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   ssh    ( 1 )

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X11 FORWARDING

If the ForwardX11 variable is set to 'yes' (or see the description
     of the -X, -x, and -Y options above) and the user is using X11 (the
     DISPLAY environment variable is set), the connection to the X11
     display is automatically forwarded to the remote side in such a way
     that any X11 programs started from the shell (or command) will go
     through the encrypted channel, and the connection to the real X
     server will be made from the local machine.  The user should not
     manually set DISPLAY.  Forwarding of X11 connections can be
     configured on the command line or in configuration files.

     The DISPLAY value set by ssh will point to the server machine, but
     with a display number greater than zero.  This is normal, and
     happens because ssh creates a 'proxy' X server on the server
     machine for forwarding the connections over the encrypted channel.

     ssh will also automatically set up Xauthority data on the server
     machine.  For this purpose, it will generate a random authorization
     cookie, store it in Xauthority on the server, and verify that any
     forwarded connections carry this cookie and replace it by the real
     cookie when the connection is opened.  The real authentication
     cookie is never sent to the server machine (and no cookies are sent
     in the plain).

     If the ForwardAgent variable is set to 'yes' (or see the
     description of the -A and -a options above) and the user is using
     an authentication agent, the connection to the agent is
     automatically forwarded to the remote side.