Create a gateway with a transparent proxy (Iptables, Squid)

You need to have at least two network interfaces. We call them eth0 and eth1.

• eth0 brings internet
• eth1 is the organization network (usually a switch)

Accept connection from inside (eth1) and forward them to (eth0)

iptables -A FORWARD -o eth0 -i eth1 -s 192.168.1.0/24 -m conntrack --ctstate NEW -j ACCEPT

 We accept to forward all already established connection

iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

Masquerading (substitute the local source ip address to the public address)

iptables -A POSTROUTING -t nat -j MASQUERADE

Force all connection to HTTP (80) to go to 8080, where Squid can handle the request

sudo iptables -t nat -A PREROUTING -i eth1 -s 192.168.1.0/24 -p tcp --dport 80 -j REDIRECT --to-port 8080

 If you need an IP to bypass Squid :

sudo iptables -t nat -I PREROUTING 1 -i eth1 -s 192.168.1.XXX -p tcp -m tcp --dport 80 -J ACCEPT