RFC 8520
Manufacturer Usage Description Specification
Part 1 of 11 🌸
RFC 8520: Manufacturer Usage Description Specification 🎀
Okay bestie, this is the RFC that gives us MUD, and the title block is already giving us a lot. Here's the tea ☕
- Publisher: Internet Engineering Task Force (IETF) 🏛️
- Request for Comments: 8520
- Category: Standards Track 👑
- ISSN: 2070-1721
- Date: March 2019 📅
- Authors: E. Lear (Cisco Systems), R. Droms (Google), and D. Romascanu 💜
- Obsoletes / Updates: there's no "Obsoletes" or "Updates" line in this header, so nothing listed here gets replaced or changed 🤷♀️
Abstract ✨
Okay so this memo sets up a component-based architecture for Manufacturer Usage Descriptions (MUDs). The whole point of MUD is to give end devices a way to tell the network what kind of access and network functionality they need to work properly 🦋. The first focus is access control, and later work can dig into other stuff.
The memo also specifies two YANG modules, IPv4 and IPv6 DHCP options, a Link Layer Discovery Protocol (LLDP) TLV, a URL, an X.509 certificate extension, and a way to sign and verify the descriptions 🧚. It's a lot, but it's giving organized.
Status of This Memo 📋
This is an Internet Standards Track document 🔥.
It's a product of the Internet Engineering Task Force (IETF), so it represents the consensus of the IETF community. It went through public review and got approved for publication by the Internet Engineering Steering Group (IESG). If you want more info on Internet Standards, check Section 2 of RFC 7841 🌈.
For the current status, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc8520 🍓.
Copyright Notice ©️
Copyright (c) 2019 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 read these carefully, because they spell out your rights and restrictions for this document 🙈. Code Components pulled out of this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in the Simplified BSD License ⭐.
Table of Contents 🗂️
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. What MUD Doesn't Do . . . . . . . . . . . . . . . . . . . 5
1.2. A Simple Example . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Determining Intended Use . . . . . . . . . . . . . . . . 6
1.5. Finding a Policy: The MUD URL . . . . . . . . . . . . . . 7
1.6. Processing of the MUD URL . . . . . . . . . . . . . . . . 8
1.7. Types of Policies . . . . . . . . . . . . . . . . . . . . 8
1.8. The Manufacturer Usage Description Architecture . . . . . 10
1.9. Order of Operations . . . . . . . . . . . . . . . . . . . 12
2. The MUD Model and Semantic Meaning . . . . . . . . . . . . . 12
2.1. The IETF-MUD YANG Module . . . . . . . . . . . . . . . . 14
3. MUD Model Definitions for the Root "mud" Container . . . . . 15
3.1. mud-version . . . . . . . . . . . . . . . . . . . . . . . 15
3.2. MUD URL . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.3. to-device-policy and from-device-policy Containers . . . 16
3.4. last-update . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. cache-validity . . . . . . . . . . . . . . . . . . . . . 16
3.6. is-supported . . . . . . . . . . . . . . . . . . . . . . 16
3.7. systeminfo . . . . . . . . . . . . . . . . . . . . . . . 16
3.8. mfg-name, software-rev, model-name, and firmware-rev . . 17
3.9. extensions . . . . . . . . . . . . . . . . . . . . . . . 17
4. Augmentation to the ACL Model . . . . . . . . . . . . . . . . 17
4.1. manufacturer . . . . . . . . . . . . . . . . . . . . . . 17
4.2. same-manufacturer . . . . . . . . . . . . . . . . . . . . 17
4.3. documentation . . . . . . . . . . . . . . . . . . . . . . 18
4.4. model . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.5. local-networks . . . . . . . . . . . . . . . . . . . . . 18
4.6. controller . . . . . . . . . . . . . . . . . . . . . . . 18
4.7. my-controller . . . . . . . . . . . . . . . . . . . . . . 19
4.8. direction-initiated . . . . . . . . . . . . . . . . . . . 19
5. Processing of the MUD File . . . . . . . . . . . . . . . . . 19
6. What Does a MUD URL Look Like? . . . . . . . . . . . . . . . 19
7. The MUD YANG Model . . . . . . . . . . . . . . . . . . . . . 20
8. The Domain Name Extension to the ACL Model . . . . . . . . . 26
8.1. src-dnsname . . . . . . . . . . . . . . . . . . . . . . . 27
8.2. dst-dnsname . . . . . . . . . . . . . . . . . . . . . . . 27
8.3. The ietf-acldns Model . . . . . . . . . . . . . . . . . . 28
9. MUD File Example . . . . . . . . . . . . . . . . . . . . . . 30
10. The MUD URL DHCP Option . . . . . . . . . . . . . . . . . . . 32
10.1. Client Behavior . . . . . . . . . . . . . . . . . . . . 33
10.2. Server Behavior . . . . . . . . . . . . . . . . . . . . 33
10.3. Relay Requirements . . . . . . . . . . . . . . . . . . . 33
11. The Manufacturer Usage Description (MUD) URL X.509 Extension 34
12. The Manufacturer Usage Description LLDP Extension . . . . . . 36
13. The Creating and Processing of Signed MUD Files . . . . . . . 38
13.1. Creating a MUD File Signature . . . . . . . . . . . . . 38
13.2. Verifying a MUD File Signature . . . . . . . . . . . . . 38
14. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 39
15. Deployment Considerations . . . . . . . . . . . . . . . . . . 39
16. Security Considerations . . . . . . . . . . . . . . . . . . . 40
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
17.1. YANG Module Registrations . . . . . . . . . . . . . . . 43
17.2. URI Registrations . . . . . . . . . . . . . . . . . . . 43
17.3. DHCPv4 and DHCPv6 Options . . . . . . . . . . . . . . . 43
17.4. PKIX Extensions . . . . . . . . . . . . . . . . . . . . 43
17.5. Media Type Registration for MUD Files . . . . . . . . . 44
17.6. IANA LLDP TLV Subtype Registry . . . . . . . . . . . . . 45
17.7. The MUD Well-Known Universal Resource Name (URNs) . . . 45
17.8. Extensions Registry . . . . . . . . . . . . . . . . . . 46
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
18.1. Normative References . . . . . . . . . . . . . . . . . . 46
18.2. Informative References . . . . . . . . . . . . . . . . . 49
Appendix A. Default MUD Nodes . . . . . . . . . . . . . . . . . 52
Appendix B. A Sample Extension: DETNET-indicator . . . . . . . . 56
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 601. Introduction ✨
The Internet was mostly built for general purpose computers, the kind of devices whose purpose is decided by whoever owns them 💻. [RFC1984] assumed that an end device would be the best at protecting itself. That made total sense when the typical device was a workstation or a mainframe, and it still makes sense for general purpose computing devices today, like laptops, smart phones, and tablets 📱.
[RFC7452] covers design patterns for smart objects and asks some questions about them 🤔. So let's imagine a group of objects that are specifically not meant for general purpose computing. This memo calls them Things 🧸. They have one specific purpose, which means every other use is, by definition, not intended. If a small number of communication patterns comes out of that small number of uses, you can combine those two ideas into a Manufacturer Usage Description (MUD) that can be applied at various points in a network 🦋. MUD mainly addresses threats to the device, not the device being a threat. In some situations, though, MUD might help with the latter case too, depending on how the MUD URL gets communicated and how devices and their communications get authenticated 🔐.
We use the word "manufacturer" pretty loosely here 🙈. It means the entity or organization that will say how a device is meant to be used. For a light bulb, that might literally be the light bulb manufacturer 💡. For a smarter device with a built-in Linux stack, it might be an integrator of that device. The big takeaways are that the device itself is assumed to have a limited purpose, and that somewhere in the supply chain there's an organization that takes responsibility for telling the network what that purpose is 🫶.
The intent of MUD is to do the following:
- Substantially reduce the threat surface on a device to just those communications the manufacturer intended 🛡️.
- Provide a way to scale network policies to the ever-increasing number of device types on the network 📈.
- Provide a way to address at least some vulnerabilities faster than it might take to update systems. This is especially true for systems that are no longer supported 🏃♀️.
- Keep the cost of implementing such a system to the bare minimum 💸.
- Provide a means of extensibility so manufacturers can express other device capabilities or requirements 🌈.
MUD is made up of three architectural building blocks 🧱:
- A URL that can be used to locate a description;
- The description itself, including how it is interpreted; and
- A means for local network management systems to retrieve the description.
MUD works best when the network can identify, in some way, the remote endpoints that Things will talk to 🎯.
In this specification, we describe each of these building blocks and how they're meant to be used together 💜. They can also be used separately, independent of this specification, by local deployments for their own purposes 🍓.
1.1. What MUD Doesn't Do 🙈
MUD is not meant for network authorization of general purpose computers, because their manufacturers can't picture one specific communication pattern to describe them 💻. Even devices with a single or small number of uses might have really broad communication patterns, and MUD on its own isn't for them either 😭.
MUD can give network administrators some extra protection when device vulnerabilities exist, but it will never replace the need for manufacturers to patch vulnerabilities 🩹. That's a hard truth, bestie.
Finally, no matter what the manufacturer puts in a MUD file, these are not directives, they're suggestions 💅. How they get put into effect locally depends on lots of factors, and it's ultimately up to the local network administrator, who must decide what's appropriate in a given circumstance 👑.
1.2. A Simple Example ✨
okay so picture a light bulb 💡 whose whole job is lighting up a room. That's it, no notes. It can be controlled remotely over the network, and it might use a rendezvous service, the kind a phone app can reach. So what can we say about this bulb? All other network access is unwanted, bestie 🙅♀️. It's not calling a news service, it's not chatting with the fridge, it doesn't need a printer or any other devices, and it has zero social networking friends 💀. So if we put an access list on it that only lets it talk to that one rendezvous service, the bulb can still do its job totally fine ✨. Meanwhile the network gets an extra layer of protection for the bulb and all its device friends 🫶.
1.3. Terminology 📖
- MUD: Manufacturer Usage Description.
