Dude i did the same thing. Trust me. I thought who would attack or care about my digital ocean droplet. But just after some time i was attacks happening. So many bots tried to use my DNS server to DDoS some websites they targeted. I can see the queries going from few thousands to hundreds of thousands. This also affected all the devices using the DNS because it became slow. Internet loaded slower. I tried changing a few settings and at last tried to allow only certain IP addresses i allowed. But just after a day, i cannot see any queries . I cannot see anything. It’s hijacked. This is entirely a bad idea. You must host your DNS over private network such as VPN