Many data centers disable direct root login to servers for security reason. You need to login first as a normal user than su to login as root.
But i like direct login as root. So i disabled this extra security on my servers. If some one can get my root password, they can also get my normal users password. So what i do is use complicated root password and keep it secure.
To enable or disable root login, you need to set this in /etc/ssh/sshd_config file.
# cd /etc/sshd
# vi sshd_config
To disable direct root SSH login, you need to add following to /etc/ssh/sshd_config
PermitRootLogin no
To enable direct root SSH login, you need to add following to /etc/ssh/sshd_config
PermitRootLogin yes
After you do the changes, save and exit. Then restart sshd.
On FreeBSD
#/etc/rc.d/sshd restart
On Linux
#/etc/init.d/sshd restart