Set up PAN networking on Raspbian Stretch (use sudo to create these files and run all commands)

bt-agent.service

# in /etc/systemd/system
[Unit]
Description=Bluetooth Agent
[Service]
ExecStart=/usr/bin/bt-agent -c NoInputNoOutput
Type=simple
[Install]
WantedBy=multi-user.target

bt-network.service

# in /etc/systemd/system
[Unit]
Description=Bluetoot PAN
After=pan0.network
[Service]
ExecStart=/usr/bin/bt-network -s nap pan0
ExecStartPost=bt-adapter --set Discoverable 1
Type=simple
[Install]
WantedBy=multi-user.target

pan0.netdev

# in /etc/systemd/network
[NetDev]
Name=pan0
Kind=bridge

pan0.network

# in /etc/systemd/network
[Match]
Name=pan0

[Network]
Address=172.20.1.1/24
DHCPServer=yes

setup-with-sudo.sh

apt-get install bluez-tools

# place other files

systemctl enable systemd-networkd
systemctl start systemd-networkd
systemctl enable bt-agent
systemctl start bt-agent
systemctl enable bt-network
systemctl start bt-network

bt-adapter --set Discoverable 1