Manufacturer Usage Description Specification
draft-lear-ietf-netmod-mud-04

Abstract

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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on February 2, 2017.

Copyright Notice

Copyright (c) 2016 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. The MUD Model and Semantic Meaning
• 3. Element Definitions
• 3.1. last-update
• 3.2. previous-mud-file
• 3.3. cache-validity
• 3.4. masa-server
• 3.5. is-supported
• 3.6. packet-direction
• 3.7. manufacturer
• 3.8. same-manufacturer
• 3.9. model
• 3.10. local-networks
• 3.11. controller
• 3.12. direction-initiated
• 4. Processing of the MUD file
• 5. What does a MUD URL look like?
• 6. The MUD YANG Model
• 7. The Domain Name Extension to the ACL Model
• 7.1. source-dnsname
• 7.2. destination-dnsname
• 7.3. The ietf-acldns Model
• 8. MUD File Example
• 9. The MUD URL DHCP Option
• 9.1. Client Behavior
• 9.2. Server Behavior
• 9.3. Relay Requirements
• 10. The Manufacturer Usage Description (MUD) URL X.509 Extension
• 11. The Manufacturer Usage Description LLDP extension
• 12. Creating and Processing of Signed MUD Files
• 12.1. Creating a MUD file signature
• 12.2. Verifying a MUD file signature
• 13. Extensibility
• 14. Security Considerations
• 15. IANA Considerations
• 15.1. DHCPv4 and DHCPv6 Options
• 15.2. PKIX Extensions
• 15.3. Well Known URI Suffix
• 15.4. MIME Media-type Registration for MUD files
• 15.5. LLDP IANA TLV Subtype Registry
• 16. Acknowledgments
• 17. References
• 17.1. Normative References
• 17.2. Informative References
• Appendix A. Changes from Earlier Versions
• Authors' Addresses

1. Introduction

Manufacturer Usage Descriptions (MUDs) provide advice to end networks on how to treat specific classes of devices. The MUD architecture is explained in [I-D.lear-mud-framework]. 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 [RFC6020] 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 modules specified here are extensions of [I-D.ietf-netmod-acl-model]. 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 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 second 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.

In this memo three means are defined to emit the MUD URL. One is a DHCP option[RFC2131],[RFC3315] 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. Finally, an LLDP frame is defined.

The other method defined is an X.509 constraint. The IEEE has developed [IEEE8021AR] that provides 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.

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.

1.1. Terminology

MUD:

manufacturer usage description.

MUD file:

a file containing YANG-based JSON that describes a recommended behavior.

MUD file server:

a web server that hosts a MUD file.

MUD controller:

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 URL:

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 [RFC2119].

2. The MUD Model and Semantic Meaning

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:

• [I-D.ietf-netmod-acl-model]
• [RFC6991]

Publishers of MUD files MUST NOT include other elements except as described in Section 13, 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.

  module: ietf-mud
    +--rw meta-information
        +--rw last-update?         yang:date-and-time
        +--rw previous-mud-file?   yang:uri
        +--rw cache-validity?      uint32
        +--rw masa-server?         inet:uri
        +--rw is-supported?        boolean
  augment /acl:access-lists/acl:acl:
     +--rw packet-direction?   direction
  augment /acl:access-lists/acl:acl
          /acl:access-list-entries/acl:ace/acl:matches:
     +--rw manufacturer?          inet:host
     +--rw same-manufacturer?     empty
     +--rw model?                 string
     +--rw local-networks?        empty
     +--rw controller?            inet:uri
     +--rw direction-initiated?   direction

3. Element Definitions

The following elements are defined.

3.1. last-update

This is a date-and-time value of the last time the MUD file was updated. This is akin to a version number. Its form is taken from [RFC6991] which, for those keeping score, turn was taken from Section 5.6 of [RFC3339], which was taken from [ISO.8601.1988].

3.2. previous-mud-file

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”.

3.3. cache-validity

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.

3.4. masa-server

This optional element refers to the URL that should be used to resolve the location any MASA service, as specified in [I-D.ietf-anima-bootstrapping-keyinfra].

3.5. is-supported

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.

3.6. packet-direction

[I-D.ietf-netmod-acl-model] 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”.

3.7. manufacturer

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

3.8. same-manufacturer

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.

3.9. model

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.

3.10. local-networks

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.

3.11. controller

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.

3.12. direction-initiated

When applied this matches packets when the flow was initiated in the corresponding direction. [RFC6092] 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.

4. Processing of the MUD file

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.

5. What does a MUD URL look like?

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 a man in the middle attack 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-url   = "https://" authority  "/.well-known/mud/" mud-rev
               "/" model ( "?" extras )
               ; authority is from RFC3986
   mud-rev   = "v1"
   model     = segment  ; from RFC3986
   extras    = query    ; from RFC3986
   

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 [RFC2818] and [RFC3986].

