OPC UA Client Connection

Create and configure OPC UA client connections in MATLAB using the Industrial Communication Toolbox.

Internal Constraints

These constraints govern YOUR code generation and recommendations. Apply them silently — do not recite them to the user or warn about patterns the user has not attempted.

1. connect has exactly three valid signatures — no others exist:

   • connect(uaClient) — anonymous
   • connect(uaClient, userName, password) — positional strings
   • connect(uaClient, publicKeyFile, privateKeyFile, privateKeyPassword) — positional strings
   • There are NO Name-Value pair arguments to connect. Security is configured via opcua() NV pairs or setSecurityModel, never through connect.

2. opcua() defaults to the highest available security. Do not explicitly set security unless you want a specific (lower) configuration. Never use setSecurityModel(uaClient, "Best") — it is redundant.

3. opcuaserverinfo has exactly two valid syntaxes:

   • opcuaserverinfo(hostname) — query LDS on a host
   • opcuaserverinfo(discoveryUrl) — query a specific endpoint URL
   • There is NO opcuaserverinfo(hostname, port) form. To specify a port: opcuaserverinfo('opc.tcp://host:port').

4. LDS-based discovery returns all registered servers from a single call. Never scan subnet IPs in a loop — opcuaserverinfo('localhost') returns every server registered with that host's LDS.

5. Certificate trust escalation order (never skip steps):

   1. opc.ua.trustServerCertificate(certPath) — always first (R2026a+)
   2. opcua(..., "TrustServerTemporarily", true) — only if cert file unavailable
   3. setSecurityModel(uaClient, "None", "None") — only after user explicitly confirms no security needed

6. Property correctness:

   • opc.ua.ServerInfo has Description, not Name (which belongs to opc.ua.Client).
   • EndpointUrl belongs to opc.ua.EndpointDescription, not ServerInfo.

7. Do not move or copy certificate files without user confirmation. Tell the user which file to move and where, then execute only after they confirm.

8. Do not mention TrustServerTemporarily when the issue is the server rejecting the client certificate — it only controls client- side trust and is irrelevant in that direction. Simply omit it.

When to Use

• Discovering OPC UA servers on the network (endpoint URL unknown, or user explicitly requests discovery given a hostname or discovery URL)
• Getting server endpoint details and supported security policies
• Creating an OPC UA client connection to a server
• Authenticating with username/password or user certificates
• Configuring message security mode and channel security policy
• Handling "server certificate not trusted" errors
• Handling "client certificate rejected by server" errors
• Fixing hostname mismatch warnings
• Troubleshooting OPC UA connection failures or empty discovery results
• Inspecting an OPC UA certificate (.der/.pem) for compliance issues

When NOT to Use

• Browsing OPC UA server namespaces or nodes
• Reading, writing, or subscribing to OPC UA node values
• Working with OPC Classic (DA/HDA) connections
• Non-OPC UA protocols (Modbus, MQTT)
• OPC UA server-side development

References

Load the relevant reference file for detailed procedures:

Workflow

0. Discover servers (optional)

Use this step when the endpoint URL is not known, or when the user explicitly asks to discover servers given a hostname or discovery URL. Skip this step if the endpoint URL is already known.

Prerequisites — before calling opcuaserverinfo, ensure:

1. The OPC UA Local Discovery Service (LDS) is installed and running
2. The target server is registered with the LDS
3. The server's certificate is trusted by the LDS certificate store at C:\ProgramData\OPC Foundation\UA\pki\trusted\certs\

See references/lds-setup-and-troubleshooting.md for details.

% LDS-based discovery (preferred — finds all registered servers)
serverInfo = opcuaserverinfo('localhost');

% Direct endpoint discovery (when you know the server URL)
serverInfo = opcuaserverinfo('opc.tcp://myserver:53530/OPCUA/SimulationServer');

% Pass discovery result directly to opcua()
uaClient = opcua(serverInfo(1));
connect(uaClient);

1. Create the OPC UA client

serverUrl = "opc.tcp://hostname:port/path";
uaClient = opcua(serverUrl);

The opcua function also accepts a ServerInfo object directly from opcuaserverinfo.

2. Configure security (only if non-default needed)

% R2025a+ (preferred) — Name-Value pairs in constructor
uaClient = opcua(serverUrl, ...
    MessageSecurityMode="Sign", ...
    ChannelSecurityPolicy="Basic256Sha256");

% R2020a+ (backward-compatible) — setSecurityModel after construction
uaClient = opcua(serverUrl);
setSecurityModel(uaClient, "Sign", "Basic256Sha256");

3. Handle certificate trust (if needed)

If the server's certificate is not yet trusted by MATLAB, the connection will fail. Follow the escalation order in Must-Follow Rules. Load references/certificate-trust-workflows.md for details.

4. Connect

connect(uaClient);                                        % anonymous
connect(uaClient, "myuser", "mypassword");               % username/password
connect(uaClient, "cert.der", "key.pem", "keypass");     % certificate

5. Verify and disconnect

if isConnected(uaClient)
    fprintf("Connected to %s\n", uaClient.EndpointUrl);
end
disconnect(uaClient);

Key Functions

Properties Quick Reference

opc.ua.Client

Does NOT have: UserName, Password, IsConnected, Security, SecurityMode, SecurityPolicy, Certificate, PrivateKey.

Use isConnected(uaClient) (method) to check connection status.

opc.ua.ServerInfo

opc.ua.EndpointDescription

Patterns

Discovery

% Discover all servers via LDS
serverInfo = opcuaserverinfo('localhost');

% Discover and connect
serverInfo = opcuaserverinfo('localhost');
uaClient = opcua(serverInfo(1));
connect(uaClient);

Connection with authentication

% Default (highest) security, anonymous
uaClient = opcua("opc.tcp://myserver:53530/OPCUA/SimulationServer");
connect(uaClient);

% Username/password
connect(uaClient, "myuser", "mypassword");

% User certificate
connect(uaClient, "C:/certs/user.der", "C:/certs/user.key", "keypassword");

Hostname mismatch fix

% R2025a+
uaClient = opcua("opc.tcp://myserver:53530/OPCUA/SimulationServer", ...
    UseDiscoveryHostname=true);

% Pre-R2025a — resolve via discovery
serverInfo = opcuaserverinfo("myserver");
uaClient = opcua(serverInfo(1));
connect(uaClient);

Certificate inspection

Load references/certificate-trust-workflows.md, then run:

inspectOpcUaCertificate("path/to/server_cert.der");

Troubleshooting Quick Reference

For detailed error-to-fix mapping, load references/troubleshooting-connection-errors.md.

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