulated, and often there are patterns that should not be emulated.
The latter are called anti-patterns, as per [antipatterns]. The latter are called anti-patterns, as per [antipatterns].
This section describes a number of things which IoT manufacturers This section describes a number of things which IoT manufacturers
have been observed to do in the field, each of which presents have been observed to do in the field, each of which presents
difficulties for MUD enforcement points. difficulties for MUD enforcement points.
4.1. Use of IP Address Literals in-protocol 4.1. Use of IP Address Literals
A common pattern for a number of devices is to look for firmware 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 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 HTTPS, sometimes with a POST, but the method is immaterial) to a
vendor system that knows whether an update is required. vendor system that knows whether an update is required.
The current firmware model of the device is sometimes provided and The current firmware model of the device is sometimes provided and
then the authoritative server provides a determination if a new then the vendor's authoritative server provides a determination if a
version is required and, if so, what version. In simpler cases, an new version is required and, if so, what version. In simpler cases,
HTTPS endpoint is queried which provides the name and URL of the most an HTTPS endpoint is queried which provides the name and URL of the
recent firmware. most recent firmware.
The authoritative upgrade server then responds with a URL of a The authoritative upgrade server then responds with a URL of a
firmware blob that the device should download and install. Best firmware blob that the device should download and install. Best
practice is that firmware is either signed internally ([RFC9019]) so practice is that firmware is either signed internally ([RFC9019]) so
that it can be verified, or a hash of the blob is provided. that it can be verified, or a hash of the blob is provided.
An authoritative server might be tempted to provide an IP address An authoritative server might be tempted to provide an IP address
literal inside the protocol: there are two arguments (anti-patterns) literal inside the protocol. An argument for doing this is that it
for doing this. eliminates problems with firmware updates that might be caused by
lack of DNS, or incompatibilities with DNS. For instance a bug that
The first is that it eliminates problems with firmware updates that causes interoperability issues with some recursive servers would
might be caused by lack of DNS, or incompatibilities with DNS. For become unpatchable for devices that were forced to use that recursive
instance a bug that causes interoperability issues with some resolver type.
recursive servers would become unpatchable for devices that were
forced to use that recursive resolver type.
The second reason to avoid an IP address literal 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 more problems with use of IP address literals for the But, there are several problems with the use of IP address literals
location of the firmware. for the location of the firmware.
The first is that the update service server must decide whether to 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 provide an IPv4 or an IPv6 literal, assuming that only one URL can be
kinds of addresses, and can also contain many different IP addresses provided. A DNS name can contain both kinds of addresses, and can
of each kind. also contain many different IP addresses of each kind. An update
server might believe that if the connection was on IPv4, that an IPv4
literate would be acceptable, but 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 arbitrary IPv4 address.
The second problem is that it forces the MUD file definition to A MUD file definition for this access would need to resolve to the
contain the exact same IP address literals. It must also contain an set of IP addresses that might be returned by the update server.
ACL for each address literal. DNS provides a useful indirection This can be done with IP address literals in the MUD file, but this
method that naturally aggregates the addresses. 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. IP address literals do A third problem involves the use of HTTPS. It is often more
not provide enough context for TLS ServerNameIndicator to be useful difficult to get TLS certificates for an IP address, and so it is
[RFC6066]. This limits the firmware repository to be a single tenant less likely that the firmware download will be protected by TLS. An
on that IP address, and for IPv4 (at least), this is no longer a IP address literal in the TLS ServerNameIndicator [RFC6066], might
sustainable use of IP addresses. not provide enough context for a web server to distinguish which of
potentially many tenants, the client wishes to reach. This then
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 Content Distribution Networks (CDNs) to Finally, it is common in some Content Distribution Networks (CDNs) to
use multiple layers of DNS CNAMEs in order to isolate the content- use multiple layers of DNS CNAMEs in order to isolate the content-
owner's naming system from changes in how the distribution network is owner's naming system from changes in how the distribution network is
organized. organized.
A non-deterministic name or address that is returned within the When a name or address is returned within an update protocol for
update protocol, the MUD controller is unable to know what the name which a MUD rule cannot be written, then the MUD controller is unable
is. It is therefore unable to make sure that the communication to to authorize the connection. In order for the connection to be
retrieve the new firmware is permitted by the MUD enforcement point. 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 authorized.
