[ssh] sync ssh key between two different machines or accounts

SRC-POINT -> DEST-POINT

1. Setting up SSH key authentication (mostly skip this step)
First, you need to make sure the DEST-POINT has the ability to user key autherntication enabled.

> vi /etc/ssh/sshd_config
RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
> /etc/init.d/sshd restart
 
2. Generation SSH public key in SRC-POINT
> ssh-keygen -t rsa
(*note: do not enter any passphrases on this, just hit enter when prompted)
 
3. Planting the public key to DEST-POINT
> vi ~/.ssh/authorized_keys
Add the public key of SRC-POINT
(*note:  user1> cat ~/.ssh/id_rsa.pub | ssh user1@DEST-POINT 'cat >> ~/.ssh/authorized_keys')
> chmod 755 ~/.ssh  (DEST-POINT)
> chmod 644 ~/.ssh/authorized_keys  (DEST-POINT)
(*note: check permissions all above directories of the "~/.ssh". If Group or Other permissions in them have "Write", sync between the two points are broken.)
 
 
* in Cluster (rocks)
In the default Rocks cluster configuration, /home is an automount directory. All nodes in the cluster system can mount the Home directory when an user accesses to each node.
Therefore, in cluster system, directories generated public key of SRC-POINT and DEST-POINT are same. "id_rsa.rsa" and "authorized_keys" files are located in same directory of an user home.
> ssh-keygen -t rsa
> cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys