Give every connected device a network permission envelope

Source fact: MUD describes intended communication

IETF RFC 8520 defines Manufacturer Usage Description (MUD) for purpose-built networked things. Its architecture has three core pieces: a URL that locates a description, the YANG-based JSON description of required access, and a mechanism for the local network to retrieve and interpret it. The initial focus is access control. The RFC says MUD primarily addresses threats to the device and may also limit how a compromised device threatens others, depending on how identity and the MUD URL are handled.

NIST SP 1800-15 demonstrates four example builds that use MUD to permit IoT devices to send and receive only the traffic required for intended operation. NIST combines that control with threat-intelligence blocking and automated secure updates. The guide explicitly presents possible solutions, not a single mandatory product architecture.

What the policy envelope can express

A MUD file can describe traffic to or from internet services identified by domain name, local networks, controllers defined by the administrator, the same model, or the same manufacturer authority, together with protocol and direction constraints. That is more maintainable than pretending every cloud service has one permanent IP address. It is also narrower than giving a device unrestricted outbound internet access because a vendor could not provide a firewall list.

MUD is not application inspection, vulnerability management, device authentication, or proof that every named destination is benign. A permissive description can be valid MUD syntax and still grant more access than an organization wants. DNS compromise, stolen device identity, an overly broad controller class, or an authorized cloud service compromise remain separate risks.

DSE recommendation: begin with an authoritative flow model

Ask the manufacturer whether the exact model and firmware emit or otherwise provide an RFC 8520 MUD URL and signed MUD file. Request the support period, version-change process, signing and retrieval behavior, failure mode, and human-readable documentation. If native MUD is absent, ask for authoritative communication requirements. A locally engineered allowlist can apply the same least-communication principle, but do not advertise it as manufacturer-authored MUD.

Map every supported workflow before enforcement:

device discovery, initial provisioning, certificate enrollment, DNS, DHCP, and time;

video, audio, access events, alarms, metadata, and controller communications;

VMS, access server, cloud relay, mobile application, remote support, syslog, monitoring, and backups;

software and signature updates, license checks, certificate revocation, failover, recovery, factory reset, and retirement;

local service tools and temporary commissioning access that should not remain in production.

Stage the policy like a production change

Bind policy to a trustworthy device identity or controlled network attachment, not merely an address that can be reused. Validate the MUD file’s retrieval and signature process according to the selected implementation. Translate the description into the actual enforcement points and inspect the generated rules for unsupported constructs, excessive local scope, name-resolution behavior, rule order, IPv4 and IPv6 consistency, and controller definitions.

Start with monitoring or a representative pilot. Exercise normal daytime and after-hours use, update and certificate renewal, server failover, internet outage, recorder recovery, mobile access, service dispatch, and device reboot. Investigate each denied flow against packet evidence, product documentation, and an accountable owner. Do not convert every deny into a permanent allow rule.

Govern exceptions and drift

Every exception should name the device class, exact purpose, source and destination, protocol, owner, approval, evidence, review date, and retirement condition. Keep the vendor MUD version, locally enforced version, generated network policy, and test result linked. Alert on failed MUD retrieval, signature failure, unexpected URL change, devices without a matching policy, repeated blocked destinations, and rules that cannot be installed.

When firmware, cloud endpoints, integrations, ownership, or product support changes, repeat the flow test. On retirement, remove the device binding, certificates, controller membership, generated policy, DNS entries, and temporary exceptions. MUD makes intended communication machine-readable; lifecycle governance keeps that intent trustworthy.

DSE recommends treating MUD as a specialized extension of segmentation and the firewall-rule lifecycle, not a replacement for them. It is especially valuable where thousands of similar purpose-built devices need a reviewable, repeatable permission envelope.

Official sources

IETF RFC 8520, Manufacturer Usage Description Specification

NIST SP 1800-15, complete practice guide

NIST SP 1800-15C, How-To Guides

IETF RFC 9238, Loading MUD URLs from QR Codes