4.2. Use of Non-deterministic DNS Names in-protocol 4.2. Use of Non-deterministic DNS Names in-protocol
A second pattern is for a control protocol to connect to a known HTTP A second pattern is for a control protocol to connect to a known HTTP
endpoint. This is easily described in MUD. References within that endpoint. This is easily described in MUD. References within that
control protocol are made to additional content at other URLs. The control protocol are made to additional content at other URLs. The
values of those URLs do not fit any easily described pattern and may values of those URLs do not fit any easily described pattern and may
point at arbitrary names. point at arbitrary DNS names.
Those names are often within some third-party CDN system, or may be
arbitrary names in a cloud-provider storage system (e.g., [AmazonS3],
or [Akamai]). Some of the name components may be specified by the
provider.
Such names may be unpredictably chosen by the content provider, and Those DNS names are often within some third-party CDN system, or may
not the content owner, and so impossible to insert into a MUD file. 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.
Even if the content provider chosen names are deterministic they may Such DNS names may be unpredictably chosen by the CDN, and not the
change at a rate much faster than MUD files can be updated. device manufacturer, and so impossible to insert into a MUD file.
Implementation of the CDN system may also involve HTTP redirections
to downstream CDN systems.
This in particular may apply to the location where firmware updates Even if the CDN provider's chosen DNS names are deterministic they
may be retrieved. may change at a rate much faster than MUD files can be updated.
A solution is to use a deterministic DNS name, within the control of This situation applies to firmware updates, but also applies to many
the firmware vendor. This may be a problem if the content other kinds of content: video content, in-game content, etc.
distribution network needs to reorganize which IP address is
responsible for which content, or if there is a desire to provide
content in geographically relevant ways.
The firmware vendor is therefore likely to be asked to point a CNAME A solution may be to use a deterministic DNS name, within the control
to the CDN, to a name that might look like "g7.a.example", with the of the device manufacturer. The device manufacturer is asked to
expectation that the CDN vendors DNS will do all the appropriate work point a CNAME to the CDN, to a name that might look like
to geolocate the transfer. This can be fine for a MUD file, as the "g7.a.example", with the expectation that the CDN vendors DNS will do
MUD controller, if located in the same geography as the IoT device, all the appropriate work to geolocate the transfer. This can be fine
can follow the CNAME, and can collect the set of resulting IP for a MUD file, as the MUD controller, if located in the same
addresses, along with the TTL for each. The MUD controller can then geography as the IoT device, can follow the CNAME, and can collect
take charge of refreshing that mapping at intervals driven by the the set of resulting IP addresses, along with the TTL for each. The
TTL. MUD controller can then take charge of refreshing that mapping at
intervals driven by the TTL.
In some cases, a complete set of geographically distributed servers In some cases, a complete set of geographically distributed servers
is known ahead of time, and the firmware vendor can list all those may be known ahead of time (or that it changes very slowly), and the
addresses in the DNS for the the name that it lists in the MUD file. device manufacturer can list all those IP addresses in the DNS for
As long as the active set of addresses used by the CDN is a strict the the name that it lists in the MUD file. As long as the active
subset of that list, then the geolocated name can be used for the set of addresses used by the CDN is a strict subset of that list,
firmware download itself. This use of two addresses is ripe for then the geolocated name can be used for the content download itself.
confusion, however.
4.3. Use of a Too Generic DNS Name 4.3. Use of a Too Generic DNS Name
Some CDNs make all customer content available at a single URL (such 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 as "s3.example.com"). This seems to be ideal from a MUD point of
view: a completely predictable URL. view: a completely predictable URL.
The problem is that a compromised device could then connect to the The problem is that a compromised device could then connect to the
contents of any bucket, potentially attacking the data from other contents of any bucket, potentially attacking the data from other
customers. customers.
skipping to change at page 9, line 50 skipping to change at page 10, line 19
unencrypted DNS (a.k.a. Do53), it is possible to outsource DNS unencrypted DNS (a.k.a. Do53), it is possible to outsource DNS
queries to other public services, such as those operated by Google, queries to other public services, such as those operated by Google,
CloudFlare, Verisign, etc. CloudFlare, Verisign, etc.
For some users and classes of devices, revealing the DNS queries to For some users and classes of devices, revealing the DNS queries to
those outside entities may constitute a privacy concern. For other those outside entities may constitute a privacy concern. For other
users the use of an insecure local resolver may constitute a privacy users the use of an insecure local resolver may constitute a privacy
concern. concern.
