Lord Vampire, короче, ловите:
-A OUTPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p icmp -j ACCEPT
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -p all -m conntrack --ctstate NEW -m owner --uid-owner root -j ACCEPT
-A OUTPUT -p all -m conntrack --ctstate NEW -m owner --uid-owner squid -j ACCEPT
...
-A OUTPUT -p all -m conntrack --ctstate NEW -m owner --uid-owner 1000 -j ACCEPT
-A OUTPUT -j REJECT --reject-with icmp-host-prohibited