RPi_Quick_Gateway.txt

Notes, since it seems I keep losing them.

RPi Quick WiFi Gateway

Let LAN interface = enxb83442385754
Let LAN IP address= 192.168.250.1/24
Let WAN interface = wlx345263464236

1. Grab a fresh Raspbian Buster Lite image.
2. Do initial configuration. Enable predictable network interface names.
3. Enable IPv4 forwarding via /etc/sysctl.conf
4. Reboot

5. Configure static IP for ethernet interface via /etc/dhcpcd.conf.
 - Just uncomment the static IP example.
 - Change interface name accordingly (e.g. eth0 -> enxb83442385754)
 - Uncomment static ip_address=192.168.250.1/24[Your LAN address here, CIDR format]
 - I guess configuring static DNS is also cool.
 - LEAVE THE REST UNTOUCHED, ESPECIALLY STATIC ROUTERS
 
6. Reboot

7. Configure iptables NAT appropriately.
iptables --table nat --append POSTROUTING --out-interface wlx345263464236 -j MASQUERADE
iptables --append FORWARD --in-interface enxb83442385754 -j ACCEPT

    By this point, if you have a device connected to RPi's ethernet such as a
    laptop (with IP address 192.168.250.2), you should already have internet.

8. Install iptables-persistent. Answer YES when asked to save current iptables entries.
9. Reboot to test everything.

    You should still have internet access on said laptop.
    
10. Anything that needs to be connected to the Pi ethernet should have 192.168.250.x as IP address.
    In this instance, I had my own WiFi router connected, with static IP 192.168.250.33. 
    CAUTION: Technically, this is triple NAT.