> For the complete documentation index, see [llms.txt](https://help.security.gallagher.com/knowledge-hub/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.security.gallagher.com/knowledge-hub/products/cc-software/configure/interface-options/api/representational-state-transfer-rest-api/rest-api-connection.md).

# REST API - connection

Verify REST API connectivity, port access, API keys, and client certificates.

Use these checks to confirm the REST API is reachable and to read what each alarm is telling you.

## Check the service is listening

Browse to `https://<server name or ip>:8904/api/` and ignore the key and certificate at first. If the service is listening and the port is open, the Command Centre **Event Viewer** shows:

* **"A REST connection was attempted with an invalid API key"** (Details tab shows the client IP). Add the correct key and try again.
* Then **"invalid client certificate"** (Details tab shows the thumbprint and IP). This means the server is now reading your key but does not yet trust your client certificate.

Working through these two alarms in order confirms the key first, then the client certificate.

## Check the port

`Telnet <ip> 8904` checks that the port is open.

## Use the sample app

Use the **REST sample app** from the Command Centre installation media. It is easier than configuring a browser.

***

🔗 **Related**

* [REST API - What is an API Key?](/knowledge-hub/products/cc-software/configure/interface-options/api/representational-state-transfer-rest-api/rest-api-what-is-an-api-key.md). The invalid-key alarm explained.
* [REST API - Certificates](/knowledge-hub/products/cc-software/configure/interface-options/api/representational-state-transfer-rest-api/rest-api-certificates.md). The invalid-client-certificate alarm explained.
* [REST API - Logs](/knowledge-hub/products/cc-software/configure/interface-options/api/representational-state-transfer-rest-api/rest-api-logs.md). Turn on logging for deeper diagnosis.
* [REST API - Troubleshooting a Technology Partner Integration](/knowledge-hub/products/cc-software/configure/interface-options/api/representational-state-transfer-rest-api/rest-api-troubleshooting-a-technology.md). When the failing client is a Technology Partner's integration.


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