title: Alertmanager: Slack Notification - x509: certificate signed by unknown authority
body: <p>I am trying to configure the <code>Prometheus alert-manager</code> with <code>slack</code> for the notification. But it fails with below error.</p>
  <pre><code>alertmanager    | level=warn ts=2021-01-11T05:53:56.925Z caller=notify.go:674 component=dispatcher receiver=slack integration=slack[0] msg=&quot;Notify attempt failed, will retry later&quot; attempts=1 err=&quot;Post \&quot;&lt;redacted&gt;\&quot;: x509: certificate signed by unknown authority&quot;
  alertmanager    | level=error ts=2021-01-11T05:54:06.019Z caller=dispatch.go:309 component=dispatcher msg=&quot;Notify for alerts failed&quot; num_alerts=1 err=&quot;slack/slack[0]: notify retry canceled after 7 attempts: Post \&quot;&lt;redacted&gt;\&quot;: x509: certificate signed by unknown authority&quot;
  </code></pre>
  <p>my <code>alertmanager.yml</code> config file as follows.</p>
  <pre><code>global:
    resolve_timeout: 5m
  
  route:
    group_by: ['alertname']
    group_wait: 10s
    group_interval: 10s
    repeat_interval: 1h
    receiver: 'slack'
  receivers:
  - name: 'slack'
    slack_configs:
    - api_url: &quot;Slack webhook api&quot;
      channel: '#slack channel'
      send_resolved: true
  </code></pre>
  <p>What is the way to resolve this in alertmanager?</p>