title: Server health check functionality of DNS server
body: <p>AWS Route 53 can do health checks of servers and thus it returns only healthy IPs as can be seen <a href="http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/dns-failover.html" rel="nofollow">here</a>.</p>
  
  <p>Which other DNS server (open source or otherwise) provides such health check functionality?</p>