- MUD file: a file containing YANG-based JSON that describes a Thing and associated suggested specific network behavior.
- MUD file server: a web server that hosts a MUD file.
- MUD manager: the system that requests and receives the MUD file from the MUD server. After it has processed a MUD file, it may direct changes to relevant network elements.
- MUD controller: a synonym that has been used in the past for MUD manager.
- MUD URL: a URL that can be used by the MUD manager to receive the MUD file.
- Thing: the device emitting a MUD URL.
- Manufacturer: the entity that configures the Thing to emit the MUD URL and the one who asserts a recommendation in a MUD file. The manufacturer might not always be the entity that constructs a Thing. It could, for instance, be a systems integrator, or even a component provider.
Now the boring-but-important rulebook 🤓:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
1.4. Determining Intended Use 🔍
The idea of intended use isn't new, honestly. Network admins apply access lists every single day to allow only that use. Chapman and Zwicky described this white listing idea really well in [FW95]. Profiling systems that use heuristics to figure out what kind of system something is have been around for years too 🧠.
A Thing could just as easily tell the network what sort of access it requires without saying what sort of system it is. That would basically be the converse of [RFC7488]. But for a general solution, we assume a device will implement the functionality it needs to fulfill its limited purpose. This is a basic economic constraint 💸. Unless the network would refuse access to such a device, its developers would have no reason to give the network any information. So far, that assumption has held up, and it's giving consistency ✨.
1.5. Finding a Policy: The MUD URL 🔗
Our work starts with the device emitting a Universal Resource Locator (URL) [RFC3986]. This URL does double duty 🙌: it classifies the device type, and it gives a way to locate a policy file.
MUD URLs MUST use the https scheme [RFC7230].
In this memo, three ways are defined to emit the MUD URL 📣:
- A DHCP option [RFC2131] [RFC8415] that the DHCP client uses to inform the DHCP server. The DHCP server may take further actions, such as acting as the MUD manager or passing the MUD URL along to the MUD manager.
- An X.509 constraint. The IEEE developed IEEE 802.1AR [IEEE8021AR] to provide a certificate-based approach to communicate device characteristics, which itself relies on [RFC5280]. The MUD URL extension is non-critical, as required by IEEE 802.1AR. Various means may be used to communicate that certificate, including the Tunnel Extensible Authentication Protocol (TEAP) [RFC7170].
- Finally, a Link Layer Discovery Protocol (LLDP) frame is defined [IEEE8021AB].
It is possible that there may be other means for a MUD URL to be learned by a network. For instance, some devices may already be fielded or have very limited ability to communicate a MUD URL, and yet they can be identified through some means, such as a serial number or a public key. In these cases, manufacturers may be able to map those identifiers to particular MUD URLs (or even the files themselves). Similarly, there may be alternative resolution mechanisms available for situations where Internet connectivity is limited or does not exist. Such mechanisms are not described in this memo, but they are possible. Implementors are encouraged to allow for the flexibility of how MUD URLs may be learned 💜.
1.6. Processing of the MUD URL 🛠️
MUD managers that are able to do so SHOULD retrieve MUD URLs and signature files as per [RFC7230], using the GET method [RFC7231]. They MUST validate the certificate using the rules in [RFC2818], Section 3.1.
Requests for MUD URLs SHOULD include an Accept header field ([RFC7231], Section 5.3.2) containing application/mud+json, an Accept-Language header field ([RFC7231], Section 5.3.5), and a User-Agent header field ([RFC7231], Section 5.5.3).
MUD managers SHOULD automatically process 3xx response status codes.
If a MUD manager is not able to fetch a MUD URL, other means MAY be used to import MUD files and associated signature files. So long as the signature of the file can be validated, the file can be used. In such environments, controllers SHOULD warn administrators when cache-validity expiry is approaching so that they may check for new files.
It may not be possible for a MUD manager to retrieve a MUD file at any given time. Should a MUD manager fail to retrieve a MUD file, it SHOULD consider the existing one safe to use, at least for a time. After some period, it SHOULD log that it has been unable to retrieve the file. There may be very good reasons for such failures, including the possibility that the MUD manager is in an offline environment, the local Internet connection has failed, or the remote Internet connection has failed.
It is also possible that an attacker is attempting to interfere with the deployment of a device. 🚨
How to handle such circumstances is a local decision 🤷♀️.
1.7. Types of Policies 📜
When the MUD URL is resolved, the MUD manager retrieves a file that describes what sort of communications a device is designed to have. The manufacturer may specify either specific hosts for cloud-based services or certain classes for access within an operational network. An example of a class might be "devices of a specified manufacturer type", where the manufacturer type itself is indicated simply by the authority component (e.g., the domain name) of the MUD URL. Another example might be to allow or disallow local access. Just like other policies, these may be combined. For example:
- Allow access to devices of the same manufacturer
- Allow access to and from controllers via the Constrained Application Protocol (COAP) [RFC7252]
- Allow access to local DNS/NTP
- Deny all other access
A printer might have a description that states:
- Allow access for port IPP or port LPD
- Allow local access for port HTTP
- Deny all other access
In this way, anyone can print to the printer, but local access would be required for the management interface 🖨️.
The files that are retrieved are intended to be closely aligned to existing network architectures so that they are easy to deploy. We make use of YANG [RFC7950] because it provides accurate and adequate models for use by network devices. JSON [RFC8259] is used as a serialization format for compactness and readability, relative to XML. Other formats may be chosen with later versions of MUD.
While the policy examples given here focus on access control, this is not intended to be the sole focus. By structuring the model described in this document with clear extension points, other descriptions could be included. One that often comes to mind is quality of service.
The YANG modules specified here are extensions of [RFC8519]. The extensions to this model allow for a manufacturer to express classes of systems that a manufacturer would find necessary for the proper function of the device. Two modules are specified. The first module specifies a means for domain names to be used in Access Control Lists (ACLs) so that devices that have their controllers in the cloud may be appropriately authorized with domain names, where the mapping of those names to addresses may rapidly change.
The other module abstracts away IP addresses into certain classes that are instantiated into actual IP addresses through local processing. Through these classes, manufacturers can specify how the device is designed to communicate, so that network elements can be configured by local systems that have local topological knowledge. That is, the deployment populates the classes that the manufacturer specifies. The abstractions below map to zero or more hosts, as follows:
Manufacturer: A device made by a particular manufacturer, as identified by the authority component of its MUD URL.same-manufacturer: Devices that have the same authority component of their MUD URL.controller: Devices that the local network administrator admits to the particular class.my-controller: Devices intended to serve as controllers for the MUD URL that the Thing emitted.local: The class of IP addresses that are scoped within some administrative boundary. By default, it is suggested that this be the local subnet.
The "manufacturer" classes can be easily specified by the manufacturer, whereas controller classes are initially envisioned to be specified by the administrator.
Because manufacturers do not know who will be using their devices, it is important for functionality referenced in usage descriptions to be relatively ubiquitous and mature. For these reasons, the YANG-based configuration in a MUD file is limited to the modules either specified or referenced in this document, or specified in documented extensions 💖.
1.8. The Manufacturer Usage Description Architecture ✨
Okay bestie, now that all the pieces are laid out, we can finally build an architecture. Cue the ASCII art! 🎨
.......................................
. ____________ . _____________
. | | . | |
. | MUD |-->get URL-->| MUD |
. | Manager | .(https) | File Server |
. End system network |____________|<-MUD file<-<|_____________|
. . .
. . .
. _______ _________ .
.| | (DHCP et al.) | router | .
.| Thing |---->MUD URL-->| or | .
.|_______| | switch | .
. |_________| .
.......................................Figure 1: MUD Architecture
In the above diagram, the switch or router collects MUD URLs and forwards them to the MUD manager (a network management system) for processing. 🦋 This happens in different ways, depending on how the URL is communicated. For instance, with DHCP, the DHCP server might receive the URL and then process it. With IEEE 802.1X [IEEE8021X], the switch would carry the URL via a certificate to the authentication server via the Extensible Authentication Protocol (EAP) over Radius [RFC3748], which would then process it. One method to do this is TEAP, as described in [RFC7170]. The certificate extension is described below.
The information returned by the MUD file server is valid for as long as the Thing is connected. There is no expiry. 🌸 However, if the MUD manager has detected that the MUD file for a Thing has changed, it SHOULD update the policy expeditiously, taking into account whatever approval flow is required in a deployment. This way, new recommendations from the manufacturer can be processed in a timely fashion.
The information returned by the MUD file server (a web server) is valid for the duration of the Thing's connection, or as specified in the description. So if the Thing is disconnected, any associated configuration in the switch can be removed. 🧹 Similarly, from time to time the description may be refreshed, based on new capabilities, communication patterns, or vulnerabilities.
The web server is typically run by or on behalf of the manufacturer. Its domain name is that of the authority found in the MUD URL. For legacy cases where Things cannot emit a URL, if the switch is able to determine the appropriate URL, it may proxy it. In a trivial case, it may hardcode a MUD URL on a switch port, or a map from some available identifier, such as an L2 address or certificate hash, to a MUD URL. Very resourceful of the switch, ngl. 💅
The role of the MUD manager in this environment is to do the following:
- receive MUD URLs,
- fetch MUD files,
- translate abstractions in the MUD files to specific network element configuration,
- maintain and update any required mappings of the abstractions, and
- update network elements with appropriate configuration.
A MUD manager may be a component of an Authentication, Authorization, and Accounting (AAA) system or a network management system. Communication within those systems, and from those systems to network elements, is beyond the scope of this memo. 🫶
1.9. Order of Operations
As mentioned above, MUD contains architectural building blocks, so the order of operation may vary. However, here is one clear intended example: 🧚
- Thing emits a URL.
- That URL is forwarded to a MUD manager by the nearest switch (how this happens depends on the way in which the MUD URL is emitted).
- The MUD manager retrieves the MUD file and signature from the MUD file server, assuming it doesn't already have copies. After validating the signature, it may test the URL against a web or domain reputation service, and it may test any hosts within the file against those reputation services, as it deems fit.
