systemctl status knockd

Loaded: loaded (/lib/systemd/system/knockd.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Thu 2019-06-27 14:36:26 CEST; 7min ago
     Docs: man:knockd(1)
  Process: 516 ExecStart=/usr/sbin/knockd $KNOCKD_OPTS (code=exited, status=1/FAILURE)
 Main PID: 516 (code=exited, status=1/FAILURE)

Jun 27 14:36:26 pins2 systemd[1]: Started Port-Knock Daemon.
Jun 27 14:36:26 pins2 knockd[516]: could not get IP address for eth0
Jun 27 14:36:26 pins2 systemd[1]: knockd.service: Main process exited, code=exited, status=1/FAILURE
Jun 27 14:36:26 pins2 systemd[1]: knockd.service: Unit entered failed state.
Jun 27 14:36:26 pins2 systemd[1]: knockd.service: Failed with result 'exit-code'.

could not get IP address for eth0

systemctl cat knockd

# /lib/systemd/system/knockd.service
[Unit]
Description=Port-Knock Daemon
Documentation=man:knockd(1)
#Requires=network-online.target
After=network-online.target

[Service]
EnvironmentFile=-/etc/default/knockd
ExecStart=/usr/sbin/knockd $KNOCKD_OPTS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=mixed
SuccessExitStatus=0 2 15
ProtectSystem=full
CapabilityBoundingSet=CAP_NET_RAW CAP_NET_ADMIN

[Install]
WantedBy=multi-user.target
Alias=knockd.service