Run the command below to get the information of various network interfaces.
ip addr show
Checking for firewalls
Checking for firewalld
sudo firewall-cmd --state
Checking for ufw
sudo ufw status
Checking iptables
sudo iptables -t nat -L -n -v
Checking iptables-legacy (needed if iptables-nft is installed
sudo iptables-legacy -t nat -L -n -v
DNS issues.
Currently the most common cause for DNS issues are ports. Ports 53 and 67 need to be allowed, as well as packet forwarding. this will be dependent on the firewall being used.
iptables-legacy has on a single case caused issues with DNS routing, the solution was upgrading to iptables-nft.