As described above in Section 3 the MUD controller needs to have As described above in Section 3 the MUD controller needs to have
access to the same resolver(s) as the IoT device. access to the same resolver(s) 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 6. Recommendations To IoT Device Manufacturers on MUD and DNS Usage
Inclusion of a MUD file with IoT devices is operationally quite Inclusion of a MUD file with IoT devices is operationally quite
simple. It requires only a few small changes to the DHCP client code 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 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 use of a QR code affixed to the packaging (see [RFC9238])
The difficult part is determining what to put into the MUD file The difficult part is determining what to put into the MUD file
itself. There are currently tools that help with the definition and itself. There are currently tools that help with the definition and
skipping to change at page 10, line 28 skipping to change at page 10, line 52
This document discusses a number of challenges that occur relating to This document discusses a number of challenges that occur relating to
how DNS requests are made and resolved, and the goal of this section 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 is to make recommendations on how to modify IoT systems to work well
with MUD. with MUD.
6.1. Consistently use DNS 6.1. Consistently use DNS
For the reasons explained in Section 4.1, the most important For the reasons explained in Section 4.1, the most important
recommendation is to avoid using IP address literals in any protocol. recommendation is to avoid using IP address literals in any protocol.
Names should always be used. DNS names should always be used.
6.2. Use Primary DNS Names Controlled By The Manufacturer 6.2. Use Primary DNS Names Controlled By The Manufacturer
The second recommendation is to allocate and use names within zones The second recommendation is to allocate and use DNS names within
controlled by the manufacturer. These names can be populated with an zones controlled by the manufacturer. These DNS names can be
alias (see [I-D.ietf-dnsop-rfc8499bis] section 2) that points to the populated with an alias (see [I-D.ietf-dnsop-rfc8499bis] section 2)
production system. Ideally, a different name is used for each that points to the production system. Ideally, a different name is
logical function, allowing for different rules in the MUD file to be used for each logical function, allowing for different rules in the
enabled and disabled. MUD file to be enabled and disabled.
While it used to be costly to have a large number of aliases in a web 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 server certificate, this is no longer the case. Wildcard
certificates are also commonly available which allow for an infinite certificates are also commonly available which allow for an infinite
number of possible names. number of possible DNS names.
6.3. Use Content-Distribution Network with Stable Names 6.3. Use Content-Distribution Network with Stable DNS Names
When aliases point to a CDN, prefer stable names that point to When aliases point to a CDN, prefer stable DNS names that point to
appropriately load balanced targets. CDNs that employ very low time- appropriately load balanced targets. CDNs that employ very low time-
to-live (TTL) values for DNS make it harder for the MUD controller to 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 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 same set of A and AAAA records, but permutes them to provide the best
one first provides a more reliable answer. one first provides a more reliable answer.
6.4. Do Not Use Tailored Responses to answer DNS Names 6.4. Do Not Use Tailored Responses to answer DNS Names
[RFC7871] defines the edns-client-subnet (ECS) EDNS0 option, and [RFC7871] defines the edns-client-subnet (ECS) EDNS0 option, and
explains how authoritative servers sometimes answer queries explains how authoritative servers sometimes answer queries
differently based upon the IP address of the end system making the differently based upon the IP address of the end system making the
request. Ultimately, the decision is based upon some topological request. Ultimately, the decision is based upon some topological
notion of closeness. This is often used to provide tailored notion of closeness. This is often used to provide tailored
responses to clients, providing them with a geographically responses to clients, providing them with a geographically
advantageous answer. advantageous answer.
When the MUD controller makes it's DNS query, it is critical that it When the MUD controller makes its DNS query, it is critical that it
receive an answer which is based upon the same topological decision receive an answer which is based upon the same topological decision
as when the IoT device makes its query. as when the IoT device makes its query.
There are probably ways in which the MUD controller could use the 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 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 treatment as when the IoT device makes its query. If this worked
then it would receive the same answer as the IoT device. then it would receive the same answer as the IoT device.
In practice it could be quite difficult if the IoT device uses a In practice it could be quite difficult if the IoT device uses a
different Internet connection, a different firewall, or a different different Internet connection, a different firewall, or a different
recursive DNS server. The edns-client-server might be ignored or recursive DNS server. The edns-client-server might be ignored or
overridden by any of the DNS infrastructure. overridden by any of the DNS infrastructure.
