Setup Pi as a router (share Wifi via Ethernet)

README.md

Use case

It turns a Raspberry Pi into a router:

1. Connect to internet via Wifi - this is a one time manual step.
2. Route traffic from devices connected via ethernet to internet - automatic step as long as devices are connected via ethernet port

Notice it's more like a wireless bridge router but do NAT for all connected devices.

How to use it

1. setup the wifi network on Pi - via either UI or editing wpa_supplicant.conf
2. sudo apt update & sudo apt install dnsmasq
3. save start-wifi-eth-route-2023-06-08.sh and stop-wifi-eth-route-2023-06-08.sh under /usr/local/bin and change ownership to root:

   sudo chown root:root /usr/local/bin/*-wifi-eth-route-2023-06-08.sh

4. save wifi-to-eth-route.service under /etc/systemd/system/ and enable it:

   sudo systemctl daemon-reload
   sudo systemctl enable wifi-to-eth-route

You can manually start it by running sudo systemctl start wifi-to-eth-route or stop it via sudo systemctl stop wifi-to-eth-route

Or simply reboot your Pi.

Reference

https://github.com/arpitjindal97/raspbian-recipes/blob/master/wifi-to-eth-route.sh

start-wifi-eth-route-2023-06-08.sh

#!/bin/bash
ip_address_and_network_mask_in_CDIR_notation="192.168.2.1/24"
dhcp_range_start="192.168.2.2"
dhcp_range_end="192.168.2.100"
dhcp_time="12h"
dns_server="1.1.1.1"
eth="eth0"
wlan="wlan0"

iptables -t nat -A POSTROUTING -o $wlan -j MASQUERADE
iptables -A FORWARD -i $wlan -o $eth -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i $eth -o $wlan -j ACCEPT

sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

ip link set $eth up
ip addr add  $ip_address_and_network_mask_in_CDIR_notation dev $eth 

# Remove default route created by dhcpcd
ip route del 0/0 dev $eth &> /dev/null

rm -rf /etc/dnsmasq.d/* &> /dev/null

tmpfile=$(mktemp)

echo -e "interface=$eth
bind-interfaces
server=$dns_server
domain-needed
bogus-priv
dhcp-range=$dhcp_range_start,$dhcp_range_end,$dhcp_time" > "$tmpfile"

cp "$tmpfile" /etc/dnsmasq.d/custom-dnsmasq.conf
rm "$tmpfile"

stop-wifi-eth-route-2023-06-08.sh

#!/bin/bash
ip_address_and_network_mask_in_CDIR_notation="192.168.2.1/24"
eth="eth0"
wlan="wlan0"

iptables -t nat -D POSTROUTING -o $wlan -j MASQUERADE
iptables -D FORWARD -i $wlan -o $eth -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -D FORWARD -i $eth -o $wlan -j ACCEPT

ip addr del $ip_address_and_network_mask_in_CDIR_notation dev $eth 

rm -f /etc/dnsmasq.d/custom-dnsmasq.conf

wifi-to-eth-route.service

[Unit]
Description=WiFi to eth route
After=network-online.target

[Service]
Type=oneshot
User=root
ExecStart=/usr/bin/sudo /bin/bash /usr/local/bin/start-wifi-eth-route-2023-06-08.sh
ExecStartPost=/bin/systemctl try-restart dnsmasq.service
ExecStop=/usr/bin/sudo /bin/bash /usr/local/bin/stop-wifi-eth-route-2023-06-08.sh
ExecStopPost=/bin/systemctl try-restart dnsmasq.service
RemainAfterExit=true

[Install]
WantedBy=multi-user.target