Maybe is too late but …
If i understand correctly, you have a router for internet access, which also give DHCP for you LAN.
In this case, the DNS forwarder/resolver for Internet access is the router. All your LAN clients have the LAN IP of the router as DNS.
You have added NethServer (ONE NIC) to prvide AD services for your LAN.
In this case, the DNS resolver for your LAN is the NS but not also forwarder!
In your case, for everythink to work well, you have some options:
1 - move your DHCP services from router to NS
- set your NS NIC as follow:
IP: 192.168.X.11 (DHCP: 192.168.X.100 - 192.168.X.250)
NM: 255.255.255.0
GW: 192.168.X.1
DNS: 192.168.X.1
In this case, the NS will act as DNS forwarder/resolver for Internet access and as DNS resolver for your LAN.
Te external DNS (8.8.8.8) is set ONLY on router!
2 - keep DHCP on router
- on router, if the router has this option, add NS as new host.
- set your NS NIC as follow:
IP: 192.168.X.11
NM: 255.255.255.0
GW: 192.168.X.1
DNS: 192.168.X.1
In this case, the router will act as DNS forwarder/resolver for Internet access and as DNS resolver for your LAN.
Te external DNS (8.8.8.8) is set ONLY on router!
3 - your solution
Gabriel