- The MUD manager may query the administrator for permission to add the Thing and associated policy. If the Thing is known or the Thing type is known, it may skip this step.
- The MUD manager instantiates local configuration based on the abstractions defined in this document.
- The MUD manager configures the switch nearest the Thing. Other systems may be configured as well.
- When the Thing disconnects, policy is removed. 👋
2. The MUD Model and Semantic Meaning 🎀
A MUD file consists of a YANG model instance that has been serialized in JSON [RFC7951]. For purposes of MUD, the nodes that can be modified are access lists as augmented by this model. The MUD file is limited to the serialization of only the following YANG schema:
Extensions may be used to add additional schema. This is described further on.
To provide the widest possible deployment, publishers of MUD files SHOULD make use of the abstractions in this memo and avoid the use of IP addresses. A MUD manager SHOULD NOT automatically implement any MUD file that contains IP addresses, especially those that might have local significance. 🙈 The addressing of one side of an access list is implicit, based on whether it is applied as to-device-policy or from-device-policy.
With the exceptions of the "name" of the ACL, "type", "name" of the Access Control Entry (ACE), and TCP and UDP source and destination port information, publishers of MUD files SHOULD limit the use of ACL model leaf nodes expressed to those found in this specification. Absent any extensions, MUD files are assumed to implement only the following ACL model features:
match-on-ipv4,match-on-ipv6,match-on-tcp,match-on-udp,match-on-icmp
Furthermore, only "accept" or "drop" actions SHOULD be included. A MUD manager MAY choose to interpret "reject" as "drop". A MUD manager SHOULD ignore all other actions. This is because manufacturers do not have sufficient context within a local deployment to know whether reject is appropriate. That is a decision that should be left to a network administrator. Respect the admin, it's their network. 👑
Given that MUD does not deal with interfaces, the support of the "ietf-interfaces" module [RFC8343] is not required. Specifically, the support of interface-related features and branches (e.g., interface-attachment and interface-stats) of the ACL YANG module is not required.
In fact, MUD managers MAY ignore any particular component of a description, or MAY ignore the description in its entirety, and they SHOULD carefully inspect all MUD descriptions. Publishers of MUD files MUST NOT include other nodes except as described in Section 3.9. See that section for more information. 🔥
2.1. The IETF-MUD YANG Module 🌈
This module is structured into three parts:
- The first component, the
"mud"container, holds information that is relevant to retrieval and validity of the MUD file itself, as well as policy intended to and from the Thing. - The second component augments the matching container of the ACL model to add several nodes that are relevant to the MUD URL, or that are otherwise abstracted for use within a local environment.
- The third component augments the
tcp-aclcontainer of the ACL model to add the ability to match on the direction of initiation of a TCP connection.
A valid MUD file will contain two root objects: a "mud" container and an "acls" container. Extensions may add additional root objects as required. As a reminder, when parsing acls, elements within a "match" block are logically ANDed. In general, a single abstraction in a match statement should be used. For instance, it makes little sense to match both "my-controller" and "controller" with an argument, since they are highly unlikely to be the same value. 🤓
A simplified graphical representation of the data models is used in this document. The meaning of the symbols in these diagrams is explained in [RFC8340].
module: ietf-mud
+--rw mud!
+--rw mud-version uint8
+--rw mud-url inet:uri
+--rw last-update yang:date-and-time
+--rw mud-signature? inet:uri
+--rw cache-validity? uint8
+--rw is-supported boolean
+--rw systeminfo? string
+--rw mfg-name? string
+--rw model-name? string
+--rw firmware-rev? string
+--rw software-rev? string
+--rw documentation? inet:uri
+--rw extensions* string
+--rw from-device-policy
| +--rw acls
| +--rw access-list* [name]
| +--rw name -> /acl:acls/acl/name
+--rw to-device-policy
+--rw acls
+--rw access-list* [name]
+--rw name -> /acl:acls/acl/name
augment /acl:acls/acl:acl/acl:aces/acl:ace/acl:matches:
+--rw mud
+--rw manufacturer? inet:host
+--rw same-manufacturer? empty
+--rw model? inet:uri
+--rw local-networks? empty
+--rw controller? inet:uri
+--rw my-controller? empty
augment
/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches
/acl:l4/acl:tcp/acl:tcp:
+--rw direction-initiated? direction3. MUD Model Definitions for the Root "mud" Container 🍓
3.1. mud-version ✨
This node specifies the integer version of the MUD specification. This memo specifies version 1.
3.2. MUD URL 💜
This URL identifies the MUD file. This is useful when the file and associated signature are manually uploaded, say, in an offline mode. 📦
3.3. to-device-policy and from-device-policy Containers 🦋
[RFC8519] describes access lists. In the case of MUD, a MUD file must be explicit in describing the communication pattern of a Thing, and that includes indicating what is to be permitted or denied in either direction of communication. Hence, each of these containers indicates the appropriate direction of a flow in association with a particular Thing. They contain references to specific access lists. 🗂️
3.4. last-update 🕐
This is a date-and-time value of when the MUD file was generated. This is akin to a version number. Its form is taken from [RFC6991].
3.5. cache-validity ⏳
This uint8 is the period of time in hours that a network management station MUST wait since its last retrieval before checking for an update. It is RECOMMENDED that this value be no less than 24, and it MUST NOT be more than 168 for any Thing that is supported. This period SHOULD be no shorter than any period determined through HTTP caching directives (e.g., "cache-control" or "Expires"). N.B., the expiring of this timer does not require the MUD manager to discard the MUD file, nor terminate access to a Thing. See Section 16 for more information. 💖
3.6. is-supported 🌸
This boolean is an indication from the manufacturer to the network administrator as to whether or not the Thing is supported. In this context, a Thing is said to not be supported if the manufacturer intends never to issue a firmware or software update to the Thing, or never to update the MUD file. A MUD manager MAY still periodically check for updates. Even unsupported Things get a check-in, how sweet. 🥹
3.7. systeminfo 💖
okay so this one is the Thing's little name tag 🎀 It's a textual UTF-8 description of the Thing being connected, so admins can glance at it and see a brief, displayable description of what they're looking at. It SHOULD NOT exceed 60 characters worth of display space ✨ so keep it short and sweet, bestie.
3.8. mfg-name, software-rev, model-name, and firmware-rev 🌸
these are optional fields and they get filled in exactly as [RFC8348] specifies 🫶 ngl the big rule here is that firmware-rev and software-rev MUST NOT be populated in a MUD file if the device can be upgraded but the MUD URL cannot be 😭 for example, that's the situation with MUD URLs that are contained in 802.1AR certificates 🔐
3.9. extensions 🧚
this optional leaf-list names the MUD extensions used in the MUD file. main character warning: MUD extensions MUST NOT be used in a MUD file without the extensions being declared 💀 and Implementations MUST ignore any node in this file that they do not understand 🙈
extensions can either extend the MUD file (like we just talked about in the previous paragraph) or reference other work. there's an extension example in Appendix B 💜
4. Augmentation to the ACL Model ✨
note that in this section, when we say "match", we're talking about the ACL model "matches" node 🎯
4.1. manufacturer 🏭
this node is a hostname that gets matched against the authority component of another Thing's MUD URL. in its simplest form, "manufacturer" and "same-manufacturer" may be implemented as access lists. in more complex forms, additional network capabilities may be used. for example, if you saw the line "manufacturer" : "flobbidy.example.com", then all Things that registered with a MUD URL containing flobbity.example.com in its authority section would match 🦋
4.2. same-manufacturer 🦋
this null-valued node is the equivalent for when the manufacturer element is used to say the authority in another Thing's MUD URL matches the authority in this Thing's MUD URL 🤝 for example, if the Thing's MUD URL were "https://b1.example.com/ThingV1", then all devices with a MUD URL whose authority section is b1.example.com would match 💅
4.3. documentation 📚
this URI is a URL pointing to documentation about the device and the MUD file. it can be super useful when the controller class is used, so its use can be explained 🌈
4.4. model 💅
this string matches the entire MUD URL, which covers the model that's unique within the authority. it can hold not only model info but versioning info too, plus any other info the manufacturer wants to add ✨ the intended use is for devices of this precise class to match, so they can permit or deny communication between one another 🫶
4.5. local-networks 🌍
this null-valued node expands to include local networks. its default expansion is that packets must not traverse toward a default route that is received from the router. administrators are free to expand the expression as is appropriate in their deployments 💖
4.6. controller 👑
this URI specifies a value that a controller registers with the MUD manager. the node then expands to the set of hosts that are registered that way 😭 this node may also be a URN. in that case, the URN describes a well-known service, such as DNS or NTP, that has been standardized. both of those URNs can be found in Section 17.7 🧚
when my-controller is used, the administrator might get prompted to populate that class for each and every model 🙈 using controller with a named class lets the user populate that class only once for many different models that a manufacturer may produce 🎀
Controller URIs MAY take the form of a URL (e.g., "http[s]://"). However, MUD managers MUST NOT resolve and retrieve such files, and it is RECOMMENDED that there be no such file at this time, since their form and function may be defined at a point in the future. for now, URLs should serve simply as class names and may be populated by the local deployment administrator 🌸
great care should be taken by MUD managers when invoking the controller class in the form of URLs. for one thing, it requires some understanding by the administrator as to when it is appropriate 💀
pre-registration in such classes by controllers with the MUD server is encouraged 🫶 the mechanism to do that is beyond the scope of this work 🦋
4.7. my-controller 🎀
this null-valued node signals to the MUD manager to use whatever mapping it has for this MUD URL to a particular group of hosts 🧚 this may require prompting the administrator for class members. future work should seek to automate membership management 👑
4.8. direction-initiated 🚦
This MUST only be applied to TCP 🙅♀️ it matches the direction in which a TCP connection is initiated. when the direction initiated is "from-device", packets that are transmitted in the direction of a Thing MUST be dropped unless the Thing has first initiated a TCP connection. by way of example, this node may be implemented in its simplest form by looking at naked SYN bits, but it may also be implemented through more stateful mechanisms 🔥
when applied, this matches packets when the flow was initiated in the corresponding direction. [RFC6092] specifies IPv6 guidance best practices. while that document is scoped specifically to IPv6, its contents are applicable for IPv4 as well 🌈
5. Processing of the MUD File 🧹
to keep things relatively simple in addition to whatever definitions exist, we also apply two extra default behaviors 💅
- Anything not explicitly permitted is denied.
