Solving 0.0.0.0 permission denied (publickey) error message on Hadoop name node

For a Hadoop cluster to work properly, all the hadoop components like NameNode, SecondaryNameNode, JobTracker and ProcessManager must be started. If you happen to create a hadoop cluster by yourself like I mentioned in my previous blog, when you start the name node by using 'start-dfs.sh', you might get the below error

'0.0.0.0 permission denied (publickey)'

This is due to the fact that, by default, the hadoop core process will try to start secondary namenode on the default ip address 0.0.0.0 which by default should be pointing to localhost. In order to do that it tries to establish an SSH connection to the ip address 0.0.0.0 and as the error message states, it fails to make the connection because the passwordless SSH is not setup correctly for the host 0.0.0.0. Below steps will help you fix this

1) If you are using a config file for passwordless ssh (mostly in case of AWS servers)

In order to setup passwordless ssh, you might have created a config file in .ssh folder of your home directory with the entries for hostname, userid and the keyfile to be used while doing ssh, as shown below

Host namenode
    User hadoopuser
    HostName namenode.xyz.com
    IdentityFile ~/.ssh/hadoop_key.pem

Host localhost
    HostName 127.0.0.1
    User hadoopuser
    IdentityFile ~/.ssh/hadoop_key.pem

Host datanode1
    User hadoopuser
    HostName datanode1.xyz.com
    IdentityFile ~/.ssh/hadoop_key.pem

Host datanode2
    User hadoopuser
    HostName datanode2.xyz.com
    IdentityFile ~/.ssh/hadoop_key.pem

Host datanode3
    User hadoopuser
    HostName datanode3.xyz.com
    IdentityFile ~/.ssh/hadoop_key.pem

just add the below entries

Host 0.0.0.0
    User hadoopuser
    IdentityFile ~/.ssh/hadoop_key.pem

and try to connect to 0.0.0.0 via ssh as below

$ ssh 0.0.0.0
The authenticity of host '0.0.0.0' can't be established.
ED25519 key fingerprint is e4:ff:65:d7:be:5d:c8:44:1d:89:6b:50:f5:50:a0:ce.
Are you sure you want to continue connecting (yes/no)? 

Type 'yes' and you should be all set.

2) If you are using ssh-keygen generated key for passwordless ssh

In this case, you dont need to do anything other than updating the known hosts
Try to connect to 0.0.0.0 via ssh as below

$ ssh 0.0.0.0
The authenticity of host '0.0.0.0' can't be established.
ED25519 key fingerprint is e4:ff:65:d7:be:5d:c8:44:1d:89:6b:50:f5:50:a0:ce.
Are you sure you want to continue connecting (yes/no)? 

Type 'yes' and you should be all set