If you tried to access your instance using SSH as root:

# ssh -i PrivateKey.pem  root@Floating-IP-Address Where Flaoting-IP-Address is the flaoting IP address associated with your instance. You might get this message: “Please login as the user “centos” rather than the user “root”.” Then the connection will close.

Steps To Allow Root Access Through SSH

1. Login as the user “centos”:

# ssh -i PrivateKey.pem  centos@Floating-IP-Address

2. Change to root:

# sudo -s

3. Edit the file “/root/.ssh/authorized_keys” (# vi /root/.ssh/authorized_keys) and keep it only contains the key (starts with “sh-rsa”) without the restrictions that exist at the begining of the file.
4. Open the file “/etc/ssh/sshd_config” and uncomment the line “PermitRootLogin yes”
5. Restart SSH daemon:

# systemctl restart sshd

Now you can SSH as root to your instance.
Note: You need to automate the above if your servers are created/ deleted on demand and SSH public keys are inserted in servers on boot by Openstack.