Thanks all for contributing to this post. I had the same issue on Centos 7 on a 2 node hadoop cluster. In my case firewall was the cause. After allowing my namenode port (in my case 8020) through firewall on namenode machine allowed datanodes to connect to namenode. $ sudo firewall-cmd --zone=public --add-port=8020/tcp --permanent $ sudo firewall-cmd --reload , Thanks all for contributing to this post. I had the same issue on Centos 7 on a 2 node hadoop cluster. In my case firewall was the cause. After allowing my namenode port (in my case 8020) through firewall on namenode machine allowed datanodes to connect to namenode. $ sudo firewall-cmd --zone=public --add-port=8020/tcp --permanent $ sudo firewall-cmd --reload ... View more