Our "catch22" situation:  we been continually getting the java.nio.channels.UnresolvedAddressException trying to do a distcp between our 2 clusters. Our setup is the following:  running CDH5, 2 separate clusters with their own internal ips and external ips do they can communicate.  The 8020 and 50010 ports are open between all of them.  I think the internal ips are the problem.   I run the following command from one of the datanodes on the namenode1 cluster distcp hdfs://namenode1:8020/somefile hdfs://namenode2:8020/ each of the namenode and datanodes on each cluster have the external ips for the other one in their /etc/hosts   The above commands fails with the following doctored error message (my system has no access to internet) .....Caused by: java.nio.channels.UnresolvedAddressException at sun.nio.ch.Net.checkAddress(Net.java:101) ...... The unresolved address appears to be the one on namenode2 cluster.  After many review of posts on similar issues it was suggested to set "dfs.client.use.datanode.hostname=false".  When I do this it gets past the above error attempts to create a file on the destination but then fails with the following error: Connection timed out at sun.nio.ch.SocketChannelImpl.checkConnect...... .... Abandoning BP-xxxxxxx- <the internal ip address>xxxxxxxx ...Excluding datanode DatanodeInfoWithStorage[<the internal ip>:50010, xxxxxx ...... Note that an empty file is created on the destination "tmp" directory.   After reviewing more posts on this error it says you NEED to have "dfs.client.use.datanode.hostname=true"  which kind of makes sense BUT puts me back to the first error.   Thanks for any assistance of where else to look.  I can provide more detail if necessary.   ... View more