Some tailored responses might only re-order the replies so that the Some tailored responses might only re-order the replies so that the
most preferred address is first. Such a system would be acceptable 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. 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 But, due to the above problems, a strong recommendation is to avoid
using tailored responses as part of the names in the MUD file. using tailored responses as part of the DNS names in the MUD file.
6.5. Prefer DNS Servers Learnt From DHCP/Route Advertisements 6.5. Prefer DNS Servers Learnt From DHCP/Router Advertisements
IoT Devices SHOULD prefer doing DNS with the DHCP provided DNS The best practice is for IoT Devices to do DNS with the DHCP provided
servers. DNS servers, or DNS servers learnt from Router Advertisements
[RFC8106].
The ADD WG has written [RFC9463] and [RFC9462] to provide information The ADD WG has written [RFC9463] and [RFC9462] to provide information
to end devices on how to find locally provisioned secure/private DNS to end devices on how to find locally provisioned secure/private DNS
servers. servers.
Use of public resolvers instead of the provided DNS resolver, whether Use of public resolvers instead of the locally provided DNS resolver,
Do53, DoQ, DoT or DoH is discouraged. Should the network provide whether Do53, DoQ, DoT or DoH is discouraged.
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 Some manufacturers would like to have a fallback to using a public
resolver to mitigate against local misconfiguration. There are a resolver to mitigate against local misconfiguration. There are a
number of reasons to avoid this, or at least do this very carefully. 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 use of non-local resolvers is only done when It is recommended that use of non-local resolvers is only done when
the locally provided resolvers provide no answers to any queries at the locally provided resolvers provide no answers to any queries at
all, and do so repeatedly. The use of the operator provided all, and do so repeatedly. The status of the operator provided
resolvers SHOULD be retried on a periodic basis, and once they resolvers needs to be re-evaluated on a periodic basis.
answer, there SHOULD be no further attempts to contact public
resolvers.
Finally, the list of public resolvers that might be contacted MUST be Finally, if a device will ever attempt to use a non-local resolvers,
listed in the MUD file as destinations that are to be permitted! then the address of that resolver needs to be listed in the MUD file
This should include the port numbers (i.e., 53, 853 for DoT, 443 for as destinations that are to be permitted. This needs to include the
DoH) that will be used as well. port numbers (i.e., 53, 853 for DoT, 443 for DoH) that will be used
as well.
7. Privacy Considerations 7. Interactions with mDNS and DNSSD
The use of non-local DNS servers exposes the list of names resolved Unicast DNS requests are not the only way to map names to IP
to a third party, including passive eavesdroppers. addresses. IoT devices might also use 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 which are not local to the link on
which they are made. This could be the result of another device
which 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, then communication will
continue as expected. As an example of this, 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
[I-D.ietf-snac-simple]), but on campus or enterprise networks, having
more than one network is not unusual. In such networks, mDNS is
being replaced with 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. 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 The use of DoT and DoH eliminates the threat from passive
eavesdropping, but still exposes the list to the operator of the DoT eavesdropping, but still exposes the list to the operator of the DoT
or DoH server. There are additional methods to help preserve or DoH server. There are additional methods to help preserve
privacy, such as described by [RFC9230]. privacy, such as described by [RFC9230].
The use of unencrypted (Do53) requests to a local DNS server exposes The use of unencrypted (Do53) requests to a local DNS server exposes
the list to any internal passive eavesdroppers, and for some the list to any internal passive eavesdroppers, and for some
situations that may be significant, particularly if unencrypted Wi-Fi situations that may be significant, particularly if unencrypted Wi-Fi
is used. Use of Encrypted DNS connection to a local DNS recursive is used.
resolver is the preferred choice.
Use of Encrypted DNS connection to a local DNS recursive resolver is
the preferred choice.
IoT devices that reach out to the manufacturer at regular intervals IoT devices that reach out to the manufacturer at regular intervals
to check for firmware updates are informing passive eavesdroppers of to check for firmware updates are informing passive eavesdroppers of
the existence of a specific manufacturer's device being present at the existence of a specific manufacturer's device being present at
the origin location. the origin location.
