Arpwatch

sudo apt install arpwatch

edit the configuration file:

sudo nano /etc/arpwatch.conf

#to get email notifications for interface eth0:
eth0    -m ractive74@gmail.com

create a systemd service:

sudo nano /etc/systemd/system/arpwatch.conf

paste this in:

[Unit]
Description=Arpwatch daemon which keeps track of ethernet/ip address pairings
After=network.target

[Service]
Type=forking
ExecStart=/usr/sbin/arpwatch

[Install]
WantedBy=multi-user.target

delete the old init.d script:

sudo rm /etc/init.d/arpwatch