Cisco Systems

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Cisco Systems

55 Cambridge Parkway Cambridge 1057 United States +1 617 621 1904 rdroms@cisco.com

Internet-Draft This memo specifies the necessary components to implement manufacturer usage descriptions (MUD). This includes a YANG model, IPv4 and IPv6 DHCP options, a URL suffix specification, an X.509 certificate extension and a means to sign and verify the descriptions.

Manufacturer Usage Descriptions (MUDs) provide advice to end networks on how to treat specific classes of devices. The MUD architecture is explained in . 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 because of the time and effort spent to develop accurate and adequate models for use by network devices. JSON is used as a serialization for compactness and readability. The YANG model specified here is an extension of . The extensions in this model allow for a manufacturer to express classes of systems that a manufacturer would find necessary for the proper function of the device. These classes are then instantiated into actual IP addresses through local processing. This provides manufacturers the opportunity to limit device exposure to that which a manufacturer would find necessary for the proper function of the device. In this memo two means are defined to emit the MUD URL. One is a DHCP option, that the DHCP client uses to inform the DHCP server. The DHCP server may take further actions, such as retrieve the URL or otherwise pass it along to network management system or controller. The other method defined is an X.509 constraint. The IEEE has developed that provides a certificate-based approach to communicate device characteristics, which itself relies on . The MUD URL extension is non-critical, as required by IEEE 802.1AR. 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 therefore, mature. Therefore, only a limited subset of NETCONF-like content is permitted.

manufacturer usage description. a file containing YANG-based JSON that describes a recommended behavior. a web server that hosts a MUD file. 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. a URL that can be used by the MUD controller to receive the MUD file. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in .

A MUD file consists of JSON based on a YANG model. For purposes of MUD, the elements that can be modified are access lists as augmented by this model. The MUD file is limited to the serialization of a small number of YANG schema, including the models specified in the following documents: Publishers of MUD files MUST NOT include other elements, and MUST only contain information relevant to the device being described. Devices parsing MUD files MUST cease processing if they find other elements. This module is structured into three parts. The first container holds information that is relevant to retrieval and validity of the MUD file itself. The second container augments the access list to indicate direction the ACL is to be applied. The final container augments the matching container of the ACL model to add several elements that are relevant to the MUD URL, or other otherwise abstracted for use within a local environment.

The following elements are defined.

This is a date-and-time value of the last time the MUD file was updated. This is akin to a version number.

This is a URL that should point to the previous MUD URL for auditing purposes. Because it should not be necessary to resign a MUD file when a new one is released, the archival location of a current MUD file should be identified prior to its release. Note the signature file MUST also be available. For example, if previous-mud-file is set to “https://example.com/.mud/v1/xxx”, the corresponding signature would be found at “https://example.com/.mud/v1/xxx.p7s”.

This uint32 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 no more than 1440 for any device that is supported.

This optional element refers to the URL that should be used to resolve the location any MASA service, as specified in .

This boolean is an indication from the manufacturer to the network administrator as to whether or not the device is supported. In this context a device is said to be supported if the manufacturer might issue an update to the device or if the manufacturer might update the MUD file.

describes access-lists but does not attempt to indicate where they are applied as that is handled elsewhere in a configuration. However, in this case, a MUD file must be explicit in describing the communcation pattern of a device, and that includes indicating what is to be permitted or denied in either direction of communication. This element takes a single value of either “to-device” or “from-device”, based on a typedef “direction”.

This element consists of a hostname that would be matched against the authority section of another device’s MUD URL.

This is an equivalent for when the manufacturer element is used to indicate the authority that is found in another device’s MUD URL matches that of the authority found in this device’s MUD URL.

This string matches the one and only segment following the authority section of the MUD URL. It refers to a model that is unique within the context of the authority. It may also include product version information. Thus how this field is constructed is entirely a local matter for the manufacturer.

This null-valued element expands to include local networks. Its default expansion is that packets must not traverse toward a default route that is received from the router.

This URI specifies a value that a controller will register with the network management station. The element then is expanded to the set of hosts that are so registered. In addition, some meta information is defined in order to determine when a usage description should be refreshed.

When applied this matches packets when the flow was initiated in the corresponding direction. provides guidance for IPv6 guidance best practices. While that document is scoped specifically to IPv6, its contents are applicable for IPv4 as well. When this flag is set, and the system has no reason to believe a flow has been initiated it MUST drop the packet. This match SHOULD be applied with specific transport parameters, such as protocol.

