I have an OpenVPN server running on my Synology NAS on local IP 192.168.9.36.
I have an application on the NAS running on local IP 192.168.9.36:1234 and I have a reverse proxy that allows me to access my app over secure HTTPS (i.e. proxies https://myNAS.app to http://192.168.9.36:1234).
I also have a DNS server on the NAS running on local IP 192.168.9.36 with a local DNS record that points myNAS.app to IP 192.168.9.36.
In my local network everything is fine: I can access my app with either http://192.168.9.36:1234 and https://myNAS.app.
The problem resides when I am away from home and connect my mobile phone via an OpenVPN client to the OpenVPN server on the NAS to access my home network. In this situation I can access my app by its IP but NOT using its domain name (https://myNAS.app).
I believe OpenVPN is failing to push my local DNS server.
In my VPN client configuration file (VPNConfig.ovpn) I have the following directives:
redirect-gateway def1
dhcp-option DNS 192.168.9.36
dhcp-option DNS 8.8.8.8
When connected to the VPN and querying my current DNS server, I am told that I am using Google (8.8.8.8) which should be the secondary and not primary DNS.
What am I doing wrong? Why is 192.168.9.36 not being pushed as my primary DNS to my mobile phone over the VPN connection?
Note: all addresses are fictional, not intended to be clicked.
From comment by OP
I am using an iPhone with iOS, cellular data when connecting to the VPN. VPN is running on Synology official VPN Server package but for DNS I'm using Pi-VPN running on the NAS in a docker container. If I remove the DNS entry for Google I am unable to reach any website by domain name, either local or public on the internet. (one note: in the OpenVPN config file it says I can have 2 entries, a primary and secondary).
Best Answer
After some digging found that Pi-Hole DNS was not listening to requests being made by the VPN, only local requests. Changing the option accordingly to the image below fixed the issue and should pose not problem since Pi-Hole is behing the router, and only one port is open on it for the VPN.