Enforce using proxy in all browsers

view story

http://serverfault.com – I've configured squid with squid with squidguard and when using proxy in browser it works fine. But I want to enforce using proxy (probably in iptables) in all browsers. Now it can be disabled in the browser settings by user. My setup is: one standalone pc with ubuntu running the squid and squidguard and on this very same device I want to somehow enforce using the proxy. Squid conf file has set: http_port 3128 transparent THX (HowTos)