I have an Ubuntu 16.04 headless system that I recently installed xfce4 and vnc4server onto. I can connect via VNC and see the desktop, but trying to launch privileged apps such as Synaptic silently fails.

Checking the synaptic.desktop entry shows use of synaptic-pkexec, which is just a small wrapper script for using pkexec.

Running the desktop entry directly shows the issue:

$ pkexec --disable-internal-agent "/usr/sbin/synaptic" "$@"
Error executing command as another user: No authentication agent found.

Question is - how do I resolve this? I'm new to PolicyKit, and I'm a bit surprised that apt install xfce4 apparently did not setup an authentication agent. Or perhaps I'm not automatically starting the desktop properly - here's my startup script:

~$ cat .vnc/xstartup

# Uncomment the following two lines for normal desktop:
# exec /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
# x-terminal-emulator -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
x-window-manager &
startxfce4 &

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