Could not determine the nameserver IP addresses for “domain.tld”. Please make sure that the domain is registered with a valid domain registrar.

Introduction:

When adding a domain, you see the error "Could not determine the nameserver IP addresses for 'domain.tld'. 

Procedure:

Follow the steps below to troubleshoot your network connectivity to port 53.

1. Within the WHM as the root user
2. Navigate to "Home / Server Configuration / Terminal"
3. Run the following to verify if the server can communicate outbound on port 53.

   for i in {a..m}; do echo -n "$i.root-servers.net: "; dig -4 "$i".root-servers.net @"$i".root-servers.net +short;done

4. If everything is okay, you should see results similar to this:

   # for i in {a..m}; do echo -n "$i.root-servers.net: "; dig -4 "$i".root-servers.net @"$i".root-servers.net +short;done
   a.root-servers.net: 198.41.0.4
   b.root-servers.net: 170.247.170.2
   c.root-servers.net: 192.33.4.12
   d.root-servers.net: 199.7.91.13
   e.root-servers.net: 192.203.230.10
   f.root-servers.net: 192.5.5.241
   g.root-servers.net: 192.112.36.4
   h.root-servers.net: 198.97.190.53
   i.root-servers.net: 192.36.148.17
   j.root-servers.net: 192.58.128.30
   k.root-servers.net: 193.0.14.129
   l.root-servers.net: 199.7.83.42
   m.root-servers.net: 202.12.27.33

5. As a temporary workaround, you can also enable the "Unregistered Domains" option within Tweak settings.

When this command is executed on a server, it iterates through all root nameservers and attempts to retrieve information using dig. The command will return the IP addresses associated with each root nameserver if the server can communicate outbound over port 53. However, if a firewall is blocking the connection or network issues exists, you will see a timeout. Timeouts should be discussed with your network provider to ensure that your server is able to reach the root nameservers.

Additional Resources:

What nameservers should I use in my resolv.conf file?

What ports should I open in my network firewall?

How to open an port with iptables

How do I open a port in firewalld?

Tweak Settings