Open display error

[Sergio Brandano]

 Hi,

 I get this message on the latest Debian potato for i386.

--> ssh -f localhost xterm
sb at localhost's password: 
--> xterm Xt error: Can't open display: 

 That happens also when connecting to another linux box:

--> ssh -f box2 /usr/X11R6/bin/xterm
--> /usr/X11R6/bin/xterm Xt error: Can't open display: 

 The $DISPLAY is
 :0.0 for box1
 undefined for box2 after plain ssh connection

 If I manually set DISPLAY in box2 to box1:0.0 then I can open
 all the windows I like, provided I ``xhost +'' box1. There are two
 problems with this:

1. box2 should set the DISPLAY by itself (how?)
2. I do not want to ``xhost +'' box1, as my email client (exmh) will
   otherwise stop working.

 Also, all my xterm proc do not appear under sshd in pstree.
 How do I fix this?

 Thank you.

 Sergio