Bash – Using BASH to determine if X11 forwarding is enabled from SSH

bashsshx11

After logging into a network via SSH, the network's default shell being used is BASH.
It is then possible to run applications which utilize graphics only if X11 forwarding has been enabled which I usually do via the -X option while logging in:

ssh -X user@network.to.connect.to

I notice that when it is enabled the DISPLAY environmental variable exists and is set as such DISPLAY=localhost:11.0, however there is nothing stopping the user from exporting an environmental variable named the same thing, making it look as if it has been enabled.

Is there a way from the bash shell or a bash script to determine if X11 Forwarding is enabled?

Best Answer

First, why? Why should a user fake DISPLAY? Nevertheless, I see two workarounds:

readonly DISPLAY in bashrc/profile. If executed right at login, the user won't be able to set DISPLAY afterwards.
xset q. Queries some X11 infos. Returns an error if no X11 session exists

Related Solutions

Ssh – Forwarding X11 not working – Error: Can’t open display: localhost:11.0

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.

Ssh – Ubuntu Windows10 App — X11 Forwarding — $DISPLAY Error

I was seeing the message debug1: X11 forwarding requested but DISPLAY not set because I was not setting the DISPLAY environment variable in the shell before connecting to the host. I am using 'Bash on Windows' with openssh.

Here is what needs to be done:

samik@mysystem:~$ export DISPLAY=localhost:0.0
samik@mysystem:~$ ssh -X samik@remotehost

Best Answer

Related Solutions

Ssh – Forwarding X11 not working – Error: Can’t open display: localhost:11.0

Ssh – Ubuntu Windows10 App — X11 Forwarding — $DISPLAY Error

Related Question