Comment 8 for bug 837954

Um... I observed that it happens because my wireless interface is down when knockd is about to start. It usually happens during boot and after the machine resumed from suspend.

I thought I'll give a try to "After=network-online.target" option for the systemd config of knockd but I didn't find any configuration. It looks like there's an old-style init script under the hood.

Finally I wrote a bash script that starts or stops knockd when the interface is up/down, made it executable and placed to /etc/NetworkManager/dispatcher.d/ - it's working now.