Independent Submission M. Richardson
Request for Comments: 9238
Sandelman Software Works
Category: Informational J. Latour
ISSN: 2070-1721 CIRA Labs
H. Habibi Gharakheili
Loading Manufacturer Usage Description (MUD) URLs from QR Codes
This informational document details a protocol to load Manufacturer
Usage Description (MUD) definitions from RFC 8520
for devices that do
not have them integrated.
This document is published to inform the Internet community of this
mechanism to allow interoperability and to serve as a basis of other
standards work if there is interest.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see 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/rfc9238
Copyright (c) 2022 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
) 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.
Table of Contents 1.
The SQRL Protocol 3.2.
Manufacturer Usage Descriptions in SQRL 3.2.1.
B000 Company Name 3.2.2.
B001 Product Name 3.2.3.
B002 Model Number 3.2.4.
MUD URL Data Record 3.2.5.
Device MAC Address 4.
Generic URL or Version-Specific URL 6.
Crowd Supply of MUD Files 7.
Privacy Considerations 8.
Security Considerations 8.1.
QR Codes Are Not Assurances 8.2.
MUD Files Can Have Signatures 8.3.
URL Shortening Services Can Change Content 8.4.
MUD QR Code Stickers Could Be Confused 8.5.
QR Code Can Include a MAC Address 9.
IANA Considerations 10.
Normative References 10.2.
The Manufacturer Usage Description (MUD) [RFC8520
] defines a YANG
data model to express what sort of access a device requires to
operate correctly. That document additionally defines three ways for
the device to communicate the MUD URL (i.e., the URL of the resulting
MUD file in JSON [RFC8259
]) to a network enforcement point: via DHCP,
within an X.509 certificate extension, and via the Link Local
Discovery Protocol (LLDP).
Each of the above mechanisms conveys the MUD URL in band and requires
modifications to the device firmware. Most small Internet of Things
(IoT) devices do not have LLDP and often have very restricted DHCP
clients. Adding LLDP or DHCP options requires at least some minimal
configuration change and possibly entirely new subsystems.
Meanwhile, use of the PKIX certification extension only makes sense
as part of a larger deployment based on an Initial Device Identifier
(IDevID) [IEEE802-1AR], for instance, as described in [RFC8995
In the above cases, these mechanisms can only be implemented by
persons with access to modify and update the firmware of the device.
In the meantime, there is a chicken or egg problem [chickenegg].
That is, manufacturers are not motivated to (and thus likely do not)
include MUD URLs in their products, as they believe that there are no
gateways using those URLs. At the same time, gateways have little
incentive to (and thus likely do not) include code that processes MUD
URLs, as it is believed that no products have or disseminate URLs.
The protocol described in this document allows any person with
physical access to the device to affix a reference to a MUD URL that
can later be scanned by an end user.
The QR-based protocol is presented as a convenient alternative when
the mechanisms from [RFC8520
] are not available to use on the device
or the gateway.
Affixing a sticker can be done by:
* the marketing department of the manufacturer,
* an outsourced assembler plant,
* value-added resellers (perhaps in response to a local request for
* a company importing the product (possibly to comply with a local
* a network administrator (perhaps before sending devices home with
employees or to remote sites), and
* a retailer as a value-added service.
QR codes are informally described in [qrcode] and formally defined in
[isoiec18004]. The protocol described in this document uses a QR
code to encode the MUD URL. Specifically, the protocol leverages the
data format from the Reverse Logistics Association's Standardized
Quick Response for Logistics [SQRL].
SQRL codes are being put on devices via a sticker or via laser
etching into the case in order to deal with many situations but
specifically for end-of-life processing for the device. An important
idea behind the effort is that clearly identifying a product permits
appropriate disposal, refurbishment, or recycling of the components
of the product.
There are also use cases for SQRL in which the codes are used as part
of regular maintenance for a product.
SQRL is an application of the 12N Data Identifier system specified by
the ANSI MH10.8.2 Committee [mh10] in a format appropriate for QR
codes, as well as other things like Normalization Form C (NFC)
QR code generators are available as web services or as programs, such
as [qrencode]. Section 5
summarizes the considerations contained in "Updating files
vs Updating MUD URLs" (Section 7.1 of [MUD-URLS]). Due to the
immutable nature of the QR code, MUD URLs in this document will need
to be non-firmware specific.
Although this document is not an IETF Standards Track publication, it
adopts the conventions for normative language to provide clarity of
instructions to the implementer. The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
", and "OPTIONAL
" in this
document are to be interpreted as described in BCP 14 [RFC2119
] when, and only when, they appear in all capitals, as shown
Readers should be familiar with the terminology in [RFC8520
including: MUD file, MUD URL, manufacturer, MUD manager, and
The QR code protocol builds upon the work by [SQRL]. That protocol
is very briefly described in Section 3.1
. Then, the list of needed
Data Records to be filled in is explained.
