# enable journaling
rpi ~$ sudo -Es
rpi ~# mkdir -p /var/log/journal
rpi ~# systemd-tmpfiles --create --prefix /var/log/journal #ignore warnings (*)

# disable classic networking
rpi ~# systemctl mask networking.service
rpi ~# systemctl mask dhcpcd.service
rpi ~# sudo mv /etc/network/interfaces /etc/network/interfaces~
rpi ~# sed -i '1i resolvconf=NO' /etc/resolvconf.conf

# enable systemd-networkd
rpi ~# systemctl enable systemd-networkd.service
rpi ~# systemctl enable systemd-resolved.service
rpi ~# ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

rpi ~# cat > /etc/wpa_supplicant/wpa_supplicant-wlan0.conf <<EOF
country=DE
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

ap_scan=2
network={
    ssid="RPiNet"
    mode=2
    key_mgmt=WPA-PSK
    psk="password"
    frequency=2437
}
EOF

rpi ~# systemctl enable wpa_supplicant@wlan0.service

rpi ~# cat > /etc/systemd/network/04-eth0.network <<EOF
[Match]
Name=eth0
[Network]
DHCP=yes
IPForward=yes
EOF

rpi ~# cat > /etc/systemd/network/08-wlan0.network <<EOF
[Match]
Name=wlan0
[Network]
Address=192.168.4.1/24
DHCPServer=yes
EOF

rpi ~# systemctl edit wpa_supplicant@wlan0.service

[Service]
ExecStartPre=/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
ExecStopPost=/sbin/iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE

rpi ~$ sudo ip route add 192.168.4.0/24 via 192.168.0.2 dev eth0

rpi ~$ sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

rpi ~$ sudo -Es
rpi ~# cat > /etc/systemd/network/02-br0.netdev <<EOF
[NetDev]
Name=br0
Kind=bridge
EOF

rpi ~# cat > /etc/systemd/network/04-br0_add-eth0.network <<EOF
[Match]
Name=eth0
[Network]
Bridge=br0
EOF

rpi ~# cat > /etc/systemd/network/12-br0_up.network <<EOF
[Match]
Name=br0
[Network]
DHCP=yes
EOF

rpi ~# systemctl edit wpa_supplicant@wlan0.service

[Service]
ExecStartPre=/sbin/iw dev wlan0 set type __ap
ExecStartPre=/bin/ip link set wlan0 master br0

ExecStart=
ExecStart=/sbin/wpa_supplicant -c/etc/wpa_supplicant/wpa_supplicant-%I.conf -Dnl80211,wext -i%I -bbr0

ExecStopPost=/bin/ip link set wlan0 nomaster
ExecStopPost=/sbin/iw dev wlan0 set type managed

rpi ~$ systemctl status systemd-networkd.service
rpi ~$ systemctl status wpa_supplicant@wlan0.service

rpi ~$ journalctl --boot --pager-end

rpi ~$ journalctl --boot --follow

rpi ~$ systemctl cat wpa_supplicant@wlan0.service

rpi ~$ systemctl show wpa_supplicant@wlan0.service

rpi ~$ sudo systemctl edit wpa_supplicant@wlan0.service
ExecStart=
ExecStart=/sbin/wpa_supplicant -c/etc/wpa_supplicant/wpa_supplicant-%I.conf -Dnl80211,wext -i%I -bbr0 -d