Passwordless SSH login

ssh-keygen -t rsa
#save the file in default path (/root/.ssh/id_rsa)
Enter Passphrase ---- make it empty by press Enter
Enter again to continue

Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
02:2c:81:1b:6d:0b:32:c0:23:3c:18:6d:fb:63:b5:d2 root@testserver
The key's randomart image is:
+--[ RSA 2048]----+
|B=. |
|O==o |
|oB+oo |
|. o. .. |
| . o..S |
| = E. |
| . o |
| |
| |
+-----------------+

Copy id_rsa.pub to remote server's /root/.ssh/authorized_keys

Done!