SSH - Configuring SSH with auto login (without a password)

[Tested On]

CentOS Linux release 7.4.1708 (Core)

Setup ssh operations between 2 or more linux machines without the need to enter password. 

* Note that the following steps should be executed for each separate user account. In this case, I am using the root account.

On machine-1
# ssh-keygen -t rsa
Condition: Enter file in which to save the key (/root/.ssh/id_rsa): press <ENTER>
Condition: Enter passphrase (empty for no passphrase): press <ENTER>
Condition: Enter same passphrase again: press <ENTER>

# ssh-copy-id -i ~/.ssh/id_rsa.pub root@machine-2
Condition: Are you sure you want to continue connecting (yes/no)? type yes <ENTER>
Condition: root@machine-2's password: type {root password} <ENTER>

# chmod 0600 ~/.ssh/authorized_keys

On machine-2
# ssh-keygen -t rsa
Condition: Enter file in which to save the key (/root/.ssh/id_rsa): press <ENTER>
Condition: Enter passphrase (empty for no passphrase): press <ENTER>
Condition: Enter same passphrase again: press <ENTER>

# ssh-copy-id -i ~/.ssh/id_rsa.pub root@machine-1
Condition: Are you sure you want to continue connecting (yes/no)? type yes <ENTER>
Condition: root@machine-1's password: type {root password} <ENTER>

# chmod 0600 ~/.ssh/authorized_keys

After completing the setup, you will now be able to ssh between machine 1 and 2 without a prompt for password.