Problem

You are trying to set up http endpoint monitoring with prometheus and blackbox_exporter by specifying something like the following:

...
 job_name: 'blackbox'
    metrics_path: /probe
    params:
      module: [http_2xx]
    static_configs:
      - targets: 
        - 'localhost:8080'
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: 127.0.0.1:9115
...

but you are getting the error (in prometheus targets) that connection was refused.

Solution

First specify the ip protocol as ip4 if you are not using ip6 like:

modules:
  http_2xx:
    prober: http
    timeout: 5s
    http:
      preferred_ip_protocol: "ip4"
      valid_status_codes: [200]
      method: GET

And then make sure that you use the container IP address in the replacement field if you have started blackbox_exporter as a docker container (tip taken from here), which you can find by looking for the container IP address (docker inspect blackbox_exporter | grep IPAddress).

So your prometheus configuration should look like the following (different replacement IP):

 job_name: 'blackbox'
    metrics_path: /probe
    params:
      module: [http_2xx]
    static_configs:
      - targets: 
        - 'localhost:8080'
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: 172.17.0.3:9115