@yas1 George is right, FOG is doing DNS queries mostly as part of a routine check to see if hosts are up. If you don’t like this you can disable the whole service in the web UI (FOG Configuration -> FOG Settings -> FOG Linux Service Enabled -> PINGHOSTGLOBALENABLED) or increase the service sleep/inactive time (FOG Configuration -> FOG Settings -> FOG Linux Service Sleep Times -> PINGHOSTSLEEPTIME) or you can take the big hammer and just disable the whole Linux service (systemctl stop FOGPingHosts ; systemctl disable FOGPingHosts).
I think that’s why every device in the database is shown as offline(“unknown”).
While this is related it’s not the fact that FOG cannot resolve it’s name but that it cannot connect to port 445 on your host(s). The word “ping” might be a bit misleading here, it’s not a standard ICMP ping we are doing but a kind of Windows specific check to see if TCP port 445 is available. Both checks definitely have drawbacks but some years ago the FOG devs decided to go the port 445 way and this is still the case today.
If you got hooked up by the idea to use the host status more there is also a plugin you can enable called “host status”.