[Grml] X11 forwarding

Maurice McCarthy moss at mythic-beasts.com
Tue Jan 27 17:37:10 CET 2009

Right so Mika sorted my wlan and I've cured a sound defect myself.
There is no urgency whatsoever in this, but I'll ask out of curiosity.

The last nuisance puzzling me is X11 forwarding inside my simple home
lan. Sounds silly perhaps but I'm just learning how to network for
myself. I can X11 forward from my secure shell account based in
London: Thus

user at laptop$ ssh -fX moss at mythic-beasts.com xclock

gives me an xclock. This tells me that at least my Xserver and ssh on
the laptop must be configured right for X11 forwarding. The home lan
is simply the laptop a wireless router BTVOYAGER2110 and my trusty old
Pentium II PC. The PII is wired into the router so communication
between the laptop and the PII is through the router. There is no
longer a monitor connected to the PII.

user at laptop$ ssh -fX user at PII xclock

gives "Error: Can't open display:" followed by a blank line. ssh has
failed to set the environmental variable $DISPLAY. It is exactly the
same if I put a wire directly between PII and the laptop and configure
a static network. The laptop is running grml-2008.11 with apt pointed
at Lenny but the PII run grml-1.0 pointed at Etch.

user at laptop$ ssh -Xvvv user at PII

gives this output after accepting the password

debug2: x11_get_proto: /usr/bin/X11/xauth  list :0.0 2>/dev/null
debug1: Requesting X11 forwarding with authentication spoofing.
debug2: channel 0: request x11-req confirm 0
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug3: tty_make_modes: ospeed 38400
debug3: tty_make_modes: ispeed 38400
debug1: Sending environment.

There is then a long list of env ignored including the display

user at PII$ echo $DISPLAY Gives a blank output.

user at PII$ export DISPLAY=:10  # This is poor practice as it can obviate
                                                 # encryption but I
tried  it anyhow as
                                                 # I'm only at home.
user at PII$ xclock &

and the xclock shows up in htop but it does not display on the laptop.
So the basic problem is that ssh is not setting the display though it
ought to. I must have a misconfiguration somewhere on the PII.
.ssh/config reads thus

Host *                               # I know this in not secure.
        ForwardAgent yes
        ForwardX11 yes
        AllowTcpForwarding yes

Once again, thanks for any help

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