To keep things relatively simple in addition to whatever definitions exist, we also apply two additional default behaviors: Anything not explicitly permitted is denied. Local DNS, DHCP, and NTP are, by default, permitted to and from the device.

To begin with, MUD takes full advantage of both the https: scheme and the use of .well-known. HTTPS is important in this case because men in the middle could otherwise harm the operation of a class of devices. .well-known is used because we wish to add additional structure to the URL. And so the URL appears as follows:

mud-rev signifies the version of the manufacturer usage description file. This memo specifies “v1” of that file. Later versions may permit additional schemas or modify the format. “model” represents a device model as the manufacturer wishes to represent it. It could be a brand name or something more specific. It also may provide a means to indicate what version the product is. Specifically if it has been updated in the field, this is the place where evidence of that update would appear. The field should be changed when the intended communication patterns of a device change. While from a controller standpoint, only comparison and matching operations are safe, it is envisioned that updates will require some administrative review. Processing of this URL occurs as specified in and .

file "ietf-mud.yang"; module ietf-mud { yang-version 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 "Cisco Systems, Inc."; contact "Eliot Lear lear@cisco.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. Copyright (c) 2016 IETF Trust and the persons identified as the document authors. 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 XXXX; see the RFC itself for full legal notices."; revision "2016-05-28" { description "A policy container for manufacturer-driven policy"; reference "RFC XXXX"; } typedef direction { type enumeration { enum to-device { description "packets or flows destined to the target device"; } enum from-device { description "packets or flows destined from the target device"; } } description "Which way are we talking about?"; } container support-information { description "Information about when support end(ed), and when to refresh"; leaf last-update { type yang:date-and-time; description "This is intended to be the time and date that the MUD file was generated."; } leaf previous-mud-file { type inet:uri; description "Use to find the previous MUD file location for auditing purposes."; } leaf cache-validity { type uint32; description "The information retrieved from the MUD server is valid for these many hours, after which it should be refreshed."; } leaf masa-server { type inet:uri; description "The URI of the MASA server that network elements should forward requests to for this device."; } leaf is-supported { type boolean; description "The element is currently supported by the manufacturer."; } } augment "/acl:access-lists/acl:acl" { description "add inbound or outbound. Normally access lists are applied in an inbound or outbound direction separately from their definition. This is not possible with MUD."; leaf packet-direction { type direction; description "inbound or outbound ACL."; } } augment "/acl:access-lists/acl:acl/" + "acl:access-list-entries/acl:ace/" + "acl:matches" { description "adding abstractions to avoid need of IP addresses"; leaf manufacturer { type inet:host; description "authority component of the manufacturer URI"; } leaf same-manufacturer { type empty; description "expand to ACEs for each device with the same origin"; } leaf model { type string; description "specific model (including version) for a given manufacturer"; } leaf local-networks { type empty; description "this string is used to indicate networks considered local in a given environment."; } leaf controller { type inet:uri; description "expands to one or more controllers for a given service that is codified by inet:uri."; } leaf direction-initiated { type direction; description "which direction a flow was initiated"; } } } ]]>

This example contains two access lists that are intended to provide outbound access to a cloud service on TCP port 443.

The IPv4 MUD URL client option has the following format:

Code OPTION_MUD_URL_V4 (TBD) is assigned by IANA. len is a single octet that indicates the length of the URL in octets. MUD URL is a URL. The length of a MUD URL does not exceed 255 bytes. The IPv6 MUD URL client option has the following format:

OPTION_MUD_URL_V6 (TBD; assigned by IANA). option-length contains the length of the URL in octets. The length MUST NOT exceed 255 octets. The intent of this option is to provide both a new device classifier to the network as well as some recommended configuration to the routers that implement policy. However, it is entirely the purview of the network system as managed by the network administrator to decide what to do with this information. The key function of this option is simply to identify the type of device to the network in a structured way such that the policy can be easily found with existing toolsets.

