Set user-wide network proxy settings on Ubuntu (without restart)

Is there any reliable way to set proxy settings **user-wide** for **all traffic** on Ubuntu? As far I'm using {http|https|ftp}_proxy variables in /home/$USER/.bashrc which is nice but there's some issues: - Many applications don't respect those settings, especially custom scripts - Restart is required to apply changes. - Doesn't work with scripts where commands are not executed in bash I need **all traffic** to go through proxy and it have to be user-wide which means I have to be able to setup different proxies for each system user profile. What comes to my mind is iptables or network-interfaces configuration but I'm not sure and I don't know how :)