ajaybnl wrote:Dns server is a Domain Name Server. You just type a website address and the DNS Server will translate it to ip.
On esp boards you can point all Websites to your Captive Portal (192.168.4.1). 
The OP knows that, and has stated that he has done that. But he asks "
Are there other situations to use a dns server?"
When I thought of this question I thought of mDNS and the reason for it. To locate local devices without the use of a DNS server. But from my experience mDNS has not been as reliable as I would want. I had thought of a ESP8266 that would be a local DNS server on my network that would handle IP address lookup for my devices.
While I thought of this, I don't know enough about DNS, what conflicts I might have, to suggest this as an answer. It is something I want to look into more but it is pretty low on my priority list.
Posted: Sat Jun 15, 2019 12:58 pm