Identifying the IoT device type empowers the attacker to launch Identifying the IoT device type empowers the attacker to launch
targeted attacks to the IoT device (e.g., Attacker can take advantage targeted attacks to the IoT device (e.g., Attacker can take advantage
of any known vulnerability on the device). of any known vulnerability on the device).
skipping to change at page 13, line 8 skipping to change at page 14, line 32
their update queries. For instance, contracting out the update their update queries. For instance, contracting out the update
notification service to a third party that deals with a large variety notification service to a third party that deals with a large variety
of devices would provide a level of defense against passive of devices would provide a level of defense against passive
eavesdropping. Other update mechanisms should be investigated, eavesdropping. Other update mechanisms should be investigated,
including use of DNSSEC signed TXT records with current version including use of DNSSEC signed TXT records with current version
information. This would permit DoT or DoH to convey the update information. This would permit DoT or DoH to convey the update
notification in a private fashion. This is particularly powerful if notification in a private fashion. This is particularly powerful if
a local recursive DoT server is used, which then communicates using a local recursive DoT server is used, which then communicates using
DoT over the Internet. DoT over the Internet.
The more complex case of section Section 4.1 postulates that the The more complex case of Section 4.1 postulates that the version
version number needs to be provided to an intelligent agent that can number needs to be provided to an intelligent agent that can decide
decide the correct route to do upgrades. [RFC9019] provides a wide the correct route to do upgrades. [RFC9019] provides a wide variety
variety of ways to accomplish the same thing without having to of ways to accomplish the same thing without having to divulge the
divulge the current version number. current version number.
The use of a publicly 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 an 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)
While a vendor proprietary scheme to distribute firmware updates
would satisfy some of these criteria, operators/Enterprises are less
likely to install one of these for every single device class. Home
(residential) users are unlikely to install any system that did not
provide service to all their devices (and came pre-installed on a
home router or other home network management system, such as a home
Network Attached Storage device), so only a system that was non-
proprietary is likely to be present.
8. Security Considerations 9. Security Considerations
This document deals with conflicting Security requirements: This document deals with conflicting Security requirements:
1. devices which an operator wants to manage using [RFC8520] 1. devices which an operator wants to manage using [RFC8520]
2. requirements for the devices to get access to network resources 2. requirements for the devices to get access to network resources
that may be critical to their continued safe operation. that may be critical to their continued safe operation.
This document takes the view that the two requirements do not need to This document takes the view that the two requirements do not need to
be in conflict, but resolving the conflict requires careful planning be in conflict, but resolving the conflict requires careful planning
on how the DNS can be safely and effectively used by MUD controllers on how the DNS can be safely and effectively used by MUD controllers
and IoT devices. and IoT devices.
9. References 10. References
10.1. Normative References
9.1. Normative References
[I-D.ietf-dnsop-rfc8499bis] [I-D.ietf-dnsop-rfc8499bis]
Hoffman, P. E. and K. Fujiwara, "DNS Terminology", Work in Hoffman, P. E. and K. Fujiwara, "DNS Terminology", Work in
Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-10, Progress, Internet-Draft, draft-ietf-dnsop-rfc8499bis-10,
25 September 2023, <https://datatracker.ietf.org/doc/html/ 25 September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-dnsop-rfc8499bis-10>. draft-ietf-dnsop-rfc8499bis-10>.
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794, [RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
DOI 10.17487/RFC1794, April 1995, DOI 10.17487/RFC1794, April 1995,
<https://www.rfc-editor.org/info/rfc1794>. <https://www.rfc-editor.org/info/rfc1794>.
skipping to change at page 14, line 29 skipping to change at page 15, line 30
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram [RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094, Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017, DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>. <https://www.rfc-editor.org/info/rfc8094>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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 [RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520, Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019, DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>. <https://www.rfc-editor.org/info/rfc8520>.
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A [RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things", Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021, RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/info/rfc9019>. <https://www.rfc-editor.org/info/rfc9019>.
9.2. Informative References 10.2. Informative References
[Akamai] "Akamai", 2019, [Akamai] "Akamai", 2019,
<https://en.wikipedia.org/wiki/Akamai_Technologies>. <https://en.wikipedia.org/wiki/Akamai_Technologies>.
[AmazonS3] "Amazon S3", 2019, [AmazonS3] "Amazon S3", 2019,
<https://en.wikipedia.org/wiki/Amazon_S3>. <https://en.wikipedia.org/wiki/Amazon_S3>.
[antipatterns] [antipatterns]
"AntiPattern", 12 July 2021, "AntiPattern", 12 July 2021,
<https://www.agilealliance.org/glossary/antipattern>. <https://www.agilealliance.org/glossary/antipattern>.