A DHCP client MAY emit either a DHCPv4 or DHCPv6 option or both. These options singletons, as specified in . Because clients are intended to have at most one MUD URL associated with them, they may emit at most one MUD URL option via DHCPv4 and one MUD URL option via DHCPv6. In the case where both v4 and v6 DHCP options are emitted, the same URL MUST be used. Clients SHOULD log or otherwise report improper acknowledgments from servers, but they MUST NOT modify their MUD URL configuration based on a server’s response. The server’s response is only an acknowledgment that the server has processed the option, and promises no specific network behavior to the client. In particular, it may not be possible for the server to retrieve the file associated with the MUD URL, or the local network administration may not wish to use the usage description. Neither of these situations should be considered in any way exceptional.

A DHCP server may ignore these options or take action based on receipt of these options. For purposes of debugging, if a server successfully parses the option and the URL, it MUST return the option with the same URL as an acknowledgment. Even in this circumstance, no specific network behavior is guaranteed. When a server consumes this option, it will either forward the URL and relevant client information to a network management system (such as the giaddr), or it will retrieve the usage description by resolving the URL. DHCP servers may implement MUD functionality themselves or they may pass along appropriate information to a network management system or controller. A DHCP server that does process the MUD URL MUST adhere to the process specified in and to validate the TLS certificate of the web server hosting the MUD file. Those servers will retrieve the file, process it, create and install the necessary configuration on the relevant network element. Servers SHOULD monitor the gateway for state changes on a given interface. A DHCP server that does not provide MUD functionality and has forwarded a MUD URL to a network management system MUST notify the network management of any corresponding change to the DHCP state of the client (such as expiration or explicit release of a network address lease).

There are no additional requirements for relays.

provides a procedure and means to specify extensions to X.509 certificates. The MUD URL is a non-critical Certificate extension that points to an on-line Manufacturer Usage Description concerning the certificate subject. This extension contains a single Uniform Resource Identifier (URI). Internationalized Resource Identifiers must be represented as URI’s in the way described in RFC 5280, section 7.4. The choice of id-pe is based on guidance found in Section 4.2.2 of :

The MUD URL is precisely that: online information about the particular subject. The new extension is identified as follows: – The MUD URI extension id-pe-mud-url OBJECT IDENTIFER ::= { id-pe TBD } The extension returns a single value: mudURLSyntax ::= IA5String – for use with mud architecture. The semantics of the URI are defined .

Because MUD files contain information that may be used to configure network access lists, they are sensitive. To insure that they have not been tampered with, it is important that they be signed. We make use of DER-encoded Cryptographic Message Syntax (CMS) for this purpose.

A MUD file MUST be signed using CMS as an opaque binary object. In order to make successful verification more likely, intermediate certificates SHOULD be included. If the device that is being described supports IEEE 802.1AR, its manufacturer certificate and the certificate in the MUD file MUST share a common trust anchor in order to insure that manufacturer of the device is also the provider of the MUD file. The signature is stored at the same location as the MUD URL but with the suffix of “.p7s”. Signatures are transferred using content-type “Application/pkcs7-signature”. For example:

Note: A MUD file may need to be resigned if the signature expires.

Prior to retrieving a MUD file the MUD controller SHOULD retrieve the MUD signature file using the MUD URL with a suffix of “.p7s”. For example, if the MUD URL is “https://example.com/.well-known/v1/modela”, the MUD signature URL will be “https://example.com/.well-known/v1/modela.p7s”. Upon retrieving a MUD file, a MUD controller MUST validate the signature of the file before continuing with further processing. A MUD controller SHOULD produce an error and it MUST cease all processing of that file if the signature cannot be validated. If the MUD controller has received the MUD URL via IEEE 802.1AR containing an IDevID (a manufacturer certificate), it MUST further confirm that the manufacturer certificate and that of the MUD file share a common trust anchor. For Example:

Note the additional step of verifying the common trust root.