3.1. The SQRL Protocol
[SQRL] documents an octet protocol that can be efficiently encoded
into QR codes using a sequence of US-ASCII bytes, plus six control
codes (see Section 3.1
* <RS> Record Separator (US-ASCII 30)
* <EoT> End of Transmission (US-ASCII 4)
* <FS> Field Separator (US-ASCII 28)
* <GS> Group Separator (US-ASCII 29)
* <US> Unit Separator (US-ASCII 31)
* Concatenation Operator (US-ASCII 43: "+")
Section 7.2 of [SQRL] gives the details, which can be summarized as: 1.
The QR code header starts with:
"[)>" <RS> "06" <GS> "12N" 2.
Include one or more Data Records. This consists of a four-letter
Field Identifier, followed by US-ASCII characters terminated with
a <Unit Separator>. 3.
Additionally, there are optional flags that may be present in every
Data Record, as described in Section 7.4 of [SQRL]. These flags have
no bearing on MUD processing. A parser that is only collecting MUD
URLs will not need to parse those flags. A general-purpose SQRL
parser will need more complexity.
Field Separator characters are used in SQRL to signify the beginning
of a new unit of data. A MUD-specific parser that encounters a Field
Separator and has not yet collected the right MUD information MUST
ignore the characters collected so far and then restart.
Environment records, as described in Section 7.4 of [SQRL], look and
act exactly as fields, with a special Field Identifier. They serve
no purpose when looking for MUD information and MAY
3.2. Manufacturer Usage Descriptions in SQRL
3.2.1. B000 Company Name
The B000 Data Record is mandatory in [SQRL]. It MUST
be in US-ASCII
representation. It should be a representation of the company or
brand name. It SHOULD
match the ietf-mud/mud/mfg-name in the MUD
file; however, the MUD file can contain arbitrary UTF-8 for this
name, while the SQRL files are expected to be 7-bit US-ASCII.
3.2.2. B001 Product Name
The B001 Data Record is optional in [SQRL]. It is the Product Name
in US-ASCII. Its presence is RECOMMENDED
. Some third parties that
create QR code stickers might not know the product name with 100%
certainty and MAY
prefer to omit this rather than create further
3.2.3. B002 Model Number
The B002 Data Record is optional in [SQRL] but is MANDATORY in this
profile. It is the Model Name in US-ASCII. It SHOULD
optional ietf-mud/mud/model-name in the MUD file if that entry is
present in the MUD file. MUD files can contain arbitrary UTF-8 for
the model-name, while the SQRL files are expected to be 7-bit US-
If a third party that is creating QR codes cannot locate an official
model number when creating their MUD file and QR code, then the third
make one up.
3.2.4. MUD URL Data Record
A new Field Identifier has been assigned by the Reverse Logistics
Association, which is "M180". This record MUST
be filled with the
Short URLs are easier to encode into a QR code because they require
fewer pixels of QR code. More content in the QR code requires a
Use of URL shortening services (see [URLshorten]) can be useful,
provided that the service is stable throughout the lifetime of the
device and QR code and that the privacy stance of the service is well
understood. The Security Considerations section of [RFC3986
applies, particularly Section 7.1. Section 8.1
of [SQRL] also has some good advice on longevity concerns
The URL provided MUST NOT
have a query (?) portion present. If one
is present, the query portion MUST
be removed before processing.
3.2.5. Device MAC Address
If a Media Access Control (MAC) address is used as a unique device
identifier (which is RECOMMENDED
if possible), then it MUST
included in this Data Record.
Section 9.10 of [SQRL] defines the Data Record "M06C" as the MAC
address. No format for the MAC address is provided in that document.
In this document, it is RECOMMENDED
that 12 (or 16) hex octets are
used with no spaces or punctuation. (16 octets are used in the IEEE
64-bit Extended Unique Identifier (EUI-64) format used in
[IEEE.802.15.4] and some next generation Ethernet proposals). In
this document, it is RECOMMENDED
that uppercase hexadecimal letters
Parsers that find punctuation (such as colons (":"), dashes ("-"),
US-ASCII Space (32), US-ASCII TAB (0), US-ASCII linefeed (10), or US-
ASCII carriage return (13)) MUST
skip over the punctuation. Parsers MUST
tolerate hexadecimal in uppercase, lowercase, and even mixed
case. Systems SHOULD
canonicalize it to uppercase.