- Local DNS and NTP are, by default, permitted to and from the Thing.
an explicit description of the defaults can be found in Appendix A 📚 these are applied AFTER all other explicit rules. so a default behavior can be changed with a "drop" action 😭✨
6. What Does a MUD URL Look Like? 🔗
MUD URLs are required to use the "https" scheme, in order to establish the MUD file server's identity and assure integrity of the MUD file 🔐
any "https://" URL can be a MUD URL. for example:
https://things.example.org/product_abc123/v5
https://www.example.net/mudfiles/temperature_sensor/
https://example.com/lightbulbs/colour/v1a manufacturer may construct a MUD URL in any way, so long as it makes use of the "https" scheme 🦋
7. The MUD YANG Model ✨
okay this is the part where the spec gets its actual YANG module, and I'm not gonna lie, it's giving main character energy 💅 every single line is a whole vibe, so read it like the code it is 🎀
<CODE BEGINS>file "ietf-mud@2019-01-28.yang"
module ietf-mud {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-mud";
prefix ietf-mud;
import ietf-access-control-list {
prefix acl;
}
import ietf-yang-types {
prefix yang;
}
import ietf-inet-types {
prefix inet;
}
organization
"IETF OPSAWG (Operations and Management Area Working Group)";
contact
"WG Web: <https://datatracker.ietf.org/wg/opsawg/>
WG List: opsawg@ietf.org
Author: Eliot Lear
lear@cisco.com
Author: Ralph Droms
rdroms@gmail.com
Author: Dan Romascanu
dromasca@gmail.com
";
description
"This YANG module defines a component that augments the
IETF description of an access list. This specific module
focuses on additional filters that include local, model,
and same-manufacturer.
This module is intended to be serialized via JSON and stored
as a file, as described in RFC 8520.
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
'MAY', and 'OPTIONAL' in this document are to be interpreted as
described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
they appear in all capitals, as shown here.
Copyright (c) 2019 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC 8520; see
the RFC itself for full legal notices.";
revision 2019-01-28 {
description
"Initial proposed standard.";
reference
"RFC 8520: Manufacturer Usage Description
Specification";
}
typedef direction {
type enumeration {
enum to-device {
description
"packets or flows destined to the target
Thing.";
}
enum from-device {
description
"packets or flows destined from
the target Thing.";
}
}
description
"Which way are we talking about?";
}
container mud {
presence "Enabled for this particular MUD URL";
description
"MUD-related information, as specified
by RFC 8520.";
uses mud-grouping;
}
grouping mud-grouping {
description
"Information about when support ends (or ended)
and when to refresh.";
leaf mud-version {
type uint8;
mandatory true;
description
"This is the version of the MUD
specification. This memo specifies version 1.";
}
leaf mud-url {
type inet:uri;
mandatory true;
description
"This is the MUD URL associated with the entry found
in a MUD file.";
}
leaf last-update {
type yang:date-and-time;
mandatory true;
description
"This is intended to be when the current MUD file
was generated. MUD managers SHOULD NOT check
for updates between this time plus cache validity.";
}
leaf mud-signature {
type inet:uri;
description
"A URI that resolves to a signature as
described in this specification.";
}
leaf cache-validity {
type uint8 {
range "1..168";
}
units "hours";
default "48";
description
"The information retrieved from the MUD server is
valid for these many hours, after which it should
be refreshed. N.B., MUD manager implementations
need not discard MUD files beyond this period.";
}
leaf is-supported {
type boolean;
mandatory true;
description
"This boolean indicates whether or not the Thing is
currently supported by the manufacturer.";
}
leaf systeminfo {
type string;
description
"A UTF-8 description of this Thing. This
should be a brief description that may be
displayed to the user to determine whether
to allow the Thing on the
network.";
}
leaf mfg-name {
type string;
description
"Manufacturer name, as described in
the ietf-hardware YANG module.";
}
leaf model-name {
type string;
description
"Model name, as described in the
ietf-hardware YANG module.";
}
leaf firmware-rev {
type string;
description
"firmware-rev, as described in the
ietf-hardware YANG module. Note that this field
MUST NOT be included when the device can be
updated but the MUD URL cannot.";
}
leaf software-rev {
type string;
description
"software-rev, as described in the
ietf-hardware YANG module. Note that this field
MUST NOT be included when the device can be
updated but the MUD URL cannot.";
}
leaf documentation {
type inet:uri;
description
"This URL points to documentation that
relates to this device and any classes that it uses
in its MUD file. A caution: MUD managers need
not resolve this URL on their own but rather simply
provide it to the administrator. Parsing HTML is
not an intended function of a MUD manager.";
}
leaf-list extensions {
type string {
length "1..40";
}
description
"A list of extension names that are used in this MUD
file. Each name is registered with the IANA and
described in an RFC.";
}
container from-device-policy {
description
"The policies that should be enforced on traffic
coming from the device. These policies are not
necessarily intended to be enforced at a single
point but may be rendered by the controller to any
relevant enforcement points in the network or
elsewhere.";
uses access-lists;
}
container to-device-policy {
description
"The policies that should be enforced on traffic
going to the device. These policies are not
necessarily intended to be enforced at a single
point but may be rendered by the controller to any
relevant enforcement points in the network or
elsewhere.";
uses access-lists;
}
}
grouping access-lists {
description
"A grouping for access lists in the context of device
policy.";
container access-lists {
description
"The access lists that should be applied to traffic
to or from the device.";
list access-list {
key "name";
description
"Each entry on this list refers to an ACL that
should be present in the overall access list
data model. Each ACL is identified by name and
type.";
leaf name {
type leafref {
path "/acl:acls/acl:acl/acl:name";
}
description
"The name of the ACL for this entry.";
}
}
}
}
augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches" {
description
"adding abstractions to avoid the need of IP addresses.";
container mud {
description
"MUD-specific matches.";
leaf manufacturer {
type inet:host;
description
"A domain that is intended to match the authority
section of the MUD URL. This node is used to specify
one or more manufacturers a device should
be authorized to access.";
}
leaf same-manufacturer {
type empty;
description
"This node matches the authority section of the MUD URL
of a Thing. It is intended to grant access to all
devices with the same authority section.";
}
leaf model {
type inet:uri;
description
"Devices of the specified model type will match if
they have an identical MUD URL.";
}
leaf local-networks {
type empty;
description
"IP addresses will match this node if they are
considered local addresses. A local address may be
a list of locally defined prefixes and masks
that indicate a particular administrative scope.";
}
leaf controller {
type inet:uri;
description
"This node names a class that has associated with it
zero or more IP addresses to match against. These
may be scoped to a manufacturer or via a standard
URN.";
}
leaf my-controller {
type empty;
description
"This node matches one or more network elements that
have been configured to be the controller for this
Thing, based on its MUD URL.";
}
}
}
augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
+ "/acl:l4/acl:tcp/acl:tcp" {
description
"add direction-initiated";
leaf direction-initiated {
type direction;
description
"This node matches based on which direction a
connection was initiated. The means by which that
is determined is discussed in this document.";
}
}
}
<CODE ENDS>8. The Domain Name Extension to the ACL Model ✨
okay so this section is all about letting domain names show up in the access list, instead of only raw IP addresses 🌸 this module extends the IETF-ACL model so that you can reference a domain name by augmenting the "matches" node 🦋
how the domain-to-IP mapping gets kept up to date is totally up to each implementation 💅 some might do it differently, and some might not need any mapping at all, idk. but the big goal is the same across the board: a resource that gets referred to by name should be authorized (or not) inside an access list 🫶
here's how the change is structured ✨
module: ietf-acldns
augment /acl:acls/acl:acl/acl:aces/acl:ace/
acl:matches/acl:l3/acl:ipv4/acl:ipv4:
+--rw src-dnsname? inet:host
+--rw dst-dnsname? inet:host
augment /acl:acls/acl:acl/acl:aces/acl:ace/
acl:matches/acl:l3/acl:ipv6/acl:ipv6:
+--rw src-dnsname? inet:host
+--rw dst-dnsname? inet:hostso why these exact spots in the ACL model? 🧚 because the whole thing assumes we're talking about IP-related resources, since that's what DNS hands back 💖 and a domain name in this context is defined in [RFC6991] 🎀 the augmentations are copied across IPv4 and IPv6 so MUD file authors can decide which IP version the Thing is allowed to use, which is honestly such a slay 👑
the following nodes are defined.
8.1. src-dnsname
This one's the source domain name, the inet:host kind 💖. There are lots of ways to resolve hosts, but what really matters is that resolution lines up with the ACLs that Things need to operate properly 🧚.
8.2. dst-dnsname
This is the destination domain name, also inet:host 🎀. Go back to the previous section (Section 8.1) for the resolution stuff.
Heads up, though! When you're using either of these in a MUD file, the access is tied to one particular Thing, so MUD files MUST NOT contain a src-dnsname in an ACL linked to from-device-policy, or a dst-dnsname linked to to-device-policy. 🚫 Not allowed, bestie 💅
8.3. The ietf-acldns Model
Here's the YANG module itself 🦋
<CODE BEGINS>file "ietf-acldns@2019-01-28.yang"
module ietf-acldns {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-acldns";
prefix ietf-acldns;
import ietf-access-control-list {
prefix acl;
}
import ietf-inet-types {
prefix inet;
}
organization
"IETF OPSAWG (Operations and Management Area Working Group)";
contact
"WG Web: <https://datatracker.ietf.org/wg/opsawg/>
WG List: opsawg@ietf.org
Author: Eliot Lear
lear@cisco.com
Author: Ralph Droms
rdroms@gmail.com
Author: Dan Romascanu
dromasca@gmail.com
";
description
"This YANG module defines a component that augments the
IETF description of an access list to allow DNS names
as matching criteria.
