Is it possible to have a subdomain from a domain without notifying the domain owner?

Question

Domain Name System

Good day. I often encounter attacks on my objects from servers like subdomain.domain-shop.ru.
Owners may not even be aware of the existence of their subdomains. What is it like? Only the human factor (given access, forgot, etc.) or is it possible to make a substitution?

There were also attacks from non-existent domains. I don't understand it at all. They don't even break through. This can exist on aliases within the network of the telecom operator (in fact, we are sitting on one large local network)

Answer 1

No, it's not real. The attack comes from the ip address, not from the domain
And which domain/subdomains are tied to this IP is secondary. There may be an overdofiga

Answer 2

You (or an attacker) can configure yourself as a PTR or specify any domain/subdomain/name in a request to the server, incl. not existing.

Answer 3

Is it possible to have a subdomain from a domain without notifying the domain owner?

No, because for a subdomain, you need to add a new A / AAAA record, as well as configure virtual hosting to work with a new subdomain. It won't work in any other way. Another question is that in many cases this process is automated, but how it is automated is a separate issue.