The use of stickers to convey MUD URLs would appear to have little
value when the stickers are applied by the end-user organization and
consumed by the same. This is particularly the case when the QR code
does not include the device MAC address. In such a situation, the
installer handling the device would scan the QR code to get the
appropriate MUD file reference and have to input the associated MAC
address as well.
In such a case, one might wonder why the installer couldn't just
enter the appropriate MAC address and select the appropriate Access
Control Lists (ACLs) for the device. Then a MUD file or QR code to
convey the MAC address would not be needed. However, the use of a
MUD file (or QR code or other way to convey the MAC address) is
advantageous because it offers several layers of indirection: 1.
The ACLs for a given device may be added or removed. 2.
The ACLs may refer to DNS names, which may map to IPv4 or IPv6
The entire file may be replaced and may also include supply chain
information, such as Software Bill of Materials (SBOM).
In addition, the mechanism to install a new device (MAC address) to
MUD file mapping does not need to permit any other network security
settings to be alterable by the person doing the installation.
5. Generic URL or Version-Specific URL
MUD URLs that are communicated in band by the device and that are
programmed into the device's firmware may provide a firmware-specific
version of the MUD URL. The advantage of this is that the resulting
ACLs enforced in the network are specific to the needs of that
version of the firmware.
A MUD URL that is affixed to the device with a sticker or etched into
the case cannot be changed.
Given the considerations of "Updating MUD URLs vs Updating MUD files"
(Section 6.1 of [MUD-URLS]), it is prudent to use a MUD URL that
points to a MUD file that will only have new features added over time
and never have features removed. To recap, if a feature is removed
from the firmware and the MUD file still permits it, then there is a
potential hole that could perhaps be exploited. The opposite
situation, where a MUD file wrongly forbids something, leads to false
positives in the security system, and the evidence is that this
results in the entire system being ignored. Preventing attacks on
core infrastructure may be more important than getting the ACL
When the firmware eventually receives built-in MUD URL support, then
a more-specific URL may be used.
Note that in many cases, it will be third parties who are generating
these QR codes, so the MUD file may be hosted by the third party.
6. Crowd Supply of MUD Files
At the time of writing, the IETF MUD is a new IETF Proposed Standard.
Hence, IoT device manufacturers have not yet provided MUD profiles
for their devices. A research group at the University of New South
Wales (UNSW Sydney) has developed an open-source tool, called MUDgee
[MUDgee], which automatically generates a MUD file (profile) for an
IoT device from its traffic trace in order to make this process
faster, easier, and more accurate. Note that the generated profile
completeness solely depends on the completeness of the input traffic
traces. MUDgee assumes that all the activity seen is intended and
UNSW researchers have applied MUDgee to about 30 consumer IoT devices
from their lab testbed and publicly released their MUD files
[MUDfiles]. MUDgee can assist IoT manufacturers in developing and
verifying MUD profiles, while also helping adopters of these devices
to ensure they are compatible with their organizational policies.
Similar processes have been done in a number of other public and
private labs. One of the strong motivations for this specification
is to allow for this work to leave the lab and to be applied in the
7. Privacy Considerations
The presence of the MUD URL in the QR code reveals the manufacturer
of the device, the type or model of the device, and possibly the
firmware version of the device.
The MAC address of the device will also need to be present, and this
is potentially Personally Identifiable Information (PII). Such QR
codes should not be placed on the outside of the packaging and only
on the device itself, ideally on a non-prominent part of the device
(e.g., the bottom).
The QR code sticker should not be placed on any part of the device
that might become visible to machine vision systems in the same area.
This includes security systems, robotic vacuum cleaners, or anyone
taking a picture with a camera. Such systems may store the
picture(s) in such a way that a future viewer of the image will be
able to decode the QR code, possibly through an assembly of multiple
pictures. Of course, the QR code is not, however, a certain
indicator that the device is present, only that the QR code sticker
that came with the device is present.
The use of URL shorting services discussed in Section 3.2.4
result in trading convenience and efficiency with privacy, since the
service provider might leverage per-device or per-customer, short
URLs to track and correlate requests.
8. Security Considerations
8.1. QR Codes Are Not Assurances
The mere presence of a QR code on a device does not in itself create
any security issues on its own. Neither an attached paper sticker
nor a laser-etched code in a plastic case will affect the device
The QR code is not active; in general, it is not able to communicate
using nearby networks. It is conceivable that something more active
is concealed in the sticker, e.g., an NFC or a Radio Frequency
Identification (RFID) tag. But, any sticker could contain such a
thing, e.g., on some university campuses, stickers are often used as
part of political campaigns and can be found attached all over the
Security issues that this protocol creates are related to assumptions
that the presence of the QR code might imply. The presence of the QR
code may imply to some owners or network operators that the behavior
of the device has been vetted by some authority. It is here that
some caution is required.