Copyright (c) 2019 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).";
revision 2019-01-28 {
description
"Base version of dnsname extension of the ACL model.";
reference
"RFC 8520: Manufacturer Usage Description
Specification";
}
grouping dns-matches {
description
"Domain names for matching.";
leaf src-dnsname {
type inet:host;
description
"domain name to be matched against.";
}
leaf dst-dnsname {
type inet:host;
description
"domain name to be matched against.";
}
}
augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
+ "/acl:l3/acl:ipv4/acl:ipv4" {
description
"Adding domain names to matching.";
uses dns-matches;
}
augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
+ "/acl:l3/acl:ipv6/acl:ipv6" {
description
"Adding domain names to matching.";
uses dns-matches;
}
}
<CODE ENDS>9. MUD File Example 🎀
This example has two access lists, and they're both meant to give outbound access to a cloud service on TCP port 443 ☁️
{
"ietf-mud:mud": {
"mud-version": 1,
"mud-url": "https://lighting.example.com/lightbulb2000",
"last-update": "2019-01-28T11:20:51+01:00",
"cache-validity": 48,
"is-supported": true,
"systeminfo": "The BMS Example Light Bulb",
"from-device-policy": {
"access-lists": {
"access-list": [
{
"name": "mud-76100-v6fr"
}
]
}
},
"to-device-policy": {
"access-lists": {
"access-list": [
{
"name": "mud-76100-v6to"
}
]
}
}
},
"ietf-access-control-list:acls": {
"acl": [
{
"name": "mud-76100-v6to",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "cl0-todev",
"matches": {
"ipv6": {
"ietf-acldns:src-dnsname": "test.example.com",
"protocol": 6
},
"tcp": {
"ietf-mud:direction-initiated": "from-device",
"source-port": {
"operator": "eq",
"port": 443
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
},
{
"name": "mud-76100-v6fr",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "cl0-frdev",
"matches": {
"ipv6": {
"ietf-acldns:dst-dnsname": "test.example.com",
"protocol": 6
},
"tcp": {
"ietf-mud:direction-initiated": "from-device",
"destination-port": {
"operator": "eq",
"port": 443
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
}
]
}
}So in this example we've got two policies 👀: one from the Thing and one to the Thing. Each policy names an access list that applies to the Thing and one that applies from it. Inside each access list, access is allowed for packets going to or coming from the Thing that can be mapped to the domain name service.bms.example.com. For each access list, the enforcement point should expect that the Thing started the connection 🫶
10. The MUD URL DHCP Option 🌸
Okay, so the IPv4 MUD URL client option is laid out like this 🍓
+------+-----+------------------------------
| code | len | MUDstring
+------+-----+------------------------------The code OPTION_MUD_URL_V4 (161) has been assigned by IANA 🎉 len is a single octet that tells you how long the MUD string is, in octets. The MUDstring is defined like this:
MUDstring = mudurl [ " " reserved ]
mudurl = URI; a URL [RFC3986] that uses the "https" scheme [RFC7230]
reserved = 1*( OCTET ) ; from [RFC5234]The whole option MUST NOT go over 255 octets 🚫📏 If a space comes after the MUD URL, then a reserved string follows, which future specs will define ✨ MUD managers that don't understand this field MUST ignore it 🙈
The IPv6 MUD URL client option looks like this 🦋
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_MUD_URL_V6 | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MUDstring |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+OPTION_MUD_URL_V6 is (112) 💜
option-length holds the length of the MUDstring, as defined above, in octets.
The point of this option is to give the network a new way to classify a Thing, plus some recommended config for the routers that carry out the policy 🧚 But what the network does with that info is totally up to the network system, as run by the network administrator. The big idea is just to tell the network what type of Thing it is, in a structured way, so the policy is easy to find with existing toolsets 💅
10.1. Client Behavior
A DHCPv4 client MAY send a DHCPv4 option, and a DHCPv6 client MAY send a DHCPv6 option ✨ These options are singletons, as specified in [RFC7227]. Clients are meant to have at most one MUD URL, so they can send at most one MUD URL option over DHCPv4 and one over DHCPv6. If both the v4 and v6 DHCP options are sent, the same URL MUST be used 🔗
10.2. Server Behavior
A DHCP server can ignore these options or act on them 🤷♀️ When a server does consume the option, it either forwards the URL and relevant client info (like the gateway address or giaddr, the requested IP address, and lease length) to a network management system, or it goes and fetches the usage description itself by resolving the URL 📬
DHCP servers can run MUD functionality themselves, or they can pass the info along to a network management system or MUD manager 💁♀️ A DHCP server that processes the MUD URL MUST follow the process in [RFC2818] and [RFC5280] to validate the TLS certificate of the web server hosting the MUD file 🔐 Those servers grab the file, process it, and then create and install the right config on the relevant network element. Servers SHOULD monitor the gateway for state changes on a given interface 👀 A DHCP server that doesn't provide MUD functionality, but has forwarded a MUD URL to a MUD manager, MUST tell the MUD manager about any matching change to the client's DHCP state (like an expired or explicitly released address lease) 📣
If the DHCP server fails while it's acting as the MUD manager, any backup mechanisms SHOULD include the MUD state, and the server SHOULD work out the status of clients when it restarts, similar to what it would do without MUD manager functionality 🔄 When the DHCP server forwards info to the MUD manager, the MUD manager will either use redundant DHCP servers for info, or clear its state based on other network info, like monitoring port status on a switch via SNMP, Radius accounting, or similar mechanisms 🦋
10.3. Relay Requirements
There are no additional requirements for relays 🌈
11. The Manufacturer Usage Description (MUD) URL X.509 Extension 🎀
okay so this bit defines a non-critical X.509 certificate extension 💅 and it carries exactly one URL that points to an online Manufacturer Usage Description about the certificate subject. The URI has to be written the way Section 7.4 of [RFC5280] describes, so no shortcuts 🫶
If you've got any Internationalized Resource Identifiers (IRIs), they MUST be mapped to URIs following Section 3.1 of [RFC3987] before they go into the certificate extension 🌸
What the URL actually means is spelled out in Section 6 of this document ✨
The choice of id-pe comes from the advice in Section 4.2.2 of [RFC5280], which says:
These extensions may be used to direct applications to on-line
information about the issuer or the subject.and honestly the MUD URL is exactly that 🦋 It's online info about the particular subject, bestie.
In addition, there's a separate new extension called id-pe-mudsigner 🔐 It contains the subject field of the signing certificate for the MUD file. How to process that field is covered in Section 13.2.
The whole point of the signature is to claim that the MUD file found on the server is valid for a given device, no matter what else is going on 👑 There are several security considerations down in Section 16, and they are not optional reading 💀
A new content-type called id-ct-mud is defined too 🧚 Signatures are detached right now, but if a MUD file ever gets sent as part of a Cryptographic Message Syntax (CMS) message, this content-type SHOULD be used.
This module imports from [RFC5912] and [RFC6268]. The new extension is identified like this:
<CODE BEGINS>
MUDURLExtnModule-2016 { iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-mod-mudURLExtn2016(88) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
-- EXPORTS ALL --
IMPORTS
-- RFC 5912
EXTENSION
FROM PKIX-CommonTypes-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57) }
-- RFC 5912
id-ct
FROM PKIXCRMF-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-crmf2005-02(55) }
-- RFC 6268
CONTENT-TYPE
FROM CryptographicMessageSyntax-2010
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }
-- RFC 5912
id-pe, Name
FROM PKIX1Explicit-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-explicit-02(51) } ;
--
-- Certificate Extensions
--
MUDCertExtensions EXTENSION ::=
{ ext-MUDURL | ext-MUDsigner, ... }
ext-MUDURL EXTENSION ::=
{ SYNTAX MUDURLSyntax IDENTIFIED BY id-pe-mud-url }
id-pe-mud-url OBJECT IDENTIFIER ::= { id-pe 25 }
MUDURLSyntax ::= IA5String
ext-MUDsigner EXTENSION ::=
{ SYNTAX MUDsignerSyntax IDENTIFIED BY id-pe-mudsigner }
id-pe-mudsigner OBJECT IDENTIFIER ::= { id-pe 30 }
MUDsignerSyntax ::= Name
--
-- CMS Content Types
--
MUDContentTypes CONTENT-TYPE ::=
{ ct-mud, ... }
ct-mud CONTENT-TYPE ::=
{ -- directly include the content
IDENTIFIED BY id-ct-mudtype }
-- The binary data that is in the form
-- "application/mud+json" is directly encoded as the
-- signed data. No additional ASN.1 encoding is added.
