CoreDNS: drop all private ip PTR lookup

For some reasons, I need to stop sending reverse lookup to public DNS. This is how:

    template IN PTR in-addr.arpa {
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>[0-9]*)[.](?P<a>10*)[.]in-addr[.]arpa[.]$
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>[1][6-9]|[2][0-9]|[3][0-1]*)[.](?P<a>172*)[.]in-addr[.]arpa[.]$
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>168*)[.](?P<a>192*)[.]in-addr[.]arpa[.]$
      rcode NXDOMAIN
      fallthrough
    }

This will take care all below IP-address:

10.0.0.0 – 10.255.255.255
172.16.0.0 – 172.31.255.255
192.168.0.0 – 192.168.255.255

Instead of NXDOMAIN, we can response with custom PTR:

    template IN PTR in-addr.arpa {
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>[0-9]*)[.](?P<a>10*)[.]in-addr[.]arpa[.]$
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>[1][6-9]|[2][0-9]|[3][0-1]*)[.](?P<a>172*)[.]in-addr[.]arpa[.]$
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>168*)[.](?P<a>192*)[.]in-addr[.]arpa[.]$
      match ^(?P<d>[0-9]*)[.](?P<c>[0-9]*)[.](?P<b>169*)[.](?P<a>192*)[.]in-addr[.]arpa[.]$
      answer "{{ .Name }} 60 IN PTR host-{{ .Group.a }}-{{ .Group.b }}-{{ .Group.c }}-{{ .Group.d }}.cluster.local."
      fallthrough
    }

Done.