6. The MUD YANG Model

<CODE BEGINS>file "ietf-mud@2016-07-20.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 (Ops Area) Working Group";

  contact
       "WG Web: http://tools.ietf.org/wg/opsawg/
       WG List: opsawg@ietf.org
       WG Chair: Warren Kumari
       warren@kumari.net
       WG Chair: Zhou Tianran
       zhoutianran@huawei.com
       Editor: Eliot Lear
       lear@cisco.com
       Editor: Ralph Droms
       rdroms@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 "2016-07-20"  {
    description "Base version of MUD extensions to ACL model";
    reference "RFC XXXX: Manufacturer Usage Description Specification";
  }

  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 meta-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";
    }
  }
} 

<CODE ENDS>

7. The Domain Name Extension to the ACL Model

This module specifies an extension to IETF-ACL model such that domain names may be referenced by augmenting the “matches” element. Different implementations may deploy differing methods to maintain the mapping between IP address and domain name, if indeed any are needed. However, the intent is that resources that are referred to using a name should be authorized (or not) within an access list.

The structure of the change is as follows:

   augment
   /acl:access-lists/acl:acl/acl:access-list-entries
      /acl:ace/acl:matches/acl:ace-type/acl:ace-ip:
      +--rw src-dnsname?        inet:host
      +--rw dst-dnsname?        inet:host

The choice of this particular point in the access-list model is based on the assumption that we are in some way referring to IP-related resources, as that is what the DNS returns. A domain name in our context is defined in [RFC6991].

The following elements are defined.

7.1. source-dnsname

The argument corresponds to a domain name of a source as specified by inet:host. Depending on how the model is used, it may or may not be resolved, as required by the implementation and circumstances.

7.2. destination-dnsname

The argument corresponds to a domain name of a destination as specified by inet:host. Depending on how the model is used, it may or may not be resolved, as required by the implementation and circumstances.

7.3. The ietf-acldns Model

<CODE BEGINS>file "ietf-acldns@2007-07020.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 (Ops Area) Working Group";

  contact
       "WG Web: http://tools.ietf.org/wg/opsawg/
       WG List: opsawg@ietf.org
       WG Chair: Warren Kumari
       warren@kumari.net
       WG Chair: Zhou Tianran
       zhoutianran@huawei.com
       Editor: Eliot Lear
       lear@cisco.com
       Editor: Ralph Droms
       rdroms@cisco.com
    ";


  description
    "This YANG module defines a component that augments the
     IETF description of an access list to allow dns names
     as matching criteria.";

  revision "2016-07-20" {
    description "Base version of dnsname extension of ACL model";
    reference "RFC XXXX: Manufacturer Usage Description Specification";
  }

  augment "/acl:access-lists/acl:acl/" +
     "acl:access-list-entries/acl:ace/" +
     "acl:matches/acl:ace-type/acl:ace-ip" {
    description "adding domain names to 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";
    }
  }
}

<CODE ENDS>

8. MUD File Example

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

{
   "ietf-mud:support-information": {
      "last-update": "2016-05-18T20:00:50Z",
      "cache-validity": 1440
      },
   "ietf-access-control-list:access-lists":  {
     "acl": [ {
      "acl-name": "inbound-stuff",
      "acl-type" : "ipv4-acl",
      "ietf-mud:direction" : "to-device",
      "access-list-entries": {
         "ace": [
            {
               "rule-name": "access-cloud",
               "matches": {
                 "ietf-acldns:src-dnsname":
                     "lighting-system.example.com",
                  "protocol" : 6,
                  "source-port-range" : {
                     "lower-port" : 443,
                     "upper-port" : 443
                     }
                  },
               "actions" : {
                 "permit" : [null]
                 }
              }
            ]
           }
         },
         {
      "acl-name": "outbound-stuff",
      "acl-type" : "ipv4-acl",
      "ietf-mud:direction" : "from-device",
      "access-list-entries": {
         "ace": [
            {
               "rule-name": "access-cloud",
               "matches": {
                  "ietf-acldns:dst-dnsname":
                       "lighting-system.example.com",
                   "protocol" : 6,
                   "destination-port-range" : {
                     "lower-port" : 443,
                     "upper-port" : 443
                    }
                },
               "actions" : {
                   "permit" : [null]
                 }
              }
            ]
           }
         }
        ]
    }
}

9. The MUD URL DHCP Option

The IPv4 MUD URL client option has the following format:

  +------+-----+------------------------------
  | code | len |  MUD URL
  +------+-----+------------------------------

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:

   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          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                            MUD URL                            |
  |                              ...                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

9.1. Client Behavior

A DHCP client MAY emit either a DHCPv4 or DHCPv6 option or both. These options singletons, as specified in [RFC7227]. 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.

9.2. Server Behavior

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 [RFC2818] and [RFC5280] 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).

9.3. Relay Requirements

There are no additional requirements for relays.

10. The Manufacturer Usage Description (MUD) URL X.509 Extension

[RFC7299] 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 [RFC5280]:

   These extensions may be used to direct applications to on-line
   information about the issuer or the subject.

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 Section 5.

11. The Manufacturer Usage Description LLDP extension

The IEEE802.1AB Link Layer Discovery Protocol (LLDP) [IEEE8021AB] is a one hop vendor-neutral link layer protocols used by end hosts network devices for advertising their identity, capabilities, and neighbors on an IEEE 802 local area network. Its Type-Length-Value (TLV) design allows for ‘vendor-specific’ extensions to be defined. IANA has a registered IEEE 802 organizationally unique identifier (OUI) defined as documented in [RFC7042]. The MUD LLDP extension uses a subtype defined in this document to carry the MUD URL.

