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A firewall can restrict which traffic crosses an OT DMZ boundary, but it cannot make the permitted traffic safe by itself. A secure design also needs deliberate data flows, controlled remote access, hardened systems, monitoring inside the zone, and checks for connections that bypass the firewall entirely.
What an OT DMZ is—and what it is for
An operational technology (OT) demilitarized zone (DMZ) is an intermediary network segment between an enterprise network and an industrial control environment. Its job is to mediate necessary exchanges, not to act as a trusted bridge between the two. CISA recommends limiting access to authorized users, applications, and services, and modeling expected traffic so defenders can monitor more than just source, destination, and service. CISA’s 2016 defense-in-depth guidance describes the DMZ as part of a layered security approach.
Why firewall rules are only one layer
A firewall enforces rules at a network boundary. It does not decide whether a permitted flow is necessary or well designed, determine whether a user is authorized for a particular task, or reveal misuse that occurs after traffic reaches a DMZ host. Nor can it protect paths that do not pass through it.
- Flow design: Rules need to reflect required applications and communication direction, not simply broad network reachability.
- Host security: A compromised or poorly maintained DMZ server can become a foothold even when its network access is restricted.
- Identity and activity: Remote users and administrators need authentication, least-privilege permissions, and oversight of their sessions.
- Visibility: Logging and monitoring help identify traffic or behavior that departs from the expected pattern.
- Complete topology: Undocumented or alternate connections can route around the intended boundary.
These controls work together; firewall rules alone cannot provide the full architecture or operating discipline that an OT environment requires.
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Design data flows to minimize trust
Start with the information an enterprise system actually needs and define how it should travel. CISA gives an example in which data is pushed from the higher-security control zone to an application in the DMZ, then enterprise users retrieve it from that application. This arrangement avoids making the enterprise network a direct path into control systems and reduces opportunities to pivot across the boundary. CISA’s guidance presents this as a way to reduce trust across the boundary, not as a universal design prescription.
For each flow, document the source, destination, purpose, protocol or service, direction, owner, and monitoring point. Permit only the communications needed for the defined purpose, and review changes when applications, vendors, or operational requirements change. A rule allowing a service is not proof that every use of that service is appropriate.
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Treat remote access as a separate security path
Remote support introduces identity, endpoint, and session risks that a perimeter rule cannot resolve. Decide who needs access, what they need to reach, and what level of privilege their work requires. Where the architecture allows it, use an authorized authentication intermediary in the DMZ rather than exposing a direct route into the control network.
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- Require multifactor authentication for remote access.
- Use role-based access and least privilege; grant only the systems and actions needed for the assigned task.
- Route sessions through hardened, managed jump systems in the DMZ, with defined connection rules.
- Log and monitor remote sessions and review access when roles or support needs change.
- Keep the intermediary systems maintained and restrict their other network connections and services.
CISA’s remote-access recommended practice emphasizes a defense-in-depth approach and notes that control-system architectures differ; no single remote-access arrangement is suitable for every site.
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Find paths that bypass the firewall
A carefully configured firewall cannot control traffic that reaches the process-control network by another route. Include physical connections, vendor access, wireless, and endpoints with multiple network interfaces in the connectivity inventory—not just the firewall’s rule set.
- Direct modems: A modem attached to a control asset can create an external route that avoids the DMZ.
- Vendor VPNs: A tunnel that terminates inside the process-control network can bypass the intended intermediary and its controls.
- Wireless access points: An access point connected to a sensitive network may provide an unreviewed entry path.
- Dual-homed machines: A host connected to both enterprise and control networks can bridge them if its configuration or security is compromised.
- Undocumented links: Temporary support connections or legacy paths may remain after their original purpose ends.
CISA’s defense-in-depth guidance identifies these kinds of alternate connectivity as risks to the intended boundary. Inventory connections, verify where they terminate, assign an owner, and remove or formally control paths that are not required.
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Choose data direction and access controls to fit operations
There is no single DMZ layout that suits every industrial process. The appropriate design depends on required services, availability needs, support roles, and whether essential communications can work with restricted directionality. The following choices illustrate the trade-offs; they are not a guarantee of security.
| Design choice | Security effect | Operational consideration |
|---|---|---|
| Bidirectional exchange through firewall rules | Allows approved traffic in both directions, but each permitted path requires justification, constraints, and monitoring. | May support applications that need two-way communication; avoid broad rules that create unnecessary reachability. |
| One-way transfer, where workable | Constrains data movement to one direction and can reduce the possibility of return traffic across the boundary. | Confirm that required services and operational workflows do not depend on communication in the opposite direction. |
| Direct remote connectivity | Can create a path around DMZ-hosted authentication and session controls. | Review whether the route is necessary and where it terminates; do not treat a VPN as sufficient protection on its own. |
| DMZ-hosted intermediary for remote access | Provides a controlled point for authentication and access restrictions, supported by hardened systems and monitoring. | Requires operational ownership, maintenance, account management, and a defined support process. |
| Firewall filtering without traffic monitoring | Enforces configured boundary rules but offers limited insight into misuse or deviations from expected patterns. | Establish expected flows and use logging and monitoring to investigate unusual activity. |
| Filtering plus traffic modeling and monitoring | Combines boundary enforcement with visibility into whether observed communications match expected behavior. | Requires staff and processes to review alerts, investigate anomalies, and maintain the expected-flow model. |
When a one-way device may help
A unidirectional network device, often called a data diode, is designed to allow data to travel in only one direction. CISA recommends considering one-way devices where possible, and its 2022 joint advisory by CISA, the FBI, and the Department of Energy also discusses their use. The advisory does not make a diode a universal requirement: the site’s communications and operational needs determine whether one-way transfer is workable. Validate that required monitoring, acknowledgments, and support functions will still operate under the chosen direction constraint.
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Review an OT DMZ as a system
A useful review examines architecture, configuration, and ownership together. Record decisions so that firewall rules and access exceptions can be assessed against actual operating needs.
- Map the zones and links. Identify enterprise, DMZ, and control networks, all connected systems, physical links, wireless equipment, modems, VPN endpoints, and dual-homed hosts.
- List required exchanges. For each data flow, record its business or operational purpose, source and destination, direction, protocol or service, and responsible owner.
- Minimize and constrain access. Remove unnecessary flows; restrict permitted connections to the required systems and services.
- Assess remote access. Identify users and vendors, confirm authentication and MFA, verify least-privilege roles, and check that sessions use authorized, hardened intermediaries where appropriate.
- Set visibility expectations. Define normal traffic patterns, log relevant activity, select monitoring points, and assign responsibility for reviewing anomalies.
- Evaluate one-way transfer. Determine whether essential workflows can operate with one-way data movement and document the reason if bidirectional communication is required.
- Govern exceptions and changes. Give each exception an owner and purpose, review it periodically, and reassess flows when systems, vendors, or operating requirements change.
Further reading
For readers seeking more detail on industrial network architecture and monitoring, Pascal Ackerman’s Industrial Cybersecurity: Efficiently Monitor the Cybersecurity Posture of Your ICS Environment, second edition, covers industrial DMZ architecture and ICS monitoring. Google Books lists the book, and Packt provides its second-edition listing. ISA’s Industrial Network Security, Second Edition covers layered defense, firewalls, VPNs, intrusion detection, and encryption; see ISA’s book page.
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