title: How come Docker Host & check_mk agent fail to addrress port 6556?
body: <p>I have Ubuntu 16.04 host and installed <code>check_mk agent</code> on it &amp; all was ok. Then I installed <code>docker</code> on it and now I can not hit port 6556 and <code>check_mk checks</code> are failing too. I tried per docker KB but no luck - Add iptables policies before Docker’s rules. How to fix this?</p>
  
  <p>Message:</p>
  
  <blockquote>
    <p>All of Docker’s iptables rules are added to the DOCKER chain. Do not manipulate this table manually. If you need to add rules which load before Docker’s rules, add them to the DOCKER-USER chain. These rules are loaded before any rules Docker creates automatically.</p>
  </blockquote>
  
  <p>I've tried:</p>
  
  <pre><code>sudo iptables -I DOCKER-USER -p tcp --dport 6556 -m conntrack --ctstate     NEW,ESTABLISHED -j ACCEPT
  </code></pre>