So for those trying to setup HDFS out there, and are struggling with this kind of error where it said datanode denied communication with namenode: 2015-05-07 08:04:19,694 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Block pool BP-1184863888-172.17.0.3-1430984962919 (Datanode Uuid null) service to dockernamenode.zanox.com/172.17.0.3:8020 beginning handshake with NN 2015-05-07 08:04:19,697 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: Initialization failed for Block pool BP-1184863888-172.17.0.3-1430984962919 (Datanode Uuid null) service to dockernamenode.zanox.com/172.17.0.3:8020 Datanode denied communication with namenode because hostname cannot be resolved (ip=172.17.0.5, hostname=172.17.0.5): DatanodeRegistration(0.