How do I add "-X" into my ssh default option? i.e., when I type ssh A.B.C.D, it should really do ssh -X A.B.C.D
I can alias it, but is there a more "elegant" solution to do that in .ssh/config file?
You need the ~/.ssh/config file ForwardX11 or ForwardX11Trusted options.
Specifically, if you want to add -X to every invocation of ssh, put something like this in your config file:
If you want to use if only for certain hosts, you need to set up a host specification for each host:
Check out the man page for ssh_config and ssh for more details.