[awss3virtualhosting] [awss3virtualhosting]
"Down to the Wire: AWS Delays 'Path-Style' S3 Deprecation "Down to the Wire: AWS Delays 'Path-Style' S3 Deprecation
at Last Minute", 12 July 2021, at Last Minute", 12 July 2021,
<https://techmonitor.ai/techonology/cloud/aws-s3-path- <https://techmonitor.ai/techonology/cloud/aws-s3-path-
deprecation>. deprecation>.
[I-D.ietf-snac-simple]
Lemon, T. and J. Hui, "Automatically Connecting Stub
Networks to Unmanaged Infrastructure", Work in Progress,
Internet-Draft, draft-ietf-snac-simple-04, 4 March 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-snac-
simple-04>.
[mudmaker] "Mud Maker", 2019, <https://mudmaker.org>. [mudmaker] "Mud Maker", 2019, <https://mudmaker.org>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>. <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., [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>. 2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W. [RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871, Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016, DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/info/rfc7871>. <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 [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>. <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. [RFC9230] Kinnear, E., McManus, P., Pauly, T., Verma, T., and C.A.
Wood, "Oblivious DNS over HTTPS", RFC 9230, Wood, "Oblivious DNS over HTTPS", RFC 9230,
DOI 10.17487/RFC9230, June 2022, DOI 10.17487/RFC9230, June 2022,
<https://www.rfc-editor.org/info/rfc9230>. <https://www.rfc-editor.org/info/rfc9230>.
[RFC9238] Richardson, M., Latour, J., and H. Habibi Gharakheili, [RFC9238] Richardson, M., Latour, J., and H. Habibi Gharakheili,
"Loading Manufacturer Usage Description (MUD) URLs from QR "Loading Manufacturer Usage Description (MUD) URLs from QR
Codes", RFC 9238, DOI 10.17487/RFC9238, May 2022, Codes", RFC 9238, DOI 10.17487/RFC9238, May 2022,
<https://www.rfc-editor.org/info/rfc9238>. <https://www.rfc-editor.org/info/rfc9238>.
skipping to change at page 16, line 13 skipping to change at page 18, line 7
<https://www.rfc-editor.org/info/rfc9462>. <https://www.rfc-editor.org/info/rfc9462>.
[RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N., [RFC9463] Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
and T. Jensen, "DHCP and Router Advertisement Options for and T. Jensen, "DHCP and Router Advertisement Options for
the Discovery of Network-designated Resolvers (DNR)", the Discovery of Network-designated Resolvers (DNR)",
RFC 9463, DOI 10.17487/RFC9463, November 2023, RFC 9463, DOI 10.17487/RFC9463, November 2023,
<https://www.rfc-editor.org/info/rfc9463>. <https://www.rfc-editor.org/info/rfc9463>.
Appendix A. A Failing Strategy --- Anti-Patterns Appendix A. A Failing Strategy --- Anti-Patterns
Attempts to map IP addresses to names in real time fails for a number Attempts to map IP addresses to DNS names in real time often fails
of reasons: for a number of reasons:
1. it can not be done fast enough, 1. it can not be done fast enough,
2. it reveals usage patterns of the devices, 2. it reveals usage patterns of the devices,
3. the mappings are often incomplete, 3. the mappings are often incomplete,
4. Even if the mapping is present, due to virtual hosting, it may 4. Even if the mapping is present, due to virtual hosting, it may
not map back to the name used in the ACL. not map back to the name used in the ACL.
This is not a successful strategy, it MUST NOT be used for the This is not a successful strategy, it MUST NOT be used for the
reasons explained below. reasons explained below.
A.1. Too Slow A.1. Too Slow
Mappings of IP addresses to names requires a DNS lookup in the in- Mappings of IP addresses to DNS names requires a DNS lookup in the
addr.arpa or ip6.arpa space. For a cold DNS cache, this will 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 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, that holds the information required. At 20 to 100 ms per round trip,
this easily adds up to significant time before the packet that caused this easily adds up to significant time before the packet that caused
the lookup can be released. the lookup can be released.
While subsequent connections to the same site (and subsequent packets While subsequent connections to the same site (and subsequent packets
in the same flow) will not be affected if the results are cached, the 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 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 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 IP address to repeated, e.g, in a few hours or days,when the cached IP address to
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A.2. Reveals Patterns of Usage A.2. Reveals Patterns of Usage
By doing the DNS lookups when the traffic occurs, then a passive 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 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 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 not. This does not require access to all on-path data, just to the
DNS requests to the bottom level of the DNS tree. DNS requests to the bottom level of the DNS tree.
