Mattermost is the self-hosted open-source alternative to Slack, which can also be provided with notifications via webhook.
1. Configuring Mattermost
If you have just set up the Mattermost server, webhooks will already be active. However you can also enable them later in the Mattermost System Console — i.e. the server’s administration interface — under Integrations > Integration Management.
Another prerequisite is that you have at least one group — otherwise you will not be able to access the web client view at all. Proceed as follows:
In the group view, open Integrations from the main menu.
Create a new webhook via Incoming Webhooks > Add Incoming Webhook.
Enter a name and description as required, and select the channel for the postings with the notifications.
With these actions the webhook URL will be generated. Copy this URL for use in the next step.
2. Configuring Checkmk
You have already learned how to set up Checkmk notifications in general in the article on notifications.
As with Slack, in Checkmk it is now sufficient to specify the URL copied above:
Create new parameters for Mattermost with Setup > Events > Notifications > Parameters for notification methods > Parameters for Slack or Mattermost > Add parameter.
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Enter the copied webhook URL:
Instead of entering the URL directly here, you can also read it from the password store - provided it was previously stored there.
Optionally, you may specify URL prefixes to control links on your Checkmk GUI within the notification.
You can find out how to test the new notification method in the article on notification rules.