Why does the Ansible uri module get 400 with valid credenitals?

We are using the uri module for querying the Graylog API the data, which is in json format.

- name: Get data
  uri:
    url: http://graylog-host:9000/api/system
    url_username: username
    url_password: password
    force_basic_auth: yes
    return_content: yes
    method: GET
    body_format: json
    validate_certs: false
  register: node_id_output

Unfortunately, with this task we get following error:

fatal: [hostname]: FAILED! => {"changed": false, "connection": "close", "content": "", "content_length": "0", "date": "Wed, 02 Sep 2020 07:21:25 GMT", "elapsed": 0, "msg": "Status code was 400 and not [200]: HTTP Error 400: Bad Request", "redirected": false, "status": 400, "url": "http://graylog-host:9000/api/system"}

Although following Curl command works fine and returns valid JSON content with status code 200:

curl -u username:password -H "Accept: application/json"  http://graylog-host:9000/api/system

What would be the cause of the error? Alternatively we can use body_format: form-urlencoded, which does not return 400, but unfortunately then the content is not parsable by json_query filter.

Solution

Using body_format: json will set the header Content-Type: application/json and send the value of the body parameter which will be null.

In your case, you want to set the header Accept: application/json to get JSON back instead of something else (XML I guess).

- name: Get data
  uri:
    url: http://graylog-host:9000/api/system
    url_username: username
    url_password: password
    force_basic_auth: yes
    return_content: yes
    method: GET
    headers:
      Accept: application/json
    validate_certs: false
  register: node_id_output