Internet Sharing

sharenet.sh

#!/usr/bin/env sh
#
# Internet Sharing
#
# Based on: https://wiki.archlinux.org/index.php/Internet_sharing
#

LAN=enp0s0
WAN=wlp0s0

## Static IP:
ip link set up dev $LAN
ip addr add 192.168.0.1/24 dev $LAN

## IP forwarding:
sysctl net.ipv4.ip_forward=1

## Making it permanent:
#| cat <<EOF >/etc/sysctl.d/30-ipforward.conf
#| net.ipv4.ip_forward=1
#| net.ipv6.conf.default.forwarding=1
#| net.ipv6.conf.all.forwarding=1
#| EOF

## NAT:
iptables -A FORWARD -i $LAN -o $WAN -j ACCEPT
iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE

## Making it permanent:
#| iptables-save >/etc/iptables/iptables.rules
#| systemctl enable iptables

## DHCP & DNS:
dnsmasq \
  --no-daemon \
  --local-service \
  --interface=$LAN \
  --dhcp-range=192.168.0.2,192.168.0.254,infinite \
  --dhcp-authoritative

## Making it permanent:
#| cat <<EOF >/etc/dnsmasq.conf
#| interface=$LAN
#| dhcp-range=192.168.0.2,192.168.0.254,infinite
#| dhcp-autheritative
#| EOF
#| systemctl enable dnsmasq

NOTE: apart from DHCP for IPv4, it is useful to enable IPv6 autoconfiguration on both endpoints. In case something breaks, that allows us to ssh into the other machine and fix things:

ssh -6 fe80::…%$LAN

It is also handy to have Wireshark open and keep an eye on the traffic.