[snip]
# DISPLAY=<that thing you got> synaptic
# echo $DISPLAY
:1.0
# DISPLAY=1.0 synaptic
(synaptic:15881): Gtk-WARNING **: cannot open display: 1.0
but: $ sux - (enter root password) # synaptic
DOES WORK.
just one try:
$ echo $DISPLAY $ sux # echo $DISPLAY