I have enabled all required ports. When i enable the firewalld service then the core-dns doesnt resolve any domain-name with command $ kubectl exec -ti busybox -- nslookup kubernetes.default
This seems to be a know case, which you can find on GitHub Fresh deploy with CoreDNS not resolving any dns lookup #1056.
There seems to be few solutions which would mean different problems.
One being:
sudo systemctl stop firewalld
sudo systemctl stop firewalldPlease remember this is not recommended.
Another solution might be:
Adding iptables -p FORWARD ACCEPT.
Also check if core dns daemon controller has enough resources, as this might be causing restarts.
You need to provide more details regarding your cluster so we can pinpoint the issue.