How to setup password-less ssh to the slaves?

For setting up Hadoop on a cluster of machines, the master should be able to do a password-less ssh to start the daemons on all the slaves.

Class MR - master starts TaskTracker and the DataNode on all the slaves.

MRv2 (next generation MR) - master starts NodeManager and the DataNode on all the slaves.

Here are the steps to setup password-ssh. Ensure that port 22 is open on all the slave (`telnet slave-hostname 22` should connect).

1) Install openssh-client on the master

sudo apt-get install openssh-client

2) Install openssh-server on all the slaves

sudo apt-get install openssh-server

3) Generate the ssh key

ssh-keygen -t rsa -P "" -f ~/.ssh/id_rsa

4) Copy the key to all the slaves (replace username appropriately as the user starting the Hadoop daemons). Will be prompted for the password.

ssh-copy-id -i $HOME/.ssh/id_rsa.pub username@slave-hostname

5) If the master also acts a slave (`ssh localhost` should work without a password)

cat $HOME/.ssh/id_rsa.pub >> $HOME/.ssh/authorized_keys 

If hdfs/mapreduce are run as different users then the steps (3,4 and 5) have to be repeated for all the users.

How to test ?

1) Run `ssh user@slave-hostname`. It should get connected without prompting for a password.