A firewall can restrict traffic between an operational technology (OT) network and the enterprise network, but it cannot make the permitted traffic safe by itself. A secure OT demilitarized zone (DMZ) also depends on deliberate data flows, controlled remote access, hardened systems, monitoring, and a search for connections that bypass the firewall entirely.
What an OT DMZ is meant to do
An OT DMZ is an intermediary network segment between an enterprise environment and the control systems that operate industrial processes. Its purpose is to mediate necessary exchanges—not to act as a trusted bridge that gives either side unrestricted access to the other.
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A firewall can enforce rules at a network boundary: for example, which source may reach which destination over which service. But a rule that permits traffic does not establish that the flow is necessary, correctly configured, or safe to use. Nor does filtering alone reveal misuse within the DMZ, secure a remote session, or prevent another connection from routing around the boundary.
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Design data flows instead of trusting the boundary
Start with the information each environment actually needs and its direction of travel. CISA describes a pattern in which the control zone pushes data to an application in the DMZ, and enterprise users retrieve the data from that application. This avoids making enterprise systems a direct route into control systems and reduces the trust placed across the boundary.
Map each required flow before writing firewall rules. Record its source, destination, direction, protocol or service, business or operational purpose, and owner. If a proposed rule cannot be tied to a specific need, it deserves review rather than automatic approval.
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When one-way transfer may fit
A one-way device can enforce data movement in a single direction. CISA’s defense-in-depth guidance defines a unidirectional device as one that allows data to travel in only one direction. CISA and its partners also recommend considering one-way devices where possible in their 2022 energy-sector advisory.
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Remote access needs a separate control plan
Remote access is not secured simply because a firewall separates the enterprise and control networks. Determine who needs access, what systems they need to reach, and what level of permission their role requires. Where remote connections are needed, use an authorized intermediary authentication system in the DMZ rather than allowing an uncontrolled direct path into the process-control network.
- Require multifactor authentication (MFA) and define which users may connect.
- Apply role-based access and least privilege; grant only the systems and actions needed for the job.
- Set explicit connection rules and use hardened, managed jump systems for approved sessions.
- Monitor and log remote activity so that access can be reviewed and unexpected behavior investigated.
- Assign responsibility for approving accounts, maintaining intermediary systems, and removing access that is no longer required.
CISA’s remote-access guidance emphasizes defense in depth and recognizes that control-system architectures differ. A remote-access design therefore needs to fit the site’s operational requirements rather than rely on a single security measure.
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Look for paths that bypass the firewall
A carefully written firewall policy cannot control a connection that does not pass through that firewall. CISA’s defense-in-depth guidance identifies potential alternate paths including direct modems, vendor VPNs that terminate inside the process-control network, wireless access points, and dual-network-interface machines.
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Compare the design choices against site needs
| Design choice | Security effect | Operational question |
|---|---|---|
| Bidirectional communication across a boundary | Allows traffic in both directions, so each permitted direction and service needs a defined purpose and controls. | Which return flows are genuinely required for operations? |
| One-way transfer where workable | Enforces a single direction of data movement. | Can the required exchange function without return traffic? |
| Direct remote connectivity | Can create a path into sensitive networks that is harder to constrain or review. | Is a direct path necessary, and does it bypass the intended DMZ controls? |
| DMZ-hosted intermediary for remote access | Provides a controlled point for authentication and access restrictions. | Who operates it, how are sessions authorized, and how are they monitored? |
| Firewall filtering alone | Enforces configured boundary rules but does not by itself provide traffic context or detect all misuse. | How will actual traffic be modeled, logged, and reviewed? |
| Filtering plus traffic monitoring | Adds visibility into expected and unexpected activity beyond the rule set. | Who investigates alerts and maintains the expected-traffic model? |
No single arrangement fits every control system. CISA’s remote-access guidance cautions that security depends on a defense-in-depth approach; the site’s required services, availability needs, support roles, and operational ownership all affect the design.
Quick Recap
Review an OT DMZ with this checklist
- Have you documented each required data flow, its direction, endpoints, service, and operational purpose?
- Are DMZ access and firewall rules limited to authorized users, applications, and services?
- Can control-side data be pushed to a DMZ application for enterprise retrieval instead of opening a direct enterprise-to-control path?
- Have you assessed whether one-way transfer can meet requirements?
- Does remote access use approved authentication, MFA, least privilege, hardened intermediary systems, and session monitoring?
- Have you inventoried modems, vendor tunnels, wireless access points, dual-homed devices, and other routes around the firewall?
- Is there a named owner for the DMZ, its rules, traffic monitoring, remote-access approvals, and exceptions?
- Are exceptions documented, approved, and reviewed when operational needs change?
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