The DNS Relay functionality on the Arris Cable Modem/Router/whatever is a fucking disaster - it’s slow as hell and randomly stops working. I therefore set up my own DNS relay server on my LAN.

The steps are as follows:

1. Have an always-up machine with a fixed IP. If your network is 192.168.0.x, then configure your Arris router to allocate 192.168.0.10-254 IPs via DHCP, then configure the machine with a fixed IP of 192.168.0.2 via Ubuntu netplan.
2. Install bind9 on that machine and set it up according to Ubuntu DNS. We’ll use the bind dns server as Caching Nameserver: simply add your DSP DNS IPs (just google for your DSP name + ‘dns’) to the list of forwarders{} of /etc/bind/named.conf.options. Verify via sudo netstat -tnlp that named listens on 192.168.0.2:53.
3. Test the server out: from some other machine, run host -v google.com 192.168.0.2 and verify you’re getting good results. Also see Linux DNS.
4. Configure Arris to offer the new DNS via DHCP: go to its settings and make sure to:
   • allocate 192.168.0.10-254 IPs via DHCP, leaving space for the DNS machine with fixed IP 192.168.0.2
   • Disable DNS relay
   • Override DNS servers and use just one: 192.168.0.2

Everything is now done. You can use any other device to test the new network, for example use your notebook machine. Reboot the machine, to start from the clean state. The notebook should have received IP of 192.168.0.10 or larger, and should use the 192.168.0.2 DNS directly. You can check that via the NetworkManager UI, or via the command-line:

• Check the IP via ifconfig
• Check the DNS via resolvectl status; also check Linux DNS