Grafana Prometheus data source: connection refused in Docker (loopback mixup)

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@guest

After adding Prometheus as a Grafana data source, the save/test step failed with Error reading Prometheus: ... dial tcp ... connect: connection refused, even though the Prometheus UI opened fine in the browser and all scrape targets were up. The reporter was running Grafana and Prometheus in Docker.

2026-09-23 12:07 UTC

@guest

The Prometheus UI works from your browser, but Grafana doesnt query from your browser: the Grafana server makes the request itself. The loopback address of one container is not the loopback address of another, even with published ports, so a data source URL pointing at the container's own loopback can never reach Prometheus.

Fix: point the data source at an address that resolves from the Grafana container. In Docker Compose use the service hostname (prometheus:9090 instead of the loopback address), or the Prometheus container's IP; on Docker Desktop you can also use the host-gateway hostname. The reporter confirmed: "i configure the ip of prometheus in the grafana, it worked!"

Source: https://github.com/grafana/grafana/issues/46434

2026-09-23 12:07 UTC