Debian network interface configuration for USB tether

enp0s21f0u1

# Debian network interface configuration for USB tether
# /etc/network/interfaces.d/enp0s21f0u1

allow-hotplug enp0s21f0u1
iface enp0s21f0u1 inet dhcp
    description usb-tether
    post-up /usr/local/sbin/firewall.sh

enp3s0

# Debian network interface configuration for NUC ethernet
# /etc/network/interfaces.d/enp3s0

auto enp3s0
iface enp3s0 inet static
    address 192.168.1.1/24

firewall.sh

#!/bin/sh
# save as: /usr/local/sbin/firewall.sh
#
# based on https://debian-administration.org/article/23/Setting_up_a_simple_Debian_gateway

LAN=enp3s0
ISP=enp0s21f0u1

# Exit if any command fails.
set -e 

PATH=/usr/sbin:/sbin:/bin:/usr/bin

# delete all existing rules.
iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -X

# Always accept loopback traffic
iptables -A INPUT -i lo -j ACCEPT

# Allow established connections, and those not coming from the outside
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW ! -i $ISP -j ACCEPT
iptables -A FORWARD -i $ISP -o $LAN -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow outgoing connections from the LAN side.
iptables -A FORWARD -i $LAN -o $ISP -j ACCEPT

# Masquerade.
iptables -t nat -A POSTROUTING -o $ISP -j MASQUERADE

# Don't forward from the outside to the inside.
iptables -A FORWARD -i $ISP -o $LAN -j REJECT

# Enable routing.
echo 1 > /proc/sys/net/ipv4/ip_forward