A.3. Mappings Are Often Incomplete A.3. Mappings Are Often Incomplete
A service provider that fails to include an A or AAAA record as part An IoT manufacturer with a cloud service provider that fails to
of their forward name publication will find that the new server is include an A or AAAA record as part of their forward name publication
simply not used. The operational feedback for that mistake is will find that the new server is simply not used. The operational
immediate. The same is not true for reverse names: they can often be feedback for that mistake is immediate. The same is not true for
incomplete or incorrect for months or even years without visible reverse DNS mappings: they can often be incomplete or incorrect for
effect on operations. months or even years without visible effect on operations.
Service providers often find it difficult to update reverse maps in a IoT manufacturer cloud service providers often find it difficult to
timely fashion, assuming that they can do it at all. Many cloud update reverse DNS maps in a timely fashion, assuming that they can
based solutions dynamically assign IP addresses to services, often as do it at all. Many cloud based solutions dynamically assign IP
the service grows and shrinks, reassigning those IP addresses to addresses to services, often as the service grows and shrinks,
other services quickly. The use of HTTP 1.1 Virtual Hosting may reassigning those IP addresses to other services quickly. The use of
allow addresses and entire front-end systems to be re-used HTTP 1.1 Virtual Hosting may allow addresses and entire front-end
dynamically without even reassigning the IP addresses. systems to be re-used dynamically without even reassigning the IP
addresses.
In some cases there are multiple layers of CNAME between the original 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 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 balancing layer in the DNS, followed by a load balancing layer at the
HTTP level. HTTP level.
The reverse name for the IP address of the load balancer usually does The reverse DNS mapping for the IP address of the load balancer
not change. If hundreds of web services are funneled through the usually does not change. If hundreds of web services are funneled
load balancer, it would require hundreds of PTR records to be through the load balancer, it would require hundreds of PTR records
deployed. This would easily exceed the UDP/DNS and EDNS0 limits, and to be deployed. This would easily exceed the UDP/DNS and EDNS0
require all queries to use TCP, which would further slow down loading limits, and require all queries to use TCP, which would further slow
of the records. down loading of the records.
The enumeration of all services/sites that have been at that load The enumeration of all services/sites that have been at that load
balancer might also constitute a security concern. To limit churn of balancer might also constitute a security concern. To limit churn of
DNS PTR records, and reduce failures of the MUD ACLs, operators would DNS PTR records, and reduce failures of the MUD ACLs, operators would
want to add all possible names for each reverse name, whether or not want to add all possible DNS names for each reverse DNS mapping,
the DNS load balancing in the forward DNS space lists that end-point whether or not the DNS load balancing in the forward DNS space lists
at that moment. that end-point at that moment.
A.4. Forward Names Can Have Wildcards A.4. Forward DNS Names Can Have Wildcards
In some large hosting providers content is hosted through a domain In some large hosting providers content is hosted through a domain
name that is published as a DNS wildcard (and uses a wildcard name that is published as a DNS wildcard (and uses a wildcard
certificate). For instance, github.io, which is used for hosted certificate). For instance, github.io, which is used for hosted
content, including the Editors' copy of internet drafts stored on content, including the Editors' copy of internet drafts stored on
github, does not actually publish any names. Instead, a wildcard github, does not actually publish any DNS names. Instead, a wildcard
exists to answer all potential names: requests are routed appropriate exists to answer all potential DNS names: requests are routed
once they are received. appropriate once they are received.
This kind of system works well for self-managed hosted content. This kind of system works well for self-managed hosted content.
However, while it is possible to insert up to a few dozen PTR However, while it is possible to insert up to a few dozen PTR
records, many thousand entries are not possible, nor is it possible records, many thousand entries are not possible, nor is it possible
to deal with the unlimited (infinite) number of possibilities that a to deal with the unlimited (infinite) number of possibilities that a
wildcard supports. wildcard supports.
It would be therefore impossible for the PTR reverse lookup to ever It would be therefore impossible for the PTR reverse lookup to ever
work with these wildcard names. work with these wildcard DNS names.
Contributors Contributors
Tirumaleswar Reddy Tirumaleswar Reddy
Nokia Nokia
Authors' Addresses Authors' Addresses
Michael Richardson Michael Richardson
Sandelman Software Works Sandelman Software Works
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