A possibly bigger risk from application of MUD file stickers to
devices is that they may begin to convey a sense of safety to users
of the device. The presence of the sticker, possibly with the logo
of the physical establishment in which the device is located, could
convey to occupants of the establishment that this device is an
official device, for instance, a university that only deploys sensors
on the university campus that have been vetted for compliance against
a MUD definition.
The risk is then of social engineering, e.g., any device with a
reasonable-looking QR code may be seen as a trusted device (even
though such trust is not justified based on that evidence). An
attacker that wishes to infiltrate their own devices need only
suitably camouflage the device with an appropriate sticker in order
to convey legitimacy.
8.2. MUD Files Can Have Signatures
The network operator who takes the MUD file designated by the QR code
needs to be careful that they are validating the signature on the MUD
file. The network operator needs to verify that the file is intact
and that the signer of the file is authorized to sign MUD files for
that vendor, or if a MUD file is a crowd-sourced definition, they
need to establish if it can be trusted. [RFC8520
] does not define
any infrastructure to authenticate or authorize MUD file signers.
8.3. URL Shortening Services Can Change Content
If a URL shortening service is used, it is possible that the MUD
controller will be redirected to another MUD file with different
content. The use of MUD signatures can detect attacks on the
integrity of the file. To do this, the MUD controller needs to be
able to verify the signature on the file.
If a Trust-On-First-Use (TOFU) policy is used for signature trust
anchors, then the URL shortening service can still attack if it
substitutes content and signature on the first use. MUD controllers
and the people operating them need to be cautious when using TOFU.
8.4. MUD QR Code Stickers Could Be Confused
Another issue with the stickers is that the wrong sticker could be
applied to a device by a reseller or another trusted party, either in
error or via some physical or socially engineered attack against that
party. The network operator now onboards a device and applies what
they think is a legitimate network policy for the device in their
hands, only it is in fact a policy for another kind of device.
Careful examination of stickers is in order!
8.5. QR Code Can Include a MAC Address
Inclusion of the device-specific MAC address (described in Section 3.2.5
) in the QR code makes use of the MUD code much easier,
as it identifies the device specifically. If the MAC address is not
included, then a network operator, having the device in their hands,
has to associate the policy with the device through some other
Despite the significant advantage of having the MAC address included,
it is unlikely that third-party stickers will include it. Including
the MAC address requires that a unique sticker with a QR code be
created for each device. This is possible if the sticker is applied
by a manufacturer; it is already common to have a serial number and
MAC address on the outside of the device. In that case, if the QR
code is part of that sticker, then the customization problem is not
For cases where a third party has produced the QR code, it is likely
that every device of a particular model will have the same QR code
applied, omitting the MAC address. This increases the possibility
that the wrong policy will be applied to a device.
9. IANA Considerations
This document has no IANA actions.
10.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", BCP 14, RFC 8174
, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174
] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520
, March 2019,
[SQRL] Reverse Logistics Association, "SQRL Codes: Standardized
Quick Response for Logistics, Using the 12N Data
Identifier", February 2017,
10.2. Informative References
Wikipedia, "Chicken or the egg", April 2022,
IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
Std. 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875,
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Secure Device Identity", IEEE Std 802.1AR-2018,
ISO/IEC, "Information technology - Automatic
identification and data capture techniques - QR Code bar
code symbology specification", ISO/IEC 18004:2015,
[mh10] ANSI, "Data Identifier and Application Identifier
Standard", ANSI MH10.8.2-2016, June 2016,
[MUD-URLS] Richardson, M., Pan, W., and E. Lear, "Authorized update
to MUD URLs", Work in Progress, Internet-Draft, draft-
ietf-opsawg-mud-acceptable-urls-04, 6 October 2021,
[MUDfiles] UNSW Sydney, "MUD Profiles",
[MUDgee] "MUDgee", commit f63a88d, July 2019,
[qrcode] Wikipedia, "QR code", April 2022,
[qrencode] "libqrencode", commit 715e29f, September 2020,
] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986
, DOI 10.17487/RFC3986
, January 2005,
Interchange Format", STD 90, RFC 8259
, December 2017,
] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995
, DOI 10.17487/RFC8995
May 2021, <https://www.rfc-editor.org/info/rfc8995
Wikipedia, "URL shortening", April 2022,
This work was supported by the Canadian Internet Registration
Sandelman Software Works
Hassan Habibi Gharakheili