Cannot connect to own DNS server (pihole on Synology NAS in Docker container)

[McAledih]

Dear all,
I have setup an OpenVPN connection from my mobile (android) to a Synology NAS. Now, I would like to use the pihole container as DNS server (to block ads).

At first, I thought it should automatically be used, since my router (Fritzbox) is using the pihole as DNS server and it works fine for all local devices in the network.
But when I open up a website on the mobile phone while connected via OpenVPN, no ads were removed.
I now learned that this has to do with the fact that the Fritzbox is the DHCP-Server for the local devices, but not for the VPN-connections, where OpenVPN is assigning the IP-addresses.

So, I think I have to enter the pihole as DNS server in the configuration of the mobile app. Is that right?

The pihole container runs in host mode.
I have tried to enable "Override DNS Settings by Server" in the "IP AND DNS" Tab and entered the NAS local network IP under "DNS Server". Not sure what to enter under "searchDomain". I changed it to "local".

So far I couldn't make it work. Any ideas?

Thanks in advance!
McAledih

[McAledih]

Thanks a lot! I appreciate your help.
Yes, the NAS runs the OpenVPN server and in a docker container pihole, so that the IP-address is the same for both (192.168.1.2).
I am connecting to the NAS through the Synology dynamic dns service at xxxx.synology.me.
Here is the client config:

dev tun
tls-client

remote xxxx.synology.me 1194

# The "float" tells OpenVPN to accept authenticated packets from any address,
# not only the address which was specified in the --remote option.
# This is useful when you are connecting to a peer which holds a dynamic address
# such as a dial-in user or DHCP client.
# (Please refer to the manual of OpenVPN for more information.)

#float

# If redirect-gateway is enabled, the client will redirect it's
# default network gateway through the VPN.
# It means the VPN connection will firstly connect to the VPN Server
# and then to the internet.
# (Please refer to the manual of OpenVPN for more information.)

redirect-gateway def1

# dhcp-option DNS: To set primary domain name server address.
# Repeat this option to set secondary DNS server addresses.

#dhcp-option DNS DNS_IP_ADDRESS

pull

# If you want to connect by Server's IPv6 address, you should use
# "proto udp6" in UDP mode or "proto tcp6-client" in TCP mode
proto udp

script-security 2


comp-lzo

reneg-sec 0

# Clients running OpenVPN 2.4 and higher will automatically upgrade from AES-256-CBC to AES-256-GCM without any configuration changes.
cipher AES-256-CBC
auth SHA512

auth-user-pass
<ca>
-----BEGIN CERTIFICATE-----

Note: I tried to replace #dhcp-option DNS DNS_IP_ADDRESS by dhcp-option DNS 192.168.1.2 but that didn't work either.

I have attached the OpenVPN.conf file as a screenshot.

Please let me know if you need more information. Thanks!

[McAledih]

Thanks a lot for your detailed help. I will try your suggestions as soon as time allows.