title: Security group for AWS NTP Server
body: <p>I just try to synchronize time with NTP-Server on EC2 instances:</p>
  
  <blockquote>
    <p>server 0.amazon.pool.ntp.org iburst<br>
    server 1.amazon.pool.ntp.org iburst<br>
    server 2.amazon.pool.ntp.org iburst<br>
    server 3.amazon.pool.ntp.org iburst</p>
  </blockquote>
  
  <p>Unfortunately, I couldn't sync with NTP-Server. My question is:  </p>
  
  <ol>
  <li>Should we have to open ports in security group to allow NTP-Server connect to EC2 instances ? </li>
  <li>If yes, then how can we know which IP is <strong>amazon.pool.ntp.org</strong> (<em>it's changing over time</em>) and we cannot open <strong>All Traffics</strong></li>
  </ol>
  
  <p>Thanks in advance,<br>
  Toan Dao</p>