id-ct-mudtype OBJECT IDENTIFIER ::= { id-ct 41 }
END
<CODE ENDS>While this extension can show up in either an 802.AR manufacturer certificate (IDevID) or a deployment certificate (LDevID), it's not guaranteed in either one, and it's not guaranteed to get carried over either 😭 So it is RECOMMENDED that MUD manager implementations keep a table that maps a Thing to its MUD URL based on IDevIDs. future-you will thank me 🍓
12. The Manufacturer Usage Description LLDP Extension 🦋
okay the IEEE802.1AB Link Layer Discovery Protocol (LLDP) is a one-hop, vendor-neutral link-layer protocol that end host network Things use to advertise their identity, capabilities, and neighbors on an IEEE 802 local area network 🌈 Its Type-Length-Value (TLV) design lets you define "vendor-specific" extensions, which is the whole reason this works. IANA has a registered IEEE 802 organizationally unique identifier (OUI), documented in [RFC7042], and the MUD LLDP extension uses a subtype defined in this document to carry the MUD URL ✨
The LLDP vendor-specific frame looks like this:
+--------+--------+----------+---------+--------------
|TLV Type| len | OUI |subtype | MUDString
| =127 | |= 00 00 5E| = 1 |
|(7 bits)|(9 bits)|(3 octets)|(1 octet)|(1-255 octets)
+--------+--------+----------+---------+--------------where:
- TLV Type =
127indicates a vendor-specific TLV - len = indicates the TLV string length
- OUI =
00 00 5Eis the organizationally unique identifier of IANA - subtype =
1(as assigned by IANA for the MUDstring) - MUDstring = the length MUST NOT exceed 255 octets
The vibe of this extension is twofold 💅 It gives the network a new Thing classifier, and it offers some recommended configuration to the routers that implement the policy. But it is entirely the purview of the network system, as managed by the network administrator, to decide what to do with the info. The key job here is simply identifying the type of Thing to the network in a structured way, so the policy can be found easily with existing toolsets 🫶
Hosts, routers, or other network elements that implement this option are intended to have at most one MUD URL associated with them, so they may transmit at most one MUD URL value 🌸
Hosts, routers, or other network elements that implement this option may ignore these options or take action based on receiving them. For example, they may fill in the respective extensions of the LLDP Management Information Base (MIB) 🧚 LLDP only goes one way, though. Link Layer Discovery Protocol Data Units (LLDPDUs) are not exchanged as information requests by one Thing and responses sent by another, and the other Things do not acknowledge LLDP info they receive from a Thing. No specific network behavior is guaranteed. When a Thing consumes this extension, it may either forward the URL and relevant remote Thing information to a MUD manager, or retrieve the usage description by resolving the URL in accordance with normal HTTP semantics 🔥
13. The Creating and Processing of Signed MUD Files 🔐
Because MUD files contain information that may be used to configure network access lists, they're sensitive 😭 To make sure they haven't been tampered with, it is important that they be signed. We use DER-encoded Cryptographic Message Syntax (CMS) [RFC5652] for this purpose 💜
13.1. Creating a MUD File Signature
A MUD file MUST be signed using CMS as an opaque binary object. To make successful verification more likely, intermediate certificates SHOULD be included. The signature gets stored at the location specified in the MUD file, and signatures are transferred using content-type application/pkcs7-signature 🦋
For example:
% openssl cms -sign -signer mancertfile -inkey mankey \
-in mudfile -binary -outform DER -binary \
-certfile intermediatecert -out mudfile.p7sNote: a MUD file may need to be re-signed if the signature expires. don't let it lapse, bestie 💀
13.2. Verifying a MUD File Signature
Before processing the rest of a MUD file, the MUD manager MUST retrieve the MUD signature file by retrieving the value of mud-signature, and then validate the signature across the MUD file 🔍 The Key Usage Extension in the signing certificate MUST be present and have the bit digitalSignature(0) set. When the id-pe-mudsigner extension is present in a device's X.509 certificate, the MUD signature file MUST have been generated by a certificate whose subject matches the contents of that id-pe-mudsigner extension. If these conditions aren't met, or if it can't validate the chain of trust to a known trust anchor, the MUD manager MUST cease processing the MUD file until an administrator has given approval 🛑
The whole point of the signature on the file is to assign accountability to an entity, and that entity's reputation can then guide administrators on whether or not to accept a given MUD file 👑 Checking web reputation based on where a server lives is already commonplace, so this is the same energy. It's likely the manufacturer will be the signer, but that's not strictly necessary, and it might not even be desirable. In some environments, integrators may install their own certificates. And honestly, what matters more is the accountability behind the recommendation, not just the relationship between the Thing and the file 🫶
An example:
% openssl cms -verify -in mudfile.p7s -inform DER -content mudfileNote the additional step of verifying the common trust root 🍓
14. Extensibility ✨
One of our design goals is for MUD files to be understood by as broad a cross-section of systems as possible 🌈 Put that together with the fact that we chose to lean on existing mechanisms, and we have no ability to negotiate extensions, plus only a limited desire for them anyway. So a two-tier extensibility framework is used, like this:
- At a coarse grain, a protocol version is included in a MUD URL. This memo specifies MUD version 1. Any and all changes are entertained when this version is bumped. Transition approaches between versions would be a matter for discussion in future versions 🧚
- At a finer grain, only extensions that would not incur additional risk to the Thing are permitted. Specifically, adding nodes to the
mudcontainer is permitted, with the understanding that such additions will be ignored by unaware implementations. Any such extensions SHALL be standardized through the IETF process and MUST be named in theextensionslist. MUD managers MUST ignore YANG nodes they do not understand and SHOULD create an exception to be resolved by an administrator, so as to avoid any policy inconsistencies 💅
15. Deployment Considerations 🚀
okay bestie, MUD is built from a bunch of architectural building blocks, so you can literally assemble different deployment setups 🧩✨. one big question is where policy gets enforced 👀. to protect the Thing from other Things in a local deployment, policy can be enforced on the nearest switch or access point 🔒. to limit unwanted traffic on a network, it may also be a good idea to enforce policy as close to the Internet as possible 🌐. sometimes policy enforcement isn't available at the closest hop 😬, and then the risk of lateral infection (infection of devices that live near one another) goes up to the number of Things that can talk to each other without protection 💀.
okay, caution time ⚠️. admitting a Thing into the "manufacturer" and "same-manufacturer" classes can affect access for other Things 👀. depending on how access is managed, it can grow the access list on switches connected to other Things. so be careful about managing that access list growth 🫶. other methods, like extra network segmentation, can help keep that growth reasonable 🧱.
because MUD is still new as of this writing, you can expect a great many devices to not have implemented it yet 🙈. it's a local deployment decision whether a device that just got connected should get broad access or limited access 🤔. and like the introduction mentioned, a deployment may choose to ignore a MUD policy entirely and just use the MUD URL as a classifier in a local policy decision 🏷️.
finally, check the info directly below about device lifetimes and use of domain names 💅.
16. Security Considerations 🔐
this one is a big deal, so buckle up 😭. depending on how a MUD URL gets emitted, a Thing may be able to lie about what it is and sneak in extra network access 👀. this can happen in a few ways when a device emits a MUD URL using DHCP or LLDP: getting inappropriately admitted into a class like "same-manufacturer", getting access to a device like "my-controller", or getting permission to reach an Internet resource that would otherwise be disallowed 🚫. whether that's a problem depends on the deployment 🫠.
Implementations SHOULD be configurable so they can disallow additive access for devices using MUD URLs that aren't emitted in a secure fashion, like in a certificate 🔒. similarly, implementations SHOULD NOT give elevated permissions (beyond those of devices presenting no MUD policy) to devices that don't strongly bind their identity to their L2/L3 transmissions 🙅♀️. when insecure methods are used by the MUD manager, the classes SHOULD NOT contain devices that use both insecure and secure methods, to prevent privilege escalation attacks 💀, and they MUST NOT contain devices with the same MUD URL that are derived from both strong and weak authentication methods 🚨.
Devices may forge source (L2/L3) information 👻. deployments should apply appropriate protections to bind communications to the authentication that has taken place ✅. for 802.1X authentication, IEEE 802.1AE (MACsec) [IEEE8021AE] is one way this can happen 🔐. a similar approach can be used with 802.11i (Wi-Fi Protected Access 2 (WPA2)) [IEEE80211i] 📶. other lower-layer technologies have their own options too 🧩. implementations using session-oriented access that isn't cryptographically bound should take care to remove state when any form of break in the session is detected 🧹.
a rogue certification authority (CA) may sign a certificate that has the same subject name as is listed in the MUDsigner field in the manufacturer certificate 😈. that seemingly permits a substitute MUD file for a device 🫣. there are two mitigations: first, if the signer changes, this may be flagged as an exception by the MUD manager 🚩. second, if the MUD file also changes, the MUD manager SHOULD seek administrator approval (it should do this in any case 🙄). in all circumstances, the MUD manager MUST maintain a cache of trusted CAs for this purpose 📚. when such a rogue is discovered, it SHOULD be removed 🗑️.
more mitigations are coming up below 👇.
when certificates aren't present, Things claiming to be from a certain manufacturer SHOULD NOT be included in that manufacturer grouping without additional validation of some form 🙅. this matters when the MUD manager uses primitives like "manufacturer" to access Things of a particular type 🔑. network management systems may also be able to fingerprint the Thing 🕵️. in that case, the MUD URL can act as a classifier that can be proven or disproven ✅❌. fingerprinting has other perks too: when 802.1AR certificates are used, they can't change themselves, so fingerprinting lets you add artifacts to the MUD string through the reserved field discussed in Section 10 ✨. the meaning of those artifacts is left as future work 🔮.
MUD managers SHOULD NOT accept a usage description for a Thing with the same Media Access Control (MAC) address that has indicated a change of the URL authority without some additional validation (like review by a network administrator) 🧐. new Things that present some form of unauthenticated MUD URL SHOULD be validated by some external means when they would be given increased network access 🛂.
it's possible for a rogue manufacturer to inappropriately exercise the MUD file parser to exploit a vulnerability 😱. there are two recommended approaches to address this. the first is to validate that the signer of the MUD file is known to and trusted by the MUD manager 🤝. the second is to have a system do a primary scan of the file to make sure it's both parseable and believable at some level 🔍. MUD files will likely be relatively small to start with 📄, and the number of ACEs used by any given Thing should be relatively small too 🔢. it may also be useful to limit retrieval of MUD URLs to only those sites with decent web or domain reputations ⭐.
using a URL means using domain names 🌐. if a domain name changes ownership, the new owner may be able to provide MUD files that MUD managers would consider valid 😬. MUD managers SHOULD cache certificates used by the MUD file server 🗄️. when a new certificate is retrieved for whatever reason, the MUD manager should check to see whether ownership of the domain has changed 🔎. a fair programmatic approximation of this is when the name servers for the domain have changed 🛠️. if the actual MUD file has changed, the MUD manager MAY check the WHOIS database to see if registration ownership of a domain has changed 📇. if a change has occurred, or it's not possible to determine whether ownership has changed, further review may be warranted 🧐. note: this remediation doesn't cover a Thing that was produced long ago and only recently fielded, or the case where a new MUD manager has been installed 📝.
the release of a MUD URL by a Thing reveals what the Thing is and gives an attacker guidance on what vulnerabilities may be present 😈.
while the MUD URL itself isn't intended to be unique to a specific Thing, releasing it may help an observer identify individuals when combined with other information 🙈. that's a privacy consideration, bestie 🫶.
for both of these concerns, implementors should take into account what other information they're advertising through mechanisms like Multicast DNS (mDNS) [RFC6872] 📡, how a Thing might otherwise be identified (maybe through how it behaves when it's connected to the network) 👀, and whether a Thing is meant to be used by individuals or carries personal identifying information 🪪. then apply appropriate data minimization techniques 🧼. one approach is to make use of TEAP [RFC7170] as the means to share information with authorized components in the network 🤝. network elements may also help limit access to the MUD URL with mechanisms like DHCPv6-Shield [RFC7610] 🛡️.
there's also the risk of the MUD manager itself being spied on to figure out what Things are connected to the network 👁️. to address that, MUD managers may choose to use TLS proxies they trust that would aggregate other information 🔐.
please note that the security considerations in Section 3.7 of [RFC8407] don't apply in this case, because the YANG serialization isn't intended to be accessed via NETCONF 🙅. however, for anyone who tries to instantiate this model in a network element via the Network Configuration Protocol (NETCONF), all objects in each model in this document exhibit similar security characteristics as [RFC8519] 📋. the basic purpose of MUD is to configure access, so by its very nature, it can be disruptive if used by unauthorized parties 💥.
