Why did the dns server cease to be accessible by name?

Question

Android

There is a VPS, among other things, it has its own dns server, which filters all sorts of muck. Previously, by specifying the dns server address on your phone as the domain name dns.xxxxxxx.ru, everything worked fine, but some time ago this configuration stopped working, there is no Internet until you specify the dns server ip address instead of a name. At the same time, nothing changed on the server itself, if you run nslookup dns.xxxxxxx.ru on your computer, then the ip address gives the correct one, a similar utility on the phone also gives the correct address. Why suddenly by name stopped working?

In many articles, the dns address is indicated by the name

, in general, it remains a mystery to me why it worked for almost half a year and suddenly stopped.

Answer 1

I would venture to suggest that if the DNS server was DoH, and the SSL certificate was issued by Let's encrypt, then either the server certificate itself or the root certificate is outdated. We need to renew the certificate.

Answer 2

In order to obtain an IP address from a domain name, the client must contact the DNS server by sending a request to the IP address of this server. But if it only knows the DNS server's domain name, how can it find the server's IP address?

Answer 3

DNS server is always indicated by IP address

Answer 4

I will assume that this works with dns.google, since its ip is most likely hardcoded on android smartphones.
Of course, this will not work with its own DNS, because the cache will go bad sooner or later and there will be nowhere to find out the current ip.
As a solution - add as an alternative DNS - the same Google or a specific ip