service/prometheus.md

@@ -1977,6 +1977,33 @@ requests, ICMP ping, etc) for a list of targets of its own. So the Prometheus
 server has one target, the host with the port for the `blackbox_exporter`, but
 that exporter in turn is set to check other hosts.
 
+This is done with the [`relabel_config`](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#relabel_config) setting in the job
+definition, like this:
+
+      relabel_configs:
+        - source_labels:
+            - '__address__'
+          target_label: '__param_target'
+        - source_labels:
+            - '__param_target'
+          target_label: 'instance'
+        - target_label: '__address__'
+          replacement: 'localhost:9115'
+
+What this does is replace the `__address__` magic label with
+`localhost:9115` so that Prometheus, instead of trying to scrape the
+`instance` directly, will scrape the `blackbox_exporter` (here running
+on `localhost:9115`), and *pass it the `instance` through the
+`__param_target` parameter. (Actually, Prometheus converts
+`__param_target` to `target=` on the scrape job.)
+
+TODO: mention blackbox alias:
+
+```
++  - source_labels: [__param_target]
++    target_label: alias
+```
+
 The [upstream documentation][] has some details that can help. We also
 have examples [above][] for how to configure it in our setup.