17. IANA Considerations 📋
17.1. YANG Module Registrations
the following YANG modules have been registered in the "YANG Module Names" registry 💅:
Name: ietf-mud
URN: urn:ietf:params:xml:ns:yang:ietf-mud
Prefix: ietf-mud
Registrant contact: The IESG
Reference: RFC 8520
Name: ietf-acldns
URI: urn:ietf:params:xml:ns:yang:ietf-acldns
Prefix: ietf-acldns
Registrant contact: The IESG
Reference: RFC 852017.2. URI Registrations
IANA has added the following entries to the "IETF XML registry" 🌸:
URI: urn:ietf:params:xml:ns:yang:ietf-acldns
Registrant Contact: The IESG.
XML: N/A. The requested URI is an XML namespace.
URI: urn:ietf:params:xml:ns:yang:ietf-mud
Registrant Contact: The IESG.
XML: N/A. The requested URI is an XML namespace.17.3. DHCPv4 and DHCPv6 Options
The IANA has allocated OPTION_MUD_URL_V4 (161) in the "Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters" registry, and OPTION_MUD_URL_V6 (112) in the "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" registry, as described in Section 10 🎀.
17.4. PKIX Extensions
IANA has made the following assignments for 🧾:
- The MUDURLExtnModule-2016 ASN.1 module (88) in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).
id-pe-mud-urlobject identifier (25) from the "SMI Security for PKIX Certificate Extension" registry (1.3.6.1.5.5.7.1).id-pe-mudsignerobject identifier (30) from the "SMI Security for PKIX Certificate Extension" registry.id-ct-mudtypeobject identifier (41) from the "SMI Security for S/MIME CMS Content Type" registry.
the use of these values is specified in Section 11 ✨.
17.5. Media Type Registration for MUD Files 🎀
Bestie, this is where we register the media type so MUD files can travel around the internet 🦋 The following media type is defined for the transfer of MUD files:
- Type name:
application - Subtype name:
mud+json - Required parameters: N/A
- Optional parameters: N/A
- Encoding considerations:
8bit; "application/mud+json" values are represented as JSON objects; UTF-8 encoding MUST be employed [RFC3629]. 🔥 - Security considerations: See Security Considerations of RFC 8520 and Section 12 of [RFC8259]. 🔒
- Interoperability considerations: N/A
- Published specification: RFC 8520
- Applications that use this media type: MUD managers as specified by RFC 8520.
- Fragment identifier considerations: N/A
- Additional information:
- Magic number(s): N/A
- File extension(s): N/A
- Macintosh file type code(s): N/A
- Person & email address to contact for further information:
Eliot Lear <lear@cisco.com>, Ralph Droms <rdroms@gmail.com>, Dan Romascanu <dromasca@gmail.com>💌 - Intended usage: COMMON
- Restrictions on usage: none
- Author:
Eliot Lear <lear@cisco.com>, Ralph Droms <rdroms@gmail.com>, Dan Romascanu <dromasca@gmail.com>✨ - Change controller: IESG
- Provisional registration? (standards tree only): No.
17.6. IANA LLDP TLV Subtype Registry 🧚
Okay so IANA made a brand new registry called "IANA Link Layer Discovery Protocol (LLDP) TLV Subtypes" and it lives under "IEEE 802 Numbers" 💜 The policy for this registry is Expert Review [RFC8126]. The maximum number of entries in the registry is 256.
IANA populated the initial registry like this:
LLDP subtype value: 1 (All the other 255 values are initially marked as "Unassigned".)Description: the Manufacturer Usage Description (MUD) Uniform Resource Locator (URL)Reference: RFC 8520
17.7. The MUD Well-Known Universal Resource Name (URNs) 🌸
The following parameter registry was added in accordance with [RFC3553]. It's giving organized vibes ✨
- Registry name: MUD Well-Known Universal Resource Name (URN)
- Specification: RFC 8520
- Repository:
https://www.iana.org/assignments/mud - Index value: Encoded identically to a TCP/UDP port service name, as specified in Section 5.1 of [RFC6335]
These entries were added to the "MUD Well-Known Universal Resource Name (URN)" registry: urn:ietf:params:mud:dns refers to the service specified by [RFC1123], and urn:ietf:params:mud:ntp refers to the service specified by [RFC5905]. 🍓
17.8. Extensions Registry 🦋
IANA set up a registry of extensions, and the details are giving very specific energy 💅
- Registry name: MUD Extensions
- Registry policy: Standards Action
- Reference: RFC 8520
- Extension name: UTF-8-encoded string, not to exceed 40 characters.
Each extension MUST follow the rules specified in this specification. 🌈 As is usual, IANA issues early allocations in accordance with [RFC7120].
18. References ✨
18.1. Normative References 📚
- [IEEE8021AB] IEEE, "IEEE Standard for Local and Metropolitan Area Networks-- Station and Media Access Control Connectivity Discovery", IEEE 802.1AB.
- [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, DOI 10.17487/RFC1123, October 1989,
https://www.rfc-editor.org/info/rfc1123. - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
https://www.rfc-editor.org/info/rfc2119. - [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997,
https://www.rfc-editor.org/info/rfc2131. - [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000,
https://www.rfc-editor.org/info/rfc2818. - [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003,
https://www.rfc-editor.org/info/rfc3629. - [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
https://www.rfc-editor.org/info/rfc3748. - [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
https://www.rfc-editor.org/info/rfc3986. - [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987, January 2005,
https://www.rfc-editor.org/info/rfc3987. - [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008,
https://www.rfc-editor.org/info/rfc5234. - [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
https://www.rfc-editor.org/info/rfc5280. - [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009,
https://www.rfc-editor.org/info/rfc5652. - [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
https://www.rfc-editor.org/info/rfc5905. - [RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, June 2010,
https://www.rfc-editor.org/info/rfc5912. - [RFC6268] Schaad, J. and S. Turner, "Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI 10.17487/RFC6268, July 2011,
https://www.rfc-editor.org/info/rfc6268. - [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011,
https://www.rfc-editor.org/info/rfc6335. - [RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013,
https://www.rfc-editor.org/info/rfc6991. - [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2014,
https://www.rfc-editor.org/info/rfc7120. - [RFC7227] Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and S. Krishnan, "Guidelines for Creating New DHCPv6 Options", BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
https://www.rfc-editor.org/info/rfc7227. - [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014,
https://www.rfc-editor.org/info/rfc7230. - [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014,
https://www.rfc-editor.org/info/rfc7231. - [RFC7610] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers", BCP 199, RFC 7610, DOI 10.17487/RFC7610, August 2015,
https://www.rfc-editor.org/info/rfc7610. - [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016,
https://www.rfc-editor.org/info/rfc7950. - [RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, August 2016,
https://www.rfc-editor.org/info/rfc7951. - [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
https://www.rfc-editor.org/info/rfc8174. - [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017,
https://www.rfc-editor.org/info/rfc8259. - [RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
https://www.rfc-editor.org/info/rfc8340. - [RFC8348] Bierman, A., Bjorklund, M., Dong, J., and D. Romascanu, "A YANG Data Model for Hardware Management", RFC 8348, DOI 10.17487/RFC8348, March 2018,
https://www.rfc-editor.org/info/rfc8348. - [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018,
https://www.rfc-editor.org/info/rfc8415. - [RFC8519] Jethanandani, M., Agarwal, S., Huang, L., and D. Blair, "YANG Data Model for Network Access Control Lists (ACLs)", RFC 8519, DOI 10.17487/RFC8519, March 2019,
https://www.rfc-editor.org/info/rfc8519.
