Introduction 

OPC UA servers created with OPC Wizard can automatically register themselves with the OPC UA Local Discovery Server (LDS). The LDS is a discovery server (a special kind of OPC UA server) that maintains a list of all OPC UA servers and gateways available on the host it runs on, and makes this list available via OPC UA to other applications.

The registration with the LDS passes the following information to the LDS:

• Application (server) URI.
• Product URI.
• Descriptive name of the server.
• Type of application.
• Gateway server URI.
• Discovery URLs.
• Indication whether the server is online.
• Server capabilities.

The registration with the LDS is repeated regardless of whether it has succeeded or failed (albeit with different rates). This assures that if the LDS itself restarts and does not persist its previous state, the registration of the server will still appear in the LDS.

Controlling the Registration 

The automatic registration with the LDS is turned on by default. You can turn it off (before you call the Start Method) by setting the RegisterWithLds Property on the EasyUAServer object.

Note that even if the LDS is not present or not working properly, the operations of the OPC server created with the OPC Wizard are not negatively affected. It is therefore not necessary to disable the registration with the LDS for that reason.

Monitoring the Registration

The OPC Wizard servers use Component Event Logging. Entries for following events are generated:

• Whether the LDS registration is effectively enabled or disabled, and when enabled, the effective parameters that control the registration with the LDS.
• LDS registration failure.
• LDS registration success.

Deploying the LDS 

The OPC UA Local Discovery Server, in general, is not a single concrete piece of software. It is a concept, a specification that can be realized by anybody by fulfilling the requirements for the LDS. Multiple implementations of the OPC UA LDS exist.

The OPC Foundation provides a popular OPC UA Local Discovery implementation for Windows. It can be downloaded from the Samples and Tools area on the OPC Foundation Web site. Its accompanying documentation is also available on the Web (notice that this is not a single page, but there is a navigation pane controlling the view, placed somewhat unusually on the right side of the window, and to get access to the most of the documentation, you need to explicitly expand the "Local Discovery Server" node on the first level).

If you have installed OPC Studio using the Setup program, the link for the download of the OPC Foundation implementation of the LDS is also available from the Launcher application: Invoke the Redistributables icon, then double-click "OPC Unified Architecture Local Discovery Server (LDS)".

Security in the LDS

In the default configuration of the OPC Foundation's LDS, the LDS must explicitly trust each server that attempts to register with it. The instructions for creating the trust are in the LDS documentation (under "Trusting a UA Application"). If you do not want to be bothered with separately creating trust for each server that wants to register with the LDS (on the same machine), you can set the following parameter in the LDS configuration file:

        [General]
        AllowLocalRegistration=yes

As the comment in the LDS configuration file says, this will disable certificate trust checks for connections coming from localhost. Note that this is a security risk. Only enable this feature if you are aware of the full implications.