I registered a couple of domains with a local hosting provider. I set up the records with the correct IP address. My servers are colocated in another data-center, not with this provider.
After filling out the necessary forms, it said:
Your changes will be visible in the next 15 minutes. For the rest of the world, it can take up to 24 hours. We have no influence on that.
This worked OK, my servers were reachable worldwide after about an hour. So far so good, everything normal, behaving as expected.
But now the strange thing happens: this hosting provider suffered from a DDoS attack. It happened two times in the last three months. The attacks put them offline for about a day, twice.
My websites were unreachable during these attacks. Even a ping, from any computer on any ISP, to my domain names didn't work, returning the message:
unknown host. My sites were perfectly reachable if the IP address was used however.
To the best of my knowledge... if some user anywhere in the world wants to connect to my website, they resolve the IP address using their ISP's DNS server. But in this case they couldn't. It appears their DNS server could not provide the correct IP address as long as my hosting provider was down.
Could it be that their DNS server actually has to resolve at my hosting provider to answer the request to resolve?