A basic transparent HTTP proxy that can sit between a client and HTTP servers, proxying requests from the client and rewriting the returned resource.
Much inspiration taken from sergio-proxy.
It is enabled on the gateway (in my case for a single client) by a simple iptables rule:
iptables -t nat -A PREROUTING -s $CLIENT_IP -p tcp --dport 80 -j REDIRECT --to-port 8080