Based on the means a URL is procured, a device may be able to lie about what it is, thus gaining additional network access. There are several means to limit risk in this case. The most obvious is to only believe devices that make use of certificate-based authentication such as IEEE 802.1AR certificates. When those certificates are not present, devices claiming to be of a certain manufacturer SHOULD NOT be included in that manufacturer grouping without additional validation of some form. This will occur when it makes use of primitives such as “manufacturer” for the purpose of accessing devices of a particular type. Network management systems SHOULD NOT deploy a usage description for a a device with the same MAC address that has indicated a change of authority without some additional validation (such as review of the class). New devices that present some form of unauthenticated MUD URL SHOULD be validated by some external means when they would be otherwise be given increased network access. It may be possible for a rogue manufacturer to inappropriately exercise the MUD file parser, in order to exploit a vulnerability. There are three recommended approaches to address this threat. The first is to validate the signature of the MUD file. The second is to have a system do a primary scan of the file to ensure that it is both parseable and believable at some level. MUD files will likely be relatively small, to start with. The number of ACEs used by any given device should be relatively small as well. Second, it may be useful to limit retrieval of MUD URLs to only those sites that are known to have decent web reputations. Use of a URL necessitates the use of domain names. If a domain name changes ownership, the new owner of that domain may be able to provide MUD files that MUD controllers would consider valid. There are a few approaches that can mitigate this attack. First, MUD file servers SHOULD cache certificates used by the MUD file server. When a new certificate is retrieved for whatever reason, the MUD controller should check to see if 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 controller MAY check the WHOIS database to see if registration ownership of a domain has changed. If a change has occured, or if for some reason it is not possible to determine whether ownership has changed, further review may be warranted. Note, this remediation does not take into account the case of a device that was produced long ago and only recently fielded, or the case where a new MUD controller has been installed.

IANA is requested to allocated the DHCPv4 and v6 options as specified in .

The IANA is requested to assign a value for id-pe-mud-uri in the “SMI Security for PKIX Certificate Extension” Registry. Its use is specified in .

The IANA is requested to register the URL suffix of “mud” as follows: o URI Suffix: “mud” o Specification documents: this document o Related information: n/a

The following media-type is defined for transfer of MUD file:

, Ralph Droms o Intended usage: COMMON o Restrictions on usage: none o Author: Eliot Lear , Ralph Droms o Change controller: IESG o Provisional registration? (standards tree only): No. ]]>

The authors would like to thank Einar Nilsen-Nygaard, Bernie Volz, Tom Gindin, Dan Romascanu, Sandeep Kumar, Thorsten Dahm, John Bashinski, and Dan Wing for their valuable advice and reviews. The remaining errors in this work are entirely the responsibility of the author.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS-TRACK] This document describes a data model of Access Control List (ACL) basic building blocks. This document specifies automated bootstrapping of a key infrastructure (BSKI) using vendor installed IEEE 802.1AR manufacturing installed certificates, in combination with a vendor based service on the Internet. Before being authenticated, a new device has only link-local connectivity, and does not require a routable address. When a vendor provides an Internet based service, devices can be forced to join only specific domains but in limited/ disconnected networks or legacy environments we describe a variety of options that allow bootstrapping to proceed. End points are commonly referenced by higher level functions through the DNS. This is especially the case in cloud-based functions, which might have hundreds of IP addresses for the same name. This brief memo extends the IETF-ACL model to allow access-control via domain names. This memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community. This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCPIP network. DHCP is based on the Bootstrap Protocol (BOOTP), adding the capability of automatic allocation of reusable network addresses and additional configuration options. [STANDARDS-TRACK] This document provides guidance to prospective DHCPv6 option developers to help them create option formats that are easily adoptable by existing DHCPv6 software. It also provides guidelines for expert reviewers to evaluate new registrations. This document updates RFC 3315. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document identifies a set of recommendations for the makers of devices and describes how to provide for "simple security" capabilities at the perimeter of local-area IPv6 networks in Internet-enabled homes and small offices. This document is not an Internet Standards Track specification; it is published for informational purposes. When the Public-Key Infrastructure using X.509 (PKIX) Working Group was chartered, an object identifier arc was allocated by IANA for use by that working group. This document describes the object identifiers that were assigned in that arc, returns control of that arc to IANA, and establishes IANA allocation policies for any future assignments within that arc. A key presumption of the Internet architecture has been that devices are general purpose computers. By constraining the set of devices that connect to the Internet to non-general purpose devices, we can introduce a set of network capabilities that provides an additional layer of protection to those devices. One such capability is the Manufacturer Usage Description (MUD). This work builds on many existing network capabilities so as to be easily deployable by all involved. The focus of this work is primarily, but not exclusively, in the realm of security; and again primarily, but not exclusively, relating to smart objects. Institute for Electrical and Electronics Engineers

RFC Editor to remove this section prior to publication. Draft -00 to -01: XML->JSON Remove device versioning information from URL Add PKIX and DHCP options Add Content-type information Clean up IANA considerations to match registration templates Ralph Droms carried over as author from DHCP option. Signing information Expanded Security Considerations Add directionality for both packets and flows. add previous-mud-file