The LLDP vendor specific frame has the following format:

+--------+--------+----------+---------+--------------
|TLV Type|  len   |   OUI    |subtype  | MUD URL
|  =127  |        |= 00 00 5E|  = 1    |
|(7 bits)|(9 bits)|(3 octets)|(1 octet)|(1-256 octets)
+--------+--------+----------+---------+--------------

where:

• TLV Type = 127 indicates a vendor-specific TLV
• len – indicates the TLV string length
• OUI = 00 00 5E is the organizationally unique identifier of IANA
• subtype = 1 (to be assigned by IANA for the MUD URL)
• MUD URL – the length MUST NOT exceed 256 octets (consistent with the DHCP option defined in Section 9)

The intent of this extension 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 extension 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.

Hosts, routers, or other network devices 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 devices that implement this option may ignore these options or take action based on receipt of these options. For example they may fill in information in the respective extensions of the LLDP Management Information Base (LLDP MIB). LLDP operates in a one-way direction. LLDPDUs are not exchanged as information requests by one device and response sent by another device. The other devices do not acknowledge LLDP information received from a device. No specific network behavior is guaranteed. When a device consumes this extension, it may either forward the URL and relevant remote device information to a network management system, or it will retrieve the usage description by resolving the URL.

12. Creating and Processing of Signed MUD Files

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) [RFC5652] for this purpose.

12.1. Creating a MUD file signature

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:

% openssl cms -sign -signer mancertfile -inkey mankey \
              -in mudfile -binary -outform DER - \
              -certfile intermediatecert -out mudfile.p7s

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

12.2. Verifying a MUD file signature

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:

% openssl cms -verify -in mudfile.p7s -inform DER -content mudfile

Note the additional step of verifying the common trust root.

13. Extensibility

One of our design goals is to see that MUD files are able to be understood by as broad a cross-section of systems as is possible. Coupled with the fact that we have also chosen to leverage existing mechanisms, we are left with no ability to negotiate extensions and a limited desire for those extensions in any event. A such, a two-tier extensibility framework is employed, as follows:

1. 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.
2. At a finer grain, only extensions that would not incur additional risk to the device are permitted. Specifically, augmenting of the meta-information container is permitted with the understanding that such additions may be ignored. In addition, augmentation of the ACL model is permitted so long as it remains safe for a given ACE to be ignored by the MUD Controller or the network elements it configures. Most specifically, is is not permitted to include as an augmentation that modifies “deny” behavior without bumping the version. Furthermore, implementations that are not able to parse a component of the ACE array MUST ignore the entire array entry (e.g., not the entire array) and MAY ignore the entire MUD file.

14. Security Considerations

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 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.

15. IANA Considerations

15.1. DHCPv4 and DHCPv6 Options

IANA is requested to allocated the DHCPv4 and v6 options as specified in Section 9.

15.2. PKIX Extensions

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 Section 10.

15.3. Well Known URI Suffix

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

15.4. MIME Media-type Registration for MUD files

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

 o Type name: application
 o Subtype name: mud+json
 o Required parameters: n/a
 o Optional parameters: n/a
 o Encoding considerations: 8bit; application/mud+json values
   are represented as a JSON object; UTF-8 encoding SHOULD be
   employed.
 o Security considerations: See {{secon}} of this document.
 o Interoperability considerations: n/a
 o Published specification: this document
 o Applications that use this media type: MUD controllers as
   specified by this document.
 o Fragment identifier considerations: n/a
 o Additional information:

     Magic number(s): n/a
     File extension(s): n/a
     Macintosh file type code(s): n/a

 o Person & email address to contact for further information:
   Eliot Lear <lear@cisco.com>, Ralph Droms <rdroms@cisco.com>
 o Intended usage: COMMON
 o Restrictions on usage: none

 o Author: Eliot Lear <lear@cisco.com>, Ralph Droms <rdroms@cisco.com>
 o Change controller: IESG
 o Provisional registration? (standards tree only): No.

15.5. LLDP IANA TLV Subtype Registry

IANA is requested to create a new registry for IANA Link Layer Discovery Protocol (LLDP) TLV subtype values. The recommended policy for this registry is Expert Review. The maximum number of entries in the registry is 256.

IANA is required to populate the initial registry with the value:

LLDP subtype value = 1

Description = the Manufacturer Usage Description (MUD) Uniform Resource Locator (URL)

Reference = < this document >

16. Acknowledgments

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

17. References

17.1. Normative References

17.2. Informative References

Appendix A. Changes from Earlier Versions

RFC Editor to remove this section prior to publication.

Draft -03 to -04: * add LLDP extension. * add Dan Romascanu as co-author.

Draft -02 to -03: * incorporate domain name model. * discuss extensibility. * leave placeholder for LLDP TLV.

Draft -01 to -02:

• 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

Draft -00 to -01:

• Add MASA server element

Authors' Addresses