REST API how to add/delete hosts to/from a host group

Hi all,

I am using the Check_MK REST API in conjunction with our own internal CMDB, to populate hosts in Check_MK based on the information in our CMDB(which is automatically updated via a discovery mechanism). The following all works well;-
Add Hosts (including folder and label assignments)
Delete Hosts
Update Hosts (including folder and label assignments)
Perform Service Discovery
Activate Changes

What I cant seem to find is a way to add hosts to a host groups. I can create host groups with the REST API without issue, but there does not appear to be a way to add hosts to the host groups, or create/delete host group rules. Is this not possible with the V2 REST API?

Any suggestions on how I might achieve this, I need to create the rules in a script as they are dynamic in nature due to the information I extract from our CMDB.



With the REST API you can create rules in the ruleset “Assignment of hosts to host groups”. There you can assign X hosts to one host group per rule. If you want to delete a host from a group you then would need to edit the rule.

Thanks for the quick response, can you point me to the documentation for this, I cannot find anything in the REST API docs that allows me to create rules?..or have I lost the plot :rofl:

Editing rules with the REST API is possible with version 2.1 onwards.

You can just go to “Help” in the lower-left corner and go to REST API documentation. There you can find anything including “Rules” under “SETUP”.

FYI I’m running 2.1.0p4 (if the documentation differs from yours)

That explains it I was on v2.0, not v2.1…thought I was going mad :upside_down_face:


Could you please explain how this should function? In the docs I can only find GET for rulesets but no POST.

I also want to create a rule for “Assignment of hosts to host groups” but can’t find how.


What doc are you using? POST to create a rule is documented.
Just go to the i on the top left corner of your checkmk instance and click on REST API documentation

I used the checkmk built-in Swagger UI and also ReDoc but on rulesets there are no POST requests available. I can find POST only on rules but there is no “Assignment of hosts to host groups” possible like in the checkmk webfrontend (if you create a hostgroup rule manually).


Edit: I have Checkmk Raw Edition 2.2.0b5, maybe this is only available on a higher tier level?

Well, you won’t find a POST in the rulesets - because you can’t create rulesets, only rules.
The “internal” name you can use with the API for Assignment of Hosts to host groups is host_groups.

An python example would be:

#!/usr/bin/env python3
import pprint
import requests

HOST_NAME = "localhost"
SITE_NAME = "cmk"
API_URL = f"http://{HOST_NAME}/{SITE_NAME}/check_mk/api/1.0"

USERNAME = "automation"
PASSWORD = "test123"

session = requests.session()
session.headers['Authorization'] = f"Bearer {USERNAME} {PASSWORD}"
session.headers['Accept'] = 'application/json'

resp =
        "Content-Type": 'application/json',  # (required) A header specifying which type of content is in the request/response body.
        'ruleset': 'host_groups',
        'folder': '~some_folder',
        'properties': {
            'disabled': False
        'value_raw': "'the_host_group'",
        'conditions': {}
if resp.status_code == 200:
elif resp.status_code == 204:
    raise RuntimeError(pprint.pformat(resp.json()))

Thanks for your reply paulr282, but with your code I get this error:

RuntimeError: {'detail': 'These fields have problems: ruleset_name',
 'fields': {'ruleset_name': ['Missing data for required field.']},
 'status': 400,
 'title': 'Bad Request'}

In bash I get:

  "title": "Bad Request",
  "status": 400,
  "detail": "These fields have problems: value_raw",
  "fields": {
    "value_raw": [
      "Not a Python data structure: 'MyHostGroup'"

Any ideas?