This can refer to hadoop main page below for setuping passphaseless ssh:

http://hadoop.apache.org/docs/r2.6.5/hadoop-project-dist/hadoop-common/SingleCluster.html

1) install ssh on ubuntu: sudo apt-get install ssh(ssh localhost)

2) ssh-keygen(or ssh-keygen -t rsa).

3) cd ~/.ssh

4) cat id_rsa.pub >> authorized_keys.

5) chmod og-wx authorized_keys(this step is optional).

setup passphaseless ssh before installing hadoop on ubuntu