To connect frontend user actions (RUM) with backend performance data (traces), the Datadog RUM SDK must inject tracing headers into outgoing requests. The allowedTracingUrls configuration parameter is set within the RUM SDK during initialization. This parameter specifies a list of backend endpoints or regular expressions to which the SDK is permitted to add these headers. This ensures that tracing context is only sent to trusted, instrumented backend services, preventing accidental data leakage to third-party domains.

A. The Datadog Agent forwards data but does not configure the client-side SDK's behavior for injecting trace headers into browser requests.

C. The Datadog Tracing Library (on the backend) receives and processes tracing headers; it does not initiate them from the browser.

D. An Application Gateway is a generic network component and is not used for this specific Datadog RUM-to-APM connection configuration.

1. Datadog Documentation, "Connect RUM and Traces", Browser Monitoring. (docs.datadoghq.com/realusermonitoring/connectrumandtraces/). This document states, "To link RUM data with backend traces, the RUM SDK adds tracing headers to the requests sent to your backend. The allowedTracingUrls initialization parameter allows you to specify the backends that can receive these headers."

2. Datadog Documentation, "RUM Browser Monitoring", Setup. (docs.datadoghq.com/realusermonitoring/browser/setup/). The setup guide shows allowedTracingUrls as a key initialization parameter for the RUM SDK.