Add default for dns_address in DNS_PING

Currently we require our user to fill in dns_address for DNS_PING. It can be obtained by scanning /etc/resolv.conf but in case users don't supply anything, we should use some sensible default.

Implementation examples:

• https://github.com/netty/netty/commit/61eea4e107357e3420d37b935856ff7290baff23