During DN registration, if the ip and the hostname in the DN InetAddress are the same (i.e. reverse name lookup falied; see the javadoc), the DN will be disallowed to register, where the DN InetAddress was obtained from the socket. It is a performance improvement added by
HDFS-3990 to avoid unnecessary DNS resolutions.
We propose to add a conf property for disabling the check. Otherwise, clusters without reverse DNS lookup support does not work.