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

On this topic: Is there an Atlas-preferred setting for this? Should the 
probe I host ideally use the ISP's DNS, or ideally a DNS from a global 
player? Or ideally an academic/non-profit DNS?

BR
Daniel AJ



On 10/5/22 18:38, Max Grobecker wrote:
> Hi,
> 
> a few days ago I wanted to debug a name resolution problem of one of our 
> domains.
> For this reason, I wanted to test if probes inside a specific ASN are 
> having difficulties to resolve a specific name (because only customers 
> of this ISP were complaining).
> This lead to very mixed results, mostly because some of the selected 
> probes did queries to a public DNS service like Google, Quad9 and so on.
> The problem existed only with the provider's DNS servers for some reason.
> 
> 
> It did take some time to make a script which tried to filter out these 
> probes, so I wondered if anyone else had the same use-case and problem.
> Is there a way to automatically tag probes, which are (seemingly) using 
> the ISP's own DNS servers, or, at least, not a well-known public service?
> This could be done maybe by querying a special DNS name which returns 
> the IP address from where the query was received (like 
> "whoami.akamai.net").
> By comparing the ASN of the probe and the ASN of the IP address returned 
> by the DNS query, one could determine, if the ISP's servers are used.
> This would also be true for people running their own recursor, but this 
> could be filtered as well very easy.
> If an ISP is using multiple ASN, this could be a problem. Maybe there's 
> an easy solution for this as well.
> 
> Probes which pass this test, could then be tagged with 
> "DNS-using-ISP-server" or something like that and explicitly be selected 
> for specific DNS resolution tests.
> 
> 
> Greetings,
>   Max
>