$ cat /etc/ssh/sshd_config | grep X11
X11Forwarding yes
X11DisplayOffset 10
#X11UseLocalhost yes
#X11Forwarding no
$ systemctl restart sshd.service
$ systemctl stop iptables.service

OSX-Macbook_wants_Xfrom_CentOS7$ ssh -v -X root@docean.com
...
debug1: X11 forwarding requested but DISPLAY not set
...

X11Forwarding yes

ssh -X user@hostname