dg-it-aid-debian:/home/wongdg/downloads/stable i386# wireshark
Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified


(wireshark:4927): Gtk-WARNING **: cannot open display:
dg-it-aid-debian:/home/wongdg/downloads/stable i386# karpski
Xlib: connection to ":0.0" refused by server
Xlib: No protocol specified


Gtk-WARNING **: cannot open display: :0

Could anyone tell me what is going on here?

Thanks,
Wilson

From what I can see there:
Your running as root; did you su to root? If so, that will not work as the screen is owned by the user that logged in.
So either login in GUI as root (not always permitted) or use xhost +; I saw that command somewhere related to a similar question; never used it however so not sure how to use it.

I don't know whether both wireshark and karpski are gui based or not. So tried them both in virtual terminal 1 logged in as root.

They returned fewer errors without the Xlib problem:
error Gtk WARNING **: cannot open display

I also tried apt-cache search for xhost but couldn't locate any.

Any advice?

They more than likely are as they refer to GTK. What do you mean by a virtual terminal; the console when you use i.e. <ctrl><alt><F1>?

PS
xhost should already be there; check man xhost; if it's there, xhost is also there.

xhost + is dangerous, it allows ANYONE to access the display. Better to use xhost +localhost.

Unless you've given blanket permission to a host (with xhost +hostname),
the X server refuses access to processes that can't provide the secret
credentials. For a normal user, the name of the file with the secret
credentials is held in the XAUTHORITY environment variable, which usually
looks something like this:

[sps@thyrsus-laptop ~]$ echo $XAUTHORITY
/tmp/.gdmB2TZKV

In Fedora 7, the XAUTHORITY environment variable is not normally
preserved by sudo(1), so commands run under sudo don't know where to find the credentials. You can use visudo(8) to edit /etc/sudoers to pass the XAUTHORITY environment variable to the commands it runs. Look for the
setting

Defaults env_keep = .......

Normally only the original user and root can read the $XAUTHORITY file; if
you're trying to do so from some other account, or the file is on a
root_squash NFS file system, you'll have to take other measure to allow
access to the file contents.