Hi Rod and all,
Thank you so much for your help. I tried what you've suggested. It works! I am so happy for that. But there is still some warnings.
What I've done as below:
My redhat is a new server running redhat enterprise server 6. There isn't ssh2_config file. So like what I did yesterday, I uncommented a line contains 'X11Forwarding yes' in sshd_conf.
Back to Mac, in terminal, type 'xterm&'. In a X terminal, using ssh login to redhad, as Rod suggested. I can login but got two warnings:
Warning: untrusted X11 forwarding setup failed: xauth key data not generated
Warning: No xauth data; using fake authentication data for X11 forwarding.
Although I got those warnings, I still typed xterm& and opened a new xterm which comes from redhat. Then try 'xclock&'. It displayed!! So great!
Now, I have an other question. I opened 'firefox' from xterm. Firefox was also booted up, but the letters on menu and top bar were blurred. And the warnings I got were:
Xlib: extension 'RANDR' missing on display 'localhost:10.0'.
Xlib: extension 'RANDR' missing on display 'localhost:10.0'.
Xlib: extension 'RANDR' missing on display 'localhost:10.0'.
Have you ever met such problem?
This is a new problem. But I already feel so happy that you help me solved a problem which had bothered me for more than three days! Thanks all your help!