[atlas] Tagging probes which are using the ISP's DNS server?

>> Inside-AS DNS
>> Outside-AS DNS
>> RFC1918 DNS 

We probably need a "localhost DNS" too

I would still argue that the "big" public resolvers warrant their own tags.

> Apart from that, should there be separate tags for RFC1918
> and ULA?

Paraphrasing a quote from may decades ago: "probe DNS tagging is like 
math - a simple idea but it can get complicated" :-)

We could separate IPv4 and IPv6 tags in this context. Which is clearer 
but also more complicated to process. Still, for each af, probes could 
get multiple of these tags.

I believe it would be informative if we assembled some statistics about 
the current state, ie. how many probes would get what kind of tags if we 
applied them today. Perhaps Max already has some up his sleeve?

Regards,
Robert