How to allow root login from one IP address with ssh public keys only

in Categories CentOS, Debian / Ubuntu, FreeBSD, Linux, OpenBSD, RedHat and Friends, Suse, UNIX last updated June 10, 2017

I am using MacOS on the desktop and Ubuntu Linux on the server. I disabled root login over ssh and enabled ssh based public key login. However, I recently added second Ubuntu server. I need to sync file between two using rsync command. Is there any way that I can log from the second server into my first server with root user from second server IP address only ({root@secondBOX }# ssh root@firstBOX1) without reducing OpenSSH server security option?