[mnordhoff]

Sharing the domain is usually critical.

Picking a random domain hosted on those nameservers, mdfs.net, it looks like, of the 4 IPs, 2 are down and 1 of the remaining ones doesn't support TCP.

http://dnsviz.net/d/mdfs.net/W48OcQ/dnssec/ https://ednscomp.isc.org/ednscomp/4040283963

1.1.1.1 is less tolerant than some resolvers of that level of breakage.

https://community.cloudflare.com/t/ipv6-timeouts-appear-to-b...

[buckminster]

Thanks for having a look. If I understand that correctly the DNS servers are returning IPv6 addresses for themselves, which aren't functioning. So he needs to get his host to stop returning the IPv6 addresses (or to fix IPv6).

[mnordhoff]

Yup. And also one of the IPv4 IPs isn't doing TCP.

I'm not sure nothing else is wrong, but the IPv6 issue is likely why 1.1.1.1 is having trouble resolving it.