SSH without password in Ubuntu 16.04

Prepare
login Ubuntu by account of root.

Using RSA:

1. Change "PermitRootLogin prohibit-password" to "PermitRootLogin yes" in /etc/ssh/sshd_config.
   Then restart the service of sshd(service sshd restart)

2. Create RSA:
   ssh-keygen -trsa -P'' -f/root/.ssh/id_rsa

3. Append Pub Key to authorized file:
   ssh-copy-id -iid_rsa root@localhost
   Need input password for root once in processing.

4. Verify SSH without password:
   ssh localhost
   Success by showing SSH into without password.

Using DSA:

1. Change "PermitRootLogin prohibit-password" to "PermitRootLogin yes" in /etc/ssh/sshd_config, and add the line of "PubkeyAcceptedKeyTypes=+ssh-dss" into it.
   Then restart the service of sshd(service sshd restart)

2. Create DSA:
   ssh-keygen -tdsa -P'' -f/root/.ssh/id_dsa

3. Create the file named config at /root/.ssh.
   Then add the line of "PubkeyAcceptedKeyTypes=+ssh-dss" into it.

4. Append Pub Key to authorized file:
   ssh-copy-id -iid_dsa root@localhost
   Need input password for root once in processing.

5. Verify SSH without password:
   ssh localhost
   Success by showing SSH into without password.