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Monitoring with Slack

You can use Slack to monitor for events that happen on the Halon MTA, like when rate limits are exceeded for example.

Setting up the webhook​

Begin by registering an incoming webhook as described here. After doing this, you'll get unique webhook URL that you can then use when posting messages to Slack.

Sending the notifications​

Below is an HSL example showing how you can post a message to Slack when a rate limit has been exceeded. Note how it's using a second rate limit to ensure that no more than one message can be generated per entry and hour. To send the messages asynchronously we're using the http-bulk plugin.

RCPT TO context
$senderdomain = $transaction["senderaddress"]["domain"];
$limit = 250;
if (rate("per-hour", $senderdomain, $limit, 3600) === false) {
    if (rate("per-hour-report", $senderdomain, 1, 3600) === true) {
        $data = ["text" => "*".gethostname().":* $senderdomain has sent more than $limit messages per hour."];
        http_bulk("slack", json_encode($data));
    }
    Defer("$senderdomain has sent more than $limit messages per hour.");
}

The http-bulk plugin can be configured as follows:

/src/config/smtpd.yaml
plugins:
  - id: http-bulk
    config:
      queues:
        - id: slack
          path: /var/log/halon/slack.jlog
          format: jsonarray
          url: "https://hooks.slack.com/services/XXXXXXXXX/XXXXXXXXXXX/XXXXXXXXXXXXXXXXXXXXXXXX"
          min_items: 1
          max_items: 1
          tls_verify: true
note

The url property should be replaced with the webhook URL you generated.