Provide an option to disable coredns forward plugin to upstream dns server

[liofko]

What would you like to be added:
I would like to have an option to completely disable the coredns's forward plugin to upstream DNS servers in both coredns and nodelocaldns config files (coredns-config.yml and nodelocaldns-config.yml)

Why is this needed:
In an fully offline environment, without connection to any upstream DNS server (both internally and externally as 8.8.8.8), when both
upstream_dns_servers and /etc/resolv.conf file are empty, the coredns and nodelocaldns are crashing with the following error:
"plugin/forward: no nameservers found"

[yankay]

HI @liofko

Thanks for your advice.

[liofko]

Hi @yankay
Thanks for your response.

#9502 won't add an option to disable upstream dns servers, right ?
It will only verify non-empty resolv.conf during pre-checks.

Thanks
liofko

[yankay]

Hi @yankay Thanks for your response.

#9502 won't add an option to disable upstream dns servers, right ? It will only verify non-empty resolv.conf during pre-checks.

Thanks liofko

HI @liofko

Yes, you are right. the PR is only to add a check to prevent the coredns crash.

And I think an option is also useful.
If glad to, feel free to provide a PR. :-)

Thank you very much.