MITM setup

mitm_setup

# flush all iptables
iptables -F
iptables -t nat -F

# enable port forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# set NAT:  intranet --> eth1 --> MITM --> eth0 --> internet
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth1 -j ACCEPT

# route HTTP/HTTPS traffic to port 8080
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 443 -j REDIRECT --to-port 8080

# start MITM proxy on port 8080
# (-T = Transparent Mode)
mitmproxy -T 

only for this scenario:
intranet --> eth1 --> MITM --> eth0 --> internet