SSH – Fix X11 Not Forwarding ($DISPLAY Not Set)

Check if Windows firewall is blocking TCP on localhost:6010.

Try the following little procedure to check if you can connect to the remote X-server (Xming) after setting up an SSH session with PuTTY. Notice port 60xx where xx is identical to the number you find in echo $DISPLAY:

$ echo $DISPLAY 
localhost:10.0
$ netstat -an | grep 6010
tcp        0      0 127.0.0.1:6010          0.0.0.0:*               LISTEN     
tcp6       0      0 ::1:6010                :::*                    LISTEN     
$ telnet 127.0.0.1:6010
telnet: could not resolve 127.0.0.1:6010/telnet: Name or service not known
$ telnet 127.0.0.1 6010
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
^]
telnet> quit
Connection closed.

When the X-server cannot be reached this is what happens:

$ telnet 127.0.0.1 6011
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused

Since $DISPLAY is correctly set and the ~/.Xauthority file is not created, this can mean that, though X11 forwarding is taken into account, xauth is not run. One reason could be that it is not in the path (I had this problem under Mac OS X, but this would be strange under Linux). You may want to do the work yourself by creating a ~/.ssh/rc file. For instance, I have the following:

if [ -n "$DISPLAY" ]; then
  echo "DISPLAY: $DISPLAY" >&2
  if read proto cookie; then
    if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
      echo add unix:`echo $DISPLAY | cut -c11-` $proto $cookie
    else
      echo add $DISPLAY $proto $cookie
    fi | $HOME/.ssh/xauth.wrapper -q -
  fi
fi

where ~/.ssh/xauth.wrapper is a wrapper to xauth that implements the locking of the ~/.Xauthority file. But you can use just xauth or the full pathname to xauth just in case... This is much like what is described in the sshd(8) man page (see Section "SSHRC").

Be careful not to do any mistake.