18.2. Informative References 💅✨
Okay so this is the reading list bit, bestie 📚 Every reference here is for context, not a must-follow rule, so think of it as the "further reading" section that helps you understand why things are the way they are 🧠🎀
[FW95] Chapman, D. and E. Zwicky, "Building Internet Firewalls", First Edition, November 1995. 🔥
[IEEE80211i] IEEE, "IEEE Standard for information technology- Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Amendment 6: Medium Access Control (MAC) Security Enhancements", IEEE 802.11i. 📶💜
[IEEE8021AE] IEEE, "IEEE Standard for Local and metropolitan area networks-Media Access Control (MAC) Security", IEEE 802.1AE. 🔐
[IEEE8021AR] IEEE, "IEEE Standard for Local and metropolitan area networks - Secure Device Identity", IEEE 802.1AR. 🪪✨
[IEEE8021X] IEEE, "IEEE Standard for Local and metropolitan area networks--Port-Based Network Access Control", IEEE 802.1X. 🚪🌸
[RFC1984] IAB and IESG, "IAB and IESG Statement on Cryptographic Technology and the Internet", BCP 200, RFC 1984, DOI 10.17487/RFC1984, August 1996, <https://www.rfc-editor.org/info/rfc1984>. 🦋
[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2003, <https://www.rfc-editor.org/info/rfc3553>. 🌈
[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service", RFC 6092, DOI 10.17487/RFC6092, January 2011, <https://www.rfc-editor.org/info/rfc6092>. 🏠💅
[RFC6872] Gurbani, V., Ed., Burger, E., Ed., Anjali, T., Abdelnur, H., and O. Festor, "The Common Log Format (CLF) for the Session Initiation Protocol (SIP): Framework and Information Model", RFC 6872, DOI 10.17487/RFC6872, February 2013, <https://www.rfc-editor.org/info/rfc6872>. 📝
[RFC7042] Eastlake 3rd, D. and J. Abley, "IANA Considerations and IETF Protocol and Documentation Usage for IEEE 802 Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042, October 2013, <https://www.rfc-editor.org/info/rfc7042>. 🧚
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna, "Tunnel Extensible Authentication Protocol (TEAP) Version 1", RFC 7170, DOI 10.17487/RFC7170, May 2014, <https://www.rfc-editor.org/info/rfc7170>. 🛡️
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/info/rfc7252>. 🪶
[RFC7452] Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson, "Architectural Considerations in Smart Object Networking", RFC 7452, DOI 10.17487/RFC7452, March 2015, <https://www.rfc-editor.org/info/rfc7452>. 🧸
[RFC7488] Boucadair, M., Penno, R., Wing, D., Patil, P., and T. Reddy, "Port Control Protocol (PCP) Server Selection", RFC 7488, DOI 10.17487/RFC7488, March 2015, <https://www.rfc-editor.org/info/rfc7488>. 🎯
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>. 📋💖
[RFC8343] Bjorklund, M., "A YANG Data Model for Interface Management", RFC 8343, DOI 10.17487/RFC8343, March 2018, <https://www.rfc-editor.org/info/rfc8343>. 🧩
[RFC8407] Bierman, A., "Guidelines for Authors and Reviewers of Documents Containing YANG Data Models", BCP 216, RFC 8407, DOI 10.17487/RFC8407, October 2018, <https://www.rfc-editor.org/info/rfc8407>. 🌷
Appendix A. Default MUD Nodes 🎀
Okay so this appendix is the "free pass" bit 🫶 It's the part of a MUD file that lets DNS traffic reach a controller registered as "urn:ietf:params:mud:dns" and lets NTP traffic reach a controller registered as "urn:ietf:params:mud:ntp". This is considered the default behavior, and these ACEs are effectively tacked onto whatever other "ace" entries a MUD file already has 💅
To block DNS or NTP, you repeat the matching statement but swap the "forwarding" action from "accept" to "drop" 🚫 Heads up though, ACEs get processed in the order they're received, so the defaults would never get reached if you don't do this. And a MUD manager might decide to optimize by just leaving the defaults out when they're overridden anyway 🧠✨
Below are four "acl" list entries that implement these default MUD nodes. Two are for IPv4 and two are for IPv6, one in each direction for both versions of IP 🌈 Quick note: the access list name and the ace name don't need to be kept or used in any way by local implementations. They're only there for completeness 💜
"ietf-access-control-list:acls": {
"acl": [
{
"name": "mud-59776-v4to",
"type": "ipv4-acl-type",
"aces": {
"ace": [
{
"name": "ent0-todev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:dns"
},
"ipv4": {
"protocol": 17
},
"udp": {
"source-port": {
"operator": "eq",
"port": 53
}
}
},
"actions": {
"forwarding": "accept"
}
},
{
"name": "ent1-todev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:ntp"
},
"ipv4": {
"protocol": 17
},
"udp": {
"source-port": {
"operator": "eq",
"port": 123
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
},
{
"name": "mud-59776-v4fr",
"type": "ipv4-acl-type",
"aces": {
"ace": [
{
"name": "ent0-frdev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:dns"
},
"ipv4": {
"protocol": 17
},
"udp": {
"destination-port": {
"operator": "eq",
"port": 53
}
}
},
"actions": {
"forwarding": "accept"
}
},
{
"name": "ent1-frdev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:ntp"
},
"ipv4": {
"protocol": 17
},
"udp": {
"destination-port": {
"operator": "eq",
"port": 123
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
},
{
"name": "mud-59776-v6to",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "ent0-todev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:dns"
},
"ipv6": {
"protocol": 17
},
"udp": {
"source-port": {
"operator": "eq",
"port": 53
}
}
},
"actions": {
"forwarding": "accept"
}
},
{
"name": "ent1-todev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:ntp"
},
"ipv6": {
"protocol": 17
},
"udp": {
"source-port": {
"operator": "eq",
"port": 123
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
},
{
"name": "mud-59776-v6fr",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "ent0-frdev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:dns"
},
"ipv6": {
"protocol": 17
},
"udp": {
"destination-port": {
"operator": "eq",
"port": 53
}
}
},
"actions": {
"forwarding": "accept"
}
},
{
"name": "ent1-frdev",
"matches": {
"ietf-mud:mud": {
"controller": "urn:ietf:params:mud:ntp"
},
"ipv6": {
"protocol": 17
},
"udp": {
"destination-port": {
"operator": "eq",
"port": 123
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
}
]
}Appendix B. A Sample Extension: DETNET-indicator 💖
okay so this appendix is a little sample extension 🎀 that adds one extra setting to the core MUD model, called is-detnet-required, to say whether the device uses DETNET 🌸. If a device says it doesn't use DETNET but then tries to anyway, a notification or exception might pop up 👀. This is just for illustration though, so don't go registering the example 🙈
Extension Name: "Example-Extension" (to be used in the extensions list) Standard: RFC 8520 (but do not register the example)
The extension adds one node to the MUD model, using this sample module. Here's its tree structure:
module: ietf-mud-detext-example
augment /ietf-mud:mud:
+--rw is-detnet-required? booleanAnd here's the module itself, all defined and everything 💅
<CODE BEGINS>file "ietf-mud-detext-example@2019-01-28.yang"
module ietf-mud-detext-example {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-mud-detext-example";
prefix ietf-mud-detext-example;
import ietf-mud {
prefix ietf-mud;
}
organization
"IETF OPSAWG (Operations and Management Area Working Group)";
contact
"WG Web: <https://datatracker.ietf.org/wg/opsawg/>
WG List: opsawg@ietf.org
Author: Eliot Lear
lear@cisco.com
Author: Ralph Droms
rdroms@gmail.com
Author: Dan Romascanu
dromasca@gmail.com
";
description
"Sample extension to a MUD module to indicate a need
for DETNET support.";
revision 2019-01-28 {
description
"Initial revision.";
reference
"RFC 8520: Manufacturer Usage Description
Specification";
}
augment "/ietf-mud:mud" {
description
"This adds a simple extension for a manufacturer
to indicate whether DETNET is required by a
device.";
leaf is-detnet-required {
type boolean;
description
"This value will equal 'true' if a device requires
DETNET to properly function.";
}
}
}
<CODE ENDS>Now let's see the extension in action 🦋 Using the module above, here's how it shows up in a MUD file:
{
"ietf-mud:mud": {
"mud-version": 1,
"mud-url": "https://lighting.example.com/lightbulb2000",
"last-update": "2019-01-28T11:20:51+01:00",
"cache-validity": 48,
"extensions": [
"ietf-mud-detext-example"
],
"ietf-mud-detext-example:is-detnet-required": "false",
"is-supported": true,
"systeminfo": "The BMS Example Light Bulb",
"from-device-policy": {
"access-lists": {
"access-list": [
{
"name": "mud-76100-v6fr"
}
]
}
},
"to-device-policy": {
"access-lists": {
"access-list": [
{
"name": "mud-76100-v6to"
}
]
}
}
},
"ietf-access-control-list:acls": {
"acl": [
{
"name": "mud-76100-v6to",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "cl0-todev",
"matches": {
"ipv6": {
"ietf-acldns:src-dnsname": "test.example.com",
"protocol": 6
},
"tcp": {
"ietf-mud:direction-initiated": "from-device",
"source-port": {
"operator": "eq",
"port": 443
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
},
{
"name": "mud-76100-v6fr",
"type": "ipv6-acl-type",
"aces": {
"ace": [
{
"name": "cl0-frdev",
"matches": {
"ipv6": {
"ietf-acldns:dst-dnsname": "test.example.com",
"protocol": 6
},
"tcp": {
"ietf-mud:direction-initiated": "from-device",
"destination-port": {
"operator": "eq",
"port": 443
}
}
},
"actions": {
"forwarding": "accept"
}
}
]
}
}
]
}
}Acknowledgments 🫶
the authors have to thank a whole squad of people 😭 Einar Nilsen-Nygaard, who single-handedly updated the model to match the updated ACL model (iconic behaviour, honestly 👑), plus Bernie Volz, Tom Gindin, Brian Weis, Sandeep Kumar, Thorsten Dahm, John Bashinski, Steve Rich, Jim Bieda, Dan Wing, Joe Clarke, Henk Birkholz, Adam Montville, Jim Schaad, and Robert Sparks, all of whom gave valuable advice and reviews 🌈 Russ Housley entirely rewrote Section 11 so it became a complete module, and Adrian Farrel provided the basis for the privacy considerations text 🧚 Kent Watsen gave a thorough review of the architecture and the YANG model 💜 And just so we're clear, any errors that are left are entirely the responsibility of the authors 🙈
Authors' Addresses ✨
Eliot Lear Cisco Systems Richtistrasse 7 Wallisellen CH-8304 Switzerland
Phone: +41 44 878 9200 Email: lear@cisco.com
Ralph Droms Google 355 Main St., 5th Floor Cambridge, MA 02142 United States of America
Phone: +1 978 376 3731 Email: rdroms@gmail.com
Dan Romascanu
Phone: +972 54 5555347 Email: dromasca@gmail.com
That's the whole RFC, bestie 🌈✨ You made it!
📦 where this page comes from
- ietf: rfc8520 2019-03
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms