Home Router

a PC with more than one ethernet devices:

• LAN: interface for internal network (10.0.0.1/24)
• WAN1: interface for main internet access (192.168.1.10/24, gateway on 192.168.1.254)
• WAN2: interface for secondary internet access (192.168.0.10/24, gateway on 192.168.0.1)
• MOBILE: emergency interface for internet access (192.168.42.10/24 gateway on 192.168.42.129)

This script:

01-nat.start

#!/bin/bash
 
source /etc/conf.d/nat
 
LAN=enp0s31f6 # internal network
WAN=
WAN_IP=
WAN_GW=
 
if [ "$D" != "" ]
then
        echo NOTICE: enabled demo mode
fi
 
if [ "$mode" = "fastweb" ]
then
        WAN=enp59s0u2u4c2
        WAN_IP=192.168.1.10
        WAN_GW=192.168.1.254
elif [ "$mode" = "vodafone" ]
then
        WAN=enp0s20f0u4u4c2
        WAN_IP=192.168.0.10
        WAN_GW=192.168.0.1
elif [ "$mode" = "mobile" ]
then
        WAN=enp0s20f0u5u3
        WAN_IP=
        WAN_GW=192.168.42.129
else
        echo ERROR: invalid value of \"$mode\" for \$mode
        echo mode must me one of: "fastweb|vodafone|mobile"
        exit 255
fi
 
if [ "$WAN" = "" ]
then
        echo invalid selection
        exit 254
fi
 
echo Extracting $WAN details...
if [ "$WAN_IP" = "" ]
then
        WAN_IP=$(ip -f inet addr show $WAN | grep -Po 'inet \K[\d.]+')
fi
echo Will use $WAN with SNAT to $WAN_IP and default gateway $WAN_GW
 
echo Flushing tables clean...
$D iptables -F
$D iptables -F -t nat
$D iptables -F -t mangle
 
echo Setting up security...
$D iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
$D iptables -A INPUT -m state --state NEW -i $LAN -j ACCEPT
$D iptables -P INPUT DROP
 
echo Enabling SNAT to $mode
$D iptables -t nat -A POSTROUTING -o $WAN -j SNAT --to $WAN_IP
 
echo Enabling IP forwarding...
echo 1 > /proc/sys/net/ipv4/ip_forward
 
echo Removing old default route...
$D ip route del to default
 
echo Setting up new default route...
$D ip route add default dev $WAN
 
echo 'All done!'

With following config file under /etc/conf.d/nat:

nat

# Valid for mode= fastweb|vodafone|mobile
mode=mobile
# unset this to actually do something:
D=echo